Modeling ion diodes with dense anode plasmas

Summary from only given. Applied-B ion diode experiments with Li/sup +/ ion sources on the PBFA II accelerator show that early in the pulse, the beam is essentially pure Li/sup +/, but is rapidly overwhelmed by impurity ions-the "parasitic load". Furthermore, the increasing parasitic current rapidly drops the diode voltage, limiting the accelerator power that can be coupled into the beam. This "impedance collapse" is believed to arise from desorption of impurity neutrals from the anode surface. These neutrals charge-exchange with the ions, rapidly expanding into the gap where they are ionized; initially by the beam ions, and later by secondary electrons as the plasma density rises. There is also evidence of dense plasmas evolving from the cathode in these experiments. To simulate these processes, we are developing a hybrid model in a new 1-D electrostatic code, DYNAID. Each species (electron, ion, or neutral) is modeled with a fluid where the density is high (>/spl sim/10/sup 14/ cm/sup -3/). and particle-in-cell (PIC) techniques where the density is lower. Simulation particles are created from, and reabsorbed into, the corresponding fluid as required.