JP2010015877A - Electron gun, electron microscope, and electron generation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electron gun capable of aiming at higher quality of electron beams in a simple way and provide an electron microscope and an electron generation method. <P>SOLUTION: The electron gun 100 in this application is an electron gun discharging electrons in a direction of incident laser beams, and is provided with a laser beam source, a hollow light guide member where the laser beam from the laser beam source enters and the incident light propagates repeating total reflections inside, a hollow lens refracting light emitted from the light guide member, and a photo-cathode where the laser beam refracted by the lens enters. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electron gun, an electron microscope, and an electron generation method, and more particularly to an electron gun using a photocathode, an electron microscope, and an electron generation method.

  Electrons such as electron microscope, X-ray free electron laser (XFEL), femtosecond X-ray pulsed light source by inverse Compton scattering, high repetition femtosecond time-resolved electron microscope, ultrashort pulse electron beam lithography system, energy recovery linac (ERL) An electron gun is used as a source. For example, the performance of an electron microscope varies depending on the electron gun. For example, the spatial resolution of an electron microscope varies depending on the emittance of an electron beam generated by an electron gun. In addition, the repetition frequency of the electron microscope changes according to the repetition frequency of the electron beam. Therefore, in order to improve the performance of the electron microscope, it is necessary to improve the quality of the electron beam emitted from the electron gun.

  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 (in some cases, in an 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, development of an electron gun that can stably generate a high-quality electron beam with low emittance and high brightness is desired.

  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 the case of using a photocathode electron gun, the electron bunch can be formed into a desired shape by shaping the pulse of the laser beam. In other words, the quality of the electron beam can be improved by shaping the pulse shape. For this reason, it is desired to shape the laser light pulse into a desired shape. For example, a technique for shaping laser light into a cylindrical shape using a pulse stacker is disclosed (Non-Patent Document 1). Here, the pulse stacker is composed of a wave plate and a polarizing beam splitter (Non-Patent Document 2). And it is branched into each polarization component (S polarization and P polarization) by the polarization beam splitter cube. Then, an optical path length difference is given to the branched pulse light, and it is combined by the combining polarization beam splitter. In the above document, since a stack of three stages is used, eight branched pulse lights are superimposed. In such a pulse stacker, for example, pulse shaping in the vertical direction (direction parallel to the optical axis) can be performed. That is, the pulse can be shaped in the vertical direction by adjusting the optical path length given to the branched pulse laser beam. Furthermore, in the processing method using pulsed laser light as shown in Patent Document 1, it is desired to shape the pulsed light into a desired shape in order to control the processing shape.

  When shaping the pulse, adjustments are made based on the actually measured pulse shape. When measuring the pulse shape of the laser light, the pulse shape in the horizontal direction (direction perpendicular to the optical axis) can be measured by, for example, a beam profiler or a CCD camera. Further, the pulse shape in the vertical direction can be measured by, for example, a streak camera.

  Short pulse ultraviolet laser light produced by harmonics is generally spatially nonuniform. That is, the distribution in the plane perpendicular to the traveling direction of light is not uniform. Further, since the laser light has coherence, fine noise appears in the electron beam profile. In Non-Patent Document 1, a compensation mirror is used to make the spatial distribution uniform.

  The compensation mirror is a deformable mirror (DM: Deformable Mirror), and has, for example, a plurality of electrodes (channels) for changing the surface (reflection surface) shape of the mirror. And the shape of a wave front changes by adjusting the voltage applied to each electrode. By optimizing the surface shape of the mirror with the aid of a genetic algorithm or the like, it is possible to correct the wavefront of the optical system and to control the direction and shape of the reflected beam.

Hiromitsu Serizawa “Aiming for a Laser Light Source That Can Meet Accelerator Requirements—Development of a Laser Light Source for Photocathode RF Electron Guns” Accelerators Vol3, No3, 2006 (251-262) Luminex Corporation [Search June 16, 2008], Internet <http://www.luminex.co.jp/> Pulse Stacker Kit JP 2002-205179 A

  In the method of Non-Patent Document 1, there is a possibility that the control becomes complicated. For example, the voltage applied to each electrode of the compensation mirror is obtained using an optimization algorithm or the like. That is, an automatic optimization program is created to control the compensation mirror. Furthermore, a configuration for supplying the voltage optimized by the automatic optimization program to each electrode based on an optimization algorithm such as a genetic algorithm becomes necessary. In addition, reproducibility in a strict sense is difficult. Therefore, the control for improving the quality of the electron beam may be complicated.

  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, an electron microscope, and an electron generation method capable of easily improving the quality of an electron beam. That is.

  An electron gun according to a first aspect of the present invention is an electron gun that emits electrons in a direction in which laser light is incident. The electron gun emits a laser light source and laser light from the laser light source. Includes a hollow light guide member that propagates inside while repeating total reflection, a hollow lens that refracts light emitted from the light guide member, and a photocathode on which the laser light refracted by the lens enters. Is. Thereby, the spatial distribution of light on the photocathode can be made uniform. Therefore, an electron beam with low emittance can be obtained.

  An electron gun according to a second aspect of the present invention is the above-described electron gun, wherein the light guide member includes a polarization-maintaining fiber bundle in which polarization-maintaining fibers that propagate while maintaining a polarization plane of light are bundled. And the direction of the polarization preserving axis on the exit end face of the light guide member is different depending on the exit position. 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 third aspect of the present invention is the electron gun described above, wherein the direction of the polarization preserving axis is symmetric with respect to the center of the emission end face. 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 fourth aspect of the present invention is the electron gun described above, wherein the polarization preserving axis direction is radial. 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 fifth aspect of the present invention is the electron gun described above, wherein the polarization plane preserving axis is arranged so that the laser light emitted from the light guide member is polarized in the radial direction as a whole. It is characterized by being. 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 sixth aspect of the present invention is the electron gun described above, wherein a metal pipe is provided inside the light guide member. Thereby, it is possible to prevent the light guide member from being colored by radiation such as an electron beam. Therefore, the laser beam can be propagated efficiently.

  An electron gun according to a seventh aspect of the present invention is the electron gun described above, wherein the metal pipe has a thickness capable of stopping electrons by its blocking ability. Thereby, it is possible to prevent the light guide member from being colored by radiation such as an electron beam. Therefore, the laser beam can be propagated efficiently.

  An electron gun according to an eighth aspect of the present invention is the electron gun described above, wherein the metal pipe is formed of copper having a thickness of 5 mm or more. Thereby, it is possible to prevent the light guide member from being colored by radiation such as an electron beam. Therefore, the laser beam can be propagated efficiently.

  An electron gun according to a ninth aspect of the present invention is the electron gun described above, wherein the lens is a hollow axicon lens. As a result, the depth of focus can be increased and the spatial distribution can be made more uniform.

  An electron gun according to a tenth aspect of the present invention is the above-described electron gun, wherein an annular beam that converts laser light from the laser light source into an annular beam between the laser light source and the light guide member. A generation means is provided. Thereby, a laser beam can be propagated efficiently.

  An electron gun according to an eleventh aspect of the present invention is the electron gun described above, wherein a metal pipe is provided in a hollow portion of the lens. Thereby, it can prevent that a lens colors. Therefore, the laser beam can be propagated efficiently.

