Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session NP12: Poster Session V (MHD, Heating, Current Drive, HBT-EP; Magnetic Reconnection; Astrophysical and Space Plasmas) |
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Room: Exhibit Hall A |
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NP12.00001: MHD, HEATING, CURRENT DRIVE, HBT-EP |
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NP12.00002: Measurement of Phase Space Structure of Fast Ions Interacting with Alfven Eigenmodes Kenichi Nagaoka, Masaki Osakabe, Mitsutaka Isobe, Kunihiro Ogawa, Yasuhiro Suzuki, Shinji Kobayashi, Satoshi Yamamoto, Yoshizumi Miyoshi, Yuto Katoh, Jose M. Fontdecaba Experimentally observed Alfven eigenmodes (AEs) shows nonlinear behaviors such as intermittency, fast sweep in frequency and so on. In order to understand such nonlinear behaviors of AEs, it is widely recognized that the phase space structure have to be taken into account. However, there are few direct measurements of phase space structure in experiments so far. Here, we propose to apply the wave-particle interaction analyzer (WPIA) technique being developed for magnetosphere plasma physics (ERG project) to magnetically confinement fusion experiments. In the meeting, we present a high speed pulse analyzer system for WPIA using the field programmable gate array (FPGA) module and discuss the phase space structures observed in the LHD experiment. [Preview Abstract] |
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NP12.00003: ABSTRACT WITHDRAWN |
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NP12.00004: Non-twist map bifurcation of drift-lines and drift-island formation in saturated 3D MHD equilibria David Pfefferle, Wilfred A. Cooper, Jonathan P. Graves Based on non-canonical perturbation theory, guiding-centre drift equations are identified as perturbed magnetic field-line equations. The topology of passing-particle orbits, called drift-lines, is completely determined by the magnetic configuration. In axisymmetric tokamak fields, drift-lines lie on shifted flux-surfaces, called drift-surfaces. Field-lines and drift-lines are subject to island structures at rational surfaces only when a non-axisymmetric component is added. The picture is different in the case of 3D saturated MHD equilibrium like the helical core associated with a non-resonant internal kink mode. In assuming nested flux-surfaces, these bifurcated states, expected for a reversed q-profile with q$_{min}$ close yet above unity and conveniently obtained in VMEC, feature integrable field-lines. The helical drift-lines however become resonant with the axisymmetric component in the region of q$_{min}$ and spontaneously generate drift-islands. Due to the locally reversed sheared q-profile, the drift-island structure follows the bifurcation/reconnection mechanism of non-twist maps. This result provides a theoretical interpretation of NBI fast ion helical hot-spots in Long-Lived Modes as well as snake-like impurity density accumulation in internal MHD activity. [Preview Abstract] |
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NP12.00005: Analysis of Island Formation Due to RMPs in D3D Plasmas Using SIESTA Steven Hirshman, Morgan Shafer, Sudip Seal, John Canik By varying the initial helical perturbation amplitude of Resonant Magnetic Perturbations (RMPs) applied to a Doublet III-D (DIII-D) plasma, a variety of meta-stable equilibrium are scanned using the SIESTA MHD equilibrium code. It is found that increasing the perturbation strength at the dominant m$=$2 resonant surface leads to lower MHD energies and significant increases in the equilibrium island widths at the m$=$2 (and sidebands) surfaces. Island overlap eventually leads to stochastic magnetic fields which correlate well with the experimentally inferred field line structure. The magnitude and spatial phase (around associated rational surfaces) of resonant (shielding) components of the parallel current is shown to be correlated with the magnetic island topology. [Preview Abstract] |
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NP12.00006: q-solver equilibrium model with fast ion orbit width, velocity anisotropy and toroidal flow effects Nikolai Gorelenkov, Steven Jardin We present a novel formulation for the plasma equilibrium problem using the q-solver framework together with the pressure coupling scheme for energetic particle (EP) contribution. The employed formulation accounts for the EP pressure anisotropy which is based on the moments of the velocity distribution function representation incorporating the finite orbit width (FOW) effects. The system of equations includes the toroidal plasma flow. These effects are important in applications for recently upgraded plasmas of NSTX-U and DIII-D where additional NBIs are installed. Strongly anisotropic beam ions accompanied by plasma rotation have to be addressed in various applications involving for example the stability of Alfvenic and internal kink modes. The anisotropy and rotational effects could be treated separately or together depending on applications. Fast ion anisotropic pressure tensor is computed using the set of basis functions. In particular we show that in the limit of zero orbit width any distribution function can satisfy the solvability requirements for Grad-Shafranov equation, which follows from the force balance along the magnetic field lines. [Preview Abstract] |
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NP12.00007: Magnetic Islands in 3D Equilibria Allan Reiman We discuss some aspects of magnetic island physics in finite $\beta $ 3D equilibria. [Preview Abstract] |
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NP12.00008: Revised numerical wrapper for PIES code Daniel Raburn, Allan Reiman, Donald Monticello A revised external numerical wrapper has been developed for the Princeton Iterative Equilibrium Solver (PIES code), which is capable of calculating 3D MHD equilibria with islands. The numerical wrapper has been demonstrated to greatly improve the rate of convergence in numerous cases corresponding to equilibria in the TFTR device where magnetic islands are present. The numerical wrapper makes use of a Jacobian-free Newton-Krylov solver along with adaptive preconditioning and a sophisticated subspace-restricted Levenberg-Marquardt backtracking algorithm. The details of the numerical wrapper and several sample results are presented. [Preview Abstract] |
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NP12.00009: Effects due to nonlinear modification of driven current on tearing mode stabilization Ge Dong, Allan Reiman, Nathaniel Fisch Neoclassical tearing modes (NTMs) can be destabilized by a helical perturbation in the boostrap current, and can result in large magnetic islands which are detrimental to confinement in toroidal plasma devices. NTM stability properties and dynamics can be strongly affected by current drive in various scenarios. The modified Rutherford equation is generally used to calculate the contributions from the current drive, without considering the self- consistent change in the driven current associated with the nonlinear effects. In this study, we evaluated the importance of such nonlinear effects as the effect of the change in Te on the current drive efficiency, and the nonlinear interaction of the current drive and the electric field. [Preview Abstract] |
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NP12.00010: Computation of Perturbed Equilibria with Resistive DCON Z.R. Wang, J.-K. Park, A.H. Glasser, Y.Q. Liu, J.E. Menard The IPEC code is widely used to compute ideal perturbed equilibria in the presence of small external 3D magnetic fields, where the plasma response is modeled by ideal MHD and computed with the ideal DCON code. It is also important to understand the resistive plasma response when 3D fields open magnetic islands near rational surfaces. The Resistive DCON (RDCON) code, which has been successfully developed to solve for resistive MHD instabilities, using the method of matched asymptotic expansions in toroidal geometry, has now been incorporated into IPEC to model the resistive plasma response. The RDCON code, with greatly improved numerical stability, solves the Euler-Lagrange equations by including both large and small power series solutions in the outer ideal region. The resistive response is obtained by matching the outer region solutions with the inner region solutions from GGJ model [1] and the external magnetic perturbations through the matching dispersion relation. Details of resistive perturbed equilibrium modeling will be presented. The interaction between the perturbed equilibrium and the effects of pressure flattening due the opened island at the rational surface will also be studied. This work is supported by U.S. DOE DE-AC02-09CH11466. \\[4pt] [1] A.H. Glasser, J.M. Greene and J.L. Johnson, Phys. Fluids 18, 875(1975) [Preview Abstract] |
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NP12.00011: Error field penetration and beta limits in two fluid toroidal plasmas D.P. Brennan, A.J. Cole, J.M. Finn This study addresses error field penetration and beta limits in low flow, low error field, two fluid regimes relevant to ITER experimental scenarios as well as current ITER-like discharges. The focus is on the Hall, Semi-Collisional, Resistive Inertial and Inertial regimes, with equilibrium stability, toroidal rotation and boundary error field otherwise varied within the regimes of the experiments. The penetration thresholds and stability limits in each of these regimes are calculated using asymptotic matching methods for the resistive modes, including toroidal effects in the resonant layers. Accurate toroidal analyses are compared to reduced models for intuitive understanding of the results. The penetrated state has recently been shown at low error field and low viscous torque to lock to a finite frequency state [Finn et al. arXiv:1507.04012]. We present the dependence of this locked state in these two fluid regimes. The dependence of the beta limits, with and without a resistive plasma and a resistive wall, and its impact on error field penetration thresholds, are also examined. The approach to the particular threshold represented in this model reduces penetration thresholds and enhances plasma response. [Preview Abstract] |
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NP12.00012: A Novel Explanation of the Greenwald Density Limit--Thermal-Resistive Tearing Mode Qian Teng, Dylan Brennan, Luis Delgado-Aparicio, David Gates, Josh Swerdlow, Roscoe White R.B. White et. al. showed that the asymmetry of a magnetic island created by a tearing mode is crucial to understand its evolution. An island is thermally isolated, the internal temperature given by the balance of radiation loss and Ohmic heating. When radiation loss from the island dominates, the cooling of the island, coupled to the asymmetry, can cause rapid growth, leading to a potential mechanism to explain the Greenwald density limit. In this work we successfully reproduced the density limit with experiment-relevant parameters using this model. Using simple equilibrium profiles and assuming internal inductance evolution with density, we simulated the equilibrium evolution as a function of density. The modeling of internal inductance is motivated by D.A. Gates and L. Delgado-Aparicio's finding that the density limit is accompanied with internal inductance increasing. Given reasonable numbers for geometric size, electric and magnetic field, we calculated power balance inside island. The simulation showed local power balance criterion agrees with the density limit within 2\%. The very sharp limit is determined by the strong dependence of radiative loss on density. Next work is to use a toroidal model and experimentally obtained equilibria to compare with experiments directly. [Preview Abstract] |
