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 high 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 both electronic and optical techniques were performed at the wiggler exit, and have clearly demonstrated beam bunching. The behavior of bunches as a function of experimental parameters is discussed.