JP2008288099A - Electron gun, electron generating method, and element whose polarization state can be controlled - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electron gun capable of generating an electron gun of high quality stably, to provide an electron generating method, and to provide an element whose polarization state can be controlled desirably. <P>SOLUTION: The electron gun has a laser beam source 11, a polarization conversion element 15 to give a phase difference corresponding to an incident position to laser beam from the laser beam source 11, a condensing lens 18 of the laser beam incident via the polarization conversion element 15 from the laser beam source 11, and a photo cathode 21 into which the laser beam condensed by the lens 18 is made incident. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electron gun, an electron generation method, and a polarization control element, and more particularly to an electron gun using a photocathode, an electron generation method, and a polarization control element suitable for the electron gun.

  An electron gun as an electron source such as an X-ray free electron laser (XFEL), a femtosecond X-ray pulsed light source by inverse Compton scattering, a femtosecond time-resolved electron microscope, an ultrashort pulse electron beam drawing apparatus, an energy recovery linac (ERL) Is used. Usually, a high-intensity electron gun or an ultrashort pulse electron gun is used for these applications.

  The electron guns are classified into three types: a hot cathode electron gun, a photocathode electron gun, and a field emission electron gun. The hot cathode electron gun emits electrons by thermal energy. The photocathode electron gun emits electrons by the photoelectric effect. The field emission electron gun emits electrons by applying a high electric field (1 GV / m or more) to the cathode. Further, when the electron gun is classified from the electron acceleration method, it is divided into an RF electron gun and a DC electron gun. An RF electron gun accelerates electrons by microwaves (RF) with the help of a resonator cavity. A DC electron gun accelerates electrons in a DC manner (impulse manner) by applying a voltage between opposing electrodes. When the beam current is large, a combination of a photocathode and an RF acceleration method and a combination of a thermal cathode and a DC acceleration method are common. This is because the properties of both can be matched and the compatibility is good. Moreover, emittance, luminance, etc. are mentioned as important characteristics of an electron beam. Therefore, it is desired to develop an electron gun that can stably generate an electron beam with low emittance and high brightness.

  The photocathode RF electron gun has (1) low emittance compared to the hot cathode DC electron gun and field emission electron gun, and (2) an electron beam easily by controlling the intensity of the light source. Therefore, it has advantages such as excellent controllability. Currently, the world's brightest pulsed electron beam is obtained by a photocathode electron gun. In the photocathode electron gun, the photocathode is irradiated with laser light to generate electrons.

  In such a photocathode, it is preferable to use a material having a small work function. That is, it is possible to use laser light having a long wavelength by reducing the work function. For example, with an alkali metal, the work function is, for example, about 2 to 3 eV. However, when the alkali metal is exposed to the atmosphere, its surface deteriorates. Therefore, maintenance in the atmosphere cannot be performed.

  In addition, cathode materials such as copper and diamond that can be used stably for a long time have a high work function. Therefore, when a metal material or the like that is stable in the atmosphere is used as the photocathode material, it is necessary to shorten the wavelength of the laser. In order to emit electrons, a deep ultraviolet light source is required. Therefore, there is a problem that the laser light source is necessarily increased in size and complexity. For example, in the case of a material that is stable even when exposed to the air, the work function becomes 4 eV or more. When the work function is 4 eV or more, the necessary laser wavelength is in the ultraviolet region. Furthermore, the conventional photocathode electron gun has a problem that the dark current is large and the contrast of the electron beam is low.

  In response to the problem of increasing the size of the laser light source device, an electron gun is disclosed in which a photocathode electron gun and a field emission electron gun are compromised (Non-Patent Document 1). In this electron gun, a high electric field perpendicular to the surface of the photocathode is applied in order to reduce the work function of the photocathode. Specifically, the photocathode is needled to increase the electric field at the tip. By doing so, the work function of the tip metal is reduced and the quantum efficiency of the photocathode is increased.

  However, with a needle photocathode, electrons are emitted even when no laser is incident. Therefore, there arises a problem that the dark current becomes large. Furthermore, since the acceleration electric field is greatly expanded from the tip of the needle, the electron beam is expanded with acceleration. There is a problem that emittance deteriorates and luminance decreases. As described above, in principle, the above-described electron gun causes the problem of dark current and the problem of the shape of the accelerating electric field. Therefore, it is very difficult to put into practical use.

  Moreover, in order to obtain a high electric field without making the cathode into a needle, a very large high voltage generator such as a spark chamber is required. For this reason, it is difficult to obtain a compact RF electron gun. Moreover, in such a high voltage generator, since the operation is performed in an impulse manner, the repeated operation becomes unstable.

  In addition, it is known that the electric field necessary to increase the quantum efficiency of the metal photocathode is about 1 GV / m or more (Non-Patent Document 2). In the spark chamber, a voltage of about 1 MV (1 nsec) is the maximum. In order to achieve the required electric field (1 GV / m), an impulse high voltage must be applied between the 1 mm electrodes. Therefore, at most, electrons are accelerated only to 1 MeV. For this reason, electrons are not accelerated to the speed of light and spread by their own space charge. Therefore, in the above acceleration system, emittance is deteriorated. Furthermore, there are problems that an acceleration system of several stages is required continuously, and the apparatus configuration becomes very large and complicated.

Y. Kawashima, "Proposal of a Photocathode Impulse-Gun and Followed by Impulse Accelerating Structures to Produce Low Emittance Electron Beam," Proceedings of the 37th ICFA Advanced Beam Dynamics Workshop on Future Light Sources, WG422 M.M. J. et al. De Loos et al. , “Production of ultra short, high charge, low emission electrobunches using a 1GV / m DC gun”, Proceedings of the 1999, 68 Acc.

  As described above, the conventional electron gun has a problem that it is difficult to stably generate a high-quality electron beam.

  The present invention has been made against the background of the above circumstances, and an object of the present invention is to provide an electron gun capable of stably generating a high-quality electron beam, an electron generation method, and a desired polarization. It is to provide a polarization control element that can be controlled to a state.

An electron gun according to a first aspect of the present invention includes a laser light source, a polarization conversion element that gives a phase difference corresponding to an incident position to the laser light from the laser light source, and the laser light source through the polarization conversion element. A lens for condensing incident laser light, and a photocathode for receiving the laser light condensed by the lens are provided. Thereby, the laser beam incident on the photocathode can be in a desired polarization state. Therefore, quantum efficiency can be improved and the brightness of the electron beam can be improved.
An electron gun according to a second aspect of the present invention is the above-described electron gun, wherein the polarization conversion element linearly polarizes laser light in a substantially radial direction across the entire cross section, and in a region facing the optical axis. The electric vector is converted so that the vibration direction is opposite.

  An electron gun according to a third aspect of the present invention is the above-described electron gun, wherein the polarization conversion element has a plurality of radially divided regions, and incident light is incident on the plurality of divided regions. The wave plates are arranged so as to emit light with their phases shifted from each other, and the optical axes of the wave plates are substantially orthogonal in the opposed divided regions. This gives an electric field in the Z direction on the cathode surface. For this reason, a work function falls and the brightness | luminance of an electron beam can be improved.

  An electron gun according to a fourth aspect of the present invention is the above-described electron gun, wherein a polarization adjusting element for electrically adjusting a polarization state of the laser light on the surface of the photocathode is provided in an optical path of the laser light. It is what. Thereby, the brightness | luminance of an electron beam can be adjusted easily.

An electron gun according to a fifth aspect of the present invention is the above-described electron gun, wherein the polarization adjusting elements are arranged along the circumferential direction and a plurality of electro-optic elements arranged along the circumferential direction. A first electrode provided on the inside of the plurality of electro-optical elements, and a second electrode provided on the outside of the plurality of electro-optical elements divided radially. The axis is different from the optical axis of the adjacent electro-optic element. Thereby, the polarization state can be adjusted precisely.
An electron gun according to a sixth aspect of the present invention is the above-described electron gun, wherein the polarization adjusting element is arranged along the circumferential direction with a plurality of magneto-optical elements arranged along the circumferential direction. A first coil provided inside each of the plurality of magneto-optical elements, and a second coil provided respectively outside the plurality of magneto-optical elements arranged along the circumferential direction, The optical axes of the plurality of magneto-optical elements are different from the optical axis of the adjacent magneto-optical element.
An electron gun according to a seventh aspect of the present invention is the above-described electron gun, wherein the polarization adjusting element is arranged along the circumferential direction and a plurality of magneto-optical elements arranged along the circumferential direction. In addition, coils provided respectively on the outer circumferences of the plurality of magneto-optical elements are provided, and the optical axes of the plurality of magneto-optical elements are different from the optical axes of the adjacent magneto-optical elements.
An electron gun according to an eighth aspect of the present invention is the above-described electron gun, in which laser light is incident on the photocathode separately from the laser light from the polarization conversion element. Thereby, the brightness | luminance of an electron beam can be improved.

  An electron gun according to a ninth aspect of the present invention is the above-described electron gun, wherein the peak positions of the profiles of the two laser beams are shifted in the photocathode. Thereby, the emittance characteristics of the electron beam can be improved.

  An electron gun according to a tenth aspect of the present invention is the above-described electron gun, wherein an annular beam converting means for converting the laser light from the laser light source into an annular light beam is provided. . Thereby, the utilization efficiency of a laser beam can be made high and the brightness | luminance of an electron beam can be improved.

  An electron gun according to an eleventh aspect of the present invention is the above-described electron gun, wherein the lens has a hollow shape, and an electron beam from the photocathode passes through a hollow portion of the lens. . Thereby, disturbance to the electron beam can be reduced. Therefore, the electron beam characteristics can be improved. In addition, the lens can be installed close to the photocathode. As a result, a lens having a larger numerical aperture can be used, and the luminance of the electron beam can be increased.

An electron gun according to a twelfth aspect of the present invention is the above-described electron gun, wherein laser light from the laser light source is incident on the photocathode from the side opposite to the electron beam emission side of the photocathode. is there. Thereby, the brightness | luminance of an electron beam can be made high with a simple structure.
An electron gun according to a thirteenth aspect of the present invention is the above-described electron gun, wherein a pulsed laser beam having a pulse width shorter than 1 psec is emitted from the laser light source.

  An electron generation method according to a fourteenth aspect of the present invention is an electron generation method for generating an electron beam by irradiating a photocathode with laser light, the step of causing linearly polarized laser light to enter a polarization conversion element, and Converting the polarization state of the laser light by the polarization conversion element so that the vibration direction of the electric vector is opposite in a region facing the optical axis of the laser light; and the polarization conversion element The method includes the steps of condensing the laser beam emitted from the laser beam and causing the laser beam to enter the photocathode, and accelerating electrons emitted from the photocathode by the laser beam. Thereby, quantum efficiency can be improved and the brightness | luminance of an electron beam can be made high.

