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 BP8: Poster Session I: Plasma Sources and Boundaries; Gyrokinetics and Turbulence; Complex, Non-neutral and Other Novel Plasmas |
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Room: Grand Hall East |
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BP8.00001: PLASMA SOURCES AND BOUNDARIES |
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BP8.00002: Modeling of tungsten surfaces bombardement with helium ions Martin Nieto, Gonzalo Ramos Tungsten is regarded as one of the top choices for the construction of plasma facing components (PFCs) in fusion reactors. As such, a good understanding of its behavior under plasma bombardment is needed. For the case of helium bombardment, the experimental evidence points to the formation of a very low density metal layer a few microns thick. In this work, Monte Carlo simulations of tungsten surfaces bombarded with helium ions is presented. The results of the simulations including ballistic effects only are contrasted against experimental observation, which may help determine, in a qualitative way, the importance of diffusive mechanisms in the formation of these structures. [Preview Abstract] |
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BP8.00003: Instability-Enhanced Collisional Friction Determines the Bohm Criterion in Multiple-Ion-Species Plasmas S.D. Baalrud, C.C. Hegna, J.D. Callen Ion-ion streaming instabilities are excited in the presheath region of plasmas with multiple ion species if the ions are much colder than the electrons. Streaming instabilities onset when the relative fluid flow between ion species exceeds a critical speed, $\Delta V_c$, of order the ion thermal speeds. Using a generalized Lenard-Balescu theory that accounts for instability-enhanced collective responses [1], one is able to show the instabilities rapidly enhance the collisional friction between ion species far beyond the contribution from Coulomb collisions alone. This strong frictional force determines the relative fluid speed between species. When this condition is combined with the Bohm criterion generalized for multiple ion species, the fluid speed of each ion species is determined at the sheath edge. For each species, this speed differs from the common ``system'' sound speed by a factor that depends on the species concentrations, masses and $\Delta V_c$.\\[4pt] [1] S.D. Baalrud, J.D. Callen, and C.C. Hegna, Phys. Plasmas {\bf 15}, 092111 (2008). [Preview Abstract] |
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BP8.00004: Dynamical Response of Continuum Regime Langmuir Probe H.L. Rappaport Probe dynamic response is sometimes used as a way to increase the amount of information obtained from Langmuir probes [1]. In this poster, the effects of frequency dependent probe capacitance and coupling of probe fields to damped Langmuir waves and damped ion acoustic waves are considered. In the continuum regime, with small Debye length to spherical probe radius ratio, the probe DC current vs. voltage characteristic displays a hard saturation at sufficiently large probe potential [2]. In this regime, the sheath thickness varies little with the applied voltage although the plasma response can still be measured. A goal of the present investigation is to show that the probe dynamical response is richer as a result of modulation of sheath thickness or shielding particularly in the larger Debye length to probe radius ratio regime. Inertia inhibits ion response at sufficiently high frequency and deviation from the DC characteristic is shown.\\[4pt] [1] D. N. Walker, R.F. Fernsler, D.D. Blackwell, and W.E. Amatucci, Phys. Plasmas 15, 123506 (2008).\\[0pt] [2] E. Baum and R.L. Chapkis, AIAA J. 8, 1073 (1970). [Preview Abstract] |
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BP8.00005: Effect of ionization on a collisionless magnetized plasma near a wall J\'er\^ome Guterl, Natalia Krasheninnikova, Vadim Roytershteyn, Xianzhu Tang Plasma-wall interactions play a key role in controlled thermonuclear fusion experiments. Wall recycling produces a stream of neutrals which are ionized in the plasma. The ionization of slow neutrals near the wall can significantly affect the sheath and pre-sheath of a magnetized plasma. This was considered previously using analytic models [1,2] for a plasma in which the system size is greater than the neutral ionization length but smaller than the Coulomb collision mean free path. The presence of this ionization source can alter electric field near the surface and affect sheath structure as well as background plasma and ionized particle flow to the wall. Here we present results of PIC simulation for such a simplified boundary plasma model with magnetic field normal to the wall. Employing the state-of-the-art VPIC[3] code, we consider 1D magnetized collisionless plasma. The ionization is modeled as a source which produces thermalized electrons and thermal ions. We examine the effects of source parameters, such as shape, intensity and spatial position on the sheath structure and plasma parameters near the wall.\\[0pt] [1] S. Krasheninnikov et al. Contrib. Plasma Phys. 34, 210 (1994). [2] A. Bailey et al. Nucl. Fusion 24, 1439 (1984). [3] K. Bowers, et al., Phys. Plasmas 15, 055703 (2008). [Preview Abstract] |
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BP8.00006: Plasma sheath dynamics for high-temperature low-density plasmas Xianzhu Tang, Natalia Krasheninnikova, Vadim Roytershteyn With an absorbing material wall, the plasma is thought to be able to access a state of high temperature but low density. This can lead to an interesting edge condition for magnetically confined plasmas and possibly new confinement regimes. The implication for tokamak plasmas has been recognized in recent years. The earlier concept of tandem mirrors depends on essentially the same physics for accessing high electron temperature. As a first step to understand such a plasma, we revisit the classical unmagnetized plasma sheath using a kinetic approach. Although kinetic simulation using VPIC [1] gives a full account of the physical processes, an emphasis of our approach is on how to interpret the unexpected plasma behavior from kinetic simulations (compared with conventional electrostatic sheath theory) using the fluid-moment formulation.\\[0pt] [1] K. Bowers, et al., Phys. Plasmas 15, 055703 (2008). [Preview Abstract] |
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BP8.00007: Are there internal sheaths in unmagnetized electronegative plasmas? Chi-Shung Yip, Noah Hershkowitz Bounded electronegative plasmas are predicted to have electropositive halos. A recent experiment [1] showed that for a negative ion to electron concentration ratio of $\alpha = 0.43 $ for an Argon-Oxygen plasma, a positive halo was formed as a consequence of negative ions satisfying a Boltzmann relation. When $T_e/T_- > 5+\sqrt{24}$ [2] and $\alpha > T_e/T_-$ [3], the negative ions are predicted to be confined by an internal sheath. Experiments are reported in $Ar-SF_6$ and $Ar-Cl_2$ plasmas aimed at finding the internal sheath by varying the gas concentrations. Experiments are carried out in a hot filament discharge in a multi-dipole chamber. Negative ions concentrations are determined from the phase velocity of C.W. Ion Acoustic Waves. Electron temperature and density are determined using Langmuir probes. Plasma potentials are determined by emissive probes. Argon drift velocities are determined by Laser Induced Flourescence. [1] Ghim, YC and Herskowitz, N, Applied Physics Letters. {\bf94}, 15, 151503 (2009) [2] N. Braithwaite and J.E.Allen, J. Physics. D: Appl. Phys {\bf 21}, 1733 (1988) [3] R. N. Franklin, Plasma Sources Sci. Technol. {\bf11}, A31, (2002) [Preview Abstract] |
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BP8.00008: Hot-filament discharge plasma in argon gas at 140 K Shannon Dickson, Scott Robertson A hot-filament discharge plasma has been created in a double-walled vacuum chamber with the inner wall cooled by liquid nitrogen vapor. The inner brass chamber (16 cm dia. x 30 cm) is wound with copper tubing for cooling. This chamber has two tungsten filaments 10 cm in length oriented axially about 2.5 cm from the wall. Plasma measurements are made using a Pt wire probe. At 300 K, 0.6 mTorr argon in the outer chamber, and 2 mA emission, the electron density is 1 x10$^{8}$ cm$^{-3}$ and the electron temperature is 0.054 eV. At 140 K, the density is 1.6 x10$^{8}$ cm$^{-3}$ and their temperature is 0.11 eV confirming that the electrons are not cooled by elastic collisions with the gas. The floating potential of the probe is -2.4 V at 300 K and -0.6 V at 140 K as a consequence of the ion current to the probe being about doubled at the lower temperature. The higher ion current may be a consequence of charge-exchange collisions producing cold ions that are more easily captured by the probe. These collisions decrease the ion losses to the wall by slowing ions accelerated by the plasma potential. Electron losses are reduced because of the requirement of quasineutrality, thus reduced evaporative cooling of electrons may be the cause of the increased electron temperature in 140 K gas. [Preview Abstract] |
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BP8.00009: Waves, Currents, Drifts and Plasma Confinement Time in a Low Temperature, Pulsed, Toroidal, ECR Plasma Michael Lindon, Arvind Thakur, P.K. Sharma, K. Satyanarayan, P.R. Parmar, Chetan Virani, Earl Scime, Saied Houshmyandar Typically the plasma in purely toroidal field experimental systems is created by a filament source or, in some cases, a helicon plasma source. Here we present measurements of plasma density, electron temperature, flow and confinement time for an electron cyclotron resonance (ECR) created plasma in a purely toroidal field at the Institute of Plasma Research (IPR) in India. A linear array of Langmuir probes was used to measure the density and temperature in a horizontal plane of the torus and a pair or probes is used to measure the vertical electric field arising from charge separation. Mach probes were used to measure bulk plasma flows in the same plane. Through measurements of the decay rate of the plasma density after the 2.45 GHz ECR microwave source turns off, the plasma confinement time as a function of initial toroidal magnetic field strength and fill pressure was investigated. This work was supported by the American Physical Society and the Indo-U.S. Science and Technology Forum (IUSSTF) through the India-U.S. Physics Visitation Program. [Preview Abstract] |
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BP8.00010: Laser Initiated, RF Sustained Air Plasmas John Scharer, Ryan Giar, Jason Hummelt, Jesse Way Measurements and analysis of air breakdown processes by focusing 193 nm, 260 mJ, 10 MW high power UV laser radiation to 18 cm and 1.3 cm zones are examined. Quantum resonant multi-photon (REMPI) and collisional cascade ionization processes affect the breakdown and plasma formation. Our spectroscopic measurements show that REMPI (2+1) processes on nitrogen play a substantial role at lower pressures due to the high photon energy (6.4 eV). The REMPI process yields high density air plasmas (5 x 10$^{16}$/cc) for the 18 cm focus with the laser flux three orders of magnitude below the classical breakdown threshold intensity. Measurements of the f = 1.3 cm core laser plasma density (8x10$^{17}$/cc) and electron temperature decay via two color laser interferometry are made. The 18 cm focal length lens and its ionizing shock wave front are utilized to produce air seed plasma to initiate a large volume (500 cc) RF sustainment discharge coupled by means of a 6 cm diameter helical coil at up to 10 kW power levels. [Preview Abstract] |
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BP8.00011: Low Pressure Discharge Initiation and LIF Diagnostic in a High Power Argon Helicon Plasma Matt Wiebold, John Scharer, He Ren A flowing argon helicon plasma is formed in a 10 cm diameter, 1.5 m long Pyrex chamber with a peaked axial magnetic field, variable up to 1 kG. Upgrades have allowed for operation at high, pulsed RF powers (up to 10 kW at 13.56 MHz) and low flow rates and pressures (as low as 1 sccm, 10$^{-5}$ Torr). Neutral-collisional plasma exists upstream of the half-turn, double helix antenna and neutral-collisionless plasma exists downstream, leading to bulk plasma acceleration due to reduced neutral drag. Microwave interferometry (105 GHz) and collisional radiative spectroscopic codes are used to measure electron density and temperature. An initial transient high-density peak ($>$10$^{14}$/cc) is seen observed followed a neutrally depleted steady state plasma ($>$10$^{12}$/cc). Neutral depletion is observed along with substantial plasma acceleration. Initial LIF results using a new MOPA system are reported. A static magnetic field threshold for discharge initiation is seen at low flow rates. Evidence is given that this is a consequence of the magnetic field quenching a multipactor discharge, which is the dominant mechanism for breakdown at low flow rates and pressures. A magnetic field ramping technique for starting discharges under these conditions is described. [Preview Abstract] |
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BP8.00012: Hollow cathode plasma electron source for beam generation C.D. Cothran, S.G. Walton, R.F. Fernsler, W.E. Amatucci The characteristics of an electron beam produced by extracting electrons from a hollow cathode plasma is described. The anode to cathode surface area ratio is less than the root of the electron to ion (Argon) mass ratio such that an electron sheath forms at the anode; a bias on this anode then accelerates the electron flux into a beam. A magnetic field assists the beam collimation. Paschen breakdown in the few Torr range at 500V initiates the hollow cathode plasma, and typical continuous operation is at 130mTorr with about 300V required to sustain the plasma at 60mA. Variation of the hollow cathode current allows direct control of the beam current. Continuous beam at up to 5kV and 80mA has been produced with this device. [Preview Abstract] |
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BP8.00013: Investigation of the Surface Wave Plasma on Cylindrical Dielectric Waveguide Rongqing Liang, Qiongrong Ou, Xijiang Chang, Shuyu Zhang, Long He, Zebing Li The surface wave plasma (SWP) generated by a cylindrical Teflon rod has been developed in our laboratory. A Teflon rod, diameter of 4cm, is adopted to be as waveguide in our experiment. The Microwave frequency is 2.45GHz, and the power is adjustable from about 100W to 800W. The pressure of working gas Argon is set around 20 Pascal. When microwave power is coupled into the cylindrical Teflon rod in a vacuum chamber, a surface wave will be conveyed along the surface of the Teflon rod in axial direction. Plasma could be excited by the electric field of surface wave. Plasma density is proportional with the input microwave power. With increasing density, plasma permittivity will decrease. The variable permittivity of plasma could result in the change of the transmitting mode of surface wave along the Teflon waveguide surrounded by the plasma. The correlative phenomena in our experiment were experimentally observed through the visible light pattern of SWP, and the involved mechanism is theoretically analyzed. [Preview Abstract] |
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BP8.00014: Plasmonic focusing and collimating of light beyond the conventional diffraction limit Kuan-Ren Chen Diffraction sets the smallest achievable product of the line width and the divergence angle of a propagating light. While we still obey the fundamental wave concept, herein we demonstrate that by utilizing a new setup of a metallic subwavelength aperture as the plasmonic lens to preserve, generate and squeeze the sub-limit wave functions an incident light can be focused in the intermediate zone to a single-line width with its value smaller than the conventional diffraction limit of half the wavelength. The fields focused by the plasma effects on the structured lens with the focusing aperture beyond (FAB) the limit are verified to be radiative as of concern and in contrast to the evanescent near-field. With a different structure and plasmonic mechanism, our FDTD simulation has yielded super-collimated light beams with almost zero divergence angles in 2-D free space, as verified by our experimental NSOM measurement. Additionally, we have built an exposure machine; the widths of the exposed patterns at different distances remain almost same and thus verify the super-collimation. The light focusing and collimating processes of the FAB lens, besides being of academic interest, is expected to open up a wide range of application possibilities. [Preview Abstract] |