  An electron gun according to a twelfth aspect of the present invention is the above-described electron gun, comprising a hollow mirror that reflects the laser light in the direction of the light guide member, and the hollow mirror is entirely covered with metal. It is constituted by a glass substrate. Thereby, an electron beam can be transported stably.

  An electron microscope according to a thirteenth aspect of the present invention includes the above-described electron gun, and an electron beam generated by the electron gun passes through the hollow portion of the lens and the light guide member and enters the sample. is there. Thereby, an electron beam can be conveyed efficiently and spatial resolution can be improved.

  An electron gun according to a fourteenth aspect of the present invention is an electron microscope in which an electron beam emitted from a photocathode is incident on a sample, and includes a laser light source and a position corresponding to the incident position of the laser light from the laser light source. A polarization conversion element that gives a phase difference, a lens that refracts laser light incident from the laser light source via the polarization conversion element, and a photocathode on which the laser light refracted by the lens enters. 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 microscope according to a fifteenth aspect of the present invention is the above-described electron microscope, wherein the polarization conversion element linearly polarizes the laser light substantially in the radial direction over the entire cross section, and in a region facing the optical axis. The electric vector is converted so that the vibration direction is opposite. 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 generation method according to a sixteenth aspect of the present invention is an electron generation method for generating electrons by irradiating light to a photocathode, and a hollow light guide in which incident light propagates through the inside while repeating total reflection. A step of causing light to enter the member; a step of refracting light emitted from the light guide member by a hollow lens; and a step of causing light refracted by the lens to enter the photocathode. Thereby, the spatial distribution of light on the photocathode can be made uniform. Therefore, an electron beam with low emittance can be obtained.

  An electron generation method according to a seventeenth aspect of the present invention is the above-described electron generation method, wherein the polarization-preserving fiber in which the light-guiding member is bundled with polarization-maintaining fibers that propagate while maintaining the polarization plane of light. It has a bundle, and the direction of the polarization preserving axis on the exit end face of the light guide member is different depending on the exit position. 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 generation method according to an eighteenth aspect of the present invention is the above-described electron generation method, characterized in that the direction of the polarization preserving axis is symmetric with respect to the center of the emission end face. is there. 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 generation method according to a nineteenth aspect of the present invention is the electron generation method described above, wherein the polarization preserving axis direction is radial. 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 generation method according to a twentieth aspect of the present invention is the above-described electron generation method, wherein the polarization plane preserving axis is arranged so that the laser beam emitted from the light guide member is polarized in the radial direction as a whole. It is characterized by being. 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 generation method according to a twenty-first aspect of the present invention is the above-described electron generation method, characterized in that a metal pipe is provided inside the light guide member. Thereby, it can prevent that a light guide member colors. Therefore, the laser beam can be propagated efficiently.

  An electron generation method according to a twenty-second aspect of the present invention is the above-described electron generation method, characterized in that the metal pipe has a thickness capable of stopping electrons by its blocking ability. Thereby, it can prevent that a light guide member colors. Therefore, the laser beam can be propagated efficiently.

  An electron generation method according to a twenty-third aspect of the present invention is the electron generation method described above, wherein the metal pipe is formed of copper having a thickness of 5 mm or more. Thereby, it can prevent that a light guide member colors. Therefore, the laser beam can be propagated efficiently.

  An electron generation method according to a twenty-fourth aspect of the present invention is the electron generation method described above, wherein the lens is a hollow axicon lens. As a result, the depth of focus can be increased and the spatial distribution can be made more uniform.

  An electron generation method according to a twenty-fifth aspect of the present invention is the electron generation method described above, wherein the light incident on the light guide member is an annular light beam. . Thereby, an electron beam can be conveyed efficiently.

An electron generating method according to a twenty-sixth aspect of the present invention is the above-described electron generating method, wherein a metal pipe is provided in a hollow portion of the lens. Thereby, it can prevent that a lens colors. Therefore, the laser beam can be propagated efficiently.
The laser beam is reflected by a hollow mirror in the direction of the light guide member,

  An electron generating method according to a twenty-seventh aspect of the present invention is the above-described electron generating method, wherein the hollow mirror is constituted by a glass substrate whose entire surface is covered with metal. Thereby, an electron beam can be transported stably.

A hollow mirror manufacturing method according to a twenty-eighth aspect of the present invention comprises a step of covering the entire surface except one surface of a glass substrate with a metal clay, and a step of removing a binder in the metal clay. Thereby, the hollow mirror which can be arrange | positioned in the path | route of an electron beam can be manufactured simply.
A hollow mirror manufacturing method according to a twenty-ninth aspect of the present invention comprises a step of providing a through hole in a glass substrate and a step of forming a metal thin film on the entire surface of the glass substrate provided with the through hole. is there. Thereby, the hollow mirror which can be arrange | positioned in the path | route of an electron beam can be manufactured simply.

  According to the present invention, it is possible to provide an electron gun, an electron microscope, and an electron generation method capable of easily improving the quality of an electron beam.

  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 is incident on the cathode. The electron beam 60 generated by the electron gun 100 is accelerated by the microwave from the microwave source 31. 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. Accordingly, electrons are emitted from the photocathode 25 in the direction in which the laser beam 50 is incident.

  The electron gun 100 includes a laser light source 11, an axicon lens 13, an axicon lens 14, a mirror 21, a light guide member 22, a lens 23, a resonator 24, a photocathode 25, and a microwave source 31. A mirror 21, a light guide member 22, a lens 23, a resonator 24, and a photocathode 25 are disposed in the vacuum chamber 20. Therefore, the mirror 21, the light guide member 22, the lens 23, the resonator 24, and the photocathode 25 are disposed in an ultrahigh vacuum.

  The laser light source 11 emits linearly polarized laser light 50. As the laser light source 11, for example, a Ti: Sapphire laser or a YAG laser with a regenerative amplifier can be used. For example, the fourth harmonic of a YAG laser or the third harmonic of a titanium sapphire laser can be used. Of course, the laser light source 11 and the laser wavelength to be used are not particularly limited, and can be selected according to the work function of the photocathode 25 and the like. Pulse light is emitted from the laser light source 11. Note that the pulse width of the pulsed light from the laser light source 11 may be extended using a pulse stacker.

  The light beam from the laser light source 11 becomes a parallel light flux and enters 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. The annular beam converting means is preferably provided in the atmosphere.

  Laser light 50 from the axicon lens 14 enters the mirror 21. The mirror 21 is inclined 45 ° with respect to the optical axis of the laser beam 50. Therefore, the mirror 21 reflects the laser beam 50 in the direction of the photocathode 25. Laser light 50 from the mirror 21 enters the light guide member 22. The mirror 21 is hollow in order to pass the electron beam 60.

  The light guide member 22 is a transparent member having a high refractive index. Accordingly, the laser light incident on the light guide member 22 is propagated by repeating total reflection inside the light guide member 22. Thereby, the spatial distribution of light can be made uniform, for example, an electron beam can be made into a flat top shape. The light guide member 22 is hollow in order to pass the electron beam 60. That is, the cross section of the light guide member 22 has a ring shape. As the light guide member 22, an annular kaleidoscope or a fiber bundle in which a plurality of fibers are bundled can be used. By using the light guide member 22 as a beam homogenizer, the profile can be made uniform. The configuration of the light guide member 22 will be described later.

