First operation of an FEL in same-cell energy recovery mode

The driver for Jefferson Lab's kW-level infrared free-electron laser (FEL) is a supcrsonducting, recirculating accelerator that recovers 75% of the el ectron-beam power and converts it to radio frequency power, As reported in FEL'98, the accelerator operated "straight-ahead" to deliver 38 MeV. 1.1 mA cw current for lasing at wavelengths in the vicinity of 5 mu m. The waste beam was sent directly to a dump, bypassing the recirculation loop. Stable operation at up to 311 W cw was achieved in this mode. The machine has now recirculated cw average current up to 4.7 mA, and has lased cw with energy recovery up to 1720 W output at 3.1 mu m. This is the first FEL to ever ope rate in the ''same-cell" energy recovery mode. Energy recovery offers sever al advantages (reduced RF power and dramatically reduced radio-nuclide prod uction at the dump) and several challenges (potential for instabilities and difficult beam transport due to large energy spreads). Solutions to these challenges will be described. We have observed heating effects in the mirro rs which will be described. We will also report on the additional performan ce measurements of the FEL that have been performed and connect those measu rements to standard models. (C) 2000 Elsevier Science B.V. All rights reser ved.