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BP8.00015: Low Ion-Velocity Slowing Down in a Strongly Magnetized Plasma Target Claude Deutsch, Romain Popoff Ion projectile stopping at velocities smaller than target electron thermal velocity in a strong magnetic field is investigated within a novel diffusion formalism, based on Green- Kubo integrands evaluated in magnetized one-component-plasma models, respectively framed on target ions and electrons. Analytic expressions are reported for slowing down parallel and orthogonal to an an arbitrary large and steady magnetic field, which are free from usual uncertainties plaguing the standard binary collision/dielectric derivations. Magnetic and temperature dependences of the low velocity slowing down are thoroughly detailed for dense plasmas of fast ignition concern and ultracold plasmas envisioned for ion beam cooling, as well. [Preview Abstract] |
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BP8.00016: GYROKINETICS AND TURBULENCE |
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BP8.00017: A Quarter-Century Later: Nonlinear Gyrokinetics Under Attack J.A. Krommes The nonlinear gyrokinetic equation (GKE) was first derived about a quarter-century ago. Subsequent technical developments have refined the GKE into a major tool for the description of both fusion and astrophysical plasmas. However, the GKE has suffered serious attacks on its veracity, two of which will be discussed: the possibilities that (i)~the asymptotic expansion for the GK variables breaks down for torsional or stochastic magnetic fields\footnote{L. E. Sugiyama, Guiding center plasma models in three dimensions, Phys.\ Plasmas \textbf{15}, 092112 (2008); J. A. Krommes, Comments on ``Guiding center plasma models\dots'' [Phys.\ Plasmas \textbf{15}, 092112 (2008)], Phys.\ Plasmas (2009, submitted); L. E. Sugiyama, Response to Comments of J. A. Krommes, Phys.\ Plasmas (2009, submitted).}; (ii)~conventional gyrokinetics is insufficiently accurate to determine the long-wavelength, axisymmetric part of the radial electric field.\footnote{F. I. Parra and P. J. Catto, Limitations of gyrokinetics on transport time scales, Plasma Phys.\ Control.\ Fusion \textbf{50}, 065014 (2008).} The relevant physical pictures and detailed mathematics will be described for both sides of each issue. For (i), local and global coordinate systems must be distinguished; for (ii), the use of Lagrangian field theory is advocated. [Preview Abstract] |
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BP8.00018: Turbulence-Driven Magnetic Reconnection William Nevins, Eric Wang, Ilon Joseph, Jeff Candy, Scott Parker, Yang Chen, Greg Rewoldt Data from finite-beta gyrokinetic simulations of ion temperature gradient turbulence show localized modifications to the magnetic shear in the neighborhood of low order rational surfaces. We analyze this data with the object of determining if these modifications result from magnetic reconnection. When magnetic reconnection occurs, we employ Poincar\'e surface-of-section plots to determine the degree to which the reconnection results in island formation or generalized magnetic stochasticity. [Preview Abstract] |
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BP8.00019: Properties of Multiscale Finite-Beta Gyrokinetic Plasmas W.W. Lee, E.A. Startsev, R.A. Kolesnikov The numerical issues arise from the finite-$\beta$ gyrokinetic Maxwell equation of the form, $ [\nabla_\perp^2 - \beta m_i / m_e -\beta] (\partial \psi \ \partial t) = - \beta (\hat{\bf b}_0 \cdot \nabla) \int v_\parallel^3 (\delta g_i - \delta g_e) dv_\parallel + (\hat{\bf b}_0 \cdot \nabla) \int v_\parallel (\delta g_i - \delta g_e)dv_\parallel + \beta \nabla \psi \times \hat{\bf b}_0 \cdot [(m_i / m_e) ({\kappa}_n + {\kappa}_{Te}) - ({\kappa}_n + {\kappa}_{Ti})/\tau)] $ will be discussed. The equation is the result of a new simulation scheme which separates out the fast particle response due to quasi-static bending of magnetic field lines by letting $\delta g=\label = F-(1 + \psi) F_0 - \int dx_{||} {\kappa} \cdot \delta{\bf B} $, so that a new full density and/or temperature gradient, which is set up by the fast particles, is transverse to the direction of the full field, background plus perturbation, where $\hat {\bf b}=\hat {\bf b}_0+\delta {\bf B}/B_0$, $\delta {\bf B} = \nabla A_\parallel \times \hat{\bf b}_0$, $\psi = \phi + \int (\partial A_\parallel / \partial t) d x_\parallel/c$, $\phi$ and $A_\parallel$ are the perturbed potentials, and $\kappa$ represents the zeroth-order inhomogeneities. For $\beta m_i/m_e (\equiv \rho_s^2 / \delta_e^2) \gg 1$, it is found that, we need to use a computational grid based on the electron skin depth $\delta_e$, which can be an order smaller than $\rho_s$, the length of interest, in agreement with the analytical perturbative methods in solving this type of singular equations. The adequacy of the above equation for the perturbations of the order of $k \sim \kappa$,$^1$ where $k$ is the perturbed wavenumber, will also be presented along with their conservation properties. The issue of transition from $\delta f$ to {\it total-F} in finite-$\beta$ PIC simulations in general geometry, based on the particle weights, will also be discussed. $^1$W. W. Lee and R. Kolesnikov, Phys. Plasmas {\bf 16}, 04506 (2009). [Preview Abstract] |
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BP8.00020: Linearized Model Collision Operators for Multiple Ion Species Plasmas Hideo Sugama, Tomohiko Watanabe, Masanori Nunami Linearized model collision operators for multiple ion species plasmas are presented, which conserve paricles, momentum, and enery, and satisfy adjointness relations and Boltzmann's H-theorem even for collisions between different particle species with unequal temperatures. The model collision operators are also written in the gyrophase-averaged form that can be applied to the gyrokinetic equation. Balance equations for the turbulent entropy density, the energy of electromagnetic fluctuations, the turbulent transport fluxes of particle and heat, and the collisional dissipation are derived from the gyrokinetic equation including the collision term and the Maxwell equations. It is shown that, in the steady turbulence, part of the heat generated by the turbulent transport fluxes produced in the unstable nonzonal-mode region is nonlinearly transferred into the stable zonal-mode region where the collisional dissipation occurs. [Preview Abstract] |
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BP8.00021: Proposal of a brand-new gyrokinetic algorithm for global MHD simulation Hiroshi Naitou, Kenichi Kobayashi, Hiroki Hashimoto, Takehisa Andachi, Wei-Li Lee, Shinji Tokuda, Masatoshi Yagi A new algorithm for the gyrokinetic PIC code is proposed. The basic equations are energy conserving and composed of (1) the gyrokinetic Vlasov (GKV) equation, (2) the Vortex equation, and (3) the generalized Ohm's law along the magnetic field. Equation (2) is used to advance electrostatic potential in time. Equation (3) is used to advance longitudinal component of vector potential in time as well as estimating longitudinal induced electric field to accelerate charged particles. The particle information is used to estimate pressure terms in equation (3). The idea was obtained in the process of reviewing the split-weight-scheme formalism. This algorithm was incorporated in the Gpic-MHD code. Preliminary results for the m=1/n=1 internal kink mode simulation in the cylindrical geometry indicate good energy conservation, quite low noise due to particle discreteness, and applicability to larger spatial scale and higher beta regimes. The advantage of new Gpic-MHD is that the lower order moments of the GKV equation are estimated by the moment equation while the particle information is used to evaluate the second order moment. [Preview Abstract] |
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BP8.00022: Radial Propagation in the ITG CYCLONE Base Case Eric Wang, William Nevins Analysis of the ITG CYCLONE benchmark [A.M. Dimits et al, Phys. Plasmas 7, 969 (2000)] in the past focused on the characteristics of plasma microturbulence located at the outboard midplane ($\theta = 0$). The outboard midplane is chosen because this is the region of the tokamak that produces the most violent instabilities, while the third spatial is ignored because plasma microturbulence in tokamaks is anisotropic, with short spatial variations perpendicular to the magnetic field and long variations parallel. In the present work we revisit the CYCLONE base case using the nonlinear gyrokinetic microturbulence code GYRO[J. Candy and R.E. Waltz, J. of Comp. Phys., Volume 186, Issue 2, 10 April 2003], observing steady state turbulence in all three spatial dimensions plus time. Unexpectedly, the perturbed quantities (electrostatic potential, density, and temperature) all exhibit radial propagation inwards for spatial positions above the outboard midplane ($\theta < 0$) and outwards below the outboard midplane ($\theta > 0$). The characteristics of this propagation are presented in detail, along with a model for the cause of these structures. [Preview Abstract] |
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BP8.00023: Comparisons of Positivity Preserving Advection Algorithms for Edge Plasma Turbulence Simulations G.W. Hammett, J.L. Peterson The steep density and temperature gradients associated with the edge and scrape off layer regions of a fusion plasma complicate the numerical simulation of plasma turbulence. Spectral methods and Arakawa finite differencing have the interesting property of exactly preserving certain conservation properties of Hamiltonian systems and work well for simulating well-resolved, small amplitude fluctuations. However, such algorithms can exhibit Gibbs phenomena, small overshoots in the vicinity of large gradients. While these overshoots are unimportant for small amplitude turbulence in the core region of tokamaks, these algorithms can lead to regions of negative density or temperature in the tokamak edge. Several finite volume methods of solving multi-dimensional hyperbolic equations can be constructed to prevent such negative solutions, and can be useful for both gyrokinetic and gyrofluid continuum simulations. We explore here different algorithms for positivity-preservation and their effects on efficiency. When combined with Strong Stability Preserving time-integration techniques, unphysical negative solutions can simply and quickly be eliminated. [Preview Abstract] |
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BP8.00024: Absolute equilibria of gyrokinetic fluctuations Jian-Zhou Zhu, Gregory Hammett A paradigm based on the absolute equilibria of Fourier-truncated inviscid systems [R. H. Kraichnan and D. Montgomery, Rep. Prog. Phys. 43, 547 (1980)] to understand turbulence is applied to study gyrokinetic plasma turbulence. This approach has been successful in understanding fundamental issues of fluid turbulence, and provides useful benchmarks for analytical theories and numerical simulations. The existence of 2 conserved quantities (energy and enstrophy) in 2-D fluids gives rise to interesting features in the equilibrium spectrum, related to the existence of inverse cascade. We have derived the class of absolute equilibria for gyrokinetic fluctuations in 2 spatial dimensions and 1 (perpendicular) velocity dimension, including finite Larmor effects with the full Bessel functions in this framework. A range of spectra types are calculated with different parameters. Extensions to absolute equilibria in fully 5-D gyrokinetics are being explored. The implication of these results for ITG and ETG plasma turbulence will also be discussed. [Preview Abstract] |
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BP8.00025: Characteristics of nonlinear interaction in entropy cascade T. Tatsuno, G.G. Plunk, W. Dorland, A.A. Schekochihin Two-dimensional electrostatic turbulence in weakly-collisional magnetized plasmas can be described as a turbulent cascade of entropy in phase space [1]. Nonlinear phase mixing [2] introduces small structures of distribution function in both position and velocity space. By invoking the Kolmogorov-type phenomenology applied in the phase space, it is argued that entropy cascades to smaller scales as energy or enstrophy does in the Navier-Stokes turbulence. In this presentation, we report the detailed characteristics of nonlinear interaction in the gyrokinetic turbulence. We diagnose the triad interaction in the wave-number as well as in the velocity dual space using the AstroGK code [3], and see how quantitatively the theoretical assumptions are satisfied, namely, (a) the scale locality of nonlinear intercation, and (b) constancy of entropy flux both in position and in velocity space. \\[4pt] [1] T. Tatsuno \textit{et al}., Phys.\ Rev.\ Lett.\ \textbf{103}, 015003 (2009).\\[0pt] [2] W. Dorland and G. Hammett, Phys.\ Fluids B \textbf{5}, 812 (1993).\\[0pt] [3] http://www.physics.uiowa.edu/\~{}ghowes/astrogk/ [Preview Abstract] |
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BP8.00026: Rotation dynamics with \& without Internal Transport Barriers Guilhem Dif-Pradalier, Patrick H. Diamond, C.S. Chang, S. Ku, Y. Sarazin, V. Grandgirard, J. Abiteboul, X. Garbet, Ph. Ghendrih, A. Strugarek We investigate the dynamics of both poloidal and toroidal flows in the presence (or absence) of a reversed safety factor profile through a scan in the incoming heat power applied to the plasma. Doing so, it incidentally also addresses the question of a power threshold for a self-consistent ITB formation in gyrokinetic modeling. As a prime candidate to drive the system away from its neoclassical prediction, we recently showed evidence of turbulence-generated poloidal rotation, consistently with earlier theories. Accurate calculation of the radial electric field is central. Accordingly, description of the mean profile dynamics, as done in {\it full--$f$} flux-driven models is shown to take on a very prominent role. The study is performed using both the \textsc{Gysela} and \textsc{Xgc--1} gyrokinetic codes with Enhanced Reverse Shear (ERS)-like parameters. [Preview Abstract] |
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BP8.00027: Free Boundary Magnetohydrodynamic Equilibria with Flow Ronald Schmitt, Gunyoung Park, Choong-Seock Chang, Luca Guazzotto, Henry Strauss, Eliezer Hameiri, Harold Weitzner Equilibria with flow are known to exhibit characteristics different from static equilibria. In particular, plasma rotation often reduces turbulent transport, as exhibited by H-mode confinement state observed in tokamaks. The equations that govern MHD equilibrium with flow are the generalized Grad-Shafranov equation and the magnetic Bernoulli equation. The finite element, free boundary equilibrium solver in the M3D code has been modified to included arbitrary toroidal and poloidal flows. The M3D+FLOW code differs from other codes in that, being a free boundary code, it includes the separatrix and the region outside the separatrix. Results for M3D+FLOW are presented for both subsonic and transonic flows. Application to data from the XGC gyrokinetic code is also presented. [Preview Abstract] |
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BP8.00028: Integrated Edge-core multiscale simulation of Full-f ITG-turbulence in realistic tokamak geometry Seung-Hoe Ku, C.S. Chang, P.H. Diamond We report ``the-tail-wagging-the-dog'' phenomenon observed from the full-f XGC1 simulation of the whole-volume ITG turbulence in realistic tokamak geometry. In H-mode experiments, the core energy confinement is promptly enhanced with steepening of the edge gradient under a strong core heating in diverted magnetic field geometry. The full-f XGC1 simulation studies the dynamical neoclassical and turbulence interaction without scale-separation. It is found that edge turbulence is nonlocally connected to the core through spatial energy propagation, modifying the core turbulence and driving a SOC. Strong core heating and self-organized ExB shear play important roles in this process. Detailed physics mechanisms and experimental implications will be discussed. [Preview Abstract] |
