Observation of Energy Gain at the BNL Inverse Free-Electron-Laser Accelerator

A 40 MeV electron beam, using the inverse free-electron-laser interaction, has been accelerated by ΔE/E0ex0ex=0ex0ex2.5% over a distance of 0.47 m. The electrons interact with a 1--2 GW CO₂ laser beam bounded by a 2.8 mm i.d. sapphire circular waveguide in the presence of a tapered wiggler with Bₘₐₓ≈1T, and a period 2.89≤λw≤3.14cm. The experimental results of ΔE/E as a function of electron energy E, peak magnetic field Bw, and laser power Wₗ compare well with analytical and 1D numerical simulations and permit scaling to higher laser power and electron energy.