JP2006344731A - Method and device for laser beam circulation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for laser beam circulation capable of confining laser beam in a specified optical path for circulation so that the same laser beam is condensed a plurality of times on the same laser beam focusing point, while the laser beam focusing point is easily and accurately fine-adjusted, resulting in significantly improved usage of laser beam. <P>SOLUTION: Laser beam 3 is introduced from outside and confined in a circulation circuit 5. It is made to pass repeatedly a laser beam focusing point 9 in the circulation circuit. The laser beam focusing point is adjusted so that the same laser beam is condensed a plurality of times on the same laser beam focusing point. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a laser beam circulating device and a laser beam circulating method for condensing the same laser beam at the same laser beam focusing point a plurality of times and improving the utilization efficiency of the laser beam.

X-ray is an electromagnetic wave having a wavelength of about of about ^{0.1~100Å (10- 11 ~10- 8 m)} , short X-ray of these wavelengths (10~100keV, λ = 1~0.1Å) hard X X-rays with long wavelengths and wavelengths (0.1 to 10 keV, λ = 100 to 1Å) are called soft X-rays. In addition, when an electron beam or the like is applied to a substance, an X-ray having a wavelength specific to the constituent element of the substance is called a characteristic X-ray.

  An X-ray transmission device, an X-ray CT device, an X-ray diffraction device, an X-ray spectroscopic device, and the like are used in a wide range of fields such as medicine, life science, and material science as devices using X-rays. For example, for the treatment of myocardial infarction, coronary artery angiography (IVCAG) using X-rays of about 50 keV is generally performed. The X-ray CT apparatus is an apparatus that obtains a two-dimensional cross-sectional image of an object by irradiating the object to be measured with X-rays from different directions, measuring absorption thereof, and reconstructing an image by a computer.

X-ray tubes and synchrotron radiation are widely known as X-ray generation sources.
An X-ray tube is an apparatus that generates X-rays by accelerating thermoelectrons obtained by heating a filament in a vacuum at a high voltage to collide with a metal anode (target). X-rays generated from the X-ray tube are composed of continuous X-rays due to bremsstrahlung of electrons and characteristic X-rays that are emission line spectra. Continuous X-rays are used as light sources for applications that do not require X-rays of a specific wavelength, for example, medical and industrial transmission methods. Characteristic X-rays are used for applications that require X-rays of a specific wavelength, such as X-ray diffraction and fluorescent X-ray spectroscopy.

On the other hand, synchrotron radiation (SR light) is an X-ray generated when the orbit of an electron beam accelerated to a speed close to the speed of light is changed by a powerful magnet in a ring accelerator (synchrotron). is there. SR light is an extremely powerful X-ray source (10 ³ times or more) compared to an X-ray tube (for example, X-ray intensity (number of photons): about 10 ¹⁴ photons / s, pulse width: about 100 ps), Used in fields that require high X-ray intensity.

  However, synchrotron facilities using synchrotrons are large facilities with a synchrotron length of, for example, 50 m or more and an orbital length of 100 m or more, and therefore cannot be easily introduced even for research and medical purposes. .

Therefore, Patent Documents 1 and 2 have been proposed as means for generating X-rays with a small apparatus.
Non-patent documents 1 and 2 are disclosed as prior arts related to the present invention.

  As shown in FIG. 15, the “X-ray generator” of Patent Document 1 defines a plurality of orbiting electron orbits that are partially shared with each other, and increases or decreases the energy of the electron beam 71 in the common part of the orbits. A microtron 73 in which an accelerator 72 is disposed, an electron beam source 74 that causes an electron beam 71 to enter the orbiting electron orbit of the microtron, and a laser that collides with electrons flying in a shared portion of the microtron orbit. And a laser light source 76 that emits a beam 75.

  Patent Document 2 “Acceleration and compression method and apparatus for short pulse electron beam” relates to a means for compressing the pulse width of a pulsed electron beam, and an accelerating tube while circulating an incident electron beam along an electron trajectory. The pulse width of the electron beam is compressed while accelerating.

As shown in FIG. 16, the “small X-ray generator” of Non-Patent Document 1 generates an X-ray 84 by colliding an electron beam 82 accelerated by a small accelerator 81 (X-band accelerator tube) with a laser 83. It is something to be made. The multi-bunch electron beam 82 generated by the RF electron gun 85 (thermal RF gun) is accelerated by the X-band accelerator tube 81 and collides with the pulse laser beam 83. The hard X-ray 84 having a time width of 10 ns is generated by Compton scattering.
This device is miniaturized by using an X band (11.424 GHz), which is four times the frequency of the S band (2.856 GHz) generally used in a linear accelerator, as RF, for example, X-ray intensity (number of photons). : About 1 × 10 ⁹ photons / s, pulse width: generation of intense hard X-rays of about 10 ps is predicted.

  In Non-Patent Document 2, as shown in FIG. 17, a laser beam is confined and circulated by using a large number of reflecting mirrors to increase the collision rate in the reaction region.

Japanese Patent Application Laid-Open No. 2002-280200, “X-ray generation apparatus and generation method” Japanese Patent No. 3151585, “Method and apparatus for accelerating and compressing short pulse electron beam”

Katsuhiro Dobashi, et al., “Development of small hard X-ray source using X-band linac”, 2002 Yasuo SUZUKI, et al. “A NEW LASER MASS SPECTROMETRY FOR CHEMICAL ULTRATRAC ANALYSIS ENHANCED WITH MULTI-MIRROR SYSTEM (RIMMPA)”, ANALYTIC SCIENCE 2001 Vol. 17 Supplement

  In the devices of Patent Documents 1 and 2, the electron beam is circulated by the microtron. However, in the method of rotating the electron beam, since the electron beam and the laser beam cannot collide head-on, the X-ray generation efficiency (that is, the laser beam) Usage efficiency is low. In addition, since a large number of large magnets are used to circulate the electron beam, the X-ray generation unit becomes large.

