Plasma Formation and Evolution on the Surface of Copper 101 & 145 Alloys Driven by a Mega-Ampere Current

Kevin Yates, University of Nevada, Reno

Thursday September 10, 2015, 2:00 – 3:00 p.m

EBUII, Room 479

ABSTRACT: The interplay between an ohmically heated conductor and a magnetic field is important for the field of Magnetized Target Fusion (MTF).  MTF compresses a magnetized fuel by imploding a flux conserving metal liner.  During compression, fields reach several megagauss, with a fraction of the flux diffusing into the metal liner.  The magnetic field induces eddy currents in the metal, leading to ionization and potential mixing of metal contaminant into the fusion fuel.  A platform has been established at the University of Nevada, Reno Terawatt Facility to study the diffusion of megagauss magnetic field into conductors.  The Zebra z-pinch provides a reproducible 1-MA, 100-ns current pulse.  The intense current produces megagauss surface magnetic fields that diffuse into the load, ohmically heating the metal until plasma forms.  With the novel “barbell” load design, plasma formation in the region of interest due to contact arcing or electron avalanche is avoided, allowing for the study of ohmically heated loads.

During the current rise, the metal is heated to temperatures that cause multiple phase changes.  When the surface magnetic field reaches a threshold, the metal ionizes and the plasma becomes pinched against the underlying cold liquid metal.  Diagnostics fielded have included visible light and EUV radiometry, two-frame shadowgraphy (266 and 532 nm wavelengths), laser interferometry, time gated EUV spectroscopy, single frame/2ns gated visible imaging, 16 frame/4ns gated visible imaging, and visible streaked imaging.  Surface temperature, expansion speeds, instability growth, time of plasma formation, and plasma uniformity are determined from the data.

Presented here is the first observation of a significant change in the threshold for plasma formation caused by a small change in material composition (metal alloy).  This is important because applications often use alloys such as Cu-145 or Al-6061 that are easier to machine, while MHD numerical simulations typically model pure elements (e.g., Cu or Al), for which tabulated material properties are available.  Here we show that, when the applied magnetic field rises linearly at 30-80 MG/μs, copper alloy Cu-145 (99.5% Cu, 0-0.7% Te, 0-0.012% P) undergoes bulk surface ionization at 3.0. MG, whereas a purer alloy, Cu-101 (>99.99% Cu), only turns to plasma when the magnetic field exceeds 3.7 MG.