  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.
