JP2552423B2 - Free electron laser oscillation method and device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a free electron laser oscillation method and apparatus for efficiently generating coherent radiation of short wavelength.

[0002]

2. Description of the Related Art Free electron laser (hereinafter referred to as FEL)
An oscillating device accelerates an electron beam obtained by an electron gun with an accelerator and introduces it into the magnetic field of a periodic electromagnetic field forming means generally called a wiggler or undulator, and changes the spatially periodically changing magnetic field to the speed of light. The electron beam is accelerated and decelerated while meandering at a close relative logical velocity, and electromagnetic waves with a uniform phase are induced and radiated from the electron beam by interaction with the electromagnetic field, and these electromagnetic waves are many in the optical resonator consisting of a pair of mirrors. The laser is oscillated by repeating the round trip.

In the above-mentioned conventional general FEL oscillator, the electron beam sent in a pulse shape is a ponder due to the beat wave of the wiggler magnetic field and the electromagnetic wave generated by the interaction with the electromagnetic field and superposed on the electromagnetic wave. The electron beam is bunched by the rotomotive force, and coherent light is oscillated by the bunched electron beam.

In this case, the electron beam has a pulse width sufficiently longer than the wavelength of the electromagnetic wave, and the length of the clustered electron beam is in the order of 1/4, which is close to the wavelength of the electromagnetic wave. .

[0005]

By the way, the FEL oscillating device has a great feature that the wavelength of the laser light generated is different from other lasers and is variable.
Therefore, it is expected to be applied to various fields.
As one attempt, various studies have been conducted on highly efficient wavelength shortening.

As a method of shortening the wavelength, since the wavelength of the electromagnetic wave generated is inversely proportional to the square of the energy intensity of the electron beam, the energy intensity of the electron beam may be increased. Has certain limits.

Another attempt is to use a pre-clustered electron beam. In a normal FEL oscillator,
The electron beam is clustered in the optical resonator, and the clustered electron beam is used to obtain laser light. Therefore,
It is presumed that coherent light can be easily obtained by using an electron beam clustered in advance. This is commonly referred to as coherent radiation.

However, it is not possible to cause laser oscillation by using an electron beam that is simply clustered in advance in a state that is not ordered. This is because the radiated light emitted by the interaction between the clustered electron beam and the wiggler magnetic field causes only one wavelength interaction in the optical resonator due to the difference in speed.

In this case, since it is premised that short-wavelength radiation is obtained, the wavelength is much smaller than the meandering pitch of the Wiggler magnetic field, and in order to be coherent radiation, the electron beam is clustered. The characteristic length must be close to the wavelength of the emitted light.

In order to obtain a clustered electron beam having a characteristic length similar to the wavelength of the emitted light, the pulse width of the introduced electron beam must be much smaller than that of the conventional one. For example, if the wavelength λs of emitted light is 100 μm,
Characteristic length lb = 300 μm (electron beam pulse width 1 ps
ec), the target state is approached and the pulse width is 10
At fsec (= 10 ⁻¹⁴ sec), λs → μm.

However, even if an electron beam having a characteristic length similar to the wavelength of the emitted light is used to obtain the emitted light, the traveling speeds of the emitted light and the electron beam are naturally different from each other.
Once the bunches of two electron beams interact with each other at a certain pitch of the wiggler magnetic field, at the next pitch, the synchrotron radiation advances one wavelength of light ahead of the bunches of the electron beams, and the pitch of the wiggler magnetic field changes. As it progresses, the gap grows larger and the interaction cannot be regenerated.

Therefore, it is not possible to oscillate the FEL simply by using the electron beam pre-bunched.

This invention pays attention to the problem of the method of obtaining a laser beam having a short wavelength by the above-mentioned conventional free electron laser oscillator, and produces an electron beam of a short pulse of the same level as that of a coherent radiation light of a short wavelength. It is an object of the present invention to provide a free electron laser oscillating method and apparatus for obtaining a coherent radiant light of a short wavelength with high efficiency in a plurality of interaction regions by pre-bunching the bunches with each other and interacting with them.

[0014]

As means for solving the above-mentioned problems, the present invention clusters an electron beam in advance so that its pulse train interval is at the same level as the period of the wavelength of light to be generated, and Introduced into a periodically changing electromagnetic field, adjusting the speed difference between the electron beam and light in the electromagnetic field to obtain coherent synchrotron radiation in multiple interaction areas, and generating radiant light many times in the optical resonator. The method was a free electron laser oscillation method that consisted of reciprocating oscillation of free electron laser light.

