An Analysis of RT Instability in Stagnated Magnetized Plasmas

The structure of stagnation regions formed in the interaction of supersonic plasmas with an obstacle can be strongly affected by the presence of the advected magnetic field in the flow. Here, we present studies of stagnation layer formed in collision of super magneto-sonic plasmas (B ∼ 5T, M_A ∼ 2, M_S ∼ 6) with planar conducting obstacles. The plasma flow (n_e ∼ 10¹⁸ cm^-3, V ∼ 70 km s^-1) is formed by the ablation of metallic wires (Al, Cu, W) using an inverse wire array configuration at the MAGPIE generator (1.4MA, 240ns).

In the case of Aluminium, we observe a well-defined stand-off shock at ∼ 3 mm from the planar wall due to the pile-up of the advected magnetic field, and the development of small-scale instabilities at the stagnated plasma. No presence of stand-off shock is instead observed in Tungsten flows, where the decelerated plasma is mainly dominated by strong density perturbations with k-vector parallel to [V x B] direction, extending for ∼ 5 mm from the obstacle in both the stagnation layer and the magnetic field pile-up region.

Preliminary analysis of the experimental data suggests that the Magnetic Rayleigh-Taylor instability in radiatively cooled plasmas is responsible for the observed structure of the stagnation layer.