Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session TI2: Flows and Turbulence in Laboratory and Space |
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Chair: William Amatucci, Naval Research Laboratory Room: Centennial I |
Thursday, November 5, 2009 9:30AM - 10:00AM |
TI2.00001: How ions really flow to objects in magnetized plasmas Invited Speaker: A new exact, 3-D, analytic solution has been obtained of the drift equations for the plasma perturbation around an ion-collecting object of essentially arbitrary shape in a flowing strongly-magnetized plasma [1]. It provides a direct rigorous theory of magnetized Langmuir probes, especially Mach probes, and of other diverse plasma-object interactions. The cross-field flow is a combination of external drifts and self-consistent perturbation by the object. This convective calculation is valid for objects smaller than the characteristic size of turbulent plasma flow structures: the usual situation. The Mach probe calibration obtained agrees with prior diffusive theory, but now accounts for cross-field diamagnetic drifts, which are not ignorable. Their contribution is non-intuitive, in that the {\it electron} diamagnetic velocity is dominant. Consequently, transverse Mach probes measure a combination of ExB and diamagnetic drifts. The analytic fluid solution compares very favorably with numerical kinetic-parallel-distribution calculations [2]. And 3-D PIC calculations for a spherical object, accounting for finite gyro-radius, also verify its regime of applicability.\\[4pt] [1] I.H. Hutchinson, Phys Plasmas, 15, 123503 (2008)\\[0pt] [2] L. Patacchini and I.H. Hutchinson submitted to Phys Rev E. [Preview Abstract] |
Thursday, November 5, 2009 10:00AM - 10:30AM |
TI2.00002: Plasma Experiments and the Environment Invited Speaker: Laboratory and Field experiments demonstrate nonlinear interactions of Electromagnetic waves and plasma waves in the presence of free energy sources. Viewing the earth's atmosphere immersed in a magnetic mirror, a number of environmental remediation concepts are presented. Waves at ion cyclotron frequencies injected into an auroral ionosphere are used to selectively excite ion species and eject them out through the ``magnetic field cusp''. The roles of the parametric decay instabilities and the auroral kilometric radiation in the presence of electron and ion loss cone distributions are considered in the presence of solar wind and radiation. There is sufficient convection of CO and CO $_{2}$ throughout the atmosphere for the sequestration of greenhouse gases in geospace. Active experiments with atmospheric platforms are presented to illustrate how charge-neutral coupling enhances transports from lower to upper atmosphere. \\[4pt] References: Wong, A. Y. , J. Chen, L. C. Lee and L. Y. Liu , Phys Rev Lett.102, 105002 (2009); Leyser, T.B. and A, Y, Wong , Rev. Geophys. 47, RG1001 (2009). [Preview Abstract] |
Thursday, November 5, 2009 10:30AM - 11:00AM |
TI2.00003: Increased Upstream Ionization Due to Spontaneous Formation of a Double Layer in an Expanding Plasma Invited Speaker: We report observations that confirm a theoretical prediction that formation of a current-free double layer in a plasma expanding into a chamber of larger diameter is accompanied by an increase in ionization upstream of the double layer. The theoretical model argues that the increased ionization is needed to balance the difference in diffusive losses upstream and downstream of the expansion region. In our expanding helicon source, we find that the upstream plasma density increases sharply at the same driving frequency at which a double layer appears. For driving frequencies at which no double layer appears, large electrostatic instabilities are observed. Time resolved laser induced fluorescence and electrostatic probe measurements suggest that the double layer initially forms for all driving frequencies. However, for particularly strong double layers, the instability appears early in the discharge and disrupts the double layer. The double layer is identified in this work through observations of an acceleration population of ions downstream of the plasma source. [Preview Abstract] |
Thursday, November 5, 2009 11:00AM - 11:30AM |
