JP4131642B2 - FEL wavelength changing method and apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength changing device and method in a free electron laser generator, and more particularly to a wavelength changing device that enables easy and accurate wavelength setting in an FEL device using a linac.
[0002]
[Prior art]
The free electron laser generator (FEL) is in principle used in many fields such as medicine, science, biotechnology, and semiconductor technology because the wavelength can be freely set and the output is high in principle.
Methods for determining the laser wavelength in the FEL include a method of adjusting the energy of an electron beam passing through the undulator and a method of adjusting the undulator gap.
Since the method of changing the energy of the electron beam is not simple, conventionally, the FEL wavelength has been changed by an undulator gap. At this time, if the gap of the undulator is changed, the magnetic field distribution in the undulator changes, so it is necessary to correct the magnetic field by providing correction magnets before and after the undulator and inside.
[0003]
When the undulator is installed in the synchrotron SR device, the reproducibility of the electron trajectory and the electron beam energy is excellent, and the position of the electron beam is processed using four or six button electrodes. By measuring, it is possible to easily measure with an accuracy within 0.05 mm. Therefore, the undulator gap required to obtain the FEL of the predetermined wavelength and the strength of each correction magnet corresponding to the undulator gap are measured in advance, and the gap corresponding to the wavelength is selected based on this information by a computer or the like. By adjusting the correction magnet strength, FEL light having an arbitrary wavelength can be obtained with high reproducibility.
[0004]
However, the electron beam generated by the linac has poor reproducibility, and there is usually a deviation of about several percent in energy every operation day, and the position of the beam may be shifted by 1 mm or more even in the vicinity of the electron gun of the electron generator. There is also a variation of about 1 mm in the beam position in the undulator.
Furthermore, it is not easy to generate FEL light by adjusting the position of the resonator mirror and the resonance condition of the electron beam.
Therefore, it has been difficult to change or adjust the wavelength while maintaining a high output, particularly in an FEL using a linac.
[0005]
[Problems to be solved by the invention]
Therefore, the problem to be solved by the present invention is to provide an FEL wavelength changing method and apparatus capable of easily selecting a wavelength in an FEL apparatus using a linac.
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the FEL wavelength changing method of the present invention first makes a reference undulator gap, aligns the electron beam positions at the entrance and exit, and adjusts the strength of the correction magnet to cause FEL oscillation. Thereafter, the FEL wavelength is adjusted by changing the undulator gap so as to correspond to the deviation from the target FEL wavelength. At this time, the strength of the correction magnet is set to a value obtained by adding a value adjusted for actual FEL oscillation to a reference adjustment value for each gap obtained in advance.
[0007]
Since the magnetic field distribution generated by the permanent magnet array of the undulator changes depending on the size of the gap, in order to cause FEL oscillation even when the undulator gap is changed, a correction magnet is attached to the undulator and the undulator gap is changed. However, it is necessary to adjust the strength of the correction magnet in accordance with the undulator gap so as to maintain the magnetic field distribution in the undulator in a predetermined state.
Since an undulator installed in a normal SR device has reproducibility in the state of the electron beam, one set of correction magnet strength adjustment values is set for each gap by appropriately changing the undulator gap using an electron beam having a predetermined energy. And the adjustment may be made corresponding to the desired wavelength.
[0008]
However, when the electron beam energy fluctuates as in the case of using a linac, the wavelength of the generated FEL is different even in the same gap, and in order to obtain a desired FEL wavelength, it is necessary to select a gap according to it. Since an appropriate correction magnet strength adjustment value also depends on the electron energy, it is necessary to provide an adjustment table based on a large amount of experimental data.
However, as a result of research by the present inventors, if the electron beam energy changes, the intensity of each correction magnet is set to an appropriate oscillation adjustment value corresponding to the energy, so that the FEL oscillation is performed unless an appropriate magnetic field distribution is formed in the undulator. However, if the FEL-oscillated magnetic field distribution in the undulator is assumed, if the incident position and the outgoing position of the electron beam do not change even if the undulator gap is changed after that, the reference energy electron beam is used. It was found that the FEL can be oscillated by maintaining the relative relationship in the intensity adjustment value of each correction magnet for each gap obtained, that is, the reference adjustment value.
