The formation of reverse shocks in magnetized high energy density supersonic plasma flows

Abstract

A new experimental platform was developed, based on the use of supersonic plasma flow from the ablation stage of an inverse wire array z-pinch, for studies of shocks in magnetized high energy density physics plasmas in a well-defined and diagnosable 1-D interaction geometry. The mechanism of flow generation ensures that the plasma flow (Re M $ 50, M S $ 5, M A $ 8, V flow % 100 km/s) has a frozen-in magnetic field at a level sufficient to affect shocks formed by its interaction with obstacles. It is found that in addition to the expected accumulation of stagnated plasma in a thin layer at the surface of a planar obstacle, the presence of the magnetic field leads to the formation of an additional detached density jump in the upstream plasma, at a distance of $c/x pi from the obstacle. Analysis of the data obtained with Thomson scattering, interferometry, and local magnetic probes suggests that the sub-shock develops due to the pile-up of the magnetic flux advected by the plasma flow. V C 2014 AIP Publishing LLC. [http://dx.

Diagnosing collisions of magnetized, high energy density plasma flows using a combination of collective Thomson scattering, Faraday rotation, and interferometry (invited)

“…This setup is interesting as it allows the study of the interaction of magnetised flows with external obstacles. 7 These data demonstrate the dynamic range of sensitivity achieved by this interferometer; the peak density in this plot, ∼5 × 10 18 cm −2 corresponds to ∼20 fringe excursions, while the accuracy of the analysis method is conservatively estimated at ∼1/5 of a fringe shift (∼4 × 10 16 cm −3 ). This corresponds to a dynamic range of ∼100.…”

“…As an example, Figure 2 contains 1053 nm interferometry data captured during an experiment carried out to measure the B field embedded in an outflow of plasma produced by an inverse cylindrical wire array; 7,19,20 (b) shows a sideon schematic view of this load configuration, (c) shows the n e dl map that results from analysis of interferograms (d) and (e) using the methods described above. The inverse wire array produces ablation streams which are accelerated radially away from the array axis due to the reversal of the standard wire array z-pinch magnetic field topology.…”

“…This is achieved using laser interferometric imaging. The interferometry diagnostic has been used extensively in previous Magpie experiments to study the interactions of wire array ablation flows, 6,8 the interactions of flows with solid targets, 7 and the interactions of jets with ambient media. 10,14 Combining the information acquired through these different diagnostic techniques allows additional constraints to be applied during analysis of experimental data, which in turn leads to a more accurate interpretation of the results.…”

“…These flows are produced using the Magpie 5 pulsed power generator at Imperial College (1.4 MA, 240 ns); the flows are accelerated by the J × B forces that arise due to the interaction of the current conducted by the plasma with the associated selfmagnetic field. Careful design of the load allows the geometry of the flows to be controlled and focused in order to produce quasi-1D flows, 6,7 cylindrically symmetric interactions, 8 or jet into ambient medium interactions. 9,10 The collisional scale lengths of the interactions can be tuned both through the choice of the element used to produce the plasmas (typically tungsten or aluminium) and through control of the timing of measurements, allowing the study of both shock formation and long-range flow interpenetration.…”

Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows

“…The ablated plasma is accelerated away from the wires, reaching a velocity of ∼50 km=s within the first 1-2 mm, and thereafter propagates with an almost constant velocity. Previous measurements [19,20] have demonstrated that the plasma flows generated by a single exploding wire array have a frozen-in, azimuthal, advected magnetic field (B ∼ 2T), and are super-fast-magnetosonic. The arrays used in the current experiments consist of 16, 40 μm-diameter Al wires, 16 mm in length, positioned on a 16 mm diameter [ Fig.…”