Two-dimensional PIC simulation of some narrowband and ultra-wideband HPM sources

We have performed two-dimensional, relativistic, electromagnetic particle-in-cell simulations for two kinds of high-power microwave systems. The first is a virtual cathode oscillator (vircator). The second is an ultra-wideband source that uses ionization in a gas-filled waveguide to cut-off the tail of an incident narrowband wave-train, leading to an increase in bandwidth and an upshift in the central frequency. These simulations have been done using a locally modified form of the XOOPIC code. With an copper foil anode, the output frequency of the vircator is found to be strongly dependent upon the variation of foil transparency with electron energy. Using an average transparency for all electron energies yields results that are markedly different from those obtained using the actual variation. However, the output power is only mildly sensitive. Hence the transparency has been calculated using a Monte Carlo code for electron transport through solids. Using the full energy-dependent form of the transparency, we get fairly good agreement with published experimental results. The second problem involves modeling the self-consistent passage of a narrowband wave-train through an argon gas-filled waveguide. The particle-in-cell model includes relativistic effects as well as the effect of a non-Maxwellian electron distribution upon the ionization process, both of which are important at the high electric fields of interest. The simulated bandwidth broadening and upshift in central frequency are reasonably in line with those reported in the literature. This paper reports on the details of the changes made in XOOPIC and our results for both systems.