JP5544598B2 - Photocathode high-frequency electron gun and electron beam apparatus provided with photocathode high-frequency electron gun - Google Patents

Description

  The present invention relates to a photocathode high-frequency electron gun and an electron beam apparatus including the photocathode high-frequency electron gun.

  The photocathode high-frequency electron gun (hereinafter referred to as “electron gun”) can control the time width of the electron beam during electron generation with a laser, so it generates a short-pulse, high-charge, high-quality, high-brightness electron beam. It is widely used as a device that can do this. Generation of an electron beam having a time width of less than 1 ps (rms) has been recognized as being very useful, and is expected to be used for, for example, a terahertz electromagnetic wave generator (for example, Patent Document 1). In other words, the electron beam width is shorter than the period of the terahertz light, and thus the generated light is coherently overlapped and strengthened. Examples of the generation method include not only orbital radiation but also transition radiation and diffracted radiation.

  In a conventional electron gun, a cathode (cathode) and an acceleration cavity are integrally formed, and a high-intensity electron beam can be obtained by immediately accelerating the generated electrons.

JP 2006-190886 A

  Although the electron beam obtained from the acceleration cavity is high in brightness, the time width of the electron is widened by the space charge effect (repulsive force between electrons) until the electron is sufficiently accelerated, and the time of the generated electron beam There was a problem that the width would be about 3 ps (rms).

  Therefore, an object of the present invention is to provide an electron gun that can easily change the time width of an electron beam, and an electron beam apparatus including the electron gun.

An electron gun according to claim 1 of the present invention includes a cathode portion, a laser irradiation port for irradiating the cathode portion with laser light, an acceleration cavity portion for accelerating electrons emitted from the cathode portion, and the acceleration sky. In the photocathode high-frequency electron gun including a high-frequency introduction unit that introduces a high frequency into the body, the acceleration cavity includes two or more basic cells, a basic iris provided between the basic cells, and the A final cell communicated with a final iris sandwiched by an outlet of the basic cell, and the basic iris includes an outlet cell provided at a terminal end of the basic cell and another basic cell adjacent to the outlet cell. Including the rear iris, the thickness of the final iris is different from the thickness of the rear iris and is formed thinner than the thickness of the rear iris .

The electron beam apparatus according to claim 2 of the present invention is characterized in that when the wavelength of the high frequency is λ, the thickness of the final iris is λ / 8 to λ / 10.

An electron beam apparatus according to a third aspect of the present invention includes the electron gun according to the first or second aspect.

According to the electron gun of the present invention, the time width of the electron beam can be easily changed by changing the thickness of the final iris and making it thinner than the thickness of the rear iris .

It is sectional drawing which shows typically the structure of the acceleration cavity main body of the electron gun which concerns on this embodiment. It is a figure which shows the result of the electromagnetic field calculation of the acceleration cavity part which concerns on this embodiment. It is a graph which shows the phase space distribution of the electron beam produced | generated with the electron gun which concerns on this embodiment. It is a graph which shows the phase space distribution of the electron beam produced | generated with the electron gun which concerns on this embodiment. It is a graph which shows the phase space distribution of the electron beam produced | generated with the electron gun which concerns on this embodiment. It is a graph which shows the phase space distribution of the electron beam produced | generated with the electron gun which concerns on a comparative example. It is a figure which shows the result of the electromagnetic field calculation of the acceleration cavity part which concerns on a modification. It is a graph which shows the phase space distribution of the electron beam produced | generated with the electron gun which concerns on the said modification. It is a schematic diagram which shows the structure of the transmission electron microscope provided with the electron gun which concerns on this embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(overall structure)
As shown in FIG. 1, the electron gun 1 according to this embodiment includes an acceleration cavity body 2. The accelerating cavity body 2 is formed of a material having a small amount of gas emission in a vacuum and having thermal conductivity and electrical conductivity, for example, oxygen-free copper. The cathode portion 3, the laser irradiation port 4, and the acceleration cavity Part 5 is provided. The electron gun 1 generates electrons 9A by accelerating the electrons 9 emitted by irradiating the laser beam 8 from the laser irradiation port 4 to the cathode portion 3 with the acceleration cavity 5, and the electrons 9A are accelerated. Irradiation is performed from the electron beam exit port 6 through the central axis 7 of the main body 2.