  Laser light 50 emitted from the light guide member 22 enters the lens 23. The lens 23 is a hollow axicon lens. Bessel beam shaping can be performed by using an axicon lens. Therefore, the depth of focus becomes deep and the spatial profile on the photocathode 25 can be made uniform. The laser beam 50 is refracted by the lens 23 and enters the photocathode 25. That is, the lens 23 collects the laser beam 50 and irradiates the photocathode 25 with it. The lens 23 condenses the laser light 50 on the optical axis. The photocathode 25 is disposed at a crossing point of the laser beam by the lens 23. That is, the annular light beam is focused on the optical axis of the light beam. At this focusing position, the spot of the light beam is minimized. And the photocathode 25 is arrange | positioned in this focusing position.

  The mirror 21, the light guide member 22, and the lens 23 have a hollow shape with a central portion cut out. In other words, the mirror 21, the light guide member 22, and the lens 23 are hollow by making a through hole along the electron beam path. Then, the electron beam passes through the through holes that are hollow portions of the mirror 21, the light guide member 22, and the lens 23. Here, the cross section of the hollow part is circular. The center of the hollow portion coincides with the optical axis of the laser beam 50 and the optical axis of the electron beam 60. 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 21, the light guide member 22, and the lens 23 are used, most of the laser light is incident on the photocathode 25. In other words, the mirror 21 and the lens 23 have a hollow portion corresponding to the annular laser beam 50. Therefore, the annular laser beam 50 does not enter the mirror 21 and the hollow portion of the lens 23. Thereby, most of the laser light 50 is incident on the photocathode 25. Accordingly, it is possible to prevent a decrease in utilization efficiency of the laser beam 50. The cross-sectional shape of the hollow portion is not limited to a circle. That is, any material that can pass through the electron beam 60 may be used. For example, the hollow portion may have a rectangular cross-sectional shape.

  The laser beam 50 that has passed through the lens 23 enters the opening of the resonator 24. The laser beam 50 passes through the cavity portion of the resonator 24 and enters the photocathode 25. The laser beam 50 is condensed on the surface of the photocathode 25 by the lens 23. That is, the surface of the photocathode 25 is disposed at the focal position of the lens 23. Therefore, the condensing point of the laser beam 50 is the surface of the photocathode 25. When the laser beam 50 is incident on the photocathode 25, 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 25. That is, the mirror 21 is inclined 45 ° with respect to the photocathode 25.

As the photocathode 25, a metal cathode such as gold, copper or magnesium can be used. Further, as the material of the photocathode 25, diamond or the like may be used. Furthermore, CS ₂ Te, Na ₂ KSb or the like may be used as the photocathode 25. Of course, the material of the photocathode 25 is not particularly limited.

  Microwaves generated by the microwave source 31 are incident on the resonator 24. The resonator 24 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 25 is generated in the resonator 24 that is an RF resonator. Electrons generated at the photocathode 25 are accelerated by an electric field in the resonator 24. That is, the electron beam 60 having a predetermined velocity is emitted from the resonator 24. Here, the timing of the laser light pulse is adjusted in accordance with the standing wave generated in the resonator 24. That is, the microwave from the microwave source 31 and the pulse of the laser beam are synchronized. Thereby, electrons are generated from the photocathode 25 at the timing when the accelerating electric field is generated in the resonator 24. Therefore, the electron beam 60 is efficiently accelerated. The accelerated electron beam 60 passes through the mirror 21, the light guide member 22, and the hollow portions of the lens 23. 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 21, the light guide member 22, and the lens 23. The mirror 21, the light guide member 22, and the lens 23 do not interfere with the electron beam 60. For this reason, deterioration of the quality of the electron beam 60 can be prevented.

  Next, the configuration of the light guide member 22 will be described with reference to FIG. 2A and 2B are diagrams showing the configuration of the light guide member 22, FIG. 2A shows a front view of the light guide member 22, and FIG. 2B shows a side sectional view of the light guide member 22. . In FIG. 2, the Z direction indicates the propagation direction of the laser light 50, and the X direction and the Y direction are perpendicular to the Z direction. That is, the Z direction is parallel to the optical axis of the laser beam 50, and the XY plane is a plane perpendicular to the optical axis. The X direction and the Y direction are directions orthogonal to each other.

  As shown in FIG. 2A, the light guide member 22 has a hollow rod shape, and the electron beam 60 passes through the hollow portion. Since the light guide member 22 has a cylindrical shape, the XY section of the light guide member 22 has a ring shape. The outer peripheral surface and inner peripheral surface of the light guide member 22 in the XY plane are circular. The centers of the circular outer peripheral surface and inner peripheral surface coincide with the optical axes of the laser beam 50 and the electron beam 60. That is, the cylindrical light guide member 22 is disposed along the optical axis of the laser beam 50. The light guide member 22 is a transparent member having a higher refractive index than vacuum. For example, as the material of the light guide member 22, quartz, glass, resin, or the like can be used. In consideration of handling in a vacuum, it is preferable to use a quartz circular pipe. For example, by using a synthetic quartz pipe, the use range can be widened from a wavelength of 190 nm to 1100 nm, and in the future, application at 150 nm or more will be possible.

  The light guide member 22 has an incident end face 22a inclined. That is, one end of the light guide member 22 is cut so that the incident end face 22a is inclined from the XY plane. Thereby, the light propagating parallel to the Z direction is refracted at the incident end face 22a. The propagation direction changes within the light guide member 22 and enters the outer peripheral surface or inner peripheral surface of the light guide member 22. And it is totally reflected in the side surface (an outer peripheral surface and an inner peripheral surface) of the light guide member 22. Incident light propagates by repeating total reflection inside the light guide member 22. Then, the light is emitted from the emission end face 22 b of the light guide member 22. Therefore, the spatial intensity distribution of the light is uniform on the emission end face 22b of the light guide member 22. The light guide member 22 is a uniformizing unit that uniformizes the spatial distribution of light. That is, the spatial distribution of the light intensity on the exit end face 22b of the light guide member 22 is uniform. Light spreads and exits from the exit end face 22b. The shape of the emission end face 22b may be bent to function as a lens. Thereby, the spread of the light emitted from the emission end face 22b of the light guide member 22 can be reduced. The inclination angle of the incident end face 22a may be set based on the incident angle to the side face. Thereby, it can be made to propagate efficiently. When the laser beam spreads and enters the incident end face 22a, the incident end face 22a does not have to be inclined.

  Further, the light guide member 22 is provided with a hollow metal pipe 27. The metal pipe 27 has a cylindrical shape, and its outer diameter is substantially equal to the inner diameter of the light guide member 22. The metal pipe 27 is disposed in the hollow portion of the light guide member 22. That is, the metal pipe 27 and the light guide member 22 are arranged coaxially. The metal pipe 27 is made of metal such as copper, for example. Then, the thickness of the metal pipe 27 is increased so that electrons passing through the inside of the metal pipe 27 do not enter the light guide member 22. For example, in the case of 6 MeV electrons, the thickness of the copper serving as the metal pipe 27 is preferably 5 mm or more. Thereby, an electron can be stopped by the stopping power of a metal. That is, the metal pipe has a thickness capable of stopping electrons by its stopping power. Therefore, the thickness of the metal pipe 27 may be selected according to the electron stopping power.

