JP2001305300A - High frequency electron gun - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high frequency electron gun capable of suppressing the back bombardment while ensuring a degree of the freedom of an accelerating cavity to extend the life of a cathode and providing a stable electron beam output. SOLUTION: In this high frequency electron gun for leading the electrons emitted from a thermion source 2 as an electron beam from an opening 3 by a high frequency electric field applied to a high frequency accelerating cavity 1, an electron guiding magnetic field such as cusp field or the like for guiding the electrons traveling in the direction within the high frequency accelerating cavity 1 so as not to collide with the thermion source 2 is formed in the high frequency accelerating cavity 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency electron gun in which back bombardment is suppressed using a thermionic electron source.

[0002]

2. Description of the Related Art A high-frequency electron gun can obtain a beam having a very low emittance as compared with a conventional electrostatic electron gun. In particular, high-frequency electron guns that use a thermionic electron source can be used in free electron laser devices and storage ring injection because they can be much smaller than systems that combine an electrostatic electron gun with a buncher. It has high utility value. In a high-frequency electron gun, electrons are accelerated in the traveling direction when the phase of the high-frequency electric field is in the forward direction, but when electrons generated at the cathode ride on the opposite phase of the high-frequency electric field, they are accelerated in the opposite direction to the cathode. A phenomenon of collision with the cathode, that is, back bombardment occurs. When back bombardment occurs, the cathode is damaged and the cathode deteriorates. Further, since the cathode temperature rises due to back bombardment, the current increases after the macro pulse, so that there is a problem that stable operation is difficult particularly when a long macro pulse is generated in a free electron laser generator.

On the other hand, Japanese Patent Application Laid-Open No. Hei 6-89680 discloses a method in which a concave wall having a cathode as a bottom is provided and one high-frequency electrode is used to apply a high-frequency current. A high-frequency electron gun that forms a high-frequency electric field that converges in the opposite phase and gradually expands in diameter from the exit port toward the concave wall in the opposite phase is disclosed. In the disclosed electron gun, the electrons accelerated in the opposite phase collide with the wall surface around the cathode and are absorbed, thereby preventing the deterioration of the cathode. However, in the disclosed method, the shape of the accelerating electric field itself is set to a specific shape by selecting the electrode shape, and the degree of freedom of the shape of the accelerating cavity is limited, so that the design is restricted. Also, since high-energy electrons are difficult to deflect,
There are certain restrictions on the effect of avoiding collision with the cathode.

[0004]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to suppress back bombardment while securing the degree of freedom of the design of the accelerating cavity, to prolong the life of the cathode, and to stably output the electron beam. Is to supply a high-frequency electron gun that can obtain

[0005]

In order to solve the above-mentioned problems, in a high-frequency electron gun, an electron emitted from a thermionic source is extracted from an opening as an electron beam by a high-frequency electric field applied to a high-frequency acceleration cavity. An electron induction magnetic field for guiding electrons traveling in the opposite direction in the cavity so as not to collide with the thermionic electron source is formed in the high-frequency acceleration cavity. The high-frequency electron gun of the present invention has an electron-induced magnetic field for guiding electrons traveling in the opposite direction so as not to collide with the thermionic electron source while maintaining the electric field properties in the high-frequency accelerating cavity as those of the conventional electron gun. Things. Electrons having a speed accelerated in the opposite direction when the high-frequency electric field is in the opposite phase are guided by the magnetic field and deflected, for example, in the radial direction of the accelerating cavity.

Accordingly, there is less concern that the thermionic electron source is heated by a large amount of collision energy to change the amount of emitted electrons, and the consumption of the electrodes is reduced and the life can be maintained. The back bombardment of high energy electrons occupies most of the heat energy given to the cathode. However, considering the effect on the cathode temperature, the low energy electrons have a small penetration depth into the cathode, so that thermal energy is localized on the cathode surface. Therefore, the effect of the low energy electrons is rather large on the cathode surface temperature. The high-frequency electron gun of the present invention has a large deflection effect on low-energy electrons, and thus efficiently contributes to stabilization of the cathode temperature. In addition, the characteristics of the high-frequency electric field are the same as those in the related art, and the electrode to which the high-frequency is applied does not need to have a special shape. Therefore, a design utilizing the accumulated knowledge of the accelerating cavity can be made.