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BP8.00029: Finite $\beta$ Effects on TEM Turbulence D.R. Ernst, J. Lang, W.M. Nevins, P.N. Guzdar, J. Candy, R.E. Waltz This work explores the $\beta$ dependence of trapped electron mode turbulence and associated electron thermal energy and particle transport, using the GS2 (continuum), GEM (particle), and GYRO (continuum) codes. Leadership class computing facilities enable us to extend to shorter wavelengths that contribute significantly to electron thermal transport, which can increase with $\beta$ in ITG dominated cases.\footnote{J. Candy, Phys. Plasmas {\bf 12}, 072307 (2005). See posters by R. Waltz and W. Nevins, this conference.} Analytic work on zonal flow modulational instabilities with finite $\beta$ suggests interesting and non-monotonic $\beta$ dependence, arising from competition between drift wave and drift-Alfv\'{e}n wave pumps.\footnote {P. N. Guzdar {\em et al.}, Phys. Plasmas 8(9) 3907 (2001).} Finally, our previous TEM zonal flow studies\footnote{D. R. Ernst, J. Lang, W. M. Nevins {\em et al.}, Phys. Plasmas 16, 055906 (2009).} found that convergence was poor for $\eta_e> 3$, despite including wavenumbers $k_\alpha\rho_s\le 4$. Additional physics, such as electromagnetic effects, could potentially resolve this. [Preview Abstract] |
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BP8.00030: Gyrokinetic study of the spatial entropy dynamics in turbulent plasmas with zonal flow Kenji Imadera, Yasuaki Kishimoto, Jiquan Li, Takayuki Utsumi We have developed a new computational algorithm based on the IDO-CF (Conservative Form of Interpolated Differential Operator) scheme [1], which is efficient in capturing sharp domain structure in long time scale, for solving full-f Gyrokineitc Vlasov-Poisson system. By using the developed code, we have performed the ITG simulation focusing on entropy dynamics and associated zonal flow formation. Here, we have introduced the modified local entropy defined as$\delta S_m (x)=\int {\left\langle {\delta f^2/\left\{ {2f_0 (-1+v_{\vert \vert }^2 /T)} \right\}} \right\rangle _{yz} dv} $, which retains the spatial information. It is found that the entropy balances with the acoustic coupling driven by ITG mode in the linear stage, and then the zonal flows expel the entropy to outside region via its convection. The spatial structure of the entropy is regulated by the zonal flows, and finally, the quasi-steady state where the entropy and zonal flows have similar structure is established. This indicates that the zonal flows couple with the entropy spatially [1] Y.Imai \textit{et al}., J. Comput. Phys. \textbf{227} (2008) 2263. [Preview Abstract] |
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BP8.00031: Catalytic Role of Zero-Frequency Zonal Modes in Saturation by Damped Eigenmodes P.W. Terry, D.R. Hatch, W.M. Nevins, F. Jenko Recent gyrokinetic simulations suggest that the effect of zonal flows on turbulence is catalytic, enabling energy to reach damped-eigenmode energy sinks in the wavenumber range of the instability. We investigate this issue, looking generically at three-wave coupling with a variety of analysis techniques. Using parametric instability analysis to study the excitation of general eigenmodes that are not linearly unstable, we find that energy transfer to damped-eigenmode energy sinks is sensitive to heavy reductions by phase mixing. Phase mixing is avoided if energy first goes to a set of catalytic modes and then to damped eigenmodes. The catalytic modes must have a zonal wavenumber (e.g., $k_{y}$=0), the linear wave frequency must be zero, but they do not have to be a flow or involve a flow, i.e., flow is not an essential aspect of the avoidance of phase mixing. Projections of ITG turbulence onto a linear-eigenmode basis indicate both the excitation of many virtually undamped, non flow-like $k_{y}$=o modes, and robustly damped $k_{y}\ne $0 eigenmodes in the instability region. [Preview Abstract] |
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BP8.00032: Gyrokinetic particle characterization of transport in tokamaks Jean-Noel Leboeuf, Viktor Decyk, David Newman, Raul Sanchez We are characterizing transport using particles in gyrokinetic simulations of ion channel turbulence in tokamaks with the 3D global toroidal nonlinear parallel particle-in-cell UCAN code. Tracking of simulation particles through space and time and especially multiple processors, including restarts with different numbers of tagged particles, is now in production mode. The particle data thus tracked and stored comprise the positions and velocities for every tracked particle at each chosen instant of time. These particle data are analyzed with tools previously applied to passive marker particles in fluid turbulence simulations. The data from electrostatic UCAN simulations with adiabatic electrons and gyrokinetic ions show that radial transport is altered fundamentally by the presence of a sheared poloidal zonal flow, changing from diffusive to anti-correlated and subdiffusive. Convergence studies with both particle number and grid size confirm the standard resolution results. To facilitate these resource-intensive calculations two-dimensional domain decomposition is being implemented in UCAN. [Preview Abstract] |
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BP8.00033: The physics behind subdiffusive transport across an externally imposed sheared flow in drift-wave turbulence D.E. Newman, R. Sanchez, Debasmita Samaddar, J.N. Leboeuf In recent gyrokinetic simulations of ITG turbulence it has been found that radial transport ceases to behave diffusively in the presence of a radially-sheared poloidal zonal flow [1]. We have observed a similar change in the character of transport across a shear flow in numerical simulations of 2D-turbulence in slab geometry using the BETA code, on which the sheared flow has been externally imposed [2]. The slab geometry and physics simplifies the search for the physics mechanism responsible for the onset of subdiffusion. The results suggest that subdiffusion is caused by the selection of a preferred sign for the vorticity carried out by the sheared flow throughout the 2D domain. This mechanism is likely to be important in a wide variety of turbulent systems.\\[4pt] [1] R. Sanchez, D.E. Newman, J.N. Leboeuf, V.K. Decyk and B.A. Carreras, Physical Review Letters \textbf{101}, 205002 (2008)\\[0pt] [2] D.E. Newman, D. Samaddar, R. Sanchez and B.A. Carreras, in Proc. of the 35th EPS Conference on Plasma Physics, Hersonissos, 9-13 June 2008, ECA Vol. 32, p. 1.044 (2008) [Preview Abstract] |
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BP8.00034: Transport suppression by shear reduction Julio Martinell, Diego del-Castillo-Negrete The relationship between transport and shear is a problem of considerable interest to magnetically confined plasmas. It is well known that there are cases in which an increase of flow shear can lead to a reduction of turbulent transport. However, this is not a generic result, and there are transport problems in which the opposite is the case. In particular, as originally discussed in Ref.~ \footnote{del-Castillo-Negrete and Morrison, Phys. Fluids A {\bf 5}, 948 (1993)}, barriers to chaotic transport typically form in regions of vanishing shear. This property, which is generic to the so-called non-twist Hamiltonian systems \footnote{del-Castillo-Negrete, Greene, and Morrison, Physica D {\bf 91}, 1 (1996)}, explains the observed resilience of transport barriers in non-monotonic zonal flows in plasmas and fluids and the robustness of shearless magnetic surfaces in reverse shear configurations. Here we study the role of finite Larmor radius (FLR) effects on the suppression of chaotic transport by shear reduction in a simplified model. Following Ref.~ \footnote{del-Castillo-Negrete, Phys. Plasmas, {\bf 7}, 1702 (2000)} we consider a model consisting of a superposition of drift waves and a non-monotonic zonal flow. The FLR effects are incorporated by gyroaveraging the ${\bf E} \times {\bf B}$ velocity, and transport is studied by following the evolution of ensembles of test particles. [Preview Abstract] |
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BP8.00035: Coupled GEM-XGC Simulations of Edge Pedestal Plasmas Scott Parker, Weigang Wan, Yang Chen, C.S. Chang, S. Ku, N. Podhorszki, S. Klasky Global GEM gyrokinetic turbulence simulations of the edge pedestal are performed assuming closed flux surfaces and using numerical profiles obtained from the XGC0 neoclassical calculation\footnote{Y. Chen and S. Parker, Phys. Plasmas {\bf 15}, 055905 (2008).}. The plasma profiles used in GEM are output from an XGC0 simulation of L- and H-mode DIII-D plasmas. For L-mode plasmas, it is found that electromagnetic effects are important and the heat diffusivities for both electrons and ions are much bigger than from equivalent simulations in the electrostatic limit. While electromagnetic ion and electron energy transport are comparable to experimental values, the particle transport is too high, and such particle diffusion coefficient would cause the pedestal crash in the XGC0 calculation. Adding carbon impurity may reduce the level of particle transport. Work is under way to incorporate the GEM/XGC coupling under the EFFIS -- ``End-to-end Framework for Fusion Integrated Simulation''. [Preview Abstract] |
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BP8.00036: Global Turbulence Calculations with GEM for Experimental Cases and Experimental Comparisons G. Rewoldt, D. Mikkelsen, Y. Chen, S. Parker The GEM code is capable of global nonlinear turbulence calculations for experimentally-realistic cases, including trapped electrons, deuterium ions, carbon impurity ions, and hot beam ions, with electron collisions and electromagnetic effects, using a model MHD equilibrium with ellipticity and triangularity. For cases for the core region only of tokamaks such as NSTX and DIII-D, when the experimental level of the equilibrium ExB velocity is included, the cases are often completely stable, while if the ExB velocity is set to zero the cases can become unstable. However, even including the experimental ExB velocity, if the edge (pedestal) region also is included in the computation domain, the cases can be unstable, grow linearly, and saturate nonlinearly. In particular, the linearly stable core region could then have a significant level of anomalous transport, which would be evidence of turbulence spreading. For one particular NSTX case where the experimental ion energy transport is anomalous, the saturated GEM level of transport matches this in the edge region, but dies off more quickly radially going into the core region than the experimental transport. Results will be presented and discussed. [Preview Abstract] |
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BP8.00037: Subdiffusive radial transport in a gyrokinetic Z-pinch plasma with zonal flows Kyle Gustafson, William Dorland We report on numerical gyrokinetic studies of a confined, magnetized plasma in a Z-pinch configuration. Building upon previous results\footnote{Ricci et al PRL 97 245001 2006} for the entropy mode in a gyrokinetic Z-pinch, we examine the details of particle transport as diagnosed by the displacements of an ensemble of tracer particles in simulations\footnote{Broemstrup, Thesis University of Maryland}. The density-gradient driven entropy mode of instability leads to vertical zonal flow structures in the turbulence that impede particle transport in the radial direction. Tracer displacements in this study point to the existence of subdiffusive radial transport, such that the spreading of the tracers proceeds more slowly than predicted by a canonical turbulent diffusion equation. The relevance of this result to the usual predictions of confinement-time scaling for fusion machines is examined. We compare the effects of collisions and several values of the density gradient for significant periods of time, measured in $v_{thermal}/L$. We use continuous-time random walk and fractional diffusion equation models to understand the transport process more generally. The importance of long-range velocity correlations and scale-free transport is considered. [Preview Abstract] |
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BP8.00038: Formation of Coherent Vortex Structures and Transport Reduction in Electron Temperature Gradient Turbulence Motoki Nakata, Tomo-Hiko Watanabe, Hideo Sugama Formation of coherent vortex structures and related transport reduction in the slab ETG turbulence has been investigated by means of gyrokinetic Vlasov simulations with high phase-space resolution. A spontaneous transition of vortex structures from a turbulent state with finer-scale fluctuations to a coherent state dominated by large-scale vortices with zonal flows are found, where the electron heat flux is significantly reduced. The transport reduction in the coherent vortex state is mainly attributed to the phase matching of the potential and temperature fluctuations in association with the structural change of the velocity distribution function. A traveling wave solution of the Hasegawa-Mima type fluid equation including electron temperature gradient successfully describes the coherent vortex structures. It is also investigated how the parameters of $\eta _{e}$ and $\Theta $=k$_{\vert \vert }$/k$_{\theta }$ affect the formation of the coherent vortex structures and the resultant transport levels. Results of the toroidal ETG turbulence simulation will be also discussed. [Preview Abstract] |
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BP8.00039: Eigenmode Analyses of Nonlinear Gyrokinetic Simulations D. Hatch, P. Terry, W. Nevins, F. Jenko, F. Merz The excitation of damped eigenmodes is shown to be the dominant saturation mechanism for gyrokinetic ITG turbulence. Stable eigenmodes are excited to significant amplitude even at linearly unstable wavenumbers, revealing the collisionless gyrokinetic plasma to be a lossy medium, quite different from the conventional, nearly conservative medium requiring a cascade to short wavelengths for energy dissipation. This observation comes from eigenmode analyses of gyrokinetic simulations using the complete linear spectrum of the numerically discretized gyrokinetic operator. A projection procedure allows for explicit observation of the importance of each eigenmode with regard to transport, energy dissipation and energy content of the turbulence. This procedure has been applied to ITG driven turbulence where it is found that a series of stable eigenmodes are strongly excited, playing a vital role in energy balance and saturation. Comparisons with simulations for which zonal modes have been artificially suppressed indicate that zonal modes facilitate energy transfer to damped eigenmodes. [Preview Abstract] |
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BP8.00040: Source formulation for fluid turbulence simulations from atomic physics differential cross sections S.H. Muller, C. Holland, G.R. Tynan, M. Xu, J.H. Yu The derivation of the correct functional form of source terms in plasma fluid theory is revisited. The relation between the fluid source terms and atomic physics differential cross sections is established for particle-impact ionization. It is shown that the interface between atomic and plasma physics is completely described by three scalar functions of the incident particle energy, which are properties of the differential cross sections only. For electron-impact ionization, the BEB and BED models [Y.-K. Kim and M. E. Rudd, Phys. Rev. A, 50 (1994) 3954] are used to calculate these functions analytically, yielding expressions that both accurately capture the physics and can be efficiently evaluated within fluid simulation codes. The source terms explain the observed electron temperature regimes in a wide variety of basic plasma physics experiments, including the trends across different gases. [Preview Abstract] |
