Numerical and experimental efforts to explain delayed gas breakdown in θ-pinch devices with bias magnetic field

A single particle model and particle-in-cell simulations have been used to elucidate the breakdown physics in a ringing theta-pinch with a bias magnetic field. Previous experimental results show that gas breakdown occurs when the bias magnetic field is nullified by the theta-pinch magnetic field. The analyses presented here agree with the experimental results and show that electron kinetic energy does not exceed the ionization threshold of deuterium until the net magnetic field is approximately zero. Despite the presence of a strong electric field, the gyromotion of electrons within the bias magnetic field prevents them from gaining energy necessary to ionize the gas. Parametric analysis of the peak electron energy as a function of the bias and pre-ionization magnetic fields reveals that: (1) when the bias magnetic field is ≈ 97% of the pre-ionization magnetic field, peak elec- tron energies are highly erratic resulting in poor overall ionization, and (2) full ionization with repeatable behavior requires a pre-ionization to bias magnetic field ratio of approximately 2 to 1 or higher. Efforts to better characterize this phenomena experimentally are ongoing. However some preliminary findings using a dual-probe cancellation technique are presented.