Modeling ion diodes with dense anode plasmas

Applied-B ion diode experiments with Li+1 ion sources on the PBFA II accelerator show that early in the pulse, the beam is essentially pure Li+1, 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 stimulate these processes, the author is developing a hybrid model in a new 1-D electrostatic code, DYNAID. Each species is modeled with a fluid where the density is high, and particle-in-cell (PIC) techniques where the density is lower. Simulation particles are created from, and reabsorbed into, the corresponding fluid as required. Preliminary results are encouraging. The fluid model is stable in a numerically harsh environment, shielding out the electric field from a dense anode plasma. Results from fluid-only simulations including only charge-exchange agree well with earlier PIC code results. Finally, adding ionization can lead to an increasing beam current at the cathode under certain conditions