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NP12.00013: Implementation of Linear Neoclassical Inner Region Model in the DCON Package A.H. Glasser, Z.R. Wang, J.K. Park We report on continuing development of the DCON package of codes for determining the stability of singular MHD modes in axisymmetric toroidal plasma, using the method of matched asymptotic expansions. We have previously reported on the development a singular Galerkin method for determining outer ideal MHD region matching data in DCON, coupled to the VACUUM code for free-boundary modes. The resistive MHD inner region of Glasser, Greene, and Johnson is implemented in the DELTAC code. Inner and outer solutions are combined with the MATCH code, which computes global eigenvalues and eigenfunctions. Verification against the MARS straight-through code indicates excellent agreement on eigenvalues and eigenfunctions, but with the DCON package running 100 times faster because of the asymptotic matching method. We now report improvement of the physics of the inner region by inclusion of the linear neoclassical effects of Connor, Hastie, and Helander[Plasma Phys. Control. Fusion 51, 0150091 (2009)], appropriate to the more realistic banana regime of low collisionality. Methods and results will be described. [Preview Abstract] |
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NP12.00014: Numerical Simulations of Hot Vertical Displacement Events K.J. Bunkers, C.R. Sovinec Loss of vertical positioning control in tokamaks leads to instability where hot confined plasma rests against the chamber wall. Resistive-MHD modeling with the NIMROD code [Sovinec, et al., JCP 195, 335] is applied to model these events. After divertor-coil current is perturbed, resistive diffusion through the non-ideal wall sets the timescale as the simulated tokamak evolves from a diverted equilibrium to a limited configuration. Results show that plasma outflow along opening magnetic surfaces, just outside the confinement zone, approaches the local ion-acoustic speed. The projection of the plasma flow velocity into the surface-normal direction ($\mathbf n\cdot\mathbf V$) near the surface exceeds the local $\mathbf E\times\mathbf B$ drift speed; near surfaces $\mathbf n\times\mathbf E$ is approximately the same as $\mathbf n\times\mathbf E_{\mathrm{wall}}$ in the nearly steady conditions. The safety factor of flux surfaces that remain intact is approximately constant over the evolution time, which is much shorter than the plasma resistive diffusion time. Assessment of external-kink stability and initial findings from 3D nonlinear computations are presented. [Preview Abstract] |
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NP12.00015: The effect of strong radial variation of the diamagnetic frequency on two-fluid stabilization of edge localized MHD instabilities Tyler Cote, Chris Hegna, Ping Zhu The conventional theory for two-fluid stabilization of ballooning instabilities in tokamaks assumes a constant diamagnetic frequency throughout the radial structure of the ballooning mode. However, this approximation is not valid in the pedestal region, due to large density and temperature gradients being present. In this work, we apply WKB theory to solve for the radial structure of the ballooning eigenmode\footnote{R. L. Dewar et al, Nucl. Fusion, 493 (1981)} in the presence of a radially varying diamagnetic frequency\footnote{R. J. Hastie et al, Phys. Plasmas, 4561 (2000)} for MHD equilibria with edge pedestal regions. A semi-classical ray tracing code is used to quantify the stabilizing influence of two-fluid physics, with and without variation in the diamagnetic frequency. Linear ideal NIMROD simulations are utilized to provide a benchmark for the ray tracing code, as well as aide studies into the validity of the WKB theory for intermediate toroidal mode number. Preliminary results for the varied diamagnetic frequency model indicate the variation in the diamagnetic frequency significantly decreases the stabilizing effect of the two-fluid physics in comparison to that of the constant diamagnetic frequency model. [Preview Abstract] |
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NP12.00016: Two-Fluid Calculations of the 1/1 Internal Kink E C Howell, C R Sovinec The nonlinear evolution of the 1/1 internal kink mode is usually responsible for sawtooth activity in tokamaks. Energetic particles and two-fluid drifts in large experiments can temporarily stabilize the kink mode. This leads to less frequent but larger giant sawtooth events. Comprehensive simulation of the giant sawtooth requires modeling and verifying both effects. Two-fluid linear calculations of the 1/1 kink are performed using the NIMROD code [Sovinec and King, JCP 229, 5803]. Cylindrical screw-pinch equilibria are used to verify the two-fluid kink calculations with respect to analytic dispersion relation [Zakahrov and Rodgers, PFB 4, 3285]. Comparisons are also made to the analytic dispersion relation in [Ara et al., AoP 112, 443], which is only valid in the low pressure limit. The stability of the two-fluid internal kink is also studied in toroidal geometry using both model equilibria and equilibria based on experimental reconstructions. [Preview Abstract] |
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NP12.00017: Generation of rotational flows in toroidally confined visco-resistive magnetohydrodynamics Jorge Morales, Wouter Bos, Kai Schneider, David Montgomery We investigate by numerical simulation the generation of rotational flows in a toroid confining a conducting magnetofluid. A current is driven by the application of externally supported electric and magnetic fields. We show how the properties and intensity of the rotations are regulated by dimensionless numbers (Lundquist and viscous Lundquist) that contain the resistivity and viscosity of the magnetofluid. At the magnetohydrodynamic level (uniform mass density and incompressible magnetofluids), rotational flows appear in toroidal, driven MHD. The evolution of these flows with the transport coefficients, geometry, and safety factor are described. Two different toroidal geometries are considered, one with an up-down symmetric and the other with an asymmetric cross section. We show that there exists a fundamental difference between both studied cases: the volume-averaged angular momentum is zero for the symmetric case, while for the asymmetric cross section a finite volume-averaged angular momentum appears. We observe a breaking in the up-down symmetry of the flow and a toroidal preferred direction emerges. Ref.: J. Morales, W.J.T. Bos, K. Schneider and D.C. Montgomery. Magnetohydrodynamically generated velocities in confined plasma. Phys. Plasmas, 22, 042515, 2015. [Preview Abstract] |
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NP12.00018: The effect of sheared toroidal flow on nonlinear pressure driven magnetic islands in tokamaks C. C. Hegna Empirically it is known that sheared toroidal rotation tends to have a stabilizing effect on neoclassical tearing modes. In this work, an analytic theory of nonlinear pressure driven magnetic island evolution is presented that accounts for sheared toroidal rotation at the rational surface. The derivation relies upon an asymptotic analysis of MHD equilibrium equations with plasma flow in the vicinity of a magnetic island assuming a small island approximation. Particular attention is paid to helical Pfirsch-Schluter current contributions to island evolution and the asymptotic matching indices. Both of these quantities are modified by sheared flow with the sheared flow enhancing the stabilizing resistive interchange contribution to the modified Rutherford theory in a tokamak. [Preview Abstract] |
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NP12.00019: NIMROD studies of RWM stability and non-linear evolution for NSTX equilibria A.L. Becerra, C.C. Hegna, C.R. Sovinec, S.E. Kruger, J.R. King, S.A. Sabbagh We make use of the generalized thin resistive wall boundary condition recently implemented in NIMROD to study the linear and nonlinear RWM stability properties of a series of reconstructed NSTX equilibria. The boundary condition operates by matching the magnetic field inside the computational domain with external fields found using the Green's function method in the GRIN vacuum-field solver at the wall, and is valid for toroidal geometries with poloidal asymmetry as well as for cylindrical geometries. Time series of NSTX equilibrium reconstructions from two shots whose normalized betas span the no-wall limit are studied. The critical beta for RWM onset found by NIMROD is compared with the stability limit predicted by ideal MHD code DCON. Scans with varying wall parameters are also performed to demonstrate the approximately linear relationship between growth rate and wall resistivity, and to test the performance limits of the boundary condition. The stability of these equilibria for n\textgreater 1 is also examined, with both linear and non-linear runs in preparation for examining the non-linear effects due to toroidal rotation. [Preview Abstract] |
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NP12.00020: Forced Magnetic Reconnection In A Tokamak Plasma J.D. Callen, C.C. Hegna The theory of forced magnetic field reconnection induced by an externally imposed resonant magnetic perturbation usually uses a sheared slab or cylindrical magnetic field model and often focuses on the potential time-asymptotic induced magnetic island state. However, tokamak plasmas have significant magnetic geometry and dynamical plasma toroidal rotation screening effects. Also, finite ion Larmor radius (FLR) and banana width (FBW) effects can damp and thus limit the width of a nascent magnetic island. A theory that is more applicable for tokamak plasmas is being developed. This new model of the dynamics of forced magnetic reconnection considers a single helicity magnetic perturbation in the tokamak magnetic field geometry [1], uses a kinetically-derived collisional parallel electron flow response, and employs a comprehensive dynamical equation for the plasma toroidal rotation frequency. It is being used to explore the dynamics of bifurcation into a magnetically reconnected state in the thin singular layer around the rational surface, evolution into a generalized Rutherford regime where the island width exceeds the singular layer width, and assess the island width limiting effects of FLR and FBW polarization currents.\\[4pt] [1] C.C. Hegna and J.D. Callen, Phys. Plasmas 1, 2308 (1994). [Preview Abstract] |
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NP12.00021: Fully Parallel MHD Stability Analysis Tool Vladimir Svidzinski, Sergei Galkin, Jin-Soo Kim, Yueqiang Liu Progress on full parallelization of the plasma stability code MARS will be reported. MARS calculates eigenmodes in 2D axisymmetric toroidal equilibria in MHD-kinetic plasma models. It is a powerful tool for studying MHD and MHD-kinetic instabilities and it is widely used by fusion community. Parallel version of MARS is intended for simulations on local parallel clusters. It will be an efficient tool for simulation of MHD instabilities with low, intermediate and high toroidal mode numbers within both fluid and kinetic plasma models, already implemented in MARS. Parallelization of the code includes parallelization of the construction of the matrix for the eigenvalue problem and parallelization of the inverse iterations algorithm, implemented in MARS for the solution of the formulated eigenvalue problem. Construction of the matrix is parallelized by distributing the load among processors assigned to different magnetic surfaces. Parallelization of the solution of the eigenvalue problem is made by repeating steps of the present MARS algorithm using parallel libraries and procedures. Results of MARS parallelization and of the development of a new fix boundary equilibrium code adapted for MARS input will be reported. [Preview Abstract] |
