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X-ray free-electron lasers (X-FELs) provide unprecedented capabilities for characterizing and controlling matter at temporal, spatial and energetic regimes which have been previously inaccessible. The quality of the electron beam is critical to X-FEL performance; a degradation of beam quality by a factor of two, for instance, can prevent the X-FEL from lasing at all, rather than yielding a simple reduction in output power. While conceptual designs for new beam sources exist, they incorporate assumptions about the behavior of the photocathode, under extreme operating conditions. The combined requirements for high bunch charge, short bunch duration, and small emission area, dictate the use of high-efficiency photocathodes operating at electric field gradients of ~140 MV/m. No suitable cathode has been operated at these gradients, however, so the success of next-generation X-FELs rests on a series of untested assumptions. We present our plans to address these knowledge gaps, including the design of a high-gradient RF cavity specifically designed for testing cathodes under MaRIE-relevant conditions.
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