  The electron gun 1 according to the present embodiment includes an electron 9A that can perform further magnetic compression (compression using an orbital difference due to a magnetic field) or velocity compression (compression using an in-bunch velocity difference) by optimally selecting an acceleration phase. It is possible to control the vertical phase space such as generation.

  The accelerating cavity main body 2 has a cylindrical shape. The cathode 3 is provided on one end surface of the accelerating cavity main body 2, and the accelerating cavity 5 is formed on the inner peripheral surface.

  The acceleration cavity portion 5 is provided in the order of the basic cell 10 and the final cell 11 in the direction of the central axis 7 from the cathode portion 3 side. The basic cell 10 is composed of a first cell 10A and a second cell 10B as an exit cell, and is constituted by a so-called 1 + 0.6 cell. In the case of the present embodiment, the first cell 10A is configured with 0.6 cells, and the second cell 10B is configured with 1 cell.

  A basic iris 12 is provided between the first cell 10A and the second cell 10B. In the present embodiment, since the basic cell 10 is composed of two pieces, the basic iris 12 is also referred to as a rear iris.

  A final iris 13 is provided between the final cell 11 and the second cell 10B. The first cell 10A, the basic iris 12, the second cell 10B, the final iris 13, and the final cell 11 are formed symmetrically with respect to the central axis 7.

  A laser irradiation port 4 is provided in the first cell 10A. The laser irradiation port 4 irradiates the cathode unit 3 with laser light 8 emitted from a laser light source (not shown).

  The final cell 11 is formed so that the axial length is the same as that of the second cell 10B. The final cell 11 is provided with a high-frequency introduction unit 15 and a vacuum suction unit 16. The high frequency introducing unit 15 introduces high frequency power supplied from a high frequency power source (not shown) into the accelerating cavity 5. As a result, a standing wave having a resonance frequency suitable for the internal structure is generated in the accelerating cavity 5. The vacuum suction unit 16 communicates with a vacuum device (not shown), and thereby the inside of the acceleration cavity body 2 is evacuated. The acceleration cavity 5 is provided with a cooling mechanism (not shown) for cooling the iris.

  In the case of this embodiment, the final iris 13 is formed with a thickness, that is, an axial length shorter than the basic iris 12. The axial length of the final iris 13 varies depending on the use example, but when a linear part is selected in the high frequency phase (when used for speed difference bunching), it is about 1/8 to 1/10 of the high frequency wavelength. It is preferable to select. For example, if the high-frequency power is about 105 mm per cycle, the axial length of the basic iris can be about 21 mm, and the axial length of the final iris can be about 13 mm. The axial length of the final iris 13 is preferably selected within a range that does not interfere with the iris cooling mechanism.

  In addition, a well-known structure is applicable about structures other than the above-mentioned structure among the structures with which the normal electron gun 1 is provided.

(Function and effect)
The laser beam 8 is irradiated from the laser irradiation port 4 to the cathode portion 3 at a phase of 20 to 30 degrees (with the high frequency phase of acceleration being 0 degree). Then, the surface of the cathode part 3 is excited and electrons 9 are emitted from the surface of the cathode part 3.

  At the same time, an electric field of about 100 MV / m is normally generated in the accelerating cavity 5 by introducing high frequency power from the high frequency introducing portion 15.

  The electrons 9 emitted from the surface of the cathode part 3 are accelerated by the electric field while propagating in the order of the first cell 10A, the basic iris 12, the second cell 10B, the final iris 13, and the final cell 11, and pass through the central axis 7. Irradiated as electrons 9A from the electron beam exit port 6.

  Normally, the electron gun 1 has a structure called 1 + 0.6 cell in order to reach a sufficient energy. By matching the half-wavelength of the high frequency with the length of the cavity, the beam is accelerated with substantially the same phase in any cell (cavity) to obtain the maximum acceleration voltage.

  On the other hand, the acceleration cavity 5 according to the present embodiment is further provided with a final cell 11 having the same axial length as the second cell 10B on the electron beam exit port 6 side of the basic cell. The axial length of the final iris 13 provided between the two cells 10B and the final cell 11 was made shorter than that of the basic iris 12. Thereby, the electron gun 1 according to the present embodiment can modulate the acceleration phase in the final cell 11 and perform bunching.

  The electron gun 1 can freely control the vertical phase space by adjusting the electric field strength in the final cell 11 by the resonance frequency of the accelerating cavity 5 or adjusting the generation phase of the initial electrons 9A, and can be used for bunching. An optimal phase space distribution can be generated, or an electron 9A having a minimum energy spread can be generated.

  In addition, since the electron gun 1 can shorten the time width of the electrons 9A only by optimizing the configuration of the accelerating cavity 5, the configuration can be simplified and is actually about 2 m × 2 m. It can be configured. Therefore, it can be applied to various electron beam apparatuses.

  In addition, since the electron gun 1 can shorten the time width of the electrons 9A, it is possible to generate a terahertz light source that can control the frequency at which the superposition is achieved by changing the time width of the electrons 9A.

  In addition, cutting-edge electron diffraction microscopes and other cutting-edge technologies require generation of electrons 9A that cuts down to 1 ps, and they can be used not only as an application but also as an electron source for building accelerators. It is possible to transport and accelerate the electrons 9A that have already become extremely short pulses.

  The electron gun 1 can generate an electron 9A having an energy of about 3 to 8 MeV (* low energy can be further achieved by changing the amount of high-frequency power), and can be used as, for example, a terahertz light source, In addition, coherent electrons 9A can be generated.

  Using the electron gun 1 according to this embodiment as a model, a simulation was performed for a case where the final iris length was shortened. In the simulation, the first cell has an inner diameter of 41.85 mm, the axial length (7λ / 30) mm, the second cell has an inner diameter of 42.04 mm, the axial length (3λ / 10) mm, the final cell has an inner diameter of 41.85 mm, An acceleration cavity having an axial length (3λ / 10), an axial length of the basic iris (2λ / 10) mm, and an axial length of the final iris (λ / 10) mm was used. Note that λ = 105 mm (2856 MHz wavelength).

  As a comparative example, a simulation was performed using an acceleration cavity having a so-called 1 + 0.6 cell structure. The acceleration cavity used in the comparative example has an inner diameter of 41.565 mm for the first cell, an axial length (7λ / 30) mm, an inner diameter of 42.0595 mm, an axial length (3λ / 10) mm, and a basic iris. Those having an axial length of (2λ / 10) mm were used. In this simulation, instead of the laser beam, an electron distribution similar to the laser shape was generated on the cathode portion. Incidentally, the inner diameter of the iris is about 12.5 mm.

  The simulation conditions in the above examples and comparative examples are as shown in Table 1.

  FIG. 3 shows the result of simulation under condition 1 in Table 1. From the result of this figure, it has confirmed that the electron 9A of the state where the energy of the front electron was low and the energy of the back electron was high could be produced | generated. Therefore, in the electron gun 1 according to the present embodiment, the electrons 9A generated based on the speed difference can be bunched.

  FIG. 4 shows the phase space distribution obtained when the final cell is used for bunching. In this figure, since the distribution of electrons with respect to the horizontal axis is narrow, it can be seen that the axial length (about 0.5 psec) of the generated electron beam is short. From the results shown in this figure, it was confirmed that by adjusting the initial acceleration phase and the like, the time width of about 0.5 psec can generate the electron 9A having a short time width without bunching due to the speed difference.

  FIG. 5 shows the result of simulation under condition 2 in Table 1. This simulation is an example when the energy difference is minimized. From this figure, it can be seen that the spread of energy is small because the distribution of electrons with respect to the vertical axis is narrow. As is apparent from the above results, the electron gun 1 can reduce not only the time width of the generated electrons 9A but also the energy difference.

  The results of the comparative example are shown in FIG. In this figure, it can be seen that the distribution of electrons with respect to the vertical axis is wide and the length of the generated electron beam in the axial direction is long.

  The simulation is merely an example of the present embodiment, and the present invention is not limited to this. In addition, the simulation can obtain the same result under the conditions shown in Table 2.