The electron induction magnetic field for guiding electrons traveling in the opposite direction of the high-frequency electron gun of the present invention can be realized by a cusp magnetic field. When an axisymmetric cusp magnetic field is formed so that the extraction axis of the electron beam coincides with the axis of the point cusp, and the line cusp plane where the magnetic field intensity is zero comes near the thermionic electron source, the cathode If the electron traveling backwards slightly deviates from the extraction axis, it is deflected so as to become entangled with the magnetic flux formed in the direction away from the axis, so that the number of electrons directly hitting the cathode is reduced.

[0008] The cusp magnetic field can be generated by a pair of solenoid coils arranged such that the same poles are adjacent to each other across the line cusp plane. A pair of solenoid coils are located outside the wall of the acceleration cavity, slightly in front of the cathode and across a plane perpendicular to the extraction axis. For example, if the same current is applied to two same solenoid coils in opposite directions,
The line cusp is located between the solenoid coils, and a plane-symmetric cusp electric field is formed across the line cusp.

Further, a cusp electric field can be formed by one solenoid coil provided outside the acceleration cavity and one electromagnet or permanent magnet provided behind the cathode on the extension of the drawing shaft. In this case, the line cap is formed between the solenoid coil and the electromagnet or the permanent magnet, and the line cusp plane where the magnetic field strength becomes zero intersects with the axis of the cusp magnetic field, that is, the extraction axis of the electron beam, near the front of the cathode. Such a caps magnetic field is formed. Even if the line cusp plane is not perpendicular to the drawing axis, it is sufficient that the trajectory of the traveling electron in the opposite direction can avoid the cathode portion.

If the solenoid coil or the electromagnet is excited by a pulse power supply that generates a pulse current equal to or smaller than the macro pulse width, and a rising or falling gradient of the pulse is used, a stronger magnetic field can be obtained. To reduce back bombardment. Further, a permanent magnet wheel having a plurality of permanent magnets connected to a plurality of permanent magnets is provided outside the acceleration cavity with the magnetic poles directed in the axial direction of the acceleration cavity, and a cusp electric field is formed by the magnet wheel and a single electromagnet or permanent magnet provided on the drawing shaft. May be generated. If a permanent magnet is used, the influence of heat generated by the solenoid coil on the acceleration cavity can be eliminated.

The solenoid coil may be charged in the wall of the acceleration cavity or in a disk provided in the cavity.
In such an arrangement, the solenoid coil approaches the drawing shaft and the coil diameter can be reduced, so that a stronger magnetic field can be efficiently formed.
Alternatively, a space may be formed at the center of the cathode to avoid collision of high-energy electrons traveling backward on the extraction shaft at high speed, thereby preventing the temperature of the cathode from rising. Furthermore, the magnetic field of the accelerating cavity is distributed so that the backward electrons are concentrated in the cavity formed in the center of the cathode,
The collision with the cathode itself may be avoided.
This space may be used as a Faraday cup to positively absorb electrons. Industrial applicability of the high-frequency electron gun of the present invention is expected by using it for a free electron laser device that requires a stable single-wavelength laser output.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments with reference to the drawings. FIG. 1 is a sectional view and a magnetic field intensity distribution diagram showing a first embodiment of the present invention, FIG. 2 is a diagram showing a magnetic flux distribution of a cusp magnetic field used in the present embodiment, and FIG. 3 is a second embodiment of the present invention. FIG. 4 is a sectional view showing a third embodiment of the present invention, FIG. 5 is a sectional view showing a permanent magnet ring used in the present embodiment, and FIG. 6 is a section showing a fourth embodiment of the present invention. FIG. 7 and FIG. 7 are a sectional view and a magnetic field intensity distribution diagram showing a fifth embodiment of the present invention.

