Research On Digital LLRF Control System For TTX Linac

Thomson scattering X-ray is a new kind of X-ray source, which become theresearching hotspot recently for its ultra-short pulse length, high brightness and lowemittance. Tsinghua Thomson scattering X-ray source is one of the earliest platform inChina that study on the Thomson scattering X-ray, and has already get someachievements on it. It’s very important to ensure the stability of accelerating field incavity for maximum X-ray yield and minimum energy spectrum. A digital Low LevelRadio Frequency (LLRF) control system has been developed for TTX to achieve thisgoal. This system is the first domestic digital LLRF control system that applies to theaccelerator with RF pulse length less than4μs. Research methods and test resultspresented in this thesis have reference significance for other accelerator projects whowork at similar state.First, mathematical models for the RF system in TTX are studied. Stability of thecontrol system that works on pulse-to-pulse feedback mode is deduced from the models.Then digital algorithms used in LLRF control systems are compared, and proper onesare chosen for TTX. RF front end, clock distribution and high-voltage measurementmodules are designed. These hardware modules and an FPGA evaluation board arefinally integrated in an LLRF control box. Off-line tests indicate that system’s phasemeasurement accuracy is about0.03°(peak to peak). Under the close-loop work mode,designed LLRF control system can ensure the phase stability at the level of0.015°(RMS) during off-line experiments and0.092°(RMS) during on-line experiments. Theresults are better than the index raised by TTX, which is below0.2°(RMS). Now thecontrol system has been running on TTX for more than half a year. Control effectduring the TTX experiments is satisfactoryIn the process of researching how to suppress RF signal’s phase jitter, this paperanalyses its main source when system works at pulse-to-pulse feedback mode, andpresents an in-pulse feed forward method based on high-voltage measurement tocompensate the phase drift caused by high-voltage variation. Compared with onlyadopting pulse-to-pulse feedback method, adding the feed forward method to the systemcan suppress the phase jitter from0.15°to0.092°（RMS）. This thesis also discusses about the applicability of Non-IQ demodulate algorithm when amplitude and phase areall changed at the sampling time, which is the actual condition in TTX due to the shortRF pulse length. A simulation indicates that Non-IQ algorithm can represent the phaseshift of RF signal exactly in TTX work situation. An RF deflecting cavity is used toestimate the phase control effect of LLRF control system. The result shows that thephase jitter is improved about30%with the help of control system. The discussion andanalysis showed above provide a valuable reference for the other LLRF control systemswork on the short RF pulse length accelerators.