On the other hand, in the apparatus of Non-Patent Document 1, since the multi-bunch electron beam and the pulse laser beam collide head-on, the utilization efficiency of the laser beam can be increased. In this case, the amount of X-ray generation is proportional to the current of the electron beam and the number of photons of the laser beam.
However, the pulse width of the pulse laser beam is very short (for example, 10 ns in the case of a Nd: YAG laser) with respect to the pulse width of the electron beam (for example, several hundred ns to several thousand ns). There is a problem that the utilization efficiency of laser light is low.
Further, for example, when laser light is used for interaction between a rare gas molecule and laser light, there is a problem that the utilization efficiency of laser light is low.

Further, as in Non-Patent Document 2, even if the laser beam is confined using a large number (for example, 32) of reflecting mirrors, the electron beam and the laser beam cannot collide head-on, so the utilization efficiency of the laser beam is low.
Also, in this case, since the light trajectory is adjusted by finely adjusting a number of reflecting mirrors, the adjustment is very difficult, the position of the electron beam and the laser beam is likely to shift, and a very thin electron beam (for example, a diameter) Similarly, there is a problem that it is almost impossible to accurately collide with a very thin laser beam (for example, about 100 μm in diameter).

  The present invention has been developed to solve the above-described problems. That is, an object of the present invention is to confine and circulate laser light in a predetermined optical path so that the same laser light can be condensed at the same laser light condensing point multiple times, and the position of the laser light condensing point. It is an object of the present invention to provide a laser beam circulating apparatus and a laser beam circulating method which can finely adjust the laser beam easily and accurately, and thereby can greatly increase the utilization efficiency of the laser beam.

According to the present invention, a laser beam circulating optical system that introduces a laser beam from the outside, confines the laser beam in a circuit that circulates the laser beam, and repeatedly passes through a laser beam condensing point in the circuit,
A laser beam condensing point adjusting device that adjusts the position of the laser beam condensing point, thereby condensing the same laser beam at the same laser beam condensing point multiple times An orbiting device is provided.

  According to a preferred embodiment of the present invention, the laser beam condensing point adjusting device includes a first convex lens located upstream of the laser beam condensing point and a second convex lens located downstream of the laser beam condensing point. And a first lens position adjusting device capable of moving the first convex lens and the second convex lens in a direction perpendicular to the optical axis.

The laser beam condensing point adjusting device further includes an upstream side in a half circuit that does not include the laser beam condensing point among the half circuit divided into two sections by the first convex lens and the second convex lens. The fourth convex lens positioned and the third convex lens positioned on the downstream side include a circumferential circuit length d ₂₃ between the second convex lens and the third convex lens, and a circumferential circuit length d ₄₁ between the fourth convex lens and the first convex lens. A second lens position adjusting device is provided that is equally provided and is capable of moving the third convex lens and the fourth convex lens in a direction orthogonal to the optical axis.

The focal length f ₄ of the fourth lens sum of the focal length f ₁ of the first convex lens is equal to the circumferential circuit length d ₄₁ between the fourth convex lens and the first convex lens, and the focal length f ₂ of the second convex lens first 3 the sum of the focal length f ₃ of the convex lens is set equal to the circumferential circuit length d ₂₃ between the second convex lens and the third lens.

In addition, the laser beam condensing point adjusting device is further provided with a third convex lens and a fourth convex lens having two same focal lengths apart from each other by a focal length, and a concave lens is provided at the center thereof.
The focal length _{f 5} of the concave lens is equal to 1/4 of the 1/4 and the focal length _{f 4} of the focal length _{f 3,} and a peripheral circuit length _{d 35} between the third convex lens and a concave lens, a concave lens and a fourth lens peripheral circuit length _{d 54} between are set equal to 1/2 of the focal length _{f 3.}

The laser beam circulating optical system passes through the first linearly polarized light as it is, and reflects the second linearly polarized light orthogonal to the polarized light beam splitter,
A plurality of reflection mirrors that reflect the laser beam emitted from the polarization beam splitter a plurality of times and circulate around the polarization beam splitter;
A Pockels cell that is located in the peripheral circuit and rotates the polarization direction of the laser beam that passes when a voltage is applied;
A control device that controls the Pockels cell so that the laser beam circulating around the polarization beam splitter is always the second linearly polarized light,
Laser light is reflected at right angles by the polarization beam splitter and confined in a circuit that circulates the laser light.

According to a preferred embodiment of the present invention, the laser beam is a pulsed laser beam,
Furthermore, a half-wave plate that rotates the polarization plane of the pulsed laser light that is located upstream of the Pockels cell and rotates 90 degrees,
The control device allows the second linearly polarized light to pass without applying a voltage to the Pockels cell during the first round of the pulsed laser beam, and applies the voltage to the Pockels cell for the second and subsequent rounds. Pass through linearly polarized light.

According to another preferred embodiment of the present invention, the laser beam is a pulsed laser beam,
The control device applies a voltage to the Pockels cell during the first round of the pulsed laser beam to change the first linearly polarized light to the second linearly polarized light, and the second and subsequent cycles without applying a voltage to the Pockels cell. Pass linearly polarized light as it is.

In addition, according to the present invention, laser light is introduced from the outside, confined in a peripheral circuit that circulates the laser light, and repeatedly passes through the laser light condensing point in the peripheral circuit,
And the position of a laser beam condensing point is adjusted and the same laser beam is condensed to the same laser beam condensing point several times, The laser beam circulation method characterized by the above-mentioned is provided.