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NP12.00022: Progress on the DPASS project Sergei A. Galkin, I.N. Bogatu, V.A. Svidzinski A novel project to develop Disruption Prediction And Simulation Suite (DPASS) of comprehensive computational tools to predict, model, and analyze disruption events in tokamaks has been recently started at FAR-TECH Inc. DPASS will eventually address the following aspects of the disruption problem: MHD, plasma edge dynamics, plasma-wall interaction, generation and losses of runaway electrons. DPASS uses the 3-D Disruption Simulation Code (DSC-3D) as a core tool and will have a modular structure. DSC is a one fluid non-linear, time-dependent 3D MHD code to simulate dynamics of tokamak plasma surrounded by pure vacuum B-field in the real geometry of a conducting tokamak vessel. DSC utilizes the adaptive meshless technique with adaptation to the moving plasma boundary, with accurate magnetic flux conservation and resolution of the plasma surface current. DSC has also an option to neglect the plasma inertia to eliminate fast magnetosonic scale. This option can be turned on/off as needed. During Phase I of the project, two modules will be developed: the computational module for modeling the massive gas injection and main plasma respond; and the module for nanoparticle plasma jet injection as an innovative disruption mitigation scheme. We will report on this development progress. [Preview Abstract] |
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NP12.00023: The locking and unlocking thresholds for tearing modes in a cylindrical tokamak Wen-long Huang, Ping Zhu The locking and unlocking thresholds for tearing modes are in general different. In this work, the physics origin for this difference is illustrated from theory analysis, and a numerical procedure is developed to find both unlocking and locking thresholds. In particular, the unlocking threshold can be determined from the lowest amplitude of the RMP (resonant magnetic perturbation) allowed for the locking mode solution. Above the unlocking threshold, a different threshold exists due to the presence of two different regimes for the locked mode states. In the first regime, the locked mode state may or may not be reachable depending on the initial conditions of the tearing mode. In the second regime, the locked mode state can always be reached regardless of the initial conditions of the tearing mode. The lowest RMP amplitude for the second regime is determined to be the mode locking threshold. Numerical examples will be presented and discussed. [Preview Abstract] |
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NP12.00024: Plasma Response to Resonant Magnetic Perturbation in a Tokamak Ping Zhu, Xing-ting Yan, Wen-long Huang Resonant magnetic perturbation (RMP) has been increasingly adopted as a practical tool to suppress or mitigate the edge localized modes in tokamak experiments. To understand the effects of RMPs on the properties of tokamak edge plasma, we calculate and study the plasma response to RMP in a circular-shaped limiter tokamak equilibrium, using both a reduced MHD model in theory and a full MHD model with anisotropic heat transport implemented in the NIMROD code. A low-$n$ RMP with single helicity is imposed as the boundary condition at the tokamak wall location, where $n$ is the toroidal mode number. Plasma responses to RMPs are obtained through solving and analyzing the steady states of the linear and nonlinear evolution of the system subject to the RMP boundary condition. The effects of toroidal rotation, plasma $\beta$, and nonlinearity on the amplitude and structure of the plasma response are examined and discussed. [Preview Abstract] |
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NP12.00025: Neoclassical Toroidal Viscosity Induced by Resonant Magnetic Perturbation in Tokamak Edge Plasma Xing-ting Yan, Ping Zhu, You-wen Sun In recent experiments, non-axisymmetric magnetic field perturbations due to external perturbations or plasma instabilities have been observed to strongly affect plasma rotation through neoclassical toroidal viscosity (NTV). In this work, we have calculated the NTV torque induced by resonant magnetic perturbation (RMP) in the edge plasma of a circular-shaped limiter tokamak, using the coupling of NIMROD and NTVTOK codes newly developed for this study. The resulting NTV torque is found to be sensitive to plasma $\beta$. In particular, when $\beta$ is increased by two orders of magnitude, NTV torque is almost increased by ten orders of magnitude. The amplitude of NTV torque also depends on the toroidal mode number $n$ of the plasma response to RMP. For a same amplitude of plasma response, the ion contribution to the resulting NTV torque increases with $n$, whereas the electron contribution decreases with $n$. This suggests the significance of nonlinear toroidal coupling in the generation of NTV torque, even when the RMP has only a single toroidal mode or helicity. [Preview Abstract] |
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NP12.00026: Kinetic MHD Simulations of Shear Alfv\'{e}n Waves Driven by Fast Particles in a Tokamak Zhao-Qing Hu, Ping Zhu, Charlson C. Kim, Zheng-Xiong Wang Shear Alfv\'{e}n waves (SAWs) such as the toroidal Alfv\'{e}n eigenmodes (TAEs) can be excited by fast particles in burning plasma tokamak experiments such as ITER. In this work, we conduct kinetic-MHD simulations of TAEs driven by fast particles in a tokamak geometry using the NIMROD code. For simplicity, a $q$-profile with a single gap in the SAW spectrum is adopted. A global SAW is observed in simulations. Its frequency is located in the gap, which is in good agreement with the ideal MHD frequency of the corresponding TAE. The SAW mode structures in velocity and magnetic fields resemble the characteristic mode pattern of a TAE. Further benchmarks with theory or other codes on the TAE scalings will be presented. [Preview Abstract] |
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NP12.00027: Role of the equilibrium and perturbative core current density in sawtoothing and non-sawtoothing discharges in KSTAR Yoonbum Nam, Hyeon Park, Minjun Choi, Gyuenghyuen Choe, Woochang Lee, Gunsu Yun, Stephen Jardin New study of multimode physics in the core of the KSTAR plasma provides an opportunity to address the long standing issue of the central current density during sawooth crash. The recent experiment [1] on excitation of the m/n$=$3/3 mode with a current blip induced by ECH and successive evolution to the m/n$=$2/2 and m/n$=$1/1 mode during one sawtooth period in the core of the sawtoothing discharge in KSTAR suggests that the central safety factor (q0) may have to change from below $\sim$ 1 (before crash) to slightly above $\sim$ 1(after crash). This interpretation is consistent with the q profile condition for MHD simulation necessary for the growth of the higher order modes which require q0 slightly above $\sim$ 1 until the 1/1 mode becomes dominant [2]. Experimental observation of a long lived higher order mode in non-sawtoothing discharge (presumably q0\textgreater 1) is consistent with the fact that the q0 has to be below $\sim$ 1 to support the growth of the m/n$=$1/1 mode.\\[4pt] [1] G. H. Choe et al, Nuclear Fusion 55 013015 (2015)\\[0pt] [2] A. Bierwage et al, Nuclear Fusion 55 013016 (2015) [Preview Abstract] |
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NP12.00028: PB3D: a new code for 3D ideal linear peeling-ballooning stability Toon Weyens, Raul Sanchez, Guido Huijsmans, Luis Garcia, Alberto Loarte Ideal peeling-ballooning modes are important for magnetic nuclear fusion devices, but most of the theoretical and computational work that has been performed over the years to gain insight into their inner workings and consequences has been limited to axisymmetric (so-called 2D) cases which limits the range of applicability of the results. For example, the proposed use of perturbation coils in tokamaks to destabilize ELMS before they have a chance to grow dangerous has an inherently non-axisymmetric (3D) nature. Furthermore, many devices, such as stellarators, are intrinsically not axisymmetric. In this contribution we present a new code, PB3D (Peeling-Ballooning in 3D), that implements the equations of a previously developed theory [1] that overcomes these stringent limitations by making no use of an axisymmetric approximation. The first benchmarking results of PB3D, dealing with the investigation of the stability properties of magnetic equilibria with nested flux surfaces obtained numerically from multiple equilibrium codes, such as HELENA and VMEC, are presented here. The results will be compared with those of various axisymmetric stability codes.\\[4pt] [1] WEYENS, T., SANCHEZ, R., GARCIA, L., LOARTE, A., AND HUIJSMANS, G. T. A., Phys. Plasmas 21, 042507 (2014). [Preview Abstract] |
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NP12.00029: Application of Smoothed Particle MHD (SPMHD) techniques to the simulation of magnetically confined plasma dynamics Luis Vela-Vela, Raul Sanchez, J. Miguel Reynolds-Barredo Magnetically confined plasmas relevant for fusion scenarios are, to first approximation, well described by ideal and resistive MHD. This includes the description of their equilibrium and stability properties, as well as their medium-to-long term nonlinear evolution under external forcing. In many of these cases, one needs to deal with magnetic topologies that include magnetic islands, stochastic regions or that require the consideration of free-moving boundaries. The present work is part of an on-going effort to develop of a numerical code capable of dealing with these situations by taking advantage of the SPMHD formalism that, although widely used in astrophysical plasmas, is not widespread within the fusion community. SPMHD is a particle (i.e., Lagrangian) method particularly well-suited to deal with complicated boundaries while retaining great parallelization benefits. Here, we will report on the adaptation of the SPMHD equations to the case of magnetically confined plasmas, several benchmarking tests typical for MHD codes, and some preliminary results obtained for more elaborate scenarios. Our results suggest that our new code (EVA) can be very advantageous to deal with problems of current interest for the fusion community, including tokamaks and stellarators. [Preview Abstract] |
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NP12.00030: Preliminary Design of the Alfv\'en Antennas on the J-TEXT Tokamak Jiyang He, Qiming Hu, Bo Rao, Linzi Liu, Ge Zhuang Research on Alfv\'en waves and Alfv\'en eigenmodes(AEs) is of importance in tokamak plasma physics, such as investigation of interaction between energetic particles and AEs, turbulence and anomalous transport due to AEs, and so on. In order to study the Alfv\'en eigenmode excitation, damping features and the interaction between AEs and plasma transport, an Alfv\'en antennas system is designed for the J-TEXT tokamak. The system can generate high frequency magnetic field aiming to excite the AEs, especially toroidal Alfv\'en eigenmodes (TAE). For a typical J-TEXT plasma (BT$=$ 1-2.2 T,ne $=$ 3-6$\times$10$^{19}$m$^{-3}$), the computed gaps in the Alfv\'en continua range from 300 to 500 kHz, with respect to the Alfv\'en waves dispersion relation [1]. Three pairs of antennas are designed at different toroidal angles respectively on the low field side. Each pair consisting of two coils installed with angles of $+$/- 45$^{\circ}$ off the mid-plane along the poloidal direction. With this system, magnetic field components of mode number m$=$1-10, n$=$1-20 can be produced. The calculations show that the magnetic field in the LCFS can reach $\sim$ 10Gs totally while about $\sim$ 0.5Gs for each mode number, with the coil current of 20A. \\[4pt] [1] Zonca F. Chen L. 1996 Physics of Plasmas 3 323. [Preview Abstract] |
