JP4250759B2 - Laser-controlled electron beam linear accelerator - Google Patents

Description

  The present invention relates to a laser-controlled electron beam linear accelerator, and more particularly to a laser-controlled electron beam linear accelerator that corrects energy spread of a multi-bunch beam due to beam loading while accelerating the electron beam.

  In the field of electron beam linear accelerators, a multi-bunch beam is generally used to accelerate a long pulse and large current electron beam. Here, the multi-bunch beam means an electron bunch row, that is, an electron beam composed of a series of electron clusters arranged in a row. When a multi-bunch beam whose electron beam pulse length is shorter than the accelerating tube feeling time, that is, the time during which high-frequency power is completely charged to the accelerating tube, is accelerated by a linear accelerator composed of a constant acceleration gradient type accelerating tube As the head bunch goes from the front bunch to the rear bunch, the energy gain decreases almost linearly by beam loading in the transient mode in the accelerating tube. There are several methods for correcting the energy difference in the multi-bunch beam, that is, the spread of energy. Typical methods include the ΔT method and the ΔF method.

  The ΔT method correction adjusts the gradient of the acceleration high-frequency power by changing the amplitude of the high-frequency that is incident on the beam during the feeling time and is supplied to the acceleration tube, and the energy in the bunch row generated by the beam loading. The difference is corrected.

  As an example of the ΔT correction, in a device including a stabilized high-frequency oscillator, an acceleration high-power pulse high-frequency source, and an acceleration tube, a high-frequency dividing unit that divides a high frequency, and a phase of each of the divided high frequencies A phase modulator for adjusting the frequency, means for synthesizing each high frequency, a high-frequency pulsed high-frequency source for acceleration by inputting the synthesized high frequency from the synthesizing means, and a pulsed electron source for outputting an electron beam to the accelerator tube, By giving phase modulation instructions to each phase modulator for each divided high frequency, amplitude modulation is applied to the rising portion of the acceleration high-frequency pulse, and acceleration energy due to the beam loading effect generated at the rising portion of the electron beam pulse is controlled. There are some that compensate for the fluctuations. (For example, refer to Patent Document 1).

In addition, ΔF correction means, for example, that an acceleration tube having a slightly different frequency (f ₁ ± Δf) with respect to the basic acceleration frequency (f ₁ ) is installed after a linear accelerator composed of f ₁ acceleration tubes. The multi-bunch is accelerated and decelerated by placing the multi-bunch at a phase near the zero cross of the frequency (f ₁ ± Δf), and the energy of the multi-bunch beam generated by the linear accelerator is corrected. In this ΔF correction, acceleration and correction are performed separately in the linear accelerator. A laser-controlled electron beam linear accelerator is designed to perform this acceleration and correction simultaneously.

  FIG. 4 shows a schematic diagram of a laser-controlled electron beam linear accelerator that corrects the energy spread of the multi-bunch beam due to beam loading while accelerating the electron beam by the ΔF method. As shown in FIG. 4, the laser-controlled electron beam linear accelerator includes a mode-locked laser 11, a photocathode high-frequency electron gun 12 and its high-frequency source 13, a high acceleration gradient type acceleration tube 14 and its high-frequency source 15. Consists of. The linear accelerator is provided with a timing system 16 for generating the frequency of the input signal supplied to the mode-locked laser 11, the frequency of the photocathode high-frequency electron gun 12, and the acceleration frequency of the accelerator tube 14. The timing system 16 is supplied with a reference signal from a reference signal generator 17. The timing system 16 includes a multiplier 18, frequency dividers 19 and 20, and an SSB (single sideband) generator 21. Here, the mode-locked laser is a laser that has the function of generating picosecond pulsed light by synchronizing several oscillation modes with very close wavelengths that the laser oscillates normally. It is. The photocathode high-frequency electron gun has a structure in which a cathode surface installed on a side surface in a high-frequency cavity is irradiated with laser light, and electrons emitted by the photoelectric effect are immediately accelerated by a high-frequency electric field.

