Simulations of H/sub 2/ layers in applied-B ion diodes

Impedance collapse and parasitic ion production have been observed on high-power ion diodes at Sandia National Laboratories with a lithium ion source. A likely cause for this behavior is the presence of contaminant material adsorbed on the anode surface. We have implemented (into the electromagnetic particle-in-cell code IVORY) new modeling to study the effects of hydrogen gas interaction and transport. Using this new capability, we have calculated degraded diode performance when >2 monolayers of hydrogen are present on the anode surface. The qualitative agreement of these simulations with experimental results suggests that a thermally-desorbed neutral layer on the anode may be responsible for the inefficient coupling of diode energy to lithium ions. This material is likely desorbed from the surface as it is heated by cathode electron bombardment. Thus, cleaning of the anode surface and bulk should improve diode performance.