(Modification)
The present invention is not limited to the above embodiment, and can be appropriately changed within the scope of the gist of the present invention.

  For example, in the above embodiment, the case where the axial length of the final iris 13 is selected at about 1/8 to 1/10 of the high-frequency wavelength has been described. However, the present invention is not limited to this, and may be smaller than the rear iris. It may be larger than 1/8 of the high frequency wavelength or smaller than 1/10.

  In the above embodiment, the case has been described in which the axial length of the final iris 13 is shorter than the axial length of the basic iris 12 that is the rear iris, but the present invention is not limited to this, and the axial length of the final iris 13 is. May be longer than the axial length of the basic iris 12, which is the rear iris. That is, as shown in FIG. 7, the acceleration cavity 20 according to this modification is formed such that the axial length 21 of the final iris is longer than that of the basic iris 12. The axial length of the final iris 13 is preferably selected within a range in which the coupling between cells is maintained. Thereby, the electron gun can intentionally irradiate an electron beam having a long time width. A simulation was performed using the electron gun according to this modification as a model. In the simulation, the first cell has an inner diameter of 41.555 mm, the axial length (7λ / 30) mm, the second cell has an inner diameter of 42.0495 mm, the axial length (3λ / 10) mm, the final cell has an inner diameter of 41.043 mm, An accelerating cavity having an axial length (3λ / 10) mm, a basic iris axial length (2λ / 10) mm, and a final iris axial length (3λ / 10) mm was used. Note that λ = 105 mm (2856 MHz wavelength). The simulation conditions are shown in Table 3.

  From the result of FIG. 8, it was confirmed that a state in which the energy of the front electrons is high and the energy of the rear electrons is low can be generated. Therefore, the electron gun according to the present modification can increase the time width of the electron beam due to the speed difference. The simulation according to the present modification is not limited to the conditions shown in Table 3 above, and similar results can be obtained even under the conditions shown in Table 4.

  Examples of the electron beam apparatus including the electron gun according to the embodiment include a transmission electron microscope, a terahertz light source, and a nondestructive inspection apparatus. Further, it can be applied to electron beam diffraction in a transmission electron microscope or the like.

  As shown in FIG. 9, the transmission electron microscope 30 includes an irradiation lens 31, a sample stage 32, an objective lens 33, and a magnifying lens system 34 along the electron beam 9 </ b> A irradiated from the electron gun 1. The electron beam 9 </ b> A emitted from the electron gun 1 is irradiated on the sample 35 held on the sample stage 32 by the irradiation lens 31. Then, the electron beam 9A that has passed through the sample 35 is magnified by the objective lens 33 and the magnifying lens system 34 and projected onto an imaging device 36 such as a CCD camera. By applying the electron gun 1 according to the present embodiment to the transmission electron microscope 30, an ultrahigh voltage electron microscope with an acceleration voltage of 100 kV or higher can be obtained.

DESCRIPTION OF SYMBOLS 1 Photocathode high frequency electron gun 3 Cathode part 4 Laser irradiation port 5 Acceleration cavity part 8 Laser beam 9 Electron 10 Basic cell 11 Final cell 12 Basic iris 13 Final iris 15 High frequency introduction part

Claims (3)

A cathode part;
A laser irradiation port for irradiating the cathode part with laser light;
An accelerating cavity for accelerating electrons emitted from the cathode,
In a photocathode high-frequency electron gun comprising a high-frequency introduction portion that introduces a high frequency into the acceleration cavity,
The acceleration cavity is
Two or more basic cells;
A basic iris provided between the basic cells;
A final cell communicated across the final iris at the outlet of the basic cell,
The basic iris includes a rear iris disposed between an outlet cell provided at the terminal of the basic cells and another basic cell adjacent to the outlet cell,
The photocathode high-frequency electron gun is characterized in that a thickness of the final iris is different from a thickness of the rear iris and is formed thinner than a thickness of the rear iris . 2. The photocathode high-frequency electron gun according to claim 1 , wherein when the wavelength of the high frequency is λ, the thickness of the final iris is λ / 8 to λ / 10. Electron beam apparatus characterized by comprising a photocathode RF electron gun according to claim 1 or 2 wherein.

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