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 JO6: Waves, Turbulence, and Boundary Effects in Laboratory and Space Plasmas I |
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Chair: Mark Gilmore, University of New Mexico Room: Hanover FG |
Tuesday, November 3, 2009 2:00PM - 2:12PM |
JO6.00001: Observations of intense whistler-mode waves and simulations of associated acceleration of electrons Cynthia Cattell, Aaron Breneman, Keith Goetz, Paul Kellogg, Kris Kersten, Lynn Wilson III, John Wygant, Stuart Bale, Ilan Roth, Milan Maksimovic Observations from waveform capture instruments on STEREO and Wind have shown that large amplitude whistler-mode wave packets occur in the Earth's radiation belts, in stream interaction regions and at shocks. Amplitudes are 1 to 2 orders of magnitude larger than previously reported from observations and than those assumed in most theoretical models of wave/particle interactions in these regions. The waves are often monochromatic, obliquely propagating, and have both large longitudinal components and components parallel to the background magnetic field. Particle tracing code results show that, in the radiation belts, the waves can result in energization by many MeV during a single wave packet encounter due to coherent, nonlinear processes including trapping. Simulation results also indicate that the waves can scatter electrons by 10s of degrees in both solar wind and magnetospheric conditions. The results suggest that coherent processes with time scales of seconds, rather than quasi-linear processes with timescales of hours to days, may sometimes dominate scattering and energization by waves. [Preview Abstract] |
Tuesday, November 3, 2009 2:12PM - 2:24PM |
JO6.00002: Warm electromagnetic lower hybrid wave dispersion Alix Verdon, Iver Cairns, Don Melrose, Peter Robinson Lower hybrid (LH) waves can interact resonantly with both electrons and ions, and so can transfer energy between electrons and ions. This property of LH waves is unusual and suggests that they may play a role in particle acceleration and heating in the wide variety of physical contexts in which they occur. Specifically, LH waves are believed to be relevant to the generation of radio emissions in the outer heliosphere, and possibly also to electron and ion acceleration in magnetic reconnection regions in Earth's magnetosphere and the solar atmosphere. In this paper a new analytic dispersion relation for LH waves is presented and compared with numerical results calculated using a fully electromagnetic code. It is shown that the new analytic dispersion relation agrees closely with the numerical results for the real part of the frequency. Surprisingly, ion magnetization effects are important for the imaginary part of the frequency (or equivalently, the damping rate), and can cause the continuous LH mode to break up into a series of segments of ion Bernstein modes. In addition, in situations where the electrons and ions have a net relative drift parallel to the magnetic field of many times the electron thermal speed, numerical results show that wave growth may occur in a mode near the LH frequency. Again, ion magnetization effects play an important role. [Preview Abstract] |
Tuesday, November 3, 2009 2:24PM - 2:36PM |
JO6.00003: Investigation of drift flute turbulence in high beta plasmas in the presence of shear flows I. Paraschiv, V.I. Sotnikov, J.N. Leboeuf, O.G. Onishchenko, E. Yasin, R. Presura, J. Kindel Laboratory experiments performed at the Nevada Terawatt Facility have studied the propagation of a plasma plume across a magnetic field. These experiments have shown that there are instabilities growing at the plasma-field interface, which could be explained by the excitation of flute drift modes. Experimental results also suggest that transverse ExB flows may have an important effect on these instabilities. Using the two-fluid equations in the low-frequency approximation, a nonlinear set of equations for the electrostatic potential, magnetic field and density perturbations was derived, taking into account finite ion Larmor radius effects. The resulting nonlinear equations describe the evolution of small-scale flute turbulence, and the generation of large scale zonal structures in the presence of shear flows. [Preview Abstract] |
Tuesday, November 3, 2009 2:36PM - 2:48PM |
JO6.00004: Intermittency in self-organized shear flows Eun-jin Kim, H.-L. Liu, J. Anderson Understanding multi-scale interactions is an outstanding problem in astrophysical plasmas. Despite complex nonlinear dynamics, coherent structures such as shear flows often form from small-scale turbulence, which then feed back on small-scales. A remarkable consequence of this mutual interaction is self-organization, which provides a powerful paradigm for understanding complexity in many systems. In this contribution, we present a novel statistical theory of self-organisation of forced shear flows, modeled by a nonlinear diffusion equation [1]. A non-perturbative method based on a coherent structure is utilized for the prediction of the PDFs, showing strong intermittency with exponential tails. We confirm these results by numerical simulations. The predicted power spectra are also in a good agreement with simulation results. Furthermore, the results reveal a significant probability of supercritical states due to stochastic perturbation. To elucidate a crucial role of relative time scales of relaxation and disturbance in the determination of the PDFs, we present results obtained in a threshold model where the diffusion is given by discontinuous values. Our results highlight the importance of the statistical description of gradients, rather than their average value as has conventionally been done. \\[0pt] [1] E Kim, H-L Liu and J Anderson, Phys. Plasmas, v16, 052304 (2009).\\[0pt] [2] E Kim and J Anderson, Phys. Plasmas, v15, 114506 (2008). [Preview Abstract] |
