Applied Axial Magnetic Field Effects on Extended Magnetohydrodynamics Laboratory Plasma Jets: Experiments and Simulation

Laboratory experiments can help validate and benchmark computer simulations. This laboratory plasma jet research focused on characterizing and quantifying the effects of extended magnetohydrodynamics (XMHD) and an externally applied axial magnetic field (Bz). In the present research, plasma jets were formed from Joule heating and ablation of radial foils (approximately 15microm thin circular disks) using a pulsed power generator (COBRA) with 1~MA peak current and 100~ns rise time. Plasma dynamics of the jet were diagnosed under a change of current polarities, which correspond to current moving either radially outward or inward from the foil's central axis. The influence of the Hall effect on the jet development was observed under opposite current polarities, which changed the jet conical structure (width and angle). Additionally, we studied the effects on jet dynamics resulting from varying the Bz from 0 to 2~T. The plasma jet formation compressed the Bz as the plasma converged toward the central z-axis. The pressure from the B z compression led to on-axis density hollowing of the jet. Experimental measurements of this compression were made using dB/dt (or ``B-dot'') magnetic probes placed in the center of the hollow plasma jet. Additionally, we found that the plasma jet formation disrupted from the ablating foil surface with a large enough applied B z, meaning the jet was no longer well-collimated but was ejected as multiple bursts of plasma. We observed that the critical B z for disruption depended upon the foil material (Al, Ti, Ni, Cu, Zn, Mo, W) and correlated with material properties of the foil such as the electrical resistivity and equation of state. Experimental results were compared with predictions made by XMHD numerical simulations (PERSEUS). This study of 1) the Hall effect, 2) applied magnetic field effects, and 3) the process of foil ablation permitted further understanding of fundamental physics topics including 1) the importance of low-density plasmas within high-density plasma environments, 2) how magnetic forces influence plasma dynamics, and 3) the impact of material properties during transitions from the solid through plasma phases.