One-dimensional particle-in-cell simulation of a current-free double layer in an expanding plasma

A one-dimensional particle-in-cell code using Monte Carlo collision techniques (MCC/PIC) for both ions and electrons is used to simulate our earlier experimental results which showed that a current-free electric double layer (DL) can form in a plasma expanding along a diverging magnetic field. These results differ from previous experimental or simulation systems where the double layers are driven by a current or by imposed potential differences. Both experiment and simulation show accelerated ions with energies up to about 60 eV on the low potential side of the plasma. A new numerical method is added to the conventional PIC scheme to simulate inductive electron heating, as distinct from the more common capacitively driven simulations. A loss process is introduced along the axis of the simulation to mimic the density decrease along the axis of an expanding plasma in a diverging magnetic field. The results from the MCC/PIC presented here suggest that the expansion rate compared to the ionization frequency is a critical parameter for the existence of the DL. For the DL to be absolutely current free, the source wall has to be allowed to charge: having both ends of the simulation at the same potential always resulted in a current flow. Also, the effect of the neutral pressure and of the size of the diffusion chamber are investigated. Finally we show that this particular type of DL has electrons in Boltzmann equilibrium and that it creates a supersonic ion beam.