The temperature rise due to the backstreaming electrons is canceled by an equal and opposite fall in temperature at the surface of the cathode due to the conduction of heat deposited at the surface immediately prior to the microwave pulse by a pulsed laser focused to uniformly illuminate the cathode surface. Variations in temperature across the surface of the cathode attributable to the non-uniform spatial distribution of the backstreaming electrons may be compensated using a second laser pulse fired during the RF pulse to maintain constant thermal power input across the surface of the cathode during the RF pulse. This second pulse can also be used to compensate for the time-dependent rate of decay of temperature due to conduction of the heat deposited by the first laser into the body of the cathode.