Secondary virtual-cathode formation in a low-voltage vircator: PIC simulations

We have performed 2-D relativistic electromagnetic axisymmetric particle-in-cell simulations for an axially extracted vircator. During periods of significant microwave emission, we observe the formation of a secondary smaller virtual cathode (VC) at some distance from the main VC. For a given vircator geometry, this appears only during low-voltage operation. The electron density in this secondary structure tends to increase with the microwave emission. We have also provided a qualitative explanation for the creation of this secondary structure and for its appearance during low-voltage operation. We find that electron trajectories in the vircator can be divided into four broad categories, based on their shapes and also on the temporal variation of electron kinetic energy. Three of these trajectories are those reported earlier by Alyokhin, while the fourth appears to be a new one. The fourth type is linked to the appearance of the secondary VC and, hence, becomes significant only at times of significant power emission.