  Radiation from an ultra-stable 240 GHz solid-state source has been injected,
through an isolator, into the cavity of the University of California Santa
Barbara (UCSB) MM-wave free-electron laser (FEL). High-power FEL emission,
normally distributed among many of the cavity's longitudinal modes, is
concentrated into the single mode to which the solid state source has been
tuned. The linewidth of the FEL emission is 0.5 MHz, consistent with the
Fourier transform limit for the 2 microsecond pulses. This demonstration of
frequency-stable, ultra-narrow-band FEL emission is a critical milestone on the
road to FEL-based pulsed electron paramagnetic resonance spectroscopy.