  A polarization control element according to a fifteenth aspect of the present invention is a polarization control element having an electro-optic element disposed between electrodes, the plurality of electro-optic elements arranged along a circumferential direction, and the circumferential direction A first electrode provided inside the plurality of electro-optic elements arranged along the plurality of electro-optic elements, and a second electrode provided outside the plurality of electro-optic elements divided radially. The optical axis of the electro-optical element is different from the adjacent optical axis. Thereby, it can be set as a desired polarization state.

  A polarization control element according to a sixteenth aspect of the present invention is the polarization control element described above, wherein the optical axes of the plurality of electro-optical elements are radial. Thereby, linearly polarized light can be brought into a polarization state close to radial polarized light.

  A polarization control element according to a seventeenth aspect of the present invention is the polarization control element described above, wherein the first electrode and the second electrode are provided so that an electric field is independently applied to the plurality of electro-optical elements. Is provided for each of the plurality of electro-optic elements. Thereby, a more precise adjustment becomes possible.

  According to the present invention, it is possible to provide an electron gun that can stably generate a high-quality electron beam, an electron generation method, and a polarization control element that can be controlled to a desired polarization state.

  Hereinafter, embodiments to which the present invention can be applied will be described. The following description is to describe the embodiment of the present invention, and the present invention is not limited to the following embodiment. For clarity of explanation, the following description is omitted and simplified as appropriate. Further, those skilled in the art will be able to easily change, add, and convert each element of the following embodiments within the scope of the present invention. In addition, what attached | subjected the same code | symbol in each figure has shown the same element, and abbreviate | omits description suitably.

Embodiment 1 of the Invention
An electron gun according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram schematically illustrating a configuration of an electron gun 100 according to the first embodiment. The electron gun 100 according to the present embodiment is a photocathode electron gun that generates electrons when a laser beam 50 enters the cathode. The electrons generated by the electron gun 100 are accelerated by the microwave from the microwave source 24. The electron gun 100 according to the present embodiment has a reflective photocathode. That is, the laser beam is irradiated from the electron beam emission side.

  The electron gun 100 includes a laser light source 11, a wavelength conversion element 12, an axicon lens 13, an axicon lens 14, a polarization conversion element 15, a polarization adjustment element 16, a mirror 17, a lens 18, a polarization adjustment power source 19, a photocathode 21, A resonator 22 and a microwave source 24 are included.

  The laser light source 11 emits linearly polarized laser light 50. As the laser light source 11, for example, a Ti: Sapphire laser with a regenerative amplifier can be used. Therefore, the laser light source 11 emits pulse laser light having a wavelength of 790 mm. The light beam from the laser light source 11 becomes a parallel light flux and enters the wavelength conversion element 12. The wavelength conversion element 12 is a nonlinear optical crystal, for example, and converts the wavelength of the laser light 50. Thereby, the wavelength of the laser beam 50 is shortened. As the wavelength conversion element 12, for example, a BBO crystal can be used. That is, the wavelength conversion element 12 generates a second harmonic wave having a wavelength of 395 nm from a fundamental wave having a wavelength of 790 nm. Of course, not only the second harmonic wave but also a third harmonic wave having a wavelength of 263 nm may be used. As described above, the wavelength conversion element 12 converts the wavelength of the laser light 50.

  The wavelength-converted laser beam 50 is incident on the pair of axicon lenses 13 and 14. The axicon lenses 13 and 14 have a conical shape. The laser beam 50 is refracted by the pair of axicon lenses 13 and 14 and converted into a ring-shaped beam. That is, the cross section of the laser beam 50 emitted from the axicon lens 14 is a hollow ring shape. Thus, the pair of axicon lenses 13 and 14 generate an annular beam from the laser light 50. A parallel light beam is emitted from the axicon lens 14. An annular beam may be generated with a configuration other than the pair of axicon lenses 13 and 14. For example, an annular beam can be generated by one axicon lens and one spherical lens. In this way, by using one or more axicon lenses, it is possible to prevent the laser light intensity from being lowered. Alternatively, a ring-shaped slit (ring zone) may be used. As described above, by providing the annular beam converting means for converting the laser light from the laser light source 11 into an annular light beam, the laser light 50 can be used efficiently.

  Laser light 50 emitted from the axicon lens 14 enters the polarization conversion element 15. The polarization conversion element 15 gives the laser light 50 a phase difference corresponding to the incident position. In other words, the polarization conversion element 15 delays light by a different phase depending on the incident position. The laser light 50 emitted from the polarization conversion element 15 has different polarization directions depending on the incident position on the polarization conversion element 15. As the polarization conversion element 15, for example, Zpol manufactured by Nanophoton can be used. The polarization conversion element 15 converts linearly polarized light into radial polarized light whose polarization axis is radial. Precisely, when the linearly polarized laser beam 50 is incident on the polarization conversion element 15, a polarization state close to radial polarization is obtained. That is, the linearly polarized light can be changed to pseudo radial polarized light that approximates the radial polarized light. By combining this polarization conversion element 15 with the lens 18, Z-polarized light having a large electric field component in the Z direction (optical axis direction) can be generated. The laser beam 50 converted into Z-polarized light oscillates in the traveling direction of the light. The polarization conversion element 15 and Z polarization will be described later.

  The laser beam 50 from the polarization conversion element 15 enters the polarization adjustment element 16. The polarization adjustment element 16 adjusts the polarization state of the laser light converted by the polarization conversion element 15. The polarization adjusting element 16 adjusts the position where Z polarized light having a large electric field in the Z direction is generated. Thereby, it can adjust so that Z polarization | polarized-light may be produced | generated in the photocathode surface, for example. The polarization adjusting element 16 is connected to a polarization adjusting power source 19. Then, the polarization can be electrically adjusted by controlling the voltage of the polarization adjusting power source 19. The polarization adjusting element 16 will be described later.

  The laser beam 50 that has passed through the polarization adjusting element 16 enters the mirror 17. The mirror 17 is inclined 45 ° with respect to the optical axis of the laser beam 50. Therefore, the mirror 17 reflects the laser beam 50 in the direction of the photocathode 21. Laser light 50 from the mirror 17 enters the lens 18. In the lens 18, the laser light 50 is refracted by the lens 18 and enters the photocathode 21. That is, the lens 18 condenses the laser light 50 and irradiates the photocathode 21.

  The mirror 17 and the lens 18 have a hollow shape with a central portion cut out. Further, the laser light 50 is formed in a ring shape by the axicon lenses 13 and 14. For this reason, even when the hollow mirror 17 and the lens 18 are used, most of the laser light is incident on the photocathode 21. In other words, the mirror 17 and the lens 18 have a hollow portion corresponding to the annular laser beam 50. Therefore, the annular laser beam 50 does not enter the mirror 17 and the hollow portion of the lens 18. As a result, most of the laser beam 50 is incident on the photocathode 21. Accordingly, it is possible to prevent a decrease in utilization efficiency of the laser beam 50.

  The laser beam 50 that has passed through the lens 18 enters the opening of the resonator 23. The laser beam 50 passes through the cavity portion of the resonator 23 and enters the photocathode 21. The laser beam 50 is condensed on the surface of the photocathode 21 by the lens 18. That is, the surface of the photocathode 21 is disposed at the focal position of the lens 18. Therefore, the condensing point of the laser beam 50 is the surface of the photocathode 21. When the laser beam 50 is incident on the photocathode 21, electrons are generated by the photoelectric effect. Note that the optical axis of the laser beam 50 is perpendicular to the surface of the photocathode 21. That is, the mirror 17 is inclined 45 ° with respect to the photocathode 21. Therefore, the laser beam 50 is directly incident on the photocathode 21.

  The microwave generated by the microwave source 24 is incident on the resonator 23. The resonator 23 is a cavity resonator, and generates a standing wave corresponding to the input microwave. That is, an electric field for accelerating electrons generated at the photocathode 21 is generated in the resonator 23 that is an RF resonator. Electrons generated at the photocathode 21 are accelerated by an electric field in the resonator 23. That is, the electron beam 60 having a predetermined velocity is emitted from the resonator 23. Here, the timing of the laser light pulse is adjusted according to the standing wave generated in the resonator 23. That is, the microwave from the microwave source 24 and the pulse of the laser beam are synchronized. Thereby, electrons are generated from the photocathode 21 at the timing when the accelerating electric field is generated in the resonator 23. Therefore, the electron beam 60 is efficiently accelerated. The accelerated electron beam 60 passes through the mirror 17 and the hollow portion of the lens 18. Thereby, it is possible to prevent a disturbance from occurring in the electron beam 60. That is, the electron beam 60 does not pass through structures such as the mirror 17 and the lens 18. The mirror 17 and the lens 18 do not interfere with the electron beam 60. For this reason, deterioration of the quality of the electron beam 60 can be prevented.

  The electron beam 60 thus obtained passes through a predetermined path, and is an X-ray free electron laser (XFEL), a femtosecond X-ray pulse light source by inverse Compton scattering, a femtosecond time-resolved electron microscope, an ultrashort pulse. It is used for electron beam drawing devices, energy recovery type linacs (ERL), and the like. Note that the mirror 17 and the lens 18 existing in the path of the electron beam 60 are disposed in a vacuum. That is, the mirror 17 and the lens 18 are disposed in a vacuum chamber. Accordingly, the laser beam 50 from the polarization adjusting element 16 enters the mirror 17 through the window provided in the vacuum chamber. As described above, the polarization adjusting element 16 and the polarization conversion element 15 are disposed in the atmosphere. Thereby, adjustment of an optical system etc. can be performed easily. Therefore, convenience can be improved.

  Here, by using the polarization conversion element 15 and the lens 18, an electric field in the Z direction is generated at the focal position of the laser light 50. By applying an electric field in the Z direction to the surface of the photocathode 21, a Schottky effect is generated. Since the effective work function of the photocathode 21 is reduced by the Schottky effect, the quantum efficiency is increased. Even when laser light having a long wavelength is used, a material that is stable in the atmosphere can be used as the photocathode 21. For example, copper having a work function of about 4 eV or copper diamond having a work function of about 5.5 eV can be used as the photocathode material. In other words, the photocathode 21 can be maintained in the atmosphere. Therefore, maintainability can be improved and the life of the electron gun can be extended. Furthermore, the selection range of the photocathode material can be widened. Further, since laser light having a long wavelength can be used, the laser light source 11 can be prevented from becoming large and complicated. That is, a small laser light source 11 can be used with a simple configuration.

  When a metal material is used for the photocathode 21, the photoelectric effect response is about 10 fsec. Therefore, by irradiating the laser beam 50 in synchronization with the microwave of the microwave source 24, electrons can be accelerated stably.