  According to a preferred embodiment of the present invention, the first convex lens is positioned upstream of the laser beam condensing point, the second convex lens is positioned downstream of the laser beam condensing point, and the first convex lens and the second convex lens are arranged. The position of the laser beam condensing point is adjusted by moving in the direction perpendicular to the optical axis.

  Further, the third convex lens is positioned upstream of the first convex lens, the fourth convex lens is positioned downstream of the second convex lens, and the first to fourth convex lenses are moved in a direction orthogonal to the optical axis to move the laser. Adjust the position of the light condensing point and the incident angle.

  Further, a concave lens is provided between the third convex lens and the fourth convex lens, thereby reducing the size of the peripheral circuit and reducing the intensity of condensing laser light between the third convex lens and the fourth convex lens.

In addition, the first linearly polarized laser light is introduced from the outside through the polarization beam splitter,
Forming a peripheral circuit for reflecting the laser beam emitted from the polarization beam splitter a plurality of times in the same plane and circulating around the polarization beam splitter;
Using the Pockels cell located in the circumference circuit, the Pockels cell is controlled so that the laser light circulating around the polarization beam splitter is always a second linearly polarized light orthogonal to the first linearly polarized light,
The laser beam is reflected at a right angle by the polarization beam splitter and confined in a circuit around the laser beam.

  According to the apparatus and method of the present invention, a laser beam is introduced from the outside by a laser beam circulating optical system, confined in a circuit that circulates the laser beam, and a laser beam condensing point in the circuit is repeated. Since the laser beam is allowed to pass, the laser beam can be confined in a predetermined optical path and circulated so that the same laser beam can be condensed at the same laser beam condensing point a plurality of times.

  In addition, since it has a laser beam condensing point adjustment device that adjusts the position of the laser beam condensing point, the position of the laser beam condensing point can be easily and accurately fine-tuned, thereby improving the laser light utilization efficiency. Can greatly increase.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In each figure, common portions are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is an overall configuration diagram of an X-ray generator equipped with a laser beam circulating apparatus according to the present invention. The X-ray generator includes an electron beam generator 10, a laser generator 20, and a laser beam circulating device 30.

The electron beam generator 10 has a function of accelerating the electron beam to generate a pulsed electron beam 1 and passing it through a predetermined linear trajectory 2.
In this example, the electron beam generator 10 includes an RF electron gun 11, an α-magnet 12, an acceleration tube 13, a bending magnet 14, a Q-magnet 15, a reduction tube 16, and a beam dump 17.

The RF electron gun 11 and the acceleration tube 13 are driven by an X-band (11.424 GHz) high-frequency power source 18. The electron beam extracted from the RF electron gun 11 changes its orbit by the α-magnet 12 and enters the acceleration tube 13. The accelerating tube 13 is a small X-band accelerating tube that accelerates the electron beam and forms a high energy electron beam, preferably around 50 MeV. This electron beam is, for example, a pulsed electron beam 1 of about 1 μs.
In particular, the pulsed electron beam 1 needs to generate an electron beam larger than the circulation time of the laser beam (about 10 ns) in order to collide the laser beam which circulates many times with one electron mass. A bunch pulsed electron beam is preferable.

  The bending magnet 14 bends the trajectory of the pulsed electron beam 1 with a magnetic field, passes the pulsed electron beam 1 through the predetermined linear trajectory 2, and guides the pulsed electron beam 1 after passing to the beam dump 17. The Q-magnet 15 adjusts the degree of convergence of the pulsed electron beam 1. The decelerating tube 16 decelerates the pulsed electron beam 1. The beam dump 17 captures the pulsed electron beam 1 after passing through the linear trajectory 2 to prevent radiation leakage.

  The synchronizer 19 synchronizes the electron beam generator 10 and the laser generator 20 to synchronize the timing of the pulsed electron beam 1 and the timing of a pulsed laser beam 3 to be described later, so that the pulsed electron beam 1 and the pulsed laser beam 3 are predetermined. It controls so that it may collide at the collision point 2a on the straight track 2 of.

  With the electron beam generator 10 described above, for example, a pulsed electron beam 1 of about 50 MeV and about 1 μs can be generated and passed through a predetermined linear trajectory 2.

The laser generator 20 has a laser device 21 and a variable beam expander 22, and has a function of generating laser light and irradiating it with a predetermined beam diameter.
The laser device 21 is an Nd-YAG laser with a wavelength of 1064 nm, for example. Further, the pulse laser beam 3 is not limited to this example, and ArF (wavelength 193 nm), KrF (wavelength 248 nm), XeCl (wavelength 308 nm), XeF (wavelength 351 nm), F2 (wavelength 157 nm), or YAG laser of an excimer laser is used. Third harmonic (wavelength 355 nm), fourth harmonic (wavelength 266 nm), fifth harmonic (wavelength 213 nm), and others may be used.

In the present invention, the laser beam may be a continuous laser beam and the laser device 21 may be a continuous laser device. However, the laser beam is a pulse laser beam 3 and the laser device 21 is a pulse laser device. preferable.
Hereinafter, the case where the laser beam is the pulse laser beam 3 and the laser device 21 is a pulse laser device will be described.

FIG. 2 is a diagram showing a first embodiment of a laser beam circulating optical system according to the present invention.
The laser beam circulating device 30 has a laser beam circulating optical system 32, and introduces the pulse laser beam 3 (p-polarized light 3 a) into the peripheral circuit 5 from the external laser generator 20 through the reflection mirror 31. The laser light 3 is confined in a peripheral circuit 5 that circulates, and the laser light condensing point 9 in the peripheral circuit is repeatedly passed. The peripheral circuit 5 is a rectangular optical path passing through positions a, b, c, and d on the same plane in this example.
The laser beam condensing point 9 corresponds to the collision position with the electron beam (collision point 2a on the straight track 2) in the example of FIG. However, the present invention is not limited to the collision with the electron beam, but can be applied to gas or the like.

