JP3810716B2 - X-ray generator and generation method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray generation apparatus, and more particularly to an X-ray generation apparatus that causes a laser beam to collide with an electron beam and generates X-rays by an inverse Compton scattering process.
[0002]
[Prior art]
As a technique for generating X-rays by the inverse Compton scattering process, a technique is known in which a single bunch electron beam (a pulsed electron beam with one electron group) collides with a single pulse laser beam. In order to increase the intensity of generated X-rays, a method of colliding a multi-bunch electron beam (a plurality of electron beams in which a plurality of electron groups are arranged at a predetermined time interval) and a pulsed laser beam has been studied. As the latter X-ray generation method, the following three methods are mainly proposed and studied.
[0003]
In the first method, a multi-pulse laser beam is emitted from a laser oscillator, and each laser pulse collides with one electron group of the multi-bunch electron beam. In order to generate a high-intensity multi-pulse laser beam by this method, the laser oscillator must be enlarged.
[0004]
The second method is disclosed in JP-A-7-110400. FIG. 3A is a schematic diagram of an X-ray generator disclosed in Japanese Patent Laid-Open No. 7-110400. The laser beam 110 is incident on the light reflecting mirror 100 from the laser light source 101 through one reflecting mirror of the optical resonator 100. The laser beam reciprocates in the optical resonator 100 to form a standing wave. The intensity of the standing wave can be increased by increasing the energy of the newly incident laser beam rather than the energy of the laser beam leaking from the optical resonator 100. A multi-bunch electron beam 111 emitted from the accelerator 102 collides with a standing wave formed in the optical resonator 100 to generate an X-ray 112.
[0005]
In this method, in order to form a standing wave in the optical resonator 100, the resonator length must be adjusted with an accuracy of 1/10 of the wavelength. For this reason, it is technically difficult to adjust the resonator length of the resonator 100.
[0006]
A third method is disclosed in Japanese Patent Laid-Open No. 11-211899. FIG. 3B is a schematic diagram of an X-ray generator disclosed in Japanese Patent Laid-Open No. 11-211899. Two reflecting mirrors 120 and 121 are arranged to face each other. The laser beam 130 emitted from the laser light source 122 travels in a zigzag manner in a direction parallel to the reflecting surfaces of the reflecting mirrors 120 and 121 while being repeatedly reflected by the reflecting mirrors 120 and 121.
[0007]
The electron beam 131 emitted from the electron beam source 123 collides with the laser beam traveling in a zigzag manner between the reflecting mirrors 120 and 121 with high frequency, and X-rays are generated. In order to condense the laser beam, a method has been proposed in which each of the reflection positions of the reflecting mirrors 120 and 121 is a concave mirror. By making it collide with an electron beam at the condensing point of the laser beam, the conversion efficiency into X-rays can be increased.
[0008]
In this method, when the number of reflections of the laser beam increases, it becomes difficult to focus the laser beam on the point to be collided with high accuracy. In addition, since there is not only one collision point, it is difficult to always converge the electron beam at the collision point.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to provide an X-ray generator capable of easily generating high-intensity X-rays.
[0010]
[Means for Solving the Problems]
According to one aspect of the present invention, an optical resonator includes a partial reflection mirror and a total reflection mirror disposed to face each other, the partial reflection mirror transmits a part of light, and the optical resonator includes An optical amplifier that amplifies the light that reciprocates in the optical resonator, a pulse laser oscillator that causes a laser pulse to enter the optical resonator through the partial reflector, and the light emitted from the pulse laser oscillator, There is provided an X-ray generator having an electron beam source that emits a plurality of electron groups at a certain time interval so as to collide with a laser pulse reciprocating in a resonator.
[0011]
Since the pulse laser beam reciprocates in the optical resonator, a plurality of electron groups and one pulse laser beam can collide a plurality of times. Thereby, the energy of the generated X-ray can be increased.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a schematic diagram of an X-ray generator according to an embodiment of the present invention. An optical resonator 3 is demarcated by the partial reflection mirror 1 and the total reflection mirror 2 which are arranged to face each other. The reflectance of the partial reflection mirror 1 is, for example, 90 to 99.9% (transmittance is 0.1 to 10%), and a laser beam can be introduced into the optical resonator 3 from the outside through the partial reflection mirror 1. it can. The reflectivity of the total reflection mirror 2 is ideally 100%, but is actually 99.5% or more, which is at least higher than the reflectivity of the partial reflection mirror 1.