  In the present embodiment, the Z-polarized light is generated using the polarization conversion element 15. Electrons are generated using an electric field in the Z direction generated by the laser beam 50. That is, Z-polarized light is incident on the surface of the photocathode 21. Thereby, emittance can be improved as compared with a needled photocathode. Furthermore, since the quantum efficiency can be improved, a high-intensity electron beam 60 can be obtained. Therefore, a high-quality electron beam 60 can be generated with a simple configuration. In addition, the effective work function can be lowered. Therefore, a photocathode material that is stable in the atmosphere, such as metal or diamond, can be used. As a result, it is possible to realize the electron gun 100 with low running cost and high maintainability.

  Next, the polarization conversion element 15 for converting linearly polarized light into Z polarized light will be described with reference to FIGS. FIG. 2A is a plan view schematically showing the configuration of the polarization conversion element 15. FIG. 2B is a diagram for explaining the polarization state of the laser light 50 that has passed through the polarization conversion element 15. FIG. 2C is a diagram for explaining another polarization state of the laser light 50 that has passed through the polarization conversion element 15. FIG. 3 is a perspective view for explaining the polarization state changed by the polarization conversion element 15 and the lens 18. FIG. 4 is a side view for explaining the polarization state changed by the polarization conversion element 15 and the lens 18. 3 and 4, only the polarization conversion element 15 and the lens 18 are shown for simplification of explanation, and other components (for example, the mirror 17 and the polarization adjustment element 16) are omitted. Yes. 2 to 4, the traveling direction of the laser light 50 is the Z direction, and the plane perpendicular to the Z direction is the XY plane. The X direction and the Y direction are directions orthogonal to each other.

  First, the configuration of the polarization conversion element 15 will be described with reference to FIG. The polarization conversion element 15 is formed, for example, by providing a wave plate on a transparent substrate made of glass or the like. The polarization conversion element 15 has four regions divided radially. As shown in FIG. 2A, these four areas are defined as a divided area 15a to a divided area 15d. That is, the polarization conversion element 15 includes four divided regions 15a to 15d. Here, the divided area 15a is arranged on the upper side, the divided area 15b is arranged on the lower side, the divided area 15c is arranged on the left side, and the divided area 15d is arranged on the right side. The divided areas 15a to 15d are divided symmetrically with respect to the center point. Accordingly, the four divided regions 15a to 15d are arranged radially. As described above, the four regions divided radially are divided regions 15a to 15d. The size of each divided area is equal. The divided areas 15a to 15d are provided over the entire circumferential direction. Accordingly, each of the divided regions 15a to 15d has a sector shape with an inner angle corresponding to the center point of 90 °.

  The divided regions 15a to 15d are provided with half-wave plates having optical axes in different directions. That is, the vibration direction of light is different for each of the divided regions 15a to 15d. In FIG. 2A, the optical axes in the divided regions 15a to 15d are indicated by arrows. Here, the optical axis of each divided region is shifted by 45 ° from the optical axis of the adjacent divided region. That is, with reference to the direction of the Y axis, as shown in FIG. 2, the angle of the optical axis of the wave plate in the divided region 15a is 0 °, the optical axis of the divided region 15b is 90 °, and the optical of the divided region 15c. The axis is −45 °, and the optical axis of the divided region 15d is 45 °.

  Accordingly, the optical axes of the pair of wave plates provided in the divided regions facing each other with respect to the center point are orthogonal to each other. For example, the optical axis of the divided area 15a is 0 °, and the optical axis of the divided area 15b facing the divided area 15a is 90 °. The optical axis of the divided region 15c and the optical axis of the divided region 15d are orthogonal to each other. In other words, in the pair of wave plates provided in the divided regions facing each other, the difference in the angle of the optical axis is 90 °. As described above, the pair of divided regions arranged diagonally across the center points of the divided regions 15a to 15d are provided with the wave plates whose optical axes are different by 90 °.

  The half-wave plate emits incident light with a half-wave phase difference. Therefore, if the angle formed by the direction of the linearly polarized light with respect to the optical axis of the half-wave plate is θ, the light passing through the half-wave plate is the linearly polarized light rotated by 2θ from the original linearly polarized light. Become. For example, when the optical axis of the half-wave plate and the polarization axis of linearly polarized light are shifted by 45 °, the half-wave plate emits linearly polarized light whose polarization axis is shifted by 90 °.

  FIG. 2B shows a case where linearly polarized light having a polarization axis along the Y direction is incident. That is, when a laser beam 50 having a polarization plane parallel to the Y direction is incident, the polarization state shown in FIG. Therefore, the polarization azimuth of the outgoing light that is emitted when linearly polarized light having an incident polarization azimuth of 0 ° is described. That is, the case where the optical axis of the divided region 15a and the polarization axis of the incident light coincide with each other will be described. In FIG. 2 (b), the polarization axis of the outgoing light emitted from each divided region is indicated by arrows. The polarization axes of the linearly polarized light emitted from the divided regions 15a to 15d are radial.

  Specifically, the polarization axes of linearly polarized light emitted from a pair of divided regions facing the center point are parallel. And a vibration direction is opposite in a pair of division area which opposes. Further, the polarization axis of the light emitted from the adjacent divided regions is shifted by 90 °. For example, the polarization axis of the light emitted from the divided region 15a and the divided region 15b is 0 °. Further, the polarization axis of the light emitted from the divided regions 15c and 15d is 90 °. Therefore, the angle of the polarization axis changes according to the incident position, and the outgoing polarization displacement becomes radial. Thus, the polarization conversion element 15 changes the polarization state of the incident light in accordance with the incident position, and controls to obtain a desired polarization state.

  By making linearly polarized light incident on the polarization conversion element 15, it is possible to control the polarization state to be close to radial polarization. Specifically, the optical axis of the laser beam 50 and the center point of the polarization conversion element 15 are matched. Then, the optical axis of the divided region 15a is matched with the polarization axis of linearly polarized light. By doing in this way, linearly polarized light can be changed to pseudo radial polarized light that approximates radial polarized light. In addition, when the linearly polarized light whose polarization axis is in the X direction is incident on the polarization conversion element 15, the polarization axis has a shape close to a circle. Therefore, a polarization state close to azimuth polarization can be obtained. That is, pseudo azimuth polarized light approximate to azimuth polarized light can be obtained. The distribution of the polarization axes on the XY plane at this time is as shown in FIG. In the above description, the 4-division polarization conversion element 15 has been described, but the number of divisions is not limited to this. For example, a two-divided or eight-divided polarization conversion element 15 can be used.

  By increasing the number of divisions of the polarization conversion element 15, the polarization state can be made closer to radial polarization or azimuth polarization. In other words, increasing the number of divided regions changes the polarization axis more smoothly. In other words, when the number of divisions is infinite, an ideal radial polarization state or an ideal azimuth polarization state can be generated. Furthermore, in order to increase the Z component of the electric field vector, the number of divisions is preferably 8 or more, and more preferably 16 or more.

  Specifically, for example, when the number of divisions is 16, the optical axis of the wave plate is shifted by 11.25 ° from the adjacent division region. As a result, the optical axes are orthogonal to each other in the divided areas facing each other. When a polarization axis having a certain angle is incident on the polarization conversion element 15, the linearly polarized light is emitted as pseudo radial polarized light or pseudo azimuth polarized light.

  Next, the point that Z polarization is generated by combining the polarization conversion element 15 and the lens 18 will be described. As shown in FIGS. 3 and 4, pseudo-radial polarization is generated by the polarization conversion element 15. That is, as shown in FIG. 2B, the vibration direction is opposite by 180 ° in the opposed divided regions. That is, the polarization axis is radial by passing through the polarization conversion element 15. The laser beam 50 in such a polarization state is condensed by the lens 18.

  When the polarization conversion element 15 is disposed on the optical path, a phase shift occurs between the light transmitted through the upper divided region 15a and the light transmitted through the lower divided region 15b. That is, the phase of light is shifted by 180 ° between the divided region 15a and the divided region 15b that are arranged to face each other vertically. Assuming that linearly polarized light is output from the laser beam 50, the phases of the orthogonal components of the electric vectors are in agreement. When linearly polarized light passes through the polarization conversion element 15, the phase of the electric vector is shifted by 180 ° between the divided region 15a and the divided region 15b. That is, the vibration direction of the electric vector is opposite between the upper divided area 15a and the lower divided area 15b. In the upper divided region 15a and the lower divided region 15b, the polarization directions are opposite to each other. That is, the light transmitted through the upper divided region 15a and the light transmitted through the lower divided region 15b are linearly polarized light on the same straight line, but their vibration directions are opposite.

  Next, a method for generating Z-polarized light will be described with reference to FIGS. 3 and 4. The arrows in FIGS. 3 and 4 schematically show the vibration direction of the electric vector at that position. As described above, since the laser light before passing through the polarization conversion element 15 is linearly polarized light, the electric vectors all vibrate in the same direction (Y direction). Then, by passing through the polarization conversion element 15, the vibration direction of the electric vector changes according to the position. As shown in FIG. 4, the electric vector of the light transmitted through the upper divided region 15a oscillates upward. On the other hand, the electric vector of the light transmitted through the lower divided region 15b oscillates downward. In FIG. 4, the vibration direction of light passing through the center is shown as an upward direction for the sake of explanation.

  Next, the state where the light transmitted through the polarization conversion element 15 is condensed on the sample by the lens 18 will be described in detail with reference to FIG. Here, the vibration direction of the electric vector of light is considered by dividing it into a component in the direction perpendicular to the traveling direction of the light (Y direction) and a component in the direction parallel to the direction (Z component). In FIG. 3, a direction perpendicular to the traveling direction of light (Y direction) is defined as an up-down direction, and a direction parallel to the traveling direction of light (Z direction) is defined as a left-right direction.

  The light transmitted through the upper divided region 15a is refracted by the lens 18 so as to be inclined downward. Therefore, the vibration direction of the electric vector of light is diagonally upward to the right as shown in FIG. Since the light transmitted through the center is not refracted by the lens 18, the vibration direction remains as it is upward. The light transmitted through the lower divided region 15b is refracted by the lens 18 so as to be inclined upward. Therefore, the vibration direction of the electric vector of light is diagonally downward to the right. In this way, light having different vibration directions depending on the position is collected on the sample.

  After passing through the lens 18, the vibration direction of the electric vector is diagonally right upward in the upper divided area 15a and diagonally lower right in the lower divided area 15b, so the components in the vertical direction are opposite to each other. Thereby, in the state condensed on the photocathode 21, the vertical components in the vibration direction of the electric vector cancel each other. Therefore, the electric vector component in the direction perpendicular to the light traveling direction is almost zero. That is, the electric vector of light does not vibrate in the direction perpendicular to the traveling direction on the sample.

