Optics matching schemes for linac-based light source

Despite its small average beam current limit, a linac has features that make it attractive as an X-ray source: high energy, ultra-low emittance and energy spread, and flexible beamline optics. Unlike a storage ring, in which an undulator radiation source necessarily is short and positioned at the electron beam waist, in a linac the undulator can be long and the electron beam can be adjustable to have a virtual waist far downstream towards the X-ray target to produce a convergent X-ray beam. Using a planned CEBAF beamline as an example, we show here that we can overcome a factor of 2000 in the beam current to produce a monochromatic hard X-ray source comparable with, or even exceeding, the performance of an X-ray line at a third-generation storage ring. Even with no monochromator, the X-ray energy bandwidth can be as small as 0.1%. We discuss the optimal conditions which produce maximum X-ray flux density by focusing the electron beam for twocases: one is based on the waist-at-target (WAT) scheme, while the other goes beyond the WAT scheme. Both cases were verified by simulations using the SRW code.