JP4431723B2 - Long wavelength coherent light generator - Google Patents

Description

  The present invention relates to an accelerator related technique using coherent radiation by an electron beam.

The submillimeter wave-millimeter wave targeted by the present invention is located in the intermediate band between the light wave and the electromagnetic wave, and there are few light sources suitable for spectroscopic measurement, and it was a region where research and development was delayed. Recently, however, this wavelength band has begun to attract attention in fields such as communication and imaging technology. Millimeter waves emitted by coherent radiation emitted from high-energy electron bunches, oscillation by backward wave tubes, and current switching of semiconductors, etc. Sub-millimeter waves are being used for remarkable research.
As a light source in this wavelength band, coherent radiation from a high-energy electron bunch is particularly promising because a continuous spectrum can be obtained with high intensity. Electron bunches accelerated by a high frequency usually have a Gaussian distribution, but there are cases where a deviation from an ideal Gaussian distribution occurs due to the crafting of the acceleration gradient or the repulsion between electrons due to high density. It is known that coherent radiation characterized by the shape of this distribution is also generated as the electron bunches pass through a deflecting magnet or the like to generate radiation. Coherent radiation has a strong continuous spectrum above the wavelength of the bunch length, and its intensity is proportional to the square of the number of electrons in the bunch. Therefore, several studies using a linac with a short bunch length and a high peak density have already been conducted, and a high-intensity submillimeter wave-millimeter wave has been obtained.

Synchrotron radiation using a storage ring is attractive as a light source because of its high repetition frequency and stable intensity, but the intensity is considerably reduced in a long wavelength region such as millimeter waves. The same applies to storage ring free electron lasers. Although broadband research from the vacuum ultraviolet region to the infrared region has been promoted in recent years (see Patent Document 1 and Patent Document 2 below), It is difficult to oscillate. Recently, attempts have been made to generate coherent radiation by irradiating an electron bunch around the storage ring with a short pulse laser and deforming the bunch waveform. However, there are still specific research policies, research equipment, and research results. There are no reports that can be used to find out, and there have been no reports of results that could lead to practical use.
Japanese Patent Laid-Open No. 06-29630 JP 09-2104070 A

Although there is coherent radiation using a linac capable of obtaining a high intensity submillimeter wave-millimeter wave of the order of millijoules, it is difficult to obtain a large amount of charge in normal multi-bunch operation, and a single bunch to increase the intensity per pulse Driving is necessary. However, when single bunch operation is performed with a linac, it is difficult to align the energy and bunch waveform of each bunch accurately and uniformly, and as a result, the intensity of coherent radiation is not stable. A special device such as a superconducting linac can obtain a sufficient strength even in multi-bunch operation, but still has a strength fluctuation of about 2-3%.
On the other hand, there is also a proposal that a short pulse laser interacts with an electron bunch of an electron storage ring to create a dip (notch) in the bunch waveform and generate a high intensity submillimeter wave-millimeter wave by coherent radiation. In order to obtain sufficient intensity, a high-intensity and high-repetition laser is required, and the electronic bunch and the laser must be synchronized with high precision. At present, a high-intensity submillimeter-millimeter wave is supplied. It is difficult. However, since the electron bunch of the storage ring is used, the intensity is highly stable, and it can be used in combination with light of other wavelengths such as synchrotron radiation.
Under the circumstances as described above, a strong and stable light source using a storage ring in the submillimeter wave-millimeter wave wavelength band is desired.

  An object of the present invention is to provide a high-intensity and stable light source using a storage ring in a submillimeter wave-millimeter wave wavelength band.

Recently, it has been reported that the storage ring-type free electron laser oscillates and strongly deforms the shape of the circulating electron bunch. Even in the free electron laser device using the electron storage ring NIJI-IV, this time change of the bunch waveform was observed, and it was observed that a strong deformation occurred particularly when the Q switch was performed. It is considered that the deformation of the bunch shape is determined by the length of the bunch and the pulse length of the free electron laser, and the degree of deformation is determined by the strength of the free electron laser. Actually, even in the macro pulse mode in which the Q switch is not performed, the form factor becomes 2-3 times larger at the time of free electron laser oscillation at the wavelength of the pulse width of the free electron laser. Until now, no consideration has been given to the emission from the electron bunch due to the deformation of the bunch shape, but for the first time, our group points out that strong coherent radiation occurs in the millimeter wave region.
Furthermore, in order to actively use this bunch waveform, a long-period wiggler is inserted into the storage ring to generate a coherent light having a large form factor at the resonance wavelength of the spontaneous emission light, and a sub milliwatt. It can be used as a submillimeter wave-millimeter wave light source.