  In this figure, the laser beam circulating optical system 32 includes a polarizing beam splitter 34, three reflecting mirrors 36a, 36b, 36c, a Pockels cell 38, and a control device 40.

The polarization beam splitter 34 passes the first linearly polarized light 3a (p-polarized light) as it is, and reflects the second linearly polarized light 3b (S-polarized light) orthogonal thereto at right angles.
The three reflecting mirrors 36a, 36b, and 36c reflect the pulsed laser light 3 emitted from the polarizing beam splitter 34 a plurality of times (in this example, three times), and circulate around the polarizing beam splitter 34 to form a rectangular shape on the same plane. The peripheral circuit 5 which is an optical path is configured.
In the present invention, the peripheral circuit 5 is not limited to a rectangular optical path on the same plane, and may be a three-dimensional optical path, a triangle, or a polygon having five or more corners.

The reflection mirrors 36a, 36b, and 36c are total reflection mirrors. When the radiation direction of the monochromatic hard X-ray 4 is the same as that of the pulsed electron beam 1, the reflection mirror 36a positioned in the radiation direction of the monochromatic hard X-ray 4 is preferably made of a material having a high X-ray transmittance (for example, quartz glass). The surface is coated with a reflective film, and is preferably made thin so that the monochromatic hard X-ray 4 can pass through and its loss is sufficiently small.
Note that this configuration is not essential. When the radiation direction of the monochromatic hard X-ray 4 is different from that of the pulsed electron beam 1, the reflection mirrors 36a, 36b, and 36c are made of a material having a low X-ray transmittance (for example, metal). May be.

The Pockels cell 38 is located between the positions d and a in the peripheral circuit 5 and rotates the polarization direction of the polarized light that passes when a voltage is applied by 90 degrees. A Pockels cell is a nonlinear optical crystal that can quickly switch the polarization direction of a light beam.
The control device 40 controls the Pockels cell 38 so that the pulsed laser light 3 that circulates and enters the polarization beam splitter 34 always becomes the second linearly polarized light 3b (S-polarized light).

  In the example of FIG. 2, the laser beam circulating optical system 32 includes a half-wave plate 41 (λ / 2 wavelength plate). In this example, the λ / 2 wavelength plate 41 is positioned between positions c and d on the upstream side of the Pockels cell 38, and always rotates the polarization plane of the passing pulse laser beam 3 (3a or 3b) by 90 degrees. .

  Further, the control device 40 passes the second linearly polarized light 3b as it is without applying a voltage to the Pockels cell 38 during the first round of the pulse laser beam 3, and applies a voltage to the Pockels cell 38 after the second round. Control is performed so that the first linearly polarized light 3a passes as the second linearly polarized light 3b.

  In the method of the present invention using the apparatus described above, the pulse laser beam 3 of the first linearly polarized light 3a is introduced from the outside through the polarization beam splitter 34, and a plurality of pulse laser beams 3 emitted from the polarization beam splitter 34 are arranged in the same plane. A pulsed laser beam that forms a circumference circuit 5 that is reflected and circulated around the polarization beam splitter 34, and circulates around the polarization beam splitter 34 using the half-wave plate 58 and the Pockels cell 38 that are located in the circumference circuit 5. 3 is controlled so that it always becomes the second linearly polarized light 3b orthogonal to the first linearly polarized light 3a, and the pulsed laser beam 3 is reflected by the polarizing beam splitter 34 at a right angle so as to circulate the pulsed laser light 3. Enclose in.

  By the above-described apparatus and method, the voltage of the second linearly polarized light 3b that has passed through the half-wave plate 58 and rotated the deflection surface by 90 degrees is not applied to the Pockels cell 38 during the first round of the pulse laser beam 3. In the second and subsequent passes, the first linearly polarized light 3a having the deflecting surface rotated 90 degrees through the half-wave plate 58 is applied to the Pockels cell 38 as a second linearly polarized light 3b. Thus, the pulsed laser light 3 entering the polarization beam splitter 34 is always the second linearly polarized light 3b (S-polarized light), and can be confined in the peripheral circuit 5 that circulates the pulsed laser light 3.

  In FIG. 2, the laser beam circulating device 30 further includes a laser beam focusing point adjusting device 50 that adjusts the position and angle of the pulse laser beam 3 at the laser beam focusing point 9 in the peripheral circuit 5.

In this example, the laser beam condensing point adjusting device 50 includes first to fourth convex lenses 51, 52, 53, and 54 and first and second lens position adjusting devices 58 and 59. The first to fourth lens 51, 52, 53 and 54 each have a focal length _{f 1, f 2, f 3} , f 4. These convex lenses may be single lenses or compound lenses.

The first convex lens 51 and the second convex lens 52 are positioned on the upstream side and the downstream side of the laser beam condensing point 9.
The third convex lens 53 and the fourth convex lens 54 are located on the upstream side and the downstream side of the Pockels cell 38.

  The first lens position adjusting device 58 is an adjustment moving mechanism that can move the first convex lens 51 and the second convex lens 52 in a direction perpendicular to the optical axis. The second lens position adjusting device 59 is an adjustment moving mechanism that can move the third convex lens 53 and the fourth convex lens 54 in a direction orthogonal to the optical axis. These mechanisms are traverse devices, and the position of each lens is finely adjusted manually or by an actuator such as an electric motor.

