Another simulation LPPFusion is working on is modeling the early stages of the current sheath within the plasma. This original simulation software is being developed by LPPFusion Simulations Researcher Dr. Warwick Dumas. It differs from the most common plasma simulations, called MHD, in accurately distinguishing the way electrons and ions behave when they are moving in the direction of the magnetic field, as compared with when they are moving across the field. The MHD approximation assumes that the particle acts the same way in all directions. This assumption leads to wrong models in our plasma focus device, because of its powerful magnetic fields. However, abandoning this simplification makes the computation of the simulation far more difficult, so Dr. Dumas has been working on the algorithms for some time.
In the past month, the simulation has been able to model the development of the current sheath in the first 220ns of the pulse, bringing the model up to the starting time of our ICCD image series. So, we will soon be able to compare simulations with observations. It is easy in the simulation to switch on or off the oscillations in the total current that we observe with our real machine. If the simulation models are validated, we may be able to get more insight into the effects of the oscillations and how to avoid them.
Fig. 4 Frames from a recent run of Dr. Dumas’ simulation show a horizontal cross section of the region within a cm of the insulator around the anode. The insulator edge is toward the bottom of the frame. Increasing heights and deeper colors show increasing levels of ionization—the proportion of atoms that have lost their electrons. As the simulation progresses, ionization levels first rise smoothly over a large region. But then a sharp ridge of ionization, which appears to be forming filaments of current, boosts the ionization toward 100%. This ridge may correspond to the bright line seen in the first frame of our ICCD video in Fig. 1. More simulations are needed to be sure.