The specific solution is
(1) In a long wavelength coherent light generator in which a free electron laser device and a Q switch control device are provided in an electron storage ring,
The Q-switch control device drives the free electron laser device based on the Q-switch method to increase the output in the resonator in the electron storage ring, and the electron bunches and optical pulses are mutually reciprocated at a high repetition rate (for example, about 10 MHz). It is characterized in that coherent radiation due to bunch-shaped distortion is induced to generate coherent light having a long wavelength from submillimeter wave to millimeter wave.
(2) The long-wavelength coherent light generator according to (1) is characterized in that the pulse length of the free electron laser is changed to change the length subjected to deformation of the bunch shape.
(3) In the long wavelength coherent light generator according to (1) or (2) above, the maximum intensity of the free electron laser is changed by the Q switch control device, and the degree of deformation of the bunch shape is changed. And
(4) In the long wavelength coherent light generator according to any one of the above (1) to (3), a wiggler having a strong magnetic field is inserted and arranged in the electron storage ring to generate coherent radiation more efficiently. It is characterized by.

The long-wavelength coherent light generator according to the present invention is a stable pulsed light having a high output, for example, about a sub-milliwatt, which has not been available in the past, in the sub-millimeter-millimeter wave region where it has been difficult to obtain strong radiation in a synchrotron radiation facility. Can supply.
As a result, in addition to imaging and spectroscopic measurement using this light source, various and unique applications such as pump probe measurement combined with other light sources of a radiant light facility such as radiated light from a deflecting magnet become possible.

Next, examples of the present invention will be described, and the effects of the present invention will be described in more detail. However, these examples do not limit the present invention.
Example:
NIJI-IV of National Institute of Advanced Industrial Science and Technology is used as the electron storage ring, and the oscillation wavelength of the free electron laser is set to 300 nm. In order to use an actual measurement example, values before the vacuum chamber modification are used as the electron beam characteristics, and main parameters used in the following calculation are shown in Table 1.

(Table 1)
Electron energy 310MeV
Current value 7.1mA
Number of electrons in the bunch 4.4 × 10 ⁹
Energy spread 4.6 × 10 ⁻⁴
Damping time 40ms

The resonator length is 14.8 m, and it takes 99 ns for a free electron laser micropulse to make one round trip between the resonators. When the current value was 7.1 mA, the free electron laser gain was about 3.6%, and the resonator loss was about 2.0%. It is assumed that the free electron laser oscillates in a macro pulse manner by a Q switch based on RF frequency modulation. The repetition of the macro pulse is 25 Hz so that sufficient intensity of the free electron laser pulse can be obtained.

When the Q-switch operation is performed, the free electron laser micropulse is rapidly amplified after the suppression of the free electron laser gain is released, so that the intensity increases and the bunch length of the electron bunch increases. As a result, the free electron laser gain also decreases and the free electron laser intensity also decreases rapidly. The bunch length decreases relatively slowly due to the radiative decay of the storage ring, which is characterized by the damping time.
FIG. 1 is a diagram showing the change over time of the bunch length during Q-switch operation by RF frequency modulation.
FIG. 1 shows an approximate straight line for plotting characteristics for reference.
It was observed that the interaction with the electron bunch was strong because the free electron laser intensity was high during Q-switch operation, and the electron bunch waveform was deformed. The waveform is shown in FIG.
FIG. 2 is a diagram showing a bunch waveform when the bunch length becomes maximum during the Q switch operation.
FIG. 2 also shows an equivalent Gaussian distribution (dotted line in FIG. 2), but notices that it intersects with the bunch waveform (for example, point A) every about 25 mm. This length roughly corresponds to the length in which the free electron laser exists during Q-switching operation (full width at half maximum is 24 mm). Furthermore, it was observed that a fine modulation of about 5 mm (for example, region B) was induced. The resolution of the measurement system was approximately 1 mm. Even in a normal pulse mode that does not use the Q switch, such deformation of the bunch waveform occurs, but it has been observed that the degree of deformation is weak.

