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

A time-frequency study of oscillation build up in an electrostatic accelerator free-electron laser (EA-FEL) oscillator is presented. The unique features of an EA-FEL are its capacity to operate in a long pulse mode and enable observation and study of linear and nonlinear processes taking place in the evolution of radiation in the EA-FEL oscillator. The experimental data recorded with the aid of a fast digital sampling oscilloscope are analyzed using fast Fourier transform (FFT) signal-processing techniques to obtain the power spectral density evolution with time. Such an analysis allows one to study the radiation excitation build up from noise to saturation, including the intermediate stages of mode build up and competition until single-mode, steady-state operation is established.