Plasma ablation and precursor column formation in wire-array Z-pinches

This paper summarises the present understanding of the processes leading to precursor column formation in cylindrical wire arrays on the 1MA MAGPIE generator at Imperial College London. The presence of precursor plasma affects the interaction of the wire array with on-axis targets, such as in the dynamic hohlraum system, and so good characterisation is essential for such experiments. Column formation parameters are directly determined by the collisionality of the plasma streams ablated from the wires as they arrive at the array axis, which depends on the array material, drive current and velocity. Direct experimental measurements of the diameter variation during the collapse and formation phase of the precursor column will be presented, along with soft x-ray emission, and quantitative radiography. The correlation of emission to column contraction indicates a non-linear collapse as a result of increasing on-axis density and radiative cooling. Differences in the formation and late-time behaviour for tungsten and aluminium arrays are observed, and characteristic values for several common materials are seen to vary according to atomic mass. Data is in good agreement with hydrodynamic code, and a recently published kinetic description of the precursor column, which predict many of the observed features.