The deformation of the bunch waveform can be expected to generate coherent radiation with the deflecting magnet. Since the inside diameter of the vacuum chamber of the deflecting magnet is 36 * 160 mm, the cutoff frequency is about 4 GHz, and light with a wavelength of 9 mm or more cannot be emitted from the vacuum chamber and can be ignored. The shape factor calculated from the bunch waveform for the time when the bunch length is maximum during Q-switch operation (as the name suggests, the shape factor expresses the degree of coherent radiation caused by the bunch shape at a certain wavelength. FIG. 3 shows that the vertical distribution function of the inner electrons is given by Fourier transform and has a value from 0 to 1. FIG. 3 is a diagram showing the wavelength dependence of the shape factor calculated from the bunch waveform shown in FIG.
The deformation of the bunch waveform approaches the Gaussian distribution (dotted line in FIG. 2) as time passes, and it is about 1 ms that is strongly deformed as shown in FIG. For comparison, the form factor immediately before the Q switch is applied is shown in FIG. FIG. 4 is a diagram showing the wavelength dependence of the shape factor calculated from the bunch waveform immediately before applying the Q switch.

Comparing FIG. 3 and FIG. 4, the shape factor immediately before applying the Q switch is relatively large, being 0.01 or more in the region exceeding the wavelength 7 cm due to the short bunch length, and gradually decreasing in the region having the wavelength 1 cm or less. It can be seen that it is about 10 ⁻⁵ . On the other hand, it can be seen that the shape factor when the bunch length is maximum has peaks (points C and D) near wavelengths of 5 mm and 25 mm. These peaks are considered to be caused by the deformation of the electron bunch waveform by the above-described free electron laser. Due to the vacuum chamber cutoff, what is actually observable is coherent radiation with a wavelength of 5 mm. At this time, since the form factor exceeds 10 ⁻⁴ , an output larger than that of normal radiation can be expected. In the case of a current value of 7.1 mA, the total radiation of the coherent radiation having a wavelength of 5 mm over the entire circumference of the ring is 68 mW with a wavelength width of 1% (the shape factor is calculated by 1.4′10 ⁻⁴ ). When the aperture is set to 30 mm, an output of about 0.27 mW can be obtained.

Since the Q switch used in the experiment was RF frequency modulation, the synchronization between the electron bunch and the free electron laser pulse was slowly shifted, and the full width at half maximum of the free electron laser was as large as 24 mm. When a Q switch that adjusts the amount of resonance light is used, the full width at half maximum can be reduced to about 1/5, which is 4-5 mm (as a means for this, the inventors' Japanese Patent Application No. 2004-055106). Can be used). In this case, the deformation of the electron bunch waveform given by the free electron laser oscillation becomes a narrow pitch as well. Therefore, the shape factor obtained in the experiment is scaled by the compression ratio of the pulse width of the free electron laser, and it is assumed that the shape factor is 1.4′10 ⁻⁴ for a wavelength of 1 mm. In general, the intensity of radiated light increases as the wavelength becomes shorter in the longer wavelength region than infrared, and is proportional to the fourth power of the wavelength. The output when the aperture is 30 mm is approximately 2.3 mW. Further, since the bunch length of the electron bunch can be adjusted by the electron energy and the harmonic cavity, the coherent radiation can be adjusted in a wide region by adjusting the pulse width of the free electron laser.
In order to efficiently obtain the output of coherent radiation, a long wavelength coherent light is strengthened by a wiggler after the bunch waveform is deformed by a free electron laser. In NIJI-IV, since there is no space in the straight line portion where the short wavelength optical klystron is inserted, a wiggler is inserted in the opposite straight line portion as shown in FIG.

FIG. 5 is a conceptual diagram in which a FEL generator and a wiggler for long-wavelength coherent light amplification are simultaneously inserted into a storage ring.
In FIG. 5, the electron storage ring 1 includes an optical klystron (ETLOK-II) 2, an RF (high frequency) cavity 3, and a long wavelength wiggler 4.
The free electron laser device 5 is composed of the electron storage ring 1, the optical klystron 2, and the optical resonators 11a and 11b, and spontaneous emission light is generated when an electron bunch passes through the optical klystron, and light having a specific wavelength. Is amplified by an optical klystron while confining in the optical resonator, thereby functioning to generate laser light.
The Q switch control device 10 includes a resonator mirror 11a, a resonance light quantity adjustment type Q switch element 12, a resonator mirror 11b, a fast time response light detector 13, and a Q switch control device 14, and its reflection optical system is the optical klystron. It is provided so as to coincide with the optical path. The Q switch control device 10 takes in the output of the high-speed time response photodetector 13 and controls the resonance light quantity adjustment type Q switch element 12,