As an apparatus for carrying out the method of the above invention, an electron beam bunching means for prebunching the electron beam so that the pulse train interval is at the same level as the period of the wavelength of the light to be generated, and a period Equipped with an electromagnetic field forming means for forming an electromagnetic field that changes with time and an optical resonator for amplifying the generated electromagnetic wave.The electromagnetic field forming means uses an electron beam and an electromagnetic field with a periodic pitch of the electromagnetic field that changes periodically as a predetermined interval. The configuration of the free electron laser oscillator is adopted in which the electromagnetic field is arranged so as to obtain the coherent radiation in a plurality of interaction regions by adjusting the speed difference with the coherent radiation generated inside.

In the above laser oscillating device, the electromagnetic field forming means arranges a periodically changing electromagnetic field strength larger than the others at every pitch, and the electromagnetic field strength is a difference in velocity between the electron beam and the emitted light. It is preferably sized to adjust the velocity of the electron beam to a velocity equal to.

In that case, further, the electromagnetic field forming means includes:
The velocity difference adjusting means may be provided between the electromagnetic field strengths which are different for each pitch, and the velocity difference adjusting means may be composed of an optical path adjuster and an electron beam orbit adjuster.

[0018]

The above method of the present invention is carried out in the apparatus of the second to fourth inventions. In that case, in the second invention, as a method of adjusting the speed difference, a constant relationship is established among the pulse interval of the electron beam, the wavelength of the emitted light, and the pitch length of the electromagnetic field forming means, and the predetermined pitch of the electromagnetic field forming means is established. The electromagnetic field forming means is arranged so that the wavelength shift due to the speed difference between the electron beam and the radiated light coincides with each other at a plurality of positions to generate an interaction region.

As a method of bunching an electron beam in advance, for example, an electron gun of a type in which a cathode is irradiated with a laser, R
An F electron gun or a buncher RF cavity can be used. In the first and second methods, the electron beam is generated in an extremely short pulse, so that the electron beam is generated in a clustered state from the beginning. In the third method, the generated electron beam is clustered in the buncher RF cavity.

In the third aspect of the invention, the electromagnetic field strength is set to be strong for each pitch, and the speed of the electron beam is delayed so that the delay is just equal to the speed difference.

In the fourth invention, the adjustment is made by delaying the progress of the emitted light by the optical path adjuster of the speed difference adjusting means.

In any of the methods, by adjusting the speed difference, a plurality of interaction regions are generated,
As a result, short-wavelength coherent radiation can be obtained with high efficiency.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a first embodiment of an apparatus for carrying out the method of the present invention. The illustrated free electron laser (hereinafter referred to as FEL) oscillation device includes an electron gun 10, a wiggler magnetic field forming means 20, and an optical resonator 30.

Unlike a normal electron gun, the electron gun 10 irradiates the heating cathode cathode 11 with the laser beam of the laser 12 to generate an electron beam, and has an extremely high electron beam generation efficiency and a laser. This is a type in which the irradiation of 12 is intermittently performed at an ultrahigh speed to obtain an electron beam in a pulse train form. The pulse train cycle of the electron beam is, for example, 1 p
sec (= 10 ^-12 sec) or 10 fsec (= 10
^-14 sec) is obtained. 13 is an anode,
14 is a cavity.

The electron gun 10 is a means for supplying an electron beam and also serves as a means for prebunching the electron beam. Very short pulse with laser 12, eg 1
When a laser beam of 0 fsec is irradiated, a short pulse electron beam is emitted from the cathode 11 correspondingly,
The electron beams generated by this are sent out in a clustered state, and these electron beams are accelerated by the anode 13 and the cavity 14 and accelerated to a relativistic velocity close to the speed of light.

The wiggler magnetic field forming means 20 is composed of a plurality of sets of electromagnets which are adjacent to each other and whose directions of magnetic fields are different from each other by 180 ° in a periodic manner, as in a normal wiggler electromagnet. However, unlike the usual case, the periodic pitch must be a specific arrangement that satisfies a predetermined relational expression, as will be described later.

The predetermined relational expression is as follows. If the pitch length λω of the wiggler magnet, the wavelength λs of the emitted light, the pulse train interval λ _{B of} the electron beam, the acceleration energy γ of the electron beam, and the resonance characteristic parameters K and M with the wiggler magnetic field are arbitrary integers, then λs = λω (1 + K ² ) / 2γ ² λ _B = Mλs.

The optical resonator 30 comprises a total reflection mirror 31 and a semi-transparent mirror 32 as in the usual case, and a beam catcher 33 for processing a used electron beam is provided outside the system.

