Development of hybrid undulator for free electron laser (FEL)

The undulator is the area where the relativistic electron beam interacts with the light field and plays a vital role in FEL research. When the electron beam quality (energy dissipation, emissivity, current intensity, etc.) is constant, the performance parameters of the undulator, such as peak field strength and good field size, will directly determine the FEL gain. According to the geometric structure, undulators can be divided into plane undulators, tapered undulators, spiral undulators, etc .; according to the materials used, they can be divided into permanent magnetic undulators, hybrid undulators, and electromagnetic undulators. The FEL of the Chinese Academy of Sciences is currently conducting 100m far-infrared FEL. The magnetic pole material is FeCoV, the saturation magnetic flux density is 2.4T; the permanent magnet block material is NdFeB, and the residual magnetism is a hybrid type undulator magnetic circuit structure. 2 The parameter index to measure the performance of the undulator 2.2.1 Peak field strength The peak field strength Chu refers to the maximum value that the magnetic field of the undulator can reach under a certain gap condition. This parameter mainly reflects the capability of the undulator in terms of magnetic performance, and it has the following relationship with the undulator parameters in particular: 々 is the period length of the undulator, 々 = 3.2.heart of the FEL hybrid undulator in this project. In addition, the peak field flux density B also has the following relationship through the undulator parameter K and the radiation wavelength 2, electron beam energy yw0c2, and undulator period length: 2.2.4 The ratio of the single-electron small signal gain to the ideal gain is an important factor of FEL The performance index is the small-signal gain, which represents the degree of energy growth after the interaction of the light field and the electron beam under the small-signal state. The small-signal gain of the planar undulator can be estimated by the following formula: For a planar undulator like the FEL hybrid undulator of the Chinese Academy of Sciences, it is necessary to replace K2 with K2 / 2. For hybrid undulators, the undulator period is N = 38, K = 0.967, then AK / K is required to be less than 1.36%. 2.2.3 Single electron trajectory simulation center offset The method of simulating the single electron trajectory of the center beam line (the symmetric center line of the undulator) is often used for judgment. Due to the effective transduction of the electron beam and the light field, the lateral movement of the electron beam must always be within the envelope of the beam, that is, the movement trajectory of the electron beam under the action of the undulator magnetic field must be within the envelope of the beam. Assuming that the electron beam line and the center line of the undulator strictly coincide, the maximum allowable deviation of the center of the trajectory of the electron beam in the action section of the undulator should be the minimum value of the difference between the spot radius at each point on the beam line and the radius of the cross section of the electron beam. For the ongoing 100m far-infrared free electron excitation, there are 10 Bz values ​​in total.

3.2 Optimized combination and sorting As shown, since the performance difference between different magnetic poles (soft magnetic) in the magnetic circuit is not large, it only serves to derive the magnetic flux. The magnetic field on the magnetic pole is mainly determined by the two permanent magnets (magnetic block) Because of the superposition of magnetic flux, the effect of the magnetic pole is ignored when optimizing the combined data processing, and the situation of 6 points on the magnetic pole when the magnetic block is directly closely connected is considered; the method of selecting the point is the same as the magnetic block test. In order to facilitate the distinction, we use the superimposed magnetic field that represents the center point of the Wth magnetic pole; the superimposed magnetic field that represents the Wth magnetic pole 15mm away from the center point in the I direction along the I direction respectively discharges two sequences, called sequence 1 and sequence 2, Corresponding to the upper and lower columns of the undulator, see the optimized combination results. The M deviation relative to the mean reflects the peak-to-peak error, and the private deviation relative to the 10,000 deviation reflects the symmetry. It can be seen that the peak-to-peak error of the optimized combination is less than 1%; the deviation of 15mm symmetrical points at the left and right of the center point is less than 5%. 4 debugging results FEL hybrid wave oscillator has now been developed and is the real wave oscillator The picture shows the debugging results under the condition that the undulator gap is 18mm. It is the single electron trajectory simulation of the central beamline magnetic field; the distribution diagram of the central beamline peak magnetic field; the distribution of the magnetic field uniformity (good field area) in the I direction; the relationship between Xi and You, especially for the FEL hybrid undulator = 0.934 x0.3235x3.2 = 0.9667, especially AX7, the result is 1.2%. It can be seen that the single-electron small-signal gain and optimized combination result is a single-electron trajectory simulation on the symmetric center line of the magnetic materials and device undulator. shift

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