(1) The output in the resonator in the electron storage ring 1 is increased, and the electron bunch and the optical pulse are caused to interact at a high repetition rate (for example, about 10 MHz) to induce coherent radiation due to the bunch-shaped distortion. It generates coherent light with a wavelength as long as millimeter waves.
(2) The pulse length of the free electron laser is changed to change the length of deformation of the bunch shape.
(3) The maximum intensity of the free electron laser is changed to change the degree of deformation of the bunch shape.
The wiggler magnet arrangement is, for example, as shown in FIG.
In the long-wavelength coherent light generator in which the electron storage ring 1 is provided with the free electron laser device 5 and the Q switch control device 10, the free electron laser device 5 is driven by the Q switch control device 10 based on the Q switch method to resonate. The coherent radiation due to the distortion of the bunch shape is induced by interacting the electron bunch and the optical pulse at a high repetition rate per unit time with a significant increase in the internal power.

Here, as shown in FIG. 5, if there is a wiggler that matches the wavelength of the fundamental wave of the radiated light to the wavelength at which the form factor of coherent radiation is maximized, the wiggler interferes with radiation from periodic oscillations of electrons. Thus, it is possible to generate a stronger, long-wavelength coherent light of submillimeter wave to millimeter wave.
Furthermore, by changing the pulse length of the free electron laser by the Q switch control device 10, the length subjected to the deformation of the bunch shape is changed, and at the same time, the resonance wavelength of the wiggler is adjusted to control the wavelength of the coherent radiation. it can. Similarly, by changing the maximum intensity of the free electron laser by the Q switch control device 10, it is possible to adjust the degree of change in the bunch shape and control the intensity of the coherent radiation generated.
The wiggler specification is assumed as shown in Table 2 below.

(Table 2)
Period length 400mm
Number of cycles 6
Peak magnetic field 1.6T
K value 60

In this case, the coherent radiation intensity per solid angle of 10 m × rad ² and energy width of 1% can be expected to be 17 μW. Since coherent radiation is proportional to the square of the current, it can be expected to be a sub-milliwatt light source when the current value is 20 mA. Further, in the submillimeter wave region, the number of wiggler periods and the K value can be reduced, so that a larger output can be expected.

It is a figure showing the time change of the bunch length at the time of Q switch operation by RF frequency modulation. It is a figure showing a bunch waveform when bunch length becomes the maximum at the time of Q switch operation. It is the figure which showed the wavelength dependence of the form factor calculated from the bunch waveform shown in FIG. It is the figure which showed the wavelength dependence of the form factor calculated from the bunch waveform just before applying Q switch. It is the conceptual diagram which inserted the wiggler for a FEL generator and long wavelength coherent optical amplification into the storage ring simultaneously.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electron storage ring 2 Optical klystron 3 RF cavity 4 Long wavelength wiggler 5 Free electron laser apparatus 10 Q switch apparatus 11a, 11b Resonator mirror 12 Resonance light quantity adjustment Q switch element 13 High-speed time response photodetector 14 Q switch control apparatus

Claims (4)

In a long wavelength coherent light generator provided with a free electron laser device and a Q switch control device in an electron storage ring,
By the Q switch control device, a free electron laser device is driven based on the Q switch method, the output in the resonator in the electron storage ring is increased, and the bunch shape generated by repeating the interaction between the electron bunch and the optical pulse is generated. A long-wavelength coherent light generator that induces coherent radiation due to distortion and generates long-wavelength coherent light of about submillimeter waves to millimeter waves. The long-wavelength coherent light generator according to claim 1, wherein the Q-switch control device changes a pulse length of a free electron laser to change a length subjected to deformation of a bunch shape. 3. The long-wavelength coherent light generator according to claim 1, wherein the Q-switch control device changes the maximum intensity of the free electron laser to change the degree of deformation of the bunch shape. 4. The long wavelength coherent light generator according to claim 1, wherein a wiggler having a strong magnetic field is inserted and arranged in the electron storage ring to generate coherent radiation more efficiently.

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