  This letter reports cavity dumping of an electrostatic-accelerator-driven
free-electron laser (FEL) while it is injection-locked to a
frequency-stabilized 240 GHz solid-state source. Cavity dumping enhances the
FEL output power by a factor of $\sim$8, and abruptly cuts off the end of the
FEL pulse. The cavity-dumped, injection-locked FEL output is used in a 240 GHz
pulsed electron spin resonance (ESR) experiment.
