Coherent and self-amplified infrared synchrotron radiation emitted by a 50-mev electron beam

We have observed and analyzed the coherent spontaneous emission (CSE) and the self-amplified spontaneous emission (SASE) emitted, in the near infrared spectral range, by short pulses of relativistic electrons passing through an undulator. The CSE is surprisingly large at wavelengths much shorter than the electron pulse length. The SASE spectral line exhibits an unexpected growth at the start-up of the process. Both phenomena are fluctuating and can be distinguished only by careful spectral measurements