Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session JO09: Fundamental: Turbulence and TransportOn Demand

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Chair: Benjamin Faber, UW Madison Room: Rooms 403405 
Tuesday, November 9, 2021 2:00PM  2:12PM 
JO09.00001: Scaledependent energy transfer and conversion in anisotropic compressible space and astrophysical plasma turbulence Senbei Du, Hui Li, Zhaoming Gan, Xiangrong Fu The crossscale transfer of kinetic and magnetic energy is an important process in plasma turbulence. For compressible magnetized plasmas that are typically found in space and astrophysical environments, the turbulence can be highly anisotropic with respect to the background magnetic field and the energy transfer parallel and perpendicular to the magnetic field needs to be considered separately. While local interactions dominate the crossscale energy transfer in typical isotropic turbulence, the locality of energy transfer in anisotropic turbulence remains poorly understood. We present a spatial filtering technique to calculate various scaledependent energy transfer and conversion terms such as kinetic, magnetic and pressure dilatation processes. The analysis presented here allows us to compute the anisotropic crossscale energy fluxes and demonstrate their locality (or nonlocality) in 3D compressible MHD simulations of turbulence. Our results shed light on the role turbulence plays in the solar wind heating and the transport of energetic particles. 
Tuesday, November 9, 2021 2:12PM  2:24PM 
JO09.00002: On the Existence of Finite Frequency Fast Modes in the Compressible MHD Turbulence Hui Li, Zhaoming Gan, Xiangrong Fu, Senbei Du Fast modes could play a significant role in turbulent magnetohydrodynamic (MHD) systems, affecting energy dissipation, density fluctuation, and energetic particle transport. We carry out a large number of 3D MHD turbulence simulations with different driving schemes at the injection scale to quantify the fraction of fast modes. We use two approaches to determine the presence of fast modes: mode decomposition based on spatial variations only and spatiotemporal 4DFFT analysis of all fluctuations. The latter method enables us to distinguish fluctuations that satisfy the dispersion relation of fast modes with finite frequency. Overall, we find that most of the fast mode fluctuations that are identified by the spatial mode decomposition approach is actually at nearly zero frequency (i.e., they do not satisfy the dispersion relationship for fast modes). These results are important for understanding the compressible fluctuations in space and astrophysics plasmas. 
Tuesday, November 9, 2021 2:24PM  2:36PM 
JO09.00003: KineticAlfvénwave turbulence in the low beta limit: tearing mediation and electron Landau damping Nuno F Loureiro, Muni Zhou, Zhuo Liu We report analytical and numerical investigations of subion scale turbulence in weakly collisional, low beta plasmas using a hybrid fluidkinetic model. Assuming that the energy cascade is mediated by the tearing instability, we derive scalings for the energy spectrum and for the eddy threedimensional anisotropy. The use of a Hermite formalism to express the velocity space dependence of the electron distribution function allows us to obtain an analytical, zeroth order solution for the Hermite moments of the distribution, which is borne out by numerical simulations. Electron Landau damping is found to play an important role in energy dissipation, enabled by the local weakening of nonlinearities in current sheets. 
Tuesday, November 9, 2021 2:36PM  2:48PM 
JO09.00004: Turbulent dynamics under two ideal invariants: dynamic phase alignment in plasmas and nonionized fluids Lucio M M Milanese, Maximilian Daschner, Nuno F Loureiro, Stanislav A Boldyrev Turbulent dynamics in the presence of two invariants, e.g., energy and kinetic helicity, is poorly understood in both plasmas and nonionized fluids. We present results of numerical studies of turbulence in low$\beta_e$ plasmas at scales below the electron skin depth, and turbulence in nonionized fluids governed by the NavierStokes equations. In both systems, the dynamics is dominated by the presence of energy and (generalized) kinetic helicity as two exact invariants. We show that, in the two systems, both invariants are subject to a forward cascade, and we demonstrate that this joint cascade is possible due to the existence of a strong dependence on scale of the Fourier phase alignment angle between, in low$\beta_e$ plasmas, fluctuations of electric and magnetic potential and, in NavierStokes turbulence, fluctuations of velocity and vorticity. This phenomenon, termed \textit{dynamic phase alignment}, thus acquires importance as a mechanism regulating the dynamics in the presence of two invariants, arising from their conservation in the joint direct cascade, regardless of the details of the physical interactions. 
Tuesday, November 9, 2021 2:48PM  3:00PM 
JO09.00005: MHD Turbulence Mediated by the Plasmoid Instability Chuanfei Dong, Liang Wang, Luca Comisso, YiMin Huang, Amitava Bhattacharjee Magnetohydrodynamic (MHD) turbulence regulates the transfer of energy from large to small scales in many astrophysical systems, from the solar corona and accretion disks to the interstellar medium and galaxy clusters. An important feature of MHD turbulence is the tendency to develop sheets of strong electric current density. These current sheets are natural sites of magnetic reconnection, leading to the formation of plasmoids that eventually disrupt the sheetlike structures in which they are born. In this presentation, we investigate the role of the plasmoid instability in both 2D and 3D MHD turbulence by means of highresolution direct numerical simulations at large magnetic Reynolds numbers. By breaking elongated current sheets into chains of small plasmoids, magnetic reconnection leads to a new range of the turbulent energy cascade in the energy spectra, where the rate of energy transfer is controlled by the growth rate of the plasmoids. The omnipresence of plasmoids and their consequences on, e.g., the solar coronal heating can be further explored with current and future satellites/telescopes. 