In the laser-controlled electron beam linear accelerator having the above-described configuration, a reference signal having a frequency of f _R = 1428.67646 MHz is output from the reference signal generator 17, and a frequency f ₀ = 2857.35291 MHz obtained by multiplying the frequency f _R by two. Is used for the high-frequency electron gun 12, the reference signal divided by 4 f _R /4=357.16911 MHz is used as the frequency f _L of the input signal of the mode-locked laser 11, and the divided signal f _R / 4 is 264 A signal of Δf = 1.52991 MHz is generated by frequency division, and this Δf and the frequency f ₀ of the high-frequency electron gun 12 are synthesized using the SSB generator 21 to produce an acceleration frequency f ₁ = 2856 MHz of the accelerating tube 14. Then, a laser output from the mode-locked laser 11 is driven into the cathode of the photocathode high-frequency electron gun 12 to generate a multi-bunch beam of 20 bunches. By changing the frequency f ₀ = 2857.35291 MHz of the beam output from the high-frequency electron gun 12 by Δf = 1.3529 MHz with respect to the reference acceleration frequency f ₁ = 2856 MHz, the acceleration phase of each bunch beam is slightly shifted, and 20 bunch The accelerating electric field is made stronger later in the beam. Accordingly, there is a technique that corrects the energy spread due to beam loading while accelerating the beam in a state where the high-frequency power is utilized to the maximum (see, for example, Non-Patent Document 1).

JP-A-11-45800 Koichiro Hirano and eight others "Development of a compact high-intensity hard X-ray source-(2) S-band linear accelerator system-", Proceedings of the 27th Linear Accelerator Meeting in Japan, Kyoto University, August 7, 2002, p. 130-132

  According to the above-described laser-controlled electron beam linear accelerator, separate high-frequency sources are used for the high-frequency electron gun and the acceleration tube, so that the entire linear accelerator is relatively large. By making the high-frequency electron gun and the high-frequency source of the accelerating tube common, it is possible to reduce the size of the entire linear accelerator, but in that case, the electric field received by the multi-bunch beam in the high-frequency electron gun is Therefore, the current of the generated electron beam becomes larger toward the rear stage. Therefore, distortion occurs in the current pulse waveform of the electron beam generated from the high-frequency electron gun.

  That is, in a normal high-frequency electron gun, the frequency of the laser uses a frequency division signal of the frequency of the high-frequency electron gun. When the current of the bunch increases, the beam loading increases even in the high-frequency electron gun as in the case of the acceleration tube, the energy decreases as the bunch in the later stage, and the energy difference between each bunch increases. In general, the acceleration energy of the accelerator tube is as large as 100 MeV compared with the acceleration energy of 5 MeV of the high-frequency electron gun, and the energy difference generated in the accelerator tube is larger than the energy difference of each bunch generated in the high-frequency electron gun. Perform energy correction with.

  If the energy correction is performed so as to cancel the beam loading in the accelerator tube, the electric field that works to cancel the beam loading generated by the high-frequency electron gun becomes stronger than the electric field lost by the beam loading. The electric field that draws out the beam standing on the inner cathode surface acts higher on the bunch in the subsequent stage, so that the current of the bunch generated from the high-frequency electron gun rises to the right. In other words, distortion occurs in the electron beam waveform generated from the high-frequency electron gun.

Further, according to the laser-controlled electron beam linear accelerator, the relationship between the frequency f ₀ of the high-frequency electron gun and the acceleration frequency f ₁ of the accelerator tube is f ₀ = f ₁ + Δf, and the high-frequency falling slope is We are using. When the high-frequency falling slope is used in this way, the bunches spread because the particles behind the first particles in each bunch receive lower energy. Therefore, it is difficult to obtain a high-performance electron beam with the conventional laser-controlled electron beam linear accelerator.

  It is an object of the present invention to reduce the size of the entire linear accelerator and correct the distortion of the current pulse waveform of the electron beam so that an electron beam having a constant bunch current is generated from a high-frequency electron gun. And

  According to the present invention, a photocathode high-frequency electron gun that immediately accelerates electrons emitted by irradiating a cathode surface with a laser beam of a mode-locked laser by a high-frequency electric field and outputs an electron beam composed of bunch rows An acceleration tube for accelerating the electron beam output from the high-frequency electron gun; a timing system for creating a frequency of the input signal of the mode-locked laser, a frequency of the high-frequency electron gun and an acceleration frequency of the acceleration tube from a reference signal; The high-frequency electron gun and a high-frequency source for supplying high-frequency power to the acceleration tube, respectively, and the frequency of the input signal of the mode-locked laser, the frequency of the high-frequency electron gun, and the acceleration frequency of the acceleration tube are within a predetermined detuning range. The phase of the frequency of the electron beam output from the high-frequency electron gun is changed to the phase of the acceleration frequency. In a laser-controlled electron beam linear accelerator that corrects the energy spread of a multi-bunch beam due to beam loading while accelerating the beam, the mode is interposed between the mode-locked laser and the high-frequency electron gun. Means for modulating the amplitude of the laser pulse output from the lock laser is provided, and the frequency of the high-frequency electron gun is made to coincide with the acceleration frequency of the acceleration tube, thereby making the high-frequency source common to the high-frequency electron gun and the acceleration tube. There is provided a laser-controlled electron beam linear accelerator characterized by being configured as a high-frequency source.