Tuesday, November 3, 2009 2:48PM - 3:00PM |
JO6.00005: Experimental test of Baalrud's model for ion velocity at the sheath edge for a two ion species plasma Noah Hershkowitz, Chi-Shung Yip, Greg Severn Recent experiments have shown that ions in plasmas containing two ion species reach a common velocity at the sheath-presheath boundary [1]. A new theory [2] suggests that collisional friction between the two ion species enhanced by two stream instability affects the drift velocity of each ion species near the sheath edge and finds that the difference in ion velocities at the sheath-presheath boundary is given by $\sqrt{\frac{1}{2\alpha} (v^2_{th1} + \alpha v^2_{th2})}$, where $\alpha = n_1M_1/(n_2 M_2)$. We report the first experimental test of this model. We measure ion velocity distribution functions (ivdfs) near sheath edge in Argon/Xenon and Argon/Helium plasmas as a function of the concentration ratios. We show that for sufficiently great relative Xenon concentration, ions do not reach a common speed at the sheath edge. The relative concentration of the two ion species, which determines $\alpha$, is inferred from Ion Acoustic Wave phase velocity measurements, the ivdfs are determined by Laser Induced Florescence. [1] Lee, D; Hershkowitz, N; Severn, GD. Appl. Phys. Lett. 91, 041505 (2007) [2] S.D. Baalrud, J.D. Callen, and C.C. Hegna, GEC 2009 [Preview Abstract] |
Tuesday, November 3, 2009 3:00PM - 3:12PM |
JO6.00006: Detailed reconstruction of the magnetic-field profile penetrating into a nearly-collisionless plasma Ramy Doron, Boaz Rubinstein, Jonatan Citrin, Ron Arad, Yitzhak Maron, Amnon Fruchtman Observations of nearly-collisionless plasmas interacting with pulsed magnetic fields reveal rich physics and intriguing, unexplained phenomena. Among these are the mechanism of rapid (non-diffusive) magnetic-field penetration, the structure of the propagating magnetic front, the fate of the dissipated magnetic energy, and the particle dynamics. We utilize highly-resolved spectroscopy to study the penetration of a $\sim $1 T magnetic field into a low resistivity plasma ($n_{e} \quad \sim $10$^{14}$ cm$^{-3}$, $T_{e} \quad \sim $ 6 eV). The magnetic-field profile is determined from Doppler-shifted line-emission of ions at the \textit{edge} of a trace-element column that are pushed by the magnetic field. This technique allows for achieving submilimeter spatial resolution, approaching the electron skin-depth scale. Reliable Doppler-shift measurements allow for determining field magnitudes as low as 0.2 T, which are extremely difficult to measure via Zeeman spectroscopy under the present conditions. Moreover, the measured ion velocities enable to calculate self-consistently the plasma compression. These calculations predict the build up of a density shock that may induce instabilities behind the penetrating magnetic front. [Preview Abstract] |
Tuesday, November 3, 2009 3:12PM - 3:24PM |
JO6.00007: Neutral depletion in magnetized plasmas Amnon Fruchtman, Gennady Makrinich The effect of neutral depletion on the steady-state of a low temperature magnetized plasma is studied. The nonlinear cross- field diffusion is assumed ambipolar. The nonlinearity results from the dependence of the transport coefficients on the plasma and neutral densities. Recent results are described. It is shown that when the dominant electron collisions are with neutrals, neutral depletion results in a peaking of the density on-axis. If, however, electrons collide mostly with ions, neutral depletion results in flattening of the radial density profile. The nonlinear dependence of the density on the plasma flux is shown to differ for the two cases. There is a fast exponential increase of the plasma density with an increase of the plasma flux, when electrons collide mostly with neutrals, while the plasma density increases only logarithmically with the plasma flux when electron collisions with ions are dominant [1]. We then show how the 2D transport along and across magnetic field in the presence of neutral depletion result in a hollow cylindrical profile of the plasma density, in which the plasma density has a minimum on the cylinder axis. \newline [1] A. Fruchtman, Plasma Sources Sci. Technol. {\bf 18}, 025033 (2009). [Preview Abstract] |
Tuesday, November 3, 2009 3:24PM - 3:36PM |
JO6.00008: Kappa: A New Universal Constant in Turbulent Behaviors C.T. Raynor, A.B. Alexander, M. Robinson, J.A. Johnson III When turbulence physics is characterized as a Ginzburg-Landau phase transformation with the tools from BCS Theory, a new universal constant is derived, defined as $\kappa $, which can be determined from measured quantities in a turbulent glow discharge plasma. Such a constant would provide a new constraint in the MHD equations for turbulent plasma simulations. Values of $\kappa$ are calculated for a wide range of noble gases subjected to an axial magnetic field in the range 0-500 gauss. We report first efforts at a determination of the sensitivity of $\kappa $ to changes in the local molecular constituent and the strength of the local magnetic field. We will also evaluate the implications from these results for new turbulence physics. [Preview Abstract] |
