Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session GO06: Fundamental Plasmas: Turbulence IILive

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Chair: M.J. Pueschel, Dutch Institute for Fundamental Energy Research 
Tuesday, November 10, 2020 9:30AM  9:42AM Live 
GO06.00001: Turbulence studies using selforganized magnetic structures in a plasma wind tunnel David Schaffner, Carlos CartagenaSanchez, Joshua Carlson An overview and recent progress of activities at the Bryn Mawr Plasma Laboratory (BMPL) is presented. The main experiment at the facility, the Bryn Mawr eXperiment (BMX), consists of a 4mF, 2kV pulseforming network that generates \textasciitilde 180us of stationary broadband fluctuations of magnetic field and plasma using a magnetized coaxial plasma gun source. These selforganized magnetized structures are launched into a 2.7m fluxconserving cylindrical wind tunnel. Singleloop magnetic pickup coils measure fluctuating magnetic field and timedelay estimated velocity. Multipoint measurements of spectra are made from linear arrays of probes along the axial direction of the chamber. [Preview Abstract] 
Tuesday, November 10, 2020 9:42AM  9:54AM Live 
GO06.00002: Spatialtemporal correlation analysis of MHD Turbulence at BMPL Carlos CartagenaSanchez, Josh Carlson, David Schaffner The Bryn Mawr Experiment (BMX) is a newly constructed experiment at the Bryn Mawr Plasma Laboratory (BMPL). BMPL is investigating magnetic turbulent generated by injecting helicity with a magnetized coaxial gun source into a flux conserving cylindrical windtunnel. This presentation represents the studies of MHD turbulent properties at BMPL. Spatialtemporal correlation analysis of magnetic fluctuations is used to estimate outer and inner scales of the energycascade inertial range. With these estimates a magnetic Reynolds number is calculated. The spatial correlation scale is used as the outer scale. The Taylor microscale from Taylor hypothesized temporal correlations is used as the inner scale. Comparisons between Taylor hypothesized temporal correlation properties and spatial correlation properties are also presented. [Preview Abstract] 
Tuesday, November 10, 2020 9:54AM  10:06AM Live 
GO06.00003: Spatial and temporal spectral dependence of magnetic structures in a plasma wind tunnel Joshua Carlson, David Schaffner, Carlos Cartagena Analysis of recent data obtained from the Bryn Mawr Magnetohydrodynamic Experiment (BMX) housed in the Bryn Mawr Plasma Laboratory (BMPL) is presented. The BMX investigates selforganized structures of magnetic field and plasma generated by a magnetized coaxial plasma gun source. These structures are discharged into a fluxconserving cylindrical wind tunnel after initial confinement within the gun region via a stuffing magnetic flux. Probes arranged along the zaxis of the tunnel measure fluctuations in magnetic field and timedelay estimated velocity as the structure moves through the system. Multipoint spectra measurements as a function of stuffing flux delay and spectral indices as a function of axial position are discussed. [Preview Abstract] 
Tuesday, November 10, 2020 10:06AM  10:30AM Live 
GO06.00004: Observation of Electromagnetic DriftAlfvén Waves in Increased $\beta$ Laboratory Plasmas (PhD Oral24) Giovanni Rossi, Troy Carter, Jeffrey Robertson, MJ Pueschel The variation of pressuregradient driven turbulence with plasma $\beta$ (up to $\beta \approx 15\%$) is investigated in linear, magnetized plasma. The magnitude of magnetic fluctuations is observed to increase substantially with increasing $\beta$. More importantly, parallel magnetic fluctuations are observed to dominate at higher $\beta$ values, with $\delta B_\parallel / \delta B_\perp \approx 2$ and $\delta B / B \approx 1\%$. Parallel magnetic fluctuations are strongly correlated with density fluctuations and the two are observed to be out of phase. The relative magnitude of and crossphase between density and parallel magnetic field fluctuations are consistent with dynamic pressure balance ($P+\frac{B_{\parallel}^2}{2\mu_0} = $ constant). A local theory of modified driftAlfvén waves, including parallel magnetic fluctuations is qualitatively and quantitatively consistent with the observations. [Preview Abstract] 
Tuesday, November 10, 2020 10:30AM  10:42AM Live 
GO06.00005: Nonlinear Interaction between Magnetic Field and Stable Eigenmodes in Forced ShearFlowDriven MHD Turbulence B. Tripathi, P.W. Terry, A.E. Fraser, M.J. Pueschel, E.G. Zweibel Transport and mixing in turbulent magnetized plasmas driven by unstable shear flows is a challenging nonlinear problem. Linearly stable (damped) eigenmodes, which are often neglected, can be crucial in saturating the instability. In gyrokinetic simulations of turbulence arising from a driven unstable shear flow, unstable and stable modes reach nearequipartition [Fraser et al. PoP (2018)]. When the flow is not driven and there is a flowaligned magnetic field, faster relaxation in stronger fields can quasilinearly flatten the profile before stable modes have time to affect the evolution. Here, we investigate turbulent saturation in MHD simulations of KelvinHelmholtzunstable flows that are forced to prevent flattening of the mean flow, employing the Dedalus code with a flowaligned magnetic field. We demonstrate that the stable modes break the hegemony of unstable modes in the nonlinear regime. The role of the magnetic field in determining the amplitudes of stable and unstable modes is quantified. With the nonlinear interaction among the magnetic field and different linear eigenmodes in hand, we pursue reduced models of Reynolds stresses, Maxwell stresses, and the ensuing transport phenomena by including the stable modes at saturation. [Preview Abstract] 
