Observation of coherent and self-amplified spontaneous emission with the CLIO free-electron laser

We have produced and analysed the “Coherent Spontaneous Emission” (CSE) and the “Self-Amplified Spontaneous Emission” (SASE) emitted by a relativistic electron beam passing through an undulator. In this spectral range, the two phenomena are both very noisy and very sensitive to the electron bunch shape. As a consequence, it is difficult to distinguish one from the other, at least when SASE is restricted to the beginning of the build-up regime. We present new measurements, showing how these two regimes differ in their spectral evolution.