Experimental study of plasma formation on an aluminum surface pulsed with megagauss magnetic field

Abstract

Plasma formation on the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, has been experimentally investigated. Aluminum rods with initial diameters ranging from 0.5 to 2.0 mm are pulsed with the 1.0 MA, 100 ns Zebra generator. Surface magnetic field rates of change vary from 20 to 80 MG/µs, with corresponding peak fields of 1 to 6 MG. For all rod diameters selected, the magnetic field penetration depth is smaller than the thickness of the conductor, enabling study of the material response in the “liner” or “thick-wire” regime. A novel mechanical connection was developed that eliminates non-thermal precursor plasma, which was produced by electric-field-driven electron avalanche and arcing electrical contacts in earlier experiments. The dynamics of the pulsed aluminum rod and resultant surface plasma are examined with time-resolved imaging, pyrometry, spectroscopy, and laser shadowgraphy. Thermal plasma forms when the surface magnetic field reaches 2.0 MG, with no clear dependence on the rise time of the applied field. Plasma forms at lower current and reaches higher peak temperature when the initial rod diameter is reduced. Surface temperature and instability growth both decrease dramatically when the initial rod diameter exceeds 1.6 mm, suggesting a transition to a regime in which highly resistive vapor forms, instead of plasma. For rods with initial diameter 1.25 mm or below, which clearly demonstrate surface plasma formation before peak current, maximum surface temperatures are of order 10 eV, and radial expansion velocities are approximately 2—4 km/s during the current rise. Emission lines from multiply ionized aluminum atoms are characterized through time gated EUV spectroscopy. The measurement of the time-evolution of the surface temperature, aluminum expansion rate, and ionization state, as a function of applied field, significantly constrains the choice of models used in radiation-magnetohydrodynamic simulations.