Design and optimization of electron accelerator system for generation of Mid-infrared Free-electron Laser at Chiang Mai University

Abstract

This thesis reports the electron beam dynamie simulation for generation of the mid-infrared free electron laser (MIR-FEL) at the PBP-CMU Electron Linac Laboratory, Chiang Mai University. The study was performed by using a software ASTRA including the space charge effects to obtain the electron beam with the shortest bunch length and the highest peak current at the undulator magnet entrance.The electrons are emitted from the thermionic cathode and accelerated with 2856-MHz RF-wave in the RF electron gun. Then, some part of electron beam was filtered and compressed in the alpha magnet. In this section, the alpha magnet's gradient and bunch charge of the electron beam were varied. The beam with selected bunch charge of 60 pC and energy range of 2.40 - 2.53 MeV was then further accelerated in the linac section. The linac accelerating gradient of 10.13 MV /m was used to accelerate electron beam to reach the maximum kinetic energy of 25 MeV at the linac exit. After that, the electrons travel through the 180* bunch compressor, which has four dipole magnets and eight quadrupole magnets. Magnetic gradients of all quadrupole magnets were varied to get the shortest bunch length at the undulator entrance. The optimization of 25-MeV electron beam shows that with the alpha magnet's gradient of 410 G/cm and proper quadrupole magnetic gradients provided the shortest bunch length of 0.21 ps and the highest peak current of 123.5 A at the undulator entrance for the bunch charge of 60 pC. The FEL simulation using these parameters with the modified GENESIS 1.3 code suggests that the MIR-FEL pulse energy of 0.42 p.J can be achieved. When ineluding the beam loading effect in the linac acceleration, it was found that for the klystron with the maximum output RF power of 6.47 MW the electron beam with the bunch charge of 60 pC can be accelerated to reach the energy of only 22 MeV. Then, the electron beam optimization for this energy was also conducted. The alpha magnet gradients of 400, 430 and 460 G/em yield similar results in term of electron bunch length at the undulator entrance. However, the case of 400 G/cm provides lower normalized emittance at the undulator entrance than other cases resulting in faster rising of the FEL power gain. This case provides the FEL pulse energy of 0.22 p.J. The optimization results of this study are used to design the position and parameters of all components in the bunch compressor and the MIR-FEL beam transport line. Currently, the installation of all components based on the suggestion from this thesis in under going.