We report on the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected ina 50 cm long undulator strongly tapered both in period and field amplitude. A CO2 10 μ m laser with power >300 GW is used as the IFEL driver. The Rayleigh range of the laser (1.8cm) is shorter than the undulator length so that the interaction is diffraction dominated. Few per cent of the injected particles are trapped in stable accelerating buckets and electrons with energies up to 35 MeV are detected on the magnetic spectrometers. Experimental results on the scaling of the accelerator characteristics versus input parameters like injection energy, laser focus position and laser power are discussed. Three dimensional simulations are in good agreement with the electron energy spectrums observed in the experiment and indicate that substantial energy exchange between laser and electron beam only occurs in the first 25-30 cm of the undulator. An energy gradient of >70 MeV is inferred. In the second section of the undulator higher harmonic IFEL interaction is observed.

Description is presented of the KIAE-2p planar undulator device manufactured for the UCLA – Kurchatov Institute IFEL project (see PAC2001 Proceedings, p.p. 4008-4010 and PAC2003 Proceedings). Physical requirements for the undulator and simulations results on the design were given earlier (Nucl. Instr. and Meth. A483 (2002) 372-382). Here we describe main technological aspects enabled to fabricate the installation responding to the stringent requirements on mechanical construction accuracy and magnetic field strong tapering. Main parameters of the tuned undulator including last results of the magnetic field measurements by different methods are given. The obtained magnetic fields were used for final simulations of the acceleration process. It is shown that capture of 30% of electrons is provided in the acceleration process with energy gain from initial 14 MeV up to 50 MeV for nominal electron beam and laser beam parameters. Special analysis of the undulator acceptances for these parameters was made. It is shown that the acceleration takes place up to energies > 30 MeV for rather wide deviations from nominal ones in laser pulse energy, Rayleigh length and e.b. emittance.