Abstract
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.
Highlights
Superconducting radio-frequency (SRF) electron guns are frequently considered to be the favorite pathway for generating the high-quality, high-current beams needed for the future high-power energy-recovery linacs
Several multipacting levels were observed with vacuum activity detected in the fundamental power coupler (FPC) and the laser cross, with the latter being a sign of stable trajectories within the cavity body
While SRF guns had been considered as a potential breakthrough in the area of high brightness electron sources, there was a very important question about compatibility of SRF cavities and high quantum efficiency (QE) photocathodes
Summary
Superconducting radio-frequency (SRF) electron guns are frequently considered to be the favorite pathway for generating the high-quality, high-current beams needed for the future high-power energy-recovery linacs. Multipacting usually starts with a few (or even a single) primary electron(s) being present inside the cavity Such electrons can appear when a cosmic x-ray strikes a molecule of residual gas or the cavity wall, or as a result of dark current/ field emission. Depending on the energy of the primary electrons and the material of the cavity walls (surface), more than one secondary electron can be emitted If these secondary electrons are released in the accelerating phase of the electric field, and the local geometry of the cavity is such that the particles can reach the surface again and return in the same phase, the process repeats. Electrons with higher energy penetrate deep into the bulk of the wall and the probability of the secondary electron emission is greatly reduced This explains the fact that multipacting is usually localized in areas with low electric fields and frequently occurs at low levels of accelerating gradient. We present our latest commissioning results, where we were able to successfully overcome the multipactor discharge problem and achieve a stable operational regime of our SRF gun
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