High spectral coherence in long-pulse and continuous free-electron laser: Measurements and theoretical limitations

Single mode operation was exhibited on the Israeli tandem free-electron las er (FEL). This enabled judicious measurements of narrow spectral linewidth, frequency chirp, and relaxation-oscillation effects. Exact 3D simulations of the FEL oscillator showed good agreement with the measurements, and perm itted an estimation of the fundamental Schawlow-Towns linewidth limit of th e FEL (including "alpha effect") as well as technical noise linewidth limit s. We estimate that with voltage-controlled stabilization high-power (10 kW ) tunable (over 60% bandwidth) quasi-cw coherent [(Delta nu/nu)(rms) approx imate to 10(-10)] mm-FIR (far infrared) radiation is attainable in the tand em FEL.