A Gridded Thermionic Injector Gun for High-Average-Power Free-Electron Lasers

High-average-power free-electron lasers (FELs) require high-average-current injectors capable of generating high-quality short-duration electron bunches with a repetition rate equal to the frequency of the energy-recovery linac (~700 MHz). We present an injector configuration that utilizes an RF-gated gridded thermionic cathode inset into a 700-MHz RF cavity. The grid is independently modulated using RF power at one or more harmonics of the main cavity frequency, in order to gate emission from the cathode surface. Two- and three-dimensional particle-in-cell simulations of this configuration were performed using the gun/collector modeling code MICHELLE. Simulations indicate that this approach can provide the necessary charge per bunch, bunch duration, beam quality, and repetition rate for high-average-power FELs operating in the infrared regime.