Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session YI3: BPP Invited III: Gyrokinetic Modeling, Basic Shocks, EOS, and Laboratory Astrophysics 
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Chair: Mark Gilmore, University of New Mexico Room: OCC Oregon Ballroom 204 
Friday, November 9, 2018 9:30AM  10:00AM 
YI3.00001: Gyrokinetic study of slab universal modes and suppression of the Gradient Drift Coupling (GDC) instability Invited Speaker: Manaure Francisquez A local linear gyrokinetic stability analysis of a collisionless, shearless slab geometry in an equilibrium pressure balance with constant $p_0+B_0^2/(8\pi)$. We focus on $k_\parallel=0$ modes, electromagnetic universal (or, entropy) modes driven by density or temperature gradients at small and large plasma $\beta$. These are smallscale nonMHD instabilities with growth rates that typically peak near $k_\perp\rho_i\sim1$ and vanish in the long wavelength limit ($k_\perp\to0$). Analytic analysis indicates that a necessary condition for instability is that at least one of $\eta_e$ or $\eta_i$ be negative, where $\eta_\alpha=L_n/L_{T\alpha}$ is the ratio of the density and temperature gradient scale lengths. That is, the density gradient must point in the opposite direction as the electron or the ion temperature gradient for this slab mode to be unstable [1]. This instability is also explored with GENE, and we discuss its relation to the Gradient Drift Coupling (GDC) instability [2,3], which arises from neglecting the pressure balance equilibrium and was described to have a finite growth rate $\gamma\simeq\sqrt{\beta/[2(1+\beta)]}C_s/L_p$, with $C_s^2=p_0/\rho_0$ at $k_\perp\to0$ (long wavelength). 
Friday, November 9, 2018 10:00AM  10:30AM 
YI3.00002: Experimental Measurements of Ion Heating in Collisional Plasma Shocks and Interpenetrating Supersonic Plasma Flows Invited Speaker: Samuel Langendorf On the Plasma Liner Experiment (PLX) at LANL, two coaxialgunformed, highly supersonic plasma jets (with initial n ∼ 10^{16} cm^{3}, T_{e} ∼ T_{i} ∼ 1.5 eV, v = 25 – 80 km/s, diameter = 8.5 cm, length ≈ 20 cm) are merged obliquely to form either a strong collisional plasma shock or semicollisional interpenetration without shock formation, depending on the merging angle. Earlier work presented detailed diagnostic measurements consistent with plasmashock formation [2]. This work [3] presents new and comprehensive measurements of ion heating due to the colliding flows, for N, Ar, Kr, and Xe. ^{2}E. C. Merritt et al., Phys. Rev. Lett. 111, 085003 (2013). *In collaboration with S. Hsu, K. Yates, C. Thoma. We acknowledge HyperV Technologies for advice on plasmagun operation. LAUR1825692. 
Friday, November 9, 2018 10:30AM  11:00AM 
YI3.00003: Magnetothermodynamics: An experimental study of the equations of state applicable to a magnetized plasma Invited Speaker: Manjit Kaur Measuring the equations of state of a compressed magnetized plasma is important both for advancing fusion experiments and understanding natural systems such as stellar winds. In this talk, I will present results from our recent experiments on the thermodynamics of compressed magnetized plasmas^{1,2}; we call these studies "magnetothermodynamics". In these experiments, we generate parcels of relaxed, magnetized plasma at one end of the linear SSX device and observe their compression in a closed conducting boundary installed at the other end. Plasma parameters are measured during compression. The instances of ion heating during compression are identified by constructing a PV diagram using measured density, temperature, and volume of the magnetized plasma. Theoretically predicted MHD and double adiabatic (CGL) equations of state are compared to experimental measurements to estimate the adiabatic nature of the compressed plasma. Since our magnetized plasmas relax to an equilibrium described by magnetohydrodynamics^{3}, one might expect their thermodynamics to be governed by the corresponding equation of state. However, we find that the magnetohydrodynamic equation of state is not supported by our data. Our results are more consistent with the parallel CGL equation of state suggesting that our weakly collisional plasmas have most of their proton energy in the parallel direction to the magnetic field. ^{1}Kaur et. al., Phys. Rev. E. 97, 011202 (2018). 
