Measurement of internal dark current in a 17 GHz accelerator structure with an elliptical sidewall

A 17 GHz single cell, standing wave, copper accelerator structure with an axisymmetric elliptical central cell sidewall was tested for internal and downstream dark current as a function of gradient up to 93 MV/m. The elliptical sidewall was predicted to suppress the internal dark current and the lower order multipactor modes as compared with a previously tested structure having a straight sidewall. During the conditioning phase of the elliptical sidewall structure, strong internal dark current generated by an N = 1 multipactor mode was observed at gradients in the 10 to 20 MV/m range. After conditioning with 2.2x10^5 pulses to 93 MV/m, the N = 1 mode was completely suppressed and no multipactor resonances were observed. The internal dark current was reduced to a comparatively low level, much smaller than in the previously studied, straight sidewall structure, in good agreement with simulations. The energy spectrum of the electrons colliding with the sidewall was measured using an isolated side dark current monitor and a bias voltage. As the conditioning progressed, the electron energy spectrum showed an increase in the concentration of lower energy electrons, also in good agreement with simulations. Studies of internal dark current may help to understand the rf conditioning and ultimate performance of high gradient accelerator structures.

本研究针对一款17吉赫兹（17 GHz）单腔驻波（standing wave）铜制加速结构开展测试，考察其内部与下游暗电流（dark current）随加速梯度的变化情况，梯度最高可达93 MV/m。相较于此前测试的直侧壁加速结构，该结构采用的轴对称椭圆中心腔侧壁被预测可抑制内部暗电流与低阶二次电子倍增（multipactor）模式。在该椭圆侧壁结构的射频老练阶段，当加速梯度处于10~20 MV/m区间时，观测到由N=1二次电子倍增模式引发的强内部暗电流。在通过2.2×10^5个脉冲将结构老练至93 MV/m后，N=1二次电子倍增模式被完全抑制，未再观测到二次电子倍增谐振现象。此时内部暗电流降至相对较低的水平，远低于此前研究的直侧壁加速结构，该结果与仿真模拟结果吻合良好。研究人员采用独立侧壁暗电流监测器与偏置电压，对轰击侧壁的电子能谱进行了测量；随着老练过程推进，电子能谱中低能电子的占比显著提升，这一结果同样与仿真模拟结果吻合良好。对内部暗电流的研究有助于理解高梯度加速结构的射频老练过程与最终性能。