Study of coherence limits and chirp control in long pulse FEL oscillator

Full Text:We report experimental studies of the spectral line width and chirp characteristics of the mm-wave RF radiation of Israeli Electrostatic Accelerator FEL (EA-FEL), along with theory and numerical simulations. The simulations matching the experimental data were carried out using space-frequency model. EA-FELs have the capacity to generate long pulses of tens microseconds and more, that in principle can be elongated indefinitely (CW operation). Since a cold beam FEL is by nature a ''homogeneously broadened laser'', EA-FEL can operate, unlike other kinds of FELs, at a Angle longitudinal mode (single frequency). This allows the generation of very coherent radiation. The current status of the Israeli Tandem Electrostatic Accelerator FEL (EA-FEL), which is based on an electrostatic Van de Graaff accelerator, allows the generation of pulses of tens microseconds duration. It has been operated recently past saturation, and produced single mode coherent radiation of record narrow inherent relative line width Δ f/f = 10-6 at frequencies near 100 GHz. A clear frequency chirp is observed during pulses of tens of microseconds (0.3 -0.5 MHz/μs). This is essentially a drifting frequency pulling effect associated with the accelerator voltage drop during the pulse. Additionally, aperiodic oscillation relaxation was experimentally measured and compared with theory both numerically, and analytically in general framework of non-harmonic oscillations. Possibilities of chirp applications and power boosting are also discussed