Experimental observation and investigation of energy exchange between an electron beam and counterpropagating laser beams in a Compton-scattering free-electron-laser scheme

We report the observation and investigation of synchronous energy exchange between nonrelativistic electrons and the ponderomotive (beat) force of two counterpropagating intense pulsed CO2 laser beams, operating at different frequencies in a stimulated Compton-scattering scheme. The interaction occurred in the nonlinear (trapping) regime, the physics of which is the same as that which occurs in laser accelerators and efficiency-enhanced free-electron lasers (FELs) with long wigglers. The experiment is a first demonstration of the principle of inverse FEL acceleration and electromagnetic pump FEL operation in the nonlinear (trapping) regime. It can also be described as a demonstration of a traveling beat-wave Kapitza-Dirac effect in the nonlinear regime. Two different mechanisms of enhanced energy transfer were observedelectron trapping and phase-area displacement. Experimental results and computer simulations of both mechanisms are presented.