Study of plasma dynamics affected by a global magnetic field in linear wire array Z pinches

Abstract

In the experiments described in this paper a linear wire array composed of several wires (e.g., four wires) was viewed as a small portion of a large cylindrical array. Comparing to cylindrical arrays, linear wire arrays have relatively simple geometry and therefore are much easier to diagnose. To simulate the global magnetic field present in a cylindrical array, a return current conductor was positioned near the linear wire array. A global magnetic field in the tens of Tesla was produced by the return current in the vicinity of the linear array. The plasma dynamics affected by the global magnetic field was studied using shearing interferometry and x-ray backlighting techniques. Experimental results on tungsten wire arrays (four wires, 1.4cm long, 12.7μm diameter, 1mm interwire gaps) are discussed. Current transfer from wire cores to the surrounding corona plasmas appeared to happen at around 10ns relative to the start of the current pulse. After that, some of the wire plasma was swept away from the wire cores by the J×B force and formed a localized, high-density peak (precursor plasma). The motion of the precursor plasma was observed to accelerate for about 20ns, indicating that current was present inside the precursor plasma during this time range. Using an x-ray backlighter and a laser interferometer, the resumption of wire core expansion, major mass ablation, and the termination of precursor plasma acceleration were observed to occur at 32–34ns. These effects can be interpreted as evidence of the transfer of the current back to the wire cores from the precursor plasma.