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.

本研究对一款17吉赫兹（GHz）单腔驻波铜加速结构开展了测试，其中心腔侧壁采用轴对称椭圆结构，测试了内部及下游暗电流随加速梯度的变化情况，梯度最高可达93兆伏每米（MV/m）。相较于此前采用直侧壁的测试结构，该椭圆侧壁结构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至20 MV/m区间时，观测到由N=1阶二次电子倍增模式产生的强内部暗电流。在通过2.2×10^5个脉冲老练至93 MV/m后，N=1阶二次电子倍增模式被完全抑制，未观测到任何二次电子倍增共振现象。内部暗电流被降至相对较低的水平，远低于此前研究中直侧壁结构的暗电流水平，该结果与仿真模拟结果吻合良好。研究人员采用独立式侧壁暗电流监测装置与偏置电压，对碰撞至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逐渐升高，该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极限性能。