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1. The structure of 3-D collisional magnetized bow shocks in pulsed-power-driven plasma flows

   https://doi.org/10.1017/S0022377822001118  

   Datta, R.; Russell, D.R.; Tang, I.; Clayson, T.; Suttle, L.G.; Chittenden, J.P.; Lebedev, S.V.; Hare, J.D. (December 2022, Journal of Plasma Physics)

   We investigate three-dimensional (3-D) bow shocks in a highly collisional magnetized aluminium plasma, generated during the ablation phase of an exploding wire array on the MAGPIE facility (1.4 MA, 240 ns). Ablation of plasma from the wire array generates radially diverging, supersonic ( $$M_S \sim 7$$ ), super-Alfvénic ( $$M_A > 1$$ ) magnetized flows with frozen-in magnetic flux ( $$R_M \gg 1$$ ). These flows collide with an inductive probe placed in the flow, which serves both as the obstacle that generates the magnetized bow shock, and as a diagnostic of the advected magnetic field. Laser interferometry along two orthogonal lines of sight is used to measure the line-integrated electron density. A detached bow shock forms ahead of the probe, with a larger opening angle in the plane parallel to the magnetic field than in the plane normal to it. Since the resistive diffusion length of the plasma is comparable to the probe size, the magnetic field decouples from the ion fluid at the shock front and generates a hydrodynamic shock, whose structure is determined by the sonic Mach number, rather than the magnetosonic Mach number of the flow. The 3-D simulations performed using the resistive magnetohydrodynamic (MHD) code Gorgon confirm this picture, but under-predict the anisotropy observed in the shape of the experimental bow shock, suggesting that non-MHD mechanisms may be important for modifying the shock structure. 

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2. Fueling limits in a cylindrical viscosity-limited reactor

   https://doi.org/10.1063/5.0101271  

   Rubin, T.; Kolmes, E. J.; Ochs, I. E.; Mlodik, M. E.; Fisch, N. J. (August 2022, Physics of Plasmas)

   Recently, a method to achieve a “natural hot-ion mode” was suggested by utilizing ion viscous heating in a rotating plasma with a fixed boundary. We explore the steady-state solution to the Braginskii equations and find the parameter regime in which a significant temperature difference between ions and electrons can be sustained in a driven steady state. The threshold for this effect occurs at [Formula: see text]. An analytic, leading order low flow solution is obtained, and a numerical, moderate Mach number [Formula: see text] is investigated. The limitation is found to be at moderate Mach numbers. 

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3. Measurement of the Taylor scale in a magnetized turbulent laboratory plasma wind-tunnel

   https://doi.org/10.1063/5.0073207  

   Cartagena-Sanchez, C. A.; Carlson, J. M.; Schaffner, D. A. (March 2022, Physics of Plasmas)

   The fluid Taylor scale is measured in the Bryn Mawr Experiment (BMX) of the Bryn Mawr Plasma Laboratory and examined as a potential dissipation scale of magnetic turbulence within the plasma. We present the first laboratory measurements of the Taylor scale of a turbulent magnetized plasma through multi-point correlations of broadband magnetic fluctuations. From spatial and temporal correlations, respectively, the measured Taylor scales are [Formula: see text] and [Formula: see text]. These measurements are on the same order of magnitude as estimated ion dissipation scales within the BMX plasma with ion inertial scales between [Formula: see text] and ion gyroscales between [Formula: see text]. From these measurements, a magnetic Reynolds number can be computed. Since Taylor scale values are determined using multi-point correlations and a Richardson extrapolation technique, an estimate of the magnetic Reynolds number can be found without the added complication of specifying a model of microscopic diffusivity, a parameter often difficult to obtain experimentally. 

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4. Electron Heating in the Transrelativistic Perpendicular Shocks of Tilted Accretion Flows

   https://doi.org/10.3847/1538-4357/ad4607  

   Sironi, Lorenzo; Tran, Aaron (June 2024, The Astrophysical Journal)

   Abstract General relativistic magnetohydrodynamic (GRMHD) simulations of black hole tilted disks—where the angular momentum of the accretion flow at large distances is misaligned with respect to the black hole spin—commonly display standing shocks within a few to tens of gravitational radii from the black hole. In GRMHD simulations of geometrically thick, optically thin accretion flows, applicable to low-luminosity sources like Sgr A* and M87*, the shocks have transrelativistic speed, moderate plasma beta (the ratio of ion thermal pressure to magnetic pressure isβ_pi1∼ 1–8), and low sonic Mach number (the ratio of shock speed to sound speed isM_s∼ 1–6). We study such shocks with 2D particle-in-cell simulations, and we quantify the efficiency and mechanisms of electron heating for the special case of preshock magnetic fields perpendicular to the shock direction of propagation. We find that the postshock electron temperatureT_e2exceeds the adiabatic expectationT_e2,adby an amount $T_{\mathrm{e}2}/T_{\mathrm{e}2,\mathrm{ad}}-1\simeq 0.0016M_s^{3.6}$, nearly independent of the plasma beta and of the preshock electron-to-ion temperature ratioT_e1/T_i1, which we vary from 0.1 to unity. We investigate the heating physics forM_s∼ 5–6 and find that electron superadiabatic heating is governed by magnetic pumping atT_e1/T_i1= 1, whereas heating byB-parallel electric fields (i.e., parallel to the local magnetic field) dominates atT_e1/T_i1= 0.1. Our results provide physically motivated subgrid prescriptions for electron heating at the collisionless shocks seen in GRMHD simulations of black hole accretion flows. 

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5. Ion shock layer formation during multi-ion-species plasma jet stagnation events

   https://doi.org/10.1063/5.0087509  

   Mohammed, A. I.; Adams, C. S. (July 2022, Physics of Plasmas)

   We report the characteristics of collisional plasma shocks formed during interactions between low density (ne≈1015 cm−3), low temperature (Te≈2 eV), high velocity (30 km s−1), plasma jets and stagnant plasma of similar parameters. This investigation seeks to probe the structure of shocks in multi-ion-species plasmas, in particular, the presence of gradient-driven ion species separation at the shock front. The railgun-accelerated jets utilized here have previously been shown to exist in a collisional regime with intra-jet collisional mean-free-path substantially smaller than jet size [Schneider et al., Plasma Sources Sci. Technol. 29, 045013 (2020)]. To induce collisions, a dielectric barrier is located downstream of the railgun to stagnate an initially supersonic plasma jet. Around the time of stagnation, the railgun emits a second jet which shortly collides with the stagnant plasma. The presence of a structure emitting in the UV-visible band is evident in high-speed photographs of the moments immediately following the arrival of the second jet at the stagnant plasma. Analysis of interferometric and spectroscopic data suggests that the observed increase in density from the jet to the post-collision plasma is consistent with the formation of a bow shock structure with a multi-millimeter-scale ion shock layer. 

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