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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. Electron Heating in 2D Particle-in-cell Simulations of Quasi-perpendicular Low-beta Shocks

   https://doi.org/10.3847/1538-4357/ad1f69  

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

   We measure the thermal electron energization in 1D and 2D particle-in-cell simulations of quasi-perpendicular, low-beta (β_p= 0.25) collisionless ion–electron shocks with mass ratiom_i/m_e= 200, fast Mach number${\mathcal{M}}_{\mathrm{ms}}=1$–4, and upstream magnetic field angleθ_Bn= 55°–85° from the shock normal$\hat{\mathit{n}}$. It is known that shock electron heating is described by an ambipolar,B-parallel electric potential jump, Δϕ_∥, that scales roughly linearly with the electron temperature jump. Our simulations have$\mathrm{\Delta }{\varphi }_{\parallel }/(0.5m_{\mathrm{i}}{u_{\mathrm{sh}}}^2)\sim 0.1$–0.2 in units of the pre-shock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measureϕ_∥, including the use of de Hoffmann–Teller frame fields, agree to tens-of-percent accuracy. Neglecting off-diagonal electron pressure tensor terms can lead to a systematic underestimate ofϕ_∥in our low-β_pshocks. We further focus on twoθ_Bn= 65° shocks: a${\mathcal{M}}_{\mathrm{s}}=4$(${\mathcal{M}}_{\mathrm{A}}=1.8$) case with a long, 30d_iprecursor of whistler waves along$\hat{\mathit{n}}$, and a${\mathcal{M}}_{\mathrm{s}}=7$(${\mathcal{M}}_{\mathrm{A}}=3.2$) case with a shorter, 5d_iprecursor of whistlers oblique to both$\hat{\mathit{n}}$andB;d_iis the ion skin depth. Within the precursors,ϕ_∥has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the${\mathcal{M}}_{\mathrm{s}}=4$,θ_Bn= 65° case,ϕ_∥shows a weak dependence on the electron plasma-to-cyclotron frequency ratioω_pe/Ω_ce, andϕ_∥decreases by a factor of 2 asm_i/m_eis raised to the true proton–electron value of 1836.

    

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3. Simulations of radiatively cooled magnetic reconnection driven by pulsed power

   https://doi.org/10.1017/S0022377824000448  

   Datta, Rishabh ; Crilly, Aidan ; Chittenden, Jeremy P. ; Chowdhry, Simran ; Chandler, Katherine ; Chaturvedi, Nikita ; Myers, Clayton E. ; Fox, William R. ; Hansen, Stephanie B. ; Jennings, Chris A. ; et al ( April 2024 , Journal of Plasma Physics)

   Magnetic reconnection is an important process in astrophysical environments, as it reconfigures magnetic field topology and converts magnetic energy into thermal and kinetic energy. In extreme astrophysical systems, such as black hole coronae and pulsar magnetospheres, radiative cooling modifies the energy partition by radiating away internal energy, which can lead to the radiative collapse of the reconnection layer. In this paper, we perform two- and three-dimensional simulations to model the MARZ (Magnetic Reconnection on Z) experiments, which are designed to access cooling rates in the laboratory necessary to investigate reconnection in a previously unexplored radiatively cooled regime. These simulations are performed in GORGON, an Eulerian two-temperature resistive magnetohydrodynamic code, which models the experimental geometry comprising two exploding wire arrays driven by 20 MA of current on the Z machine (Sandia National Laboratories). Radiative losses are implemented using non-local thermodynamic equilibrium tables computed using the atomic code Spk, and we probe the effects of radiation transport by implementing both a local radiation loss model and$P_{1/3}$multi-group radiation transport. The load produces highly collisional, super-Alfvénic (Alfvén Mach number$M_A \approx 1.5$), supersonic (Sonic Mach number$M_S \approx 4-5$) strongly driven plasma flows which generate an elongated reconnection layer (Aspect Ratio$L/\delta \approx 100$, Lundquist number$S_L \approx 400$). The reconnection layer undergoes radiative collapse when the radiative losses exceed the rates of ohmic and compressional heating (cooling rate/hydrodynamic transit rate =$\tau _{\text {cool}}^{-1}/\tau _{H}^{-1}\approx 100$); this generates a cold strongly compressed current sheet, leading to an accelerated reconnection rate, consistent with theoretical predictions. Finally, the current sheet is also unstable to the plasmoid instability, but the magnetic islands are extinguished by strong radiative cooling before ejection from the layer.

