| Free Electron Laser(FEL)Sources driven by high energy electron beam can generate high-power,frequency-tunable terahertz radiation,and play an important role in obtaining high average power terahertz radiation.The quality of the electron beam produced by the accelerator in FEL directly affects the performance parameters of terahertz radiation such as power,waveform,spectrum,et al.A stable microwave power system is an important guarantee for producing high-quality electron beams and the basic condition for accurate measurement of the waveform of FEL output terahertz pulse.In this paper,relying on the compact THz-FEL source in Huazhong University of Science and Technology(HUST THzFEL),the influence of the stability of the microwave power system on electron beam,terahertz radiation and the detection of THz time-domain waveform is studied,and the stability control targets of microwave power system are proposed.Further,a digital LowLevel-Radio-Frequency(LLRF)system has been built to improve stability of electron beam and FEL pulses.Meanwhile,optimized design and testing of the THz-FEL time-domain measurement system based on Electro-Optical sampling has also been completed.According to the theory of Beam dynamics and FEL,the acceleration process of electron beam in microwave field and the generation,gain and saturation process of THzFEL in the undulator are calculated.The influence of microwave electric field amplitude jitter,which has a particularly obvious impact on FEL output power,and phase jitter,that causes serious distortion of the THz time domain measurement waveform,on electron beam,FEL output power and THz time domain measurement waveform is obtained as well.Therefore,the LLRF control targets of microwave electric field amplitude and phase jitter are proposed.The mathematical model of the LLRF system is established,the baseband transfer function of the microwave system including the traveling wave accelerating tube is derived,and the stability of the RF control system is analyzed.The appropriate Proportional Integral coefficient is selected to meet the requirements of control accuracy.Further,in order to improve speed of system response and suppress overshoot,a fuzzy controller is added to the PI control of normal LLRF in other facility,and a fuzzy control strategy has been designed to apply in a variety of LLRF operation modes.In this paper,a FPGA-based digital LLRF system has been built.Relying on HUST THz-FEL,RF front-end and digital signal processing modules have been developed.Based on Verilog HDL language,hardware algorithms executed in FPGA has been developed.The LLRF system is able to operate in Reference-Follow mode and Parameter-Set mode,and it meets the requirements of RF jitter control targets.The modules of LLRF and control effect of RF jitter have been tested.In the testing,output signals of both RF front-end and digital signal processing module have good performance(signal-to-noise ratio and phase noise),and the control loop that uses the klystron output as the feedback signal limits the RF jitter of amplitude and phase within the range of ±0.7% and ±1° respectively.In order to measure the THz pulse waveform in wide frequency adjustment range of FEL output radiation,the THz-FEL radiation time-domain detection system requires a large detection time-window and wide bandwidth.Thus,a 10 fs broad-spectrum probe laser and a thick electro-optic crystal for electro-optic sampling of THz pulses have been utilized in THz-FEL radiation time-domain detection.However,due to the dispersion effect of wideband pulse in thick EO crystals,the efficiency of EO sampling will decrease.In order to compensate the dispersion of the broad-spectrum probe laser in thick crystals,a frequency-dependent optimization for EO sampling has been proposed,and a chirped optical path has been designed to optimize the time-domain detection system,which increases the positive half-cycle amplitude of the EO signal by 71.29%,and the SNR by 7.5d B. |
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