[0013]
The partial reflection mirror 1 and the total reflection mirror 2 are both concave mirrors, and are arranged so that the focal points of the both coincide. For example, a laser beam directed from the total reflection mirror 2 toward the partial reflection mirror 1 is collected at the focal point 11 and then enters the partial reflection mirror 1 as a divergent beam. What is the laser beam reflected by the partial reflector 1? The light is condensed again at the focal point 11, becomes a divergent beam, and enters the total reflection mirror 2.
[0014]
The moving mechanism 7 can move the total reflection mirror 2 in the optical axis direction of the optical resonator 3. By moving the total reflection mirror 2, the resonator length of the optical resonator 3 can be adjusted.
[0015]
An optical amplifier 4 is disposed in the optical resonator 3. The optical amplifier 4 includes, for example, a Ti sapphire laser medium and an excitation light source for exciting the laser medium. For example, an Nd: YAG laser oscillator is used as the excitation light source. The optical amplifier 4 amplifies the laser beam that reciprocates in the optical resonator 3.
[0016]
A laser light source 5 emits a single pulse laser beam 10. The laser light source 5 includes, for example, a Ti sapphire mode-locked laser oscillator and a regenerative amplifier. The Ti sapphire laser oscillator emits a multi-pulse laser beam having a pulse frequency of 119 MHz, for example. The regenerative amplifier cuts out and amplifies one pulse from the multi-pulse laser beam emitted from the Ti sapphire laser oscillator. For example, a single pulse laser beam having a pulse energy of 500 μJ / pulse and a pulse width of 200 ps is emitted from the regenerative amplifier.
[0017]
A single pulse laser beam 10 emitted from the laser light source 5 passes through the partial reflection mirror 1 and is introduced into the optical resonator 3. When the pulse energy of the pulse laser beam 10 emitted from the laser light source 1 is 500 μJ / pulse and the transmittance of the partial reflection mirror 1 is 1%, the pulse of the single pulse laser beam introduced into the optical resonator 3 The energy is about 5 μJ / pulse.
[0018]
When the laser medium of the optical amplifier 4 is excited in synchronization with the single pulse laser beam 10 emitted from the laser light source 5, the single pulse laser beam introduced into the optical resonator 3 reciprocates within the resonator 3. Then, it is amplified by the optical amplifier 4. As the single pulse laser beam is amplified, the amplification factor of the optical amplifier 4 decreases, so that the pulse energy of the single pulse laser beam becomes maximum at a certain point and then gradually decreases. For example, amplification can be performed until the maximum value of pulse energy reaches 44 mJ / pulse. Further, the single pulse laser beam reciprocates about 20 times in the optical resonator 3 until the pulse energy is amplified to 22 mJ / pulse or more and then decreased to 22 mJ / pulse.
[0019]
An electron beam source 6 emits a multi-bunch electron beam 12. The electron beam source 6 includes, for example, an electron gun having a photocathode and an acceleration cavity, and a linear accelerator. The linac accelerates the multi-bunch electron beam to a relativistic speed (eg, more than 0.5 times the speed of light).
[0020]
The first electron emitted from the electron beam source 6 in the period when the pulse energy of the single pulse laser beam reciprocating in the optical resonator 3 exceeds 22 mJ / pulse and is first directed from the total reflection mirror 2 to the partial reflection mirror 1. The laser light source 5 and the electron beam source 6 are synchronously controlled by the synchronizer 20 so that the group collides with the single pulse laser beam at the condensing point 11.
[0021]
The optical resonator 3 is moved by the moving mechanism 7 so that the time for the single pulse laser beam to reciprocate once in the optical resonator 3 is equal to the time interval of the electron group of the multi-bunch electron beam 12 emitted from the electron beam source 6. The resonator length is adjusted. Therefore, the single pulse laser beam colliding with the electron group at the condensing point 11 collides with the next electron group when the single pulse laser beam reciprocates once in the optical resonator 3 and is condensed again at the condensing point 11.
