Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session YO4: Turbulence and Transport 
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Chair: Jason TenBarge, University of Iowa Room: 551AB 
Friday, November 2, 2012 9:30AM  9:42AM 
YO4.00001: 3D Global TwoFluid Simulations of Turbulence in LAPD Dustin Fisher, Barrett Rogers, Paolo Ricci 3D global twofluid simulations are presented in an ongoing effort to identify and understand the physics of instabilities that arise in the Large Plasma Device (LAPD) at UCLA's Basic Science Facility. The LAPD, with its wide range of tunable parameters and device configurations, is ideally suited for studying space and laboratory plasmas. Moreover, the highly detailed and reproducible measurements of the LAPD lend themselves amicably to comparisons with simulations. Ongoing modeling is done using a modified version of the Global Braginskii Solver (GBS) [1] that models the plasma from source to edge region in a fully 3D twofluid code. The reduced Braginskii equations are solved on a fieldaligned grid using a finite difference method and 4th order RungeKutta time stepping and are parallelized on Dartmouth's Discovery cluster. Recent progress has been made to account for the thermionic cathode emission of fast electrons at the source, the axial dependence of the plasma source, and it is now possible to vary the potential on the front and side walls. Preliminary results, seen from the density and temperature profiles, show that the low frequency Kelvin Helmholtz instability still dominates the turbulence in the device.\\[4pt] [1] B. Rogers and P. Ricci. Phys. Rev. Lett. 104:225002, 2010 [Preview Abstract] 
Friday, November 2, 2012 9:42AM  9:54AM 
YO4.00002: LES models for incompressible magnetohydrodynamics derived from the variational multiscale formulation David Sondak, Assad Oberai Novel large eddy simulation (LES) models are developed for incompressible magnetohydrodynamics (MHD). These models include the application of the variational multiscale formulation (VMS) of LES to the equations of incompressible MHD, a new residualbased eddy viscosity model (RBEVM,) and a mixed LES model that combines the strengths of both of these models. The new models result in a consistent numerical method that is relatively simple to implement. A dynamic procedure for determining model coefficients is no longer required. The new LES models are tested on a decaying TaylorGreen vortex generalized to MHD and benchmarked against classical and stateofthe art LES turbulence models as well as direct numerical simulations (DNS). These new models are able to account for the essential MHD physics which is demonstrated via comparisons of energy spectra. We also compare the performance of our models to a DNS simulation by A. Pouquet et al., for which the ratio of DNS modes to LES modes is 262,144. Additionally, we extend these models to a finite element setting in which boundary conditions play a role. A classic problem on which we test these models is turbulent channel flow, which in the case of MHD, is called Hartmann flow. [Preview Abstract] 
Friday, November 2, 2012 9:54AM  10:06AM 
YO4.00003: Largescale magnetic field generation via the KelvinHelmholtz instability in unmagnetized scenarios Eduardo Alves, Thomas Grismayer, Ricardo Fonseca, Luis Silva Recent particleincell simulations of the KelvinHelmholtz instability (KHI) reveal the development of a strong and largescale DC magnetic field component at the shear interface, which is not captured by the standard linear twofluid theory. We show that the DC magnetic field arises from electron mixing across the shear surface. The mixing process can be modeled by an electron thermal expansion in the case of a warm shear flow, and we link this picture to the case of a cold shear flow where the development of the standard cold fluid KHI produces an effective temperature that drives the electron expansion. We present a simple analytical model that describes the early evolution and saturation of the DC magnetic field. Simulations reveal a long living DC magnetic field (well into the ion timescale) reaching equipartition values of 10\^{}3, which are in good agreement with our estimates. Finally, we discuss the possibility of probing the KHI and associated DC magnetic field via the interaction of an intense laser pulse with overcritical targets. [Preview Abstract] 
Friday, November 2, 2012 10:06AM  10:18AM 