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BP8.00041: Physics behind the role of shear flow in turbulent transport of magnetic fields A. Newton, E. Kim Fundamental processes governing the dynamics of coherent structures and their interplay with turbulence in magnetized fluids present some of the most outstanding problems in classical physics. In particular, various observations indicate that typical magnetic activities in astrophysical plasmas must involve the fast transport of magnetic fields on time scales much shorter than the Ohmic diffusion time scale. In this contribution, we report on the first comprehensive direct numerical simulations of 2D sheared MHD turbulence to elucidate fundamental physical processes which accelerate or moderate turbulent transport of magnetic fields [1]. We show (i) that transport quenching by shear flows and resonant interactions are vitally important; (ii) that a shear flow plays a dual role of quenching transport by shearing and enhancing it by resonance and the overlap of resonant layers; (iii) that a strong suppression of transport by shear flow (magnetic fields) occurs when the shearing (Alfv\'enic) timescale is shortest among all the characteristic timescales. Specifically, without resonance, turbulent magnetic diffusivity $\eta_T$ is quenched as $\eta_T \propto B_0^{-4}$ for weak shear ($\Omega$) and strong magnetic field ($B_0$) while $\eta_T\propto \Omega^{-2.7}$ for strong shear and weak magnetic fields. In comparison, $\eta_T$ is less severely quenched with resonance with the scaling $\eta_T \propto B_0^{-2} \propto \Omega^{-2}$ for strong shear and magnetic fields. \\[4pt] [1] A. Newton and E. Kim, Phys. Rev. Lett., v102, 165002 (2009). [Preview Abstract] |
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BP8.00042: Universal Probability Distribution Function for Bursty Transport in Plasma Turbulence Ingmar Sandberg, Sadruddin Benkadda, Xavier Garbet, George Ropokis, Kyriakos Hizanidis Bursty transport phenomena associated with convective motion present universal statistical characteristics among different physical systems. A stochastic univariate model and the associated probability distribution function for the description of bursty transport in plasma turbulence is presented. The proposed stochastic process recovers the universal distribution of density fluctuations observed in plasma edge of several magnetic confinement devices and the remarkable scaling between their skewness $S$ and kurtosis $K$. Similar statistical characteristics of variabilities have been also observed in other physical systems that are characterized by convection such as the X-ray fluctuations emitted by the Cygnus X-1 accretion disc plasmas and the sea surface temperature fluctuations. [Preview Abstract] |
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BP8.00043: The effects of non-uniform magnetic field strength on test particle transport in drift wave turbulence Richard Dendy, Joseph Dewhurst, Bogdan Hnat Our model of drift turbulence is a modified form of the Hasegawa-Wakatani equations, extended to include magnetic field inhomogeneity in the radial direction, thus incorporating interchange modes. Direct numerical simulation of this system yields local time series for: the turbulent E x B radial density flux $\Gamma $, whose probability density function (PDF) is analyzed in terms of skewness and kurtosis; and the relative phase and amplitude of fluctuations in density n, electrostatic potential $\phi $ and radial velocity v. We investigate how changes in the magnitude C of the magnetic field inhomogeneity affect the relative phases of n, $\phi $ and v and in consequence the skewness of the PDF of $\Gamma $. This is a consequence of the shift from drift to drift-interchange turbulence. The challenge is then to identify a Fickian expression linking $\Gamma $ to the radial diffusivity that embodies C as a parameter, while noting the conservation of potential vorticity. This is achieved, assisted and confirmed by statistical analysis of the transport of ensembles of test particles in stationary turbulence and by measurements of the decay of correlation in potential vorticity. [Preview Abstract] |
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BP8.00044: Turbulence Spreading and Zonal Flow Evolution - A Non-Perturbative Theory A. Ulvestad, M. Malkov, P.H. Diamond The interplay of turbulence spreading and zonal flows has been a subject of controversy in recent years. Previous work on this subject has utilized perturbative, weak-coupling approaches (i.e. turbulence closures, envelope expansions, etc) to address this subject. Many of these calculations do not properly conserve momentum between fluctuations and flows. In this work, we study solutions of an asymptotic (i.e. eikonal) model without the use of a perturbative expansion. The model conserves momentum between fluctuations and zonal flows by treating the radial flux of turbulence potential enstrophy in the flow momentum balance. Results indicate intensity propagation depends strongly on fluctuation and flow dissipation profiles. [Preview Abstract] |
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BP8.00045: Particle in cell simulations of wave turbulence in solar wind in ion cyclotron frequency range Vladimir Svidzinski, Hui Li, Harvey Rose, Brian Albright, Kevin Bowers Fully electromagnetic particle in cell simulations of nonlinear waves propagation and interaction is performed in two-dimensional plane geometry in magnetized plasma in ion cyclotron frequency range. A spectrum of fast wave modes with different total energies with wave numbers parallel and perpendicular to uniform equilibrium magnetic field is launched into plasma and the nonlinear dynamics of these waves is analyzed. Results show that the wave magnetic energy spectrum cascades to smaller scales exhibiting strong anysotropy, it is wider in direction perpendicular to the equilibrium magnetic field. The shape of the cascade is established after a few ion cyclotron periods and most of the energy in the cascade stays in the fast wave oscillations. Collisionless damping on electrons is the main dissipation chanel in these results. [Preview Abstract] |
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BP8.00046: Role of linearly damped eigenmodes in several two field turbulence models K.D. Makwana, P.W. Terry Linearly damped eigenmodes have been shown to provide a significant finite-amplitude-induced energy sink for saturation in collision-less trapped electron turbulence(CTEM). Based on an analytic criterion, similar behaviour is predicted for seven other distinct fluid models in certain parameter regimes. These models (TEM, Rayleigh-Taylor, Local Resistive g-mode, Drift thermal, Micro-tearing with time dependent thermal force, Hasegawa-Wakatani and Ionization driven drift wave) are solved numerically and the solutions are projected onto the linear eigenmodes. The analytic criterion is explicitly evaluated for various parameter regimes and used to predict whether or not damped modes will significantly affect saturation. These regimes are investigated in the simulations and the predictions are verified by comparing stable mode amplitudes with unstable mode amplitudes in saturation. Fluxes are calculated and it is shown that quasi-linear flux overestimates the true flux whenever damped modes contribute significantly to saturation. This invalidates the quasi-linear approximation and shows that damped modes cannot be ignored in these models. [Preview Abstract] |
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BP8.00047: A Simple Dynamical Model of Flux-Driven Turbulence and Profile Evolution Z.H. Wang, P.H. Diamond, C.S. Chang, S. Ku, X.G. Wang We study nonlocal, flux driven turbulence and profile evolution using a simple model of coupled nonlinear reaction-diffusion equation, heat transport equation and density source-diffusion equation. We study temperature profile evolution in the presence of turbulence produced by a strong edge source, which spreads inward and interacts with both heat pulses and locally driven core turbulence. Basic results are: 1)propagation of intensity and heat pulse differs in that the speed of the former grows and then decays as heat flux Q increases, while the latter grows and saturates at a value set by neoclassical transport. 2) speed of inward propagating turbulence is sensitive to Q. It first increase as $\surd $Q and then decreases as 1/Q, following the formation of ITB. It suggests ITB location is determined by \underline {both} heat flux and near edge conditions and ITB works as much by keeping turbulence \underline {out} as by keeping heat \underline {in}! 3) collisions of in to out and out to in pulses trigger local profile steeping, and (in some cases) ITB formation. Moreover, the interaction point varies with Q. [Preview Abstract] |
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BP8.00048: Zonal flow generation from trapped electron mode turbulence Lu Wang, T.S. Hahm Most existing zonal flow generation theory [1,2] has been developed with a usual assumption of $q_r \rho _{i\theta } <<1$ ($q_r $ is the radial wave number of zonal flow, and $\rho _{i\theta } $ is the ion poloidal gyroradius). However, recent nonlinear gyrokinetic simulations of trapped electron mode (TEM) turbulence exhibit a relatively short radial scale of the zonal flows with $q_r \rho _{i\theta } \sim 1$ [3,4,5]. This work reports an extension of zonal flow growth calculation to this short wavelength regime via the wave kinetics approach. A generalized expression for the polarization shielding for arbitrary radial wavelength [6] which extends the Rosenbluth-Hinton formula in the long wavelength limit [7] is applied. The electron nonlinearity effects on zonal flow are investigated by using GTC simulation. This work was supported by the China Scholarship Council (LW), U.S. DoE Contract No. DE--AC02--09CH11466 (TSH, LW), the U. S. DOE SciDAC center for Gyrokinetic Particle Simulation of Turbulent Transport in Burning Plasmas, and the U. S. DOE SciDAC-FSP Center for Plasma Edge Simulation (TSH). [1] P. H. Diamond et al., IAEA-CN-69/TH3/1 (1998). [2] L. Chen, Z. Lin, and R. White, Phys. Plasmas \textbf{7}, 3129 (2000). [3] Z. Lin et al., IAEA-CN-138/TH/P2-8 (2006). [4] D. Ernst et al., Phys. Plasmas \textbf{16}, 055906 (2009). [5] Y. Xiao and Z. Lin, ``Turbulent transport of trapped electron modes in collisionless plasmas'', submitted to Phys. Rev. Lett. (2009). [6] Lu Wang and T.S. Hahm, Phys. Plasmas \textbf{16}, 062309 (2009). [7] M. N. Rosenbluth and F. L. Hinton, Phys. Rev. Lett. \textbf{80}, 724 (1998). [Preview Abstract] |
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BP8.00049: Transport studies of Toroidal Angular Momentum carried by Drift Resonance of Toroidal Ion Temperature Gradient Instability near Marginality E.S. Yoon, T.S. Hahm We calculate transport of parallel momentum carried by drift resonance of toroidal ITG instability near marginality using quasilinear theory starting from a phase space conserving gyrokinetic equation [1] for ions.~The principal results are: TEP pinch remains inward as the most robust term of pinch. In addition, ion temperature gradient driven pinch is inward for typical parameters. Interestingly,~density gradient driven pinch is outward as in the previous~kinetic results in slab geometry [2], but opposite to the results in the fluid regime [3]. ~Parameter~dependency of momentum diffusivity and pinch velocity on the electron to ion temperature ratio ($\tau =T_e /T_i )$ has been examined over~a wide range of density profiles. \\[4pt] [1] T.S. Hahm, Phys. Fluids, \textbf{31} 2670 (1988) \\[0pt] [2] P.H. Diamond, C.J. McDevitt, and .D. G\"{u}rcan, \textit{et. al.}, Nuclear Fusion, \textbf{15} 012303 (2008) \\[0pt] [3] A.G. Peeters, C. Angioni and D. Strintzi, Phys. Rev. Lett., \textbf{98} 072302 (2007) [Preview Abstract] |
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BP8.00050: Construction of momentum theorem using cross moments T.S. Hahm, Lu Wang, P.H. Diamond Charney-Drazin theorem has been extended to Hasegawa Wakatani system~for zonal flow problem in magnetic fusion [P.H. Diamond, et al., Plasma Phys. Control. Fusion \textbf{50}, 124018 (2008)]. For this model, the guiding center density is the potential vorticity and zonal flow is influenced by the particle flux. In this work we construct momentum theorems in terms of a hierarchy of cross moments $\left\langle {n_G ^2} \right\rangle $, $\left\langle {u_\phi n_G } \right\rangle $, and $\left\langle {p_i n_G } \right\rangle $. Then we show that the particle flux, momentum flux, and heat flux influence the zonal flow for each system respectively. This work was supported by U. S. Department of Energy Contract No. DE--AC02--09CH11466 (TSH, LW), China Scholarship Council (LW), U. S. DOE SciDAC center for Gyrokinetic Particle Simulation of Turbulent Transport in Burning Plasmas, and the U. S. DOE SciDAC-FSP Center for Plasma Edge Simulation (TSH). [Preview Abstract] |
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BP8.00051: Role of stable eignmodes in ETG-driven turbulence Juhyung Kim, Paul T. Terry The role of stable eigenmodes in the saturation of plasma turbulence is investigated in the framework of the ETG fluid model. Previously, Kelvin-Helmholtz instability has been involved as a saturation mechanism, applying to the transition from the linear to nonlinear regime where the structure of linearly unstable modes breaks down. This saturation mechanism is investigated from the ``energetic'' point of view with the damped eigenmodes. The nonlinear energy transfer between the unstable and damped eigenmodes is traced in the simulations. It is observed that the energy of the linearly unstable modes is transferred to the damped eigenmodes at the corresponding wavelength at the time of the linear structure break-down. Additional details of energy balances among eigenmodes in the nonlinear state are presented for a 2D sheared slab geometry. Extension to 3D has also been developed and results will be given. The analysis of energetics that complement dynamical approaches, such as the analysis of phase mixing, will be considered and described. [Preview Abstract] |
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BP8.00052: Angular Momentum Ejection and Recoil$*$ O. Ohia, B. Coppi The spontaneous rotation phenomenon observed in axisymmetric magnetically confined plasmas has been explained by the ``accretion theory'' [1] that considers the plasma angular momentum as gained from its interaction with the magnetic field and the surrounding material wall. The ejection of angular momentum to the wall, and the consequent recoil are attributed to modes excited at the edge while the transport of the (recoil) angular momentum from the edge toward the center is attributed to a different kind of mode. The toroidal phase velocity of the edge mode, to which the sign of the ejected angular momentum is related, is considered to change its direction in the transition from the H-regime to the L-regime. For the latter case, edge modes with phase velocity in the direction of $\textup{v}_{di}$ are driven by the temperature gradient of a cold ion population at the edge and damped on the ``hot'' ion population. The ``balanced'' double interaction [2] of the mode with the two populations, corresponding to a condition of marginal stability, leads to ejection of hot ions and loss of angular momentum in the direction of $\textup{v}_{di}$ while the cold population acquires angular momentum in the opposite direction. In the H-regime resistive ballooning modes with phase velocities in the direction of $\textup{v}_{de}$ are viewed as the best candidates for the excited edge modes. *Sponsored in part by the U.S. DOE. [1] B. Coppi, \textit{Nucl. Fusion} \textbf{42}, 1 (2002) [2] B. Coppi and F. Pegoraro, \textit{Nucl. Fusion} \textbf{17}, 969 (1977) [Preview Abstract] |
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BP8.00053: Wave Momentum, Radial Current and Intrinsic Rotation P.H. Diamond, O.D. Gurcan In this paper, we extend ongoing work on the origins of intrinsic rotation in tokamaks to address general novel mechanisms for producing a non-diffusive Reynolds stress which drives intrinsic rotation. A unifying approach employs the calculation of momentum fluxes via consideration of transport of resonant particle and wave momentum. The latter is calculated via wave kinetics. Specific results include the calculations of radial current $\langle T_r \rangle$ and the associated toroidal acceleration due to wave turbulence and a more general calculation of parallel spin-up due to $k_\parallel$-symmetry breaking. The latter is shown to occur for any intensity gradient. [Preview Abstract] |
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BP8.00054: Momentum Theorems for Vlasov and Gyrokinetic Turbulence L.T. Neko, P.H. Diamond, C. McDevitt, Y. Kosuga, X. Garbet, O.D. Gurcan, T.S. Hahm In this work, we extend Charney-Drazin type momentum theorems to encompass Vlasov and gyrokinetic systems. The common key element of these and their fluid antecedents is the existence of a Kelvin's Theorem for a conserved circulation in the relevant phase space. A pseudomomentum, quadratic in the perturbed distribution function, is identified. In the non-resonant limit, we show that this pseudomomentum reduces to the familiar wave momentum density. Constraint relations for anomalous resistivity (1D) and GK zonal flow growth are derived. The relation to phase space density granulations is discussed. [Preview Abstract] |
