JP3436352B2 - High frequency electron gun - Google Patents

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 by using a thermoelectron source.

[0002]

2. Description of the Related Art A high frequency electron gun can obtain a beam having a very small emittance as compared with an electrostatic type electron gun which has been conventionally used. In particular, a high-frequency electron gun using a thermoelectron source can be made much smaller than a system in which a buncher is combined with an electrostatic electron gun. As a high utility value. In the high-frequency electron gun, when the phase of the high-frequency electric field is in the forward direction, the electrons are accelerated in the traveling direction.However, when the electrons generated at the cathode get in the opposite phase of the high-frequency electric field, they are accelerated in the opposite direction to the cathode. Back bombardment occurs. When back bombardment occurs, the cathode is damaged and the cathode is deteriorated. Further, since the cathode temperature rises due to the back bombardment, the current increases toward the end of the macro pulse, so that there is a problem that stable operation is difficult especially when a long macro pulse is generated in the free electron laser generator.

On the other hand, in Japanese Unexamined Patent Publication No. 6-89680, a concave wall having a cathode as a bottom is provided to serve as one of the high frequency electrodes, and a high frequency current is applied to the high frequency electrode. There is disclosed a high-frequency electron gun that forms a high-frequency electric field that converges and then gradually expands in diameter from the exit port toward the concave wall in the opposite phase. In the disclosed electron gun, the electrons accelerated in the opposite phase collide with the wall surface around the cathode and are absorbed, so that the deterioration of the cathode can be prevented. However, in the disclosed method, the accelerating electric field itself is made into a specific shape by selecting the electrode shape, and the degree of freedom of the accelerating cavity shape 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]

Therefore, the problem to be solved by the present invention is to suppress back bombardment while ensuring the degree of freedom in the design of the accelerating cavity to extend the life of the cathode and to provide a stable electron beam output. Is to supply a high frequency electron gun.

[0005]

In order to solve the above problems, in a high frequency electron gun in which electrons emitted from a thermoelectron source are extracted as an electron beam from an aperture by a high frequency electric field applied to a high frequency acceleration cavity, high frequency acceleration is performed. It is characterized in that an electron induction magnetic field for guiding electrons traveling in the opposite direction in the cavity so as not to collide with the thermionic source is formed in the high frequency acceleration cavity. The high frequency electron gun of the present invention is provided with an electron induction magnetic field for guiding electrons traveling in the opposite direction so as not to collide with the thermionic source while keeping the electric field property in the high frequency acceleration cavity the same as that of the conventional electron gun. It is a thing. Electrons accelerated in the opposite direction and having a velocity when the high-frequency electric field is in the opposite phase are guided to the magnetic field and deflected, for example, in the radial direction of the accelerating cavity, so that the ratio of collision with the cathode electrode, which is the thermionic source, decreases.

Therefore, it is less likely that the thermoelectron source is heated by a large amount of collision energy to change the amount of electron emission, and the consumption of the electrode is reduced to maintain the life. The back bombardment of high energy electrons occupies most of the thermal energy given to the cathode. However, considering the influence 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 low-energy electrons have a great influence on the cathode surface temperature. Since the high-frequency electron gun of the present invention has a large deflection effect on low-energy electrons, it effectively contributes to stabilization of the cathode temperature. Further, the property of the high-frequency electric field is the same as the conventional one, and the electrode for applying the high-frequency does not need to have a special shape, so that the design utilizing the accumulated knowledge of the accelerating cavity is possible.

The electron induction magnetic field for guiding the electrons traveling in the opposite direction of the high frequency electron gun of the present invention can be realized by a cusp magnetic field. An axisymmetric cusp magnetic field is formed so that the extraction axis of the electron beam and the axis of the point cusp coincide with each other, and the line cusp surface where the magnetic field strength becomes zero is close to the thermionic source. Even a slight amount of electrons traveling in the opposite direction from the extraction axis is deflected so as to be entangled with the magnetic flux formed in the direction away from the extraction axis, so that the number of electrons directly hitting the cathode is reduced.

The cusp magnetic field can be generated by a pair of solenoid coils arranged so that the same poles are adjacent to each other with the line cusp surface sandwiched therebetween. A pair of solenoid coils are arranged outside the wall of the acceleration cavity, slightly in front of the cathode and sandwiching 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 between the solenoid coils, and a plane-symmetric cusp electric field is formed with the line cusp sandwiched.

