Linearization Of The Input And Output Characteristic Curve Of A Klystron Based On The LLRF Control System Of SXFEL

The undesired influence of the high power amplifiers nonlinearities on telecommunication systems and high energy physics experiments has been researched since the last decades of previous century.This phenomena is also present in the RF amplifiers which are working in the linear accelerators.Linear accelerators are generally used in modern particle physics experiments or user facilities such as free electron lasers.Shanghai soft X-ray free electron laser(SXFEL)device as the first domestic fourth generation light source has a high demand on the electron beam of the energy stability and divergence.The SXFEL LLRF measurement and control system of the output signal of klystrons is to ensure the stable operation of the amplitude and phase of the electric field of a radio frequency(RF)cavity in the linear accelerator(LINAC).However,the nonlinearity of the input-to-output characteristic curve of the klystron makes the radio frequency regulation of the LLRF control system inefficient.This dissertation is dedicated to the high power amplifiers which can be encountered in the accelerators build and developed in SXFEL.This work focuses on the amplifiers work performance diagnostic and linearization in the RF stations of SXFEL accelerator.We report the development and implementation of three kinds of FPGA based predistortion type klystron linearization algorithms.The linearization algorithms described in the following are predistortion type linearizations,implemented in the firmware of the LLRF would not disturb other controllers.Digital predistortion method is of high flexibility and precise linearization,but needs careful and adequate measurements of the characteristics of the klystron in order to calculate the accurate inverse of the nonlinear characteristics of the klystron coefficients of the predistortion function.So we first test and analyze the nonlinear characteristic of the klystron,through its nonlinear characteristic,we obtain its predistortion function.We use the polynomial algorithm,the direct lookup table algorithm and the lookup table with linear interpolation algorithm to simulate the predistortion function and implement and compare them in the FPGA.Finally we choose the lookup table with linear interpolation algorithm for linear design.The study aims to design an online system using the FPGA-based lookup table with linear interpolation algorithm to compensate the nonlinearity problem brought up by the klystrons in SXFEL.In the on line experiment with the S-band acceleration device and its corresponding Toshiba E3730 A klystron and high-voltage modulationlow-level system.The input-to-output characteristic curve of the klystron is linearized,and the system effectively compensates the nonlinearity problem introduced by the klystron.Therefore we conclude that this online system improves the precision and accuracy of the Low Level RF(LLRF)control system,which meets the requirements of the SXFEL and it also portability.Through analyzing the predistortion function of the nonlinear compensation of klystron under different high pressures.We also conclude that when the input and output characteristic of the klystron under a certain high pressure can be obtained,we can use the research methods mentioned in this paper to solve the nonlinear problem,but when the input and output characteristic curve of the klystron under a certain high pressure is not measured,the predistortion function can be deduced from the known predistortion function at adjacent high voltage,and then to linearize it.In the case when the difference between adjacent high voltages is small enough,the relationship between the predistortion function is considered to be linear.