Abstract:
Background In the hard X-ray free electron laser (Shanghai high repetition rate XFEL and extreme light facility, SHINE), the normal-conducting L-band buncher plays a critical role in the compression of electron bunches, significantly improving beam quality and meeting the stringent injection requirements of low emittance and low energy spread.
Purpose Due to its 2-cell structure, a dedicated digital low-level RF control system was developed.
Methods This system, based on an architecture comprising FPGA and RF front-end boards, and adopts I/Q demodulation techniques. It incorporates amplitude and phase feedback, frequency tuning, and multi-motor coordination for field flatness control.
Results During 10 kW continuous-wave (CW) operation, the amplitude stability (peak-to-peak) improved from ±0.17% in open-loop mode to within ±0.03% under closed-loop mode, while the phase stability (peak-to-peak) was controlled within ±0.05°, and field flatness was maintained within ±2%, fully meeting design specifications. Additionally, a radio-frequency (RF) power calibration method based on ADC acquisition of LLRF was proposed.
Conclusions Experimental results showed calibration error was within ±2% when compared with a power meter, demonstrating reliability of this method as an alternative solution for RF power calibration.
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