Study on Low-Level Control of the Buncher in the Hard X-ray Free Electron Laser Facility
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摘要: 硬X射线自由电子激光装置（SHINE）中常温L波段聚束器在电子束团进行压缩过程中发挥了关键作用，有效提升了束流品质，满足了SHINE对低发射度和低能散的注入要求。由于聚束器采用了2-cell的设计，特研制了一套数字化低电平控制系统。该系统基于FPGA板卡和上下变频板卡的架构，采用I/Q解调技术，集成了幅度相位反馈、频率调谐及场平坦度多电机协调控制功能。在10 kW连续波运行中，聚束器腔压的幅度稳定度（peak-to-peak）由开环的±0.17%提高到闭环的±0.03%，相位稳定度（peak-to-peak）控制在±0.05°以内，场平坦度保持在±2%以内，满足了设计指标要求。此外，还提出了一种基于模拟数字转换（ADC）采集的射频信号功率校准方法，通过与功率计对比实测误差在±2%以内，验证了该方法的可行性，为射频功率标定提供了一种可选方案。Abstract:
Background In the Hard X-ray Free Electron Laser (SHINE), the normal-conducting L-band buncher plays a critical role in the compression of electron bunches, significantly improving beam quality 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 coordinated 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 to within ±0.03% under closed-loop, 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 within ±2% when compared with power meter, demonstrating this method’s reliability as an alternative solution for RF power calibration. -
图 1 SHINE 常温L波段聚束器系统框图(LLRF:低电平控制; SSA:固态功率源)
Figure 1. Block diagram of RF system at SHINE buncher (LLRF: low level radio frequency; SSA: solid-states amplifier)
图 6 腔压幅度稳定性比较((a)：开环≤±0.17%；(b)：闭环≤±0.03%)
Figure 6. Compare with normalized amplitude stability of voltage ((a): open-loop within ±0.17%; (b): closed-loop within ±0.03%)
图 7 闭环状态下腔压相位稳定性(peak-to-peak≤±0.05o)
Figure 7. Phase stability in closed-loop(peak-to-peak≤±0.05o)
图 9 聚束器低电平控制器外部设备标定与内部ADC功率校准框图
Figure 9. Block diagram of calibrating the buncher LLRF
图 10 输入功率与ADC采集信号的幅度拟合图
Figure 10. Relationship between input power and ADC signal amplitude
表 1 SHINE常温L波段聚束器系统设备功能简介
Table 1. Equipment function of SHINE buncher
equipment function buncher compressed electron beam, reduce the beam emittance and energy spread LLRF control the RF field SSA amplifier the RF power motor driver tuning the resonance and field flatness circulator isolate the RF power from the buncher 
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表 2 SHINE注入器聚束器低电平技术指标
Table 2. Parameters of LLRF for SHINE buncher
parameter operation mode Frequency/MHz quantity of feeding amplitude stability(RMS)/(%) phase stability(RMS)/(°) value continuous wave 1300 single 0.02 0.02
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表 3 SHINE频调环路失谐角与电机动作解析
Table 3. Analysis the relationship between the tuning and status of motor in SHINE
Area Analysis (a1) motor is moving (a2) motor is moving (b) motor is stopping (c1) 1、motor is moving when changing from Th2 towards 0 within c1
2、 motor is stopping when changing from 0 to Th2 within c1(c2) 1、motor is moving when changing from -Th2 towards 0 within c2
2、motor is stopping when changing from 0 to -Th2 within c2
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表 4 聚束器幅度稳定性测试结果(peak-to-peak)
Table 4. Results of normalized amplitude stability for buncher at SHINE(peak-to-peak)
parameter open-loop closed-loop max 0.6395 0.6450 min 0.6384 0.6446 mean 0.6389 0.6448 amplitude stability ≤±0.17% ≤±0.03% amplitude (RMS) 0.019% 0.00517 %
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表 5 测试功率比较
Table 5. Comparison list of test power
PPower-meter/W PSSA/W error ratio1/% PADC/W error ratio2/% 2001 1766 −11.74 2033 1.6 4004 3721 −7.07 3989 −0.4 5995 5722 −4.55 5990 −0.08 9023 8764 −2.87 9152 1.4 11398 10205 −10.47 11783 3.4
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