In the FEL oscillator having the above structure, laser oscillation is performed as follows. This will be described below with reference to FIGS. 2 and 3. First, as shown in FIG. 3, the electron beam sent into the electromagnetic field of the wiggler magnetic field forming means is supplied in advance as a grouped beam having a predetermined short pulse interval by the electron gun 10 or the like.

If the pulse width of this short pulsed electron beam is, for example, 1 psec as described above,
Characteristic length of clustered beam

[0031]

## EQU1 ## 1b 300 μm

The wavelength of the coherent radiant light generated by the interaction between the clustered beam and the electromagnetic field is λs.
= 100 μm.

Now, assuming that the electron beam advances on the electron orbit of (b) at the leftmost position of the Wiggler magnet shown in (a) of FIG. 3, a group of one electron beam becomes ( As shown in c), a plurality of columns (indicated as 1 to 5 in the figure) are clustered in a certain characteristic length lb, which emits radiated light by the interaction with the electromagnetic field by the Wiggler magnet.

The one electron beam thus clustered advances in the same direction as the emitted light, but the electron beam is delayed by one wavelength and one wavelength at the pitch position of the next wiggler magnet because the speed of light is faster. It will shift. Two wavelengths are offset at the third pitch. Therefore, this wavelength shift increases as the wiggler pitch increases.

If there is no regular relationship between the pulse train period of the electron beam and the wavelength of the emitted light with respect to the large number of pitches of the above-mentioned wiggler magnet, the electron beam interacts once. Since it is out of phase with the emitted light, the interaction with the wiggler magnet does not occur again.

However, in this embodiment, the pitch length λω of the wiggler magnet, the wavelength λs of the emitted light, and the pulse train interval λ _B of the electron beam have a relationship satisfying the above-mentioned relational expression. Therefore, as shown in (c) of FIG. 2, if interaction occurs at the first pitch (numbers shown above the magnets) of the wiggler magnet, it gradually shifts to λs at the second pitch and 2λs at the third pitch. Although increases in the first M + 1 pitch match the pulse train of the wavelength and the electron beam again synchrotron radiation becomes Mλs = λ _B, so that the interaction occurs.

For example, assuming that the wiggler magnet of the wiggler magnetic field forming means 20 is 100 and M = 10, nine interaction regions are generated, and coherent radiation light is generated much more efficiently than when there is only one interaction region. Will occur.

In the above-mentioned embodiment, the electron gun 1 using the laser irradiation type cathode as a means for clustering the electron beam in advance.
Although 0 has been described as an example, a method using an RF electron gun or a buncher RF cavity can be used instead.

When an RF electron gun is used, for example, when a large-current electron beam is generated from the cathode, the current waveform of the electron beam has an extremely short wavelength (for example, 100 f) due to vibration due to self-charge.
s to 10 fs), and the electron beam is clustered by this extremely short wavelength.

In the case of the buncher RF cavity, velocity modulation is applied to the electron beam generated by the cw electron gun in the buncher RF cavity, and the electron beam is clustered by energy dispersion.

FIG. 4 is a schematic configuration diagram of the second embodiment.
In the first embodiment, the wavelength shift due to the speed difference between the electron beam and the synchrotron radiation was made to generate a plurality of interaction regions by making the arrangement of the Wiggler magnets satisfy a specific relationship. Unlike the first embodiment, the example is an example in which a wavelength shift due to a speed difference is not generated.

The wiggler magnetic field forming means 20 are arranged at the same pitch as in the first embodiment, but a wiggler magnet (reference numeral 1X) which can give a stronger magnetic field between the first pitch and the second pitch of the wiggler magnet. , 2X ...) are arranged. Therefore, the trajectory of the electron beam in this case is as shown in FIG.

The strength of the strong magnetic field depends on the first pitch and the second pitch.
The speed difference between the electron beam and the emitted light between the pitches is set to be exactly the wavelength λs of the emitted light. That is, the electron beam is delayed by the wavelength λs at the position of the second pitch, and the emitted light catches up with the bunch row in front of the electron beam row, and spatially coincides temporally with each other to cause interaction.

This is because, when the magnetic field strength of the wiggler magnet is increased, the electron beam is largely swung at right angles to the traveling direction as shown in the figure, and the velocity in the traveling direction is decelerated accordingly.

FIG. 5 shows a schematic block diagram of the third embodiment. This embodiment is also an example of a method for adjusting the speed difference as in the second embodiment.

In this embodiment, the first group of the wiggler magnetic field forming means 20 in which every other strong wiggler magnet as in the second embodiment shown in FIG. 4 is provided, and the second group of the wiggler magnetic field forming means having exactly the same configuration as this. A speed difference adjusting means including an optical path adjuster 40 and a beam path adjuster 50 is provided between the means 20 '.