    

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4. Dynamic Formation of a Transient Jet from Arched Magnetized Laboratory Plasma

   https://doi.org/10.3847/1538-4357/acdf47  

   Sklodowski, Kamil D. ; Tripathi, Shreekrishna ; Carter, Troy ( July 2023 , The Astrophysical Journal)

   A laboratory plasma experiment was built to explore the eruptive behavior of arched magnetized plasmas with dimensionless parameters relevant to the Sun’s photosphere (β≈ 10⁻³, Lundquist number ≈10⁴, plasma radius/ion gyroradius ≈20, ion–neutral collision frequency ≫ion cyclotron frequency). Dynamic formation of a transient plasma jet was observed in the presence of the strapping magnetic field. The eruption leading to the jet is unintuitive because the arched plasma is both kink- and torus-stable. The jet structure erupts within a few Alfvén transit times from the formation of the arched plasma. Extensive measurements of plasma temperature, density, magnetic field, and flows are presented. In its early stages, the jet plasma flows away from the arch with supersonic speeds (Mach 1.5). This high-speed flow persists up to the resistive diffusion time in the arched plasma and is driven by large gradients in the magnetic and thermal pressures near the birthplace of jets. There are two distinct electric current channels within the jet, one consisting of outgoing electrons and another composed of electrons returning to the anode footpoint. Significant current density around the jet is a consequence of the diamagnetic current produced by a large thermal pressure gradient in the jet. Ion–neutral charge-exchange collisions provide an efficient mechanism to produce the cross-field current and control the dynamics of the complex current channels of the jet.

    

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5. Compressible two-dimensional turbulence: cascade reversal and sensitivity to imposed magnetic field

   https://doi.org/10.1088/1367-2630/ad0172  

   Fouxon, Itzhak ; Kritsuk, Alexei G. ; Mond, Michael ( November 2023 , New Journal of Physics)

   We study the impact of compressibility on two-dimensional turbulent flows, such as those modeling astrophysical disks. We demonstrate that the direction of cascade undergoes continuous transition as the Mach numberMaincreases, from inverse atMa = 0, to direct at$Ma=\mathrm{\infty }$. Thus, at$Ma\sim 1$comparable amounts of energy flow from the pumping scale to large and small scales, in accord with previous data. For supersonic turbulence with$Ma\gg 1$the cascade is direct, as in three dimensions, which results in multifractal density field. For that regime ($Ma\gg 1$) we derive a Kolmogorov-type law for potential forcing and obtain an explicit expression for the third order correlation tensor of the velocity. We further show that all third order structure functions are zero up to first order in the inertial range scales, which is in sharp contrast with incompressible turbulence where the third order structure function, that describes the energy flux associated with the energy cascade is non-zero. The properties of compressible turbulence have significant implications on the amplification of magnetic fields in conducting fluids. We thus demonstrate that imposing external magnetic field on compressible flows of conducting fluids allows to manipulate the flow producing possibly large changes even at small Mach numbers. Thus Zeldovich’s antidynamo theorem, by which atMa = 0 the magnetic field is zero in the steady state, must be used with caution. Real flows have finiteMaand, however small it is, for large enough values ofI, the magnetic flux through the disk, the magnetic field changes the flow appreciably, or rearranges it completely. This renders the limitMa → 0 singular for non-zero values ofI. Of particular interest is the effect of the density multifractality, at$Ma\gg 1$which is relevant for astrophysical disks. We demonstrate that in that regime, in the presence of non-zeroIthe magnetic field energy is enhanced by a large factor as compared to its estimates based on the mean field. Finally, based on the insights described above, we propose a novel two-dimensional Burgers’ turbulence, whose three-dimensional counterpart is used for studies of the large-scale structure of the Universe, as a model for supersonic two-dimensional magnetohydrodynamic flows.

    

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