  On the other hand, the vibration direction of the electric vector is diagonally right upward in the upper divided area 15a and diagonally lower right in the lower divided area 15b, so that the left and right components are the same right direction. As a result, the horizontal component of the electric vector is strengthened by the upper divided region 15a and the lower divided region 15b. Accordingly, the electric vector component parallel to the light traveling direction is emphasized in the right direction. That is, the electric vector of light is oscillating in a direction parallel to the traveling direction. By condensing the laser beam whose phase is shifted by the polarization conversion element 15 with the lens 18 in this way, the laser beam 50 is irradiated on the photocathode 21 in a state where the electric vector vibrates in a direction parallel to the traveling direction. Can do. The component in the Z direction changes to the focal length and NA (numerical aperture) of the lens 18. That is, the component in the Z direction can be increased by using the lens 18 having a short focal length and a large NA.

  In this way, the Z-polarized laser light 50 is incident on the photocathode 21. Therefore, a strong electric field is generated in the Z direction on the photocathode surface. Thereby, the effective work function of the photocathode 21 can be reduced. For example, when an electric field of about 1 GV / m is generated on the photocathode surface, the effective work function can be lowered in units of eV. Therefore, even when a stable metal material such as copper is used for the cathode material, electrons are generated by the laser light having a long wavelength. The quantum efficiency can be improved. For example, the beam diameter is 30 mm, the focal length of the lens 18 is 300 mm, and the laser wavelength is 263 nm. In this case, if the laser beam has a peak intensity of 11 MW, an electric field of 1 GV / m is generated. That is, when laser light having a peak intensity of 11 MW is converted into radial polarized light, an electric field of 1 GV / m is generated at the focal point. In addition, said value is a calculation result.

  The above calculation will be described below. First, the relationship between the light intensity I and the electric field E is given by equation (1).

  When the X-polarized laser beam is collected by the lens, the X-polarized light intensity Ix at the focal position is given by the equation (2) by the power P of the light incident on the lens.

  Here, a is the diameter of the lens, f is the focal length of the lens, and λ is the wavelength of light. In addition, the equation (2) Born AND E.E. WOLF, “Principles of optics,” 7Th ed. , Cambridge University Press "8.8 The three-dimensional light distribution near focus" can be derived from Equation 22 and Equation 33.

  Here, the intensity of the Z-polarized light component at the focal point when the eight-divided polarization conversion element 15 is used is Iz. Further, the intensity of the X-polarized component when the 8-divided polarization conversion element 15 is not used is Ix. Note that Iz and Ix are intensities at the focal point of a lens having a diameter of 30 mm and a focal length of 300 mm, respectively. The ratio between the intensity Iz of the Z-polarized component and the intensity Ix of the X-polarized component is as shown in equation (3).

  Equation (4) is obtained from Equations (1) to (3).

Here, Ez = 1 × 10 ⁹ , ε = 8.85 × 10 ⁻¹² , c = 3.00 × 10 ⁸ , λ = 2.63 × 10 ⁻⁷ , f = 0.300, a = 1.5 Substituting × 10 ⁻² into Equation 4. Then, the power required to obtain an electric field of 1 GV / m is P = 1.1 × 10 ⁷ (W). By using 11 MW laser light, the work function can be lowered. The intensity of the Z-polarized component at the focal point is inversely proportional to the square of the laser wavelength. Therefore, an electric field of 1 GV / m can be obtained by using a 25 MW laser beam when the wavelength is 395 nm, and a 100 MW laser beam when the wavelength is 790 nm, and the work function can be lowered.

  Next, an example of a specific configuration of the polarization conversion element 15 will be described with reference to FIG. FIG. 5A is a plan view showing the configuration of the polarization conversion element 15, and FIG. 5B is a side sectional view. FIG. 5C is a diagram schematically illustrating a wave plate used for the polarization conversion element 15. In FIG. 5, unlike in FIGS. 2 to 4, an eight-divided polarization conversion element 15 is shown. Therefore, as shown in FIG. 5C, eight wave plates are prepared corresponding to the divided areas of eight. FIG. 5A shows a state before the wave plate is arranged. FIG. 5B shows a state in which the wavelength plate is partially arranged.

  As shown in FIG. 5, the polarization conversion element 15 includes a holder 51 and a wave plate 52. The holder 51 holds eight wave plates 52. That is, the polarization conversion element 15 is completed by disposing the wave plate 52 shown in FIG. 5C on the holder 51 shown in FIG. As shown on the left side of FIG. 5B, the holder 51 is provided with a storage portion 53 for storing the wave plate 52. Further, a through hole 54 is formed below the storage portion 53. That is, the upper side of the through hole 54 is a storage portion 53 for storing the wave plate 52. Here, since the eight-divided polarization conversion element 15 is used, eight through holes 54 are formed. The eight through holes 54 are arranged along the circumferential direction. And the accommodating part 53 is provided in the upper part of each through-hole 54. FIG. The storage part 53 is formed in a ring shape. A side wall of the holder 51 is disposed outside the storage portion 53. Here, the storage portion 53 is formed by changing the size of the through hole 54 in the middle of the thickness direction (Z direction) of the holder 51.

  A wave plate 52 having different optical axes is disposed in the storage unit 53. That is, the wave plate 52 is placed at a position where the size of the through hole 54 is changed. For example, the through hole 54 is formed in a substantially trapezoidal shape. The wave plate 52 is formed in a trapezoidal shape that is slightly larger than the through hole 54. The optical axis of the wave plate 52 is as shown by the arrow in FIG. The eight wave plates 52 have the same trapezoidal shape. The optical axis of each wave plate 52 changes by a certain angle. The eight wave plates 52 are arranged along the circumferential direction. When eight wave plates 52 are arranged, a ring shape corresponding to the cross-sectional shape of the laser light is obtained. More specifically, when eight wave plates 52 are connected in the circumferential direction, a hollow regular octagon is formed.

  Furthermore, screw holes 58 are provided on the outer sides of the respective storage portions 53. As shown on the right side of FIG. 5B, the screw 55 is inserted into the screw hole 58. That is, the screw 55 is put into the screw hole 58. Then, the tip of the screw 55 comes into contact with the spacer 57. The spacer 57 is in contact with the outer side surface of the wave plate 52. Accordingly, when the screw 55 is screwed into the screw hole 58, the wave plate 52 is fixed to the holder 51. That is, the screw 55 presses the wave plate 52 against the holder 51 via the spacer 57. Thus, the screw 55 presses the side surface of the wave plate 52 from the outside via the spacer 57. Thereby, the inner side surface of the wave plate 52 contacts the side surface of the storage portion 53 of the holder 51. Therefore, the wave plate 52 is sandwiched between the holder 51 and the spacer 57. In this way, each wave plate 52 is held in the holder 51. Thus, by fixing the wave plate 52 from the side surface, it is possible to prevent the structure from being arranged on the optical path.

  As the spacer 57, for example, an elastic body such as rubber or resin can be used. Thereby, it is possible to prevent the wave plate 52 from being damaged. In the present embodiment, as shown in FIG. 1, the cross section of the laser light has a ring shape. For this reason, the holder 51 is arranged in the central portion. That is, since the laser beam does not enter the center of the polarization conversion element 15, it is not necessary to arrange the wave plate 52 on the optical axis. In addition, the size of each wave plate 52 can be determined according to the cross-sectional shape of the laser light. In addition, the use efficiency of the laser beam can be improved by providing the through hole 54. Of course, when the holder 51 is formed of a transparent material, the through hole 54 may not be provided.

  In the above description, the polarization conversion element 15 has a plurality of divided wave plates, but the configuration of the polarization conversion element 15 is not limited to this. For example, as the polarization conversion element 15, a photonic crystal grown on a transparent substrate can be used. Specifically, a groove pattern is formed on a glass substrate. The groove pattern is formed in different directions for each of the divided areas that are radially divided into two or more. A photonic crystal having a periodic structure of a plurality of dielectric films is formed on the substrate provided with the groove pattern. Thereby, a wave plate whose optical axes are substantially orthogonal to each other can be formed in the opposed divided regions. A polarization control element capable of realizing pseudo radial polarization or pseudo azimuth polarization can be manufactured by a simple method. Alternatively, a Mach-Zehnder interferometer using a helical phase delay element and a wave plate can be used as the polarization conversion element 15. In this case, a near-ideal radial or azimuth polarization can be generated instead of pseudo. [Steve C. et al. "Generating radially polarized beams interferometrically," Applied Optics Vol. 29, p. 2234]

  Next, the configuration of the polarization adjusting element 16 will be described with reference to FIG. FIG. 6 is a plan view showing the configuration of the polarization adjusting element 16. The polarization adjusting element 16 is a polarization control element having an electro-optic element disposed between electrodes. Then, the polarization state is controlled by changing the voltage applied between the electrodes.

  The polarization adjusting element 16 has a plurality of electro-optical elements 81 arranged along the circumferential direction. The electro-optic element 81 is a transparent electro-optic crystal such as BBO. The refractive index of the electro-optic element 81 changes according to the magnitude of the electric field. Therefore, the polarization adjusting element 16 is provided with a Pockels cell whose refractive index changes in proportion to the strength of the electric field. Here, the eight electro-optical elements 81 are arranged along the circumferential direction. Each electro-optic element 81 has substantially the same size. Accordingly, the eight electro-optic elements 81 are arranged at regular intervals in the circumferential direction.

  Each electro-optic element 81 has a hollow sector shape. Here, eight electro-optical elements 81 are provided. Therefore, each electro-optical element 81 has a 45 ° fan shape. The optical axis (c-axis) of each electro-optical element 81 is indicated by an arrow in FIG. The optical axes of the plurality of electro-optical elements 81 are radial. That is, the optical axes of the adjacent electro-optical elements 81 are in different directions. Here, since eight electro-optical elements 81 are provided, the angle formed by the optical axes of the adjacent electro-optical elements 81 is 45 °. The plurality of electro-optic elements 81 are arranged in the circumferential direction so as to be shifted by 45 ° from the optical axis of the adjacent electro-optic element 81. The electro-optical element 81 is held by, for example, a holder having the same configuration as the holder 51 shown in FIG. Laser light 50 is incident on the electro-optic element 81.

  A first electrode 82 is provided inside the eight electro-optic elements 81. The first electrode 82 is provided in a hollow ring shape. Then, it comes into contact with the inner surface of each electro-optic element 81. Therefore, the inside of the first electrode 82 is hollow. Furthermore, a second electrode 83 is provided outside the eight electro-optic elements 81. The second electrode 83 is provided in a hollow ring shape. Then, it contacts the outer surface of each electro-optic element 81. A plurality of electro-optic elements 81 are arranged inside the second electrode 83. Accordingly, the first electrode 82 has a size corresponding to the inner diameters of the eight electro-optical elements 81, and the second electrode 83 has a size corresponding to the outer shapes of the eight electro-optical elements 81.

