Thermal ionization of an aluminum surface by pulsed megagauss field

Abstract

When, where, and how plasma forms on metal surfaces driven by intense current are important questions for both basic science and applications. The question of the conductivity of a metal surface under pulsed megagauss magnetic field has been posed since at least 1959, when Fowler et al.1 produced fields above 10 MG. The thermal ionization of the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, is being investigated with well-characterized experiments2,3 and detailed 1-D and 2-D numerical modeling4–6. Aluminum rods with radii larger than the magnetic skin depth are pulsed with the 1.0-MA, 100-ns Zebra generator. A novel mechanical connection eliminates nonthermal precursor plasma, which in earlier experiments was produced by electric-field-driven electron avalanche and arcing electrical contacts. The surface was examined with time-resolved imaging, pyrometry, spectroscopy, and laser shadowgraphy. Thermal plasma forms when the surface magnetic field reaches 2.0 MG, in agreement with recent theoretical results4. Measurement of the surface temperature, expansion velocity, and ionization state, as a function of applied field, constrains the choice of models used in the radiation-magnetohydrodynamic simulations, which include the Eulerian MHRDR code and the Lagrangian RAVEN and UP codes. Numerical predictions can vary by orders of magnitude, but, for MHRDR modeling, the computed times of plasma formation agree well with observations if a standard SESAME Maxwell-construct EOS is used in conjunction with a VNIIEF resistivity model. An analytic calculation indicates ohmic heating should produce plasma, consistent with numerical and experimental observations.