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BP8.00055: Current-Driven Drift Wave Turbulence and Electron Thermal Transport in Tokamaks C. Lee, P.H. Diamond, M. Porkolab Recent analyses (Y. Lin, M. Porkolab; 2009) have indicated that the ``usual suspects'' for the mechanism of electron thermal transport, such as ITG, ETG, CTEM modes, etc, cannot explain results from modest density, $T_e>T_i$ plasmas, in either OH or ECH heating regimes. Interestingly, such plasmas exhibit very large toroidal current drift parameters $v_d/c_s$, thus naturally suggesting a re-visitation of current driven drift waves. In this paper, we discuss the linear, quasilinear and non-linear theory of current driven drift waves in tokamaks. Parallel electron velocity scattering, a critical effect beyond the capacity of most, gyrokinetic codes is a major focus of investigations. The coupled transport of current density and heat are considered. Work is ongoing and results will be presented. [Preview Abstract] |
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BP8.00056: Mesoscopic Lattice Boltzmann Representation for Quantum to Classical Turbulence Bo Zhang, George Vahala, Linda Vahala Here we present a lattice Boltzmann (LB) mesoscopic representation of the Gross-Pitaevskii equation to examine the multi-scale physics of turbulence spanning the quantum vortex core scales to the classical large scales were the quantization of vortices is unimportant. While the unitary mesoscopic algorithm faithfully represents the Hamiltonian structure of GP, the LB representation is at first glance dissipative. The macroscopic nonlinearities are recovered by phase shifts -- just as in the mesoscopic qubit unitary algorithm. However, this dissipative effect can be rescaled in time. Soliton collisions as well as turbulence will be considered and compared to the unitary algorithm. [Preview Abstract] |
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BP8.00057: Scalar Magnetic Field Distribution Function Approach to MHD Turbulence Tao Wang, George Vahala, Linda Vahala Lattice Boltzmann (LB) representations are mesoscopic algorithms that exploit a simple collide-stream scheme that is ideal for parallelization -- even for non-periodic boundary conditions. Moreover, in LB one can enforce to machine accuracy. Typically one has introduced a vector distribution function for the magnetic field to account for the asymmetry tensor in the magnetic field evolution as opposed to the symmetric stress tensor in velocity evolution. Here we investigate 2D MHD turbulence by working with a scalar magnetic distribution function representation. A major advantage of the scalar representation is the much reduced computational memory requirements, simpler boundary condition enforcement, and simple entropic stabilization schemes. The Orszag-Tang vortex will be examined as well as some LES closure schemes using Elsasser variables. [Preview Abstract] |
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BP8.00058: Kolmogorov to Kelvin Wave Cascades in Turbulence: from large scales to quantum vortex core scales George Vahala, Jeffrey Yepez, Min Soe, Linda Vahala A novel unitary mesoscopic lattice algorithm with low memory requirements, permits simulations of the nonlinear Schrodinger equation (NLS) on spatial grids up to 5760$^{3}$. The algorithm is built from the collisional unitary entanglement of 2 qubits at each spatial node and then unitary streaming of this entangled state to neighboring sites. The algorithm scales perfectly -- even to the full 163840 processors on Blue Gene P/Intrepid. Our simulations have determined 3 distinct power laws in the incompressible kinetic energy spectrum: a classical Kolmogorov k$^{-5/3}$ spectrum at large scales, and a quantum Kelvin wave cascade spectrum of at scales of the order of the quantum cores. In the adjoining semiclassical regime there is a non-universal steeper spectral decay adjoining the classical and quantum regimes. Our unitary (reversible) algorithm fully respects the Hamiltonian nature of the GP equation and approaches pseudo-spectral accuracy. Somewhat unexpectedly, we find a set of initial conditions that exhibit very short Poincare recurrence times. [Preview Abstract] |
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BP8.00059: Candidates for the Observed Electron Thermal Energy Transport and Micro-reconnecting Modes* C. Crabtree, B. Coppi Two plausible candidates to explain the observed (anomalous) electron thermal energy transport in collisionless or weakly collisional plasmas are shown to be the trapped electron mode[1] (TEM) and the micro-reconnecting mode[2]. The first mode is driven by the combined effects of the plasma pressure and the magnetic field curvature. The second produces a string of magnetic islands and requires that the relative electron temperature gradient to be significantly larger than the relative temperature gradient. Both modes, when considering the largest transverse wavelengths for which they can be excited ($1/k_{\bot} \sim\ \rho_{i}$, for the first mode, $\rho_{i}=$ ion gyro-radius, and $1/k_{\bot} \sim c/\omega_{pe}$ for the second mode) require a phase space (non-fluid) description. For the second mode the transverse electron energy effective diffusion can be represented by $D^{th}_{e\bot} \sim (de/r_{te})cT_{e}/(eB)$ which does not involve a strong degradation of the energy confinement time as a function of the heating power. Here $r_{te}=-1/(d \textup{ln}T_{e}/dr$). A more complete form of the composite transport coefficient that is suitable for the numerical simulation of the transport properties of experimentally produced plasmas with different degrees of collisionality has been derived from the characteristics of the modes. *Supported in part by the U.S. D.O.E. [1] B. Coppi and G. Rewoldt, \textit {Phys. Rev. Letts.} \textbf{33} 1329 (1974) [2] B. Coppi, in \textit {Collective Phenomena in Macroscopic Systems}, p. 59 publ. \textit {World Scientific} (2007). [Preview Abstract] |
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BP8.00060: A Green's function method for local and non-local parallel transport in general magnetic fields Diego del-Castillo-Negrete, Luis Chac\'on The study of transport in magnetized plasmas is a problem of fundamental interest in controlled fusion and astrophysics research. Three issues make this problem particularly challenging: (i) The {\em extreme anisotropy} between the parallel (i.e., along the magnetic field), $\chi_\parallel$, and the perpendicular, $\chi_\perp$, conductivities ($\chi_\parallel/\chi_\perp$ may exceed $10^{10}$ in fusion plasmas); (ii) Magnetic {\em field lines chaos} which in general complicates (and may preclude) the construction of magnetic field line coordinates; and (iii) {\em Nonlocal parallel transport} in the limit of small collisionality. Motivated by these issues, we present a Lagrangian Green's function method to solve the local and non-local parallel transport equation applicable to integrable and chaotic magnetic fields. The numerical implementation employs a volume-preserving field-line integrator [Finn and Chac\'on, {\em Phys. Plasmas}, {\bf 12} (2005)] for an accurate representation of the magnetic field lines regardless of the level of stochasticity. The general formalism and its algorithmic properties are discussed along with illustrative analytical and numerical examples. Problems of particular interest include: the departures from the Rochester--Rosenbluth diffusive scaling in the weak magnetic chaos regime, the interplay between non-locality and chaos, and the robustness of transport barriers in reverse shear configurations. [Preview Abstract] |
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BP8.00061: Self Consistently Calculated Zonal Flow Shears and Stability and Its Implications Y. Kosuga, P.H. Diamond In this work, we derive an \textit{exact} expression for the zonal flow velocity profile using conservation of potential enstrophy for drift wave models, in a \textit{stationary state}. This result extends the Charney-Drazin theorem, familiar from geophysical fluid dynamics, and should be contrasted to previous zonal flow models in that it: a.) is derived for the \textit{stationary} turbulence-flow system, rather than for the transient growth phase. b.) links the zonal flow directly to the driving transport \textit{flux}, which is \textit{fixed}. c.) is formulated in \textit{real} space, instead of Fourier space, which is critical to determine the strength of the shear and curvature of zonal flow (n.b. the former controls turbulent transport and the latter controls Kelvin-Helmholtz instability of zonal flows). We have obtained results for the flow shear profile and the flow curvature profile. Results indicate that: a.) zonal flow \textit{shear} is determined primarily by the driving flux and the profile of the flow damping, which allows determination of the critical flux for reduction of turbulent transport. b.) zonal flow \textit{curvature} is determined by the flow damping curvature, along with the profile of potential enstrophy dissipation, which determines a condition for KH stability. This research was supported by U.S Department of Energy Grant Nos. DE-FG02-04ER54738, DE-FC02-08ER54959 and DE-FC02-08ER54983. [Preview Abstract] |
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BP8.00062: On the mechanism responsible for subdiffusive transport across poloidal zonal flows in gyro-kinetic simulations of tokamak ITG turbulence Raul Sanchez, David Newman, Jean-Noel Leboeuf, Viktor Decyk It has been recently found that radial transport ceases to behave diffusively in the presence of a radially-sheared poloidal zonal flow, becoming instead strongly subdiffusive [1]. The same behavior is observed in other simulations, suggesting that the mechanism responsible is rather general. In numerical simulations of 2D-turbulence, the change in character seems to be related to the selection by the sheared flow of a preferred sign of the axial vorticity (that of the sheared flow), further reinforced by the tilting of the turbulent eddies carried out by the shear [2]. In this contribution we look for evidence of the same mechanism in gyrokinetic simulations of tokamak ITG turbulence carried out by the UCAN code. The way in which these concepts must be modified to accommodate the toroidal geometry is discussed.\\[4pt] [1] R. Sanchez et al, Physical Review Letters \textbf{101}, 205002 (2008)\\[0pt] [2] D.E. Newman et al, in Proc. of the 35th EPS Conference on Plasma Physics, Hersonissos, 9-13 June 2008, ECA Vol. 32, p. 1.044 (2008) [Preview Abstract] |
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BP8.00063: COMPLEX, NON-NEUTRAL AND OTHER NOVEL PLASMAS |
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BP8.00064: Low Velocity Ion Stopping in Dense and Multicomponent Plasmas Bekbolat Tashev, Claude Deutsch, Patrice Fromy We focus attention on low velocity ion slowing down (LIVSD) in dense and multicomponent plasmas of ICF and astrophysical interest. The target is treated in a dielectric formalism with classical electrons neutralizing binary ionic mixtures (BIM) of any relative proportion.We consider first charge symmetric BIM such as deuterium-tritium, proton-heliumlike iron ions in the solar interior or proton-helium ions considered in planetology as well as other mixtures of relevance to the heavy ion production of warm dense matter at Bragg peak conditions. We single out ion projectile velocities (so-called critical) at which target electron LIVSD matches the sum of target ion ones. Proton stopping in the white dwarf crust (carbon-helium BIM) is also considered. [Preview Abstract] |
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BP8.00065: Weibel instability in positron-electron plasma Barbara Shrauner The Weibel or filamentation instability of counter-flowing beams described by a recent model is investigated. The model combined computer simulations and an analytic one-particle distribution function for the quasi-equilibrium of a positron-electron plasma. An electron-ion plasma is not considered. This instability is conjectured to give rise to turbulent magnetic fields in collisionless shocks. The linear growth rate and time dependence of the instability is determined from the Vlasov-Maxwell equations and confirms that the dominant mode for the magnetic field has the spatial dependence of the initial perturbed magnetic field as found by the simulations. The quasi-equilibrium distribution function is expanded in Hermite polynomials varying in two spatial variables, a generalization of earlier results for nonlinear transverse waves. The quasi-equilibrium in reference 1 is further constrained by the conservation of particle number and total energy. That quasi-equilibrium agrees only qualitatively with the simulation results. A nonlinear model is proposed with the sinh-Poisson equation. An analytic form for the vector potential is found for this nonlinear model in terms of Jacobian elliptic functions. A. Suzuki and T. Shigeyama, Astrophys. J. \textbf{695}, 1550-1558 (2009). B. Abraham-Shrauner, Phys. Fluids, \textbf{11}, 1162-1167 (1968). [Preview Abstract] |
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BP8.00066: Emittance Preservation of Positron Beams Traveling in Hollow Plasma Channels Xiaoying Li, Patric Muggli, Samuel Martins, Warren Mori, Wayne D. Mori Emittance preservation of the incoming beam is essential for all accelerators. In the blowout regime of plasma wakefield accelerator (PWFA), the electron bunch propagates in a uniform ion column and its emittance is preserved along the plasma. For a positron bunch propagating in a uniform plasma, the beam suffers emittance growth [P. Muggli et al., Phys. Rev. Lett. 2008]. However, it has been shown that when a positron bunch propagates in a hollow plasma, the axial accelerating field can be enhanced when compared to the uniform plasma case [S. Lee, PRE, 64, 045501(R), 2001]. We explore through numerical simulations the possibility of preserving emittance of positron beams by using a hollow plasma channel instead of a uniform plasma. We consider in particular the case of a drive/witness bunch train, necessary to accelerate the witness bunch with a high gradient and a narrow final energy spread. We explore a range of beam and channel parameters to maximize the acceleration and minimize the final energy spread and emittance of the witness bunch. Detailed simulation results will be presented. [Preview Abstract] |
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BP8.00067: Metal Wire Explosions in Underwater Discharges Deok-Kyu Kim, Sung-Hyun Baek, Inho Kim Magnetohydrodynamics of metal wires exploding in underwater electrical discharges have been simulated and the results are compared with the experimental observations by streak camera. We use a one-dimensional time-dependent magnetohydrodynamic simulation code which employs an equation-of-state model and an electrical conductivity model for dense plasmas [1].~The rapidly expanding boundaries of metal plasmas and the resulting shock fronts propagating in the water are computed as function of time for comparison with the streak camera images, which shows fair agreements. In addition, the electrical conductivities of metal plasmas are reproduced from the measured current and voltage profiles and then discussed for verification of the conductivity model used in the simulation.\\[4pt] [1] Deok-Kyu Kim and Inho Kim, Phys. Rev. E \textbf{68}, 056410 (2003); Contrib. Plasma Phys. \textbf{47}, 173 (2007). [Preview Abstract] |
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BP8.00068: ABSTRACT WITHDRAWN |
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BP8.00069: X-ray absorption spectroscopy using short pulse laser isochoric heated targets Sophia Chen, R. Shepherd, P. Audebert, P. Renaudin, B. Loupias, L. Lecherbourg, B. Wilson, C. Iglesias, R. Majoribanks, H. Chen, P. Beiersdorfer We present a series of experiments to help quantify the effects of density on bounds states in plasmas. The experiments are performed by x-ray back-lighting aero-gel targets heated with an ultrashort pulse laser. The density and temperature is determined by assuming an isothermal expansion and using frequency domain interferometry measure the time-dependent position of the critical surface. Additional temperature verification is made using the target emissivity. The aero-gel target density is systematically increased and absorption spectra is used to monitor the effect of the increased density on the bound states. Preliminary experimental results will be presented. [Preview Abstract] |