[0009]
The FEL wavelength changing method of the present invention is based on this finding, and the conditions of the undulator magnetic field for FEL oscillation differ depending on the electron beam energy, but the correction magnet strength is adjusted using the reference undulator gap in the state of the electron beam. Once FEL oscillation is secured, the wavelength is changed while maintaining a high output so that the relative strength between the correction magnets is maintained in a fixed relationship even when the FEL wavelength is changed by changing the gap thereafter. It is what I did.
The table of reference adjustment values used for adjusting the strength of the correction magnet is based on the state of FEL oscillation using an electron beam having a predetermined energy under the reference undulator gap, and the FEL is changed by changing the gap. It is possible to prepare and use in advance the intensity of each correction magnet obtained by adjusting so as to oscillate as a difference from the reference intensity.
[0010]
Conventionally, since it is extremely difficult to adjust the FEL wavelength to a predetermined value when there is a change in the energy of the electron beam, the practicality of a linac-based FEL device, which should be smaller and simpler, has become a problem. However, by using the method of the present invention, it has become possible to easily generate an FEL having a target wavelength even when the electron beam energy varies from day to day.
The FEL wavelength changing method of the present invention measures the FEL wavelength when oscillating at a reference undulator gap, calculates the electron beam energy from the FEL wavelength, and uses the calculated electron beam energy to obtain a desired FEL. An undulator gap for obtaining a wavelength may be determined.
[0011]
In order to solve the above problems, the FEL wavelength changing device of the present invention corrects the magnetic field distribution of the undulator, the gap adjusting device for adjusting the undulator gap, the correction electromagnet for adjusting the trajectory of the electron beam incident on the undulator. A correction electromagnet, a correction electromagnet that adjusts the trajectory of the electron beam emitted from the undulator, a beam position monitor provided at the entrance and exit of the undulator, and an oscillation using the electron beam in the reference state for each undulator gap A storage device that records the deviation between a storage device that records the FEL wavelength, a reference gap state of the correction electromagnet intensity that can cause FEL oscillation for each undulator gap, and a correction magnet strength adjustment device; Undulator gap at the entrance and exit The electron beam position is adjusted to a predetermined position, the intensity of the correction electromagnet is adjusted to cause FEL oscillation, and then the undulator gap is changed so as to correspond to the deviation from the target FEL wavelength. The FEL wavelength is adjusted by setting to a value obtained by adding a value adjusted for actual FEL oscillation to the obtained reference adjustment value for each gap.
[0012]
The apparatus of the present invention can automatically change the FEL wavelength according to the FEL wavelength changing method of the present invention even when an electron beam from a linac that tends to fluctuate is used. If the correction electromagnets interfere with each other, it is not necessary to independently set the electron beam incident position, emission position, and electron beam trajectory in the undulator during FEL oscillation. Needless to say.
The storage device and the correction magnet strength adjustment device may be configured at least partially by a computer.
Moreover, the set values of the undulator gap and the correction electromagnet strength can be obtained by interpolation from the values described in each table. By using the interpolation method, the required number of experiments can be reduced and the required set value can be efficiently obtained from a small data table. Since these relationships are simple, it goes without saying that interpolation for estimation exceeding the upper and lower limits of the acquired data can also be used.
[0013]
When the correction electromagnet is divided into a part that is first adjusted to oscillate the FEL and a part that performs magnetic field correction to optimize the magnetic field distribution for each gap, different adjustments based on different factors are performed independently. This is convenient.
For this purpose, the correction electromagnets may be provided separately, or a single electromagnet may be provided with a separately controllable coil. Alternatively, the current component of the oscillation adjustment value and the reference adjustment value may be added and applied by the power supply device using a single coil.
In particular, it is preferable that the correction electromagnet for correcting the magnetic field distribution of the undulator can be set independently.
[0014]
Furthermore, if the beam position monitor is of a type that performs position determination by image processing using a sensor such as a button electrode or an OTR target that generates an electric signal corresponding to the irradiation position of the electron beam, the FEL wavelength is directly It can be used for an automatic adjustment device.
