| | The most demanding component of a cw injector is cw
operating RF-gun delivering highly populated (~1 nC)
low emittance bunches. RF-guns, both working at room
temperature and superconducting, when they generate
highly populated low emittance bunches have to be
operated at high accelerating gradients to suppress space
charge effects diluting emittance. Superconducting RFguns
(SRF-guns) are technically superior to the normal
conducting devices, because they dissipate orders of
magnitude less power when operating at very high
gradients. They can easily work at high duty factor but
the challenge here is integration of a non superconducting
photo-cathode material with a superconducting cavity in
a way preserving its original high intrinsic quality factor
Qo (small cryogenic losses). An alternative approach,
technically feasible for miliampere-class SRF-guns, is to
use superconducting metal as the photo-cathode. In this
case difficulty arises from moderate quantum efficiency
(QE) of the superconducting materials. Relatively low
QE must be compensated with higher energy pulses of
the illuminating laser. This approach has been studied
first for niobium cathode at BNL, but the measured QE at
4K was very low (~10-5). The recent progress in a
complementary approach, with the lead cathode [1], is
discussed in the last section of this overview. | |