[0022]
The single pulse laser beam reciprocates about 20 times in the optical resonator 3 while maintaining a pulse energy of 22 mJ / pulse or more. For this reason, a pulse laser beam having a pulse energy of 22 mJ / pulse or more can collide with an electron group about 20 times. Due to this collision, pulsed X-rays 13 are generated by the inverse Compton scattering process.
[0023]
Although the pulse laser beam 10 emitted from the laser light source 5 is a single pulse, the pulse laser beam and the electron beam can collide a plurality of times by reciprocating the pulse laser beam 10 while amplifying it in the optical resonator. Thereby, the energy utilization efficiency of a pulse laser beam can be improved. In addition, the overall energy of the generated pulsed X-ray 13 can be increased.
[0024]
In order to generate resonance efficiently in the optical resonator 3, the laser beam transmitted through the partial reflection mirror 1 and introduced into the optical resonator 3 matches the specific beam trajectory of the optical resonator 3. I have to let it. Specifically, the inherent beam trajectory in the optical resonator 3 is extended to the outside of the partial reflector 1. A laser beam that coincides with the extended virtual beam trajectory may be incident on the partial reflection mirror 1. When the laser beam emitted from the laser light source 5 is a parallel beam, a convex lens can be used to obtain a laser beam that matches the virtual beam trajectory.
[0025]
In the X-ray generator according to the above embodiment, the collision angle between the single pulse laser beam and the electron beam is constant. For this reason, the spatial monochromaticity of the generated X-rays is not impaired.
[0026]
Further, in the above embodiment, the resonator length of the optical resonator 3 may be adjusted so that the time for which the single pulse laser beam makes one round trip and the time interval of the electron group of the multi-bunch electron beam 12 are equal. This adjustment does not require the accuracy of the wavelength order of the laser beam. For this reason, the resonator length of the optical resonator 3 can be adjusted relatively easily.
[0027]
In the above embodiment, when the single pulse laser beam 10 emitted from the laser light source 5 enters the optical resonator 3, the laser medium of the optical amplifier 4 is excited only once. Even with only one excitation, as described above, the pulse energy of 22 mJ / pulse or more can be maintained and the single pulse laser beam can be reciprocated about 20 times in the optical resonator 3.
[0028]
When the pulse energy of the single pulse laser beam reciprocating in the optical resonator 3 decreases, the pulse energy can be increased again by exciting the laser medium of the optical amplifier 4 again. Thereby, the collision of many times of 20 times or more can be performed.
[0029]
In the above embodiment, the pulse laser beam collides with the electron beam once every time it makes a round trip in the optical resonator 3 in FIG. 1, but every time it makes a round trip, it collides twice (on the forward path and once on the return path). It is also possible to cause a single collision). The optical path length from the focal point 11 to the partial reflecting mirror 1 and the optical path length from the focal point 11 to the total reflecting mirror 2 are made equal to each other, and the pulse interval of the electron beam is set to 1 of the time during which the pulse laser beam makes one round trip through the resonator 3. / 2. In the case of adopting this method, it is preferable that the partial reflection mirror 1 is also movably held by the moving mechanism 7a in order to finely adjust the optical path length.
[0030]
In the above embodiment, the reflecting mirrors 1 and 3 at both ends defining the optical resonator 3 are concave mirrors, but other condensing optical systems may be added in the optical resonator 3.
[0031]
FIG. 2A shows another configuration example of the optical resonator and the condensing optical system. A flat partial reflecting mirror 1a and a concave mirror 2a having a focal length of about 10 m define an optical resonator 3a. In the optical resonator 3a, two convex lenses 30 and 31 are arranged such that the central axis thereof coincides with the optical axis of the optical resonator 3a and the focal points of the two convex lenses coincide with each other. The focal length of the convex lens 30 on the partial reflecting mirror 1a side is 600 mm, for example, and the focal length of the convex lens 31 on the concave mirror 2a side is 150 mm, for example. The optical amplifier 4 is disposed between the partial reflection mirror 1a and the convex lens 30.
