A compact wakefield accelerator for a high repetition rate multi user X-ray free-electron laser facility

Abstract

A concept is presented for a multi beamline x-ray FEL user facility driven by an array of highly efficient compact collinear wakefield accelerators (CWA) where Cˇerenkov radiation of a 400 MeV high charge drive bunch is used to accelerate a low charge witness bunch to 2 GeV to produce soft x-rays in the FEL. The CWA design is based on a cylindrical corrugated waveguide with a 2 mm internal diameter embedded into a focusing channel of quadrupole magnets with a 3 mm aperture. The accelerating gradient for the witness bunch ~ 100 MeV/m is limited by a beam break up instability of the drive bunch and hybrid permanent magnet quadrupole lenses with the maximum field gradient ~ 0.9 T/mm are used to facilitate Balakin-Novokhatsky-Smirnov damping adapted here to a case of strong longitudinal and transverse wakefields. A self-consistent design of a CWA with an optimal lattice for the focusing channel, bunch shape for the drive bunch, and Green's function for the corrugated waveguide has been developed and verified numerically. Obtaining the witness bunch with a minimum energy spread is also discussed. Finally, results are presented for the expected FEL performance using a carefully selected undulator taper.