By finely adjusting the first convex lens 51 and the second convex lens 52 in the direction orthogonal to the peripheral circuit 5 by the first lens position adjusting device 58, the position of the pulse laser beam 3 passing through the laser beam condensing point 9 can be easily adjusted. And fine adjustment can be made accurately.
Further, the first to fourth convex lenses 51, 52, 53, 54 are finely adjusted in the direction orthogonal to the peripheral circuit 5 by the first lens position adjusting device 58 and the second lens position adjusting device 59, so that the incident angle and position are adjusted. Both of these can be finely adjusted easily and accurately, whereby the X-ray generation efficiency (that is, the utilization efficiency of laser light) can be greatly increased.

In this example, the fourth convex lens 54 located on the upstream side in the half-circular circuit that does not include the laser light condensing point 9 out of the half-circular circuit divided into two sections by the first convex lens 51 and the second convex lens 52. And a third convex lens 53 located on the downstream side, a peripheral circuit length d ₂₃ between the second convex lens 52 and the third convex lens 53, and a peripheral circuit length d ₄₁ between the fourth convex lens 54 and the first convex lens 51. Equally provided.
In this example, the sum (f ₄ + f ₁ ) of the focal length f ₄ of the fourth convex lens and the focal length f ₁ of the first convex lens is equal to the peripheral circuit length d ₄₁ between the fourth convex lens and the first convex lens. Is set.
Similarly, the sum (f ₂ + f ₃ ) of the focal length f ₂ of the second convex lens and the focal length f ₃ of the third convex lens is set equal to the peripheral circuit length d ₂₃ between the second convex lens and the third convex lens. Has been.
These two relations are the laser light circulation conditions.

For example, as in an example to be described later, the focal lengths f ₁ and f ₂ are set to 1000 mm, the focal lengths f ₃ and f ₄ are set to 500 mm, and the laser beam condensing point 9 is positioned at 1000 mm from the first convex lens 51 and the second convex lens 52. Is set, the laser beam can be condensed at two locations between the laser beam condensing point 9, the third convex lens 53, and the fourth convex lens 54.
By condensing the laser light at the laser light condensing point 9, the intensity of X-rays generated by the collision with the pulsed electron beam 1 (that is, the utilization efficiency of the laser light) can be increased.
Further, by condensing the laser light between the third convex lens 53 and the fourth convex lens 54, the responsiveness of the Pockels cell 38 can be enhanced. The position of the Pockels cell 38 can be arbitrarily shifted from the focal position for protection.

FIG. 3 is a diagram showing a second embodiment of the laser beam circulating optical system according to the present invention.
In this example, the λ / 2 wavelength plate 41 is located between the Pockels cell 38 and the position d on the upstream side thereof, and always rotates the polarization plane of the passing pulse laser beam 3 (3a or 3b) by 90 degrees. It has become. Other configurations are the same as those in the first embodiment.
Even with this configuration, the second linearly polarized light 3b that has passed through the half-wave plate 58 and whose deflection surface is rotated by 90 degrees is applied as it is without applying a voltage to the Pockels cell 38 during the first round of the pulse laser beam 3. Through the second and subsequent rounds, by applying a voltage to the Pockels cell 38 and passing the first linearly polarized light 3a having the deflection surface rotated 90 degrees after passing through the half-wave plate 58, the second linearly polarized light 3b is passed, The pulse laser beam 3 entering the polarization beam splitter 34 always becomes the second linearly polarized light 3b (S-polarized light), and can be confined in the peripheral circuit 5 that circulates the pulse laser beam 3.

  Further, since the λ / 2 wave plate 41 is located between the Pockels cell 38 and the position d on the upstream side, the second linearly polarized light 3b (S-polarized light) is always applied to each mirror 36a, 36b, after the second round. It is possible to reflect at 36c. As a result, since the S-polarized light has a higher reflectance than the P-polarized light, the energy efficiency at the time of rotation can be increased.

FIG. 4 is a diagram showing a third embodiment of the laser beam circulating optical system according to the present invention.
In this example, the laser beam condensing point adjusting device 50 of the present invention further includes two third convex lenses 53 and fourth convex lenses 54 having the same focal length separated from each other by a focal length, and a concave lens 55 at the center thereof. The concave lens 55 may be a single lens or a compound lens.
In this example, the focal length _{f 5} of the concave lens 55 is equal to 1/4 of the 1/4 and the focal length _{f 4} of the focal length _{f 3.} Further, the peripheral circuit length d ₃₅ between the third convex lens 53 and the concave lens 55 and the peripheral circuit length d ₅₄ between the concave lens 55 and the fourth convex lens 54 are set equal to ½ of the focal length f _3. Yes. These two relationships are the laser light circulation conditions in this configuration.
Other parts of the configuration are omitted, but are the same as those in the first embodiment.

  With this configuration, the concave lens 55 can prevent the laser light from focusing between the third convex lens 53 and the fourth convex lens 54, and in particular, air plasma that may be generated when the laser light has a high output. Can be prevented.

FIG. 5 is a diagram showing a fourth embodiment of the laser beam circulating optical system according to the present invention.
In this example, the positions of the polarizing beam splitter 34 and the reflection mirror 36c in the fourth embodiment of FIG. 4 are reversed.

The polarization beam splitter 34 passes the first linearly polarized light 3a (p-polarized light) as it is, and reflects the second linearly polarized light 3b (S-polarized light) orthogonal thereto at right angles.
The three reflecting mirrors 36a, 36b, and 36c reflect the pulse laser beam 3 emitted from the polarizing beam splitter 34 a plurality of times (in this example, three times), and circulate around the polarizing beam splitter 34 so that they are on the same plane. The peripheral circuit 5 is a rectangular optical path.

In this example, the controller 40 applies a voltage to the Pockels cell during the first round of the pulsed laser light to pass the first linearly polarized light 3a (p-polarized light) as the second linearly polarized light 3b (S-polarized light). From the second round onward, the second linearly polarized light is passed as it is without applying a voltage to the Pockels cell.
Other parts of the configuration are omitted, but are the same as those in the third embodiment.