As for the position of the electron beam, for example, if four or six button electrodes are used for signal processing, measurement accuracy within 0.05 mm can be easily obtained.
Since the FEL wavelength is determined by the electron beam energy and the state of the undulator, the energy of the electron beam can be obtained from the wavelength of the FEL generated by making an unstable electron beam incident on the reference undulator. Accordingly, a spectroscope may be provided to observe the wavelength of the FEL oscillated at the reference gap to obtain electron beam energy, and based on this, an undulator gap for obtaining a desired FEL wavelength may be obtained.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to examples.
1 is a block diagram of an FEL apparatus including an FEL wavelength changing apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram showing a control system of the FEL wavelength changing apparatus of the present embodiment, and FIG. 3 is an adjustment of a correction electromagnet. FIG. 4 is an explanatory diagram for explaining a coil adjustment method for the correction electromagnet.
[0016]
The FEL apparatus shown in FIG. 1 is a linac type FEL apparatus, which generates an electron beam with an RF electron gun 1, enters the generated electron beam into an α electromagnet 2, selects a wavelength, and further accelerates with an acceleration tube 3. Then, it is guided to the undulator 6 by the deflecting electromagnets 4 and 5 and interacts with the laser light existing between the resonator mirrors 9 and 10 in the undulator 6 to enhance the laser light, and is deflected by the 90-degree deflecting electromagnet 7. To be absorbed by the beam dump 8. The FEL laser light generated in the undulator 6 is taken out from the resonator mirror 10 and used. The generated FEL laser light can be incident on the spectroscope 11 or the joule meter 12 to measure the characteristics.
[0017]
The FEL apparatus is provided with correction electromagnets (steering electromagnets) everywhere so that the properties of the electron beam can be adjusted and the position of the orbit can be finely adjusted.
The correction electromagnet according to the FEL wavelength changing device of the present embodiment particularly has a first electromagnet group used for guiding the electron beam emitted from the accelerating tube 3 to a predetermined position of the undulator 6 and the magnetic field distribution of the undulator 6. This is a second correction electromagnet group to be corrected.
The first correction electromagnet group includes a correction electromagnet 21 attached to the deflection electromagnet 4 provided at the end of the accelerating tube 3 and a correction electromagnet 22 provided in the electron beam trajectory to the entrance of the undulator 6 and a deflection electromagnet at the entrance of the undulator 6. 5 is included.
[0018]
Further, the second correction electromagnet group includes, for example, correction electromagnets provided at various positions where electron beams inside and outside the undulator 6 pass, as indicated by reference numerals 24, 25, and 26 in FIG.
In addition, since the electron beam trajectory in the undulator 6 changes when the magnet strength of the second correction electromagnet group is adjusted, the position of the electron beam at the exit of the undulator 6 can also be adjusted.
[0019]
As shown in FIG. 2, each of the correction electromagnets 31, 32, 33, and 34 is connected to a power supply device 35, 36, 37, and 38 that is provided corresponding to each of the correction electromagnets 31, 32, 33, and 34, and supplies current independently. A correction magnetic field can be generated.
The gap of the undulator 39 can be set by the gap control device 40.
The correction electromagnet power supply devices 35, 36, 37, and 38 and the gap control device 40 are cooperatively controlled by a controller 41 that bundles them together, and the controller 41 is further controlled by a computer 42 that has an arithmetic processing capability.
As the controller 41, a sequence controller capable of logical operation or a composite control device can be used.
The computer 41 is accompanied by a data file 43 that stores data relating to the correction magnet strength.
[0020]
The contents of the data file 43 are a table for correcting the beam position of the undulator as shown in FIG.
This table assumes that the undulator gap can be changed from 8 mm to 20 mm, and suppose that the magnetic field distribution in the undulator is finely adjusted by four correction electromagnets a, b, c, and d, and the gap reference is set to 12 mm. Is. Needless to say, these values vary depending on the apparatus.
