Solving the Polarity Riddle in 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 cathode) causes an order of magnitude decrease in the neutron yield when using deuterium as a fill gas. This polarity riddle has previously been studied empirically through several experiments, and is yet not well understood. To understand the puzzle, we use the hybrid particle-in-cell (PIC) code Chicago to simulate both polarities and are able to reproduce this drastic reduction in yield. The PIC method captures electric fields and the acceleration of deuterons that produce neutrons in a DPF. We find 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. This knowledge is used to design a better inner electrode configuration, which allows reverse polarity to create a high-quality ion beam as well as a high-density target. Such a design is shown to generate yield comparable to standard polarity. Further, the results suggest that for some applications, making the inner electrode negative can be advantageous.