A LOW-ENERGY-SPREAD RF ACCELERATOR FOR A FAR-INFRARED FREE-ELECTRON LASER

A high electron current and a small energy spread are essential for th e operation of a free electron laser (FEL). In this paper we discuss t he design and performance of the accelerator for FELIX, the free elect ron laser for infrared experiments. The system consists of a thermioni c gun, a prebuncher, a buncher and two standard commercial linac secti ons. The gun is operated with a pulse duration of 280 ps and a bunch c harge of 200 pC. After compression to 35 ps by the prebuncher, the bun ches are accelerated to 4 MeV in the buncher and simultaneously compre ssed to 6 ps. The principle of the method is that the order of the ele ctrons is conserved in the buncher, so that the resulting more or less linear energy-phase relationship along each bunch can be compensated effectively against space charge forces and the accelerating field gra dient in the linacs, via an appropriate choice of the phase of the rf wave. Behind the linacs an rms energy spread of 0.30% has been measure d.