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NP12.00031: Relation between n=0 vertical MHD instabilities and neighboring Grad-Shafranov equilibria Antoine Cerfon, Jungpyo Lee, Jeffrey Freidberg In tokamaks, the n = 0 vertical instability limits the maximum elongation that can be achieved, even with feedback control. Since plasma beta and energy confinement time depend strongly on the elongation, the axisymmetric vertical instability plays a crucial role in overall tokamak performance. We derive an intuitive form of $\delta W_{F}$, the plasma contribution to the change in total potential energy associated with the n = 0 instability. Using this expression, we show that the differential equation for the perturbed flux $\psi$ obtained by setting $\delta W_{F}=0$ is identical to the neighboring equilibrium equation obtained by letting $\Psi = \Psi+\psi$ in the Grad-Shafranov equation, and setting the first order contribution to zero. Here $\Psi$ and $\psi$ are the equilibrium and perturbed flux. In other words, n = 0 modes at marginal stability can be viewed as neighboring equilibria of the Grad-Shafranov equation.This relationship was long believed to be true, but to the authors' knowledge, never explicitly derived in the literature. [Preview Abstract] |
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NP12.00032: Teaching an Old Dog an Old Trick: FREE-FIX and Free-Boundary Axisymmetric MHD Equilibrium Luca Guazzotto A common task in plasma physics research is the calculation of an axisymmetric equilibrium for tokamak modeling. The main unknown of the problem is the magnetic poloidal flux $\psi$. The easiest approach is to assign the shape of the plasma and only solve the equilibrium problem in the plasma / closed-field-lines region (the ``fixed-boundary approach''). Often, one may also need the vacuum fields, i.e. the equilibrium in the open-field-lines region, requiring either coil currents or $\psi$ on some closed curve outside the plasma to be assigned (the ``free-boundary approach''). Going from one approach to the other is a textbook problem [1], involving the calculation of Green's functions and surface integrals in the plasma. However, no tools are readily available to perform this task. Here we present a code (FREE-FIX) to compute a boundary condition for a free-boundary equilibrium given only the corresponding fixed-boundary equilibrium. An improvement to the standard solution method, allowing for much faster calculations, is presented. Applications are discussed. \\[4pt] [1] S.~Jardin, {\em Computational Methods in Plasma Physics}, Taylor \& Francis, Boca Raton, FL, 2010. [Preview Abstract] |
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NP12.00033: MHD stability of ITER H-mode confinement with pedestal bootstrap current effects taken into account L.J. Zheng, M.T. Kotschenreuther, P. Valanju, S.M. Mahajan, D. Hatch, X. Liu We have shown that the bootstrap current can have significant effects both on tokamak equilibrium and stability (Nucl. Fusion {\bf 53}, 063009 (2013)). For ITER H-mode discharges pedestal density is low and consequently bootstrap current is large. We reconstruct numerically ITER equilibria with bootstrap current taken into account. Especially, we have considered a more realistic scenario in which density and temperature profiles can be different. The direct consequence of bootstrap current effects on equilibrium is the modification of local safety factor profile at pedestal. This results in a dramatic change of MHD mode behavior. The stability of ITER numerical equilibria is investigated with AEGIS code. Both low-n and peeling-ballooning modes are investigated. Note that pressure gradient at pedestal is steep. High resolution computation is needed. Since AEGIS code is an adaptive code, it can well handle this problem. Also, the analytical continuation technique based on the Cauchy-Riemann condition of dispersion relation is applied, so that the marginal stability conditions can be determined. Both numerical scheme and results will be presented. The effects of different density and temperature profiles on ITER H-mode discharges will be discussed. [Preview Abstract] |
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NP12.00034: Braking of Tearing Mode Rotation by Ferromagnetic Conducting Walls in Tokamaks Richard Fitzpatrick An in-depth investigation of the braking of tearing mode rotation in tokamak plasmas via eddy currents induced in external ferromagnetic conducting structures is performed. In general, there is a ``forbidden band'' of tearing mode rotation frequencies that separates a branch of high-frequency solutions from a branch of low-frequency solutions. When a high-frequency solution crosses the upper boundary of the forbidden band there is a bifurcation to a low-frequency solution, and vice versa. The bifurcation thresholds predicted by simple torque-balance theory (which takes into account the electromagnetic braking torque acting on the plasma, as well as the plasma viscous restoring torque, but neglects plasma inertia) are found to be essentially the same as those predicted by more complicated time-dependent mode braking theory (which takes inertia into account). Significant ferromagnetism causes otherwise electromagnetically thin conducting structures to become electromagnetically thick, and also markedly decreases the critical tearing mode amplitude above which the mode ``locks'' to the conducting structures (i.e., the high-frequency to low-frequency bifurcation is triggered). This research was funded by the U.S. Department of Energy under contract DE-FG02-04ER-54742. [Preview Abstract] |
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NP12.00035: Continuum absorption in the vicinity of the toroidicity-induced Alfv\'en gap Meng Li, Boris Breizman, Linjin Zheng This work examines the resonant dissipative response of the Alfv\'en continuum to an oscillating driving current when the driving frequency is slightly outside the edges of the toroidicity-induced spectral gap. The problem is motivated by the need to the describe the continuum absorption in the frequency chirping events for energetic-particle-driven modes. A key element of this problem is the negative interference of the two closely spaced continuum crossing points. We explain why the continuum absorption can have very different features. This difference is closely related to the Toroidicity-induced Alfv\'en Eigenmode(TAE) theory that the eigenmode frequency can be arbitrarily close to the upper edge of the gap, whereas the lower edge of the gap is always a finite distance away from the closest eigenmode. [Preview Abstract] |
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NP12.00036: Energetic Ion Interactions with Tearing Mode Stability Michael Halfmoon, Dylan Brennan This study focuses on the interactions between energetic ions and pressure-driven, slow growing tearing modes in high beta tokamaks. Previous studies have shown that energetic ions interact with and affect the tearing mode stability, in a mechanism similar to those of ideal MHD instabilities and resistive wall modes. The 2/1 tearing mode is found to be damped or stabilized in the presence of energetic ions, with the most significant effects on the slow-growing resistive mode. To gain an understanding of the underlying physics of these effects, we have investigated a combination of reduced analytics and numerical simulations. In the reduced model, a high aspect ratio, step function equilibrium is investigated, where the dynamics of high-energy ions interacting with the tearing mode is implemented through integration over the pressure step. In the simulations, a series of experimentally relevant D-shaped equilibria with fixed monotonic safety factor and varying peaked pressure profiles is analyzed using the $\delta $f hybrid kinetic-mhd code in NIMROD. Results show a damping effect from the ions that is consistent between the reduced model and the simulations. The stabilizing effect is mainly due to trapped particle resonance, causing the tearing mode to have a finite frequency. [Preview Abstract] |
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NP12.00037: Measuring Properties of Magnetic Reconnection in Nonlinear Resistive and Two-Fluid Toroidal Simulations of Sawteeth Matthew Beidler, Paul Cassak, Stephen Jardin, Nathaniel Ferraro The sawtooth crash in tokamaks limits the core temperature, harms confinement, and seeds disruptions. A predictive capability of its ramifications has been elusive. Extended-MHD physics is needed to properly analyze the magnetic reconnection that occurs during the crash phase, but it has only recently been integrated into codes using a toroidal geometry. In this study, we employ the three-dimensional toroidal, extended-MHD code M3D-C1 to study reconnection during the sawtooth crash. We study the nonlinear evolution of a test equilibrium in a non-reduced field representation for resistive-MHD and the two-fluid model. We find that the toroidal mode growth rates for the two-fluid reconnection process exhibit a nonlinear acceleration and greatly exceed that of a similar resistive MHD model, more closely in line with experimental results. Furthermore, by sampling the two-fluid simulation data in the plane perpendicular to the helical (m,n)$=$(1,1) mode, we present the first observation of the quadrupole out-of-plane magnetic field appearing during sawtooth reconnection with the Hall term. We also explore how reconnection as viewed in the helically perpendicular plane varies toroidally, which affects the symmetry of the reconnection geometry and the local diamagnetic effects. [Preview Abstract] |
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NP12.00038: MHD edge instabilities in toroidal plasmas Linda Sugiyama Different types of MHD edge instabilities in different toroidal magnetically confined plasmas are compared. Large scale numerical simulations show that the nonlinear evolution of an unstable edge mode in a shaped plasma with a single X-point and a surrounding open field line region has a number of common features in the full resistive MHD model for strongly unstable and weaker instabilities. These include the relation of the nonlinear mode structure and dominant toroidal harmonics to the linear eigenmode spectrum, the effects of the mode on reducing the edge pressure or density gradient, the inward penetration of a ballooning-type perturbation into the plasma interior, and the potential to drive a coherent axisymmetric poloidal rotation of the outer part of the plasma, exhibited at different strengths. The results can be compared to experiment to estimate the usefulness and validity of the MHD model for predicting edge stability and instability properties. [Preview Abstract] |
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NP12.00039: NIMROD Modeling of Sawtooth Modes Using Hot-Particle Closures Scott Kruger, T.G. Jenkins, E.D. Held, J.R. King In DIII-D shot 96043, RF heating gives rise to an energetic ion population that alters the sawtooth stability boundary, replacing conventional sawtooth cycles by longer-period, larger-amplitude `giant sawtooth' oscillations. We explore the use of particle-in-cell closures within the NIMROD code to numerically represent the RF-induced hot-particle distribution, and investigate the role of this distribution in determining the altered mode onset threshold and subsequent nonlinear evolution. Equilibrium reconstructions from the experimental data are used to enable these detailed validation studies. Effects of other parameters on the sawtooth behavior, such as the plasma Lundquist number and hot-particle beta-fraction, are also considered. The fast energetic particles present many challenges for the PIC closure. We review new algorithm and performance improvements to address these challenges, and provide a preliminary assessment of the efficacy of the PIC closure versus a continuum model for energetic particle modeling. We also compare our results with those of Choi {\it et al.} [Phys. Plasmas {\bf 14}, 112517 (2007)], and discuss plans for a more complete validation campaign for this discharge. [Preview Abstract] |
