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-accelera tor free-electron laser (EA-FEL) oscillator is presented. The unique featur es 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 th e evolution of radiation in the EA-FEL oscillator. The experimental data re corded with the aid of a fast digital sampling oscilloscope are analyzed us ing 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, includin g the intermediate stages of mode build up and competition until single-mod e, steady-state operation is established.