JPH04359900A - Microtron - Google Patents

Abstract

PURPOSE:To provide a magnetron containing a magnetic field such that an electron beams are focused during circular motion so as to reduce electrons such that they collide against incient holes of a cavity resonator and an electron extracting tube due to the thickness growth of the electron beams in the process of accelerating electrons in the microtron. CONSTITUTION:The magnetic pole sectional view of a microtron is formed in a serrate-shaped as shown in Fig.1 magnetic field intensity is set in such a way that respective ideal circular orbits 2 are located in the center between respective vertexes of the serrate-shaped magnetic poles. Then, the magnetic field has a gradient in the radial direction of the circular orbit, so that electrons being deviated from an ideal orbit are focused by adequately selecting the magnitude of the gradient. According to this invention, as the electron beams can be focused on a desired point, electrons, which collide against incient holes of a cavity resonator and an extracting tube, can be reduced so that an accelerating efficiency of the microtron can be increased.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam accelerator, and more particularly to an accelerator structure suitable for efficiently and easily extracting electron beams by focusing the electron beams using a magnetic field.

0002

[Prior Art] "Medical Imaging and Radiology Equipment Handbook"
As described in Japan Radiation Equipment Manufacturers Association, pages 200 to 205 (Densoku Shuppansha, published on March 10, 1989), a microtron is composed of a cavity resonator 3 and a microwave source 5 as shown in FIG. It is an accelerator that repeatedly accelerates electrons by moving them in a circular motion in a uniform magnetic field. That is, the electron beam emitted from the electron gun is guided to the cavity resonator 3 excited by microwaves, accelerated by the high frequency electric field inside the resonator, moves circularly in a uniform magnetic field, and returns to the cavity resonator 3. 3 and is accelerated again. A microtron is an accelerator that accelerates to the required energy by repeating this process. The electron beam is extracted by inserting a magnetism-shielding iron extraction tube 7 into an appropriate position. Therefore, by moving this extraction tube, it becomes possible to extract an electron beam of any energy from any orbit.

0003

[Problem to be solved by the invention] After the electron beam accelerated by the high-frequency electric field within the cavity resonator exits from the cavity resonator 3, it must move circularly in a uniform magnetic field and enter the cavity again. The thickness of the electron beam must not be thicker than the entrance hole of the cavity resonator 3 so that the electron beam does not spread. Further, since the iron extraction tube 7 is used when taking out the electron beam to the outside, the electron beam cannot pass through the tube unless it is thinner than the inner diameter of the extraction tube 7. However, due to positional deviations when accelerating in the cavity resonator 3, some electrons are emitted in a direction that deviates from the ideal trajectory, resulting in an increase in the thickness of the electron beam. There was a problem that it could not be focused inside. The purpose of the present invention is to solve the above-mentioned problem, and to avoid collision with the entrance hole of the cavity resonator 3,
It is an object of the present invention to provide a microtron in which the loss of electrons caused by the inability to pass through the extraction tube 7 is reduced.

0004

[Means for solving the problem] In order to solve the above problem, the magnetic field strength corresponding to the electron orbit should be set to the magnetic field strength on an ideal circular orbit, and the magnetic field strength at a portion with a larger radius than the circular orbit. By increasing the strength of the magnetic field in the part where the radius is weak and smaller than the circular orbit, the electrons emitted in a direction away from the ideal circular orbit can be made to vibrate at an appropriate period around the orbit, thereby converting the electron beam. All you have to do is focus.

[0005]

[Operation] In order to make the electrons emitted in a direction deviated from the ideal circular orbit vibrate around the ideal circular orbit, the theory of betatron oscillation (Experimental Physics Course "Accelerator" edited by Hiroo Kumagai p313~ p316), the magnetic field strength may be given a gradient on the inside and outside of the circular orbit. Then, this magnetic field may be provided on each circular orbit having a different radius for each energy. At this time, if we make the inside of the ideal orbit weak and the outside strong, the radius of curvature of the circular motion of the electrons outside the ideal orbit will gradually become smaller, and the radius of curvature of the circular motion of the electrons inside the ideal orbit will become larger. Therefore, it vibrates around an ideal orbit in the radial direction.

However, in this case, since the magnetic field outside the raceway plane has a radial component, it is subject to a diverging effect in a direction perpendicular to the raceway plane. In order to prevent this divergence, the direction of the gradient of the magnetic field strength may be reversed, and the magnetic field may be made stronger on the inside of the orbit and weaker on the outside. In this case, if the gradient is too large, no vibration will occur, but by providing an appropriate gradient, vibration can be obtained. In other words, electrons emitted outward from the ideal orbit perform circular motion while increasing the radius of curvature, but the increase is small and the speed of increase is proportional to the increase in distance from the center of the ideal circular orbit. If it is slower, it will be as if a circle with the same radius as the ideal circular orbit is shifted sideways, and it will return to the ideal circular orbit as shown in Figure 4. Electrons emitted inside the ideal orbit perform circular motion while decreasing the radius of curvature, but this also returns to the ideal circular orbit. In addition, since a convergent force acts in a direction perpendicular to the orbital surface, it vibrates around the ideal orbit. Therefore, if such a magnetic field gradient is provided along each ideal circular orbit with a different radius for each energy, electrons can be focused.