Further, the 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 due to extension of the extraction shaft. In this case, a line cusp is formed between the solenoid coil and the electromagnet or permanent magnet, and the line cusp surface where the magnetic field strength becomes zero intersects the axis of the cusp magnetic field, that is, the extraction axis of the electron beam in the vicinity of the front of the cathode. Such a Kaps magnetic field is formed. Even if the line cusp surface is not perpendicular to the extraction axis, it is sufficient if the trajectory of electrons traveling in the opposite direction can avoid the cathode portion.

If the excitation of the solenoid coil or electromagnet is performed by a pulse power source that generates a pulse current equal to or smaller than the macro pulse width and the rising or falling gradient of the pulse is used, a stronger magnetic field is obtained. Can be generated to reduce back bombardment. Furthermore, a permanent magnet ring in which a plurality of permanent magnets are connected to each other with the magnetic poles directed in the axial direction of the acceleration cavity is provided outside the acceleration cavity, and the cusp electric field is provided by this magnet ring and one electromagnet or permanent magnet provided on the extraction shaft. May be generated. By using a permanent magnet, it is possible to eliminate the influence of heat generated in the solenoid coil on the acceleration cavity.

The solenoid coil may be placed in the wall of the acceleration cavity or a disk provided in the cavity.
With such an arrangement, the solenoid coil approaches the extraction shaft and the coil diameter can be reduced, so that a stronger magnetic field can be efficiently formed.
Alternatively, the temperature rise of the cathode may be prevented by forming a space in the center of the cathode and avoiding collision of high-energy electrons traveling backward on the extraction shaft at high speed. 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.
Electrons may be positively absorbed by using this space as a Faraday cup. The high-frequency electron gun of the present invention is expected to be industrially useful when used in a free electron laser device which requires a particularly stable single wavelength laser output.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail based on embodiments with reference to the drawings. 1 is a sectional view showing a first embodiment of the present invention and a magnetic field strength distribution diagram, FIG. 2 is a drawing showing a magnetic flux distribution of a cusp magnetic field used in this embodiment, and FIG. 3 is a second embodiment of the present invention. 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 this embodiment, and FIG. 6 is a sectional view showing a fourth embodiment of the present invention. FIG. 7 and FIG. 7 are a sectional view and a magnetic field strength distribution diagram showing a fifth embodiment of the present invention.

[0013]

Embodiment 1 The high frequency electron gun of this embodiment is one in which 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 conceptually shown in FIG. 1, the high frequency electron gun has an accelerating cavity 1 for introducing a high frequency electric field, a cathode 2 for emitting thermoelectrons, and an electron beam extraction opening 3 formed 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 in the vicinity of the front of the cathode 2 so as to face each other with a surface orthogonal to the extraction axis interposed therebetween, and a current is applied so as to magnetize in opposite directions. Then, the opposing surfaces of the solenoid coils become the same pole, and a magnetic field line distribution state called a cusp magnetic field as shown in FIG. 2 appears.

In the cusp magnetic field in this embodiment, point cusps appear in the direction of the electron beam extraction axis z, and line cusps appear on the 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 extracted 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 that it approaches zero again 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 radiated from the aperture 3 as an electron beam. If the electron beam is emitted along the extraction axis z through the aperture 3, the cusp magnetic field rather exerts a force in the direction of focusing the electron beam, which is not an obstacle. When the emitted electrons are decelerated or inversely 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 while being entangled with the magnetic flux and further away from the axis z. Especially for low energy electrons, the deflection effect by the magnetic field is great.

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, whereas in the high frequency electron gun of this embodiment in which the cusp magnetic field exists in the high frequency acceleration cavity, The cusp magnetic field acts as an electron induction magnetic field, and most of the electrons traveling in the opposite direction avoid the cathode 2 and collide with the wall around the cathode. Therefore, the heat generation due to the back bombardment to the cathode 2 is small and thermions are not excessively generated, so that the electron beam does not gradually increase and a stable macro pulse with a predetermined intensity is obtained. Can occur. In the free electron laser, the stability of the electron beam is required because the laser wavelength changes as the electron energy changes. However, by using the electron gun of this embodiment in the free electron laser device, the electron beam in the 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 accelerating 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 installing the permanent magnet 7 in the second embodiment and the other configurations are the same as those in the first embodiment, only different parts will be described while avoiding duplication. In the cusp magnetic field in this embodiment, point cusps appear in the direction of the electron beam extraction axis, and line cusps appear on the surface sandwiched by the solenoid coil and the permanent magnet 7. The curved surface where the magnetic field strength is zero has a shape similar to a paraboloid of revolution having a line cusp as a skirt and an apex near the front of the cathode 2.