  The link-like first electrode 82 and second electrode 83 are connected to the polarization adjusting power source 19 shown in FIG. The polarization adjusting power source 19 applies a voltage between the electrodes. For example, the first electrode 82 is supplied with ground (ground potential), and the second electrode 83 is supplied with a predetermined potential. The polarization adjusting power source 19 changes the applied voltage to adjust the polarization. When the applied voltage changes, the electric field applied to the electro-optical element 81 changes. Accordingly, the refractive index of the plurality of electro-optical elements 81 changes. The polarization adjusting power source 19 outputs an applied voltage so that an electric field of several kV / m is applied to the electro-optic element 81.

  Here, the first electrode 82 and the second electrode 83 are shared by the eight electro-optic elements 81. That is, the applied voltage applied to each electro-optical element 81 is the same. The electric field applied to each electro-optical element 81 is equal. In addition, a radial electric field is generated. Here, an electric field is applied in a direction perpendicular to the traveling direction of the laser beam 50. The polarization adjusting element 16 has a radial optical axis. That is, the optical axis changes according to the incident position of the laser beam 50. A voltage is applied between the electrodes arranged on both sides of the electro-optic element 81. Therefore, an electric field along the direction of the optical axis is applied. The electro-optic element 81 has birefringence corresponding to the applied voltage.

  The polarization state of the laser beam 50 can be adjusted by changing the applied voltage. Thereby, the electric field in the Z direction of the laser beam 50 can be maximized on the surface of the photocathode 21. For example, the optical axis of the polarization conversion element 15 may be displaced due to manufacturing errors. In such a case, the phase difference of the light that has passed through the opposing divided regions will deviate from 180 °. However, by adjusting the applied voltage, the phase difference of the light that has passed through the opposing electro-optic element 81 can be brought close to 180 °.

  Arrangement is made so that the vibration direction of the laser beam 50 that has passed through the polarization conversion element 15 coincides with the optical axis of the polarization adjustment element 16. That is, the angle between the polarization adjusting element 16 and the polarization converting element 15 is adjusted so that the divided region of the polarization converting element 15 and the electro-optic element 81 of the polarization adjusting element 16 are arranged in the same manner. In the XY plane, the boundary line of the divided region is matched with the boundary line of the electro-optic element 81. Accordingly, the electro-optical element 81a is disposed at a position corresponding to the wave plate 52a on the XY plane.

  When the plane of polarization of the laser beam 50 and the optical axis of the polarization adjusting element 16 coincide, only the phase of the light changes as described above. Therefore, even if the laser beam 50 passes through the electro-optic element 81 having an optical axis parallel to the polarization plane, the polarization plane does not change. That is, when the laser beam 50 having a polarization plane parallel to the optical axis of all the electro-optic elements 81 is incident, the polarization state does not change. However, when the laser light 50 passes through the electro-optic element 81 having an optical axis that is not parallel to the polarization plane, the polarization plane changes. Accordingly, the polarization axis of the laser beam 50 rotates in the XY plane. When the polarization axis is deviated from the optical axis of the electro-optic element 81, the polarization plane of the laser light 50 changes. In other words, only the electro-optic element 81 having the optical axis inclined from the polarization plane changes the polarization plane of the laser light 50. Therefore, only when the optical axis and the plane of polarization are shifted, the plane of polarization changes depending on the applied voltage. The angle of the polarization plane in the XY plane can be changed by adjusting the applied voltage.

  Then, the applied voltage is changed while monitoring the current of the electron beam 60 with a Faraday cup or the like. That is, the applied voltage is adjusted so that the current of the electron beam 60 is maximized. The polarization state of the laser light changes according to the applied voltage. For this reason, the brightness of the electron beam 60 can be increased by applying an optimum voltage to the polarization adjusting element 16. Thereby, the high quality electron beam 60 can be obtained. That is, the number of electrons contained in one bunch can be increased. Furthermore, the luminance of the electron beam 60 can be easily increased by electrically adjusting the polarization state.

  Furthermore, as shown in FIG. 7, the second electrode 83 may be divided. FIG. 7 is a plan view showing the configuration of the polarization adjusting element 16 having a mode different from that in FIG. In the polarization adjusting element 16 of FIG. 7, different second electrodes 83 are provided for the eight electro-optical elements 81. Accordingly, the eight second electrodes 83 are provided apart from each other. The eight second electrodes 83 are arranged at equal intervals in the circumferential direction. Then, the polarization adjusting power source 19 supplies different potentials to the respective second electrodes 83. Electric fields of different strengths can be applied to the pair of electro-optic elements 81 arranged opposite to each other. Thereby, an electric field can be independently applied to each electro-optical element 81, and finer adjustment can be performed.

  Specifically, as shown in FIG. 7, an electro-optical element 81a is disposed between the first electrode 82 and the second electrode 83a, and the electro-optical element 81b is disposed between the first electrode 82 and the second electrode 83b. Suppose it is placed. Here, the first electrode 82 is set to the same ground, and the second electrode 83a and the second electrode 83b are set to different potentials. As a result, different electric fields are applied to the electro-optical element 81a and the electro-optical element 81b. The polarization plane of the laser light from each electro-optic element 81 can be adjusted independently. That is, it is possible to adjust the polarization state precisely. In this way, fine adjustment is possible by independently controlling the electric field applied to the plurality of electro-optic elements 81. The phase of the laser light emitted from the opposing electro-optic element 81 can be surely shifted by 180 °. That is, the electric vector of the laser beam from the opposing electro-optic element 81 oscillates in the opposite direction by 180 °. Of course, instead of the second electrode 83, the first electrode 82 may be divided. As described above, by dividing at least one of the first electrode 82 and the second electrode 83, more precise adjustment is possible.

Embodiment 2 of the Invention
The electron gun according to this embodiment will be described with reference to FIG. FIG. 8 is a diagram showing a configuration of the electron gun 200 according to the present embodiment. The basic configuration of the electron gun 200 is the same as that of the electron gun 100 shown in FIG. The electron gun 200 according to the present embodiment is an electron gun having a reflective photocathode. That is, the laser beam is irradiated from the electron beam emission side.

  In the present embodiment, in addition to the laser light source 11 for generating a Z-direction electric field, a laser light source 31 for generating a photoelectric effect is provided. That is, two laser beams are applied to the photocathode. An electric field in the Z direction is applied to the surface of the photocathode 21 by the laser light source 11. Thereby, the work function of the photocathode 21 can be reduced. In this state, the laser light from the laser light source 31 is irradiated. Therefore, quantum efficiency can be improved and the brightness of the electron beam 60 can be improved. Since the photoelectric effect is generated by the laser beams from different laser light sources 31, emittance can be improved.

  As shown in FIG. 8, a laser beam 70 is emitted from the laser light source 31. The laser light source 31 is the same light source as the laser light source 11. A pair of axicon lenses 33 and 34, a polarization conversion element 35, a polarization adjustment element 36, a mirror 37, a beam splitter 38, and a profile adjustment mechanism 40 are provided in the optical path of the laser light 70. The pair of axicon lenses 33 and 34, the polarization conversion element 35, and the polarization adjustment element 36 are the same as the pair of axicon lenses 13 and 14, the polarization conversion element 15, and the polarization adjustment element 16 described in the first embodiment. It is the same composition. For this reason, the detailed description with respect to these is abbreviate | omitted. That is, the laser beam 70 becomes a ring-shaped light beam by the pair of axicon lenses 33 and 34. The annular laser beam 70 is brought into a desired polarization state by the polarization conversion element 15 and the polarization adjustment element 16. The polarization adjustment element 36 is connected to a polarization adjustment power source 39 in the same manner as the polarization adjustment element 16. The polarization state of the laser beam 70 may be different from that of the laser beam 50. In addition, when the laser light 70 is incident on the photocathode 21 as normal linearly polarized light, the polarization conversion element 35 and the polarization adjustment element 36 may not be provided.

  Laser light 70 from the polarization adjusting element 36 enters the mirror 37. The laser beam 70 reflected by the mirror 37 enters the beam splitter 38 via the profile adjustment mechanism 40. The profile adjusting mechanism 40 adjusts the profile of the laser beam on the photocathode surface. The profile adjustment mechanism 40 will be described later. The beam splitter 38 is a half mirror or the like, and reflects a part of the laser light 70. Further, the beam splitter 38 is disposed in the optical path of the laser light 50. That is, the beam splitter 38 is disposed at a location where the optical path of the laser beam 50 and the optical path of the laser beam 70 merge. A part of the laser light 50 incident on the beam splitter 38 passes through the beam splitter 38. Thereby, the laser beam 50 and the laser beam 70 are synthesized. Here, the optical axes of the laser beam 50 and the laser beam 70 coincide.

  The synthesized laser beam 50 and laser beam 70 are reflected by the mirror 17 and enter the lens 18 as in the first embodiment. Then, the lens 18 condenses the laser light 50 and the laser light 70 on the surface of the photocathode 21. As a result, electrons are generated from the photocathode 21. That is, since the laser beam 50 is radially polarized, an electric field in the Z direction is applied to the surface of the photocathode 21. Then, the laser beam 70 is incident on the photocathode surface while an electric field in the Z direction is applied. Electrons are generated by the photoelectric effect of the laser beam 70. The electrons are accelerated by the resonator 23 to become an electron beam 60.

  Since an electric field in the Z direction is generated by the laser light 50, the effective work function can be lowered. Thereby, the long wavelength laser beam 70 can be used. That is, the laser light source 31 can be selected according to the work function lowered by the electric field in the Z direction. The laser light source 31 only needs to have a function of emitting laser light higher than energy corresponding to the lowered work function. Thereby, the selection range of the laser light source 31 becomes wide. Moreover, the laser light source 11 should just have the function to produce | generate the electric field of a Z direction. For this reason, the range of wavelength selection becomes wide. For example, the laser light source 11 and the laser light source 31 can be light sources having different wavelengths. Thus, the laser beam 50 that generates an electric field in the Z direction and the laser beam 70 that generates electrons by the photoelectric effect are different laser beams. Thereby, a laser light source suitable for each function can be used, and convenience can be improved.

Further, a profile adjusting mechanism 40 is provided in the optical path of the laser light 70. Thereby, the emittance of the electron beam 60 can be improved. The reason why the following emittance can be improved will be described with reference to FIG. FIG. 9A is a diagram showing a profile of the electric field strength Ez in the Z direction. FIG. 9B is a diagram showing a profile of the laser beam intensity I. FIG. 9C is a diagram showing a profile of the electron beam intensity _Ie . In FIGS. 9A to 9C, the horizontal axis indicates the position in the X direction.

  When radially polarized light is collected, the electric field intensity Ez in the Z direction is maximized at the center of the light beam spot. Here, the quantum efficiency QE of electron generation by the photoelectric effect is proportional to the route of the electric field strength Ez. That is, the Schottky effect is likely to occur at the center of the light beam spot, and the quantum efficiency QE is the highest.

