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Experimental Investigation of the Properties and Phase State of Thick Aluminum Surfaces Pulsed to Megagauss Level Magnetic Field in a Z-Pinch Geometry
AuthorAwe, Thomas J.
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Thermal transformation to plasma of an aluminum surface pulsed to multi-megagauss magnetic field is observed to occur when the surface field reaches a threshold level of 2.2 MG. Magnetic field is pulsed on the surface of cylindrical metallic rods. Rods are thick, with radii exceeding the magnetic field penetration depth. Ohmic heating is confined to a skin layer, with the magnetic field penetration depth determined by diffusion and hydrodynamic processes. Initial rod diameters ranging from 2.00 to 0.50 mm are pulsed with 1.0 MA peak current by the Zebra z-pinch. Due to the high transmission line impedance (1.9 ohm) of Zebra, the current waveform is insensitive to the initial rod diameter. The Zebra current consistently rises exponentially to 100 kA (with rise time, tau=13 ns), and then linearly from 100 to 900 kA for 70 ns to a maximum current of 1.0 MA. By altering the initial rod diameter, a variety of magnetic field and current density profiles are examined. For initial rod diameters of 2.00 and 0.50 mm, magnetic field rise rates vary from 30 and 80 MG/microsecond, and peak surface fields reach 1.5 and 4 MG, respectively. Novel contact configurations and load surface profiles mitigate plasma formation from contact arcing or electric-field-driven electron avalanche, ensuring that plasma forms thermally, as a result of ohmic or compression heating.Aluminum plasma is observed through a variety of independently measured phenomena. First, for rod surfaces pulsed above the magnetic field threshold of 2.2 MG, multi eV brightness temperatures are observed, clearly indicating plasma for aluminum. For example, peak brightness temperatures reach 20 and 36 eV for 1.00 and 0.50 mm rods, respectively. Plasma forms at lower current and reaches higher temperatures as the intial rod diameter is decreased. Second, aluminum ion species are distinguished via extreme ultraviolet (EUV) spectroscopy. Line spectra from Al-3-plus and Al-4-plus ions are obtained. The average ion charge and line ratios depend strongly upon temperature, and taking the ratio of line intensities results in an estimated peak plasma temperature of 15 eV for 1.00-mm-diameter rods. Third, EUV photon flux consistent with multi-eV temperature is recorded by Al or Si/Zr filtered photodiodes sensitive to photon energies from 16 to 73 eV, or 60 to 100 eV, respectively. Fourth, magnetohydrodynamic (MHD) instabilities form. Instability development depends on the conductivity of the low density expanding surface material. High resistivity vapor interacts weakly with magnetic field; therefore, flute instabilities are attributed to surface plasma. For those rods which do not reach the 2.2 MG magnetic field threshold, no evidence of surface plasma is obtained. For 2.00-mm-diameter rods, which reach peak surface field of only 1.7 MG, surface temperatures remain cool (peak brightness temperatures reach 0.7 eV), no EUV emission can be measured, and even while carrying 1.0 MA of current, and after significant radial expansion, no surface instability is observed. The experiment offers the first detailed study of plasma formation by pulsed magnetic field on a thick metallic surface carrying a skin current. The magnetic field threshold for plasma formation, surface brightness temperature, radial expansion velocity, instability growth, and ionization state have been measured. The effects of hardware design, load geometry, Al alloy, and surface smoothness have been carefully examined, creating a dataset that can be used for the design of practical systems. The experiment has achieved thermal, uniform, and symmetric plasma formation, providing a meaningful comparison for MHD simulations.