  According to the present invention, the means for modulating the amplitude of the laser pulse may have a function of cutting out a bunch row having a predetermined number of bunches by quickly switching the polarization direction of the light beam.

  According to the present invention, the means for modulating the amplitude of the laser pulse may comprise a Pockel cell and a pair of polarizers.

According to the present invention, the timing system creates a signal of a frequency Δf and f ₀ by multiplying or dividing the reference signal, the side band of f ₀ by mixing signals of Δf and f ₀ An acceleration frequency f ₁ is created, and Δf is a frequency within a range of several hundred kHz to several MHz with respect to the acceleration frequency f ₁ , and f ₀ has a relationship of f ₀ = f ₁ −Δf. The input signal of the mode-locked laser may be a signal obtained by dividing the f ₀ signal.

According to the present invention, the timing system generates a signal of frequency Δf and acceleration frequency f ₁ by multiplying or dividing the reference signal, and mixes the signals of Δf and f ₁ to thereby generate the side of f ₁ . A signal having a frequency f ₀ that is a band is created, and Δf is a frequency within a range of several hundred kHz to several MHz with respect to the acceleration frequency f ₁ , and f ₀ is a relationship of f ₀ = f ₁ −Δf. The input signal of the mode-locked laser may be a signal obtained by dividing the signal of f ₀ .

According to the present invention, since the common high-frequency source is used for the high-frequency electron gun and the acceleration tube, the entire linear accelerator can be miniaturized. In addition, a means for modulating the amplitude of the laser pulse output from the mode-locked laser is provided between the mode-locked laser and the high-frequency electron gun, thereby modulating the amplitude of the laser pulse so that a constant value is obtained from the high-frequency electron gun. An electron beam having a bunch current of Further, according to the present invention, by selecting the relationship of f ₀ = f ₁ −Δf, it is possible to use the rising slope of the high frequency, and hence the particles behind the head particles in each bunch are higher. Since the energy is received, the bunch is hardened and an electron beam with good performance can be obtained.

FIG. 1 is a schematic diagram showing the configuration of a laser-controlled electron beam linear accelerator according to the present invention. The same reference numerals as those in FIG. 4 denote the same elements. The laser-controlled electron beam linear accelerator of the present invention is similar to the linear accelerator shown in FIG. 4. The mode-locked laser 11, photocathode high-frequency electron gun 12, accelerator tube 14, and accelerator tube high-frequency source 15 are used. And a timing system 16 and a reference signal generator 17, and the timing system 16 includes a multiplier 18, frequency dividers 19 and 20, and an SSB generator 21. However, the linear accelerator of FIG. 1 is provided with a laser pulse amplitude modulator 22 between the mode-locked laser 11 and the photocathode high-frequency electron gun 12, and the high-frequency source 15 of the accelerator tube 14 is a high-frequency electron gun. The difference is that twelve high-frequency sources are also used. The frequency f _{R of the} reference signal generated from the reference signal generator 17 is also different.

The reference signal generator 17 generates a reference signal having a frequency f _R = 1427.831624 MHz. The reference signal is divided by 4 by the frequency divider 19 to become a signal of f _R /4=356.957906 MHz, and is used as the frequency f _L of the input signal of the mode-locked laser 11. Further, the signal of f _R / 4 is divided by 4N by the frequency divider 20 (N = 265) to produce a signal of Δf = 0.36736753 MHz. On the other hand, the reference signal having the frequency f _R is multiplied by 2 by the multiplier 18 to generate a signal having the frequency f ₀ = 2855.66324 MHz. The SSB generator 21 synthesizes the signal of f ₀ = 2855.66324 MHz and the signal of Δf = 0.336753 MHz, that is, mixes them to produce the acceleration frequency f ₁ = 2856 MHz of the high frequency source 15. Here, f ₁ = f ₀ + Δf, and f ₁ is a side band of f ₀ .