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BP8.00070: Strongly Coupled Plasma Dynamics Using the Particle-in-Cell Methodology D.V. Rose, T.C. Genoni, D.R. Welch, R.E. Clark, T.A. Mehlhorn, R.B. Campbell, D.G. Flicker, W.A. Stygar Three-dimensional simulations of moderately to strongly coupled electron-ion and multi-component plasmas using the particle-in- cell method are presented. The simulations resolve sub-Debye- length inter-particle spacing to accurately model the dynamics of these systems. We consider realistic mass ratios and quasi- equilibrium conditions with different component temperatures which are relevant on short time scales. The simulation results are in very good agreement with classical hypernetted chain calculations for dense electron-ion and ion-ion plasmas [1]. Our results demonstrate the feasibility and utility of large- scale particle-in-cell simulations for the modeling and analysis of multi-component moderately and strongly coupled plasmas. Application of the simulation model to conductivity [2] and mass-stopping power of energetic ions in strongly coupled plasmas is discussed. \\[4pt] [1] V. Schwarz, et al., Contrib. Plasma Phys. 47, 324 (2007). \\[0pt] [2] W. A. Stygar, G. A. Gerdin, and D. L. Fehl, Phys. Rev. E 66, 046417 (2002). [Preview Abstract] |
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BP8.00071: Degenerate Mixing of Electrostatic Modes on a Finite-Length Nonneutral Plasma Column M.W. Anderson, T.M. O'Neil Using cold fluid theory, we discuss the structure of standing electron plasma waves on a magnetized, nonneutral plasma column of finite length. Such eigenmodes can be surprisingly complex, involving a superposition of many component waves with different axial and transverse wavenumbers $k_z$ and $k_\perp$. The dispersion relation\footnote{A.W. Trivelpiece and R.W. Gould, J. Appl. Phys. {\bf 30}, 1784 (1959).} for the individual components [{\it i.e.}, $\omega = \omega_p k_z / \sqrt{k_z^2 + k_\perp^2}$] implies that waves with small $k_z$ and $k_\perp$ can be degenerate with waves with large $k_z$ and $k_\perp$. Reflection at the column ends mixes these degenerate components, yielding the complicated structure. We have in mind eigenmodes on a cryogenic plasma column, where cold fluid theory is valid even for waves with large $k_z$ and $k_\perp$. In a warmer plasma, kinetic effects ({\it e.g.}, Landau damping of the large wavenumber components) spoils the degeneracy and kills the mixing. [Preview Abstract] |
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BP8.00072: Electron Acoustic Waves in Pure Ion Plasmas F. Anderegg, C.F. Driscoll, D.H.E. Dubin, T.M. O'Neil Electron Acoustic Waves (EAW) are the low frequency branch of electrostatic plasma waves. These waves exist in neutralized plasmas, pure electron plasmas and in pure ion plasmas\footnote{F. Anderegg {\it et al.}, PRL {\bf 102}, 095001 (2009) and PoP {\bf 16}, 055705 (2009).} (where the name is deceptive). Here, we observe standing $m_\theta = 0$ $m_z = 1$ EAWs in a pure ion plasma column. At small amplitude, the EAWs have a phase velocity $\mathrm{v}_{\mathrm{ph}} \simeq 1.4 \overline{\mathrm{v}}$, and the frequencies are in close agreement with theory. At moderate amplitudes, waves can be excited over a broad range of frequencies, with observed phase velocities in the range of $1.4 \overline{\mathrm{v}} \leq \mathrm{v}_{\mathrm{ph}} \leq 2.1 \overline{\mathrm{v}}$. This frequency variability comes from the plasma adjusting its velocity distribution so as to make the EAW resonant with the drive frequency. Our wave-coherent laser-induced fluorescence diagnostic shows that particles slower than $\mathrm{v}_{\mathrm{ph}}$ oscillate in phase with the wave, while particles moving faster than $\mathrm{v}_{\mathrm{ph}}$ oscillate 180$^\circ$ out of phase with the wave. From a fluid perspective, this gives an unusual negative dynamical compressibility. That is, the wave pressure oscillations are 180$^\circ$ out of phase from the density oscillations, almost fully canceling the electrostatic restoring force, giving the low and malleable frequency. [Preview Abstract] |
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BP8.00073: Enhanced Neoclassical Transport and Mode Damping Caused by Chaos Near an Asymmetric Separatrix D.H.E. Dubin, Yu.A. Tsidulko Plasma loss due to apparatus asymmetries is a ubiquitous phenomenon in magnetic plasma confinement. Recent experiments have investigated the loss rate when a central squeeze potential is applied to a magnetized plasma column, creating two trapped particle populations separated by a separatrix.\footnote{D.H.E. Dubin, Phys. Plasmas {\bf 15}, 072112 (2008); D.H.E. Dubin {\it et al.}, Neoclassical transport and plasma mode damping caused by collisionless scattering across an asymmetric separatrix, in preparation.} These populations react differently to the asymmetries, leading to a collisional boundary layer at the separatrix. A loss rate scaling as $\sqrt{\nu / B}$ due to the boundary layer is expected theoretically,$^1$ provided that the separatrix itself is axisymmetric. However, when the separatrix is {\it asymmetric}, particles become trapped and detrapped as they follow collisionless orbits. This can lead to a chaotic region around the separatrix, giving enhanced transport scaling as $\nu^0 B^{-1}$. This effect also damps certain plasma modes. Predictions for damping of trapped particle diocotron modes will be compared to experiments.\footnote{A.A. Kabantsev, adjacent poster.} [Preview Abstract] |
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BP8.00074: Damping of Diocotron Modes by Chaotic Dissipation on a $\theta$-ruffled Separatrix A.A. Kabantsev, D.H.E. Dubin, Yu.A. Tsidulko, C.F. Driscoll Diocotron mode damping measurements on pure electron plasma columns clearly distinguish the novel ``chaotic'' dissipation on a $\theta$-ruffled separatrix from the usual collisional dissipation on a $\theta$-symmetric separatrix. Here, an applied electrostatic ``squeeze'' barrier $\Phi_0 (r)$ makes a separatrix between fast and slow electrons, and separatrix dissipation\footnote{A.A. Kabantsev {\it et al.}, Phys. Rev. Lett. {\bf 101}, 065002 (2008).} causes the observed mode damping. We add controlled $\theta$-ruffles $\Phi_m \cos (m \theta )$ to the separatrix by either static wall voltages or by other launched waves, and we quantify the enhanced damping in both cases. The chaotic dissipation results from electrons becoming trapped and untrapped as the plasma $E \times B$ drift rotates; and it is observed to scale as $\Phi_m^1 B^{-1} $. In contrast, the collisional dissipation scales as $ B^{-1/2}$, and so tends to dominate at high magnetic fields. The transition from collisional to chaotic is unambiguously observed as the strength of the separatrix ruffle is increased. These experimental results are in quantitative agreement with nascent theory analyses\footnote{D.H.E. Dubin, adjacent abstract} which treat the neoclassical transport and damping from $\theta$-ruffled separatrices. [Preview Abstract] |
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BP8.00075: Separatrix-Induced Transport, Damping, and Wave-Coupling C.F. Driscoll, A.A. Kabantsev, T.M. O'Neil, D.H.E. Dubin, Yu.A. Tsidulko Recent experiments have characterized a broad range of transport, damping, and wave-coupling effects caused by trapping separatrices in both the ``collisional'' and ``chaotic'' regimes. Here, the pure electron plasma columns have an electrostatic trapping separatrix created by an applied ``squeeze'' voltage; and this separatrix may have ``$\theta$-ruffles'' from the applied voltages or from other $m_\theta \not= 0$ waves. For a $\theta$-symmetric separatrix, traditional neoclassical theory predicts collisional dissipative effects scaling as $\nu_{ee}^{1/2} B^{- 1/2}$. For a $\theta$-ruffled separatrix, recent theory predicts chaotic dissipative effects scaling as $\nu_{ee}^0 B^{-1}$. The experiments have characterized the wave damping for both diocotron (drift) waves and Langmuir (plasma) waves, as well as the bulk plasma expansion that occurs when external confinement asymmetries (such as magnetic tilt) are present. For $B > 10$kG, the $E \times B$ drift rotation is slow and collisional effects tend to dominate; for $B < 1$kG, rapid plasma rotation around $\theta$-ruffled separatrices causes chaotic effects to dominate. This novel chaotic transport may have applicability to low-collisionality stellarator and tokamak separatrices also, especially given the ubiquity of wave-induced separatrix ruffles. [Preview Abstract] |
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BP8.00076: Progress Towards a Multicell Trap for Positron Storage J.R. Danielson, T.R. Weber, C.M. Surko There are many potential applications of high-capacity and/or portable antimatter traps. Here, we describe the latest progress toward the development of a novel multicell Penning-Malmberg trap that will be capable of accumulating orders of magnitude more positrons than is possible presently.\footnote{J. R. Danielson, T. R. Weber and C. M. Surko, {\it Phys. Plasmas} {\bf 13}, 123502 (2006).} This design represents the next major step in antimatter storage technology. Described here is the design for a 21 cell trap capable of accumulating and storing more than $5 \times 10^{11}$ positrons using 1 kV confinement potentials. Experiments with electron plasmas establishing techniques critical to the implementation of a practical multicell trap are discussed. This trap could be used to multiplex the output of the new generation of positron sources, either operating now or currently under development, as well as to provide record-high bursts of positrons for a variety of applications. [Preview Abstract] |
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BP8.00077: Magnetic multipole induced zero-rotation frequency bounce-resonant loss in a Penning-Malmberg trap used for antihydrogen trapping Joel Fajans In many antihydrogen trapping schemes, antiprotons held in a short-well Penning-Malmberg trap are released into a longer well. This process necessarily causes the bounce-averaged rotation frequency $\Omega_r$ of the antiprotons around the trap axis to pass through zero. In the presence of a transverse magnetic multipole, experiments show that many antiprotons (over 30{\%} in some cases) can be lost to a hitherto unidentified bounce-resonant process when $\Omega_r$ is close to zero. The results of these experiments will be presented, as well as an analytic model and numeric simulations [Preview Abstract] |
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BP8.00078: Antihydrogen Induced Radial Redistribution of Antiprotons and Detection of Weakly Bound Antihydrogen Niels Madsen Attempts to trap antihydrogen ($\bar{\mbox{H}}$) in a magnetic minimum trap are ongoing. The $\bar{\mbox{H}}$s is typically formed through direct mixing of plasmas of positrons ($\mbox{e}^+$) and antiprotons ($\bar{\mbox{p}}$), as this, thus far, has shown the highest yields of $\bar{\mbox{H}}$. The formation is believed to be dominated by a process in which two $\mbox{e}^+$s interact with a $\bar{\mbox{p}}$, such that one $\mbox{e}^+$ becomes bound and the second carries away excess momentum and energy. This process is expected to generate relatively weakly bound $\bar{\mbox{H}}$. We have found that, when using direct mixing, field-ionization of the weakly-bound $\bar{\mbox{H}}$ by the external electric fields, leads to radial redistribution of the $\bar{\mbox{p}}$s. We can use this effect to efficiently detect these weakly bound $\bar{\mbox{H}}$, by intentionally causing $\bar{\mbox{p}}$s at large radii in our trap to be lost on the introduction of a strong transverse octupole magnetic field and monitoring the losses. This technique, together with the direct detection of strongly-bound $\bar{\mbox{H}}$ (bound enough that it survives the various trap electric fields), allows us to measure the fraction of $\bar{\mbox{H}}$ formed in strongly-bound states. Since it is these states which survive the electric fields necessary for synthesis, they are those available for trapping if cold enough. Using this technique we can therefore optimize the synthesis for maximum generation of strongly-bound, potentially-trappable states, and hence increase the likelihood of trapping. [Preview Abstract] |
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BP8.00079: ABSTRACT WITHDRAWN |
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BP8.00080: Overview of recent results from non-neutral plasmas in the CNT stellarator T. Sunn Pedersen, A.H. Boozer, P.W. Brenner, B. Durand de Gevigney, M.S. Hahn, X. Sarasola, A. Senter An overview of recent results from the Columbia Non-neutral Torus (CNT) will be given. CNT is a stellarator dedicated to studies of non-neutral and electron-positron plasmas [1]. CNT operates with a surplus of electrons -- most of the time with only a trace amount of ions (ni/ne $<$1{\%}), creating negative space potentials of several hundred volts despite having Te $\approx $5 eV [2]. This allows the study of neoclassical confinement with negative radial electric fields much larger than those that would arise from ambipolarity constraints, and also allows the study of pure electron plasmas in a minimum energy state. The overview will include results from studies of partially neutralized plasmas [3-5], recent improvements in confinement times, observations of transport jumps [6] and will give an update on our plans for the creation and study of pure electron plasmas. [1] T. Sunn Pedersen and A. H. Boozer, PRL \textbf{88 }(2002) 205002 [2] J. P. Kremer \textit{et al., }PRL \textbf{97}, (2006) 095003 [3] P. W. Brenner \textit{et al., }this poster session [4] Q. R. Marksteiner \textit{et al.,} PRL \textbf{100 }(2008) 065002 [5] X. Sarasola Martin \textit{et al., }this poster session [6] M. S. Hahn et al., Phys. Plasmas \textbf{16} (2009) 022105 [Preview Abstract] |
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BP8.00081: Confinement of Pure Electron Plasmas in the Columbia Non-neutral Torus P.W. Brenner, T. Sunn Pedersen, M.S. Hahn, X. Sarasola The plasmas studied in the Columbia Non-neutral Torus are uniquely different from non-neutral plasmas in other magnetic geometries. Confinement is primarily limited by transport resulting from insulating probes inserted into the plasma and electron neutral collisions. Recently the best measured confinement time has been increased by over an order of magnitude to 323 ms. The magnetic geometry allows for confinement of plasmas with arbitrary degree of neutralization but the geometry's complexity also introduces new challenges and sources of transport. An improved conducting boundary is being designed and installed to minimize potential variation along magnetic surfaces and externally diagnose the plasma. Three stellarator configurations can be studied in CNT by varying the angle between coils. Progress toward a comparison of each configuration will be presented. Methods to create plasmas unperturbed by internal rods and diagnose the plasma at or outside the edge are described and initial results are presented. A thorough understanding of these methods is a significant step towards the goal of studying electron-positron plasmas. [Preview Abstract] |
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BP8.00082: Numerical Studies of Transport in the Columbia Non-neutral Torus Benoit Durand de Gevigney, Thomas Sunn Pedersen, Allen H. Boozer The Columbia Non-neutral Torus (CNT) is a stellarator dedicated to the study of non-neutral plasmas on magnetic surfaces. Due to space charge imbalance such plasmas exhibit a very large radial electric field. In the absence of parallel electric fields, the induced $\vec E \times \vec B$ rotation dominates over the radial magnetic drifts and closes the orbits. However the confinement of trajectories is sensitive to the electrostatic boundary conditions at the plasma edge. Variations of the electric potential on magnetic surfaces, inherent to the CNT equilibrium, can also lead to bad orbits. Insulated probes inserted into the plasma charge up negative and create large, localized potential variations leading to $\vec E \times \vec B$ plasma flow out of the confining region. A code was written to solve for the electric potential in CNT allowing for a large variety of boundary conditions and capable of resolving the perturbation of the probes. Using results from this code as input for a Monte-Carlo electron drift orbit code we study how these perturbations affect the loss rate of electrons at different magnetic field strengths, and we compare our results to experimental data. [Preview Abstract] |