Even with this configuration, during the first round of the pulsed laser beam, a voltage is applied to the Pockels cell to pass the first linearly polarized light 3a (p-polarized light) into the second linearly polarized light 3b (S-polarized light), and the Pockels cell after the second round. By passing the second linearly polarized light as it is without applying a voltage to the pulse laser beam 3, the pulsed laser light 3 entering the polarization beam splitter 34 always becomes the second linearly polarized light 3 b (S-polarized light), and the circuit 5 that circulates the pulsed laser light 3. Can be trapped inside.

Examples of the present invention will be described below.
An electron beam and a high-intensity pulsed laser beam from an X-band (11.424 GHz, wavelength 2.6 cm) linac, which is a quarter wavelength of S-band (2856 MHz, wavelength 10.5 cm) that has been conventionally used When generating monochromatic hard X-rays by collision (inverse Compton scattering) of the laser beam, as described above, the pulse width (10 ns, FWHM) of the laser beam is shorter than the pulse width (1 μs) of the electron beam. There was a problem that only a part of the electron beam contributed.
Therefore, the laser beam circulating apparatus and method of the present invention have been invented as a method for enhancing the X-ray intensity by confining a laser pulse in a certain circuit and repeatedly using it.

  FIG. 6 is an image diagram of multiple collisions according to the present invention. When the circulating distance (optical path length) of the laser beam circulating device is 6 m as in an example described later, for example, the period of the laser beam is about 20 ns. Therefore, as shown in this figure, the same pulsed laser beam 3 can be caused to collide with the same pulsed electron beam 2 a plurality of times (for example, 50 times).

(Principle experiment)
A block diagram of the principle proof experiment is shown in FIG. In this figure, each reference numeral is the same as in FIG. 1, 6 is a CCD, and 7 is a biplanar phototube.
The pulse laser beam 3 used in the experiment is Q-switch Nd: YAG second harmonic (λ = 532 nm), energy is 25 mJ / pulse, and pulse width is 8 ns (FWHM). In the proof-of-principle experiment, one round is 6m. Therefore, the time for one round of the laser pulse is 20 ns.

What is confirmed in this experiment is (1) energy amplification by laser light circulation and (2) position adjustment of laser light by an optical system.
The principle of lap is described. The laser beam 3 emitted from the laser device 21 is adjusted in size inclination by a beam steerer and an incident lens group. The expanded laser light is incident on the peripheral circuit 5 through the polarization beam splitter 34. The laser beam 3 is converted to S-polarized light 3b by the λ / 2 wavelength plate 41 and guided to the collision point 2a, and is again directed to the polarization beam splitter 34. Since the S-polarized light 3 b is reflected by the polarization beam splitter 34, the laser light 3 returns to the peripheral circuit 5. Again, since it passes through the λ / 2 wavelength plate 41 and becomes p-polarized light 3a, it passes through the polarizing beam splitter and does not go around. Therefore, in the second and subsequent rounds, a voltage is applied to the Pockels cell 38, p-polarized light is reflected as S-polarized light by the polarization beam splitter, and the laser light is circulated.

(Energy amplification by laser beam circulation)
In the laser beam circulating apparatus and method of the present invention, the same pulsed laser beam 3 is caused to collide with the same pulsed electron beam 2 a plurality of times as shown in FIG. The amplification of the X-ray intensity due to this was estimated.

FIG. 8 shows the result of measuring the laser light intensity of the light transmitted through the third mirror 36 b in FIG. 7 using the biplanar phototube 7.
Since the energy of the laser pulse is attenuated in a geometric sequence by the circulation, the energy I _{N in the Nth cycle} becomes I _N = A ^N−1 I ₀ if the energy in the first cycle is I ₀ . Therefore, when the transmittance A is obtained from FIG. 8, A = 0: 91, and the apparent total energy amplification is obtained with the pulse width of 1 μs of the electron beam from the formula (1) (shown in Formula 1) of the geometric sequence sum. Therefore, when N = 50 is calculated, it is estimated to be about 10 times. Since the collision between the single laser pulse and the electron beam is calculated to be 10 ⁸ photons / sec, it is expected that the amount of X-ray generation will reach 10 ⁹ photons / sec by introducing the laser light circulation system.

(Position adjustment of laser beam by optical system)
In order to generate X-rays efficiently, the position of the laser beam at the collision point, the tilt of the optical axis, the spot size, and the spread (divergence) must be adjusted according to the electron beam. Furthermore, the circulating laser beam needs to be repeatedly guided to the collision point under the same conditions. Hereinafter, this is referred to as “circulation condition”.

In order to satisfy the condition of the circulation, the transfer matrix (transfermatrix) of the circumference of the circumference circuit may be a unit matrix. Since the transfer matrix of FIG. 7 can be expressed by the formula (2) of Formula 2 with the first convex lens 51 as a starting point, the lens interval d (the optical path length of the second convex lens 52 and the third convex lens 53 or the fourth convex lens 54 and the first convex lens 51). ) Needs to be the sum of the focal lengths f _CP and f _PC of the two types of convex lenses in order to satisfy the condition of the circulation.

In other words, Equation (3) of Equation 2 is sufficient, and the experimental system is set to f _CP = 1000 mm, f _PC = 500 mm, and d = 1500 mm in order to satisfy this condition.

FIG. 9 and FIG. 10 are adjustment methods that do not break the conditions of the circulation. FIG. 9 is an explanatory view of the optical axis inclination adjusting means according to the present invention. Adjustment is performed at the incident position on the first convex lens 51, but the beam size and the like are performed outside the peripheral circuit, so that the conditions for the circulation are not lost.
FIG. 10 is an explanatory diagram of the collision point position adjusting means according to the present invention. Adjustment is performed at the position of the lens in the circuit.

