The structure of bow shocks formed by the interaction of pulsed-power driven magnetised plasma flows with conducting obstacles

Abstract

We present an experimental study of the development and structure of bow shocks produced by the interaction of a magnetised, collisional, super-Alfv enic plasma flow with conducting cylindrical obstacles. The plasma flow with an embedded, frozen-in magnetic field (Re M $ 20) is produced by the current-driven ablation of fine aluminium wires in an inverse, exploding wire array z-pinch. We show that the orientation of the embedded field with respect to the obstacles has a dramatic effect on the bow shock structure. When the field is aligned with the obstacle, a sharp bow shock is formed with a global structure that is determined simply by the fast magneto-sonic Mach number. When the field is orthogonal to the obstacle, magnetic draping occurs. This leads to the growth of a magnetic precursor and the subsequent development of a magnetised bow shock that is mediated by two-fluid effects, with an opening angle and a stand-off distance, that are both many times larger than in the parallel geometry. By changing the field orientation, we change the fluid regime and physical mechanisms that are responsible for the development of the bow shocks. MHD simulations show good agreement with the structure of well-developed bow shocks. However, collisionless, two-fluid effects will need to be included within models to accurately reproduce the development of the shock with an orthogonal B-field. Published by AIP Publishing.

Interactions of magnetized plasma flows in pulsed-power driven experiments

“…The supersonic and super-Alfvènic velocities which can be achieved by the flow from an inverse wire array (see table 1) make it suitable for creating magnetized shock structures. Experiments were carried out to study magnetized shocks in a simple interaction geometry by placing either a planar or cylindrical obstacle normal to the path of the plasma flow [figure 4(a)] [19,26,43]. In order to ensure that the magnetic field played a role in the structure of this shock, a conducting (metallic) obstacle was used, as this provided a boundary to prevent the magnetic flux diffusing through the obstacle surface, therefore forcing the flux to accumulate with the post-shock plasma.…”

“…Parameters of the plasma flow from inverse arrays of carbon (C), aluminium (Al) and tungsten (W) wires[13,19,20,[24][25][26][27]. Ranges indicate variability of the flow parameters over radial distance from the wires (~5-15 mm) and over the experimental timescale (~100-400 ns).…”

Collective optical Thomson scattering in pulsed-power driven high energy density physics experiments (invited)

“…In the experimental setup careful choice is made of the probe and collection directions (̂ and ̂ respectively), to select the most appropriate direction of K (= − ) for the velocity component of interest, as well as to optimize sensitivity to the temperature parameters through the magnitude of Kwhich affects both the spectral resolution by broadening of the spectra ( = • ) and the TS regime by adjustment of the α-parameter 17 . For the typical parameters obtained in our experiments, α-values lie in the range 0.5 -3 [18][19][20][21][22][23][24] .…”

Hydrodynamic and magnetohydrodynamic simulations of wire turbulence

“…This is especially true since at least some degree of ablation could occur experimentally. While ablated mass in experiments such as Burdiak et al (2017) would probably have been small on the time-scale of the experiment, the situation could be quite different in the case where the wires are "preconditioned," e.g. by a separate current pulse, and converted into a lower density vapor state.…”

“…Earlier work by Meinecke et al (2014) showed that MHD turbulence could be achieved after a laser driven plasma flow generated from a carbon rod had been passed through a grid. Experiments to generate MHD turbulence could also be performed using magnetized plasma flows generated with pulsed power drivers (Lebedev et al 2014;Bott-Suzuki et al 2015;Burdiak et al 2017;Lebedev et al ress) In this paper, we report on simulations that also used a grid to generate turbulence from an initially laminar flow. Using the adaptive mesh refinement (AMR), MHD code As-troBEAR, we tracked the evolution of the flow to explore the conditions under which turbulence could be generated.…”