IFEL Experiment at the Neptune Lab

We present a two stage Inverse Free Electron Laser accelerator proposed for construction at the UCLA Neptune Lab. Proof-of -principle experiments on the IFEL scheme have been carried out successfully. This experiment is intended to achieve a 100 MeV energy gain, staging two IFEL modules. It will use a 16 MeV electron beam, a 1 TW CO2 laser and two different tapered helical undulators. The problem of refocusing both laser and electron beam is analyzed in detail. A preliminary beam-line layout and numerical simulation are presented. INTRODUCTION One of the most appealing possibilities for the acceleration of charged particle is to make them interact with the very large high electric fields easily available in today's high power lasers. One important advantage of far field accelerator with respect to other advanced accelerator scheme, is that the acceleration takes place in vacuum and the interaction does not require the presence of a plasma or other media at a wavelength distance from the beam, thus avoiding problems of electrical breakdown, beam intensity limitations due to electromagnetic interaction with material boundary, and beam quality degradation due to the interaction with a plasma. In principle every reverse process of a charged particle radiation can be used for acceleration. In this paper we study the inverse process of the Free Electron Laser, namely the interaction of a quasi monochromatic electromagnetic wave, with a relativistic electron beam inside an oscillating static magnetic field. This idea has been proposed initially by Palmer [1] and then extensively explored by Courant, Pellegrini and Zakowicz [2] and others [3-4]. Proof-of-principle Inverse Free Electron Laser experiments have already been carried out successfully and recently also the possibility of staging of different IFEL modules has been proved [5]. In particular a system with many accelerating regions can be obtained either by using a number of laser beams each focused only once, or by multiple focusing of one laser beam. In the first case the main problem is to keep the phase coherence of the different laser beams so that the particles remain in step with the accelerating field [6]. We explore the second case, where the main problem is the transport and focusing of a high power laser beam. The goal of the proposed experiment is to realize an IFEL accelerator raising the beam energy from about 14 MeV, to about 100 MeV, and to test the feasibility of a staging scheme using only one laser beam. CP569, Advanced Accelerator Concepts: Ninth Workshop, edited by P. L. Colestock and S. Kelley © 2001 American Institute of Physics 0-7354-0005-9/017$ 18.00 249 The Neptune Laboratory at UCLA has already a high brightness split photoinjector [6], and the high power MARS laser. The electron beam and laser parameters are given in Table 1. Table 1. Initial parameters Electron beam energy Electron beam charge Electron beam pulse length Electron beam normalized emittance Laser wavelength Laser energy Laser pulse duration 14MeV InC 6ps 5 mm-mrad 10.6(1 100 J 100 ps In the first part of this paper we propose a solution to the problem of focusing and transporting a laser pulse with 3-4 order of magnitude more energy respect to other IFEL experiments. A particular study of the IFEL interaction including the effect of the laser diffraction is also presented. The Guoy phase shift that a Gaussian beam experiences going through a waist is compensated by a gap between two halfundulators to allow re-phasing of electrons and photons. With this new scheme it is particularly important to control the effect of the wigglers on the transverse beam dynamics. At the end we present the results of 3 dimensional simulations of the beam phase space dynamics. DEALING WITH TERAWATT LASER We describe the laser beam with a Gaussian approximation: i k[x +y ] (z-zw -,\ kz-ox+<p0 + \_/ l-arctg\ w(z) (i) The best possible optical configuration for an IFEL application would be a laser beam focused at the center of the undulator to a spot size such that the Raleigh range is comparable with the length of the interaction region, that is the undulator length. To reach this optimum situation is complicated by the limit set by the damage threshold of the materials used in the transport system (2J/cm) [6]. In fact the spot size on the focusing lens cannot be smaller than 50 cm and the focal distance is limited by the fact that for practical space reasons, the lens cannot be more that 2-3 m away from the waist point. Using these numbers in the relation valid for Gaussian beams: