  Recent studies of the performance of radio-frequency (RF) copper cavities
operated at cryogenic temperatures have shown a dramatic increase in the
maximum achievable surface electric field. We propose to exploit this
development to enable a new generation of photoinjectors operated at cryogenic
temperatures that may attain, through enhancement of the launch field at the
photocathode, a significant increase in five-dimensional electron beam
brightness. We present detailed studies of the beam dynamics associated with
such a system, by examining an S-band photoinjector operated at 250 MV/m peak
electric field that reaches normalized emittances in the 40 nm-rad range at
charges (100-200 pC) suitable for use in a hard X-ray free-electron laser
(XFEL) scenario based on the LCLS. In this case, we show by start-to-end
simulations that the properties of this source may give rise to high efficiency
operation of an XFEL, and permit extension of the photon energy reach by an
order of magnitude, to over 80 keV. The brightness needed for such XFELs is
achieved through low source emittances in tandem with high current after
compression. In the XFEL examples analyzed, the emittances during final
compression are preserved using micro-bunching techniques. Extreme low
emittance scenarios obtained at pC charge, appropriate for significantly
extending temporal resolution limits of ultrafast electron diffraction and
microscopy experiments, are also reviewed. While the increase in brightness in
a cryogenic photoinjector is mainly due to the augmentation of the emission
current density via field enhancement, further possible increases in
performance arising from lowering the intrinsic cathode emittance in cryogenic
operation are also analyzed. Issues in experimental implementation, including
cavity optimization for lowering cryogenic thermal dissipation, external
coupling, and cryo-cooler system are discussed.