TI2.00004: Strong magnetohydrodynamic turbulence with cross helicity Invited Speaker: Magnetohydrodynamics (MHD) is invoked to model different turbulent phenomena in magnetized plasmas. For instance, the energy spectrum of velocity and magnetic fluctuations in the solar wind obeys a power law spanning a broad range of scales, which has been associated to an inertial range of incompressible MHD turbulence. In addition to the total energy, ideal MHD equations conserve cross helicity which also undergoes a turbulent cascade to small scales. However, most of the current understanding of MHD turbulence is limited to the zero cross helicity case. Cross helicity quantifies the energy imbalance in Alfv\'enic fluctuations oppositely moving along the background magnetic field. Although it is well established that turbulence in the solar wind is dominated by fluctuations moving away from the sun, only in recent years there have been an increased interest in addressing the role of cross helicity in MHD turbulence. Moreover, numerical simulations have also revealed the fundamental role that cross helicity plays in the turbulent cascade. In this talk, we present results from high resolution numerical simulations of steady-state incompressible MHD turbulence, with and without cross helicity. We show that in the zero cross helicity case the configuration space spontaneously develops regions of positive and negative cross-helicity. The obtained scaling of the energy spectrum ($E^\pm$) of fluctuations moving in opposite directions is consistent with simulations of incompressible MHD \emph{without} cross-helicity by other groups. When cross helicity is injected, $E^+$ and $E^-$ maintain the same inertial range scaling, but with differing amplitudes depending on the amount of injected cross-helicity. It is argued that in each of these correlated regions of \emph{imbalance}, the scaling of $E^+$ and $E^-$ is the same. When the total energy spectrum $E^++E^-$ is averaged over those regions, the inertial range scaling is the same regardless of the amount of cross helicity in the system. [Preview Abstract] |
Thursday, November 5, 2009 11:30AM - 12:00PM |
TI2.00005: Modification of turbulence and turbulent transport associated with a confinement transition in LAPD Invited Speaker: Azimuthal flow is driven in the edge of the Large Plasma Device (LAPD) through biasing a section of the vacuum vessel relative to the plasma source cathode. As the applied bias exceeds a threshold, a transition in radial particle confinement is observed, evidenced by a dramatic steepening in the density profile, similar to the L- to H-mode transition in toroidal confinement devices. The threshold behavior and dynamic behavior of radial transport is related to flow penetration and the degree of spatial overlap between the flow shear and density gradient profiles. An investigation of the changes in turbulence and turbulent particle transport associated with the confinement transition is presented. Two-dimensional cross-correlation measurements show that the spatial coherence of edge turbulence in LAPD changes significantly with biasing. The azimuthal correlation in the turbulence increases dramatically, while the radial correlation length is little altered. Turbulent amplitude is reduced at the transition, particularly in electric field fluctuations, but the dominant change observed is in the cross-phase between density and electric field fluctuations. The changes in cross-phase lead to a suppression and then apparent reversal of turbulent particle flux as the threshold is exceeded. [Preview Abstract] |
Thursday, November 5, 2009 12:00PM - 12:30PM |
TI2.00006: A second look at zonal flows Invited Speaker: Despite the fact that the paradigm of self-regulating drift wave-zonal flow turbulence is over fifteen years old, many relevant fundamental questions remain. In particular, nearly all the theoretical work on zonal flow generation has approached the problem via an essentially linear modulational analysis of some pre-existing turbulent state. In this talk, we report on exact momentum theorems which constrain the zonal flow and wave momentum (pseudomomentum), and relate flow evolution to the driving flux, potential enstrophy, dissipation, etc., as well as flow drag. These theorems are derived for both reduced fluid and gyrokinetic models, and severely constrain possible zonal flow growth, via the zonal momentum budget. Since the zonal momentum balance includes potential enstrophy convergence, turbulence spreading dynamics is intrinsically coupled to zonal flow momentum. Results for spreading, collisionless saturation and zonal flow evolution in flux-driven systems will be discussed. The relation between kinetic pseudomomentum and dynamic pressure in kinetic energy principles for self-gravitating systems has been determined and will be presented, along with implications for phase space granulation evolution. [Preview Abstract] |
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