[0032]
When the single pulse laser beam 10 is transmitted through the partial reflection mirror 1a and enters the optical resonator 3a, the light is condensed at the focal point by the convex lens 30. Thereafter, the light beam becomes a divergent light beam, is made into a substantially parallel light beam by the other convex lens 31, and enters the concave mirror 2a. The parallel light beam reflected by the concave mirror 2a is condensed at the focal point by the convex lens 31. Thereafter, it becomes a divergent light beam, becomes a parallel light beam by the convex lens 30, and enters the partial reflecting mirror 1a.
[0033]
The multi-bunch electron beam 12 collides with the single pulse laser beam at the confocal position of the convex lenses 30 and 31.
[0034]
Since the focal length of the convex lens 30 is longer than the focal length of the other convex lens 31, the beam diameter of the laser beam between the partial reflecting mirror 1a and the convex lens 30 is larger than the beam diameter at other positions in the optical resonator 3a. large. Since the optical amplifier 4 is arranged at a position where the beam diameter becomes large, damage to the laser medium of the optical amplifier 4 can be avoided. In addition, since the power density of the laser beam in the laser medium is reduced, nonlinear effects (self-phase modulation, self-convergence) can be reduced. Furthermore, since the amplification gain can be reduced, it is possible to delay the consumption of energy stored in the amplifier. As a result, the number of reciprocations of the pulse laser beam can be increased.
[0035]
As shown in FIG. 2B, concave mirrors 30a and 31a can be used instead of the convex lenses 30 and 31 of FIG.
[0036]
Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.
[0037]
【The invention's effect】
As described above, according to the present invention, a single pulse laser beam can be introduced into an optical resonator through the partial reflection mirror by using one of the reflection mirrors of the optical resonator as a partial reflection mirror. By arranging an optical amplifier in the optical resonator, the pulse energy of the introduced single pulse laser beam can be increased. Multipulse X-rays can be generated by increasing the pulse energy and causing the multi-bunch electron beam to collide with a single pulse laser beam reciprocating in the optical resonator.
[Brief description of the drawings]
FIG. 1 is a schematic view of an X-ray generator according to an embodiment of the present invention.
FIG. 2 is a schematic diagram illustrating another configuration example of the optical resonator and the focusing optical system of the X-ray generator according to the embodiment.
FIG. 3 is a schematic view of a conventional X-ray generator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Partial reflection mirror 2 Total reflection mirror 3 Optical resonator 4 Optical amplifier 5 Laser light source 6 Electron beam source 7 Moving mechanism 10 Single pulse laser beam 11 Condensing point 12 Multi bunch electron beam 13 Multi pulse X-ray 20 Synchronizer 30,31 convex lens

Claims (6)

An optical resonator including a partial reflection mirror and a total reflection mirror disposed to face each other, wherein the partial reflection mirror transmits a part of the light;
An optical amplifier disposed in the optical resonator for amplifying light reciprocating in the optical resonator;
A pulsed laser oscillator for injecting a laser pulse into the optical resonator through the partial reflection mirror;
An X-ray generator comprising: an electron beam source that emits a plurality of electron groups at a certain time interval so as to collide with a laser pulse emitted from the pulse laser oscillator and reciprocating in the optical resonator. The X-ray generator according to claim 1, further comprising a moving mechanism that moves at least one of the partial reflection mirror and the total reflection mirror in an optical axis direction of the optical resonator. The X-ray generator according to claim 1, wherein the optical resonator includes a condensing optical element that condenses a laser pulse reciprocating in the optical resonator at one condensing point. The X-ray generation device according to claim 3, wherein the partial reflection mirror and the total reflection mirror are concave mirrors and also serve as the condensing optical element. The electron beam emitted from the optical axis of the optical resonator and the electron beam source so that the electron group emitted from the electron beam source collides with the pulse laser beam at the condensing point in the optical resonator. The X-ray generator according to claim 3, wherein the path of the X-ray is adjusted. An optical resonator including a partial reflection mirror and a total reflection mirror arranged opposite to each other, wherein the partial reflection mirror allows a laser pulse to be incident through the partial reflection mirror to transmit a part of the light. Reciprocating the inside and amplifying the light reciprocating in the optical resonator with an optical amplifier disposed in the optical resonator;
And a step of causing a plurality of electron groups to collide with a laser pulse reciprocating in the optical resonator at a certain time interval.

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