Regenerative gain in a Raman free-electron laser oscillator

A parametric study of gain in a millimeter-wave Raman free-electron laser oscillator and comparisons to linear theory are carried out. The intense (1 kA/cm2), relativistic (600-800 keV), cold [(δγ/γ)∥ < 1percent] electron beam employed is guided by a 9.45 kG magnetic field through a 1.45 cm period, 49.5 cm long uniform undulator. Operation atυ⊥/υ∥) velocity. The laser power output has been mea sured at ∼ 3 MW corresponding to an efficiency of 4 percent, and tunability in the 90-170 GHz range has been achieved with a narrow linewidth (Δ λ/λ ≤ 1percent). Using a new technique, linear small-signal growth rates have been unfolded from the oscillator startup delays. Excellent agreement is found with three-dimensional small-signal calculations for both the spatial growth rate and the resonance frequency. One-dimensional theory was found to predict shorter wavelength laser output than that observed.