Regenerative Gain and Sideband Instability in a Raman Free Electron Laser.

A parametric study of gain in a millimeter-wave Raman free electron laser oscillator and comparisons with linear theory are carried out. The intense (1kA/cm('2)), relativistic (600-800keV), cold (((delta)(gamma)/(gamma))(,(PARLL)) < 1% electron beam employed is guided by a 9.45kG magnetic field through a 1.45cm period, 50cm long uniform undulator. Operation at <1kG pump field results in a <10% electron quiver ((nu)(,(PERP))/(nu)(,(PARLL))) velocity. The laser power output has been measured at (DBLTURN)3MW corresponding to an efficiency of 4% and tunability in the 90 to 170GHz frequency range has been achieved with a narrow linewidth ((DELTA)(lamda)/(lamda) (TURNEQ) 1%). Using a new technique, linear small-signal growth rates have been unfolded from oscillator startup delays. Excellent agreement is found with three dimensional small-signal calculations. The first experimental evidence of a predicted sideband instability in the saturated Raman FEL spectrum is presented. A new method for the control and possible suppression of the sidebands by elimination of pulse slippage is demonstrated.