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NP12.00040: Resistive Wall Tearing Mode Locking H. Strauss The most common type of disruption is caused a rotating tearing mode with poloidal and toroidal mode number $(2,1)$. When mode rotation slows down, in a process called mode locking, a disruption almost invariably ensues. The mechanisms involved in this process are not well understood. Simulations with M3D are presented in which an ITER equilibrium was modified by peaking the current so that $q$ was somewhat greater than $2$ at the edge of the current channel. The equilibrium is then unstable to a resistive wall tearing mode (RWTM). If the ratio of the resistive wall penetration time to Alfven time $S_{wall}$ is large, the instability saturates at a low amplitude, and the asymmetric wall force $F_x$ remains small. If $S_{wall}$ is low, the instability grows to a larger amplitude, and the force $F_x$ is an order of magnitude larger. Shear free and sheared toroidal rotation are modeled If the rotation is sufficiently fast, the wall acts as if $S_{wall}$ were high, while if it is slow, $S_{wall}$ is not affected. Implications for mode locking disruptions will be be discussed. [Preview Abstract] |
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NP12.00041: Error field penetration and locking to the backward wave John Finn, Andrew Cole, Dylan Brennan Error field penetration involves driving a stable tearing mode in a rotating toroidal plasma. In this paper it is shown that locking for modes with real frequencies $\omega_{r}$ differ from conventional results. The reconnected flux for modes with frequencies $\pm\omega_{r}$ in the plasma frame is maximized when the frequency of the stable backward mode ($-\omega_{r}$) in the lab frame is zero, i.e. when $v=\omega_{r}/k$. Notably, the locking torque is exactly zero at $v=\omega_{r}/k$, with a pronounced peak at just higher rotation, leading to a locked state with plasma velocity just above $\omega_{r}/k$. Real frequencies are known to occur due to the Glasser effect for modes in the resistive-inertial (RI) regime. This therefore leads to locking of the plasma velocity to just above $v=\omega_{r}/k$. Also, similar real frequencies can occur in the visco-resistive (VR) regime with pressure, and the locking torque is similar to the RI result. Real frequencies occur due to diamagnetic effects in other tearing mode regimes and also show this effect. Nonlinear effects on the mode amplitude and torque for weakly stable modes or large error fields are discussed. We discuss the possibility of applying external fields of different helicities to drive sheared flows. [Preview Abstract] |
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NP12.00042: Excitation of low-frequency waves via coupling between slow Alfven waves in the GAMMA 10 tandem mirror R. Ikezoe, M. Ichimura, T. Okada, M. Hirata, M. Sakamoto, Y. Iwamoto, S. Sumida, S. Jang, J. Itagaki, Y. Onodera, M. Yoshikawa, J. Kohagura, Y. Shima, X. Wang, Y. Nakashima In normal discharges of the GAMMA 10 tandem mirror, confined energy is saturated against heating power and unstable slow Alfven wave named as Alfven-Ion-Cyclotron (AIC) wave is observed in the saturated phase. This saturation may be partly related to (1) the decay of ICRF heating power, which is the main power source in GAMMA 10, due to the coupling with the AIC waves to produce difference-frequency waves and (2) the enhancement of axial transport of high-energy ions owing to nonlinearly excited low-frequency waves. To investigate these phenomena precisely, reflectometry is applied, which can provide assessment of nonlinear process at the location where the nonlinear process are taking place without any disturbance. Bispectral analysis applied to the density fluctuations measured at a wide radial region clearly shows the occurrence of various wave-wave couplings among the heating ICRF wave and the AIC waves. Generation of low-frequency waves via the coupling between coexisting AIC waves is found to be significant only near the core region. Details of measured nonlinear couplings are presented along with the observation showing the clear relation of generated low-frequency waves with the axial transport of high-energy ions. [Preview Abstract] |
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NP12.00043: Measurement of the ICRF wave propagation in the internal region of plasmas by using reflectometers on GAMMA10 T. Okada, R. Ikezoe, M. Ichimura, M. Hirata, M. Sakamoto, S. Sumida, Y. Iwamoto, S. Jang, J. Itagaki, Y. Onodera, M. Yoshikawa, J. Kohagura, Y. Shima, Y. Nakashima ICRF waves is one of valuable tools for producing and heating plasmas. On GAMMA10, ions are mainly heated by the ICRF waves with the absorption of the cyclotron resonance layers. ICRF waves of 6.36, 9.9 and 10.3 MHz are normally used to be compatible with the magnetic mirror configuration and damped at the resonance layers of the central cell, east and west anchor cells, respectively. These waves are usually excited by ICRF antennas installed in the central cell and propagate to each resonance layer. It is essential for the ongoing divertor simulation experiments on GAMMA 10 to investigate wave excitation, propagation and absorption. We observe the electron density fluctuations accompanied with the ICRF waves by using microwave reflectometer systems. It is confirmed that the wave of 6.36 MHz is further damped near the resonance layer in the internal region. The waves of 9.9 / 10.3 MHz excited in the east / west anchor cells interferes with the wave from the central cell. The interfered wave is controlled with antenna phasing by the phase difference between both antennas in the central and the anchor cell. The wave intensity measured by reflectometers depends clearly on the phase difference. In this talk, the availability of wave measurement with reflectometers is shown, and the wave propagation in the internal region of plasmas on GAMMA 10 is reported. [Preview Abstract] |
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NP12.00044: Wave Heating in Ion Cyclotron Ranges of Frequencies in RT-1 M. Nishiura, Z. Yoshida, Y. Yano, Y. Kawazura, T. Mushiake, H. Saitoh, M. Yamasaki, A. Kashyap, N. Takahashi, M. Nakatsuka, A. Fukuyama The magnetosphere plasma device RT-1 has been developed for the studies on magnetosphere and advanced fusion plasmas. A levitated superconducting coil produces magnetic dipole fields that realize a high confinement state. The electron cyclotron resonance heating (ECRH) with 8.2 GHz and 50 kW produces the plasmas with hot electrons in a few ten keV range. We reported that the local electron beta exceeded 1 in RT-1 plasmas. In such situation, the ions still remain cold at a few ten eV. Heating ions is expected to access high ion beta state and to improve the plasma confinement theoretically. Therefore the ion cyclotron range of frequencies (ICRF) heating with 2-4 MHz and 10 kW is being prepared in RT-1. Based on the results of the TASK-WF2 code, the $\cap $ shape loop antenna was designed for a slow wave excitation, and was implemented in the RT-1. In the ICRF heating experiments, a base plasma was sustained by ECRH. We observed the clear increase in diamagnetic signals and impurity ion temperature (CIII) in helium plasmas at the neutral gas pressure of 3 mPa, if the ICRF power of 10 kW is comparable to the ECRH one. This result is the first time in a magnetosphere plasma device. The results related to the ICRF heating will be presented in detail. [Preview Abstract] |
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NP12.00045: Solid-State Radio Frequency Plasma Heating Using a Nonlinear Transmission Line Kenneth Miller, Timothy Ziemba, James Prager, Ilia Slobodov Radio Frequency heating systems are rarely used by the small-scale validation platform experiments due to the high cost and complexity of these systems, which typically require high power gyrotrons or klystrons, associated power supplies, waveguides and vacuum systems. The cost and complexity of these systems can potentially be reduced with a nonlinear transmission line (NLTL) based system. In the past, NLTLs have lacked a high voltage driver that could produce long duration high voltage pulses with fast rise times at high pulse repetition frequency. Eagle Harbor Technologies, Inc. (EHT) has created new high voltage nanosecond pulser, which combined with NLTL technology will produce a low-cost, fully solid-state architecture for the generation of the RF frequencies (0.5 to 10 GHz) and peak power levels ($\sim$ 10 MW) necessary for plasma heating and diagnostic systems for the validation platform experiments within the fusion science community. The proposed system does not require the use of vacuum tube technology, is inherently lower cost, and is more robust than traditional high power RF heating schemes. Design details and initial bench testing results for the new RF system will be presented. [Preview Abstract] |
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NP12.00046: Survey of the OXB mode conversion window using fast scanning O-mode radiation in the LHD Yuki Goto, Shin Kubo, Hiroe Igami, Takashi Shimozum, Yasuo Yoshimura, Hiromi Takahashi, Tohru Tujimura, Ryohhei Makino ECRH using an Electron Bernstein Wave (EBW) is an effective method for the heating of the over-dense plasma. The EBW can be excited via the ordinary(O)-extraordinary(X)-EBW(B) (OXB) mode conversion process with launching the O-mode from the low field side toward the OXB mode conversion window, that is, the scope of injection direction to get high OXB mode conversion rate. In the Large Helical Device (LHD), EBW heating experiments using the OXB mode conversion process have been carried out. For improving the heating performance and the experimental efficiency, it is essential to tune the injection direction precisely to aim the OXB mode conversion window experimentally. Since a thermally excited EBW in the plasma core region can be emitted as the O-mode via the inverse OXB mode conversion mechanism, we have tried to find out the window by measuring the O-mode emission intensity using a fast steerable antenna. The increase of radiation intensity near this region is confirmed experimentally when an over-dence plasma is sustained. A ray-tracing code is extended to simulate the ECE including the emission originated from the EBW (EBE) to understand the observed change in the radiation using experimentally obtained plasma profiles for each time slice during the antenna steering. [Preview Abstract] |
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NP12.00047: A Diagnostic for Electric Field Measurements in the Near/Far-Field Regions of ICRF Antenna E. H. Martin, J. B. O. Caughman, R. C. Isler The physics mechanisms of wave heating and current drive processes in the bulk hot plasma are generally well identified. However, details of the wave-plasma interaction with a material surface in the cold plasma edge are still not fully understood. The driver behind this interaction is the time-periodic wave electric field and is referred to as the near/far-field depending on the location with respect to the antenna. Various models have been formulated to capture the near/far-field physics but have not been tested experimentally. Thus, a diagnostic capable of measuring the electric field with temporal and 3D-spatial resolution is critical for confidence in the codes used to design next generation ICRF antennas. This research is focused on the development of a laser based spectroscopic technique, Doppler-free saturation spectroscopy (DFSS), and its implementation to study near/far-field physics. Using DFSS the spectra line profile of various electronic transitions are measured and fit to a quantum mechanical model incorporating both magnetic and dynamic electric field operators. The electric field direction and magnitude are extracted from the fit. The experimental setup and planned experiments will be discussed. Additionally, initial measurements of fitted H$_{\mathrm{\delta }}$ spectrum under the influence of known electric and magnetic fields will be presented. [Preview Abstract] |