  Since the inner peripheral surface of the light guide member 22 is protected by the metal pipe 27, it is possible to prevent scattered electrons from entering the light guide member 22. Thereby, it can prevent that a color center is made in quartz. That is, the light can be prevented from being absorbed by the light guide member 22 being colored. Therefore, light can be efficiently incident on the photocathode 25. In addition, in a normal high-frequency electron gun, the energy of electrons is about 6 MeV at the maximum, so if copper having a thickness of 5 mm or more is used, the light guide member 22 can be reliably prevented from being colored due to the incidence of electrons. Therefore, the laser beam can be propagated efficiently. Thereby, the brightness | luminance of an electron beam can be improved.

  The lens 23 and the mirror 21 are also preferably protected from the electron beam. Furthermore, it is necessary to prevent dark current from entering the lens 23, the mirror 21, and the light guide member 22. Therefore, the hollow portion is enlarged so that the transparent optical member comes to an area where the electron beam trajectory and the dark current trajectory do not hit. In particular, the lens 23 is closest to the cathode 25. For this reason, it is important to prevent the electron beam or dark current from hitting the lens 23. For example, the lens 23 can also be protected by providing a metal pipe in the hollow portion. That is, by using the protection pipe, it is possible to prevent an electron beam or dark current electrons from entering the lens 23. The protective pipe of the lens 23 is also preferably formed of copper having a thickness of 5 mm or more. In this way, a metal pipe is provided to allow electrons to pass through the hollow portion of the metal pipe. By doing so, even when a transmission optical system made of a transparent material is used, the occurrence of color centers can be reduced.

  Further, it is preferable that the entire surface of the mirror 21 is made of metal, and the hollow portion of the mirror 21 is separated from the optical axis of the electron beam by 9 mm or more. Thereby, the influence of a wake field can be reduced. It is possible to prevent the quality from deteriorating due to the force of the electron beam. For example, when an electron beam passes through a place where there is a metal wall, a mirror image charge runs through the metal wall in the opposite direction to the electrons. For this reason, the electron beam is affected by them. Although this influence can be reduced by setting the boundary condition such that there is no level difference in the middle of the metal pipe that is the target of the axis in the electron beam traveling direction, this is difficult. Therefore, in this embodiment, the metal walls are sufficiently separated to avoid this influence. That is, the mirror 21 covered with metal is disposed so as to be separated from the optical axis of the electron beam. Although depending on the charge density and β function of the electron beam bunches, the diameter of the hollow portion viewed from the electron beam is 20 mm here. Thereby, an electron beam can be transported stably.

  Note that the influence of the wake field can be reduced by using a dielectric multilayer mirror instead of a metal mirror as the mirror 21. However, since a color center is generated, it is difficult to use a dielectric multilayer mirror. That is, even when the directivity of the electron beam is high, the charge-up occurs when the dark current is incident on the dielectric multilayer mirror. As a result, the electron beam may be bent due to the Coulomb force. Furthermore, there is a possibility that the electric discharge charged on the mirror 23 causes discharge. When the discharge occurs, the trajectory of the electron beam changes so that it rebounds. Since the position of the electron beam changes so as to jump, it is not suitable for application to an electron microscope or the like that requires high resolution.

  In order to prevent these, it is necessary to cover all the glass substrate constituting the mirror 23 with metal or to make the substrate with all metal. An all-metal mirror with improved polishing accuracy is ideal in nature. However, when a mirror is made by polishing metal, it is difficult to obtain a mirror with good surface accuracy. Therefore, in this embodiment, a method of covering all the glass substrates with metal is adopted. This is because the organic solvent is degassed in a vacuum when a gold foil or the like is pasted using an adhesive. This is because there is a possibility that the vacuum worsens.

  How to make a hollow mirror that covers the entire surface with metal will be explained. For example, metal clay (http://www.silver-clay.com/) can be used. Then, silver and gold with a purity of 99.9% can be made by flying a metal clay binder (binder). In a specific process, an optical metal mirror surface is formed by evaporating metal on a hollow transparent glass substrate. Cover all surfaces with metal clay except the optical metal mirror surface. Here, the inner and outer side surfaces of the glass substrate are covered with clay. That is, metal clay is applied to the inner peripheral surface of the hollow portion. As a result, the entire surface of the glass substrate is covered with a metal such as silver or gold. Then, using an oven, for example, in the case of silver, baking is performed at 600 ° C., and the binder in the metal clay is removed. Thereafter, an optical coating of an aluminum mirror is applied to the mirror for the ultraviolet laser, and a gold coat is applied to the mirror for visible to infrared. As a result, an all-metal hollow mirror can be created. This hollow mirror is used as the mirror 21. Thus, the hollow mirror is constituted by a glass substrate whose entire surface is covered with metal. Alternatively, an aluminum-deposited mirror without a protective coating may be made first, and a gold foil may be attached to a portion of the mirror substrate where the mirror does not come out to cover the metal. As a result, the mirror 21 can be prevented from being charged up, and stable beam transportation is possible.

Thus, when creating the hollow mirror 23, a hollow glass substrate is prepared. That is, a glass substrate hollowed out in a circle is prepared. And the whole surface except one surface used as the reflective surface of a glass substrate is covered with a metal clay. Remove the binder in the metal clay. And a metal film is vapor-deposited on the surface used as a reflective surface. By doing so, a hollow mirror can be easily created.
Note that the entire surface of the glass substrate may be covered with a metal thin film. In this case, a through hole is provided in the glass substrate. Then, a metal thin film is formed on the entire surface of the glass substrate having the through holes by vapor deposition or the like. Thereby, the hollow mirror which can be arrange | positioned in the path | route of an electron beam can be manufactured simply.

  The light guide member 22 has a length that can make the spatial distribution uniform. Further, the outer diameter and inner diameter of the light guide member 22 are set according to the spot diameter of the laser light 50. That is, the ring-shaped laser light is prevented from protruding from the light guide member 22. The inner diameters of the light guide member 22 and the metal pipe 27 are set such that the electron beam 60 can pass through. That is, the inner diameter of the metal pipe 27 is made sufficiently larger than the spot diameter of the electron beam 60.

  By providing the light guide member 22 in the optical path, the spatial distribution of the laser light can be made uniform. The influence of speckle or the like can be reduced, and the intensity distribution of the laser beam on the XY plane becomes uniform. Then, the uniformed light is collected and incident on the photocathode. Thereby, the emittance of the electron beam can be reduced. That is, since uniform light is incident on the photocathode 25, the spatial distribution of electrons emitted from the photocathode 25 can be made uniform. Thereby, the space charge effect can be suppressed and emittance can be reduced.

  Furthermore, the pulse width can be extended by the wavelength dispersion of the light guide member 22. For example, in the case where the light guide member 22 is a quartz rod having a nonlinear dispersion effect, the band is broadened by the nonlinear dispersion effect of quartz, and the pulse width is increased in proportion to the length of the rod. By utilizing the dispersion of the light guide member 22 and the optical path difference, the pulsed light can be extended to a required pulse width. The length of the light guide member 22 is selected so that the optimum pulse width is obtained.

  By making the light guide member 22 hollow, the light guide member 22 can be disposed in the path of the electron beam 60. That is, the light guide member 22 can be disposed in the vacuum chamber 20. Thereby, since the light guide member 22 can be arrange | positioned in a vacuum, the light guide member 22 can be arrange | positioned near the photocathode 25. FIG. The distance from the light guide member 22 to the photocathode 25 can be shortened, and the apparatus configuration can be simplified. Light that spreads and exits from the light guide member 22 can be efficiently collected on the photocathode 25. That is, by disposing the light guide member 22 in a vacuum, the distance from the light guide member 22 to the lens 23 can be shortened even when light spreads from the light guide member 22 and is emitted. In other words, the distance from the lens 23 to the photocathode 25 can be increased. Thereby, the lens 23 with a long working distance can be used. Therefore, the high-quality electron beam 60 can be easily obtained.