Tuesday, November 9, 2021 3:00PM  3:12PM 
JO09.00006: Turbulence and particle energization in strongly magnetized pair plasmas Cristian S Vega, Stanislav A Boldyrev, Vadim S Roytershteyn Plasma turbulence is ubiquitous in astrophysical systems. In some applications, plasma may be relativistically hot. We present phenomenological and numerical studies of Alfvenic turbulence in a collisionless, relativistic pair plasma with an imposed largescale magnetic field. We consider the regime when the equation of state is relativistic while the plasma fluctuations are mildly relativistic. We discuss the spectra of turbulence and intermittency effects. In collisionless plasmas, particles that interact with turbulent fluctuations do not fully relax to a thermal distribution and end up forming nonMaxwellian tails. We perform particleincell simulations of relativistic decaying turbulence in a pair plasma to study particle energization. We discuss the turbulent cascade and the global particle energy distribution functions. 
Tuesday, November 9, 2021 3:12PM  3:24PM 
JO09.00007: What Limits Zonal Flow Saturation in (Nearly) Collisionless DriftWave Turbulence? Taurean Zhang, Patrick H Diamond, Robin Heinonen Drift wave  zonal flow turbulence has long been known as a selfregulating system. A key question that remains is how are zonal shears regulated with weak frictional damping. 
Tuesday, November 9, 2021 3:24PM  3:36PM 
JO09.00008: Physics of Turbulence Spreading and Explicit Nonlocality Qinghao Yan, Patrick H Diamond Turbulence can spread from the linear unstable region into the stable or weakly unstable region. Thus, turbulence spreading can introduce more turbulence than expected from the linear stability, therefore breaks the scenario of local turbulence. From a simplified gyrophase and bouncephase averaged kinetic equation, using the 2point correlation function, 2point quasilinear approximation and Green’s function, we systematically derived an explicitly nonlocal model for turbulence spreading. Explicit nonlocality means that the evolution of quantities at r are explicitly affected by other positions, as characterized by the Green’s function convolution. The Green’s function comes from the inverting of potential vorticity to electric potential, and has the kernel width of several δ_{b} (banana orbit width). Our model recovers the usual spreading model when δ_{b} is small. Results show that the nonlocal effects, especially the nonlocal growth, thicken the spreading front and speed up front propagating. Penetration into the stable region Δ_{p} linearly grows with δ_{b}. Convolution reduces the growth rate in the unstable region, thus decreases the saturation level of turbulence, and lead to a simple linear relation I/l_{r*}^{2}=1  δ_{b} for the total turbulence intensity in the unstable region. 
Tuesday, November 9, 2021 3:36PM  3:48PM 
JO09.00009: On the collisional transport of plasma using a 13moment model Jason Hamilton, Charles E Seyler Plasmas, being complex systems of a large number of nonlinear processes, do not usually allow simple or even analytical solutions. Their evolution over time is described by different sets of equations depending on the parameters of the given plasma at hand. The goal of this field, as in any other theoretical field of physics, is to bridge these various descriptions so that a complete understanding of these complex nonlinear processes can be obtained. In this talk, plasma transport such as thermal & electrical conductivity, thermoelectric effects, and viscosity are studied with a model consisting of the 13moments of the FokkerPlanck equation. In the collisional regime this model is compared against ChapmanEnskog expansions, such as Braginskii's (1965) transport equations. Comparisons to the results of Davies et al. (2021) confirm that the 13moment model possesses the correct physical behaviour of the transport coefficients at low magnetization, unlike Epperlein & Haines (1986). Validation tests using the XMHD code PERSEUS show that a 13moment plasma model can both reasonably approximate Braginskii's near equilibrium transport while maintaining a physical hyperbolic formulation that has finite propagation speeds, as well as offer a natural extension to nonequilibrium systems. 
Tuesday, November 9, 2021 3:48PM  4:00PM 
JO09.00010: Kinetic Stress and Particle Transport by Stochastic Fields and Turbulence ChangChun Chen, Patrick H Diamond, Rameswar Singh, Steven Tobias It is general wisdom that stochastic magnetic fields affect the electron heat transport in a tokamak. In the LH transition, with resonant magnetic perturbation (RMP), the transport of ion heat, particles, toroidal momentum can also be influenced by stochastic fields due to RMP. A wellknown model from Finn et al. discussed how toroidal flow is affected under the influence of stochastic fields. However, they merely have an Elsässerlike variable of parallel rotation and particle density—no explicit momentum and particle density transport presented. Moreover, they didn’t mention the kinetic stress which is critical in toroidal flow damping. 
