  Recent investigations of RF copper structures operated at cryogenic
temperatures performed by a SLAC-UCLA collaboration have shown a dramatic
increase in the maximum surface electric field, to 500 MV/m. We examine use of
these fields to enable very high field cryogenic photoinjectors that can attain
over an order of magnitude increase in peak electron beam brightness. We
present beam dynamics studies relevant to X-ray FEL injectors, using
start-to-end simulations that show the high brightness and low emittance of
this source enables operation of a compact FEL reaching a photon energy of 80
keV. The preservation of beam brightness in compression, exploiting
micro-bunching techniques is discussed. While the gain in brightness at high
field is due to increase of the emission current density, further increases in
brightness due to lowering of the intrinsic cathode emittance in cryogenic
operation are also enabled. While the original proposal for this type of
cryogenic, ultra-high field photoinjector has emphasized S-band designs, there
are numerous potential advantages that may be conferred by operation in C-band.
We examine issues related to experimental implementation in C-band, and
expected performance of this type of device in a future hard X-ray FEL such as
MaRIE.
