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 TI2: Fundamental Plasma Physics II |
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Chair: Yu Lin, Auburn University Room: Ballroom DE |
Thursday, November 1, 2012 9:30AM - 10:00AM |
TI2.00001: Quantum physics of classical waves in plasma Invited Speaker: I.Y. Dodin The Lagrangian approach to plasma wave physics is extended to a universal nonlinear theory which yields generic equations invariant with respect to the wave nature. The traditional understanding of waves as solutions of the Maxwell-Vlasov system is abandoned. Oscillations are rather treated as physical entities, namely, abstract vectors $|\psi\rangle$ in a specific Hilbert space. The invariant product $\langle\psi|\psi\rangle$ is the total action and has the sign of the oscillation energy. The action density is then an operator. Projections of the corresponding operator equation generate assorted wave kinetic equations; the nonlinear Wigner-Moyal equation is just one example and, in fact, may be more delicate than commonly assumed. The linear adiabatic limit of this classical theory leads to quantum mechanics in its general form. The action conservation theorem, together with its avatars such as Manley-Rowe relations, then becomes manifest and in partial equilibrium can modify statistical properties of plasma fluctuations. In the quasi-monochromatic limit geometrical optics (GO) is recovered and can as well be understood as a particular field theory in its own right. For linear waves, the energy-momentum equations, in \textit{both} canonical and (often) kinetic form, then follow automatically, even without a reference to electromagnetism. Yet for waves in plasma the general GO Lagrangian is also derived explicitly, in terms of single-particle oscillation-center Hamiltonians. Applications to various plasma waves are then discussed with an emphasis on the advantages of an abstract theory. Specifically covered are nonlinear dispersion, dynamics, and stability of BGK modes, and also other wave transformations in laboratory and cosmological plasmas. [Preview Abstract] |
Thursday, November 1, 2012 10:00AM - 10:30AM |
TI2.00002: Observation of improved and degraded confinement with driven flow on the LAPD Invited Speaker: David Schaffner External continuous control over azimuthal flow and flow shear has been achieved in a linear plasma device for the first time allowing for a careful study of the effect of flow shear on pressure-gradient-driven turbulence and transport in the edge of the Large Plasma Device (LAPD). The flow is controlled using biasable iris-like limiters situated axially between the cathode source and main plasma chamber. LAPD rotates spontaneously in the ion diamagnetic direction (IDD); positive limiter bias first reduces, then minimizes (producing a near-zero shear state), and finally reverses the flow into the electron diamagnetic direction (EDD). Degradation of particle confinement is observed in the minimum shearing state and reduction in turbulent particle flux is observed with increasing shearing in both flow directions. Near-complete suppression of turbulent particle flux is observed for shearing rates comparable to the turbulent autocorrelation rate measured in the minimum shear state. Turbulent flux suppression is dominated by amplitude reduction in low-frequency ($>10$kHz) density fluctuations and a reduction in the radial correlation length. An increase in fluctuations for the highest shearing states is observed with the emergence of a coherent mode which does not lead to net particle transport. Magnetic field is varied in order to explore whether and how field effects transport modification. Calculations of transport equations are used to predict density profiles given source and temperature profiles and can show the level of transport predicted to be necessary in order to produce the experimental density profiles observed. Finally, the variations of density fluctuations and radial correlation length are fit well with power-laws and compare favorably to simple models of shear suppression of transport. [Preview Abstract] |
Thursday, November 1, 2012 10:30AM - 11:00AM |
TI2.00003: Energy dynamics in a simulation of LAPD turbulence Invited Speaker: Brett Friedman It is often assumed that linear instabilities maintain turbulence in plasmas and some fluids, but this is not always the case. It is well known that many fluids display subcritical turbulence at a Reynolds number well below the threashold of linear instability. Certain plasma models such as drift waves in a sheared slab also exhibit subcritical turbulence [1]. In other instances such as drift-ballooning turbulence in tokamak edge plasmas, linear instabilities exist in a system, but they become subdominant to more robust nonlinear mechanisms that sustain a turbulent state [2, 3]. In our simulation of LAPD turbulence, which was previously analyzed in [4], we diagnose the results using an energy dynamics analysis [5]. This allows us to track energy input into turbulent fluctuations and energy dissipation out of them. We also track conservative energy transfer between different energy types (e.g. from potential to kinetic energy) and between different Fourier waves of the system. The result is that a nonlinear instability drives and maintains the turbulence in the steady state saturated phase of the simulation. While a linear restistive drift wave instability resides in the system, the nonlinear drift wave instability dominates when the fluctuation amplitude becomes large enough. The nonlinear instability is identified by its energy growth rate spectrum, which varies significantly from the linear growth rate spectrum. The main differences are the presence of positive growth rates when k$_{\vert \vert }$ = 0 and negative growth rates for nonzero k$_{\vert \vert }$, which is opposite that of the linear growth rate spectrum.\\[4pt] [1] B. D. Scott, Phys. Rev. Lett., 65, 3289 (1990).\\[0pt] [2] A. Zeiler et al, Phys. Plasmas, 3, 2951 (1996).\\[0pt] [3] B. D. Scott, Phys. Plasmas, 12, 062314 (2005).\\[0pt] [4] P. Popovich et al, Phys. Plasmas, 17, 122312 (2010).\\[0pt] [5] [physics.plasm-ph]. [Preview Abstract] |
Thursday, November 1, 2012 11:00AM - 11:30AM |