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NP12.00048: Experimental Study of RF Sheath Formation on a Fast Wave Antenna and Limiter in the LAPD Michael Martin, Walter Gekelman, Patrick Pribyl, Bart Van Compernolle, Troy Carter Ion cyclotron resonance heating (ICRH) will be an essential component of heating power in ITER. During ICRH, radio frequency (RF) sheaths may form both at the exciting antenna and further away, e.g. in the divertor region, and may cause wall material sputtering and decreased RF power coupling to the plasma. It is important to do detailed laboratory experiments that fully diagnose the sheaths and wave fields. This is not possible in fusion devices. A new RF system has recently been constructed for performing such studies in the LAPD plasma column ($n_{e} \sim 10^{12}-10^{13}cm^{-3},$ $T_{e} \sim 1-10eV, \quad B_{0} \sim 400-2000G,$ diameter $\sim 60cm,$ length $\sim 18m)$. The RF system is capable of pulsing at the 1 Hz rep. rate of the LAPD plasma and operating between 2 -- 6 MHz (1$^{\mathrm{st}}$ -- 9$^{\mathrm{th}}$ harmonic of f$_{\mathrm{ci}}$ in H) with a power output of 200 kW. First results of this system driving a single-strap fast wave antenna will be presented. Emissive and Langmuir probe measurements in the vicinity of both the antenna and a remote limiter and wave coupling measured by magnetic pickup loops will be presented. [Preview Abstract] |
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NP12.00049: High field side lower hybrid launch leads to wave amplification on alpha particles Ian Ochs, Nicola Bertelli, Nathaniel Fisch Although lower hybrid waves have been shown to be effective in driving plasma current in present-day tokamaks, they are predicted to strongly interact with the energetic $\alpha$ particles born from fusion reactions in eventual tokamak reactors. However, in the presence of the expected steep $\alpha$ particle birth gradient, this interaction can produce wave amplification rather than wave damping. Here, we identify the flexibilities in achieving this amplification effect through a consideration of symmetries in the channeling interaction, in the wave propagation, and in the tokamak field configuration. Interestingly, for current drive that supports the poloidal magnetic field, we find that wave amplification through $\alpha$ channeling is fundamentally coupled to the elusive $| kl |$ upshift. In so doing, we show that wave launch from the tokamak high-field side is favorable both for $\alpha$-channeling and for achieving the $| kl |$ upshift. We then present a simple linear model to calculate the required radial gradients to achieve amplification. Combining this model with ray tracing simulations, we demonstrate the potential for substantial wave amplification in a regime consistent with a hot-ion-mode fusion reactor. [Preview Abstract] |
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NP12.00050: Full-wave simulations of lower hybrid wave propagation in the EAST tokamak P.T. Bonoli, J.P. Lee, S. Shiraiwa, J.C. Wright, B. Ding, C. Yang Studies of lower hybrid (LH) wave propagation have been conducted in the EAST tokamak where electron Landau damping (ELD) of the wave is typically weak, resulting in multiple passes of the wave front prior to its being absorbed in the plasma core. Under these conditions it is interesting to investigate full-wave effects that can become important at the plasma cut-off where the wave is reflected at the edge, as well as full-wave effects such as caustic formation in the core. High fidelity LH full-wave simulations were performed for EAST using the TORLH field solver [1]. These simulations used sufficient poloidal mode resolution to resolve the perpendicular wavelengths associated with electron Landau damping of the LH wave at the plasma periphery, thus achieving fully converged electric field solutions at all radii of the plasma. Comparison of these results with ray tracing simulations [2, 3] will also be presented. \\[4pt] [1] J. C. Wright et al, Physics of Plasmas 16, 072502 (2009).\\[0pt] [2] C. Yang et al, Plasma Physics and Controlled Fusion 56 125003 (2014).\\[0pt] [3] S. Shiraiwa et al, 21st Topical Conference on Radio-frequency Power in Plasmas, 27-29 April 2015, Lake ArrowHead, CA. [Preview Abstract] |
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NP12.00051: Wave solutions of ion cyclotron heated plasmas with self-consistent velocity distributions in a tokamak Jungpyo Lee, John Wright, Paul Bonoli, Robert Harvey We describe a numerical model for the propagation and absorption of ion cyclotron waves in a tokamak with a non-Maxwellian velocity space distribution function. The non-Maxwellian distribution is calculated by solving Maxwell's equations and the Fokker-Plank equation self-consistently. This approach will be useful to interpret measurements of minority hydrogen tail formation during ICRF heating experiments in Alcator C-Mod [A. Bader et al, Nuclear Fusion \textbf{52}, 094019 (2012)]. To couple the Maxwell equation solver with Fokker-Plank equation solver, the quasilinear diffusion coefficients for the fundamental ion cyclotron absorption and the first harmonic absorption are calculated. In a previous study [Jaeger et. Al,Nuclear Fusion \textbf{46}, S396 (2006)], the all-orders spectral algorithm wave solver (AORSA) was coupled with the Fokker-Plank code (CQL3D) to find the self-consistent non-Maxwellian distribution. We derive the modified quasilinear diffusion coefficients for the finite Larmor radius (FLR) approximation using a significantly faster wave solver (TORIC) following the approach by Jaeger.~The coupled TORIC-CQL3D model will be compared against results from AORSA-CQL3D in order to verify the accuracy of the reduced FLR physics in TORIC. [Preview Abstract] |
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NP12.00052: Coupling an ICRF core spectral solver to an edge FEM code John Wright, Syunichi Shirwaiwa The finite element method (FEM) and the spectral approaches to simulation of ion cyclotron (IC) waves in toroidal plasmas each have strengths and weaknesses. For example, the spectral approach (eg TORIC) has a natural algebraic representation of the parallel wavenumber and hence the wave dispersion but does not easily represent complex geometries outside the last closed flux surface, whereas the FEM approach (eg LHEAF) naturally represents arbitrary geometries but does not easily represent thermal corrections to the plasma dispersion. The two domains: thermal core with flux surfaces and cold edge plasma with open field lines may be combined in such as way that each approach is used where it works naturally. Among the possible ways of doing this, we demonstrate the method of mode matching. This method provides an easy way of combining the two linear systems without significant modifications to the separate codes. We will present proof of principal cases and initial applications to minority heating. [Preview Abstract] |
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NP12.00053: GPU enabled kinetic effects in radio-frequency heating simulation David Green In previous work we have demonstrated [1] the iterative addition of parallel kinetic effects to finite-difference frequency-domain simulation of radio-frequency (RF) wave propagation in fusion relevant plasmas. Such iterative addition in configuration space bypasses several of the difficulties with traditional spectral methods for kinetic RF simulation when applied to problems that exhibit non-periodic geometries. Furthermore, the direct numerical integration of particle trajectories in real magnetic field geometries removes violations of the stationary phase approximation inherent in the spectral approach [2]. Here we extend this method to include perpendicular kinetics by relying on the massively parallel capability of GPUs to enable resolution of 3 velocity-space dimensions. We present results for a mode converted ion Bernstein wave scenario in 1-space plus 3-velocity dimensions case relevant to fusion plasmas. \\[4pt] [1] D. L. Green and L. A. Berry, Comp. Phys. Comm., 185(3), pg. 736-743 (2014); doi:10.1016/j.cpc.2013.10.032\\[0pt] [2] D. L. Green and L. A. Berry, http://meetings.aps.org/link/BAPS.2013.DPP.BP8.70 [Preview Abstract] |
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NP12.00054: Allowing for Slow Evolution of Background Plasma in the 3D FDTD Plasma, Sheath, and Antenna Model David Smithe, Thomas Jenkins, Jake King We are working to include a slow-time evolution capability for what has previously been the static background plasma parameters, in the 3D finite-difference time-domain (FDTD) plasma and sheath model used to model ICRF antennas in fusion plasmas [1]. A key aspect of this is SOL-density time-evolution driven by ponderomotive rarefaction from the strong fields in the vicinity of the antenna. We demonstrate and benchmark a Scalar Ponderomotive Potential method [2], based on local field amplitudes, which is included in the 3D simulation. And present a more advanced Tensor Ponderomotive Potential approach [3], which we hope to employ in the future, which should improve the physical fidelity in the highly anisotropic environment of the SOL. Finally, we demonstrate and benchmark slow time (non-linear) evolution of the RF sheath, and include realistic collisional effects from the neutral gas. [1] ``Benchmarking sheath sub-grid boundary conditions for macroscopic-scale simulations,'' T. G. Jenkins and D. N. Smithe, Plasma Sources Science and Technology 24 (2015). [2] ``Nonlinear ICRF-plasma interactions,'' J. R. Myra, et. al., Nucl. Fusion 46, S455 (2006). [3] ``Improvements to the ICRH Antenna Time-Domain 3D Plasma Simulation Model,'' D. N. Smithe, T. G. Jenkins, J.R. King, 21st Topical Conf. on RF Power in Plasmas (2015). [Preview Abstract] |
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NP12.00055: Plasma--Surface Interactions and RF Antennas Thomas Jenkins, D.N. Smithe, K. Beckwith, B.D. Davidson, S.E. Kruger, A.Y. Pankin, C.M. Roark Implementation of recently developed finite-difference time-domain (FDTD) modeling techniques on high-performance computing platforms allows RF power flow, and antenna near- and far-field behavior, to be studied in realistic experimental ion-cyclotron resonance heating scenarios at previously inaccessible levels of resolution. We present results and 3D animations of high-performance (10k--100k core) FDTD simulations of Alcator C-Mod's field-aligned ICRF antenna on the Titan supercomputer, considering (a) the physics of slow wave excitation in the immediate vicinity of the antenna hardware and in the scrape-off layer for various edge densities, and (b) sputtering and impurity production, as driven by self-consistent sheath potentials at antenna surfaces. Related research efforts in low-temperature plasma modeling, including the use of proper orthogonal decomposition methods for PIC/fluid modeling and the development of plasma chemistry tools (e.g. a robust and flexible reaction database, principal path reduction analysis capabilities, and improved visualization options), will also be summarized. [Preview Abstract] |
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NP12.00056: Time-Dependent Distribution Functions in C-Mod Calculated with the CQL3D-Hybrid-FOW, AORSA Full-Wave, and DC Lorentz Codes R.W. (Bob) Harvey, Yu.V. Petrov, E.F. Jaeger, L.A. Berry, P.T. Bonoli, A. Bader A time-dependent simulation of C-Mod pulsed ICRF power is made calculating minority hydrogen ion distribution functions with the CQL3D-Hybrid-FOW finite-orbit-width Fokker-Planck code. ICRF fields are calculated with the AORSA full wave code, and RF diffusion coefficients are obtained from these fields using the DC Lorentz gyro-orbit code. Prior results with a zero-banana-width simulation using the CQL3D/AORSA/DC time-cycles showed a pronounced enhancement of the H distribution in the perpendicular velocity direction compared to results obtained from Stix's quasilinear theory, in general agreement with experiment. The present study compares the new FOW results, including relevant gyro-radius effects, to determine the importance of these effects on the the NPA synthetic diagnostic time-dependence. The new NPA results give increased agreement with experiment, particularly in the ramp-down time after the ICRF pulse. [Preview Abstract] |
