A pure permanent magnet — two plane focusing — tapered wiggler for a high average power FEL

Abstract

A high-average power FEL is under construction at Los Alamos. The FEL will have aspects of both an oscillator and a SASE (self-amplified spontaneous emission) device. That is, a high-gain and high-extraction efficiency wiggler will be used with a very low- Q optical resonator. FEL simulations reveal that a tapered wiggler with two-plane focusing is required to obtain desired performance. The wiggler is comprised of a 1 m long untapered section followed by a 1 m tapered section. The taper is achieved with the magnetic gap and not the wiggler period which is constant at 2 cm. The gap is tapered from 5.9 to 8.8 mm. The gap, rather than the period, is tapered to avoid vignetting of the 16 μm optical beam. Two-plane focusing is necessary to maintain high current density and thus high gain through out the 2 m long wiggler. Several magnetic designs have been considered for the wiggler. The leading candidate approach is a pure permanent wiggler with pole faces that are cut to roughly approximate the classical parabolic pole face design. Focusing is provided by the sextupole component of the wiggler magnetic field and is often called “natural” or “betatron” focusing. Details of the design will be presented.