This table is created by repeated field tests. In the practical test, first, the gap is set to a reference value of 12 mm, an electron beam having a predetermined energy is introduced into the undulator, and the intensity of the correction electromagnet is adjusted to cause FEL oscillation. The adjustment value in each correction electromagnet when the satisfactory FEL oscillation is successful is recorded as the oscillation adjustment value.
[0021]
Next, the gap is changed at an appropriate interval such as 1 mm, and adjustment values of the correction electromagnets a, b, c, and d are obtained so that satisfactory FEL oscillation can be performed again. When the gap changes, the shape of the field in which the magnetic field is generated changes and the magnetic field distribution in the undulator changes, so that the appropriate adjustment value also changes. Therefore, an appropriate magnet strength adjustment value obtained for each gap, that is, a reference adjustment value is recorded. As shown in FIG. 3, if the reference adjustment values are recorded by taking differences A, B, C, and D from the adjustment value at 12 mm as the reference gap, it is convenient for subsequent processing. In the table, the subscripts attached to the magnet adjustment values A, B, C, and D represent the gap amount.
[0022]
The value indicated by (+ X) in the table is an adjustment value and an oscillation adjustment value of each correction electromagnet when the electron beam of the day whose energy changes every day is incident on the undulator of the reference gap to cause FEL oscillation. is there. The subscript represents the corresponding correction electromagnet. That is, the adjustment values of the correction electromagnets a, b, c, and d when the FEL oscillation is performed with a gap of 12 mm are Xa, Xb, Xc, and Xd, respectively.
In order to cause FEL oscillation in each gap using the electron beam, the adjustment value of each correction electromagnet may be set to a value obtained by adding the reference adjustment value and the oscillation adjustment value as shown in the table. For example, the adjustment value in the correction magnet b when a 9 mm gap is selected is B9 + Xb.
[0023]
In order to obtain an FEL having a desired wavelength by an electron beam generated from the linac 1 on a certain day using the FEL generator of this embodiment, first, the gap of the undulator 6 is set to a reference of 12 mm, The beam is incident. The condition of the linac 1 changes every time it is adjusted, but if there is a deviation of several percent in the beam energy, the electron beam trajectory on the way may have a deviation of about 1 mm.
Therefore, while observing the beam position monitor inserted into the entrance portion of the undulator 6, it is on the electron beam locus to the correction electromagnet 21 at the position of the deflection electromagnet 4 at the end of the accelerating tube 3 and the deflection electromagnet 5 of the undulator portion. The correction electromagnet 22 and the like are adjusted so that the positional deviation of the electron beam is suppressed within about 0.3 mm at the entrance position.
[0024]
Further, while observing a beam position monitor installed where the electron beam exits the undulator, the exit is adjusted by adjusting the magnetic field strength of the correction electromagnets 23, 24, 25, 26, etc. provided in or around the undulator. As with the entrance, the position of the electron beam is adjusted to an accuracy of about 0.3 mm and FEL oscillation is performed.
Note that the position of the electron beam can be measured with an accuracy of 0.05 mm or less if it is obtained by signal processing using four or six button electrodes. In addition, a mirror-finished metal foil such as aluminum is inserted into the beam trajectory, the generated OTR light is captured by a camera, image processing is performed, and the center position of the beam is calculated to determine the accuracy within 0.1 mm. be able to.
[0025]
Since the positions of the electron beams at the entrance and the exit of the undulator 6 are reproduced, if the adjustment is made by trial and error with reference to the oscillation adjustment value oscillated using the electron beam having the reference energy in the reference gap, the comparison is made. The optimum value can be easily reached.
Of course, these adjustments affect each other and are not simple. However, if the adjustment cannot be made mechanically, it may be performed by a skilled person.
In this way, adjustment values Xa, Xb, Xc, Xd,... Of the correction electromagnet that enable FEL oscillation are recorded as oscillation adjustment values.
[0026]
Next, an undulator gap that generates the target FEL wavelength is selected. When an electron beam having a certain energy is used, the gap and the wavelength have a one-to-one correspondence. Therefore, the selection may be made based on a table prepared for the FEL wavelength for each gap measured in advance. If the target wavelength is not in the table, an appropriate gap can be obtained by interpolation by interpolation or extrapolation.