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BP8.00083: First observations of partially neutralized and quasineutral plasmas in the Columbia Non-neutral Torus Xabier Sarasola, Paul Brenner, Michael Hahn, Thomas Pedersen The Columbia Non-neutral Torus (CNT) is the first stellarator devoted to the study of pure electron, partially neutralized and positron-electron plasmas. To date, CNT usually operates with electron rich plasmas (with negligible ion density) [1], but a stellarator can also confine plasmas of arbitrary degree of neutralization. In CNT the accumulation of ions alters the equilibrium of electron plasmas and a global instability has been observed when the ion fraction exceeds $10 \%$. A characterization of this instability is presented in [2], analyzing its parameter dependence and spatial structure (non- resonant with rational surfaces). A new set of experiments is currently underway studying plasmas of arbitrary degree of neutralization, ranging from pure electron to quasineutral plasmas. Basic observations show that the plasma potential decouples from emitter bias when we increase the degree of the neutralization of our plasmas. Partially neutralized plasmas are also characterized by multiple mode behavior with dominant modes between 20 and 200 kHz. When the plasma becomes quasineutral, it reverts to single mode behavior. The first results on partially neutralized plasmas confined on magnetic surfaces will be presented. [1] J. Kremer, PRL 97, (2006) 095003 [2] Q. Marksteiner, PRL 100 (2008) 065002 [Preview Abstract] |
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BP8.00084: Electrostatic Charging of Insulating Rods and Resulting Particle Transport in the Columbia Non-neutral Torus Aaron Senter, Nikolaus Rath, Paul Brenner, Xabier Sarasola, Thomas Pedersen The Columbia Non-neutral Torus (CNT) is a stellarator created to study non-neutral plasmas confined on magnetic surfaces. To create and diagnose the electron plasma, filaments supported by ceramic rods are inserted into the plasma. These rods charge negatively allowing particles to $\vec E \times \vec B$ drift across the confining magnetic surfaces and out of the plasma. A simple model of this process has yielded good qualitative agreement with experimentally observed radial transport[1]. However, when the rod is retracted, it perturbs the plasma more than expected. To better understand the rod perturbations, externally biased conducting rods are now being used. We find the effect of the ceramic rods by measuring the increase in filament emission current with a rod installed. Comparing the radial transport rates of the ceramic and conducting rods, we aim to understand the charge distribution on the former, and to minimize the rod driven transport. Initial experiments show that a uniformly biased rod does not reproduce the transport observed. Experiments using a rod with a varying radial potential profile are being conducted and will be reported on. [1] J.W. Berkery \emph{et al.}, Phys. Plasmas \textbf{14}, 062503 (2007). [Preview Abstract] |
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BP8.00085: Simulating the FTICR-MS signal of a decaying $^7$Be Ion Plasma using a 2D PIC code M. Takeshi Nakata, Grant W. Hart, Bryan G. Peterson, Ross L. Spencer Beryllium-7 ($^7$Be) decays only by electron capture into lithium-7 ($^7$Li) with a half life of 53 days. As a result, its decay rate varies with its environment. We desire to study the effect of ionization on its decay rate. We will do this by trapping a $^7$Be ion plasma in a cylindrical Malmberg-Penning trap and measuring its and $^7$Li's concentration as a function of time by using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). We use this ratio as a function of time to directly measure the decay rate of the confined ion plasma rather than using gamma detection. We have simulated these signals in a 2-dimensional electrostatic particle-in-cell (PIC) code. The two spectrum peaks merge at high densities and at low densities they can be resolved. The merged peak linearly shifts with the relative abundances of these species. We have also simulated singly-ionized beryllium-7 hydride ($^7$BeH$^+$) and $^7$Li ion plasmas at high densities. These two separate peaks shift according to their relative abundances. We have developed an analytical model that explains how these peaks shift with their relative abundances. [Preview Abstract] |
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BP8.00086: Theory and simulation of m=0 Bernstein modes in a non-neutral plasma Grant W. Hart, Ross L. Spencer, M. Takeshi Nakata Axisymmetric upper-hybrid oscillations have been known to exist in non-neutral plasmas and FTICR/MS devices for a number of years. However, because they are electrostatic in nature and axisymmetric, they are self-shielding and therefore difficult to detect in long systems. When non-zero temperature is included the modes become analogous to Bernstein modes in slab geometry. Using a kinetic theory model We have analyzed the theory of these modes in a rigid-rotor thermal equilibrium. We find that in the central region (where the density is constant) the perturbed velocity is proportional to the Bessel function J$_{1}(\alpha r)$, with $\alpha$ having a distinct value for each mode. We have simulated these modes in our $r-\theta$ particle-in-cell code that includes a full Lorentz-force mover and find that in a mostly flat-top plasma the perturbed velocity closely matches this theoretical prediction. The dispersion relation derived from the theory also matches the values of $\omega$ and $\alpha$ seen in the simulation. There are still a few unresolved difficulties with the model. First, it is difficult to find the appropriate boundary condition to apply at the plasma edge. Second, the fundamental mode is undamped but all higher modes appear to be Landau damped. Progress in understanding these issues will be discussed. [Preview Abstract] |
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BP8.00087: Characterization of an ion trap to be used to determine the decay rate of ionized beryllium-7 Bryan G. Peterson, Daniel Erickson, Grant W. Hart Beryllium-7 is an isotope with a 53-day half-life that decays exclusively by electron capture. The decay rate can be modified by changing the electron configuration, and the availability of electrons for capture. All previous measurements of the half-life were made with the Be-7 atoms embedded in a matrix of some kind that resulted in significant and not well characterized modifications to the electron configuration. We are building an ion trap to study the rate of decay of singly-ionized Be-7 to determine the rate when the electron configuration is well known. We are using a boron carbide plasma to characterize the behavior of the trap. The presence of boron-10, boron-11, and carbon-12 will also allow us to determine the resolution and sensitivity of the Fourier Transform Ion Cyclotron Resonance Mass Spectrometry technique that will be used to monitor the conversion of the Be-7 to Li-7 and to determine the half-life. The results of this characterization will be presented. [Preview Abstract] |
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BP8.00088: Autoresonant transition in the presence of noise and self-fields Ido Barth A sharp threshold on the driving amplitude for transition to adiabatic nonlinear phase-locking (autoresonance) is characteristic of a single, electrostatically trapped particle starting in equilibrium and driven by a chirped frequency perturbation. In the presence of a weak noise or a small temperature ($T$) initial distribution of trapped particles, the threshold develops a width, scaling as $T^{1/2}$. The inclusion of repulsive self-fields makes the width of the autoresonant transition narrower. [Preview Abstract] |
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BP8.00089: Measuring transport in a toroidal electron plasma J. Smoniewski, M.R. Stoneking, F. Choudhury, E. Frater Global confinement times in excess of one second have been measured in the Lawrence Non-neutral Torus II using the frequency of the $m=1$ diocotron mode as a diagnostic, coupled with modeling of the toroidal modifications of that mode. We now report measurements (and modeling) of the $m=2$ mode frequency, which yields a measure of the average electron number density. The combined information provided using both modes permits the first measurements of transport rates in this experiment. Transport is initially faster than the previously reported global confinement times. We also report on initial experiments in which the plasma is allowed to expand from a $270^\circ$ toroidal arc (in which all previous experiments were conducted in this device) into a fully toroidal (closed field line) trap. [Preview Abstract] |
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BP8.00090: Constraints on an empirical flux equation for asymmetry-induced transport D.L. Eggleston Despite a large body of experimental work on asymmetry-induced transport, the correct theory remains elusive. We are currently developing an empirical model of the transport with an eye toward providing guidance for further theoretical development. In previous work\footnote{D.~L. Eggleston and J.~M. Williams, Phys. Plasmas 15, 032305 (2008).} we have shown that the flux equation for the transport is empirically constrained to be of the form $\Gamma(\epsilon) = -(B_0/B)^{1.33}D(\epsilon)[\nabla n_0+f(\epsilon)]$, where $\epsilon=\omega -l\omega_R$, $\omega$ is the asymmetry frequency, $\omega_R$ the plasma rotation frequency, $l$ the azimuthal mode number, $B$ the magnetic field, $n_0$ the density, $B_0$ an empirical constant, and $D (\epsilon)$ and $f(\epsilon)$ are unknown functions. To gain information about $D(\epsilon)$ and $f(\epsilon)$, we have examined data near the $\epsilon=0$ point and compared it to a first order expansion of $\Gamma(\epsilon)$. This analysis shows that $dD/d\epsilon(0)\not=0$, in contradiction to resonant particle theory\footnote{D.~L. Eggleston and T.~M. O'Neil, Phys. Plasmas 6, 2699 (1999).}. We also find that $f(\epsilon)$ can only be a fraction of the size predicted by that theory, and that $dD/d\epsilon(0)$ is an increasing function of radius and scales with the inverse of the center wire bias. This last result suggests that $\epsilon$ may be scaled by $\omega_R$ rather than the axial bounce frequency. [Preview Abstract] |
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BP8.00091: Experimental conditions to realize characteristic parameters of strongly coupled fine particle plasmas: An inverse problem Hiroo Totsuji Statistical properties of fine particle plasmas are characterized by dimensionless parameters such as strength of coupling. When the experimental conditions of density and temperatures are known, the dimensionless parameters are readily computed. However, it is straightforward to determine experimental conditions to realize a given combination of dimensionless parameters including charging condition of fine particles. In this presentation, we solve this inverse problem as much as possible analytically and give typical example by finally resorting to numerical methods[1]. The dependency of experimental conditions on various dimensionless characteristic parameters is obtained and it is shown that there exists a domain of dimensionless parameters where experimental conditions do not exist. We also analyze the dependency on species of neutral atoms. The results may be helpful to observe various phenomena related to the strong coupling between fine particles including possible existence of the critical point. \\[4pt] [1] Earlier results have been given in, H. Totsuji, Plasma and Fusion Research, 3, 046(2008). [Preview Abstract] |
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BP8.00092: Theory of arbitrary amplitude dust ion-acoustic shock waves in a multi-ion dusty plasma A.A. Mamun, Bengt Eliasson, Padma K. Shukla We present a theory for arbitrary amplitude dust ion-acoustic shock waves in a multi-ion dusty Plasma, composed of electrons, light positive ions, heavy negative ions, and stationary massive dust grains. For this purpose, the coupled Poisson and dust-charging equations, which accounts for the fluctuation of charges on static dust, is solved numerically. We find that large amplitude shocks are associated with a sudden decrease of the electrostatic potential and of the dust grain charge. In the lower speed limit small amplitude shocks, which are smoother in space, are formed, but in the larger speed limit large amplitude shocks with sharper fronts are formed. It is anticipated that the profiles and amplitudes of the DIA shocks predicted here will be observed in forthcoming laboratory and space experiments. [Preview Abstract] |
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BP8.00093: Direct measurement of the out-of-plane (optical) phonon spectrum in a 2D complex plasma crystal L. Couedel, V. Nosenko, S.K. Zhdanov, A.V. Ivlev, H.M. Thomas, G.E. Morfill The out-of-plane phonon spectrum of a 2D complex plasma crystal in the sheath of a RF discharge was measured using a novel imaging technique that allowed us to resolve the particle motion in the 3 dimensions. The out-of-plane mode was proven to have negative optical dispersion, as predicted a long time ago. Comparison with theory showed a remarkably good agreement. The effect of the plasma wakes on the dispersion relation is discussed. [Preview Abstract] |
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BP8.00094: Effect of a floating circular aperture on a dc glow discharge dusty plasma Jonathon R. Heinrich, Su-Hyun Kim, Robert L. Merlino We have investigated novel effects observed when a floating aperture, either 6 mm or 8 mm in diameter, is placed 1-2 cm in front of an anode disk (4 cm diameter) that is used to form a dc glow discharge dusty plasma. Dust is incorporated into the anode glow plasma from a tray located below the anode which contained kaolin powder. The glow discharge traps particles with an average size of 1 micron. When the aperture is placed in front of the disk, well-defined pear-shaped or spherical dust clouds are formed, depending on the diameter of the aperture and its distance from the anode. The dust interacts with the aperture through the potential structure associated with the floating (negative) plate in which the aperture is located. The dust cloud is imaged using a CCD camera and a thin sheet of 532 nm laser light. Some of the effects observed include: outwardly expanding spherical dust acoustic waves and shocks, dust rotation around a void formed at the aperture, and a dust/discharge instability in which the discharge is periodically quenched and reignited while the dust cloud expands and contracts, with the dust retaining a residual charge. [Preview Abstract] |
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BP8.00095: Thermal Characteristics of Dust Particles in Complex Plasmas with Dust Acoustic Waves R. Fisher, E. Thomas In a complex (dusty) plasma, charged micron-sized particles are suspended in a dc glow discharge plasma. The velocity space distribution of particles in the dust cloud was measured using stereoscopic particle image velocimetry (StereoPIV). As has been shown previously [J. Williams and E. Thomas, Jr,. \textit{Phys Plasmas}, \textbf{14}, 063702 (2007)] the velocity distribution of stable dust clouds can be modeled with a maxwellian distribution function, from which a characteristic kinetic temperature of the dust cloud can be obtained. In this study, the velocity distribution of dust particles was measured in dusty plasmas which contained driven dust acoustic waves (DAW). It was found that the measured velocity distributions of regions of the dust cloud containing DAW could not be adequately described by a maxwellian distribution function. This presentation describes the first attempts to find and interpret a suitable alternative velocity distribution model. [Preview Abstract] |
