The Formation of Precursor Structures in Cylindrical and “4 $\times$ 4” Wire Arrays

This paper summarizes observations of precursor-column formation in cylindrical wire arrays on the 1-MA MAGPIE generator at Imperial College London. Direct experimental measurements of the diameter variation during the collapse and formation phase of the precursor column taken from gated soft X-ray images are presented. Three stages in precursor-column formation are identifiable from the data: broad initial density profile, rapid contraction to small diameter, and slow expansion after formation. The variation in the minimum diameter is measured for several materials, and data show good agreement with a pressure-balance model. Stability calculations suggest that the fraction of the drive current flowing the precursor column is $<$ 1% for Al arrays and $\sim$2% for W. “4 $\times$ 4” arrays show the formation of local precursor columns close to the wire position, which exhibit differences in stability depending on the wire separation. This may be related to deceleration of the plasma flow by the B-field or current convection in the flow.