YO4.00004: Energy Transfer in the RichtmyerMeshkov instability Ben Thornber, Ye Zhou The variable density spectral kinetic energy budget for the RichtmyerMeshkov induced turbulent mixing layer is presented using results from a 512$^{3}$ Implicit Large Eddy Simulation (ILES). The budget is presented at several time instants and as a function of the inhomogeneous direction as the layer transitions from the initial impulse through to selfsimilarity. There are clear parallels in the development of the mixing layer with a previous analysis for the RayleighTaylor instability. The transfer spectra are clearly asymmetric, where the majority of the activity is occurring on the spike side. The quadratic and pressure components are of most often of opposite sign and almost cancel each other out in this region. In the core of the layer, the quadratic terms are largely negative in the energy containing scales. The dilatational terms are negligiblein comparison to the difference between the quadratic and pressure transfer. A notable result is that vortex rings are identified as the key source of alternating fields of negative and positive energy transfer within the mixing layer. [Preview Abstract] 
Friday, November 2, 2012 10:18AM  10:30AM 
YO4.00005: Impurity transport due to electromagnetic drift wave turbulence Sara Moradi, Istvan Pusztai, Albert Moll\'en, T\"unde F\"ul\"op In the view of an increasing interest in high $\beta$ operation scenarios, such as hybrid scenarios for ITER the question of finite $\beta$ effects on the impurity transport is a critical issue due to possible fuel dilution and radiative cooling in the core. Here, electromagnetic effects at finite $\beta$ on impurity transport are studied through local linear gyrokinetic simulations with {\sc gyro} [J.~Candy and E. Belli, General Atomics Report GAA26818 (2011)]; in particular we investigate the parametric dependences of the impurity peaking factor (zeroflux density gradient) and the onset of the kinetic ballooning modes (KBM) and microtearing modes (MTM) in spherical (NSTX) and standard tokamaks (AUG and JET). [Preview Abstract] 
Friday, November 2, 2012 10:30AM  10:42AM 
YO4.00006: Scales and the critical Reynolds numbers of the minimum state magnetohydrodynamic turbulence Ye Zhou, Sean Oughton Magnetohydrodynamic (MHD) systems can be strongly nonlinear (turbulent) when their kinetic and magnetic Reynolds numbers are high, as is the case in many astrophysical and space plasma flows. Unfortunately these high Reynolds numbers are typically much greater than those currently attainable in numerical simulations of MHD turbulence. A natural question to ask is how can researchers be sure that their simulations have reproduced all of the most influential physics of the flows and magnetic fields? In this paper, a metric is defined to indicate whether the necessary physics of interest has been captured. It is found that current computing resources will typically not be sufficient to achieve this minimum state metric. [Preview Abstract] 
Friday, November 2, 2012 10:42AM  10:54AM 
YO4.00007: Asymmetric diffusion in turbulent magnetized plasmas with a mean field gradient M.V. Medvedev We analyze particle transport in plasmas with turbulent magnetic fields in the presence of a gradient of the mean magnetic field and weak pitchangle diffusion. We demonstrate that such transport is described by asymmetric diffusion: the generalization of the conventional diffusion process to the case of random walk with unequal probabilities. Using a Markov chain analysis of a toy 1D model, we demonstrate that the particle density distribution becomes exponential in distance, instead of linear as is the case for the standard diffusion process. Implications of our results for the transport of Cosmic Rays in the Galaxy are discussed. [Preview Abstract] 
Friday, November 2, 2012 10:54AM  11:06AM 
YO4.00008: Statistics of fieldaligned intermittent electron flux in a linear ECR plasma Shinji Yoshimura, Kenichiro Terasaka, Mitsutoshi Aramaki, Masayoshi Y. Tanaka Spontaneous emission of fieldaligned intermittent highenergy electron flux has been observed in a linear electroncyclotronresonance (ECR) plasma produced in the HYPERI device (NIFS, Japan). We utilized the temporal variation of probe's floating potential due to electron influx as an index of the intermittent events. Time series of the floating potential fluctuation have been analyzed statistically. The probability density function (PDF) exhibits a nonGaussian distribution with a long tail in the negative amplitude side, indicating that the signal is dominated by large amplitude negative spikes. The frequency distribution of waiting time, which is defined by the time interval between two consecutive spikes, is well fitted by an exponential distribution, implying a probable connection to the stationary Poisson process. Although a powerlaw dependence is found in the duration distribution, its relation to the selforganized criticality has not been clear. The effect of ion species on the statistics above will also be discussed. [Preview Abstract] 
Friday, November 2, 2012 11:06AM  11:18AM 
