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. The authors have studied this open-quotes multipactingclose quotes 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 open-quotes bunching,close quotes 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