Tuesday, November 3, 2009 3:36PM - 3:48PM |
JO6.00009: Characteristic Frequencies and Critical Energies in Magnetized Turbulent Glow Discharge Plasmas M. Robinson, A.B. Alexander, C.T. Raynor, J.A. Johnson III Previous studies have revealed that dynamic axial magnetic fields can significantly influence the critical turbulent energy and the characteristic turbulent frequencies in glow discharge plasmas. To this end, we analyze the light signatures of several noble gases at constant pressure of 500mtorr and under the influence of static axial magnetic fields ranging from 0 to 500 Gauss. We find that there may be a relationship between the mass of the gas and the slope of its critical turbulent energy when plotted versus magnetic field. We find that this behavior is echoed in the characteristic turbulent frequencies. Further, for all masses and all magnetic fields, we find a high degree of correlation between the two parameters, with important implications for new turbulence physics. [Preview Abstract] |
Tuesday, November 3, 2009 3:48PM - 4:00PM |
JO6.00010: Evidence of New Turbulence Physics in Madison Symmetric Torus Plasmas using a Second Order Phase Transformation Interpretation E.D. Mezonlin, J.B. Titus, J.A. Johnson III, K.M. Williams, C.T. Raynor, C.A. Weatherford When turbulence physics is characterized as a Ginzburg-Landau phase transformation with the tools from BCS Theory, a new universal constant is derived, defined as $\kappa $, which can be determined from measured quantities in the turbulent MST plasmas. Such a constant would provide a new constraint in the MHD equations for fusion plasma simulations. Values of $\kappa $ are calculated from light and magnetic field fluctuations for a wide range of MST operating conditions. The implications from these results are evaluated for new turbulence physics and new fusion simulation methodologies. We would like to thank the MST group for providing data used in these analyses. [Preview Abstract] |
Tuesday, November 3, 2009 4:00PM - 4:12PM |
JO6.00011: Turbulent Parameters during Magnetic Field and Particle Diffusion Fluctuations in the Madison Symmetric Torus J.B. Titus, E.D. Mezonlin, J.A. Johnson III, K.M. Williams, C.T. Raynor Turbulence physics may be characterized as a Ginzburg-Landau phase transformation with the tools from BCS Theory. Such a characterization predicts anomalous enhancements of transport at critical turbulent energies. The standard turbulent parameters have been studied during the magnetic field fluctuations and the particle and thermal diffusion coefficient fluctuations. We will determine correlations between the associated transport coefficients and the turbulent parameters in search of evidence of turbulence driven heating and diffusion enhancements at MST. We would like to thank the MST group for providing data used in these analyses. [Preview Abstract] |
Tuesday, November 3, 2009 4:12PM - 4:24PM |
JO6.00012: First Evidence of Inverse Bremsstrahlung in Laser Enhanced Laser Induced Plasmas D.L. Wiggins, C.T. Raynor, E.D. Mezonlin, J.A. Johnson III Plasmas are created using a Nd-YAG 1020mJ laser at 532nm in air at atmospheric pressures. We bathed the plasma with a1080nm laser beam from a continuous wave fiber laser with powers of 50 W to 1 kW. We have observed the apparent effects of inverse bremsstrahlung in the plasma as the cw laser power increases. Specifically, with increasing cw fiber laser power the electron temperature increases and then begins to saturate after a threshold as predicted due to inverse bremsstrahlung. Furthermore, after a threshold, the signal to noise in the ion lines increases and the signal to noise in the neutral lines decreases as predicted. Implications from these results for astrophysical plasmas as well as plasmas produced for remote sensing in laser induced breakdown spectroscopy are discussed. [Preview Abstract] |
Tuesday, November 3, 2009 4:24PM - 4:36PM |
JO6.00013: Pre- and Post-Shock Wave Self-Induced Magnetic Field Geometry in a Hypersonic Arc Driven Shock Tube K.M. Willams, J.B. Titus, A.B. Alexander, C.T. Raynor, M. Scott, J.A. Johnson Using a version of the motional Stark Effect method based on relative line intensities to determine the internal magnetic field of the arc driven shock tube, a study comparing the magnetic field geometry before and after the primary and reflected shock wave plasma is conducted for different Noble gases. Using spectral line emissions from He, Ar, Kr, and other Noble gases measurements of the internal magnetic field of the shock tube are obtained. When the data for pre-primary shock region, post-primary shock region, pre-reflected and post-reflected shock regions are compared evidence of reconnection events is confirmed. When the magnetic field intensity and the direction evolve with turbulent energy, the B-field direction displays a discontinuity similar to a 2$^{nd}$-order phase transition. [Preview Abstract] |
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