Study of radiation build-up and spectral evolution in the Israeli electrostatic accelerator free-electron laser oscillator

Time-frequency study of oscillation build-up in an electrostatic accelerator free-electron laser (EA-FEL) oscillator is presented. The unique features of EA-FEL and its capacity to operate in a long pulse mode, enable observation and study of linear and non-linear processes taking place in the evolution of radiation in the EA-FEL oscillator. The experimental data recorded with the aid of a digital 4 GS/s fast sampling oscilloscope, was analyzed using various signal processing techniques to obtain time-frequency phase space presentation of the power spectral density evolution. This presentation make it possible to follow the radiation field coherence evolution from noise to saturation including intermediate stages of mode build-up from noise and competition between longitudinal modes until single-mode steady-state operation is established. The experimental results were also compared with the results of a FEL oscillator simulation code. The code predicts well the behavior of the oscillator in its various regimes of operation.