Tuesday, November 10, 2020 10:42AM  10:54AM Live 
GO06.00006: HeatFlux Suppression by Kinetic Instabilities in High$\beta$, Weakly Collisional, Magnetized Plasma Evan Yerger, Matthew Kunz, Anatoly Spitkovsky High$\beta$ plasmas can be highly magnetized ($\rho/H\ll 1$) at the largest astrophysical scales, e.g., in the intracluster medium of galaxy clusters. If the plasma is furthermore weakly collisional, the transport of momentum and heat is highly anisotropic with respect to the magnetic field direction. Such transport can result in significant parallel heat flux and pressure anisotropy, which trigger kinetic instabilities that backreact on the transport. In this work, we use the particleincell code TristanMP to calculate the steadystate heat flux through a stratified, high$\beta$, collisionless, magnetized plasma. The consequent departures from a Maxwellian distribution function excite the heatfluxdriven whistler instability and the pressureanisotropydriven mirror instability. Both instabilities reduce heat transport by scattering and/or trapping particles (e.g., RobergClark et al. 2018; Komarov et al. 2016, 2018). By tracking thousands of particles and simulating across a range of $\beta$ and ionelectron mass ratio, we construct a space and timeresolved, energydependent collision operator which quantitatively describes the effect of the saturated instabilities on particle motion, and therefore on the transport properties of the plasma. [Preview Abstract] 
Tuesday, November 10, 2020 10:54AM  11:06AM Live 
GO06.00007: Dynamic Phase Alignment in Inertial Alfv´en Turbulence Lucio Milanese, Nuno Loureiro, Maximilian Daschner, Stanislav Boldyrev In weaklycollisional plasma environments with sufficiently low electron beta, Alfv\'enic turbulence transforms into inertial Alfv\'enic turbulence at scales below the electron skindepth, $k_\perp d_e > 1$. We argue that, in inertial Alfv\'enic turbulence, both energy and generalized kinetic helicity exhibit direct cascades. We demonstrate that the two cascades are compatible due to the existence of a strong scaledependence of the phase alignment angle between velocity and magnetic field fluctuations, with the phase alignment angle scaling as $\cos\alpha_k\propto k_{\perp}^{1}$. As a result of the dual direct cascade, the generalizedhelicity spectrum scales as $\propto k^{5/3}_{\perp}$, implying progressive balancing of the turbulence as the cascade proceeds to smaller scales in the $k_{\perp} d_e \gg 1$ range. Our results may be applicable to a variety of geophysical, space, and astrophysical environments, including the Earth's magnetosheath and ionosphere, solar corona, nonrelativistic pair plasmas, as well as to strongly rotating nonionized fluids. [Preview Abstract] 
Tuesday, November 10, 2020 11:06AM  11:18AM Live 
GO06.00008: Elasticity of tangled magnetic fields David Hosking, Alexander Schekochihin, Steven Balbus The fundamental difference between incompressible ideal magnetohydrodynamics and the dynamics of a nonconducting fluid is that magnetic fields exert a tension force that opposes their bending; magnetic fields behave like elastic strings threading the fluid. It is natural, therefore, to expect that a magnetic field tangled at small length scales should resist a largescale shear in an elastic way, much as a ball of tangled elastic strings responds elastically to an impulse. In this talk, I will describe a treatment of magnetoelasticity motivated by the need to understand the largescale dynamics of the hot, rarified plasma of the intracluster medium (ICM). In contrast to previous work, the treatment I present explicitly accounts for intermittency of the Maxwell stress. I will show, via analytical theory and supporting numerical work, that this intermittency necessarily decreases the frequency of `magnetoelastic waves' propagating through a tangledmagneticfield equilibrium, and results in anomalous viscous damping. I will also present numerical simulations of sporadicallydriven MHD turbulence, elucidating the possible role of magnetoelastic waves in facilitating energy transport in the turbulent ICM. [Preview Abstract] 
Tuesday, November 10, 2020 11:18AM  11:30AM Live 