  The change in the inclination of the optical axis caused by the adjustment of the first convex lens 51 is corrected by moving the second convex lens 52 by the same amount. Further, the third convex lens 53 and the fourth convex lens 54 are moved by the same amount to cancel the position change. With this configuration, the state of the laser beam always coincides with the time of incidence of the first convex lens 51, and the circulation condition can be maintained.

  FIG. 11 shows the result of adjustment of the position (center of gravity) of the laser beam at the collision point using four convex lenses in the circumferential circuit with respect to the x-axis direction (horizontal direction). The amount of adjustment of the first convex lens 51 and the amount of movement of the position of the laser light correspond to approximately 1: 1, and the laser light remains in a collected state even after adjustment. Similar results are also obtained in the y-axis direction (vertical direction).

Furthermore, FIG. 12 shows the result of measuring the laser beam position at a point 50 cm from the first convex lens 51 when the position adjustment at the collision point is performed.
The laser beam position was proportional to the adjustment amount of the first convex lens 51, and the laser beam was kept in a gathered state.
From the results of these two position adjustments, it was confirmed that the position of the laser beam can be made the same in all the turns by moving the four convex lenses by the same amount.
From the above, it can be said that this method has established an adjustment method that does not break the conditions of the laser light circulation.

(Demonstration system introduction system)
The laser beam introduced into the demonstration machine is Q-switch Nd: YAG double wave (λ = 532 nm), energy is 1.4 J / pulse, pulse width is 10 ns (FWHM), and high output. Then, in a system similar to the experimental system, the plasma of air becomes a problem at the focal point of the convex lens on the Pockels cell side.

Therefore, it is preferable to incorporate a concave lens on the Pockels cell side as shown in FIG. In this configuration, one round is 7 m.
In this configuration, as shown in FIG. 14, a concave lens is inserted in the center between the convex lenses on the Pockels cell side. Condensation of laser light can be prevented by the concave lens. Furthermore, by setting the focal length of the concave lens to ¼ of the focal length of the convex lens, the inter-lens distance d becomes 2d in the transfer matrix, and the size can be reduced.
However, in this system, since the transfer matrix for one round is expressed by Equation (4), the read circuit needs to be readjusted when the condition of the incident light is changed. However, since there are four convex lenses, the degree of freedom of adjustment is the same as in the experimental system, and the adjustment method used in the experiment is possible.

As described above, in the proof-of-principle experiment of the laser beam circulation system for enhancing the X-ray intensity, the X-ray generation amount is expected to be 10 ⁹ photons / sec, which is 10 times that when the laser beam circulation system is not introduced. It was.
In addition, it was possible to establish a means for guiding the laser beam to the collision point without breaking the circulation condition by moving the convex lens in the circuit around by the same amount. By this means, the laser beam can collide with the electron beam relatively easily, and X-rays can be generated.
Furthermore, in a laser light circulation system that is introduced into an actual monochromatic hard X-ray source, as a measure to prevent the impact of air impact on the collision and circulation, by inserting a concave lens between the convex lenses, An adjustment method that does not break the conditions can be used.
Therefore, the introduction of this laser beam circulation system makes it possible to amplify the X-ray intensity, and it can be expected that the application range of X-rays will be widened.

  In addition, this invention is not limited to embodiment mentioned above, Of course, it can change variously in the range which does not deviate from the summary of this invention.

1 is an overall configuration diagram of an X-ray generator provided with a laser beam circulating apparatus according to the present invention. It is 1st Embodiment figure of the laser beam circulation optical system by this invention. It is 2nd Embodiment figure of the laser beam circulation optical system by this invention. It is 3rd Embodiment figure of the laser beam circulation optical system by this invention. It is a 4th embodiment figure of a laser beam circulation optical system by the present invention. It is an image figure of the multiple collision by this invention. It is a block diagram of a principle proof experiment. It is a laser beam intensity | strength measurement result of the transmitted light of a 3rd mirror. It is explanatory drawing of the adjustment means of the inclination of the optical axis by this invention. It is explanatory drawing of the position adjustment means of the collision point by this invention. It is a relationship diagram of the position and beam size of the laser beam at the collision point, and the x-axis direction lens adjustment amount. It is a relationship figure of the position and beam size of a laser beam in the point of 50 cm from a 1st convex lens, and an x-axis direction lens adjustment amount. It is the block diagram which incorporated the concave lens in the Pockels cell side. FIG. 14 is a detailed view of FIG. 13. 1 is a schematic diagram of an “X-ray generator” in Patent Document 1. FIG. 1 is a schematic diagram of a “small X-ray generator” in Non-Patent Document 1. FIG. It is a schematic diagram of the circulation device of Non-Patent Document 2.

Explanation of symbols

1 pulse electron beam, 2 linear trajectory, 2a collision point,
3 Laser light (pulse laser light), 3a First linearly polarized light (p-polarized light),
3b Second linearly polarized light (S-polarized light), 4 monochromatic hard X-ray, 5 circuit,
6 CCD, 7 Biplanar phototube, 9 Laser light focusing point 10 Electron beam generator, 11 RF electron gun, 12 α-magnet,
13 acceleration tube, 14 bending magnet, 15 Q-magnet,
16 decelerator, 17 beam dump, 18 high frequency power supply,
19 Synchronizer, 20 Laser generator,
21 laser equipment, 22 variable beam expander,
30 laser beam circulating device, 31 reflecting mirror,
32 laser beam circulating optical system, 34 polarization beam splitter,
36a, 36b, 36c Reflective mirror, 38 Pockels cell,
40 control device, 41 half-wave plate (λ / 2 wave plate),
50 Laser beam condensing point adjustment device,
51 1st convex lens, 52 2nd convex lens, 53 3rd convex lens,
54 4th convex lens, 55 concave lens,
58 First lens position adjusting device (adjustment moving mechanism)
59 Second lens position adjustment device (adjustment movement mechanism)

