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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 microbunching 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.

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Recent initiatives in ultra-short, GeV electron beam generation have focused on achieving sub-fs pulses for driving X-ray free-electron lasers (FELs) in single-spike mode. This scheme employs very low charge beams, which may allow existing FEL injectors to produce few-100 as pulses, with high brightness. Towards this end, recent experiments at SLAC have produced ∼2 fs rms, low transverse emittance, 20 pC electron pulses. Here we examine use of such pulses to excite plasma wakefields exceeding 1 TV/m. We present a focusing scheme capable of producing <200 nm beam sizes, where the surface Coulomb fields are also ∼TV/m. These conditions access a new regime for high field atomic physics, allowing frontier experiments, including sub-fs plasma formation for wake excitation.Use of low charge, in the pC range, has been recently proposed for enabling GeV-class beams to be compressed to the hundreds of attosecond level [1]. Further, these beams are predicted to have very low normalized transverse emittance ǫ n , and thus unprecedented brightness. This proposal was generated in the context of planning future X-ray self-amplified spontaneous emission free-electron lasers ( SASE FELs [2]). The scheme addresses two challenges in the X-ray SASE FEL: it breaches the fs frontier in X-ray pulse length, and it allows single spike SASE FEL performance. Both properties permit exploitation of the revolutionary aspects of coherent X-ray FEL light, as one may resolve properties of atomic and molecular systems at the spatial and temporal scales relevant to electronic motion [3].