[0013]

Embodiment 1 In the high-frequency electron gun of this embodiment, two solenoid coils are installed so that the same poles are adjacent to each other, and a cusp magnetic field is formed near the cathode. As shown conceptually in FIG. 1, the high-frequency electron gun includes an accelerating cavity 1 for introducing a high-frequency electric field, a cathode 2 for emitting thermoelectrons, and an electron beam extraction opening 3 opened at a position facing the cathode. Prepare. In the electron gun of this embodiment, a pair of solenoid coils 5 and 6 are arranged outside the outer wall 4 of the acceleration cavity 1. The two solenoid coils 5 and 6 are located near the front of the cathode 2 with a plane perpendicular to the extraction axis interposed therebetween, and a current flows so as to be magnetized in directions opposite to each other. Then, the opposing surfaces of the solenoid coils have the same pole, and a magnetic field distribution state called a cusp magnetic field as shown in FIG. 2 appears.

In the cusp magnetic field in the present embodiment, a point cusp appears in the direction of the electron beam extraction axis z, and a line cusp appears on a surface sandwiched by the solenoid coils. The line cusp is on a plane passing near the front of the cathode 2. Note that the magnetic field strength B along the electron beam axis z drawn from the cathode 2 crosses zero on the line cusp plane as shown in the lower diagram of FIG. The sign is different, and the absolute value takes the maximum value, and changes so as to approach zero at a distance on the axis.

When a high-frequency electric field is applied to the accelerating cavity under such a magnetic field distribution, the electrons emitted from the cathode 2 are accelerated in the order phase of the high-frequency electric field and emitted from the opening 3 as an electron beam. If the electron beam is emitted along the extraction axis z passing through the aperture 3, the cusp magnetic field will not be a hindrance because it exerts a force in a direction to converge the electron beam. When the emitted electrons are decelerated or accelerated in the opposite phase of the high-frequency electric field, the magnetic flux of the cusp magnetic field appears in the axial direction toward the cathode 2 in the direction of diffusing toward the line cusp. The electrons move further away from the axis z as they become involved in the magnetic flux. Particularly for low energy electrons, the deflection effect by the magnetic field is large.

Therefore, if there is no cusp magnetic field, most of the electrons accelerated in the opposite phase of the high-frequency electric field collide with the cathode 2. On the other hand, in the high-frequency electron gun of the present embodiment in which the cusp magnetic field exists in the high-frequency accelerating cavity, The cusp magnetic field acts as an electron induction magnetic field, and many of the electrons traveling in the opposite direction avoid the cathode 2 and collide with the wall around the cathode. For this reason, heat generation due to back bombardment with respect to the cathode 2 is small and thermoelectrons are not generated excessively, so that the electron beam does not gradually increase and a stable macro pulse with a predetermined intensity is obtained. Can occur. In a free electron laser, the stability of the electron beam is required because the laser wavelength changes when the electron energy changes. However, by using the electron gun of the present embodiment in a free electron laser device, the electron beam in a macro pulse becomes more stable. Stabilize.

[0017]

Embodiment 2 In the high-frequency electron gun of this embodiment, as shown in FIG. 3, one solenoid coil 5 is installed outside the acceleration cavity 1 so as to be located slightly in front of the cathode 2, and behind the cathode 2. Since the cusp magnetic field is formed by disposing the permanent magnet 7 in the first embodiment, the other configuration is the same as that of the first embodiment. Therefore, only different portions will be described to avoid duplication. In the cusp magnetic field in this embodiment, a point cusp appears in the direction of the electron beam extraction axis, and a line cusp appears on a surface sandwiched between the solenoid coil and the permanent magnet 7. The curved surface where the magnetic field intensity becomes zero has a shape similar to a paraboloid of revolution having a line cusp as a skirt and having a vertex near the front of the cathode 2.

In the magnetic field distribution of the present embodiment, the deflecting force received by the inversely accelerated electrons deviating from the extraction axis is smaller than in the case of the first embodiment, but has a sufficient effect for the purpose of reducing back bombardment to the cathode 2. However, it can be realized with a simpler configuration. Needless to say, the operation does not change even if an electromagnet is provided instead of the permanent magnet behind the cathode 2. In addition, if a pulse power source whose excitation time is about the same as or smaller than the macro pulse width is used as the excitation power source for the electromagnet and is synchronized with the high-frequency electric field, the steepness of the rising and falling edges will be used to further increase the efficiency. Can well reduce back bombardment. Also, when using a solenoid coil, the heat generated by the exciting current affects the temperature of the accelerating cavity, which may be a hindrance to obtaining a stable electron beam. There is also an advantage that the problem of heat generated is reduced.