Claims (14)

A laser beam circulating optical system that introduces a laser beam from the outside, confines the laser beam in a circular circuit that circulates the laser beam, and repeatedly passes through a laser beam condensing point in the circular circuit;
A laser beam condensing point adjusting device that adjusts the position of the laser beam condensing point, thereby condensing the same laser beam at the same laser beam condensing point multiple times Orbiting device.   The laser beam condensing point adjusting device includes a first convex lens positioned upstream of the laser beam condensing point, a second convex lens positioned downstream of the laser beam condensing point, the first convex lens, and the second convex lens. And a first lens position adjusting device capable of moving in a direction orthogonal to the optical axis. The laser beam condensing point adjusting device further includes an upstream side in a half circuit that does not include the laser beam condensing point among the half circuit divided into two sections by the first convex lens and the second convex lens. The fourth convex lens positioned and the third convex lens positioned on the downstream side include a circumferential circuit length d ₂₃ between the second convex lens and the third convex lens, and a circumferential circuit length d ₄₁ between the fourth convex lens and the first convex lens. 3. The laser beam circulating apparatus according to claim 2, further comprising a second lens position adjusting device that is provided equally and is capable of moving the third convex lens and the fourth convex lens in a direction orthogonal to the optical axis. The focal length f ₄ of the fourth lens sum of the focal length f ₁ of the first convex lens is equal to the circumferential circuit length d ₄₁ between the fourth convex lens and the first convex lens, and the focal length f ₂ of the second convex lens first 4. The laser beam circulating apparatus according to claim _{3, wherein} the sum of the focal lengths f ₃ of the three convex lenses is set equal to a circumferential circuit length d ₂₃ between the second convex lens and the third convex lens. The laser beam condensing point adjusting device further provides two third and fourth convex lenses having the same focal length apart from each other by a focal length, and a concave lens at the center thereof.
The focal length f ₅ of the concave lens is equal to 1/4 of the 1/4 and the focal length f ₄ of the focal length f _3, that the laser light circulator device according to claim 3, characterized in. The circumferential circuit length d ₃₅ between the third convex lens and the concave lens and the circumferential circuit length d ₅₄ between the concave lens and the fourth convex lens are set equal to ½ of the focal length f _3. The laser beam circulating apparatus according to claim 5. The laser beam circulating optical system passes through the first linearly polarized light as it is, and reflects the second linearly polarized light orthogonal to the polarized light beam splitter,
A plurality of reflection mirrors that reflect the laser beam emitted from the polarization beam splitter a plurality of times and circulate around the polarization beam splitter;
A Pockels cell that is located in the peripheral circuit and rotates the polarization direction of the laser beam that passes when a voltage is applied;
A control device that controls the Pockels cell so that the laser beam circulating around the polarization beam splitter is always the second linearly polarized light,
The laser beam circulating apparatus according to claim 1, wherein the polarized beam splitter reflects the laser beam at a right angle and confines the laser beam in a circuit that circulates the laser beam. The laser beam is a pulsed laser beam,
Furthermore, a half-wave plate that rotates the polarization plane of the pulsed laser light that is located upstream of the Pockels cell and rotates 90 degrees,
The control device allows the second linearly polarized light to pass without applying a voltage to the Pockels cell during the first round of the pulsed laser beam, and applies the voltage to the Pockels cell for the second and subsequent rounds. 8. The laser beam circulating device according to claim 7, wherein the laser beam circulating device is linearly polarized light. The laser beam is a pulsed laser beam,
The control device applies a voltage to the Pockels cell during the first round of the pulsed laser beam to change the first linearly polarized light to the second linearly polarized light, and the second and subsequent cycles without applying a voltage to the Pockels cell. The laser beam circulating apparatus according to claim 7, wherein linearly polarized light is passed as it is. Laser light is introduced from the outside, confined in a circuit that circulates the laser light, and repeatedly passes through a laser beam condensing point in the circuit,
A laser beam circulation method characterized by adjusting the position of the laser beam focusing point and focusing the same laser beam on the same laser beam focusing point multiple times.   The first convex lens is positioned upstream of the laser beam condensing point, the second convex lens is positioned downstream of the laser beam condensing point, and the first convex lens and the second convex lens are moved in a direction perpendicular to the optical axis. The laser beam circulating method according to claim 10, wherein the position of the laser beam focusing point is adjusted.   Further, the third convex lens is positioned upstream of the first convex lens, the fourth convex lens is positioned downstream of the second convex lens, and the first to fourth convex lenses are moved in a direction orthogonal to the optical axis to move the laser. The laser light circulating method according to claim 11, wherein the position of the light condensing point and the incident angle are adjusted.   A concave lens is provided between the third convex lens and the fourth convex lens, whereby the peripheral circuit is miniaturized, and the light collecting intensity of the laser light between the third convex lens and the fourth convex lens is reduced. Item 13. A laser beam circulating method according to Item 12. The first linearly polarized laser beam is introduced from the outside through the polarization beam splitter,
Forming a peripheral circuit for reflecting the laser beam emitted from the polarization beam splitter a plurality of times in the same plane and circulating around the polarization beam splitter;
Using the Pockels cell located in the circumference circuit, the Pockels cell is controlled so that the laser light circulating around the polarization beam splitter is always a second linearly polarized light orthogonal to the first linearly polarized light,
11. The laser beam circulating method according to claim 10, wherein the laser beam is reflected at a right angle by the polarizing beam splitter and confined in a circuit that circulates the laser beam.

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