  The mirror 21, the light guide member 22, and the lens 23 existing in the path of the electron beam 60 are disposed in a vacuum. That is, the mirror 21, the light guide member 22, and the lens 23 are disposed in the vacuum chamber 20. Accordingly, the laser beam 50 from the axicon lens 14 is incident on the mirror 21 through a window (not shown) provided on the side wall of the vacuum chamber 20.

  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. Further, the electron gun 100 may be used in a laser processing apparatus or a laser etching apparatus. For example, the spatial resolution can be improved by using the electron gun 100 for an electron microscope. In this case, the sample is irradiated with the electron beam 60 as a probe beam. If the emittance of the electron beam 60 is small, the spot size can be reduced. Therefore, since the spot size of the electron beam 60 on the sample can be reduced, the spatial resolution can be further improved.

  The electron gun according to the present embodiment is particularly suitable for a single shot time-resolved electron microscope. In the single-shot time-resolved electron microscope, the effect of the space charge effect is large. Therefore, the space charge effect can be reduced by irradiating spatially uniform light. Therefore, an electron beam having a flat top profile can be generated.

  A laser beam from the light guide member 22 is shaped into a Bessel beam using an annular axicon lens. For this reason, a spatially uniform flat top can be achieved. Therefore, an optimization algorithm for making the spatial profile uniform is unnecessary. Since the deformable mirror and its optimization algorithm are not necessary, the apparatus configuration can be simplified. That is, the spatial distribution can be made uniform without complicated control. Thus, a high-quality electron beam can be obtained by the above-described electron generation method.

Embodiment 2 of the Invention
An electron gun 100 according to the present embodiment will be described with reference to FIG. FIG. 3 is a diagram showing a configuration of the electron gun 100 according to the present embodiment. In the present embodiment, a polarization conversion element 15 and a polarization adjustment element 16 are provided between the axicon lens 14 and the mirror 21. The polarization conversion element 15 and the polarization adjustment element 16 are provided in the atmosphere. In the present embodiment, the light guide member 22 is not provided. Since the basic configuration other than these is the same as that of the first embodiment, the description of the overlapping parts is omitted.

  From the laser light source 11, linearly polarized laser light 50 is emitted. The laser light 50 emitted from the laser light source 11 is refracted by the axicon lenses 13 and 14 as in the first embodiment, and becomes an annular beam. 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 23, it is possible to generate Z-polarized light having a large electric field component in the Z direction (optical axis direction). 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 21. Then, the laser beam reflected by the mirror 21 enters the lens 23 as in the first embodiment. In the present embodiment, a hollow spherical lens or aspherical lens is used as the lens 23 instead of an axicon lens. The laser light collected by the lens 23 enters the photocathode 25.

  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 25. 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. 4A is a plan view schematically showing the configuration of the polarization conversion element 15. FIG. 4B is a diagram for explaining the polarization state of the laser light 50 that has passed through the polarization conversion element 15. FIG. 4C is a diagram for explaining another polarization state of the laser light 50 that has passed through the polarization conversion element 15. FIG. 5 is a perspective view for explaining the polarization state changed by the polarization conversion element 15 and the lens 23. FIG. 6 is a side view for explaining the polarization state changed by the polarization conversion element 15 and the lens 23. In FIGS. 5 and 6, only the polarization conversion element 15 and the lens 23 are shown for simplification of explanation, and other components (for example, the mirror 21 and the polarization adjustment element 16) are omitted. Yes. 4 to 6, the traveling direction of the laser beam 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. 4A, 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. 4A, 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. 4, 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. 4B 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. 4B, the polarization axes of the emitted light emitted from each divided region are 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 23 will be described. As shown in FIGS. 5 and 6, the pseudo-radial polarization is generated by the polarization conversion element 15. That is, as shown in FIG. 4B, 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 collected by the lens 23.

  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. The polarization direction is symmetric with respect to the center of the exit end face.

  Next, a method for generating Z-polarized light will be described with reference to FIGS. 5 and 6. The arrows in FIG. 5 and FIG. 6 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. 6, 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. 6, the lens 23 is not hollow, but is actually hollow.

  Next, the state in which the light transmitted through the polarization conversion element 15 is condensed on the photocathode by the lens 23 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. 6, a direction perpendicular to the light traveling direction (Y direction) is defined as an up-down direction, and a direction parallel to the light traveling direction (Z direction) is defined as a left-right direction.

  The light transmitted through the upper divided region 15a is refracted by the lens 23 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 23, the vibration direction remains as it is upward. The light transmitted through the lower divided region 15b is refracted by the lens 23 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 photocathode.

  After passing through the lens 23, 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 25, 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, on the photocathode, the electric vector of light does not vibrate in the direction perpendicular to the traveling direction.

  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 23 in this manner, the laser beam 50 is irradiated on the photocathode 25 in a state where the electric vector vibrates in a direction parallel to the traveling direction. Can do. Note that the electric field strength component in the Z direction varies depending on the focal length and NA (numerical aperture) of the lens 23. For example, the electric field strength component in the Z direction is proportional to the fourth power of the numerical aperture. That is, the electric field strength component in the Z direction can be increased by using the lens 23 having a short focal length and a large NA. Therefore, it is preferable to use a spherical lens or an aspheric lens rather than an axicon lens.

  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 25. 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.

  In this way, the Z-polarized laser beam 50 is incident on the photocathode 25. Therefore, a strong electric field is generated in the Z direction on the photocathode surface. Thereby, the effective work function of the photocathode 25 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.

  Next, the polarization adjusting element 16 will be described. The polarization adjustment element 16 is an electro-optic element, and adjusts the polarization state generated by the polarization conversion element 15 based on the voltage applied from the polarization adjustment power source 19. The polarization adjusting element 16 has a transparent electro-optic crystal such as BBO, for example. Then, the electro-optic crystal is arranged for each divided region shown in FIG. 4A, and a voltage is applied independently for each region. Specifically, a Pockels cell whose refractive index changes in proportion to the strength of the electric field is provided. Alternatively, a liquid crystal optical element or the like may be used as the polarization adjusting element 16. The polarization state can be adjusted by controlling the voltage applied by the polarization adjusting power source 19. Thereby, the shift | offset | difference of a polarization direction can be reduced and it can approximate more to radial polarization. That is, the polarization direction in each divided region can be brought close to the polarization direction shown in FIG. In addition, when only the polarization conversion element 15 can increase the electric field in the Z direction on the photocathode 25, the polarization adjustment element 16 may not be used.

  In the electron gun 100 according to the present embodiment, the effective work function can be reduced. Thereby, low-power laser light can be used, and damage to the photocathode 25 can be reduced. That is, even when the number of electrons contained in one bunch is high, it is not necessary to use high-power laser light. Damage to the photocathode 25 can be reduced. Thereby, a repetition frequency can be made high. It is possible to prevent the photocathode 25 from being damaged. Therefore, the life of the photocathode 25 can be extended. In this embodiment mode, a transmissive photocathode may be used instead of the reflective photocathode. The light incident on the lens 23 does not have to be radial polarized light. That is, it is only necessary that the polarization conversion element 15 approaches the radially polarized light.