Tuesday, November 9, 2021 4:00PM  4:12PM 
JO09.00011: Intermittency and hysteresis in a stochastic model of the LowtoHigh confinement mode transition EunJin Kim The need for a proper statistical theory for understanding fusion plasmas has grown significantly, with experiments and simulations revealing ample evidence for nonGaussian fluctuations, anomalous transport, or intermittency. The latter questions the validity of the meanfieldtype theory based on small Gaussian fluctuations, necessitating the calculation of an entire probability density function (PDF). In this paper, we show the importance of intermittency and timedependent PDF approach in the LowtoHigh confinement mode (LH) transition. In particular, PDFs are shown to be strongly nonGaussian with convoluted structures and multiple peaks, and that intermittency (rare events of large amplitude) of zonal flows can play an important role in promoting the LH transition. We also propose a new information geometric method by using information length, dynamical time scale, and information phase portrait, and show their utility in forecasting transitions and selfregulation between turbulence and zonal flows. Implications for hysteresis in the LH and HL transition are discussed. 
Tuesday, November 9, 2021 4:12PM  4:24PM 
JO09.00012: dc Electrical Conductivity of Strongly Magnetized Plasmas Scott D Baalrud, Trevor Lafleur A generalized Ohm's law is derived to treat strongly magnetized plasmas in which the electron gyrofrequency significantly exceeds the electron plasma frequency. The frictional drag due to Coulomb collisions between electrons and ions is found to acquire a component perpendicular to their relative drift, producing an additional transverse resistivity term in the generalized Ohm's law that is perpendicular to both the current and the Hall directions. In the limit of very strong magnetization, the parallel resistivity is found to increase by a factor of 3/2, and the perpendicular resistivity to scale as a logarithmic function of the Hall parameter. Correspondingly, the parallel conductivity coefficient is reduced by a factor of 2/3, and the perpendicular conductivity scales proportionally to the logarithm of the Hall parameter divided by the square of the Hall parameter. These results suggest that strong magnetization significantly changes the magnetohydrodynamic evolution of a plasma. 
Tuesday, November 9, 2021 4:24PM  4:36PM 
JO09.00013: A Method to Determine the ElectronIon Thermal Exchange Rate from Force Distributions of Test Ions David J Bernstein, Louis Jose, Scott D Baalrud Temperature relaxation between species, e.g. electrons and ions, is an important transport process in plasma applications such as fusion experiments. However, these relaxation rates can be difficult to measure in the laboratory. A sparse population of test particles, e.g. fusion products, is often present and may be used as a diagnostic. Here, we demonstrate that the velocity distributions and distribution of forces on these test particles can be used to measure electronion energy exchange densities, which are directly proportional to temperature relaxation rates. It is well known that the average force on test particles is the friction force, which is essentially the stopping power. Here, we show that the thermal energy exchange is related to the covariance between the test particle velocity and force distributions. Firstprinciples molecular dynamics (MD) simulations are used to test this under a range of conditions: when the background species with which the test particles interact is strongly coupled (the average interparticle potential energy exceeds the average thermal energy), when the background species is strongly magnetized (particle gyroradii are smaller than length scales associated with collisions), and when the test particle has opposite charge as that of the background particles. These conditions are known to significantly affect the friction, and their effects on the energy exchange density are assessed with the proposed covariance method. 
Tuesday, November 9, 2021 4:36PM  4:48PM 
JO09.00014: NearResonant Heat Flux Reduction in Gyrokinetic DimitsShift Analysis and Applications to a Quasilinear Model PingYu Li, Paul W Terry, MJ Pueschel, ShuWei Tsao, Garth G Whelan The onset of turbulent heat flux at a higher temperature gradient than the critical gradient of linear instability (known as the Dimits shift) is a wellknown feature in fusion plasmas. It is investigated in relation to the saturation mechanism for toroidal Ion Temperature Gradient (ITG) instability using a fluid model with threshold physics. It is shown that resonance in the nonlinear coupling between the modes that dominate energy transfer in saturation can lead to suppression of turbulence and transport just above the linear critical gradient. Resonant effects occur in both the triplet correlation time and the nonlinear coupling coefficients. The triplet correlation time is sensitive to the eddy turnover rate, which broadens the resonance, reduces the triplet correlation time and increases the heat flux. The resonance concept is then tested in gyrokinetic simulations with ITG turbulence which has a Dimits shift, by use of artificial complex frequencies to break the resonance. 
Tuesday, November 9, 2021 4:48PM  5:00PM 
JO09.00015: Staircase Formation by an Array of Stationary Convective Cells Fredy R Ramirez, Patrick H Diamond We show that a staircase profile of scalar concentration forms in a simple system of stationary convective cells set in a fixed array. This case study is a particularly simple example of layer formation. Here, for Peclet number much greater than one, layering results from the interplay of the two disparate time scales in the problem, namely the cell turnover time and the diffusion time. They key physics is that celltocell transport is controlled by diffusion, and thus slow. This system has properties in common with those with richer dynamics near marginality. We study staircase resilience by exploring the consequences of imposing: i.) a spatially varying crossprofile shear flow (N.B. The shearing rate introduces a third time scale) and ii.) noisy concentration deposition and the consequent stochastic avalanching. Results concerning the response of the staircase structure will be discussed. 
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