Friday, November 9, 2018 11:00AM  11:30AM 
YI3.00004: Current Singularity Formation in Linetied Magnetic Fields: the Parker Problem Invited Speaker: Yao Zhou Coronal heating has been a longstanding problem in solar physics. Parker’s conjecture that the current density in the corona is distributed in the form of singular current sheets that produce the required heating has been controversial. In ideal MHD, can genuine current singularities emerge from a smooth 3D linetied magnetic field? To resolve this issue computationally, the numerical scheme must preserve magnetic topology exactly to avoid artificial reconnection. We develop a novel variational integrator for ideal MHD by discretizing Newcomb’s ideal MHD Lagrangian on a moving mesh using discrete exterior calculus (Zhou et al. 2014, PoP 21, 102109). The motion of the mesh advects the discrete magnetic flux exactly, such that the scheme is free of artificial reconnection. With this method, we confirm that the nonlinear solution to the ideal HahmKulsrudTaylor (HKT) problem in 2D yields a singular current sheet (Zhou et al. 2016, PRE 93, 023205), agreeing well with an analytical solution we obtain using the boundary layer approach developed by Rosenbluth et al. (1973) for the m=1 internal kink problem. We then extend the ideal HKT problem to 3D linetied geometry (Zhou et al. 2018, ApJ 852, 3), the effect of which is crucial in the controversy over Parker's conjecture. The linear solution, which is singular in 2D, is found to be smooth. However, with finite amplitude, it can become pathological when the system is sufficiently long. The nonlinear solution turns out to be smooth for short systems. Nonetheless, the scaling of peak current density vs. system length suggests that the nonlinear solution may become singular at a finite length. Albeit not yet a conclusive resolution, our results contribute to our understanding of the Parker problem by exemplifying how the linetied geometry influences the formation of current singularities. 
Friday, November 9, 2018 11:30AM  12:00PM 
YI3.00005: The Barkas Effect in Plasma Transport Invited Speaker: Nathaniel R Shaffer Molecular dynamics simulations reveal that a fundamental symmetry of plasma kinetic theory is broken at moderate to strong Coulomb coupling: the collision rate depends on the signs of the colliding charges. This breaking of chargesign symmetry is analogous to the ``Barkas effect'' observed in chargedparticle stopping experiments. It gives rise to significantly enhanced electronion collision rates and is expected to affect any neutral plasma with moderate to strong Coulomb coupling such as ultracold neutral plasmas (UNP) and the dense plasmas of ICF and lasermatter interaction experiments. The physical mechanism responsible for the Barkas effect is screening of binary collisions by the correlated plasma medium. By including screening directly in the interaction potential governing collisions, it is shown that the Barkas effect arises in the close interactions that lead to largeangle scattering. Because the effect hinges on how screening affects close  not distant  interactions, it is a phenomenon beyond what is predicted by traditional transport models based on cutoff Coulomb collisions or meanfield dielectric response. A model for the effective screened interaction potential is presented that is suitable for the coupling strengths achieved in UNP experiments. Transport calculations using this potential agree with simulated relaxation rates and predict that the Barkas effect can cause up to a 70\% increase in the electronion collision rate at the conditions of present UNP experiments, where the electron coupling parameter can range from $\Gamma_e\approx0.1$ to $0.5$. The influence of the Barkas effect in other transport processes is also considered. 
Friday, November 9, 2018 12:00PM  12:30PM 
YI3.00006: The Magnetorotational Instability (MRI): Observation in a Mass/Spring System and the Effects of Conductive Boundaries on a Free StewartsonShercliff Layer as a Step Towards MRI in a Liquid Metal Invited Speaker: Erik P Gilson The magnetorotational instability (MRI) has been proposed as a powerful mechanism for rapid angular momentum transport in many accretion disks, but has not been confirmed by observation or experiment. The PPPL apparatus was designed to identify the MRI mechanism in a magnetized liquid metal in a TaylorCouette flow. A waterfilled device was used to directly observe the MRI mechanism by measuring the angular momentum growth of a mass tethered to a spring, confirming the validity of the picture of MRI that is often offered as an explanation of the mechanism^{1}. The liquidmetalfilled apparatus operates with conductive endcaps to reinforce the MRIunstable mean flow, increasing the saturation amplitude of the MRI. SFEMaNS code results are presented that demonstrate this improvement and suggest how to distinguish MRI from residual Ekman flows^{2}. The stronger field/galinstan coupling has been experimentally observed to modify the Shercliff layer instability^{3}. Previous measurements, using insulating endcaps, showed that the instability occurs when the Elsasser number exceeds unity^{4}. New measurements, using conducting endcaps, are presented showing that stronger coupling reduces the threshold field for the instability. However, the linetying of the field to the endcaps and the galinstan supports the shear layer, resulting in a highlysheared mean flow with significant fluctuations. Guided by simulations, the shear profile of the experiment can be changed to mitigate the centrifugal instability during the search for the MRI signature. Recent progress on identifying the MRI mechanism in liquid metal experiments will also be presented. ^{1} D. M. H. Hung et al., arXiv:1801.03569 ^{2} X. Wei et al., PRE 94, 063107 (2016) ^{3} K. Caspary et al., PRE 97, 063110 (2018) ^{4} A. Roach et al., PRL 108, 154502 (2012)

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