Embodiment 3 of the Invention
An electron gun according to this embodiment will be described with reference to FIG. FIG. 7 is a diagram showing the overall configuration of the electron gun 100. In the electron gun 100 according to the present embodiment, a polarization-maintaining fiber bundle in which polarization-maintaining fibers are bundled is used as the light guide member 22. The Z-polarized light is generated using not the polarization conversion element 15 but the polarization plane preserving fiber bundle. That is, the light guide member 22 functions as the polarization conversion element 15. The configuration other than the light guide member 22 is the same as that of the electron gun 100 shown in the first embodiment. Therefore, description of the same parts as those described in the first and second embodiments is omitted.

  As in the first embodiment, the linearly polarized laser beam 50 is reflected by the mirror 21 and enters the light guide member 22. The light guide member 22 converts linearly polarized light into radial polarized light. Then, the laser light that has been radially polarized by the light guide member 22 enters the lens 23. Then, the light is condensed by the lens 23 and enters the photocathode 25. On the photocathode 25, the laser beam 50 is Z-polarized light. The lens 23 is not an axicon lens but a refractive index distribution type lens such as GRADIUM (registered trademark), an aspherical lens, or the like in order to increase the electric field in the Z direction.

  Next, the polarization plane preserving fiber bundle that is the light guide member 22 will be described with reference to FIGS. 8 and 9. FIG. 8 is a perspective view showing the light guide member 22 and schematically showing the polarization state of the laser beam 50 passing therethrough. FIG. 9 is a plan view showing the incident end face 22 a and the outgoing end face 22 b of the light guide member 22, and schematically shows the direction of the polarization plane at each end face of the light guide member 22. That is, the directions of the polarization preserving axes at the incident end face 22a and the outgoing end face 22b are indicated by arrows. The linearly polarized light in the direction of the polarization axis propagates through the polarization plane preserving fiber.

  As shown in FIG. 8, a cylindrical polarization plane preserving fiber bundle is the light guide member 22. That is, a polarization plane preserving fiber bundle is configured by bundling a plurality of polarization plane preserving fibers. The light guide member 22 is hollow as in the first embodiment. Therefore, no polarization plane preserving fiber is provided on the optical axis 51 of the laser beam 50. Light incident on the polarization preserving fiber propagates with the polarization plane preserved. That is, the laser beam having a polarization plane parallel to the polarization preserving axis of the polarization preserving fiber propagates.

  Then, by gradually changing the polarization preserving axis of the polarization plane preserving fiber, it becomes radial polarization. That is, the direction of the polarization preserving axis can be changed by twisting the polarization preserving fiber. When the polarization plane preserving fiber is twisted from the incident end face 22a toward the outgoing end face 22b, the direction of the polarization preserving axis rotates. Therefore, the direction of the polarization preserving axis can be changed between the incident end face 22a and the outgoing end face 22b. Here, the direction of the polarization preserving axis is adjusted according to the position on the XY plane. That is, the angle at which the polarization-preserving fiber is twisted is changed according to the position in the XY plane. The direction of the polarization plane at the exit end face 22b of the light guide member 22 differs depending on the exit position. Thereby, the polarization state of light can be controlled so as to be radial polarization.

  Here, the direction of the polarization plane stored by the polarization plane maintaining fiber will be described with reference to FIG. As shown in FIG. 9A, the incident end face 22a is described by being divided into four divided regions 28a to 28d. The divided area 28a is arranged on the upper side, the divided area 28b is arranged on the lower side, the divided area 28c is arranged on the left side, and the divided area 28d is arranged on the right side. The divided areas 28a to 28d are divided symmetrically with respect to the center point. Accordingly, the four divided regions 28a to 28d are arranged radially. In this way, the four regions divided radially are divided regions 28a to 28d. The size of each divided area is equal. The divided regions 28a to 28d are provided over the entire circumferential direction. Accordingly, each of the divided regions 28a to 28d has a sector shape with an inner angle corresponding to the center point of 90 °.

  Similarly, the emission end face 22b is divided into divided areas 29a to 29d and will be described. That is, as shown in FIG. 9B, the emission end face 22b is divided into four. The divided area 29a is arranged on the upper side, the divided area 29b is arranged on the lower side, the divided area 28c is arranged on the left side, and the divided area 28d is arranged on the right side. Therefore, the divided area 28a of the incident end face 22a corresponds to the divided area 29a of the outgoing end face 22b. That is, the light incident on the divided area 28a is emitted from the divided area 29a. Similarly, the divided area 28b of the incident end face 22a corresponds to the divided area 29b of the outgoing end face 22b, the divided area 28c of the incident end face 22a corresponds to the divided area 29c of the outgoing end face 22b, and the divided area 28d of the incident end face 22a is emitted. This corresponds to the divided region 29d of the end face 22b. The light incident on the divided area 28b is emitted from the divided area 29b, the light incident on the divided area 28c is emitted from the divided area 29c, and the light incident on the divided area 28d is emitted from the divided area 29d.

  In the incident end face 22a, the directions of the polarization preserving axes are the same in all the divided regions 28a to 28d. That is, on the incident end face 22a, the polarization preserving axes of all the polarization preserving fibers are in the same direction. Here, the polarization preserving axis is in the upward direction (+ Y direction). This direction coincides with the polarization direction of the laser beam 50. That is, the polarization plane of the laser beam 50 and the polarization preserving axis directions of the divided regions 28a to 28d are parallel to each other. Thereby, light propagates efficiently.

  In the output end face 22b, the polarization preserving axes of the divided regions 29a to 29d are different. In the divided area 29a, the polarization preserving axis is in the upward direction (+ Y direction), and in the divided area 29b, the polarization preserving axis is in the downward direction (−Y direction). In the divided region 29c, the polarization preserving axis is in the left direction (−X direction), and in the divided region 29d, the polarization preserving axis is in the right direction (+ X direction). Therefore, similarly to the polarization conversion element 15, radial polarization can be obtained. That is, at the positions facing each other across the optical axis, the polarization direction is opposite. The direction of the polarization preserving axis is symmetric with respect to the center of the exit end face.

  For example, between the incident end face 22a and the outgoing end face 22b, the polarization plane preserving fiber included in the divided regions 28b and 29b is twisted by 180 °, and the polarization preserving fiber included in the divided regions 28c and 29c is twisted by −90 ° and divided. A polarization preserving axis as shown in FIG. 9 can be obtained by twisting the polarization preserving fiber included in the regions 28d and 29d by + 90 °. Therefore, radial polarization can be easily obtained.

  Then, when the radially polarized laser light is condensed by the lens 23, as described in the second embodiment, it becomes Z-polarized light on the photocathode 25. 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 25. 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. For this reason, a stable photocathode material can be used even if it is opened to the atmosphere, such as metal or diamond. As a result, it is possible to realize the electron gun 100 with low running cost and high maintainability. In addition, the repetition frequency can be increased. Thus, by using the polarization plane preserving fiber bundle, the same effect as that of the second embodiment can be obtained.

  Further, in the polarization plane preserving fiber included in the light guide member 22, the laser light propagates by repeating total reflection. Thereby, the spatial distribution of light on the emission end face 22b can be made uniform. By using the polarization plane preserving fiber bundle, the low emittance electron beam 60 can be generated as in the first embodiment. The light incident on the lens 23 does not have to be radial polarized light. That is, it is only necessary that the polarization conversion element 15 approaches the radially polarized light. In the present embodiment, the incident end face 22a may not be inclined.

