Coherent spontaneous radiation from highly bunched electron beams

Coherent spontaneous undulator radiation has now been observed in several FELs, and is a subject of special importance to the design of self-amplified spontaneous emission (SASE) devices. We report observations of coherent spontaneous radiation at the Stanford Picosecond FEL Center at wavelengths as short as 5 microns. Enhancement of spontaneous radiation over predicted incoherent levels by as much as a factor of 6 × 104 has been observed at longer wavelengths when the electron bunches are compressed after off-peak acceleration. We discuss the possible structure responsible for these enhancements and present direct measurements of the electron distributions using transition radiation techniques.