Perpendicular subcritical shock structure in a collisional plasma experiment

Abstract

We present a study of perpendicular subcritical shocks in a collisional laboratory plasma. Shocks are produced by placing obstacles into the super-magnetosonic outflow from an inverse wire array z-pinch. We demonstrate the existence of subcritical shocks in this regime and find that secondary shocks form in the downstream. Detailed measurements of the subcritical shock structure confirm the absence of a hydrodynamic jump. We calculate the classical (Spitzer) resistive diffusion length and show that it is approximately equal to the shock width. We measure little heating across the shock (< 10% of the ion kinetic energy) which is consistent with an absence of viscous dissipation.

Time-resolved velocity and ion sound speed measurements from simultaneous bow shock imaging and inductive probe measurements

“…The j × B force accelerates the plasma radially outwards, creating radially-diverging flows of collisional (λ mfp /L 1) plasma. 1,8,9,20,21 The magnetic Reynolds number (computed at the plasma length scale ∼ 1 cm) is typically large (R M > 10), 8,9 so the ablating plasma advects magnetic field from inside the array to the outside, creating flows with frozen-in azimuthally-oriented field lines.…”

“…These measurements are acquired using optical Thompson scattering (OTS) in experiments with aluminum exploding wire arrays of similar dimensions, also fielded on MAGPIE, and at roughly 5 mm from the wires. 8,20,21 OTS measures spatially-localized flow velocity from the Doppler shift in the spectra of light scattered by the plasma. 11 The corrected flow velocity estimated from inductive probe measurements agrees, within experimental uncertainty, with OTS measurements reported in literature.…”

“…11 The corrected flow velocity estimated from inductive probe measurements agrees, within experimental uncertainty, with OTS measurements reported in literature. 8,20,21 The ion feature of OTS also allows for the measurement of ZT e from the wavelength separation of the ion acoustic peaks in the scattered spectrum. 11 Values of ZT e inferred from OTS in similar plasmas lie in the range ZT e ∼ 28 − 40 eV, 8 which includes the experimental estimate of ZT e ∼ 22 ± 9 eV.…”