Experimental results of nonrelativistic electron beam trapping and energy transfer in a compton scattering free electron laser scheme

We report the first observation of electron trapping and the energy transfer effect induced by two counter-propagating laser beams. The beam sources were two CO₂ transversely excited lasers, operating at 9.3 μm, and 10.6 μm, respectively, generating a ponderomotive potential with a short period (about 5 μm). The electron beam had an energy of 1-1.3 keV, corresponding to a velocity in resonance with the ponderomotive potential created by the laser beams. A decelerating axial electric field of 62 V/m was applied in the interaction region. Trapped or quasi-trapped electrons were not fully decelerated by the axial field, allowing energy differentiation between these and the untrapped electrons. Trapping efficiencies of up to 25% were measured, and the laser-induced increase in energy (nondeceleration) ranged between 0 and 5 eV. The measured laser to electron energy transfer resonance curve may be explained by either multimode quasi-trapping or by the phase displacement energy transfer and energy spread processes.