However, since the electron beam energy is not constant in the FEL apparatus using linac, the spectroscope 11 measures and uses the actual oscillation wavelength at the reference gap of 12 mm, and selects an appropriate gap.
Since the change in the gap and the change in the FEL wavelength correspond to some extent, an appropriate undulator gap can be estimated based on the above correspondence table using the difference between the oscillation wavelength at the target wavelength and the reference gap.
[0027]
Further, an appropriate correction electromagnet adjustment value in the new undulator gap is calculated using the oscillation adjustment value and the reference adjustment value, and the coil current of the correction electromagnet is adjusted based on this value to ensure FEL oscillation.
When the adjusted undulator gap is not in the table, an appropriate correction electromagnet adjustment value can be calculated by interpolation by interpolation or extrapolation.
The wavelength of the FEL light oscillated with the set undulator gap can be confirmed by measuring with the spectroscope 11.
[0028]
By the way, the reference adjustment value is a value inherent to the gap, whereas the oscillation adjustment value is a value determined every day based on the state of the electron beam, and the correction electromagnet is adjusted using a value obtained by adding both. It is convenient to be able to set each separately.
Therefore, as shown in FIG. 4 (a), for each of the coils 52 wound around each correction electromagnet 51, two power supply devices that can set the supply current independently are installed, respectively. One power supply 53 is set with a reference adjustment value corresponding to the gap, and the other power supply 54 is set with the oscillation adjustment value for the day, and the outputs of both power supplies are added to flow through the coil 52. Thus, the strength of the correction electromagnet 51 can be adjusted.
Further, as shown in FIG. 4B, each correction electromagnet 51 is provided with substantially two coils 55 and 56, and is connected to the power supply devices 53 and 54, respectively. The strength of the correction electromagnet 51 can be adjusted by passing a current corresponding to the value and a current corresponding to the oscillation adjustment value from the other power source 54. Furthermore, the correction electromagnets themselves may be separately installed to generate correction magnetic fields corresponding to the oscillation adjustment value and the reference adjustment value, respectively.
[0029]
Note that the oscillation wavelength λ _FEL of the _FEL is obtained by using the period length λ _W of the undulator magnet, the undulator parameter K, and the Lorentz factor γ,
λ _FEL = λ _W (K ² +1) / γ ²
It can be expressed as.
Therefore, by measuring the oscillated FEL wavelength λ _FEL , the Lorentz factor γ, which is an expression of electron beam energy, can be obtained. Using the Lorentz factor γ thus obtained, the undulator parameter K corresponding to the target FEL wavelength λ _FEL is obtained, the gap is calculated and set, and the appropriate correction electromagnet adjustment is performed to obtain the actually required FEL wavelength. Obtainable.
[0030]
In addition, in the FEL wavelength changing method of the present embodiment, it is not necessary to replace the data set every time the electron beam energy changes when obtaining the reference adjustment value, so that not only the storage capacity of a computer or the like can be saved, Since the number of cases in the field test for obtaining the adjustment value can be greatly reduced as compared with the conventional case, it is extremely advantageous in manufacturing an actual machine.
The above embodiment is applied to the case where an electron beam from a linac is used. However, even if an unstable electron beam supplied from another device is used, a stable electron such as a synchrotron is used. Needless to say, even when a beam generator is used, it can be effectively utilized.
[0031]
【The invention's effect】
As described above, by using the FEL wavelength changing method and apparatus of the present invention, even when the energy of the electron beam guided to the undulator is not constant, the FEL apparatus can be adjusted more easily to the target FEL wavelength. As a result, high-power FEL light having an arbitrary wavelength can be obtained and used in various fields.
[Brief description of the drawings]
FIG. 1 is a block diagram of an FEL apparatus including an FEL wavelength changing apparatus according to an embodiment of the present invention.
FIG. 2 is a block diagram illustrating a control system of the FEL wavelength changing device according to the present embodiment.
FIG. 3 is a table showing adjustment values of correction electromagnets in the present embodiment.