41-46 are all mirrors, 41, 4
6 is a fixed type, and the others are movable. Reference numerals 51 to 54 are bending electromagnets for changing the trajectory of the electron beam.

In this embodiment, contrary to the case of the second embodiment, the optical path adjuster 40 of the speed difference adjusting means delays the progress of light with respect to the electron beam to adjust the speed difference.

In the optical path difference adjuster 40, a large number of mirrors 41 to 41 are arranged.
46, eg mirrors 42, 43 and 44, 45
Is moved to make the optical path longer, the light travels later than the electron beam in the second group of Wiggler magnetic field forming means 20 ', so that the delay is set so as to match the traveling speed difference between the electron beam and the emitted light. .

The beam orbit path adjuster 50 is provided because it is necessary to change the orbit because it cannot move straight on the optical path by the mirrors 41 and 46 on the way.

The other constituent elements are the same as those in the first embodiment, and the same constituent elements are designated by the same reference numerals and the description thereof is omitted.

In the above-mentioned embodiment, the pitch of the magnetic field forming means, the magnitude of the magnetic field strength, and the optical path length are mainly adjusted as a method of adjusting the speed difference. However, in addition to this, resonance with the wiggler magnetic field is performed. It is also possible to adopt a method of adjusting the characteristic parameter K by providing a difference.

[0053]

As described in detail above, in the laser oscillation method according to the present invention, an electron beam having an extremely short pulse interval is used to pre-bundle the electron beam, and the coherent radiant light generated by the interaction between the electron beam and the electromagnetic field is converted into an electron beam. Since the speed difference between the beam and the light is adjusted so as to be obtained in a plurality of interaction regions, there is a great advantage that coherent radiation light having an extremely short wavelength can be efficiently obtained.

In the apparatus of the second invention for carrying out the above method, the speed difference is adjusted by the arrangement of the electromagnetic field forming means. In the third invention, the electromagnetic field forming means is arranged so that the intensity of the electromagnetic field is the above speed. The speed of the electron beam is adjusted so as to be equal to the difference, and in the fourth invention, the speed of the emitted light is adjusted by the optical path adjuster of the speed difference adjusting means provided between the electromagnetic field forming means of the third invention. Therefore, in any of the inventions, there is an effect that the speed difference is adjusted and the short wavelength coherent radiation light is efficiently obtained in the plurality of interaction regions.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a free electron laser oscillator according to a first embodiment.

FIG. 2 is a diagram for explaining the operation

FIG. 3 is a diagram for explaining the operation.

FIG. 4 is a schematic configuration diagram of a free electron laser oscillator according to a second embodiment.

FIG. 5 is a schematic configuration diagram of a free electron laser oscillator according to a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Electron gun 11 Cathode 12 Laser 13 Anode 14 Cavity 20 Wiggler magnetic field forming means 30 Optical resonator 31 Total reflection mirror 32 Semi-transparent mirror 33 Beam catcher 40 Optical path adjuster 50 Beam orbit adjuster

Claims (4)

(57) [Claims] 1. An electron beam is pre-grouped so that its pulse train interval has the same level as the period of the wavelength of light to be generated, and this is introduced into a periodically changing electromagnetic field, Free electrons consisting of oscillating free electron laser light by adjusting the speed difference between the electron beam and light to obtain coherent radiation in multiple interaction areas and reciprocating the generated radiation many times in the optical resonator Laser oscillation method. 2. An electron beam bunching means for prebunching an electron beam so that its pulse train interval is at the same level as the period of the wavelength of light to be generated, and an electromagnetic field forming a periodically changing electromagnetic field. The electromagnetic field forming means includes a forming means and an optical resonator for amplifying the generated electromagnetic wave, and the electromagnetic field forming means forms an electron beam and coherent radiation light generated in the electromagnetic field with a periodic pitch of the electromagnetic field that periodically changes as a predetermined interval. A free-electron laser oscillator in which the electromagnetic fields are arranged to adjust the speed difference and obtain coherent radiation in a plurality of interaction regions. 3. The electromagnetic field forming means arranges the intensity of the electromagnetic field that periodically changes is larger than the others at every pitch, and the intensity of the electromagnetic field is equal to the velocity difference between the electron beam and the emitted light. The free electron laser oscillator according to claim 2, wherein the free electron laser oscillator has a size adjusted to adjust a speed of an electron beam. 4. The speed difference adjusting means is provided between the electromagnetic field forming means in which the strength of the electromagnetic field is different for each pitch, and the speed difference adjusting means comprises an optical path adjuster and an electron beam orbit adjuster. The free electron laser oscillation device according to claim 3, wherein