Since the high frequency source 15 is a high frequency source common to the photocathode high frequency electron gun 12 and the acceleration tube 14, the high frequency of the frequency f ₁ output from the high frequency source 15 is applied to the photocathode high frequency electron gun 12 and the acceleration tube 14. Supplied to both sides. A klystron power supply may be used as the high frequency source 15. Alternatively, a 1.6-cell cavity BNL type photocathode high-frequency electron gun may be used, and Cs ₂ Te may be used for the photocathode. As the mode-locked laser, an Nd: YVO laser (266 nm, 1 μJ / pulse for a charge amount of 2 nC / bunch) may be used. Furthermore, a 3 m constant acceleration gradient traveling wave accelerator tube (average high acceleration gradient 33 MeV / m) may be used as the acceleration tube.

The frequency f _L of the signal used as the input signal of the mode-locked laser is the frequency f _{R of the} reference signal divided by 4, ie, f _L = f _R / 4, and the frequency f ₀ is the frequency of the reference signal. Since the frequency is doubled, that is, f ₀ = 2f _R , the frequency f _L = f _0/8 . Therefore, the frequency f _L is the frequency f _{0 divided} by eight. In this way, by irradiating the cathode of the high-frequency electron gun with the pulse output from the mode-locked laser locked with the signal obtained by dividing the frequency f ₀ lower than the acceleration frequency f ₁ of the high-frequency source by Δf, FIG. As shown, the timing of each bunch of multi-beams generated from the high-frequency electron gun can be shifted little by little with respect to the high-frequency acceleration phase.

In FIG. 2, eight vertical broken lines L indicate the timing of laser pulse generation, and one bunch B is formed for each pulse. Since the frequency of the laser pulse generation timing L is f _L , the electron beam is output in synchronization with f _L. For example, in order to emit a 20 bunched beam with an interval of 2.8 nsec from a high-frequency electron gun, a laser with a micropulse width of 10 psec is driven into the photocathode at a micropulse repetition frequency f _L = 356.957906 MHz and a macropulse width of 56 nsec. . That is, since the period of the micro pulse of the laser is 2.8 nsec, 20 electron bunches are included in the macro pulse width 56 nsec at intervals of 2.8 nsec.

In FIG. 2, A shows a waveform of an acceleration high frequency having an acceleration frequency f ₁ = 2856 MHz. As described above, an f _0/8 = f _L, since it is f ₀ = f ₁ -Δf, acceleration frequency f ₁ = 2856 MHz is approximately 8 times the frequency f _L = 356.957906MHz laser. Therefore, almost one laser pulse is generated in 8 periods of the acceleration high frequency A. For this reason, only one of the eight accelerated high-frequency A waves is used for acceleration, and the remaining seven are idle. Therefore, in FIG. 2, for each laser frequency f _L , only the first one of the eight cycles of the acceleration high frequency A is shown by a solid line, and the waveforms for the subsequent seven cycles are omitted. . Moreover, although the actual bunches are 20 bunches, only eight of them are shown in the figure.

As shown in FIG. 2, the timing of each bunch of the multi-bunch beam is changed by changing the frequency f _{L of} the laser, and hence the frequency of the multi-bunch beam, by Δf = 0.336753 MHz with respect to the acceleration frequency f ₁ . It can be shifted little by little with respect to the high frequency acceleration phase. Therefore, the acceleration electric field at the rear stage of the multi-bunch beam can be increased. That is, correction is performed so that a higher acceleration energy gain is obtained for the bunch beam at the rear stage. This correction curve is indicated by a dotted line 1. As a result, the acceleration energy drop due to the beam loading effect (indicated by the dotted line 2) is corrected to obtain a constant acceleration gain (indicated by the alternate long and short dash line 3). The relationship between the laser frequency f _L , the acceleration frequency f ₁ and Δf is Δf = f ₁ −f ₀ = f ₁ −8f _L.

  When the correction according to FIG. 2 is performed, a constant acceleration gain is obtained, but the electric field that works to cancel the beam loading generated by the high-frequency electron gun is stronger than the electric field lost by the beam loading. As a result, a higher electric field acts on the bunch at the rear stage, and an energy difference opposite to that of a normal high-frequency electron gun occurs. Therefore, the current of the generated electron beam becomes larger in the rear stage. That is, the output current of the high frequency electron gun rises to the right. In addition, the directionality of the electron beam also deteriorates. For example, a high-frequency electron gun exit beam current pulse waveform output from the high-frequency electron gun 12 in response to a laser pulse having a macro pulse width of 56 nsec has a current intensity at the falling portion of the pulse as shown in FIG. It increases by 2.8% compared to the current intensity. Therefore, according to the present invention, in order to correct this increase, a laser pulse amplitude modulator 22 is provided between the mode-locked laser 11 and the photocathode high-frequency electron gun 12 as shown in FIG.