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BP8.00096: A Proposed Magnetized Dusty Plasma User Facility E. Thomas, R.L. Merlino, M. Rosenberg As the experimental study of dusty (complex) plasmas has advanced over the last two decades, a great deal of new insight has been gained on the complex interaction between the background plasma and charged microparticles. Even through the charged dust grains in a typical experiment can acquire several thousand elementary charges, the large mass of the grains ensures that the charge-to-mass ratio is quite low. As a result, it has been considered experimentally challenging to design an experiment that can achieve full magnetization of ions, electrons, and the charged dust grains. However, with continuing improvements in magnet design and sub-micron particle imaging technologies, it is now possible to contemplate the development of a Magnetized Dusty Plasma Facility. This presentation discusses the design, experimental parameters, and scientific motivation for a flexible, superconducting, 4 Tesla magnetic field user facility for the study of magnetized dusty plasmas. This work is supported by NSF grant number PHY-0936470 (AU), DOE Grant No. DE-FG01-04ER54795 (UI) and DOE Grant No. DE-FG02-04ER54804 (UCSD) [Preview Abstract] |
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BP8.00097: Applying PIV analysis techniques to complex plasmas: reduced gravity and phase transition experiments C. R\"ath, E. Thomas, J. Williams, M. Thoma, S. Mitic, L. Couedel, H. Thomas, M. Chaudhari Particle image velocimetry (PIV) techniques have been applied to studies of complex (dusty) plasmas for over a decade. In these previous investigations, measurements were performed using specialized hardware configured specifically for PIV. However, with the increasing use of higher speed video imaging, i.e., in excess of 100 frames per second, it is possible to apply PIV analysis techniques directly to video data. This presentation will discuss techniques used to benchmark and validate the use of PIV in this manner. Then, it will describe recent applications of PIV to reduced gravity and phase transition measurements in complex plasmas. It is shown that PIV can reveal detailed information on particle flows and waves in these systems. [Preview Abstract] |
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BP8.00098: Preliminary measurements of thermal effects in the dust acoustic wave Jeremiah Williams A complex (dusty) plasma (CDP) is a four-component system composed of ions, electrons, neutral particles and charged microparticles. The presence of the microparticles gives rise to new plasma phenomena, including collective modes such as the dust acoustic wave. Recent measurements of the dispersion relationship of this wave mode [E. Thomas, Jr., \textit{et. al.}, Phys. Plasmas 14, 123701 (2007), J.D. Williams, \textit{et. al.}, Phys. Plasmas 15, 043704 (2008)] have shown that, over a range of neutral gas pressures, it is necessary to include thermal effects to accurately fit the measured dispersion relations. In this work, initial measurements of the dispersion relation in a new dusty plasma experiment, the Wittenberg University DUsty Plasma Experiment (WUDUPE), will be presented. In particular, the dependence of the kinetic dust temperature on the neutral gas pressure will be presented. [Preview Abstract] |
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BP8.00099: Non-ideal Effects on Drift Wave Instabilities in Magnetized Inhomogeneous Dusty Plasma Enrique Castro, Julio Puerta, Pablo Martin Low-frequency drift wave instabilities, for perpendicular propagation, on magnetized inhomogeneous dusty plasmas are analyzed, considering a temperature gradient and using a kinetic treatment. The growth rate of the instabilities has been determined by using a four poles two-point quasi-rational approximation for the plasma dispersion function. For large grain radius or high particle densities, the non-ideal effects in the state equation can be considered and described by the Ree and Hoover Pade's type state equation. The contribution of non-ideal effects is analyzed for different plasma parameters such as temperature ratios, particle drift velocities, dust charge and mass values. Fluctuating charge effects are not considered in this work. Our results are compared with those where ideal contributions are considered. [Preview Abstract] |
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BP8.00100: Phase Transition Studies for Conducting Dust in a GEC Reference Cell Jorge Carmona Reyes, Truell Hyde, Lorin Matthews, David George, Mike Cook, Jimmy Schmoke Dust particles immersed in plasma typically acquire a negative charge. The resulting Yukawa interaction between grains in a two-dimensional horizontal layer leads to the formation of disordered or ordered structures depending on whether short or long range ordering dominates, as determined by the ratio of the particle's interparticle potential energy to its average kinetic energy. Various stable crystalline phases have been observed experimentally for dust particles residing within such two-dimensionally extended lattice planes with system dynamics driven in large part by particle charge. Although the charging process for insulating materials has been examined in some detail, conducting materials have not yet been fully investigated. This work experimentally examines the phases and phase transitions for both conductive (gold coated) and non-conductive (melamine formaldehyde) particles. Phase maps for each type of particle are obtained using data from pair correlation functions and voronoi diagrams for dust structures formed over a range of pressures and powers within a standard GEC reference cell. [Preview Abstract] |
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BP8.00101: Dust Particle Probes in a Complex Plasma Angela Douglass, Truell Hyde, Lorin Matthews, Michael Pope, Bernard Smith In dusty plasma experiments within a GEC rf reference cell, the confining potential is generally assumed to be parabolic. The validity of this assumption can be tested in a number of ways. One noninvasive method is to utilize the particles themselves as system probes. In this experiment a cw laser (0.1-5W, 532nm) was used to apply a radiation pressure force on a single MF particle. The particle's trajectory while the laser was on and off was recorded and the confining potential calculated. System power, pressure, DC bias, and cutout size of the plate placed on the powered electrode were varied in order to better understand their effects on the shape of the confining potential. [Preview Abstract] |
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BP8.00102: Measurements within a GEC rf Reference Cell James Creel, Truell Hyde, Lorin Matthews, David GEorge, Jorge Carmona Reyes, Brooks McMaster, Ke Qiao, Mike Cook, Jimmy Schmoke Since its introduction, the GEC rf Reference Cell has provided a baseline for comparison among various experiments performed within the complex (dusty) plasma community. The GEC cell, while providing for data comparison between systems due to its standardized design, does not exist without some variation between cells. In this work, two GEC rf Reference Cells located within the CASPER Hypervelocity Impacts {\&} Dusty Plasmas Lab will be utilized to examine variations in operating parameters. Both standard analysis and Langmuir probe techniques will be employed in an attempt to gain insight into the experimental workings of a standard GEC rf Reference Cell. [Preview Abstract] |
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BP8.00103: Studying the interaction between two dust clouds of differently sized particles in complex plasma Stephen Pickett, Victor Land, Divyaprakash Singh, Diana Bolser, Lorin Matthews, Truell Hyde Dust particles levitate in the sheath of an argon RF discharge in a modified GEC cell. Radial confinement is created by a circular cutout in a top-plate covering the lower electrode. By using differently sized MF particles, two 2D dust crystals are formed, levitated at different heights. By heating or cooling the lower electrode, the crystals are moved relative to each other by thermophoresis. When the crystals are far apart, two vertically aligned layers are visible. Upon approach,~layer-splitting occurs and a \textit{zig-zag} pattern appears. Upon further compression, longer aligned chains form. This behavior is studied using experimental data, as well as by using a combination of a fluid model and N-body code. [Preview Abstract] |
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BP8.00104: Effects of rotating dust grains on the instability of dust-acoustic waves in a complex Lorentzian plasma Myoung-Jae Lee, Kyu-Sun Chung Charged non-spherical dust grains can rotate due to the interaction with surrounding plasmas or oscillating electric field. The rotational effect will modify the conventional plasma wave dispersion relations. In this work, the growing of dust-acoustic wave is investigated in the presence of the rotating dust grains in a complex Lorentzian plasma. Full spectrums of the growth rate of the dust-acoustic wave is obtained and the effect of dust grain rotation is discussed. The Lorentzian plasma effect on the growth rate is also analyzed. The growth rate was found to be enhanced by the rotation of dust grains, but suppressed by the Lorentzian plasmas. [Preview Abstract] |
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BP8.00105: The Dust Accelerator Facility at CCLDAS Anthony Shu, E. Grun, M. Horanyi, S. LeBlanc, T. Munsat, S. Robertson, R. Srama, Z. Sternovsky, E. Thomas, M. Wagner, T. Wingfield The lunar surface is continually bombarded with micrometeorites, primarily within the 0.1-1 $\mu$m and $\le$ 100 km/s range. The impacts of such particles at the lunar surface introduce significant potential hazards to humans and instruments, but also create a scientifically rich complex system. Upon impact into the lunar regolith, cratering and micro-plasma creation can lead to liberation of many families of materials in to the charged lunar dusty plasma. To address the many scientific and technical questions surrounding the hypervelocity micrometeorites at the lunar surface, we describe an accelerator facility based on nuclear accelerator techniques, to be constructed at the Colorado Center for Lunar Dust and Atmospheric Studies at the University of Colorado. Key technical features of the 3 MV electrostatic accelerator include 1) high achievable charge and mass, up to realistic micrometeorite parameters, 2) precise selection of particle size and velocity for high experimental control, 3) high repetition and data acquisition rates. We present technical details of the accelerator and a description of the scientific questions to be addressed at such a facility within the Center. [Preview Abstract] |
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BP8.00106: ABSTRACT WITHDRAWN |
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BP8.00107: Time-resolved Schlieren imaging of electrohydrodynamic interaction induced by a dielectric barrier discharge Sirous Nourgostar, Noah Hershkowitz Commercially available transparent conducting electrodes provide an inexpensive way to investigate different configurations of dielectric barrier discharges (DBD), including so called plasma actuators. Although operation of atmospheric DBDs in a diffuse mode has been reported, they ares limited to very restricted range of operational parameters including the gas flow rate. In addition to diffuse and filamentary modes, there exists a patterned mode which like the diffuse one, is also severely limited by operational constraints. In the experiment, spatial and temporal evolution of both flow and plasma fields in parallel, tilted plate and asymmetric plasma actuator geometries are analyzed by the Schlieren method, and using gated intensified CCD. [Preview Abstract] |
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BP8.00108: Enhancement of electric force on ions by ion - neutral collisions Gennady Makrinich, Amnon Fruchtman The force exerted by a radially-outward plasma and neutral flow from a Radial Plasma Source (RPS) [1], is measured. From the measured force, the electric force exerted on the ion flow by the applied electric field is deduced. This force is found to be larger than the electric force that can be exerted if the ions are collisionless. In addition, the increase of the gas pressure is found to result in an increase of the electric force despite a simultaneous decrease of the deposited electric power. Employing a simple model, we argue that these experimental findings result from the electric force being felt by the ions for a longer time; their residence time in the acceleration region is increased due to their slowing-down collisions with neutrals. In separate experiments, when no magnetic field is present, a plasma ball is formed, attached to the anode. We measure the force exerted by the plasma and neutrals flowing away from the ball. We relate the measured force to the estimated density and temperature of the plasma of the plasma ball. \newline [1] G. Makrinich and A. Fruchtman, Phys. Plasmas {\bf 16}, 043507 (2009). [Preview Abstract] |
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BP8.00109: Controlling synthesis of carbon nanostructures by plasma means in arc discharge Olga Volotskova, Alexey Shashurin, Jon Torrey, Yevgeny Raitses, Michael Keidar Thermal stability of SWNTs at conditions of atmospheric arc is crucial for determination of region of their synthesis in arc and in general for clarification of the thermal regime of SWNT in arc plasmas. We investigated electrical resistance dependence on temperature of mats of SWNTs under variable pressures in helium atmosphere, in the air and in vacuum in high temperature ranges (300-1200K) which closely mimic conditions during the synthesis in arc discharge. Dependence of SWNT resistance on temperature exhibits similar ``V-shape'' behavior for all applied conditions which characterized by two temperatures: Tmin (temperature of the minimum of resistance) and Tcr (temperature of destruction of SWNT bundles). It is found that Tmin and Tcr increased with helium pressure, so that at 500 Torr Tcr was 1100K, while Tmin -900K. This is the temperature that corresponds to buffer region between the arc plasma and helium background in arc discharge. Based on that it can be suggested that region of formation of SWNTs in arc should be close to arc periphery. Our study also demonstrates a strong effect of electric and magnetic fields on properties and growth conditions of SWNTs and other carbon nanostructures such as graphene. These effects are quantified by variety of diagnostics tools: SEM, TEM, AFM - microcopies, TGA, RAMAN and UV-vis-NIR. [Preview Abstract] |
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BP8.00110: Particle-in-cell simulations of electron transport in complex shape dc discharges Alexander V. Khrabrov, Igor D. Kaganovich, Yevgeny Raitses, Vladimir I. Demidov, Dmytro Sydorenko A region of dc discharge near cathode, or negative glow, exists in very nonequlibrium state. Three distinct groups of electrons play different roles in discharge self-organization [1]: 1) fast electrons from cathode produce ionization; 2) very cold trapped electrons make up the plasma density; and 3) intermediate electrons conduct the current. These non-equilibrium conditions provide considerable freedom to choose optimal plasma parameters for many applications by controlling electron energy distribution function (EEDF). The EEDF modification is achieved by making use of additional biased electrodes or cathode voltage forms in afterglow [2]. We have performed particle-in-cell simulations in 1 and 2D geometry to demonstrate possible control of EEDF. This work was supported by AFOSR through STTR contract. \\[4pt] [1] V. I. Kolobov and L. D. Tsendin, Phys. Rev. A \textbf{46,} 7837 (1992). \\[0pt] [2] V.I. Demidov, C.A. DeJoseph, and V. Ya. Simonov, Appl. Phys. Lett. \textbf{91}, 201503 (2007). [Preview Abstract] |
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BP8.00111: Structure and stability of relativistic two-fluid electron-positron jets Jesse Pino, Hui Li, Swadesh Mahajan Magnetized, rotating compact objects can launch jets with large Lorentz factors. Using a temperature-transformed magnetofluid coupling formalism, we derive the relativistic two-fluid equations in vortex form. For the case of homentropic pair plasmas, we find analytic self-organized solutions in cylindrical geometry. Various temperature, density and Lorentz factor profiles are considered. We compare these solutions to previous jet models, focusing on magnetic tower type jets with an external return current. Finally, we investigate the stability of the equilibria to MHD and two-fluid perturbations. [Preview Abstract] |
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