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NP12.00057: Modeling of ICRF wave propagation and heating in EAST with the full-wave code TORIC E.M. Edlund, P.T. Bonoli, M. Porkolab, S.J. Wukitch Access to advanced tokamak (AT) scenarios in EAST depends on efficient coupling of the launched ion-cyclotron range of frequency (ICRF) power for heating and lower-hybrid waves to the plasma for steady-state current drive. This work builds on recent predictions from the full-wave code TORIC that have shown significant reductions in loading, resulting in improved heating efficiency, by operating with smaller phasing between antenna straps [1]. The density regime of typical EAST experiments produces perpendicular wavelengths of the fast-wave that are comparable to the minor radius of the plasma, resulting in cavity-resonance effects and requiring full-spectrum analysis for accurate calculations of the antenna coupling. This study examines the effects of antenna phasing, as well as plasma density, temperature and current as control parameters for achieving good coupling of the ICRF power in the pursuit of the optimal conditions for AT plasmas. This work is funded by the US DOE under contract DoE Grant DE-SC0010492. \\[4pt] [1] E. M. Edlund, P. T. Bonoli, M. Porkolab, S. J. Wukitch, 21st Topical Conference on Radiofrequency Power in Plasmas (2015) [Preview Abstract] |
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NP12.00058: Simulations of Electron Bernstein Wave Heating in Field-Reversed Configuration Plasmas Xiaokang Yang, Yuri Petrov, Alf Koehn, Francesco Ceccherini, Laura Galeotti It is extremely challenging to use microwaves to heat electrons effectively in high-beta Field-Reversed Configurations (FRCs) such as the C-2U experiment [1]. For a fixed two dimensional profile of C-2U equilibrium field, electron density and temperature, feasibility studies of electron Bernstein wave (EBW) heating via O-X-B mode conversion, have recently been conducted with use of the Genray ray-tracing code for six selected frequencies which cover the frequency range from fundamental electron cyclotron resonance (ECR) up to more than 20 harmonics of ECR. Very promising and also physically interesting simulation results, which are strongly related to the unique C-2U configuration, will be presented in detail \\[4pt] [1] M.W. Binderbauer \textit{et al.}, Physics of Plasma \textbf{22}, 056110 (2015). [Preview Abstract] |
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NP12.00059: Kinetic simulations of X-B and O-X-B mode conversion A. Arefiev, E.J. Du Toit, A. Kohn, E. Holzhauer, V.F. Shevchenko, R.G.L. Vann High-performance spherical tokamaks are usually overdense (typically $\omega_{pe}/\omega_{ce} ~ 4$ in the core) and so regular electron cyclotron emission is blocked. However, electron Bernstein waves, generated at the local cyclotron frequency (and its harmonics) in the core may be observed outside the plasma via a mode conversion process that takes place typically in the plasma edge between an electromagnetic mode and the (electrostatic) electron Bernstein wave. Understanding the details of this mode conversion process is important in tokamaks with over-dense plasmas both for the interpretation of microwave diagnostic data and to assess the feasibility of EBW heating and/or current drive. To this end, we have performed the first ever 2-D fully-kinetic simulations of O-X-B mode conversion using the particle-in-cell code EPOCH. In addition to benchmarking these numerical results against the linear dispersion relation, we have also investigated nonlinearities associated with a larger incident intensity and the effect of a steeper (and more realistic) density gradient at the mode conversion layer. Simulations were performed on the HELIOS supercomputer at the IFERC-CSC, Rokkasho, Japan and on TACC supercomputers at the University of Texas at Austin. [Preview Abstract] |
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NP12.00060: RF Wave Propagation and Scattering in Turbulent Tokamak Plasmas W. Horton, A. Arefiev, Y. Peysson, M. Goniche The propagation, scattering and absorption of the lower hybrid and electron cyclotron RF waves used to control fusion plasmas is reviewed. Drift wave turbulence driven by the steep ion and electron temperature gradients in H-mode divertor tokamaks produces strong scattering of the RF waves used for heating and plasma currents drive [W. Horton, et al., Physics of Plasmas 20, 112508 (2013)]. Both the 3-5GHz lower-hybrid (LH) and the 170GHZ electron cyclotron (EC) waves experience scattering and diffraction as propagating through the statistically complex density of the plasma. Ray equations are used to calculate the spread of the rays and the associated change in the parallel phase velocity of the RF waves and their change of polarization in the propagation through the fusion plasma. A Fokker Planck equation for the phase space of the RF plasmons is used to describe the spread of the RF wave power in the complex geometry of a divertor tokamak using the ray tracing codes. The evolution of the electron distribution function from the resonant electron-wave interactions is reviewed for several scenarios. The resulting X-ray spectrum is broaden giving better agreement with the measured X-ray spectrum than that calculated in the absence of the turbulent scattering of the RF waves. [Preview Abstract] |
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NP12.00061: Scattering of radio frequency waves by density fluctuations A.K. Ram, K. Hizanidis, Z. Ioannidis, I. Tigelis The scattering of radio frequency waves by density fluctuations in magnetized fusion plasmas is studied theoretically and computationally. For coherent fluctuations, such as filaments in the edge region, we use a full-wave model for which the theory is similar to that for Mie scattering of electromagnetic waves by dielectric objects [1]. The filaments are considered to be cylindrical with their axes aligned along the magnetic field. The results from the theoretical model are compared with numerical simulations using COMSOL. The simulations are extended to plasma conditions that are beyond the scope of the theoretical model, e.g., multiple filaments and filaments with density gradients. For incoherent planar fluctuations, which can be either in the core of the plasma or in the edge region, our theory is based on the Kirchhoff approach in tandem with Huygen's principle. The coherent and incoherent fluctuations scatter the incident plane wave, as well as couple some of the power to different plasma waves. The scattered spectrum is affected by the size of the fluctuations, the frequency, and the direction of propagation of the incident wave.\\[4pt] [1] A. K. Ram, K. Hizanidis, and Y. Kominis, \textit{Phys. Plasmas} \textbf{20}, 056110 (2013). [Preview Abstract] |
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NP12.00062: Multiple scattering of radio frequency waves by blobs: homogenization of a mixture of blobs and the Waterman-Truell approach K. Hizanidis, F. Bairaktaris, S.I. Valvis, A.K. Ram Radio frequency waves are of particular importance for heating and current drive in magnetized fusion plasmas. The scattering of these waves by a multitude of density fluctuations, such as blobs in the edge region, is studied by homogenizing the edge region populated by an ensemble of ellipsoidal plasma blobs immersed in an ambient background plasma. The effective permittivity tensor is formulated on the basis of a depolarization dyadic. In general, the interface between the homogenized slab and the ambient plasma is not necessarily aligned with the magnetic field line. The misalignment leads to changes in the propagation characteristics of the RF waves. The scattering of an incident wave is treated by considering the reflection and transmission through a composite plasma slab. This study is a generalization of [1]; it applies to RF waves in plasmas interacting with ellipsoidal blobs of arbitrary shapes and sizes.\\[4pt] [1] Ari Sihvola, Homogenization of a dielectric mixture with anisotropic spheres in anisotropic background, Lund Institute of Technology, Sweden (1996). [Preview Abstract] |
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NP12.00063: Full Wave Parallel Code for Modeling RF Fields in Hot Plasmas Joseph Spencer, Vladimir Svidzinski, Evstati Evstatiev, Sergei Galkin, Jin-Soo Kim FAR-TECH, Inc. is developing a suite of full wave RF codes in hot plasmas. It is based on a formulation in configuration space with grid adaptation capability. The conductivity kernel (which includes a nonlocal dielectric response) is calculated by integrating the linearized Vlasov equation along unperturbed test particle orbits. For Tokamak applications a 2-D version of the code is being developed. Progress of this work will be reported. This suite of codes has the following advantages over existing spectral codes: 1) It utilizes the localized nature of plasma dielectric response to the RF field and calculates this response numerically without approximations. 2) It uses an adaptive grid to better resolve resonances in plasma and antenna structures. 3) It uses an efficient sparse matrix solver to solve the formulated linear equations. The linear wave equation is formulated using two approaches: for cold plasmas the local cold plasma dielectric tensor is used (resolving resonances by particle collisions), while for hot plasmas the conductivity kernel is calculated. [Preview Abstract] |
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NP12.00064: Verification of nonlinear particle simulation of radio frequency waves in fusion plasmas Animesh Kuley, Jian Bao, Zhihong Lin Nonlinear global particle simulation model has been developed in GTC to study the nonlinear interactions of radio frequency (RF) waves with plasmas in tokamak. In this model, ions are considered as fully kinetic particles using the Vlasov equation and electrons are treated as guiding centers using the drift kinetic [Kuley et al., Phys. Plasmas, 20, 102515 (2013)]. Boris push scheme for the ion motion has been implemented in the toroidal geometry using magnetic coordinates and successfully verified for the ion cyclotron, ion Bernstein and lower hybrid waves. The nonlinear GTC simulation of the lower hybrid wave shows that the amplitude of the electrostatic potential is oscillatory due to the trapping of resonant electrons by the electric field of the lower hybrid wave. The nonresonant parametric decay is observed an IBW sideband and an ion cyclotron quasimode (ICQM). The ICQM induces an ion perpendicular heating with a heating rate proportional to the pump wave intensity. [Preview Abstract] |
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NP12.00065: HBT-EP Program: Active MHD Mode Dynamics and Control G.A. Navratil, J. Bialek, A.H. Boozer, P.J. Byrne, G.V. Donald, P.E. Hughes, J.P. Levesque, M.E. Mauel, Q. Peng, D.J. Rhodes, C.C. Stoafer, C.J. Hansen The HBT-EP active mode control research program aims to: (i) quantify external kink dynamics and multimode response to magnetic perturbations, (ii) understand the relationship between control coil configuration, conducting and ferritic wall effects, and active feedback control, and (iii) explore advanced feedback algorithms. Biorthogonal decomposition is used to observe multiple simultaneous resistive wall modes (RWM). A 512 core GPU-based low latency (14$\mu$s) MIMO control system uses 96 inputs and 64 outputs for Adaptive Control of RWMs. An in-vessel adjustable ferritic wall is used to study ferritic RWMs with increased growth rates, RMP response, and disruptivity. A biased electrode in the plasma is used to control the rotation of external kinks and evaluate error fields. A Thomson scattering diagnostic measures $T_e$ and $n_e$ at 3 spatial points, soon to be extended to 10 points. A quasi-linear sharp-boundary model of the plasma's multimode response to error fields is developed to determine harmful error field structures and associated NTV and resonant torques. Upcoming machine upgrades will allow measurements and control of scrape-off-layer currents, and control of kink modes using optical diagnostics. [Preview Abstract] |
