Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session GO5: Turbulence and Transport |
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Chair: Mark Gilmore, University of New Mexico Room: Governor's Square 10 |
Tuesday, November 12, 2013 9:30AM - 9:42AM |
GO5.00001: Identification of chaotic and stochastic processes by permutation entropy analysis J.E. Maggs, G.J. Morales The dynamical nature of time signals can be determined by the simultaneous use of entropy and statistical complexity [O. A. Rosso, \textit{et al.}, \textit{Phys. Rev. Lett. }99 154102 (2007)]. These key measures can be implemented using the amplitude permutation probability introduced by Bandt and Pompe [C. Bandt and B. Pompe, \textit{Phys. Rev. Lett.}, 88 174102 (2002)]. Stochastic and chaotic processes are distinguished because they occupy different regions of the entropy-complexity plane. Permutation entropy analysis is used to demonstrate that temperature fluctuations observed in a basic heat transport experiment arise from chaotic dynamics [J. E. Maggs and G. J. Morales, \textit{Plasma Phys. and Control. Fusion}, 55 085015 (2013)]. Locations of various known stochastic and chaotic processes in the entropy-complexity plane are presented and the important technique of 'sub-sampling' for the amelioration of noise is discussed. The permutation entropy analysis can be applied to any time signal as no pre-processing or \textit{a priori} conditions are required. This signal analysis technique has the potential to uncover new features in a wide range of fusion and basic plasma experiments. [Preview Abstract] |
Tuesday, November 12, 2013 9:42AM - 9:54AM |
GO5.00002: Gyrokinetic Simulation of Lorentzian Pulse Generation and Anomalous Transport in a Magnetized Temperature Filament R. Sydora, J. Maggs, G. Morales, B. Van Compernolle 3D gyrokinetic particle simulations with self-consistent electrostatic and electromagnetic fields have been carried out for plasma conditions similar to those in the Large Plasma Device (LAPD) temperature filament experiments at UCLA. The observed electron temperature gradient-driven drift-Alfven fluctuations closely match the eigenmodes predicted by linear theory and in the nonlinear regime the spatio-temporal pattern of the electric potential and density fluctuations (spiral-like) also agree with experiments. The temporal behavior of the electric potential, magnetic field and temperature fluctuations at fixed spatial position contain non-sinusoidal pulses which are fit with a Lorentzian shape. These are consistent with an exponential frequency spectrum in the broadband turbulence as has also been found in the LAPD and other experiments. The temperature pulses were found to be approximately four times narrower than the electric and magnetic field pulses. A permutation entropy analysis (J. Maggs, G. Morales, PPCF, 55, 085015 (2013)) was also performed confirming that the underlying dynamics is chaotic rather than stochastic. [Preview Abstract] |
Tuesday, November 12, 2013 9:54AM - 10:06AM |
GO5.00003: A Study of Biasing and Magnetic Field on Intrinsic Fluctuations in a Linear Plasma Device Tiffany Hayes, Mark Gilmore, Josh Plank Biasing experiments are currently being conducted in the linear Helicon-Cathode (HelCat) Device. Using a set of concentric electrode rings, biasing is used to suppress intrinsic drift fluctuations. It is seen that the fluctuation magnitude is strongly dependent on the magnetic field strength, B. Depending on the magnetic field, modes with varying m numbers are observed, and at high B, fluctuations are turbulent. The change from coherent at low field, to turbulent at high field, is non-monotonic. At some B-field strengths, no instability exists, while chaos is observed at others. With the change in magnetic field, and fluctuation amplitude, the bias needed to affect the instability also changes. At a low B, a bias of approximately 3T$_{\mathrm{e}}$ is sufficient to suppress fluctuations, whereas at B, 40T$_{\mathrm{e}}$ or greater bias is needed to have an effect. Additionally, biasing has been performed using a semitransparent grid electrode near the source. A grid bias $\sim$ 10-15 T$_{\mathrm{e}}$ causes the plasma to develop large scale ($\sim$ 100{\%}) fluctuations in both density and potential across the entire plasma column on a timescale $\sim$ 10L/cs. This plasma collapse and rebound may indicate the presence of a bistable potential profile. Experimental results and 1D simulations will be presented. [Preview Abstract] |
Tuesday, November 12, 2013 10:06AM - 10:18AM |
