The Research On Microwave Power System Of The Linac In HUST-FEL

The research of tunable, high-power THz sources based on lianc has been carried out by HUST. Microwave power system which provides the linac with the energy is an important part of the linac. Its stability is related to the quality of the electron beam and ultimately affects the wavelength and output power of the terahertz radiation.To improve the efficiency of beam tuning,adaptive control system of microwave based on maximizing energy and adaptive optimization system of microwave based on minimizing energy spread is designed. The phase and amplitude of input microwave power determines the energy and beam quality of the electron beam, therefore adjustable phase shifters and adjustable attenuators are added to waveguide transmission system. Based on the LabVIEW, program is written for remote control of the phase shifter, attenuator. Automatic adjustment program is designed on the premise of knowing the current microwave power. On the basis of the microwave detector calibration interpolation method is used to measure power and IQ demodulation is used to measure phase. Microwave power measurement program is written based on LabVIEW. In addition, the optimization of beam is based on the data transmission from beam measurement system to the microwave power measurement system, so the TCP protocol is used to implement the communication.The beam loading effect of the accelerating tube affects electromagnetic field distribution and coupled power. From the power flow equation, the transient beam loading effect general analytical solution is deduced. Based on parameters from accelerating tube of HUST-FEL simulation, we get electron beam energy loss cause by beam loading effect in the different injection time. According to the above analysis, the reasonable sequence of the electron gun and the microwave power source is needed to maximize the beam energy and the three stages of self-consistent distribution of microwave power which consist of undercoupling, optimum coupling and overcoupling is further analyzed starting from the equivalent circuit of the accelerator.Stability of microwave power affects beam stability. Starting from the theoretical analysis, we acquire the reasons which cause measurement errors of phase and power measurement. In addition, the voltage jitter of modulator, the frequency instability of microwave power source and temperature changes of waveguide which affects the beam quality is also analyzed.