Inclusion of non-ideal effects in 3-D PIC simulations of ion diodes

The 3-D particle-in-cell code QUICKSILVER has been used extensively to simulate ion diodes. These simulations use an idealized setup; perfectly azimuthally-symmetric geometric and applied magnetic field, electrons incident on the anode are simply removed from the system, and the entire anode emission region emits only a single ion species -- directly from the physical surface throughout the simulation. Results from SABRE experiments exhibit significant differences with such ideal simulations. First, the beam has a time-dependent composition; early in time it is mostly the desired Li+ ions, but these are rapidly replaced by protons and C+n (the parasitic load). Second, the diode impedance drops rapidly after peak lithium beam power. Third, there are substantial current losses in the diode (i.e., current not carried by the ion beam). The first two discrepancies may result from formation of dense plasmas on the anode. These are believed to arise from ionization of neutral hydro-carbon impurities desorbed from the anode surface by the intense flux of high-energy electrons from the cathode size. QUICKSILVER has been modified to carefully treat electrons incident on the anode, scattering a fraction back into the diode gap, and updating the local temperature rise from electron deposition. When the temperature of an emission cell exceeds a threshold, emission switches from Li+ to parasitic ions. The model reproduces the main features of the parasitic load. The authors have also studied sources of the diode loss currents