GO5.00004: Effects of ExB Velocity Shear on Nonlinear Coupling between Electron Temperature Gradient Mode and Drift Wave Mode in Magnetized Plasmas Chanho Moon, Rikizo Hatakeyama, Toshiro Kaneko In our previous work, we have observed that the multi-scale nonlinear coupling between the electron temperature gradient (ETG) mode ($\sim$0.4 MHz) and the drift wave mode ($\sim$7 kHz) is enhanced when the ETG mode exceeds a certain threshold. In this paper, we report the effects of ExB velocity shear on the ETG mode and the drift wave mode in linear magnetized plasmas. The ExB velocity shear owing to the radial electric field can be controlled independently of the ETG by changing the bias voltages of the electron emitter [1]. When the radial electric field becomes slightly positive, the ETG mode amplitude is increased, because a bicoherence between the ETG and drift wave modes is decreasing with an increase in the ExB velocity shear, which reduces the energy transfer of ETG mode and increase the ETG mode amplitude. On the other hand, the ETG mode is suppressed with the strong radial electric field regardless of its signs. It can be figured out for the first time that the ExB velocity shear affects not only the suppression but also the nonlinear coupling of the ETG mode. \\[4pt] [1] C. Moon et al., Rev. Sci. Instrum.\textbf{ 81}, 053506 (2010). [Preview Abstract] |
Tuesday, November 12, 2013 10:18AM - 10:30AM |
GO5.00005: Experimental investigation of the fast-ion transport in the ASDEX Upgrade tokamak Benedikt Geiger, Ralph Dux, Francois Ryter, Giovanni Tardini, Manuel Garcia-Munoz The radial transport of fast-ions is an active field of investigation in fusion devices. In particular, in the presence of MHD instabilities, fast-ions can be redistributed and even ejected from the plasma. This reduces the plasma heating and current drive efficiencies and must consequently be investigated and avoided in view of future fusion devices. In ASDEX Upgrade, sawtooth crashes in NBI heated plasmas have been observed to induce a very strong radial redistribution of the fast-ion population, as measured by fast-ion D-alpha (FIDA) spectroscopy. Modelling done with TRANSP assuming the Kadomstev sawtooth model very well reproduces the experimental measurements. In contrast to the strong anomalous fast-ion transport due to sawtooth crashes, the transport of the fast ions is found to be neo-classical in the absence of significant MHD activity. This is shown by the measurement of the redistributed fast-ions in the time interval following the crashes and by dedicated experiments with off-axis NBI deposition. All the measurements in MHD quiescent plasmas are well reproduced by the neo-classical fast-ion distribution functions from the TRANSP code. [Preview Abstract] |
Tuesday, November 12, 2013 10:30AM - 10:42AM |
GO5.00006: Alpha particle transport in the presence of toroidal driftwaves Y. Nishimura, B. Huang, C.Z. Cheng Transport of fusion born $\alpha$ particles is investigated in the presence of poloidally mode coupled ballooning type driftwaves.\footnote{ C.Z.Cheng, Phys. Fluids {\bf 25}, 1020 (1982); ibid., {\bf 23}, 1770 (1980).} The onset of orbit stochasticity is understood as an overlapping of electric islands\footnote{B.Weyssow, J.H.Misguich, R.Balescu, Plasma Phys. Controlled Fusion {\bf 33}, 763 (1991).} produced by the driftwaves, whose overlapping threshold is lower for the thermal particles than for the $\alpha$ particles (high energy particles). For the trapped particles, transport is determined by the particles' sensitive response to the fluctuation at the banana tip where the parallel velocity decreases drastically. Time dependent turbulent signals (finite $\omega_{\star}$ effects) give rise to the shift of the resonant radial locations, which again is larger for the thermal particles than the high energy particles. The transport process is influenced by the microscopic structure of the islands, which deviates from the Gaussian process. This work is supported by National Science Council of Taiwan, NSC 100-2112-M-006-021-MY3 and NCKU Top University Project. [Preview Abstract] |
Tuesday, November 12, 2013 10:42AM - 10:54AM |
GO5.00007: Characterization of particle confinement properties in RFX-mod at high Ip Fulvio Auriemma, Matteo Agostini, Paolo Franz, Rita Lorenzini, Paolo Innocente, Paolo Scarin A wide range of plasma density has been explored in order to study the particle confinement properties of RFX-mod plasma at Ip \textgreater\ 1MA. At low density (n/n$_{\mathrm{G}}$ \textless\ 0.25) the MHD spectrum peaks and 1 mode prevails against the others leading to the SHAx state. The PWI results distorted according to the peculiar 3D magnetic topology of the plasma. A particular effort has been devoted to its reconstruction in order to describe the asymmetries of the particle influx, as highlighted by the H$_{\alpha}$ emission distribution. Exploiting such description of the particle influx, transport analysis has been carried out with the ASTRA code, comparing different densities and equilibria. At low density, when the SHAx state is achieved, the core diffusivity results one order of magnitude lower than the non-SHAx configuration, whereas large particle transport is still present at the edge. At higher plasma density a better global particle confinement is found, thanks to the transport reduction of a factor 5 at the edge. The relative role on the particle transport of the magnetic diffusion, acting in the plasma core, and of the electrostatic turbulence, localized in the edge region, will be discussed. [Preview Abstract] |
