Computer simulated optimization of THz Free-electron laser light sources using Thermionic-cathode and Pho- tocathode RF Electron Guns

Abstract

A superradiant terahertz free-electron laser (THz-FEL) source has been developed for the first time in Thailand and the South-East Asia at the PBP-CMU Electron Linac Laboratory (PCELL) of Chiang Mai University. The radiation source requires relativis- tic ultra-short electron bunches to produce intense coherent THz pulses. The PCELL accelerator system has three electron bunch compression processes using an alpha mag- net, a velocity bunching process in radio-frequency linear accelerator (RF linac), and a 180-degree magnetic bunch compressor. The compression of electron bunch in the mag- netic compressor system is considerably challenging using four dipole magnets and three quadrupole doublets. The quadrupole field strengths affect significantly on the rotation of longitudinal phase-space distribution of electron beam along the bunch compressor. Start-to-end beam dynamic simulations using ASTRA code were performed to optimize electron beam properties for generating the superradiant THz-FEL. Operating parameters considered in the simulations are field gradient of the alpha magnet, linac RF phase, and quadrupole feld strengths. Optimization results reveal that 10 - 16 MeV femtosecond electron bunches with low energy spread (~0.2%), small emittance (0.3 - 0.4 mm.mrad) and high peak current (165 - 247 A) can be produced from the PCELL accelerator system operating at optimum parameters. The superradiant THz-FEL with sub-uJ pulse energy can be obtained from these optimized electron beam properties. Physical and conceptual design of the THz-FEL beamline has been done based on the results of beam dynamic simulations from this research. Another part of this thesis is the study of superradiant THz-FEL at the Photo Injector Test Facility at DESY, Zeuthen site (PITZ). The PITZ facility can produce high brightness electron beams with high charge and small emittance. Currently, a study on development of a tunable high-power THz SASE FEL for supporting THz-pump, X-ray- probe experiments at the European XFEL is underway. The linac coherent light source I (LCLS-I) undulator, a magnetic chicane bunch compressor, and THz pulse diagnostics have been installed downstream the previously existing setup of the PITZ beamline. Additional to the SASE FEL, a possibility to generate superradiant THz undulator radiation from short electron bunches is under investigation, which is the focus in this part of thesis. Numerical simulations of the superradiant THz radiation by using sub-picosecond electron bunches with energy of 17 MeV and bunch charge of up to 2 nC produced from the PITZ accelerator are performed. The results show that the radiation with a spectral frequency of 3 THz and a pulse energy in the order of sub-pJ can be accomplished. The results from this study can be applied for the future development of the system. Finally, electron beam properties and superradiant FEL characteristics obtaining from the PCELL laboratory and the PITZ facility were compared and discussed in this thesis.