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 $\Delta E/E=2.5\mathrm{}\%$ over a distance of 0.47 m. The electrons interact with a 1–2 GW ${\mathrm{CO}}_2$ laser beam bounded by a 2.8 mm i.d. sapphire circular waveguide in the presence of a tapered wiggler with $B_{\max }\approx 1\mathrm{T}$, and a period $2.89\le {\lambda }_w\le 3.14\mathrm{cm}$. The experimental results of $\Delta E/E$ as a function of electron energy $E$, peak magnetic field $B_w$, and laser power $W_l$ compare well with analytical and 1D numerical simulations and permit scaling to higher laser power and electron energy.