Tuesday, November 12, 2013 10:54AM - 11:06AM |
GO5.00008: An ICF Mix Model with Experimental Input James Glimm, Baolian Cheng, Douglas Wilson, David Sharp, Hyunkyung Lim, Jeremy Melvin, Verinder Rana A buoyancy drag type mix model is adapted to the study of mix in the deceleration phase of an ICF capsule. The time dependent Atwood number and acceleration are essential inputs to the model. A range of values for these parameters reflecting experimental data and 1D simulations are studied. The simulations are based on HYDRA, FLASH and the Stony Brook code FronTier. Physical quantities from HYDRA at deceleration time provide inputs to the other simulations. Time dependent density profiles and accelerations taken from HYDRA simulations suggest only a minor level of mix for the deceleration phase. Results from FLASH and FronTier simulations with modified input and having an Atwood number closer to experiment at the time of maximum neutron production, suggest a significant level of mixing. [Preview Abstract] |
Tuesday, November 12, 2013 11:06AM - 11:18AM |
GO5.00009: Energy Transfer and Turbulence in Hypersonic and Laser-Induced Plasmas Kyron Williams, L.E. Johnson, A.B. Alexander, C. Akpovo, J. Martinez, J. Titus It has been previously reported that hypersonic weakly ionized Argon and Krypton plasmas created by electric discharge can display Stark and Zeeman profiles that are self-induced. Now evidence suggests that the internal plasma dynamics that contribute to the Stark and Zeeman profiles exhibit a lower level of complexity than profiles that are not Stark and Zeeman lines. Energy transport in the system is also studied and found to exhibit a dependence on Stark/Zeeman profiles that is not observed in non-Stark/Zeeman profiles. In addition, the evidence of Stark and Zeeman profiles are seen in laser induced plasmas as well. Analysis of the Stark/Zeeman spectra in laser-induced plasmas indicates turbulent signatures correlated with pulse frequency and width, which are not seen in non-Stark/Zeeman profiles in such plasmas. [Preview Abstract] |
Tuesday, November 12, 2013 11:18AM - 11:30AM |
GO5.00010: Astrophysically Relevant Turbulent Magnetic Fields in Table-Top Intense-Laser Experiments G. Ravindra Kumar, Gourab Chatterjee, Amit D. Lad, Prashant K. Singh, Amitava Adak, P. Brijesh, Z.M. Sheng, Amita Das, Sudip Sengupta, Predhiman K. Kaw We present experiments on the spatio-temporal dynamics of megagauss magnetic fields created in solid-density plasmas by intense femtosecond laser pulses. Our results display distinctive signatures of the Kolmogorov -5/3 scaling in the k-spectra of the turbulent magnetic fields. About 50 picoseconds after the incidence of the main interaction laser pulse, the spectrum displays two distinct turbulent regimes, characterized by a -5/3 scaling (for smaller values of k) and a -7/3 scaling (for larger values of k), separated by a spectral ``kink.'' Similar spectral ``knees'' have been observed previously in Alfven ion cyclotron waves in the turbulent magnetosheath of the earth as well as in turbulent processes in the solar photosphere. [Preview Abstract] |
Tuesday, November 12, 2013 11:30AM - 11:42AM |
GO5.00011: Transition Between Saturation Regimes of Gyrokinetic Turbulence David Hatch, Frank Jenko, Alejandro Banon Navarro, Vasil Bratanov We examine the injection, spectral redistribution, and dissipation of free energy in a turbulent reduced-gyrokinetic system over a broad range of background gradients and collision frequencies. A Hermite representation is used for the parallel velocity coordinate, allowing for a detailed study of the scales of free energy dynamics over the entire phase-space. A type of \textit{critical balance}---an equilibration of the parallel streaming time and the nonlinear correlation time---is observed for each order Hermite polynomial. This critical balance produces steep Hermite spectra, causing the collisional dissipation in parallel velocity space to peak at large velocity space scales even for very small collision frequencies. The dimensionless parameter L$_{\mathrm{T}}$/L$_{\mathrm{C}}$ (the ratio of the temperature gradient scale length to the collisional mean free path) determines a transition between saturation regimes. For very small values of L$_{\mathrm{T}}$/L$_{\mathrm{C}}$ the dissipation occurs mostly at small perpendicular scales. As L$_{\mathrm{T}}$/L$_{\mathrm{C}}$ increases (but still much less than unity), the dissipation is dominated by large scales in \textit{all} phase-space dimensions. [Preview Abstract] |
Tuesday, November 12, 2013 11:42AM - 11:54AM |
