Three-dimensional particle-in-cell simulations of applied-B ion diodes on PBFA II

Summary form only given. We are using the 3-D, particle-in-cell code QUICKSILVER to simulate applied-B ion diodes on the PBFA II accelerator at Sandia. Typical diodes use an A-K gap of /spl sim/2 cm, anode area of /spl sim/1000 cm/sup 2/, and insulating fields of /spl sim/3 T. The accelerator delivers a 20 TW, 40 ns pulse, producing a /spl sim/10 MeV, /spl sim/1 MA Li/sup +/ beam. Spectroscopic diagnostics indicate that the LiF ion source used on most shots is not a space-charge-limited emitter, the normal electric field at the anode is 8-10 MV/cm. The field emission algorithm in QUICKSILVER has been modified to model this. We have also developed a transmission line algorithm, allowing us to crudely model the accelerator. The simulations show good agreement with experiment early in the pulse, but differ substantially as the ion beam current increases. Experimentally, the lithium beam is contaminated with hydrocarbon impurities that grow in time. Furthermore, there is strong evidence that plasmas are formed in the A-K gap from ionization of neutrals. These effects not addressed with the current simulations, which focus on idealized diode performance assuming emission of only lithium ions, directly from the anode surface.