Session TM11: Mini-Conference on Recent Advances in Magnetic Fields in High Energy Density Plasmas II

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The interactions of fast-streaming, magnetized plasmas can result in a wide range of fundamental plasma physics processes such as the formation of MHD shocks, magnetic turbulence, reconnection and wave-particle interactions. We present experiments from a versatile platform, where supersonic plasma flows generated by the ablation of pulsed-power driven wire arrays are used to study a wide range of magnetized plasma interactions [1,2]. The setup allows a control over the global system parameters, including the drive strength, magnetization, magnetic field topology and interaction geometry. The plasma composition (wire material) can also be chosen to vary the collisionality of the plasma and introduce dynamically significant radiative cooling. The detailed structure of the interactions is measured using optical collective Thomson scattering, laser interferometry and Faraday rotation diagnostics, providing measurements of the flow velocities, plasma temperature, electron density and magnetic field distributions of the plasma. [1] Suttle et al., PRL (2016) [2] Burdiak et al., PoP (2017)   

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Magnetic field dynamics and its coupling with HED plasmas play a key role in magnetized fusion schemes, magnetic reconnection, and laboratory astrophysics experiments. The understanding of magnetic field transport properties in HED plasma is thus a primary goal for these applications. We present recent experiments designed to study cross-field transport in magnetized HED plasmas at the OMEGA laser facility, generated by expanding a laser-produced plasma into a background magnetic field and observing the expansion and collapse dynamics of the associated diamagnetic bubble. A laser-ablated plasma expanded into a pre-existing magnetic field powered by MIFEDS. The evolution of the 2D global topology of the magnetic fields was imaged with proton radiography by 3 and 15 MeV protons, acquired at different plasma expansion times. The interactions were explored for different laser energies and target orientations relative to initial field. The corresponding local electron temperature and density were measured with 2$\omega $ Thomson scattering.   

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We present experimental results investigating the reflection of oblique shocks in magnetised, high density plasmas. The shocks are produced by placing multiple obstacles into the supersonic, super-Alfv\'{e}nic~outflow from an ablating inverse wire array~z-pinch. The advected magnetic field (5T) is large enough to affect the shock structure via pile-up of the field at the obstacle, which depends on the obstacle size and resistivity and its orientation with respect to the field.~ The downstream plasma parameters are determined by the compressibility of the magnetic field and two-fluid effects, and we observe a modest density jump by a factor of 2 at the shock. Thomson scattering data show very little heating at the shock when there is magnetic field pile-up, which is consistent with the downstream pressure being provided by the magnetic field. At the oblique shock reflection, magnetic field compression leads to a clear difference in the reflection geometry when compared with experiments with no field pile-up, and an absence of reflected shocks in some geometries.   

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Three-dimensional FLASH simulations are carried out to study the hydrodynamics and magnetic fields in the shock-shear derived platform. Simulations indicate that fields of tens of Tesla can be generated via the Biermann battery effect due to vortices and mix in the counterpropagating shock-induced shear layer. Synthetic proton radiography simulations using MPRAD and synthetic X-ray image simulations using SPECT3D are carried out to predict the observable features in the diagnostics.   

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The identification of sources of high-energy cosmic rays (CRs) requires the understanding of how CRs are deflected by the stochastic, spatially intermittent intergalactic magnetic field. We discuss a set of recently published laser-driven experiments of the TDYNO collaboration, which measure the propagation of energetic charged particles through a magnetized plasma with these properties. These experiments were designed using the FLASH code and were executed on the Omega Laser Facility at the Laboratory for Laser Energetics of the University of Rochester. The diffusive transport is characterized experimentally. The results show that the transport is diffusive and that, for the regime of interest for the highest-energy CRs, the diffusion coefficient is unaffected by the spatial intermittency of the magnetic field.   

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Magnetized collisionless shocks are common features in space and astrophysical systems where supersonic plasma flows interact, such as in the solar wind, the heliopause, and supernova remnants. Recent experimental capabilities and diagnostics allow detailed laboratory investigations of high-Mach number shocks. Using particle-in-cell simulations, we demonstrate the mechanism and the associated requirements of experiments for generation of energetic electron populations in laboratory high-Mach number collisionless shocks. We show through a parameter study that electron acceleration by magnetized collisionless shocks is feasible in laboratory experiments. Conditions for experimental observation of pre-accelerated electrons are formulated.   

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LANL has a broad range of High Energy Density hydrodynamic platforms which have been used for studying self-generated fields in shock tubes and creating turbulent dynamos, as well as hydro platforms to which we have applied magnetic fields for ICF as well as HED and Laboratory Astrophysics research. An overview of the platforms, some exciting preliminary results and future experiments will be discussed.