中文摘要：

在上海软X射线自由电子激光（Soft X-ray Free-electron Laser,SXFEL）中的微波低电平控制系统中,通过对各速调管的输出信号进行测量和控制,保证加速器的稳定运行并且达到技术指标。另一方面,SXFEL中的脉冲速调管的输入输出特性存在非线性的关系,这种电子管放大器不理想的非线性特性会导致微波低电平（Low Level Radio Frequency,LLRF）控制系统调节的效率降低。基于SXFEL装置,采用基于现场可编程门阵列（Field Programmable Gate Array,FPGA）的数字预失真方法,在LLRF控制系统中设计了一套实现高功率速调管放大器线性化的在线系统。S波段加速装置和其相应的东芝E3730A速调管及高压调制器和低电平系统实地测试结果表明,在LLRF控制系统中建立的在线系统能够有效补偿速调管带来的非线性问题,提高微波低电平控制系统的精度及准确性。

英文摘要：

Background： The output signal of klystrons for the soft X-ray free-electron laser （SXFEL） is processed by the low level radio frequency （LLRF） measurement and control system to ensure the stable operation of the amplitude and phase of the electric field of an radio frequency （RF） structure 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. Purpose： The study aims to design an online system to compensate the nonlinearity characteristic from klystrons in SXFEL. Methods： The online system uses the FPGA-based digital predistortion method, including three algorithms： third order polynomial, direct lookup table, and lookup table with linear interpolation. Results： In the on line experiment, the input-to-output characteristic curve of the klystron is linearized, and the system effectively compensates the nonlinearity characteristic introduced by the klystron. Conclusion： The online system improves the precision and accuracy of the LLRF control system, which meets the requirements of the SXFEL.