Embodiment 4 of the Invention
An electron microscope according to this embodiment will be described with reference to FIG. FIG. 10 is a diagram illustrating an overall configuration of the electron microscope 200. In the present embodiment, the electron gun 100 shown in the first embodiment is used. That is, the electron microscope 200 includes the electron gun 100 according to the first embodiment. Here, description will be made assuming that the electron microscope 200 is a transmission electron microscope (TEM). The electron microscope 200 includes an electron lens 41, a deflector 42, and a detector 44 in addition to the electron gun 100 in order to observe the sample 43.

  Electrons generated at the photocathode 25 of the electron gun 100 are accelerated by the resonator 24. The accelerated electron beam 60 passes through the lens 23, the light guide member 22, and the hollow portion of the mirror 21. The electron beam 60 is focused by an electron lens 41 provided in the electron microscope 200. The electron lens 41 converges the electron beam 60 by generating an electric field or a magnetic field in the path of the electron beam 60.

  The deflector 42 deflects the electron beam 60 by generating an electric field or a magnetic field in the path of the electron beam 60. The electron beam 60 is deflected by the deflector 42 and enters the sample 43 to be observed. That is, the deflector 42 deflects the electron beam 60 in the X direction and the Y direction. As a result, the electron beam 60 can be incident on a desired position of the sample 43. That is, observation at an arbitrary position of the sample 43 becomes possible.

  Depending on the component and structure of the sample 43, the amount of electron transmission differs. Therefore, the sample 43 can be observed by the transmission image of the electron beam 60. The electron beam 60 that has passed through the sample 43 is focused by the two electron lenses 41. The deflector 42 deflects the electron beam 60. Then, the electron beam 60 deflected by the deflector 42 is focused by the electron lens 41. The electron beam 60 transmitted through the sample 43 is enlarged and projected by the three electron lenses 41. An electron beam 60 focused by the electron lens 41 enters the detector 44. The detector 44 includes a light emitter and a camera. For example, when the electron beam 60 enters the fluorescent plate, fluorescence is generated. The fluorescence is detected by a CCD camera or the like. Time-sharing data is acquired by sorting the data into the CCD array by the deflector 42 and reconstructing the data later. A CCD with a gate is preferably used for the CCD camera. Thereby, the sample 43 can be enlarged and observed. As the detector 44, a back-illuminated CCD, an HPD (Hybrid Photo Detector), a MAPs (Monolithic Active Pixel sensor), or the like may be used.

  In the electron microscope 200 described above, the electron gun emits a low emittance electron beam 60. Therefore, the beam spot of the electron beam 60 can be reduced on the sample 43. For example, the spot diameter of the electron beam can be about 10 μm. Thereby, the resolution of the electron microscope 200 can be improved.

  The configuration of the electron microscope 200 is not limited to the above. For example, in FIG. 10, four electron lenses 41 are provided, but the number of electron lenses 41 is not particularly limited. That is, it is sufficient that one or a plurality of electron lenses 41 are disposed in the vacuum chamber 20.

The electron microscope 200 may be a single shot time-resolved TEM. That is, observation is performed using electrons generated when one pulse of laser light is incident. In order to improve the time resolution with a single shot, it is necessary to increase the number of electrons in one bunch. For this reason, the space charge effect becomes a problem, but in the electron microscope 200 according to the present embodiment, light having a uniform spatial distribution is incident on the photocathode 25, and therefore the space charge effect can be reduced. For example, in Sample 43 surface, it is possible to realize the electron density of approximately 10 ⁷ cells / 1 [mu] m square. In the case of time-resolved TEM, it is preferable to use a photocathode 25 having a high quantum efficiency (QE) and a wavelength of 300 to 400 nm. In order to suppress the space charge effect, the light guide member 22 may extend the pulse width. In the case of a single shot, for example, it becomes possible to observe with a time resolution in the range of msec to μsec.

Embodiment 5 of the Invention
An electron microscope 200 according to the present embodiment will be described with reference to FIG. FIG. 11 is a diagram illustrating an overall configuration of the electron microscope 200. In this embodiment, the electron gun according to the second embodiment is used. That is, the electron microscope 200 includes the electron gun 100 according to the second embodiment. Here, description will be made assuming that the electron microscope 200 is a transmission electron microscope (TEM). Since the path of the electron beam 60 is the same as that of the electron microscope 200 according to the fourth embodiment, the description of overlapping parts is omitted.

  In the electron microscope 200 according to the present embodiment, a configuration for realizing high repetition will be described. Here, as described in Embodiment 2, since the Z deflection is used, the repetition frequency of the laser light can be increased. Therefore, the repetition frequency of electrons emitted from the photocathode 25 can be increased.

  In the present embodiment, the electron beam 60 transmitted through the sample 43 is deflected by the deflector 42. Thereby, the incident position of the electron beam 60 in the detector 44 changes. The deflector 42 is controlled by a controller 45. The controller 45 controls the deflector 42 so as to synchronize with the pulsed light emitted from the laser light source 11. Thereby, different bunches are incident on different positions on the CCD camera of the detector 44. Enlarged images of different locations on the sample 43 can be acquired with one frame of the camera. Therefore, observation can be performed in a time division manner, and the repetition frequency can be increased. As described above, in the case of time division, the final stage deflector 42 assigns images to each part of the light receiving element array of the detector 44 and images. That is, a time series number is given to each address, and the data is rearranged later. Thereby, it is possible to capture images with high repetition.

  In order to increase the repetition frequency, the electron microscope 200 is a superconducting electron microscope. That is, the resonator 24 is a superconductive RF cavity. For superconducting RF cavities, the L band (1.3 GHz) where technology has been accumulated is selected. In this case, continuous repeated operation at a maximum of 1.3 GHz is possible. As the continuous operation cavity, a single cavity having an acceleration performance of 30 to 40 MV / m or a multiple cavity of 20 MV / m is used. Z-polarized light is used to realize a high repetition rate, large current, and high-quality electron source. Two types of electron guns for accelerating electrons from the photocathode 25 are a superconducting radio frequency (RF) photocathode electron source and a photocathode direct current (DC) electron source. In order to realize this electron source, the high-efficiency and long-life photocathode 25 is a common technical development issue for both, but the high efficiency (high quantum efficiency) is due to the Schottky effect induced by laser Z polarization. Realized and long life is achieved by using metal cathode.

  It should be noted that if the Schottky effect induced by the laser Z polarization is used effectively, the laser to be irradiated must have a pulse of 100 femtoseconds or less. Without using Z-polarized light, irradiate the cathode with a laser pulse with a three-dimensional ideal shape of 10 to 40 ps (the ideal pulse width depends on the frequency used (S band, L band, etc. in the case of a high-frequency photocathode electron source)) However, at present, the intensity of the individual pulse energy of the 1.3 GHz high repetition laser becomes insufficient. In this case, the Schottky effect may not be expected, and a high quantum efficiency (high QE) cathode having a quantum efficiency of about 1 to 10% is required.