GO5.00012: Global Gyrokinetic Particle Simulations of Electromagnetic Drift-Wave Turbulence and Transport Ihor Holod Gyrokinetic simulation of the finite-$\beta $ drift-wave microturbulence in global tokamak geometry, using GTC code, has been reported. In the GTC the electron dynamics is treated using the fluid-kinetic hybrid electron model developed to enhance numerical efficiency. In this model, the electron response is separated into the lowest order adiabatic part, treated as a massless fluid, and the high-order kinetic correction described by the drift-kinetic equation. The fluid-kinetic hybrid electron model is verified by running linear simulations using Cyclone base case parameters with different values of $\beta_{\mathrm{e}}$ (ratio of electron kinetic pressure to magnetic pressure), obtained by varying the equilibrium electron density. The dependence of mode real frequency and linear growth rate on $\beta _{\mathrm{e}}$ recovers the effect of $\beta $-stabilization of the ion-temperature gradient mode, transition to the trapped electron mode and the onset of the kinetic-ballooning mode. The nonlinear simulations address the effect of zonal flow and zonal current on mode saturation amplitude, and heat transport properties. [Preview Abstract] |
Tuesday, November 12, 2013 11:54AM - 12:06PM |
GO5.00013: Directional multi-scale statistics of quasi-static magnetohydrodynamic turbulence Naoya Okamoto, Katsunori Yoshimatsu, Kai Schneider, Marie Farge Anisotropy and intermittency of quasi-static magnetohydrodynamic (MHD) turbulence in an imposed magnetic field are examined, using three-dimensional orthonormal wavelet analysis. This analysis is applied to two turbulent MHD flows computed by direct numerical simulation with $512^3$ grid points and with different intensities of the imposed magnetic field. It is found that the imposed magnetic field leads to a substantial amplification of intermittency in the velocity field, especially in the direction of the imposed magnetic field. The Eulerian and Lagrangian accelerations are also examined by applying directional multi-scale analyses. [Preview Abstract] |
Tuesday, November 12, 2013 12:06PM - 12:18PM |
GO5.00014: Dynamics and turbulence in electron MHD Maxim Lyutikov We consider dynamics and turbulent interaction of whistler modes within the framework of inertialess electron MHD (EMHD). We argue there is no energy principle in EMHD: any stationary closed configuration is neutrally stable. We consider the turbulent cascade of whistler modes. We show that (i) harmonic whistlers are exact non-linear solutions; (ii) co-linear whistlers do not interact (including counter- propagating); (iii) whistler modes have a dispersion that allows a three-wave decay, including into a zero frequency mode; (iv) the three-wave interaction effectively couples modes with highly different wave numbers and propagation angles. In addition, linear interaction of a whistler with a single zero-mode can lead to spatially divergent structures via parametric instability. We derive the Hamiltonian formulation of EMHD, and using Bogolyubov transformation reduce it to a canonical form; we calculate the matrix elements for the three-wave interaction of whistlers. We solve numerically the kinetic equation and show that, generally, the EMHD cascade depends on the forcing and often fails to reach a steady state. Analytical estimates predict the spectrum of magnetic fluctuations for the quasi-isotropic cascade $\sim$k$^{-2}$. The cascade remains weak (not critically-balanced). The cascade is UV-local, while the infrared locality is weakly (logarithmically) violated. [Preview Abstract] |
Tuesday, November 12, 2013 12:18PM - 12:30PM |
GO5.00015: Electron Fluid Description of Wave-Particle Interactions in Strong Buneman Turbulence Haihong Che To understand the nature of anomalous resistivity in magnetic reconnection, we investigate turbulence-induced momentum transport and energy dissipation associated with electron heating in Buneman instability. Using 3D particle-in-cell simulations, we find that the macroscopic effects generated by wave-particle interactions can be described by a set of electron fluid equations. These equations show that the energy dissipation and momentum transports in Buneman instability are locally quasi-static but globally non-static and irreversible. Turbulence drag dissipates both the bulk energy of electron streams and the associated magnetic energy. The decrease of magnetic field maintains an inductive electric field that re-accelerates electrons. The net loss of streaming energy is converted into electron heat and increases the electron Boltzmann entropy. The growth of self-sustained Buneman waves satisfies a Bernoulli-like equation which relates the turbulence-induced convective momentum transport and thermal momentum transport. Electron trapping and de-trapping drives local momentum transports, while phase mixing converts convective momentum into thermal momentum.These two local momentum transports sustain the Buneman waves and act as the micro-macro link in the anomalous heating process. [Preview Abstract] |
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