Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session GI3: Plasma Turbulence |
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Chair: Masaaki Yamada, Princeton Plasma Physics Laboratory Room: Ballroom AC |
Tuesday, November 15, 2011 9:30AM - 10:00AM |
GI3.00001: Dissipation Range Turbulent Cascades in Plasmas Invited Speaker: Recent spectral measurements of turbulence in the interstellar medium, solar wind, and laboratory experiments reach small scales where dissipation likely plays a role. The spectra show breaks in power laws and steepening of spectral indices from either small-scale inertial effects or dissipation. These observations motivated formulation of the first theoretical description of dissipation range cascades in MHD turbulence, with generalizations to other types of dissipation range plasma turbulence with scalable dissipation rates, including tokamak micorturbulence. The central construct is the scaled attenuation in wavenumber space of the spectral energy transfer rate. With closure, this yields spectra characterized by the product of a power law and exponential fall-off. Both functional forms apply to all scales. Spectral indices of the power law and exponential fall-off depend on the scaling of the dissipation, the strength of the nonlinearity, and alignment of vector fields. The theory describes three very different spectral measurements. Turbulence at low magnetic Prandtl number in the liquid metal Madison Dynamo Experiment has a magnetic dissipation range/flow inertial range dominated by power law behavior, well matched by the MHD dissipation range theory. Magnetic turbulence in the MST reversed field pinch plasma has spectra that are exponential or power law, depending on propagation direction and scale. The dominant small-scale turbulence fits a product of a power law and exponential, but the implied dissipation is larger than expected for classical dissipation, hinting at kinetic effects. Gyrokinetic ion- temperature-gradient turbulence has a spectrum in which the exponential component and dissipation become weaker at small scale, giving a power law asymptotically. Strong dissipation at large scale and the asymptotic power law are observed in simulation. [Preview Abstract] |
Tuesday, November 15, 2011 10:00AM - 10:30AM |
GI3.00002: Gyrokinetic Simulations of Solar Wind Turbulence Invited Speaker: Recent high sampling rate solar wind turbulence observations of have been extended well into the frequency regime above the observed magnetic energy spectral break at $1Hz$, where a nearly power law spectrum with a spectral index around $-2.8$ is observed. The range of scales above the spectral break is typically referred to as the dissipation range and corresponds to the range of spatial scales between the Doppler- shifted ion and electron gyro-radii. One of the proposed theories to explain the dissipation range of magnetic turbulence is a cascade of low frequency kinetic Alfv\'{e}n waves. We present the results of the first three-dimensional, non-linear gyrokinetic simulation of plasma turbulence resolving scales from the ion to the electron gyro-radius, where all dissipation is provided by resolved physical mechanisms. The simulation employs parameters comparable to solar wind plasma and yields a magnetic spectral index of $-2.8$, in excellent quantitative agreement with observations. Since the simulation fully resolves the physical dissipation, we are able to explore a variety of novel aspects of turbulence, including the first evidence of the proposed ion entropy cascade in a three-dimensional, electromagnetic turbulence simulation. [Preview Abstract] |
Tuesday, November 15, 2011 10:30AM - 11:00AM |
GI3.00003: Fast Ion and Thermal Plasma Transport in Turbulent Waves in the Large Plasma Device (LAPD) Invited Speaker: The transport of fast ions and thermal plasmas in electrostatic microturbulence is studied. Strong density and potential fluctuations ($\delta n/n\sim \delta \varphi /kT_e \sim 0.5$, f$\sim $5-50 kHz) are observed in the LAPD in density gradient regions produced by obstacles with slab or cylindrical geometry. Wave characteristics and the associated plasma transport are modified by driving sheared E$\times $B drift through biasing the obstacle, and by modification of the axial magnetic fields (B$_{z})$ and the plasma species. Cross-field plasma transport is suppressed with small bias and large B$_{z}$, and is enhanced with large bias and small B$_{z}$. Suppressed cross-field thermal transport coincides with a 180\r{ } phase shift between the density and potential fluctuations in the radial direction, while the enhanced thermal transport is associated with modes having low mode number (m=1) and long radial correlation length. Large gyroradius lithium ions ($\rho _{fast} /\rho _s \sim 10)$ orbit through the turbulent region. Scans with a collimated analyzer and with Langmuir probes give detailed profiles of the fast ion spatial-temporal distribution and of the fluctuating fields. Fast-ion transport decreases rapidly with increasing fast-ion gyroradius. Background waves with different scale lengths also alter the fast ion transport: Beam diffusion is smaller in waves with smaller structures (higher mode number); also, coherent waves with long correlation length cause less beam diffusion than turbulent waves. Experimental results agree well with gyro-averaging theory. When the fast ion interacts with the wave for most of a wave period, a transition from super-diffusive to sub-diffusive transport is observed, as predicted by diffusion theory. A Monte Carlo trajectory-following code simulates the interaction of the fast ions with the measured turbulent fields. Good agreement between observation and modeling is observed. [Preview Abstract] |
