An undulator with non-adiabatic tapering for the IFEL project

Abstract

We describe the design of a planar undulator with unusually strong tapering, for the inverse FEL experiment (on the IFEL experiment at the UCLA Neptune Lab. (1998) 437) at UCLA. A powerful TW CO 2 laser will be used to accelerate electrons up to 50-60 MeV in 50 cm long undulator. A strong undulator tapering is needed because of the short Rayleigh length of the laser beam. Both the magnetic field and the undulator period are tapered to provide synchronicity of the laser beam interaction with a captured electron bunch along the whole undulator length. The most critical part of the undulator is the region near the laser focus. The main characteristics of the IFEL, such as the percentage of trapped electrons, energy of accelerated electrons and sensitivity to the laser focus transverse position, are given. The general principles of the design of this undulator construction can also be useful for high efficiency FEL amplifiers of intense laser modes. r

Ultrashort laser pulse driven inverse free electron laser accelerator experiment

“…The 50 cm long UCLA/Kurchatov undulator magnet is the same one that was previously used in the Neptune IFEL experiment [11]. In this strongly tapered planar undulator both the period and magnetic field are increased along the axis to maintain the IFEL resonant condition with a violently accelerating beam.…”

High Energy Gain of Trapped Electrons in a Tapered, Diffraction-Dominated Inverse-Free-Electron Laser

“…1. A 14.5 MeV energy electron beam with a charge of 300 pC, a normalized emittance of 5 mm mrad, and a rms pulse length of 6 ps was injected in the IFEL undulator [11] using the Neptune rf photoinjector [12]. Final focus quadrupoles focused the e beam to the spot size of 120 m (rms) in the middle of the undulator.…”

“…The laser beam exiting the undulator was sent to the streak camera for timing measurements. The IFEL hybrid planar undulator KIAE-2p [11] was designed and built at the Kurchatov Institute. This magnet was optimized for a 400 GW, 3.6 cm Rayleigh range driver laser beam.…”

“…In order to maintain synchronism through the focal region, two undulator sections were built with the field in the central region adjusted using a corrector Vanadium-Permandur plate. The undulator period and field strength were specified using simulation results [11] obtained with the three-dimensional code TREDI [14].…”