[0019]

Embodiment 3 In the high-frequency electron gun of this embodiment, as shown in FIG. 4, instead of the solenoid coil of Embodiment 2, a permanent magnet 8 is positioned outside the acceleration cavity 4 and slightly in front of the cathode 2. It was installed. A permanent magnet 7 is provided behind the cathode 2, and a cusp magnetic field is formed by these magnetic field generators. Since other configurations are the same as those of the first embodiment, only different portions will be described to avoid duplication. As shown in FIG. 5, the permanent magnets 8 installed outside the accelerating cavity 1 have a plurality of permanent magnets 8 surrounding the accelerating cavity 1 in the form of radiation with the same magnetic pole directed in the axial direction. The magnet ring is arranged so as to bite into the outer wall 4, and has a form in which the opposite magnetic poles of the permanent magnet 8 are connected by an annular yoke 9. The permanent magnet 7 provided on the cathode 2 has the same magnetic pole that faces forward of the cathode 2 as the magnetic pole that faces in the axial direction of the magnet wheel. The cusp magnetic field of the present embodiment has substantially the same magnetic field distribution as that of the second embodiment, and also has the effect of reducing the back bombardment of backward electrons to the cathode 2.

[0020]

Fourth Embodiment As shown in FIG. 6, the high-frequency electron gun of the present embodiment is configured such that the solenoid coils 5 and 6 of the first embodiment are connected to the acceleration cavity 1.
Is provided inside the wall of the acceleration cavity 4 instead of being provided on the outside of the acceleration cavity. Other configurations are the same as those of the first embodiment, and only different portions will be described. Cathode 2
The solenoid coil 6 which should be located behind the acceleration cavity 2 is embedded in the rear wall of the acceleration cavity 2, and the solenoid coil 5 which should be located in front of the cathode 2 is charged and embedded in a disk 10 which projects axially from the outer wall 4 of the acceleration cavity. Have been. According to the arrangement of the present embodiment, the magnetic field generating mechanism can be arranged near the extraction axis, so that a strong magnetic field can be generated near the axis even if the magnetization intensity is small, and the required electron An induction magnetic field can be efficiently formed in the acceleration cavity 1. Similarly, in the second and third embodiments, a strong magnetic field can be efficiently formed by embedding the solenoid coil 5, the electromagnet, the permanent magnets 7, 8 and the like in the outer wall 4 of the acceleration cavity, the disk 10, and the like. it can.

[0021]

Embodiment 5 The high-frequency electron gun of this embodiment has a space provided at the center of the cathode 2 as shown in FIG.
The description of the same parts as those in the above embodiment is omitted. Backward electrons having high energy do not largely deflect due to the action of the electron induction magnetic field as well as the absence of the electron induction magnetic field of each of the above embodiments, but are back bombarded to the cathode 2 to raise the temperature of the cathode 2. And abrasion. The high-frequency electron gun according to the present embodiment is characterized in that a hole is provided in the center of the cathode 2 to reduce the rate at which high-energy electrons that are reversely accelerated and return collide with the electrode.

According to the high-frequency electron gun of this embodiment, the amount of thermal electrons emitted around the cathode 2 is reduced during the macropulse because the thermal energy given to the cathode 2 is small even when back bombardment of electrons occurs. It is possible to generate a stable electron beam by preventing the change. If a hole provided in the center of the cathode 2 is formed as a Faraday cup, the colliding electrons are trapped and released as a current, thereby preventing secondary electrons and secondary ions from being released. The influence can be suppressed more effectively.