GO06.00009: Stochastic webs formation and anomalous chaotic crossfield particle transport in Hallthruster by ExB electron drift instability Debraj Mandal, Yves Elskens, Xavier Leoncini, Nicolas Lemoine, Fabrice Doveil, Devendra Sharma The ExB electron drift instability is,observed in many magnetized plasma devices, an important agent in crossfield particle transport. The collisionless electron transport mechanism is analyse, due to presence of a single electrostatic mode generated fromthis instability, by considering a reduced twodegreesoffreedom Hamiltonian. It helps to simplify the original dynamics complexity. In the presence of this electrostatic wave the magnetized charged particle dynamics becomes chaotic, and for different parameter values it generates Halloweenmask like and other different stochastic webs in the phasespace. A scaling exponent is defined to characterise transport in such phasespace, and find anomalous transport, of superdiffusive type. The trajectories stick to different kinds of islands in phase space, and their different sticking time powerlaw statistics generate successive regimes of the superdiffusive transport. In the next part we are intending to generate the ExB drift instability selfconsistently, in Hallthruster geometry using a 2D PICMCC simulation, and compare the crossfield transport coefficient value with that coming from the simplified model. [Preview Abstract] 
Tuesday, November 10, 2020 11:30AM  11:42AM Live 
GO06.00010: Dynamics of Shear Layer Collapse in Modified HasegawaWakatani Channel Flows Mikhail Malkov, Patrick Diamond Density limit phenomenology has been associated with the collapse of edge shear layers at high density. Theoretical work has suggested that the onset of such collapse occurs when adiabaticity $\alpha=k_{\parallel}^{2}V_{th}^{2}/\omega\nu$ drops below $\alpha_{crit}\approx1$. Here, we explored shear flow dynamics in a spatially varying $\alpha\left(x\right)$ profile. $\alpha<\alpha_{crit}$ on the outer boundary, and $\alpha>\alpha_{crit}$ on the inner one. The gradient in $\alpha$ triggers the formation of a barrier shear layer, which separates the regions of isotropic turbulence and zonal flows. The barrier is pinned to the location of $\alpha_{crit}$ and does not propagate. Work on the effect of a spatially profiled neutral drag (reflecting a neutral profile) is ongoing. More generally, we report on some interesting differences between zonal flow phenomena in the conventional doubly periodic box and in channel flows. Specifically: i) Zonal flows are more coherent in the channel flow. ii) A transition curve of $R=\frac{ZF Energy}{Total Kinetic Energy}$ is largely monotonic in for the box. More complex behavior is shown in the case of the channel flow. iii) Time histories are different for the box and channel cases. Work on all the issues discussed above is ongoing. [Preview Abstract] 
Tuesday, November 10, 2020 11:42AM  11:54AM Live 
GO06.00011: Zonal flow dynamics with noise and modulations Rameswar Singh, Patrick Diamond Spectral equations for zonal flow and turbulence energy have been derived for the drift wave turbulence. The spectral equation for the zonal flow energy shows two distinct mechanisms of zonal flow excitation modulational growth and zonal noise drive. The two are of comparable strength. The zonal noise term scales as spectral Reynolds stress squared times the triad interaction time. The zonal noise is positive definite and is insensitive to the spectral slope. Zonal nonlinear noise generates the zonal flow even when the drift waves are modulationally stable. Noise eliminates the threshold in the linear growth rate of turbulence for zonal flow excitation within the predator prey model. This is consistent with the observation of zonal flows without a critical power in experiments. The zonal intensity increases and turbulence intensity decreases with the strength of zonal noise. A 0D model is advanced to study the effect of zonal noise on LH transition dynamics. With noise, substantial zonal flows appear much below the threshold for the modulational excitation without noise. This reduces the predicted threshold power for the transition to the H mode. Iphase persist, but with lower onset threshold and reduced oscillations. [Preview Abstract] 
Tuesday, November 10, 2020 11:54AM  12:06PM Live 
GO06.00012: Zonal Flow Driven by Electron Temperature Gradient Driven Drift Mode in a Linear Magnetized Plasma N. Wakde, S. Bose, R. Roy, P.K. Chattopadhyay, R. Singh, J. Ghosh Zonal flow (ZF) is a poloidally symmetric band like shear flow, a secondary mode driven by drift waves, which regulates turbulence and transport. Here, we report observation of ZF driven by electron temperature gradient (ETG) driven drift wave turbulence in a linear machine. A low frequency (0.20.3 kHz) mode is identified as ZF by measuring density and potential fluctuations, and radial, poloidal and axial wave numbers. In radius, maxima of ZF amplitudes tracks the minima of the ETG scale length ($L_{Te}$) on controlled variation of the location of minimum of the later. The theoretical criteria $L_{Te} / L_{n} < 1.5$ for excitation of ETG driven mode is well satisfied in the experiment. Here, $L_{n}$ is the density gradient scale length. Electron density fluctuations has a broadband spectrum above 300 kHz which is close to theoretically predicted frequency of ETG driven mode. Bicoherence analysis shows coupling of ZF with ETG mode. We have found that ZF becomes stronger when collision frequency is decreased. This further proves that ZF is driven by ETG as ETG driven drift is reactive in nature. [Preview Abstract] 
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