  Further, the quantum efficiency QE is proportional to the intensity of the laser light 70. Here, the profile of the laser beam 70 is adjusted to the shape shown in FIG. That is, adjustment is performed so that a depression exists at the center of the light beam spot and peaks exist on both sides thereof. For example, the focus position of the laser beam is slightly shifted in the Z direction by the profile adjustment mechanism 40. Since the laser beam 70 is an annular beam, the profile shown in FIG. 9B can be obtained by shifting the focal position. Specifically, the profile adjustment mechanism 40 has a lens or the like. The focal position is converted by adjusting the position of the lens in the Z direction. Alternatively, the profile may be adjusted by adjusting the beam diameter of the light beam using a diaphragm. Furthermore, the profile may be adjusted using a two-dimensional spatial phase modulation element, a deformable mirror, or the like.

The intensity I _e of the electron beam 60 changes according to the electric field intensity Ez in the Z direction and the intensity I of the laser light 70. That is, the profile of the electron beam 60 depends on the electric field intensity Ez in the Z direction and the intensity I of the laser beam 70. Simply, the intensity I _e of the electron beam is proportional to the product of the root of the electric field intensity Ez in the Z direction and the intensity I of the laser beam 70. In the state where the electric field intensity Ez in the Z direction shown in FIG. 9A exists, the laser beam 70 having the profile shown in FIG. 9B is irradiated. Then, as shown in FIG. 9C, the profile of the electron beam 60 becomes a flat top. Thus, by adjusting the profile of the laser beam 70, the spread of the electron beam 60 can be reduced. Therefore, the profile of the electron beam 60 can be improved and the emittance can be improved. Further, even with the configuration according to the first embodiment, emittance can be improved. That is, for example, the focal position is shifted from the photocathode surface. Thereby, the profile of the electron beam 60 can be improved, and emittance can be improved.

Thus, the peak position of the intensity I profile of the laser beam 70 is shifted from the peak position of the intensity profile of the laser beam 50. Thereby, the profile of the intensity _Ie of the electron beam 60 can be optimized. Therefore, a high quality electron beam can be obtained.

  As described above, the two laser beams 50 and 70 are caused to enter the photocathode 21 in synchronization. Note that the laser beam from the same laser light source 11 may be divided into two and incident on the photocathode 21. For example, when the pulse width is sub-picosecond, the laser beam from the same laser light source is divided into two. Then, one of the divided laser beams is incident on the polarization conversion element 15 to generate an electric field in the Z direction. The other laser beam may or may not be incident on the polarization conversion element 35. That is, at least one laser beam may be incident on the polarization conversion element 15. Thereby, two laser beams can be incident simultaneously with a simple configuration. Thereby, the number of generated electrons can be increased. When two laser beams are incident, the profile of the electron beam can be improved by shifting the focal position.

  Further, the laser beam 70 can be obliquely incident on the photocathode 21. That is, the laser beam 70 is incident on the photocathode 21 surface at an angle. At this time, the optical axis of the laser beam 70 is tilted from the perpendicular to the surface of the photocathode 21. Here, depending on the material of the photocathode 21, the quantum efficiency of electron generation may differ between S-polarized light and P-polarized light. For example, P-polarized light may have higher quantum efficiency than S-polarized light. In such a case, the luminance of the electron beam can be increased by controlling the polarization state of the laser light 70. Only one of the polarization conversion element 15 and the polarization adjustment element 16 can control the polarization state. Alternatively, the polarization state may be controlled using an element other than the polarization conversion element 15 or the polarization adjustment element 16. That is, the polarization state may be controlled using an optical element instead of the polarization conversion element 15 and the polarization adjustment element 16.

Embodiment 3 of the Invention
An electron gun according to this embodiment will be described with reference to FIG. FIG. 10 is a diagram showing a main configuration of the electron gun 300 according to the present embodiment. The electron gun 300 according to the present embodiment is an electron gun having a transmissive photocathode. That is, the laser beam is irradiated from the side opposite to the electron beam emission side. The configurations of the laser light source 11, the polarization conversion element 15, the photocathode 21, the resonator 23, and the microwave source 24 are the same as those in the first embodiment, and are not illustrated.

  As shown in FIG. 10, the laser beam 50 enters the photocathode 21 from the side opposite to the resonator 23. That is, the laser beam 50 is incident on the photocathode 21 without passing through the resonator 23. In the case of the transmissive photocathode 21, the optical path of the laser beam 50 and the path of the electron beam 60 do not overlap. Therefore, unlike the first embodiment, it is not necessary to use a pair of axicon lenses 13 and 14 to form a laser beam 50 in a ring shape. Moreover, it is not necessary to make the mirror 17 and the lens 18 hollow. That is, it is not necessary to arrange the mirror 17 and the lens 18 in the vacuum that is the path of the electron beam 60. Therefore, the mirror 17 and the lens 18 can be disposed in the atmosphere.

  In the present embodiment, the lens 18 is disposed on the side opposite to the resonator 23. For this reason, the lens 18 does not interfere with the structure in the resonator 23. Therefore, the lens 18 can be brought close to the photocathode 21. When the reflective photocathode 21 is used, the lens 18 can be disposed in the atmosphere. Accordingly, the lens 18 can be disposed close to the photocathode 21. As a result, the lens 18 can have a high NA. The electric field strength in the Z direction can be increased. In this case, the beam diameter of the laser beam 50 may be expanded with a beam expander or the like.

  FIG. 11 shows the relationship between the NA (numerical aperture) of the lens 18 and the amplitude of the electric field in the Z direction. FIG. 11 is a graph showing a calculation result obtained by calculating the amplitude of the electric field in the Z direction by changing the numerical aperture of the lens 18. As shown in FIG. 11, increasing the NA of the lens 18 dramatically increases the amplitude of the electric field in the Z direction. Therefore, the electron gun 300 having a transmissive photocathode has an advantage that the brightness of the electron beam can be improved as compared with the electron guns 100 and 200 having a reflective photocathode.

  Furthermore, the position of the lens 18 can be easily adjusted by arranging the lens 18 in the atmosphere. When the lens 18 is moved along the Z direction, the focal position changes in the Z direction. The focal position of the laser beam can be adjusted to be on the surface of the photocathode 21. Even when a reflective photocathode is used, a normal lens and mirror can be used by slightly tilting the incident angle of the laser beam to the photocathode. In this case as well, the annular beam need not be generated.

  In the first and second embodiments, the laser light is formed into an annular shape by the axicon lens. That is, the cross-sectional shape of the laser beam is annular due to the pair of axicon lenses 13 and 14. Thereby, the electric field strength in the Z direction can be further increased. Therefore, not only the first and second embodiments but also the third embodiment preferably uses an annular laser beam. That is, a pair of axicon lenses 13 and 14 may be used in the optical system of the transmissive photocathode shown in FIG. Laser light incident on the objective lens is converted into an annular shape by a pair of axicon lenses. Thereby, the electric field strength in the Z direction can be increased.

  The reason why the electric field strength in the Z direction can be increased will be described with reference to FIG. FIG. 12 is a side view schematically showing how the annular laser beam is collected by the lens 18.

  As shown in FIG. 12, the radially polarized laser beam is collected by the lens 18. The lens 18 condenses the laser beam at the focal point. Accordingly, the light is refracted more greatly toward the outer end side of the lens 18. That is, the angle refracted by the lens 18 changes depending on the distance from the optical axis in the Y direction. For example, no light is refracted by the lens 18 on the optical axis. Therefore, the light incident on the center of the lens 18 propagates in parallel with the optical axis. On the other hand, in the vicinity of the outer edge of the lens 18, the light is largely refracted by the lens. Therefore, when it is incident on the outer end side of the lens 18, it is refracted at a large angle. The light refracted in the vicinity of the outer edge of the lens 18 is greatly inclined from the optical axis. As the angle of refraction increases, the Z component of the velocity decreases and the Y component increases. Note that the surface of the photocathode is the focal position of the lens 18.

  Here, the greater the angle refracted by the lens 18, the higher the electric field in the Z direction. That is, when light travels in the Z direction, the Z component of the electric field becomes zero. When light travels in the Y direction, the Z component of the electric field is maximized. Therefore, the light component refracted by the lens 18 increases the Z component of the electric field. Here, the radially polarized laser beam is focused on the focal point. Therefore, the Z component of the electric field is not canceled at the focal position. Then, the Z component of the electric field increases as the outer portion of the lens 18 increases.

  In an annular laser beam, the laser beam propagates at a position away from the optical axis. In other words, when the ring shape is formed by the pair of axicon lenses 13 and 14, it does not pass near the optical axis. Therefore, the angle refracted by the lens is increased. This increases the Z component of the electric field. At the focal position, the electric field strength in the Z direction can be increased. In other words, when the ring is formed, the laser beam does not pass on the optical axis and passes through the outer end side of the lens 18. Therefore, the Z component of the speed of the laser beam decreases and the Y component increases. The light beam that has passed through the portion near the outer diameter end of the lens 18 has a greater contribution to the Z-polarized light generated at the focal point than the light beam that has passed through the vicinity of the optical axis. By converting to an annular beam, light passes only through the portion of the lens 18 that is close to the short outer diameter. Therefore, the electric field strength in the Z direction at the focal position can be increased.

Thus, the Z-polarized component can be increased depending on the incident position of the light in the lens 18. The Z-polarized component can be increased by changing the ratio of the inner diameter and the outer diameter of the annular laser beam. Here, as shown in FIG. 13, the inner diameter is r ₁ and the outer diameter is r ₀ . The inner diameter (r ₁ ) / outer diameter (r ₀ ) is the ratio of the ring. FIG. 14 shows a graph showing the intensity of Z-polarized light when the ratio of the ring is changed. That is, FIG. 14 is a graph showing the relationship between the ratio of the ring and the intensity of Z-polarized light at the focal position obtained by calculation. FIG. 14 shows a graph in the case of an annular shape using an axicon lens and a mask. In FIG. 13, the horizontal axis represents the ratio of the ring, and the vertical axis represents the intensity of Z-polarized light. For example, when an axicon lens is used, the intensity of Z-polarized light can be increased by changing the ratio of the ring. Furthermore, the ratio of the ring is preferably about 30 to 50%, more preferably 40%. For example, the ratio of the ring can be adjusted by changing the distance between the pair of axicon lenses 13 and 14. Increasing the intensity of Z-polarized light causes a Schottky effect due to the Z-direction electric field. For this reason, a work function can be reduced. Furthermore, the number of electrons in one bunch can be increased. Moreover, the use efficiency of a laser beam can be made high by using a pair of axicon lenses. This effect is particularly effective when the ring ratio r1 / r0 is large.