In the magnetic field distribution of this embodiment, although the deflection force received by the inversely accelerated electrons from the extraction axis is smaller than that in the first embodiment, it has a sufficient action 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 behind the cathode 2 instead of the permanent magnet. If a pulsed power supply with a rise time that is equal to or smaller than the macro pulse width is used as the excitation power supply for the electromagnet and is synchronized with the high-frequency electric field, a steep rise and fall slope is used to further improve efficiency. Can reduce back bombardment well. Also, when a solenoid coil is used, the heat generated by the exciting current affects the temperature of the accelerating cavity, which may be an obstacle in obtaining a stable electron beam. There is also an advantage that the problem of heat generated is alleviated.

[0019]

Third Embodiment As shown in FIG. 4, in the high frequency electron gun of this embodiment, instead of the solenoid coil of the second embodiment, a permanent magnet 8 is located outside the acceleration cavity 4 and slightly in front of the cathode 2. It was installed. A permanent magnet 7 is installed 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 parts will be described while avoiding duplication. As shown in FIG. 5, the permanent magnet 8 installed outside the acceleration cavity 1 has a plurality of permanent magnets 8 that surround the acceleration cavity 1 in a radial direction with the same magnetic pole oriented in the axial direction, and the magnetic pole portion is the acceleration cavity. The magnet ring is arranged so as to bite into the outer wall 4 and has magnetic poles on the opposite side of the permanent magnet 8 connected to each other by an annular yoke 9. The permanent magnet 7 provided in the cathode 2 portion has a magnetic pole that faces the front of the cathode 2 and is the same as a magnetic pole that faces the axial direction of the magnet ring. The cusp magnetic field of this embodiment has almost the same magnetic field distribution as that of the second embodiment, and also has the effect of reducing the back bombardment of the backward electrons to the cathode 2.

[0020]

Fourth Embodiment As shown in FIG. 6, in the high frequency electron gun of the present embodiment, the solenoid coils 5 and 6 of the first embodiment are connected to the acceleration cavity 1.
Instead of being provided on the outside of, the inside of the wall of the acceleration cavity 4 is charged. Since other configurations are the same as those of the first embodiment, only different parts will be described. Cathode 2
The solenoid coil 6 which should be located behind the accelerating cavity 2 is embedded in the rear wall of the accelerating cavity 2, and the solenoid coil 5 which is located in front of the cathode 2 is embedded and embedded in a disk 10 projecting axially from the accelerating cavity outer wall 4. Has been done. According to the arrangement of this embodiment, since the magnetic field generating mechanism can be arranged near the extraction axis, a strong magnetic field can be generated near the axis even if the strength of magnetization is small, and the required electron The 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 and 8 in the outer wall 4 of the accelerating cavity, the disk 10 and the like. it can.

[0021]

[Fifth Embodiment] As shown in FIG. 7, the high frequency electron gun of the present embodiment has a space provided at the center of the cathode 2.
The description of the same parts as those in the above embodiment will be omitted. The backward electrons having high energy are not largely deflected by the action of the electron induction magnetic field not only when the electron induction magnetic field of each of the above embodiments does not exist, but are back bombarded to the cathode 2 and the temperature of the cathode 2 rises. It may cause abrasion and abrasion. The high frequency electron gun of the present embodiment is characterized in that a hole is provided in the central portion of the cathode 2 so that the ratio of high-energy electrons that are reversely accelerated and returned to the electrode is reduced.

According to the high frequency electron gun of this embodiment, since the thermal energy applied to the cathode 2 is small even if there is electron back bombardment, the amount of thermoelectrons emitted to the periphery of the cathode 2 is reduced during the macro pulse. It is possible to prevent a change and generate a stable electron beam. If the hole provided at the center of the cathode 2 is formed as a Faraday cup, the colliding electrons are captured and discharged as a current, and secondary electrons and secondary ions can be prevented from being emitted. The influence can be suppressed more effectively.