  The amplitude modulator 22 modulates the intensity, ie, amplitude, of the laser pulse output from the mode-locked laser 11, thereby correcting the distortion of the pulse waveform as shown by C in FIG. To do. The amplitude modulator 22 includes a Pockel cell and a pair of polarizers. The Pockel cell is an element used for quickly switching the polarization direction of a laser beam. By combining this Pockel cell and a pair of polarizers, a laser pulse can be cut out with a predetermined width and the light intensity can be modulated. The Pockel cell can modulate the amplitude of the laser pulse by forming a laser pulse with a macro pulse width of 56 nsec and controlling the tilt of the polarizer with respect to the laser pulse. In the example of FIG. 3, as indicated by E, the laser pulse intensity is controlled to decrease by 2.8% during 56 nsec. As a result, the distortion of 2.8% of the current pulse waveform of the high frequency electron gun exit beam is corrected.

  The laser-controlled linear accelerator of the present invention can be configured with a compact X-ray radiation source in combination with the electron beam generator and light storage device of the radiation source storage ring device. Moreover, a small high-intensity pulsed neutron source can be configured in combination with a neutron generation converter.

It is the schematic of the laser control type | mold linear acceleration apparatus of this invention. It is a figure explaining the (DELTA) F system multi-bunch energy correction method in this invention. It is a figure explaining the high frequency electron gun exit beam current correction method in this invention. It is the schematic of the conventional laser control type | mold linear acceleration apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Mode-locked laser 12 Photocathode high frequency electron gun 13, 15 High frequency source 14 Accelerator tube 16 Timing system 17 Reference signal generator 18 Multiplier 19, 20 Divider 21 SSB generator 22 Amplitude modulator A Acceleration high frequency waveform B Electronic bunch C Corrected high-frequency electron gun exit beam current pulse waveform D Corrected high-frequency electron gun exit beam current pulse waveform E Modulated laser pulse intensity L Laser pulse timing

Claims (4)

A photocathode high-frequency electron gun that immediately accelerates electrons emitted by irradiating the cathode surface with a mode-locked laser beam by a high-frequency electric field and outputs an electron beam composed of bunch rows, and the high-frequency electron gun An acceleration tube for accelerating the electron beam output from the above, a timing system for creating the frequency of the input signal of the mode-locked laser, the frequency of the high-frequency electron gun, and the acceleration frequency of the acceleration tube from the reference signal, and A high-frequency source for supplying high-frequency power to each of the tubes, and by setting the frequency of the input signal of the mode-locked laser, the frequency of the high-frequency electron gun, and the acceleration frequency of the accelerator tube to a predetermined detuning range, The phase of the frequency of the electron beam output from the gun is slightly different from the phase of the acceleration frequency. Rashi, whereby the laser-controlled electron beam linear accelerator for correcting the energy spread of the multi-bunch beam by beam loading while accelerating the beam,
Means for modulating the amplitude of a laser pulse output from the mode-locked laser is provided between the mode-locked laser and the high-frequency electron gun, and the frequency of the high-frequency electron gun is matched with the acceleration frequency of the accelerator tube A laser-controlled electron beam linear acceleration device characterized in that the high-frequency source is configured as a high-frequency source common to the high-frequency electron gun and the acceleration tube.   The laser-controlled electron according to claim 1, wherein the means for modulating the amplitude of the laser pulse has a function of cutting out a bunch row having a predetermined number of bunches by quickly switching the polarization direction of the light beam. Beam linear accelerator.   3. The laser-controlled electron beam linear accelerator according to claim 2, wherein the means for modulating the amplitude of the laser pulse comprises a Pockel cell and a pair of polarizers.   The timing system multiplies or divides the reference signal to generate a signal of frequency Δf and f0, and mixes the signal of Δf and f0 to generate an acceleration frequency f1 that is a sideband of f0. Δf is a frequency within the range of several hundred kHz to several MHz with respect to the acceleration frequency f1, f0 has a relationship of f0 = f1−Δf, and the input signal of the mode-locked laser is the signal of f0. 4. The laser-controlled electron beam linear acceleration device according to claim 1, wherein the signal is a frequency-divided signal.

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