  Further, when the radially polarized laser beam is condensed by the lens 18, only the Z-polarized component exists at the focal position. That is, at the focal position, the Y-polarized component is canceled and the X-polarized component is canceled. However, when deviating from the focal position, there are also a Y-polarized component and an X-polarized component. Here, by using an annular laser beam, the intensity of the Z-polarized component can be relatively increased compared to the X-polarized component and the Y-polarized component.

  FIG. 15 is used to compare the electric field intensity when a circular laser beam is used and the light intensity when an annular laser beam is used. FIG. 15A shows the light intensity in the vicinity of the focal point generated by a laser beam that is not in an annular shape, that is, a circular laser beam. FIG. 15B shows the light intensity near the focal point generated by the annular laser beam. In FIG. 15, the horizontal axis indicates the position on the XY plane. Here, the position of the optical axis in the XY plane is set to zero. The vertical axis represents the intensity of the Z-polarized component and the intensity of the Y-polarized component obtained by calculation. Y-polarized light indicates linearly polarized light in which the vibration plane of the electric vector is parallel to the Z direction. Therefore, the light intensity of Z-polarized light corresponds to the Z component of the electric field, and the light intensity of Y-polarized light corresponds to the Y component of the electric field.

  In this calculation, the wavelength is 790 nm and the lens NA is 0.8. Further, the laser light having the same light intensity is used in FIGS. 15A and 15B. The light intensity by the annular beam is calculated with the ratio of the annular ring being 0.8. The intensity of the Z-polarized component has a peak at the 0 position, that is, on the optical axis. The intensity of the Y-polarized component has a peak at a position away from the optical axis. Further, the intensity of the Y-polarized component has a minimum value on the optical axis. By using an annular beam, it can be made sufficiently larger than Y-polarized light. Therefore, the ratio of the Z polarization component to the Y polarization component can be made larger than that of the circular laser beam. Of course, not only the ratio of the Z-polarized light component to the Y-polarized light component but also the Z-polarized light component to the X-polarized light component are similarly increased. That is, the intensity of the Z-polarized component can be relatively increased compared to the X-polarized component and the Y-polarized component. Increasing the intensity of Z-polarized light causes a Schottky effect due to the Z-direction electric field. For this reason, a work function can be reduced. Furthermore, the number of electrons in one bunch can be increased.

  As described above, in the first to third embodiments, radial polarized light is generated by the polarization conversion element 15. Then, the radially polarized light is collected by the lens 18. Therefore, an electric field in the Z direction is formed on the photocathode surface. Since the Schottky effect due to the Z-direction electric field occurs, the work function can be lowered. Therefore, a long wavelength laser beam can be used. Therefore, the configuration can be simplified. Furthermore, quantum efficiency can be improved by the Schottky effect. Thereby, the brightness | luminance of an electron beam can be improved. Therefore, a high-quality electron beam can be generated with a simple configuration. Therefore, the electron gun is suitable for an electron beam application apparatus such as an electron beam accelerator or an electron beam drawing apparatus.

  In this way, the linearly polarized laser beam 50 is incident on the polarization conversion element 15. Then, the polarization state of the laser beam 50 is converted by the polarization conversion element 15 so that the vibration direction of the electric vector is opposite in the region facing the optical axis of the laser beam. The laser beam 50 from the polarization conversion element 15 is condensed and incident on the photocathode 21. The electrons emitted from the photocathode 21 by the laser light 50 are accelerated by the electric field of the resonator 23. By generating electrons by such a method, an electron beam with low emittance and high brightness can be obtained. The bunch length of the electron beam can be about 20 psec.

  Irradiation with a laser beam generates a high electric field on the surface of the photocathode 21 in an impulse manner. At this time, since the laser beam 50 is Z-polarized light, an electric field in a direction perpendicular to the photocathode surface is generated. As a result, the work function is reduced and electrons are easily emitted. The brightness of the electron beam can be increased. In addition, an increase in discharge and dark current can be prevented. For this reason, the quantum efficiency of the photocathode 21 can be effectively increased.

  In addition, the work function can be lowered in units of eV. For example, assuming that the work function is usually about 4 eV, the work function is lowered to 2 to 3 eV by laser irradiation. For this reason, a laser having a near infrared wavelength range longer than the conventional ultraviolet wavelength can be directly used as a light source. The configuration of the laser light source 11 can be simplified and the reliability can be improved. For example, in the photocathode 21 made of copper, a laser light source having a wavelength shorter than 270 to 288 nm is conventionally required. On the other hand, the electron gun having the above configuration can use the second harmonic (395 nm) or the fundamental wave (790 nm) of a titanium sapphire laser. Further, in the photocathode 21 made of diamond, conventionally, a wavelength shorter than 226 nm is required. In contrast, the electron gun having the above-described configuration can use the third harmonic (263 nm) or the second harmonic (395 nm) of a titanium sapphire laser.

  In the first to third embodiments, the polarization state is adjusted by the polarization adjusting element 16 having an electro-optical element, but the polarization state can also be adjusted by the polarization adjusting element 16 having a magneto-optical element. The configuration of the polarization adjusting element 16 having a magneto-optical element will be described with reference to FIG. FIG. 16A is a plan view showing the configuration of the polarization adjusting element 16 using a magneto-optical element. FIG. 16B is a perspective view showing the configuration of the magneto-optical element used for the polarization adjusting element 16. In FIG. 16, the Faraday arrangement is such that the traveling direction of light and the magnetic field are collinear. Accordingly, in FIG. 16A, the direction of the magnetic field is in a direction perpendicular to the paper surface. In FIG. 16B, the direction of the magnetic field is indicated by a white arrow, and the traveling direction of the laser light is indicated by an arrow.

  As shown in FIG. 16A, the polarization adjusting element 16 has a plurality of magneto-optical elements 85. Here, eight magneto-optical elements 85 are provided. The plurality of magneto-optical elements 85 are arranged along the circumferential direction. More specifically, one magneto-optical element 85 is disposed outside each side of the regular octagon. The centers of the eight magneto-optical elements 85 coincide with the optical axis. That is, the optical axis is arranged between two magneto-optical elements arranged diagonally. Accordingly, the eight magneto-optical elements 85 provided in the polarization adjusting element 16 correspond to the eight wavelength plates 52 of the polarization conversion element 15 shown in FIG. Accordingly, the pulse laser beam that has passed through one wave plate 52 is incident on one magneto-optical element 85. The optical axis (crystal axis) of the adjacent magneto-optical element 85 is in a different direction. Here, the optical axes of the eight magneto-optical elements 85 are radial as in FIG. In each magneto-optical element 85, the direction of the optical axis coincides with the direction of the magnetic field. The polarization state of the pulse laser beam that has passed through one wavelength plate 52 is adjusted by one magneto-optical element 85. As the magneto-optical element 85, for example, a non-magnetic glass rod can be used. For example, the magneto-optical element 85 is formed by a rectangular parallelepiped glass rod. And the longitudinal direction of the glass rod is parallel to the Z direction.

  A coil 86 is wound around each magneto-optical element 85. That is, the coil 86 is formed by winding a conducting wire a plurality of times so as to surround the magneto-optical element 85. The coil 86 is a solenoid coil. As shown in FIG. 16B, the coil 86 is provided so as to surround the outer periphery of the magneto-optical element 85. Accordingly, the coil 86 generates a magnetic field in the Z direction. When a current flows through the coil 86, a magnetic field is generated according to Bio-Savart's law. The magnetic field is generated in the longitudinal direction of the glass rod. Accordingly, when linearly polarized light is passed through the glass rod in a state where an electric current is passed, the plane of polarization of the laser light (vibration direction of the electric vector) rotates. That is, the polarization plane of light rotates due to the Faraday effect. The polarization plane of light changes according to the strength of the magnetic field. That is, the change in the polarization state passing through the magneto-optical element 85 corresponds to the strength of the magnetic field. The strength of the magnetic field changes depending on the number of turns of the coil 86 and the current. Therefore, the polarization state changes according to the current. In other words, the polarization state can be adjusted by adjusting the current with the polarization adjusting power source 39. Thereby, the polarization state of the laser light that has passed through each wave plate 52 can be adjusted.

  Furthermore, the arrangement of the magneto-optical element 85 is not limited to the Faraday arrangement. As shown in FIG. 17, a forked arrangement is also possible. FIG. 17A is a plan view showing a configuration of the polarization adjusting element 16 using a magneto-optical element in a forked arrangement. FIG. 17B is a perspective view showing a configuration of a forked magneto-optical element used for the polarization adjusting element 16. The description of the same configuration as that of the polarization adjusting element 16 shown in FIG. 17 is omitted. Therefore, in FIG. 17A, the direction of the magnetic field is in a direction perpendicular to the paper surface. In FIG. 17B, the direction of the magnetic field is indicated by a white arrow, and the traveling direction of the laser light is indicated by an arrow.

  When using a Forked arrangement, the direction of the magnetic field and the traveling direction of the light are orthogonal. That is, the coils are arranged on both sides of the magneto-optical element 85. For example, as shown in FIG. 17A, a coil 86 disposed on the optical axis side (inner side) of the magneto-optical element 85 and a coil 86 disposed on the opposite side (outer side) of the optical axis are provided. It has been. Specifically, as shown in FIG. 17B, the magneto-optical element 85 is sandwiched between the coil 86a and the coil 86b. The coil 86 on the optical axis side of the magneto-optical element 85 and the coil 86 outside the magneto-optical element 85 are connected, and the same current flows. In FIG. 17A, the direction of the magnetic field is radial. The optical axis (crystal axis) of the adjacent magneto-optical element 85 is in a different direction. Here, the optical axes of the eight magneto-optical elements 85 are radial as in FIG. Accordingly, in each magneto-optical element 85, the optical axis and the direction of the magnetic field coincide. The polarization state can be adjusted by adjusting the current flowing through each coil 86. As described above, either a forked arrangement or a Faraday arrangement may be used.

  Furthermore, in Embodiments 1 to 3, it is preferable to use it for femtosecond pulse laser light. That is, the pulse width of the pulse laser beam emitted from the laser light source 11 is made shorter than 1 psec. Thereby, a low emittance electron beam can be generated. In other words, the space charge effect can be suppressed by devising the three-dimensional shape of the electron beam bunch. Below, the effect of using a femtosecond pulse laser beam is demonstrated.

  First, consider the case where the bunch length is infinitely long. In this case, by using an infinitely long cylindrical beam, only a space charge effect linear in the radial direction (radial direction) can be eliminated. That is, it becomes possible to eliminate the non-linear space charge effect in the radial direction. If there is only a linear space charge effect in the radial direction, the increase in emittance can be reduced by a solenoid electromagnet. For example, the electron beam is focused by a solenoid electromagnet provided in the electron gun. Thereby, the space charge effect in the radial direction can be compensated. That is, as a whole, the photocathode as a whole can suppress an increase in the emittance of the electron beam due to the space charge effect in the radial direction.