Further, for example, the acceleration cavity 1 in the first embodiment
Pair of solenoid coils 5 and 6 arranged outside the outer wall 4 of the
When an electric current in the same direction is made to flow in the magnetic field to generate an axial magnetic field in the same direction, the electron beam extraction axis z shown in FIG.
The magnetic field strength B distribution as shown in the lower part of FIG. 2 is generated, and the magnetic flux is gathered in the vicinity of 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 accelerating cavity 1, most of the back bombardment electrons are concentrated near the center of the cathode 2 and are absorbed by the Faraday cup 11, so that the effect of back bombardment is significantly suppressed. You can Instead of the pair of solenoid coils 5 and 6,
It goes without saying that the same effect can be obtained by using individual solenoid coils or by using a magnet ring or the like.

[0024]

As described above, according to the high frequency electron gun of the present invention, the back bombardment electrons are prevented from colliding with the wall around the cathode by the action of the electron-induced magnetic field, or the center of the cathode is collided. It is possible to prevent the Faraday cup provided in the section from absorbing and colliding with the cathode. Therefore, as a result of stabilizing the amount of thermoelectrons emitted from the cathode, the electron beam is stable, and the laser wavelength in the macropulse is stable even when used in a free electron laser generator or the like. Further, since the cathode is protected from damage due to back bombardment electrons, the life can be extended and the operation and maintenance cost can be reduced.

[Brief description of drawings]

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

FIG. 2 is a drawing 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 showing a third embodiment of the present invention.

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

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

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

[Explanation of symbols]

1 High-frequency acceleration cavity 2 cathode 3 Electron beam extraction aperture 4 Acceleration cavity outer wall 5 solenoid coil 6 solenoid coil 7 Permanent magnet 8 permanent magnets 9 York 10 discs 11 Faraday cup

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-82254 (JP, A) JP-A-11-176600 (JP, A) JP-A-6-89680 (JP, A) JP-A-2001-305300 (JP, A) JP-B 47-15200 (JP, B1) (58) Fields investigated (Int.Cl. ⁷ , DB name) G21K 5/04 H01J 37/07

Claims (8)

(57) [Claims] 1. A high-frequency electron gun by a high frequency electric field is provided thermionic source at a position opposed to the opening of the high-frequency accelerating cavity having an opening with an electron beam was set to withdraw from the opening, electrons in said high-frequency accelerating cavity Beam extraction
A line cusp that has a point cusp on the horizontal axis and the magnetic field strength is zero
A cusp field whose surface is near the thermionic source
To form an electron induced magnetic field, a high frequency electron gun, wherein the guide wolfberry to not collide electrons traveling in the opposite direction to the heat electron source. Wherein said cusp magnetic field, a high frequency electron gun according to claim 1, wherein the same pole across the line cusp surface is generated by a pair of solenoid coils disposed so as to be adjacent . 3. The cusp magnetic field is generated by one solenoid coil provided so as to surround the acceleration cavity and one electromagnet or permanent magnet provided on the extraction shaft. The high frequency electron gun according to claim 1 . 4. The high frequency electron gun according to claim 2, wherein the solenoid coil or the electromagnet is excited by using a pulse power source having a rising edge that is equal to or shorter than the frequency of the high frequency electric field. 5. The cusp magnetic field is generated by a permanent magnet ring provided so as to surround the acceleration cavity with a pole facing inward, and one electromagnet or a permanent magnet provided on the extraction shaft. The high frequency electron gun according to claim 1, wherein Wherein said solenoid coil, high-frequency electron gun according to any one of claims 1 to 5, characterized in that embedded in or disk disk within the wall forming the accelerating cavities. 7. An opening in a high frequency accelerating cavity having an opening.
A thermoelectron source is installed at a position facing the
A high-frequency electron gun in which the beam is extracted from the opening
In the high-frequency accelerating cavity,
Electron-induced magnetic field that guides the child so that it does not collide with the thermionic source
Is formed in the high-frequency accelerating cavity, and a Faraday cup is arranged at the center of the thermoelectron source, and the electron induction magnetic field has a symmetry axis in the extraction axis of the electron beam and a magnetic field near the thermoelectron source. high-frequency electron gun you wherein the strength of a magnetic field that maximizes. 8. free electron laser apparatus using a high-frequency electron gun according to any one of claims 1 to 7.