YO4.00009: Turbulence and Dynamos in a Rotating Disks Krista Martocci, Fausto Cattaneo, Paul Fischer, Aleksandr Obabko We present numerical evidence for largescale dynamo action in a rotating disk. The results are from our threedimensional, global simulations of magnetized cylindrical Couette flow at high Reynolds numbers. The rotation profile of our set up is hydrodynamically stable, but hydromagnetically unstable. Therefore, only if a disk is magnetized will drive turbulence, which is necessary for efficient angular momentum transport. This leads to the question of whether these unstable disks can self magnetize or sustain dynamo action. In other words, can this turbulence generate and increase the magnetic fields necessary to continue driving the turbulence? The nonlinear evolution of the system leads to a sustained turbulent state capable of generating strong, coherent azimuthal magnetic structures. Cyclic behavior, in which these structures are formed and destroyed, is apparent in the simulations. The Maxwell stresses associated with the magnetic structures are largely responsible for the outward transport of angular momentum. We will discuss how this turbulent transport is affected by changes in the geometry, in particular, flattening to a more disklike shape. The implications for astrophysical disks will also be discussed. [Preview Abstract] 
Friday, November 2, 2012 11:18AM  11:30AM 
YO4.00010: Not Much Helicity is Needed to Drive Large Scale Dynamos Jonathan Pietarila Graham, Eric Blackman, Pablo Mininni, Annick Pouquet Understanding the in situ amplification of large scale magnetic fields in turbulent astrophysical rotators has been a core subject of dynamo theory. When turbulent velocities are helical, large scale dynamos that substantially amplify fields on scales that exceed the turbulent forcing scale arise, but the minimum sufficient fractional kinetic helicity $f_{h,C}$ has not been previously well quantified. Using direct numerical simulations for a simple helical dynamo, we show that $f_{h,C}$ decreases as the ratio of forcing to large scale wave numbers $k_F/k_{min}$ increases. From the condition that a large scale helical dynamo must overcome the backreaction from any nonhelical field on the large scales, we develop a theory that can explain the simulations. For {$k_F/k_{min}\ge8$ we find $f_{h,C}< 3\%$,} implying that very small helicity fractions strongly influence magnetic spectra for {even moderate} scale separation. [Preview Abstract] 
Friday, November 2, 2012 11:30AM  11:42AM 
YO4.00011: On the transition from electromagnetic to electrostatic shocks Anne Stockem, Frederico Fiuza, Elisabetta Boella, Ricardo Fonseca, Luis Silva Electrostatic and electromagnetic shocks are relevant in various unmagnetized scenarios. The first can be produced in the laboratory by the interaction of a laser with a nearcritical density target and are of interest for the generation of quasimonoenergetic ion beams, e.g. for cancer therapy, whereas electromagnetic shocks are more relevant in astrophysical scenarios. We explore the conditions under which these shocks are generated in a scenario of two colliding plasma slabs, each consisting of cold ions and electrons with nonzero temperature. The main features of the shock character are discussed as a function of the initial fluid velocity and the electron temperature, and the governing regimes are theoretically predicted, by considering the shock formation time scales and the relevant scales for the instabilities mediating the shock formation. Particleincell simulations confirm the theoretical findings and show the transition between both regimes. [Preview Abstract] 

YO4.00012: ABSTRACT WITHDRAWN 
Friday, November 2, 2012 11:54AM  12:06PM 
YO4.00013: Langmuir Turbulence and Suprathermal Electron Acceleration Peter Yoon The suprathermal electron velocity distribution functions (VDFs) with nonMaxwellian tails are commonly detected in the space environment as well as in the laboratory. For instance, since the early days of laboratory beamplasma experiment, it was known that suprathermal electrons were generated during the experiment. Typical {\em in situ} spacecraft measurements in the solar wind show that electron (and also ion) VDFs contain quasi powerlaw velocity tails. These are often empirically modeled by the socalled kappa distribution, but their physical origin is not clearly understood. In this paper a selfconsistent theory for the formation of kappalike distributions is discussed in the context of the electronLangmuir turbulence interaction process. It is shown that the balance of spontaneous and induced emissions and scattering processes not only lead to the selfconsistent determination of asymptotic turbulence spectrum and electron VDF, but also the powerlaw index associated with the energetic tail is uniquely determined as well. Theoretical predictions of the velocity powerlaw index is compared against the superhalo electron velocity distribution detected by WIND and STEREO spacecraft. It is shown that the theoretical prediction is in good agreement with the observation. [Preview Abstract] 
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