113 MHz超导RF光注入器中的多重放电抑制
Mitigation of multipacting in 113 MHz superconducting RF photo-injector
摘要：
超导射频（SRF）光注入器是产生高亮度连续波电子束的最有前途的器件之一。SRF枪的超高高真空提供高量子效率（QE）光电阴极的长寿命，而SRF技术提供超过10 MV/m的高加速梯度。对于低频SRF电子枪尤其如此，在低频SRF电子枪中，电子在加速电压的峰值处在光电阴极处产生。SRF枪的两个主要物理挑战是它们与高QE光电阴极的兼容性和二次放电。第一个是有关的光阴极材料（如Cs）的SRF腔的墙壁上的沉积的可能性，这可能会导致增加的暗电流通过减少体Nb的功函数和增强的次级电子发射产额（SEY）。SEY在电子枪的二次放电过程中起着关键作用，它不仅破坏了电子枪的真空度，而且加速了阴极材料在SRF腔壁上的沉积。简而言之，超导加速单元中的微电子倍增管行为必须被很好地理解，以成功地操作SRF光注入器。在本文中，我们介绍了我们的研究，1.2 MV 113 MHz的四分之一波长SRF光注入器作为电子束源的相干电子冷却实验（CeC）在BNL。在操作SRF枪的三年中，我们遇到了许多微碰撞区。我们还观察到，$\textrm{CsK}_{2}\textrm{Sb}$光阴极的存在下，在枪可以创建额外的二次放电障碍。我们对SRF电子枪中的二次电子碰撞放电进行了全面的数值和实验研究，并开发了一种在不影响光阴极寿命和提高二次电子碰撞势垒强度的情况下从零到工作电压跨越二次电子碰撞势垒的过程。
英文摘要：
Superconducting RF (SRF) photo-injectors are one of the most promising devices for generating continuous wave (CW) electron beams with record high brightness. Ultra-high vacuum of SRF guns provides for long lifetime of the high quantum efficiency (QE) photocathodes, while SRF technology provides for high accelerating gradients exceeding 10 MV/m. It is especially true for low frequency SRF guns where electrons are generated at photocathodes at the crest of accelerating voltage. Two main physics challenges of SRF guns are their compatibility with high QE photocathodes and multipacting. The first is related to a possibility of deposition of photocathode materials (such as Cs) on the walls of the SRF cavity, which can result in increased dark current via reduction of the bulk Nb work function and in enhancing of a secondary electron emission yield (SEY). SEY plays critical role in multipacting, which could both spoil the gun vacuum and speed up the deposition of the cathode material on the walls of the SRF cavity. In short, the multipactor behavior in superconducting accelerating units must be well understood for successful operation of an SRF photo-injector. In this paper we present our studies of 1.2 MV 113 MHz quarter-wave SRF photo-injector serving as a source of electron beam for the Coherent electron Cooling experiment (CeC) at BNL. During three years of operating our SRF gun we encountered a number of multipacting zones. We also observed that presence of $\textrm{CsK}_{2}\textrm{Sb}$ photocathode in the gun could create additional multipacting barriers. We had conducted a comprehensive numerical and experimental study of the multipactor discharge in our SRF gun, and had developed a process of crossing the multipacting barriers from zero to the operational voltage without affecting the lifetime of our photocathode and enhancing the strength of multipacting barriers.
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