Variable-Wiggler Free-Electron-Laser Experiment

First results are presented of a free-electron-laser experiment that utilizes a tapered wiggler for efficiency enhancement. The spontaneous spectrum and the electron-beam energy loss due to the free-electron-laser interaction are measured and compared with theory. With a 2.25% magnetic field taper, 12% of the electrons were decelerated by 0.6%, corresponding to a gain of 2.7% and an efficiency of 0.07%, which is 10 times higher than that calculated for a zero taper and otherwise identical conditions.