Effect of polarity on beam and plasma target formation in a dense plasma focus
Dense plasma focus (DPF) devices are conventionally operated with a polarity such that the inner electrode (IE) is the anode. It has been found that interchanging the polarity of the electrodes (i.e., IE as the cathode) can cause an order of magnitude decrease in the neutron yield. This polarity riddle has previously been studied empirically through several experiments and is yet not well understood. Herein, we have performed kinetic simulations using the particle-in-cell modeling to investigate the problem. This is the first time that both polarities have been studied with simulations in great detail. In our simulations, we have modeled the entire beam and plasma target formation processes, but we did not consider differences in break-down conditions caused by the two polarities. We have found that when using reverse polarity ions are still accelerated and, in fact, attain similar energy spectra as in the standard polarity case. The difference is that the fields are flipped and thus ions are accelerated in the opposite direction. So, in the reverse polarity case, the majority of the “plasma target” (formed by the imploding plasma) is in the opposite direction of the beam, and thus, the beam hits the IE and produces few neutrons. With a better inner electrode configuration, reverse polarity is able to create a high-quality ion beam as well as a high-density target. Both can be comparable to that generated by standard polarity. Furthermore, we will show that it is easier to add an additional solid catcher target to a DPF device with reverse polarity, potentially enabling it to generate more neutrons than standard polarity.