  The 1.3 GHz high repetition rate laser light source 11 is based on a fiber laser, but supercontinuum or femtosecond titanium using a photonic crystal crystal with pump harmonics of the second harmonic (SHG) of a CW Yb: fiber laser. Using NOPA with sapphire as seed light, wideband amplification and compression generate a femtosecond 1.3 GHz high repetition rate laser. This laser is described in the following document. (H. Tomizawa and Virtual laboratory LAAA (Laser-aided Accelerator Association), <Proposal of Fiber-laser-based photocathode light source for both ERL & ILC projectsc, Internal Report 2006-003, SPring8 Report Series, August 2006 (in Japanese) .

  Although it is inferior to Yb: fiber laser in terms of energy efficiency and long wavelength, an Er: fiber laser can oscillate in femtoseconds as it is. These contrasts are summarized in the following documents. H. Tomizawa, S. Kawato, S. Matsubara, <Proposal of laser light source for energy recovery linac (ERL) e, 2006, Sr.

  A high-current micro-electron bunch beam array composed of pulses with an electron beam pulse width of 10 to 40 psec and a charge amount of 200 pC or less is maintained at high quality (standardized emittance to 0.2 mm · mrad), and is an integral fraction of 1.3 GHz. Can be generated with high repetition. A laser-driven time-division electron microscope using such a high repetition rate, large current, high quality electron source will be realized. Continuous operation that takes advantage of the high efficiency of superconductivity is essential for obtaining high-quality beams with high repetition. By using high-repetition high-intensity light pulses that can be realized for the first time by continuous operation of a superconducting high-frequency cavity, it is possible to continuously observe in detail, like frame-by-frame, how extremely fine objects change very quickly. Can do. "

  The configuration of the electron microscope 200 is not limited to that shown in the fourth and fifth embodiments. That is, the electron microscope 100 shown in Embodiments 1 to 3 can be applied to various types of electron microscopes. Furthermore, you may combine each embodiment suitably.

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 typically the structure of the light guide member used for the electron gun concerning Embodiment 1. FIG. It is a figure which shows typically the structure of the electron gun concerning Embodiment 1 of this invention. FIG. 5 is a diagram illustrating a configuration of a polarization conversion element used in an electron gun according to a second embodiment. 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 typically the structure of the electron gun concerning Embodiment 3 of this invention. It is a figure which shows typically the structure of the light guide member used for the electron gun concerning Embodiment 3. It is a figure which shows typically the polarization direction of the light guide member used for the electron gun concerning Embodiment 3. FIG. It is a figure which shows typically the structure of the electron microscope concerning Embodiment 4 of this invention. It is a figure which shows typically the structure of the electron microscope concerning Embodiment 5 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Laser light source 13 Axicon lens 14 Axicon lens 15 Polarization conversion element 15a-15h Division | segmentation area | region 16 Polarization adjustment element 19 Power supply for polarization adjustment 20 Vacuum chamber 21 Mirror 22 Light guide member 22a Incidence end surface 22b Output end surface 23 Lens 24 Resonator 25 Photocathode 27 Metal pipe 28a to 28h Divided region 31 Microwave source 41 Electron lens 42 Deflector 43 Sample 44 Detector 45 Controller 50 Laser beam 60 Electron beam 100 Electron gun 200 Electron microscope

Claims (29)

An electron gun that emits electrons in the direction in which the laser beam has entered,
A laser light source;
A hollow light guide member in which laser light from the laser light source enters, and the incident light propagates inside while repeating total reflection;
A hollow lens that refracts light emitted from the light guide member;
An electron gun comprising: a photocathode on which the laser light refracted by the lens is incident. The light guide member has a polarization plane preserving fiber bundle in which polarization plane preserving fibers that propagate while maintaining the polarization plane of light are bundled,
The electron gun according to claim 1, wherein a direction of the polarization preserving axis on an emission end face of the light guide member is different depending on an emission position.   3. The electron gun according to claim 2, wherein the direction of the polarization preserving axis is symmetric with respect to the center of the emission end face.   4. The electron gun according to claim 2, wherein the direction of the polarization preserving axis is radial.   5. The electron gun according to claim 2, wherein the polarization plane preserving axis is arranged so that the laser light emitted from the light guide member is polarized in a radial direction as a whole. .   6. The electron gun according to claim 1, wherein a metal pipe is provided inside the light guide member.   The electron gun according to claim 6, wherein the metal pipe has a thickness capable of stopping electrons by its stopping power.   The electron gun according to claim 6 or 7, wherein the metal pipe is made of copper having a thickness of 5 mm or more.   9. The electron gun according to claim 1, wherein the lens is a hollow axicon lens.   10. An annular beam generating means for converting the laser light from the laser light source into an annular beam is provided between the laser light source and the light guide member. The electron gun according to the item.   The electron gun according to any one of claims 1 to 10, wherein a metal pipe is provided in a hollow portion of the lens. A hollow mirror that reflects the laser light in the direction of the light guide member;
The electron gun according to claim 1, wherein the hollow mirror is formed of a glass substrate whose entire surface is covered with metal. The electron gun according to any one of claims 1 to 12, comprising:
An electron microscope in which an electron beam generated by the electron gun passes through the lens and a hollow portion of a light guide member and is incident on a sample. An electron microscope that injects an electron beam emitted from a photocathode into a sample,
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 hollow lens that refracts laser light incident from the laser light source via the polarization conversion element;
An electron microscope comprising: a photocathode on which the laser light refracted by the lens is incident.   15. The polarization conversion element linearly polarizes laser light 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. electronic microscope. An electron generation method for generating electrons by irradiating light to a photocathode,
Incident light is incident on a hollow light guide member that propagates inside while repeating total reflection;
Refracting light emitted from the light guide member by a hollow lens;
Allowing the light refracted by the lens to enter a photocathode. The light guide member has a polarization plane preserving fiber bundle in which polarization plane preserving fibers that propagate while maintaining the polarization plane of light are bundled,
The electron generation method according to claim 16, wherein a direction of the polarization preserving axis on an emission end face of the light guide member is different depending on an emission position.   18. The electron generating method according to claim 17, wherein the direction of the polarization preserving axis is symmetric with respect to the center of the emission end face.   18. The electron generation method according to claim 16, wherein the direction of the polarization preserving axis is radial.   The electron generation according to any one of claims 17 to 19, wherein the polarization plane preserving axis is arranged so that the laser light emitted from the light guide member is polarized in a radial direction as a whole. Method.   The method for generating electrons according to any one of claims 16 to 20, wherein a metal pipe is provided inside the light guide member.   The electron generating method according to claim 21, wherein the metal pipe has a thickness capable of stopping electrons by its stopping power.   23. The electron generating method according to claim 21, wherein the metal pipe is formed of copper having a thickness of 5 mm or more.   24. The electron generating method according to any one of claims 16 to 23, wherein the lens is a hollow axicon lens.   The electron generation method according to any one of claims 16 to 24, wherein the light incident on the light guide member is an annular light beam.   The electron generation method according to any one of claims 16 to 25, wherein a metal pipe is provided in a hollow portion of the lens. The laser beam is reflected by a hollow mirror in the direction of the light guide member,
27. The electron generating method according to any one of claims 16 to 26, wherein the hollow mirror is constituted by a glass substrate whose entire surface is covered with metal. Covering the entire surface except one surface of the glass substrate with metal clay,
Removing the binder in the metal clay, and a method of manufacturing a hollow mirror. Providing a through hole in the glass substrate;
Forming a metal thin film on the entire surface of the glass substrate having the through hole.

Images