Also, for example, the acceleration cavity 1 in the first embodiment
A pair of solenoid coils 5, 6 disposed outside the outer wall 4 of
When an axial magnetic field is generated in the same direction by supplying a current in the same direction to the
A magnetic field intensity B distribution as shown in the lower part of FIG. 2 is generated, and magnetic flux comes to gather near the cathode 2, so that the electron beam accelerated in the opposite direction can be converged. When such an electron induced magnetic field exists in the acceleration cavity 1, most of the back bombardment electrons concentrate near the center of the cathode 2 and are absorbed by the Faraday cup 11, so that the influence of the back bombardment is significantly suppressed. Can be. Note that, instead of the pair of solenoid coils 5 and 6, 1
Needless to say, the same effect can be obtained by using a single solenoid coil, or by using a magnet wheel or the like.

[0024]

As described above, according to the high-frequency electron gun of the present invention, the back bombardment electrons are caused to collide with the peripheral wall avoiding the cathode by the action of the electron induced magnetic field, or the center of the cathode. It can be absorbed by the Faraday cup provided in the section and can be prevented from colliding with the cathode. As a result, the amount of thermionic electrons emitted from the cathode is stabilized, so that the electron beam is stabilized, and the laser wavelength in the macro pulse is stabilized even when used in a free electron laser generator or the like. Further, the cathode is prevented from being damaged by back bombardment electrons, so that the service life is prolonged and the operation and maintenance costs can be reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view and a magnetic field intensity distribution diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a magnetic flux distribution of a cusp magnetic field in the first embodiment.

FIG. 3 is a sectional view showing a second embodiment of the present invention.

FIG. 4 is a sectional view illustrating a third embodiment of the present invention.

FIG. 5 is a sectional view showing a permanent magnet wheel used in a third embodiment.

FIG. 6 is a sectional view illustrating a fourth embodiment of the present invention.

FIG. 7 is a sectional view and a magnetic field intensity distribution diagram showing a fifth embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 High frequency acceleration cavity 2 Cathode 3 Electron beam extraction opening 4 Acceleration cavity outer wall 5 Solenoid coil 6 Solenoid coil 7 Permanent magnet 8 Permanent magnet 9 Yoke 10 Disk 11 Faraday cup

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masaaki Byojima 118 Futatsuka, Noda City, Chiba Prefecture Kawasaki Heavy Industries Noda Plant F-term (reference) 5C030 BB17

Claims (9)

[Claims] A high-frequency electron gun in which a thermoelectron source is provided in a high-frequency acceleration cavity having an opening at a position facing the opening so that an electron beam is extracted from the opening by a high-frequency electric field. A high frequency electron gun characterized in that an electron induction magnetic field for guiding electrons traveling in a direction so as not to collide with the thermoelectron source is formed in the high frequency acceleration cavity. 2. The method according to claim 1, wherein the electron induced magnetic field is a cusp magnetic field having a point cusp on an extraction axis of an electron beam and having a line cusp plane at which the magnetic field intensity becomes zero near the thermionic electron source. The high-frequency electron gun according to claim 1, wherein: 3. The high-frequency electron gun according to claim 2, wherein said cusp magnetic field is generated by a pair of solenoid coils arranged such that the same poles are adjacent to each other across said line cusp plane. . 4. The cusp magnetic field is generated by one solenoid coil provided to surround the accelerating cavity and one electromagnet or permanent magnet provided on the drawing shaft. The high-frequency electron gun according to claim 2. 5. The high-frequency electron gun according to claim 3, wherein the excitation of the solenoid coil or the electromagnet is performed using a pulse power supply having a rising time equal to or shorter than the frequency of the high-frequency electric field. 6. The cusp magnetic field is generated by a permanent magnet wheel provided so as to surround the accelerating cavity with a pole facing inward and a single electromagnet or permanent magnet provided on the drawing shaft. 3. The high-frequency electron gun according to claim 2, wherein: 7. The high-frequency electron gun according to claim 2, wherein the solenoid coil is embedded in a wall forming an acceleration cavity or in a disk. 8. A Faraday cup is provided at a central portion of the thermoelectron source, wherein the electron induction magnetic field has an axis of symmetry in an extraction axis of an electron beam, and the intensity of the magnetic field is maximized near the thermoelectron source. 2. The magnetic field according to claim 1, wherein
High frequency electron gun as described. 9. A free electron laser device using the high-frequency electron gun according to claim 1.