DIRECT OBSERVATION OF BEAM BUNCHING PRODUCED BY A HIGH-POWER MICROWAVE FREE-ELECTRON LASER

In this Letter we present a direct measurement of beam bunching by a h igh power free-electron laser (FEL). An induction linac delivered a 1 kA, 2.2 MeV electron beam to a FEL in which a 35 GHz input signal was amplified to power levels of the order of 10 MW. Measurements using bo th electronic and optical techniques were performed at the wiggler exi t, and have clearly demonstrated beam bunching. The behavior of bunche s as a function of experimental parameters is discussed.