TI2.00004: Nonlinear instabilities driven by coherent phase-space structures Invited Speaker: Maxime Lesur Coherent phase-space (PS) structures are an important feature of plasma turbulence. They can drive nonlinear instabilities [1], intermittency in drift-wave turbulence [2], and transport [3]. We aim at a comprehensive understanding of turbulence, not just as an ensemble of waves, as quasilinear theory implies, but as a mixture of coupled waves and localized structures. This work, which focuses on isolated PS structures, is a fundamental advance in this direction. We analyze the effects of self-binding negative fluctuations (PS holes) on stability, intermittency and anomalous resistivity, both analytically and numerically. We present a new theory which describes the growth of a hole or clump [4]. We find that PS holes grow nonlinearly, independently of linear stability. Numerical simulations clarify the physics of nonlinear instabilities in both subcritical and supercritical conditions. When many resonances are unstable, several holes can coalesce into one main macro-scale structure, which survives much longer than a quasilinear diffusion time, suggesting that it may be crucial to resolve phase-space turbulence in analytical and numerical studies of transport. These findings are applied to two fundamental paradigms of plasma physics: bump-on-tail instabilities in 1D electronic plasma and current-driven ion-acoustic instabilities electron-ion plasma. Our results expose important limits of routinely-used linear and quasilinear theories.\\[4pt] [1] T.H. Dupree, Phys. Fluids 15, 334 (1972); R.H. Berman et al., Phys. Rev. Lett. 48, 1249 (1982).\\[0pt] [2] P.W. Terry, P.H. Diamond, and T.S. Hahm, Phys. Fluids B 2, 2048 (1990).\\[0pt] [3] H. Biglari et al., Phys. Fluids 31, 2644 (1988); Y. Kosuga et al., Phys. Plasmas 18, 122305 (2011).\\[0pt] [4] M. Lesur, P.H. Diamond, submitted to Phys. Rev. Lett. [Preview Abstract] |
Thursday, November 1, 2012 11:30AM - 12:00PM |
TI2.00005: Weak Turbulence Effects in Space Plasmas Invited Speaker: Chris Crabtree With the advent of multi-satellite missions such as Cluster and the Radiation Belt Storm Probes (RBSP) space plasmas have become a rich laboratory for the detailed and fundamental study of plasma turbulence. Space offers a diversity of plasma environments to directly test theory and simulation, from high-$\beta$ plasmas in the solar-wind and the Earth's magnetotail, to low-$\beta$ multi-species plasmas in the radiation belts and ionosphere. Recent theoretical work has demonstrated that by considering the effects of induced non-linear scattering (non-linear Landau damping, to be referred to as NL scattering) of electromagnetic waves leads to testable predictions in both storm-time radiation belt plasmas and the solar wind turbulent spectrum at scales below the ion gyroradius. In the radiation belts, VLF waves (with frequencies between the ion and electron gyrofrequencies) of sufficient amplitude may be nonlinearly scattered near the lower-hybrid surface inside the plasmasphere. Upon scattering a portion of these waves can return to the ionosphere where they may be reflected. This process can lead to the formation of a VLF wave cavity [1] that can efficiently resonate with the energetic (MeV) trapped electron population and quickly precipitate these particles into the ionosphere [2]. In the solar wind, the large-scale Alfvenic fluctuations can be shown to lead to a plateau in the electron distribution function that reduces the Landau damping of kinetic Alfven waves (KaWs). With the reduction of the linear damping the NL scattering of KAWs becomes important and leads to a non-local redistribution of energy in k-space and results in a steeper turbulent spectrum [3]. The edges of the plateaus are also unstable to electromagnetic left hand polarized ion cyclotron-Alfven waves as well as right hand polarized magnetosonic-whistler waves. These waves can pitch angle scatter the ion super-thermal velocity component to provide perpendicular ion heating [4]. \\[4pt] [1] C. Crabtree, L. Rudakov, G. Ganguli, M. Mithaiwala, V. Galinsky, V. Shevchenko, Phys. Plasmas 19, 032903, (2012).\\[0pt] [2] G. Ganguli, L. Rudakov, C. Crabtree, and M. Mithaiwala, Submitted to Geophys Res. Lett. (2012).\\[0pt] [3] L. Rudakov, M. Mithaiwala, G. Ganguli, and C. Crabtree, Phys. Plasmas 18, 012307 (2011).\\[0pt] [4] L. Rudakov, C. Crabtree, G. Ganguli, and M. Mithaiwala, Phys. Plasmas 19, 042704 (2012). [Preview Abstract] |
Thursday, November 1, 2012 12:00PM - 12:30PM |
TI2.00006: Guide Field Effects on Hall Reconnection in a Laboratory Plasma Invited Speaker: Tim Tharp Guide field effects are a major outstanding research issue for magnetic reconnection. Recent results from simulation and observation indicate that guide field can strongly impact reconnection dynamics in collisionless two-fluid regimes such as the earth's magnetosphere and magnetotail. In particular, simulations agree that the addition of guide field can produce effects such as a tilting of the current sheet, reduction or destruction of the quadrupole field, and a reduced reconnection rate. In the present work, the effect of guide field on magnetic reconnection has been quantitatively studied in the two-fluid regime by systematically varying an externally applied guide field in the Magnetic Reconnection Experiment (MRX). The quadrupole field, a signature of two-fluid reconnection, is significantly altered by the presence of guide field. The structure of the layer is twisted in the plane by $J\times B$ forces, and the resulting modified Hall fields are very similar to those observed in simulation. Additionally, the in-plane Hall currents are reduced by an amount consistent with the scaling $E_{rec} \approx \frac{J\times B}{ne}$, indicating that two-fluid physics is critical to the local reconnection dynamics. The reconnection rate, $E_{rec}$, is strongly reduced with increasing guide field. This dependence is explained by the advection and compression of guide field, which produces an increased magnetic pressure both within and downstream of the reconnection region. These observations indicate that a dynamic guide field is capable of influencing reconnection through both the local physics of two-fluid reconnection and the global physics of pressure balance. [Preview Abstract] |
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