We characterize the plasma flows generated during the ablation stage of an
over-massed exploding planar wire array, fielded on the COBRA pulsed-power
facility (1 MA peak current, 250 ns rise time). The planar wire array is
designed to provide a driving magnetic field (80-100 T) and current per wire
distribution (about 60 kA), similar to that in a 10 MA cylindrical exploding
wire array fielded on the Z machine. Over-massing the arrays enables continuous
plasma ablation over the duration of the experiment. The requirement to
over-mass on the Z machine necessitates wires with diameters of 75-100 μm,
which are thicker than wires usually fielded on wire array experiments. To test
ablation with thicker wires, we perform a parametric study by varying the
initial wire diameter between 33-100 μm. The largest wire diameter (100
μm) array exhibits early closure of the AK gap, while the gap remains open
during the duration of the experiment for wire diameters between 33-75 μm.
Laser plasma interferometry and time-gated XUV imaging are used to probe the
plasma flows ablating from the wires. The plasma flows from the wires converge
to generate a pinch, which appears as a fast-moving (V≈100
kms−1) column of increased plasma density (nˉe​≈2×1018 cm−3) and strong XUV emission. Finally, we compare the results
with three-dimensional resistive-magnetohydrodynamic (MHD) simulations
performed using the code GORGON, the results of which reproduce the dynamics of
the experiment reasonably well.Comment: 14 pages; 14 figure