Thomson scattering measurements of supersonic tungsten plasma flow interpenetration in wire array z-pinches

We present direct Thomson Scattering measurements of flow interpenetration and heating in the axial region of tungsten wire array z-pinches. These experiments were carried on the MAGPIE (1.4MA, 240ns) Pulsed Power generator at Imperial College London. Measurements were taken at early times in the evolution of the arrays (~120ns), before the formation of the dense precursor column. At this time the array is producing a set of eight cylindrically converging supersonic plasma streams (v~1.5×107cms-1, ni~1017cm-3). On axis, the collision of these streams results in the formation of an azimuthally isotropic interaction region. No shock structures are observed to form at the points where the flows collide; instead the density profile (as measured by end-on interferometry) varies smoothly between the ablation streams and the inter-wire regions. The Thomson scattering geometry used in these experiments allowed independent measurements of the radial and axial velocity distribution of the plasma through the axis, based on the analysis of the form-factor of the ion feature of the TS spectra. These measurements were both temporally (4ns) and spatially resolved across the array diameter. The spectra appear to be formed of two “peaks”, corresponding to counter-streaming interpenetrating flows. The flows were observed to decelerate over an extended distance (~1.5mm), gradually heating one another to Ti ~30keV as they stagnate. The independent measurements of the axial flow velocities show, for the first time, the appearance of a significant axial flow of the ions directed from cathode to anode, against the applied electric field. A magnetic field of ~20T is required to produce the observed deflection of the W ions.