[0007]

[Embodiment] An embodiment of the present invention will be explained with reference to FIGS. 1 and 2. The distance between the magnetic poles at each ideal circular orbit position is d0
Assuming that the magnetic field strength is B0, this is 0.122[T]
, the acceleration energy for each acceleration is 535 [keV]. At this time, the radius r0 of the ideal circular orbit is, for example, the 20th
The orbit is about 0.319 [m], and the time required for one revolution, that is, the rotation period T, is about 6.68 [ns]. Then, the cross section of the magnetic pole is made into a sawtooth shape as shown in FIG. 1, and each ideal circular orbit is set to be located at the center between the vertices of the sawtooth magnetic pole. In this way, the apex of the sawtooth magnetic pole will be at the position indicated by the dashed line in FIG. At this time, since the magnetic field strength is inversely proportional to the distance between the magnetic poles, it is necessary to set sawtooth magnetic poles on each orbit so that the distance d between the magnetic poles is a function of the radius r of the circular orbit as shown in (Equation 1). For example, a gradient of magnetic field strength can be provided, and the period of betatron oscillation is twice the period of rotation of the electron, i.e., about 1 in the 20th orbit.
It becomes 3.4 [ns]. Therefore, the electrons whose emission direction from the cavity resonator deviates from the ideal trajectory return to the ideal trajectory when they enter the cavity again, so the electron beam is focused and the loss of electrons is reduced. As a result, more current can be obtained. Note that when the apex of the magnetic pole is provided as shown in FIG. 2, the cavity resonator portion does not need to be saw-toothed, and the surface of the magnetic pole may be flat.

The above is just one embodiment of the present invention. For example, if the position of the electron beam extraction tube entrance hole moves along a line inclined at 30 degrees with respect to the central axis of the orbit as shown in Figure 2, if you want the electron beam to be focused at the entrance hole, use the Betatron. The half period of vibration may be set to 210°/360° of the rotation period, that is, 7/12, and in this case, the period of betatron oscillation is shortened. Therefore, the gradient of the magnetic field is made smaller than when focusing using a cavity resonator, and the curve between the two vertices of the sawtooth magnetic poles is set so that the distance between the magnetic poles changes as shown in Equation 2.

[0009] Furthermore, if the extraction trajectory is limited to a few, it is possible to set the curve (Equation 2) only for the extraction trajectory and the curve (Equation 1) for the other trajectories, so that the extraction tube entrance hole Electron loss can be prevented both at the entrance hole of the cavity and at the entrance hole of the cavity, and more efficient acceleration can be achieved. In short, it is a structure in which the magnetic pole shape has a gradient along each ideal orbit determined by the magnetic field strength and accelerating electric field strength, and the magnetic field distribution is such that the electrons oscillate around the ideal orbit. If there is, it does not impair the essence of the present invention. Furthermore, such a magnetic field distribution may be created not only from the shape of the magnetic pole, but also by attaching a coil to the surface of the magnetic pole.

0010

[Effects of the Invention] According to the configuration of the present invention, the electron beam can be focused at a necessary point, so that it is possible to prevent electrons from colliding with the entrance hole of the cavity resonator or from being unable to pass through the extraction tube. Loss can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a magnetic pole of a microtron showing an embodiment of the present invention.

FIG. 2 is a basic configuration diagram of a microtron showing an embodiment of the present invention.

FIG. 3 is a basic configuration diagram of a conventional microtron.

FIG. 4 is a conceptual diagram showing electron orbits in one embodiment of the present invention.

[Explanation of symbols]

1 Magnetic pole 2 Ideal electron orbit 3 Cavity resonator 4 Excitation coil 5 Microwave source 6. Apex of sawtooth magnetic pole 7 Take-out tube 8. Central axis of ideal orbit 9 Line of movement at the entrance of the extraction tube 10 Line of magnetic force 11 Electron orbit

Claims (6)

[Claims] Claim 1: In a microtron, the electrons are accelerated by microwaves in a cavity resonator and then repeatedly moved in a circular motion in a static magnetic field, and the accelerated electrons are taken out to the outside using a metal tube. For each electron orbit that has energy, the electrons that deviate from the ideal circular orbit in each electron orbit perform a circular motion while vibrating around the circular orbit, and the magnetic field strength corresponding to the electron orbit is adjusted to the circular orbit. A microtron is characterized by having a weaker magnetic field strength in a portion with a larger radius than a circular orbit, and a stronger magnetic field strength in a portion with a smaller radius than a circular orbit. 2. The microtron according to claim 1, wherein the surface shape of the magnetic pole that creates a magnetic field intensity distribution corresponding to the electron orbit is serrated. 3. The magnetic field intensity distribution corresponding to the electron orbit is
3. The microtron according to claim 1, wherein the microtron is configured to focus the electron beam by causing the electrons that have deviated from the circular orbit to move in a circular motion while vibrating around the circular orbit. 4. A magnetic field distribution corresponding to the electron orbit causes the electrons that have deviated from the circular orbit to move in a circular motion while vibrating around the circular orbit, thereby focusing them at a tube entrance hole for taking out the electron beam. 3. The microtron according to claim 1, wherein the microtron is configured as follows. 5. A magnetic field distribution corresponding to the electron orbit focuses the electron beam at the entrance hole of the cavity resonator by causing the electrons that have deviated from the circular orbit to move in a circular motion while vibrating around the circular orbit. The microtron according to claim 1 or 2, characterized in that it is configured as follows. 6. The microtron according to claim 5, wherein the magnetic pole shape of the cavity resonator portion is flat.