A high-current micro-pulse electron gun

A novel concept for a high-current-density micro-pulse electron gun has been studied. The concept employs the resonant amplification of an electron current by secondary electron emission in an RF cavity. We have studied this “multipacting” process in detail, including space charge, and have found the parameters necessary to make use of the phenomenon to produce a micro-pulsed electron beam. One wall of the cavity is made partially transparent to electrons but opaque to the input RF electric field. It is shown from self-consistent analytic theory and two-dimensional, fully relativistic, electromagnetic particle-in-cell (PIC) simulations that the current density scales with frequency cubed. The natural “bunching”, or resonant phase selection of the particles gives rise to high current densities (20-5600 A/cm²), high charge bunches (up to 100 nC/bunch for a solid beam and up to 1000 nC for a hollow beam), and short pulses (36-3.5 ps) for frequencies from 1.3 to 8 GHz. The beam pulse width is found to be typically 4.6% of the RF period. Beam extraction through the transparent wall was studied using a 2-1/2 dimensional PIC code. Good beam transmission (52-85%) with low normalized emittance (9-18 mm-mrad) could be achieved. The best normalized emittance per charge is 3 mm-mrad/nC. Tuning to the resonant parameters has been shown to be very tolerant. Electrical breakdown is not a problem, and materials are available to satisfy the secondary emission yield requirements for this device. Applications are accelerator injectors and RF generation at a high harmonic of the fundamental frequency