Prospects for high power linac coherent light source (LCLS) development in the 1000 Å−1 Å wavelength range

Abstract

Electron bunch requirements for single‐pass saturation of a Free‐Electron Laser (FEL) operating at full transverse coherence in the Self‐Amplified Spontaneous Emission (SASE) mode include: 1) a high peak current, 2) a sufficiently low relative energy spread, and 3) a transverse emittance e[r‐m] satisfying the condition e≤λ/4π, where λ[m] is the output wavelength of the FEL. In the insertion device that induces the coherent amplification, the prepared electron bunch must be kept on a trajector sufficiently collinear with the amplified photons without significant dilution of its transverse density. In this paper we discuss a Linac Coherent Light Source (LCLS) based on a high energy accelerator such as, e.g., the 3 km S‐band structure at the Stanford Linear Accelerator Center (SLAC), followed by a long high‐precision undulator with superimposed quadrupole (FODO) focusing, to fulfill the given requirements for SASE operation in the 1000 A−1 A range. The electron source for the linac, an RF gun with a laser‐excited photocathode featuring a normalized emittance in the 1–3 mm‐mrad range, a longitudinal bunch duration of the order of 3 ps, and approximately 10−9 C/bunch, is a primary determinant of the required low transverse and longitudinal emittances. Acceleration of the injected bunch to energies in the 5–25 GeV range is used to reduce the relative longitudinal energy spread in the bunch, as well as to reduce the transverse emittance to values consistent with the cited wavelength regime. Two longitudinal compression stages are employed to increase the peak bunch current to the 2–5 kA levels required for sufficiently rapid saturation. The output radiation is delivered, via a grazing‐incidence mirror bank, to optical instrumentation and a multi‐user beam line system. Technological requirements for LCLS operation at 40 A, 4.5 A, and 1.5 A are examined.