超快电子衍射装置的束流能量反馈控制

上海交通大学的超快电子衍射(UED)装置由一台电子直线加速器作为驱动,电子枪为一台光阴极微波电子枪。加速器在运行中电子枪会偶尔打火,腔体失谐,造成束流损失,束流能量产生变化,束流需要很长时间才能恢复到初始状态,影响了用户的使用。为此,对低电平控制器(LLRF)的幅度相位控制环路进行了改进,增加了能量反馈,代替了幅度反馈,通过对束流的中心位置进行实时的反馈以控制低电平控制器输出信号的幅度,保证了电子束流的能量稳定和电子枪加速场强的稳定。长时间的稳定性测试表明,电子枪在打火产生时,束流能量可以很快恢复,能量抖动由4.2933×10^-4(RMS)提高到2.8557×10^-4(RMS),实现了束流能量的长期稳定。

A beam energy feedback for ultrafast electron diffraction facility

The ultrafast electron diffraction(UED)facility located in Shanghai Jiao Tong University,driven by a linear electron accelerator,has a photocathode RF gun.Sometimes an RF gun arc might happen during the accelerator running,causing a cavity detuning and beam loss,then resulting in a beam energy change.It will take a long time for the beam to restore its previous energy,which will influence utilization of the facility.An energy feedback is applied to low level RF(LLRF)system after improvement of amplitude-phase loop,using a real-time feedback of the beam center position to regulate the output amplitude of LLRF,to ensure the stability of beam energy and RF gun accelerating field.A long period of stability testing indicates,that beam energy can return to its original value quickly after arc occurence,energy jitter is improved from 4.2933×10^-4(RMS)to 2.8557×10^-4(RMS),realizing a long term stability of beam energy.