Tuesday, November 15, 2011 11:00AM - 11:30AM |
GI3.00004: Highly localized, fully 3-D disruptions of the reconnection layer in the Magnetic Reconnection Experiment Invited Speaker: Magnetic reconnection is a fundamental process in plasmas which converts magnetic energy to plasma kinetic and thermal energy through topological changes. One of the important goals in magnetic reconnection research is to explain the fast reconnection rate observed in real three-dimensional laboratory and astrophysical systems. In the Magnetic Reconnection Experiment (MRX), an enhancement of the reconnection electric field is often associated with a wholesale disruption of the reconnection current layer, an intrinsically 3-D phenomena observed in the presence of out-of-plane gradients of local quantities such as reconnection layer current and density. During a disruption, the out-of-plane current decreases as current carrying electrons are redirected in the outflow direction. Observed ``O-point'' signatures and density striations suggest that this redirection often occurs though the ejection of 3-D flux rope structures. Large fluctuations in the lower hybrid frequency range are also routinely seen, but the ratio of the phase speed to the diamagnetic drift speed does not match what is predicted by 3-D kinetic simulations without disruptions. A 2-D Hall MHD analysis of the out-of-plane gradients is consistent with the buildup of magnetic energy leading to the event [1], but variation in all three spacial dimensions is required in order to obtain results in agreement with the disruptive behavior observed. Analysis and comparison with 3-D simulations is ongoing to determine if the fluctuations and/or disruptive behavior are responsible for the corresponding discrepancies in the layer structure between the experiments and 2-D kinetic simulations [2,3,4]. Supported by DOE, NASA, and NSF. \\[4pt] [1] J.D. Huba and L.I. Rudakov, Phys. Plasmas 10, 3139 (2003).\\[0pt] [2] Y. Ren, et al., Phys. Plasmas 15, 082113 (2008).\\[0pt] [3] S. Dorfman, et al., Phys. Plasmas 15, 102107 (2008).\\[0pt] [4] V. Roytershteyn, et al., Phys. Plasmas 17, 055706 (2010). [Preview Abstract] |
Tuesday, November 15, 2011 11:30AM - 12:00PM |
GI3.00005: Numerical Simulations of Strong Incompressible Magnetohydrodynamic Turbulence Invited Speaker: Magnetized turbulence pervades the universe and is likely to play an important role in a variety of astrophysical processes. Magnetohydrodynamics provides the simplest theoretical framework in which phenomenological models for the turbulent dynamics can be built. Numerical simulations are widely used to guide and test the theoretical predictions; however, simulating MHD turbulence is not without its difficulties. Computational power limits the simulations to parameter regimes that are much less extreme than those in astrophysics and often simplifying assumptions are made in order that a wider range of scales can be accessed. After describing the competing theoretical predictions and the numerical approaches that are often employed in studying strong incompressible MHD turbulence, I will present the findings of a series of high-resolution direct numerical simulations. I will discuss the effects that physically motivated simplifying assumptions can have on the numerical solution and its physical interpretation.\\[4pt] Collaborators: Stanislav Boldyrev (U. Wisconsin - Madison), Fausto Cattaneo (U. Chicago), Jean C. Perez (U. New Hampshire). [Preview Abstract] |
Tuesday, November 15, 2011 12:00PM - 12:30PM |
GI3.00006: Reduction of Large-scale Turbulence and Optimization of Flows in the Madison Dynamo Experiment Invited Speaker: The Madison Dynamo Experiment seeks to observe a magnetic field grow at the expense of kinetic energy in a flow of liquid sodium. The enormous Reynolds numbers of the experiment and its two vortex geometry creates strong turbulence, which in turn leads to transport of magnetic flux consistent with an increase of the effective resistivity. The increased effective resistivity implies that faster flows are required for the dynamo to operate. Three major results from the experiment will be reported in this talk. 1) A new probe technique has been developed for measuring both the fluctuating velocity and magnetic fields which has allowed a direct measurement of the turbulent EMF from $<$ v x b $>$. 2) The scale of the largest eddies in the experiment has been reduced by an equatorial baffle on the vessel boundary. This modification of the flow at the boundary results in strong field generation and amplification by the mean velocity of the flow, and the role of turbulence in generating currents is reduced. The motor power required to drive a given flow speed is reduced by 20{\%}, the effective Rm, as measured by the toroidal windup of the field(omega effect), increased by a factor of $\sim $2.4, and the turbulent EMF (previously measured to be as large as the induction by the mean flow) is eliminated. These results all indicate that the equatorial baffle has eliminated the largest-scale eddies in the flow. 3) Flow optimization is now possible by adjusting the pitch of vanes installed on the vessel wall. An analysis of the kinematic prediction for dynamo excitation reveals that the threshold for excitation is quite sensitive to the helical pitch of the flow. Computational fluid dynamics simulations of the flow showed that by adjusting the angle of the vanes on the vessel wall (which control the helical pitch of the flow) we should be able to minimize the critical velocity at which the dynamo onset occurs. Experiments are now underway to exploit this new capability in tailoring the large-scale flow. [Preview Abstract] |
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