FIG. 4 is an explanatory diagram of a coil adjustment method of a correction electromagnet in the present embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 RF electron gun 2 (alpha) electromagnet 3 Acceleration tube 4, 5 Bending electromagnet 6 Undulator 9, 10 Resonator mirror 7 90 degree deflection electromagnet 8 Beam dump 11 Spectrometer 12 Joulemeter 21, 22, 23, 24, 25, 26 Correction electromagnet 31, 32, 33, 34 Correction electromagnet 35, 36, 37, 38 Power supply 39 Undulator 40 Gap control device 41 Controller 42 Computer 43 Data file 51 Correction electromagnet 52, 55, 56 Coil 53, 54 Power supply

Claims (6)

A gap adjusting device that adjusts the undulator gap, a first correction electromagnet group that adjusts the trajectory of the electron beam incident on the undulator, a second correction electromagnet group that corrects the magnetic field distribution of the undulator, and an inlet and an outlet of the undulator In an FEL wavelength changing device for a FEL device using a linac equipped with a provided beam position monitor, a reference undulator gap is first selected, and a first correction electromagnet group is adjusted so that electron beams at an entrance and an exit Adjust the position to a predetermined position, adjust the strength of the second correction magnet group to cause FEL oscillation, measure the FEL wavelength when oscillating at the reference undulator gap, and calculate the electron beam energy from the FEL wavelength Then, using the calculated electron beam energy, the angle to obtain the target FEL wavelength Decide regulator gap, along with changing the undulator gap so as to correspond to the deviation between the FEL wavelength of the target, a first correction magnet group strength of the second correction magnet group, undulator each gap obtained in advance The FEL wavelength is adjusted by setting the value obtained by adding the oscillation adjustment value adjusted for actual FEL oscillation in the reference undulator gap to the reference adjustment value representing the relative relationship in the intensity adjustment value of each correction magnet. FEL wavelength changing method characterized by the above-mentioned. A gap adjusting device that adjusts the undulator gap, a first correction electromagnet group that adjusts the trajectory of the electron beam incident on the undulator, a second correction electromagnet group that corrects the magnetic field distribution of the undulator, and an inlet and an outlet of the undulator A beam position monitor provided; a first storage device that records a table of FEL wavelengths when oscillation is performed using an electron beam in a reference state for each undulator gap; and FEL oscillation is performed for each undulator gap. And a second storage device that records a table of deviations of the corrected electromagnet strength from the reference gap state, and a correction magnet strength adjustment device. First, the gap adjustment device serves as a reference undulator gap, and the correction magnet strength adjustment is performed. A device based on the output of the beam position monitor; Adjust the magnet strength of the correction electromagnet group and the second correction electromagnet group to adjust the electron beam positions at the entrance and exit of the undulator to a predetermined position, and further adjust the magnet strength of the second correction electromagnet group to cause FEL oscillation, Thereafter, based on the FEL wavelength table recorded in the first storage device, the undulator gap is changed so as to correspond to the deviation from the target FEL wavelength, and based on the deviation table recorded in the second storage device The FEL wavelength is adjusted by setting the intensity of the second correction electromagnet to a value obtained by adding the value adjusted for actual FEL oscillation to the reference adjustment value for each gap obtained in advance. FEL wavelength changing device for FEL device using linac . 3. The FEL wavelength changing device according to claim 2, wherein the second correction electromagnet includes an electromagnet adjusted for FEL oscillation and an electromagnet used for correcting a magnetic field deviation based on a difference between undulator gaps. 3. The FEL wavelength changing device according to claim 2, wherein the second correction electromagnet includes a coil used for correcting a magnetic field deviation based on a difference between a coil adjusted for FEL oscillation and an undulator gap. 4. 5. The beam position monitor performs position determination by image processing using a sensor that generates an electric signal corresponding to an irradiation position of an electron beam such as a button electrode or an OTR target . FEL wavelength change device according to one. Furthermore, a spectroscope is provided, the wavelength of the FEL oscillated in the reference undulator gap is measured, the energy of the electron beam is calculated using the measurement result, and the undulator gap when changing to the desired wavelength is calculated. The FEL wavelength changing device according to any one of claims 2 to 5, wherein the FEL wavelength changing device is characterized in that:

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