Plasma Jet Formation Disruption From a Critical Applied Uniform Axial Magnetic Field

We examine the effects of varying the magnitude of an applied, uniform, axial magnetic field ( B_{z} ) on the formation of laboratory plasma jets produced by a 1-MA 100-ns rise time pulsed power generator in a radial foil configuration. A Helmholtz coil applies the external magnetic field (0 to 2 T). With a small-enough applied field, the foil surface ablation is relatively azimuthally uniform, and J\times B forces drive the ablated plasma inward and upward to form a well-defined azimuthally symmetric jet. Using applied field strengths larger than a critical B_{z} = 1.1\pm 0.1 T with aluminum foils, the plasma ablation is not azimuthally uniform, discrete bursts of plasma initiate from the foil surface, and the formation of a well-defined jet is disrupted. The critical B_{z} for this plasma jet formation disruption correlates with the foil material's electrical resistivity and equation of state (EOS). To better understand the material-dependent phenomenon, we compare the experimental results with 3-D extended magnetohydrodynamics simulations of ablation of a 2 mm \times 2 mm by 25- \mu \text{m} slab that represents a section of the radial foil. The simulations initialize the slab in the solid state and include the material resistivity and EOS from the solid to plasma phases. As is observed in the experimental disruption, these simulations show enhanced nonuniform plasma ablation with an applied B_{z} , which can inhibit the azimuthal uniformity necessary to produce a well-defined plasma jet. Furthermore, the simulations also show a material and resistivity dependence on the ablation process similar to the trend shown by the experiments. These results demonstrate the necessity of accurate, detailed modeling of material properties during the transition from solid to plasma phases.