  The electron guns of the first to third embodiments are RF electron guns. For this reason, the bunch has a pulse shape with a finite length. When a single bunch is characterized, for example, the pulse width (bunch width) of the electron beam is set to about 10 psec. Further, the spot size of the pulse laser beam on the cathode surface is about 1 to 2 mm. Therefore, the aspect ratio of the bunch is about 1 to 2 times. Here, the aspect ratio is (length in the vertical direction) / (length in the horizontal direction) of the bunch.

  When the actual three-dimensional shape of the bunch is cylindrical, emittance increases at both end edges. Therefore, various edge shapes have been studied. For example, it has been found that the emittance is reduced by making the bunch shape into an ellipsoid (a rugby ball shape which is a spheroid). As a method for forming an ellipsoid shape, a proposal by shaping a fiber bundle and a proposal by a diffractive optical element are known. Among them, the ellipsoid shaping has been demonstrated only by the proposal by the fiber bundle shaping. In these methods, the bunch is compensated to have an ellipsoidal shape by shaping the incident pulsed laser light (Tomizawa et al., “Adaptive shaping system for both spacial and temporal profiles of af- for A photocathode RF gun "Nuclear Instruments AND Methods In Physics Research A 557 (2006) 117-123).

  Such ellipsoid shaping is difficult to shape with an adaptive optical element, and the time response of the spatial profile shaping method is slow. For this reason, it is difficult to make a complete ellipsoid shape. Both of the above two proposals are pseudo shaping by a fixed optical element. However, the object to be actually shaped is not a pulse laser beam but an electron beam bunch. For this reason, a simulation result has been announced that an ellipsoidal bunch can be formed by condensing an ultrashort pulse laser beam of 10 fsec on a photocathode and utilizing the property of an electron beam that is expanded by the space charge effect. (See, eg, J. B. Rosenzweig et al., “Emittance compensation with dynamically optimized photoelectron beam profiles 87” Nuclear Instruments AND Meth 7).

  Although the ultra low emittance can be obtained by using the method described in this document, there is no demonstration example. In the electron gun described above, when the polarization conversion element 15 that converts linearly polarized light into radial polarized light is used, it is ideal to use a femtosecond ultrashort pulse laser in order to increase the Z-direction electric field. Therefore, an ellipsoidal shape can be easily obtained by irradiating the photocathode with a femtosecond laser pulse. That is, the combination of the polarization conversion element 15 and the femtosecond pulse laser beam is compatible. The femtosecond pulse laser beam can be irradiated only with the femtosecond laser oscillator. Therefore, emittance can be reduced with a compact laser light source. Thus, it is preferable to use a femtosecond laser pulse having a pulse width of 0 to 1 psec.

  The polarization conversion element 15 may be disposed between the axicon lens 13 and the wavelength conversion element 12. In this case, since the polarization conversion element 24 can be made small, the component cost of the polarization conversion element 24 can be reduced.

  A relay optical system may be disposed between the polarization conversion element 15 and the photocathode 21. Thereby, the influence by the change of distribution of a polarization state can be reduced. That is, when the pulse laser beam propagates, the distribution of the polarization state in the beam cross section changes due to the diffraction effect. In this case, a change in the distribution due to diffraction may affect the electric field distribution at the focal plane. In order to reduce such influence, for example, two relay lenses are arranged.

For example, in the configuration of the first embodiment, as shown in FIG. 18, the focal length relay lens 91 of f _1, the focal length to place the relay lens 92 of f _1. Further, the focal length of the lens 18 and _{f 2.} The polarization conversion element 15 and the relay lens 91 are spaced apart by f ₁ . The relay lens 91 and the relay lens 92 are spaced apart by 2 × f ₁ . The relay lens 92 and the lens 18 are spaced apart by f ₁ . The lens 18 and the photo cathode 21 spaced apart by _{f 2.} By forming an image in this way, the polarization state immediately after passing through the polarization conversion element 15 and the polarization state at the position of the pupil of the lens 18 can be made substantially the same. Therefore, changes in the polarization state distribution due to the diffraction effect can be reduced.

  In the above description, the second harmonic is used, but the present invention is not limited to this. That is, the fundamental wave can be used depending on the photocathode material and the wavelength of the laser light source 11. Thereby, it becomes unnecessary to use the wavelength conversion element which produces | generates a 2nd harmonic and a 3rd harmonic. The wavelength conversion element 12 becomes unnecessary, and a smaller laser device can be used. Furthermore, since the optical path of the laser beam 50 can be shortened, the stability can be improved. In addition, the configurations of Embodiments 1 to 3 can be partially combined as appropriate.

It is a figure which shows typically the structure of the electron gun concerning Embodiment 1 of this invention. It is a figure which shows the structure of the polarization conversion element used for an electron gun. It is a perspective view which shows the polarization state of the laser beam which passed the polarization conversion element. It is a side view which shows the polarization state of the laser beam which passed the polarization conversion element. It is a figure which shows the specific structural example of a polarization converting element. It is a top view which shows the structure of a polarization adjusting element. It is a top view which shows the structure of the modification of a polarization adjusting element. It is a figure which shows typically the structure of the electron gun concerning Embodiment 2 of this invention. It is a figure for demonstrating the profile of the electron beam obtained by the electron gun concerning Embodiment 2 of this invention. It is a figure which shows typically the structure of the electron gun concerning Embodiment 1 of this invention. It is a graph which shows the relationship between the numerical aperture of a lens, and the amplitude of the electric field of a Z direction. FIG. 6 is a side view schematically showing how an annular laser beam is collected by a lens 18. It is a figure for demonstrating the ratio of a ring. It is a graph which shows the relationship between the ratio of a ring, and the intensity | strength of Z polarized light. It is a graph which shows the intensity | strength of the Y polarization component and Z polarization component in the focus position vicinity. It is a figure for demonstrating the other structure of the polarization adjusting element using the magneto-optical element of a Faraday arrangement. It is a figure for demonstrating the other structure of the polarization adjusting element using the magneto-optical element of a forked arrangement | positioning. It is a figure which shows the structure of the relay lens system arrange | positioned between a photocathode and a polarization conversion element.

Explanation of symbols

11 laser light source, 12 wavelength conversion element, 13 axicon lens,
14 Axicon lens, 15 Polarization conversion element, 16 Polarization adjustment element, 17 Mirror,
18 lens, 19 power supply for polarization adjustment,
21 Photocathode, 22 Resonator, 24 Microwave source 31 Laser light source, 33 Axicon lens, 34 Axicon lens,
35 polarization conversion element, 36 polarization adjustment element, 37 mirror, 38 lens,
39 power supply for polarization adjustment, 40 profile adjustment mechanism,
50 laser beam, 51 holder, 52 wavelength plate, 53 storage section, 54 through hole,
55 screws, 57 spacers, 58 screw holes,
60 Electron beam, 70 Laser light, 71 Magneto-optic element, 72 Coil 81 Electro-optic element, 82 First electrode, 82 Second electrode,
85 magneto-optic elements, 86 coils,
91 relay lens, 91 relay lens,
100 electron gun, 200 electron gun, 300 electron gun,

Claims (17)

A laser light source;
A polarization conversion element that gives a phase difference corresponding to an incident position to the laser light from the laser light source;
A lens that condenses laser light incident from the laser light source via the polarization conversion element;
An electron gun comprising: a photocathode on which a laser beam condensed by the lens is incident.   2. The polarization converter according to claim 1, wherein the polarization conversion element linearly polarizes the laser beam substantially in the radial direction over the entire cross section and converts the vibration direction of the electric vector to be opposite in a region facing the optical axis. Electron gun. The polarization conversion element has a plurality of radially divided regions,
The plurality of divided regions are each provided with a wave plate that emits light with a phase shifted, and the optical axes of the wave plates are substantially orthogonal to each other in the opposed divided regions. The electron gun described.   4. The electron gun according to claim 1, wherein a polarization adjusting element that electrically adjusts a polarization state of the laser light on the surface of the photocathode is provided in an optical path of the laser light. A plurality of electro-optic elements in which the polarization adjusting element is arranged along a circumferential direction;
A first electrode provided inside a plurality of electro-optic elements arranged along the circumferential direction;
A second electrode provided on the outside of the plurality of electro-optical elements divided radially,
The electron gun according to claim 4, wherein the optical axes of the plurality of electro-optical elements are different from the optical axis of an adjacent electro-optical element. The polarization adjusting element is
A plurality of magneto-optical elements arranged along the circumferential direction;
A first coil provided inside a plurality of magneto-optical elements arranged along the circumferential direction;
A second coil provided outside each of the plurality of magneto-optical elements arranged along the circumferential direction,
The electron gun according to claim 4, wherein an optical axis of the plurality of magneto-optical elements is different from an optical axis of an adjacent magneto-optical element. The polarization adjusting element is
A plurality of magneto-optical elements arranged along the circumferential direction;
A coil provided on the outer periphery of each of the plurality of magneto-optical elements arranged along the circumferential direction;
The electron gun according to claim 4, wherein an optical axis of the plurality of magneto-optical elements is different from an optical axis of an adjacent magneto-optical element.   The electron gun according to claim 1, wherein laser light is incident on the photocathode separately from laser light from the polarization conversion element.   The electron gun according to claim 8, wherein the peak positions of the profiles of the two laser beams are shifted in the photocathode.   The electron gun according to any one of claims 1 to 9, further comprising an annular beam converting means for converting laser light from the laser light source into an annular light beam. The lens has a hollow shape,
The electron gun according to claim 10, wherein an electron beam from the photocathode passes through a hollow portion of the lens.   The electron gun according to claim 1, wherein a laser beam from the laser light source is incident on the photocathode from a side opposite to an electron beam emitting side of the photocathode.   The electron gun according to claim 1, wherein a pulsed laser beam having a pulse width shorter than 1 psec is emitted from the laser light source. An electron generation method for generating electrons by irradiating a photocathode with laser light,
Entering linearly polarized laser light into the polarization conversion element;
The polarization conversion element converts the laser light into light that is linearly polarized in a substantially radial direction across the entire cross section, and in the region facing the optical axis, the vibration direction of the electric vector is in the opposite direction. Converting the polarization state of the laser light;
Collecting the laser light emitted from the polarization conversion element and causing the laser light to enter the photocathode. A polarization control element having an electro-optic element disposed between electrodes,
A plurality of electro-optic elements arranged along the circumferential direction;
A first electrode provided inside a plurality of electro-optic elements arranged along the circumferential direction;
A second electrode provided on the outside of the plurality of electro-optical elements divided radially,
In the adjacent electro-optical elements, the polarization control elements are different in optical axis direction.   The polarization control element according to claim 15, wherein optical axes of the plurality of electro-optical elements are radial.   The at least one of the first electrode and the second electrode is provided for each of the plurality of electro-optic elements so that an electric field is independently applied to the plurality of electro-optic elements. Or the polarization control element of 16.

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