Significant change in threshold for plasma formation and evolution with small variation in copper alloys driven by a mega-ampere current pulse

Abstract

We report the first observation of a significant change in plasma formation and evolution caused by a small change in the material composition (metal alloy). Thick copper rod alloys were varied in the initial diameter from 0.5 mm to 1.59 mm and pulsed to 1 mega-ampere (MA) peak current in 100 ns with a surface magnetic field rising nearly linearly at 45–80 MG/μs. The copper rods were diagnosed with surface radiometry, two-frame laser shadowgraphy, two-frame surface emission imaging, and time resolved load current measurements. Diagnostics determined time resolved surface temperature, surface expansion rates, time of surface plasma formation, and the growth rates of surface instabilities. Data indicate that copper alloy Cu-145 (99.5% Cu, 0%–0.7% Te, 0%–0.012% P) undergoes surface plasma formation when the surface magnetic field reaches 3.0 ± 0.1 MG whereas Cu-101 (>99.99% Cu) forms surface plasma at 3.5 ± 0.2 MG. The Cu-145 alloys also reach higher peak temperatures and form surface instabilities earlier in the current pulse.