External Magnetic Field Effects on Foil Ablation Relating to Plasma Jet Disruption

In the present research, plasma jets form from Joule heating and ablation of a radial foil (approximately 15 μm thin disk) using a pulsed power generator (COBRA) with 1 MA peak current and 100 ns rise time. We study the effects on jet dynamics resulting from varying an applied uniform axial magnetic field (Bz) from 0 to 2 T. We empirically observe a disruption of the plasma jet collimation, for which plasma is ej ected from the foil as discrete bursts and does not form an azimuthally symmetric plasma jet. The critical Bz for the disruption depends upon the foil material (Al, Ti, Ni, Cu, Zn, Mo, W). The disruption initiates from the foil surface and is dependent upon material properties such as electrical conductivity and equation of state. The applied Bz acts with the higher conductivity materials to facilitate nonuniform plasma and current filamentation that breaks the azimuthal symmetry needed to form the plasma jet. The plasma filamentation likely originates from initial perturbations (in density and resistivity) in the higher-density solid-liquid phases. 3D numerical simulations in Cartesian coordinates (using the extended magnetohydrodynamics code, PERSEUS) of a section of the ablating foil reproduce some of the experimental trends including the external magnetic field dependence and material resistivity dependence of the generation of plasma and current nonuniformities.