characterization of electron source and rf power system for mid-infrared free electron laser at Chiang Mai University

Abstract

At the PBP-CMU Electron Linac Laboratory (PCELL), significant progress is being made in the development and commissioning of an electron accelerator and mid-infrared (MIR) and terahertz (THz) free-electron laser (FEL) systems. The electron accelerator system includes a thermionic radio-frequency (RF) gun as an electron source, an alpha magnet as a pre-bunch compressor and energy filter, and an RF linear accelerator (linac) structure for further energy acceleration. Downstream the accelerator system, 180-degree magnetic bunch compressor systems and two beamlines are installed specifically designed for the generation of MIR and THz FEL. The aim of this thesis is to characterization of electron source and RF power system for production of MIR FEL. The scope of this research is categorized into two parts. The first part is characterization of RF power systems for both the RF electron gun and the RF linac. The other part is generation and characterization of electron beam produced from the RF gun. For the first part, the RF equipment used in RF power measurement systems, which are crystal detectors, directional couplers, coaxial cables, and attenuators, were calibrated and characterized their functions before performing RF power measurements. The results of the RF power measurements revealed that the input RF powers were 2.2 and 4.2 MW for the gun and the linac, respectively. With these RF powers, the accelerator system is capable of generating the electron beam with a maximum energy 2.6 MeV after leaving the gun and 15.0 MeV after leaving the linac. This leads to the final energy of 17.6 MeV. This energy is still too low for the desired energy of 22-25 MeV needed for the MIR-FEL production. It is important to note that the RF output power of the linac’s klystron have not been confirmed yet due to the failure of the modulator transformer during the measurement period. Further RF power measurement of the linac’s klystron will be conducted after the broken modulator transformer is replaced with a new one. According to the klystron’s specifications, it should be able to produce the RF power of up to 7 MW. With this RF power value, the calculation suggested that the accelerator system should be able to generate the electron beam with an energy of 23.9 MeV, which meets the MIR-FEL requirements. In the second part, the operation of the RF gun was studied. The electron beams were generated at different cathode filament powers with different RF input powers. The electron current was measured with the first current transformer downstream the RF gun. Then, the energy spectra of electron beam were measured using the alpha magnet and its downstream current transformer. The results reveal that the cathode filament power strongly affects the beam current, while the RF power significantly affects the beam energy. The appropriate cathode filament power also depends on the conditions of RF power, the pressure inside the gun cavity, and the cathode lifetime. The proper cathode filament power can be found by slowly increasing the filament power to survey the point that gives the maximum and stable beam current. The optimal conditions for generating electron beam during this study was achieved at a cathode filament power of 21.3 W with an RF input power of 2.0 MW and an RF gun’s temperature of about 24.0°C. With these conditions, the electron beam has a beam current of 225 mA with an average energy of 1.9 MeV and an energy spread of about 19%. The average energy of electron beam is quite uniform with a deviation of less than 5% within a length 2.25 μs for the RF pulse width of 4.20 μs. The results of this research indicate the properties of electron beams produced from the electron gun and the linac with optimized RF power properties. Consequently, this investigation reveals the limitations of the accelerator system for generating the electron beam for production of MIR FEL, thereby offering valuable information for future beam operation. Moreover, this research provides the procedures for measuring the RF power of the RF system and measuring the energy spectrum of the electron beams, which are valuable for the operation of our facility.