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NP12.00066: Scrape-off-layer current and EUV diagnostics and control on the HBT-EP tokamak J.P. Levesque, M.E. Mauel, J. Bialek, G.A. Navratil, L. Delgado-Aparicio, C.J. Hansen Non-axisymmetric currents in the scrape-off-layer (SOL) and conducting structure of a tokamak can produce severe forces at high plasma performance, compromising the device's structural integrity. Diagnosing these currents during disruptions is important for extrapolating forces in future machines including ITER. Progress on designing components to measure and control SOL and vessel currents in the HBT-EP tokamak is presented. Movable tiles positioned around limiting surfaces will measure SOL and vessel currents during mode activity and disruptions. Biasable plates at divertor strike points will allow control of field-aligned SOL currents for kink mode control studies and will drive convection in the plasma edge. In-vessel Rogowski coils will measure currents in wall components with high spatial resolution. A planned EUV diagnostic upgrade is also presented. Four sets of 16 poloidal views will allow tomographic reconstruction of plasma emissivity and internal kink mode structure. A separate two-color, 16-chord tangential system will allow reconstruction of temperature profiles versus time. Measurements will be input to HBT-EP's GPU-based feedback system, providing active feedback for kink modes using only optical sensors and both magnetic and edge current actuators. [Preview Abstract] |
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NP12.00067: Comparing Plasma Response to Magnetic Perturbations Applied to Limited and Diverted Discharges in HBT-EP Patrick Byrne, J.P. Levesque, C. Stoafer, Q. Peng, M. Mauel, J. Bialek, G. Navratil We report~experiments on and~ideal MHD models of the multi-mode plasma response to magnetic~perturbations applied to stable discharges that are limited with circular cross-section and diverted. We focus on the excitation of both ``stiff'' non-resonant perturbations with n $=$ 1 and 2 and on the resonant perturbations of marginally stable external kink modes. DCON is used to compute the multi-mode ideal plasma response. We find the ``stiff,'' non-resonant modes have low poloidal wavenumber - \textbar m\textbar $\approx $ 0. Consequently,~they have a ten-fold higher coupling coefficient to the HBT-EP magnetic control coils. ~For circular, limited plasmas, the mode is maximum at the high-field side of the~plasma; whereas in diverted geometry, the mode creates a highly localized perturbation near the x-point of the plasma. By driving this mode with external coils, it may be possible to actively control the heat deposited by a fusion plasma across a larger surface, relaxing the materials requirements of the~strike point. Additionally, the low level of spectral overlap between these modes and the edge-resonant kink in a typical tokamak (2/1 or greater) means that this mode can be strongly driven with minimal excitation of unstable edge modes. [Preview Abstract] |
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NP12.00068: Ongoing Ferritic Wall Mode studies on HBT-EP P.E. Hughes, M.E. Mauel, J.P. Levesque, G.A. Navratil Low-activation ferritic steels are leading material candidates for use in next-generation fusion development experiments such as a prospective US component test facility and DEMO [1]. Understanding the interaction of plasmas with a ferromagnetic wall will provide crucial physics for these experiments. Although the ferritic wall mode (FWM) was seen in a linear machine [2], ferritic steel was observed to be compatible with high-performance operation in JFT-2M [3]. Using its high-resolution magnetic diagnostics and adjustable wall segments, HBT-EP now operates successfully with a high-permeability tiled ferritic first wall. Initial measurements showed the ferritic wall enhances the growth rate of the m/n = 3/1 kink mode [4]. In this poster, we report results of our study of the evolution of naturally rotating modes, increased plasma response to phase-flip resonant magnetic perturbations (RMPs), and enhanced plasma disruptivity as the walls are adjusted from stainless wall to ferritic wall configuration. \\[4pt] [1] Kurtz, R.J., et. al. J Nucl Mater 386-388 (2009)\\[0pt] [2] Bergerson, W., et. al. Phys Rev Lett 101 (2008)\\[0pt] [3] Tsuzuki, K., et. al. Nucl Fus 46 (2006)\\[0pt] [4] Levesque, J., et al. Phys. Plasmas 22, 056102 (2015) [Preview Abstract] |
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NP12.00069: Improved feedback control of wall-stabilized kink modes with different plasma-wall couplings and mode rotation Q. Peng, J.P. Levesque, C.C. Stoafer, D.J. Rhodes, P.E. Hughes, P.J. Byrne, M.E. Mauel, G.A. Navratil The HBT-EP tokamak can excite strong, saturated kink modes whose growth rates and rotation frequencies evolve on a millisecond timescale. To control such modes, HBT-EP uses a GPU-based feedback system in a low latency architecture [1]. When feedback is applied, the mode amplitude and rotation frequency can change quickly. We describe an improved algorithm that captures the rapid phase changes in the mode while also removing transient amplitude jumps. Additionally, the control coil driving signal is implemented using a current-controller instead of a voltage-controller. The feedback performance is improved and has been tested under more unstable regimes, including different wall configurations and plasmas slowed by a bias probe. Feedback suppression is observed in all cases and the feedback parameters' dependency on different experimental conditions is studied.\\[4pt] [1] N. Rath, \emph{et al.}, Fusion Eng. Des., \textbf{87}, 1895 (2012). [Preview Abstract] |
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NP12.00070: Non-Ideal Error-Field Response Model with a Ferritic and Resistive Wall Dov Rhodes, A.J. Cole, G.A. Navratil, J.P. Levesque, M.E. Mauel, R. Fitzpatrick A sharp boundary MHD model including non-ideal tearing layer physics [1] has been extended to include an up-down asymmetric equilibrium, with resistive and ferritic wall boundary conditions in the vacuum region outside the plasma. The model is designed to explore two related problems in strongly shaped geometry: the intrinsic stability limit of a resistive plasma in the presence of a resistive and/or ferritic wall, and the response of the plasma to error-fields near marginal stability. Presented results include an extension of the recent cylindrical 4-beta calculations of Brennan and Finn [2] to shaped toroidal equilibrium, and the response to error-fields near each beta limit. Future applications include the study of multi-mode feedback control, verification with larger codes such as NIMROD, PEST3, and VALEN, and comparison with recent ferritic wall experiments at HBT-EP [3].\\[4pt] [1] R. Fitzpatrick, Phys. Plasmas 17, 112502 (2010).\\[0pt] [2] D. P. Brennan and J. M. Finn, Phys. Plasmas 21, 102507 (2014).\\[0pt] [3] J.P. Levesque et. al., Phys. Plasmas 22, 056102 (2015). [Preview Abstract] |
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NP12.00071: Study of Kink Modes and Error Fields using Rotation Control with a Biased Probe Chris C. Stoafer, J.P. Levesque, Q. Peng, M.E. Mauel, G.A. Navratil A bias probe has been installed in the High Beta Tokamak - Extended Pulse (HBT-EP) for studying MHD mode rotation and stability. When the probe is inserted into the edge of the plasma and a voltage applied, the rotation of long-wavelength kink instabilities is strongly modified. A large poloidal plasma flow results, measured with a bi-directional Mach probe, and changes in plasma flow correlate to changes in edge kink mode rotation. An active controller is used to adjust the probe voltage in real time for controlling both the plasma flow and mode rotation. Bias probe voltages are generated through an active GPU-based digital feedback system. Mode rotation control is desirable and allows for MHD stability studies under conditions of varying mode rotation rates. At large positive biases, the probe current induces a torque that opposes the natural direction of mode rotation. We are able to apply sufficiently large torque to induce a transition to a fast rotation state (both mode and plasma rotation). The bias required to induce the transition is shown to depend on an applied error field, establishing a technique to determine the natural error field on HBT-EP. [Preview Abstract] |
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NP12.00072: Multi-Point Thomson Scattering System Calibration and Measurements on HBT-EP G.V. Donald, J.P. Levesque, C.C. Stoafer, M.E. Mauel A Thomson scattering (TS) system has been successfully installed and calibrated for diagnostics of HBT-EP. The TS system provides three spatial point measurements and has significantly improved upon the previous single point system. Analysis of Rayleigh Scattering was performed for an absolute density calibration of the TS system. Energy fluctuations in the output pulse from the Nd:YAG laser are individually recorded and accounted for using an integrating sphere and photodetector. T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$ profiles have been investigated for varying wall configuration changes, including insertion of ferritic wall elements. We report our results, for the three spatial points, and measurements of the T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$ evolution through typical HBT-EP discharges. The three fiber bundle system will be upgraded within the next grant period to allow measurement of ten spatial points. The ten point system will enhance our equilibrium reconstruction capability, improve stability analysis of the HBT-EP discharges, and allow for further understanding of the plasma characteristics during resistive wall mode (RWM) activity and active control experiments. [Preview Abstract] |
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NP12.00073: MAGNETIC RECONNECTION |
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NP12.00074: High Power Heating of Magnetic Reconnection in UTokyo Spherical Tokamak Merging Experiment: TS-U Y. Ono, M. kawanami, K. Kimura, R. Nakai, K. Nishida, R. Ishida, H. Yamanaka, A. Kuwahata, H. Tanabe, M. Inomoto, C.Z. Cheng Significant ion heating of magnetic reconnection up to 0.2keV and 1.2keV were documented in two tokamak merging experiments: TS-3 and MAST [1,2], leading us to a new high-field merging experiment: TS-U in University of Tokyo. 1D and 2D contours of ion and electron temperatures measured in TS-3 already revealed clear energy-conversion of magnetic reconnection: huge outflow heating of ions in the downstream and electron heating localized at the X-point[1]. It is noted that the ion heating energy is proportional to square of the reconnecting (poloidal) magnetic field B$_{\mathrm{rec}}$ [1,2]. It is because the reconnection outflow accelerates ions up to the poloidal Alfven speed [1]. The accelerated ions are thermalized by shock-like density pileups in the downstreams. These results agree qualitatively with recent solar satellite observations and PIC simulation results [2]. Based on those results, our poster will show the design of upscaled high-field tokamak merging experiment: TS-U. The high-power heating of tokamak merging is useful not only for laboratory study of reconnection heating mechanisms but also for economical startup and heating of tokamak plasmas. The tokamak merging with B$_{\mathrm{rec}}$\textgreater 0.3T will enables us to heat the tokamak plasma to the burning regime: T$_{\mathrm{i}}$\textgreater 5keV without using any additional heating facility. [1] Y. Ono et al., Phy. Rev. Lett. 107, 185001 (2011), [2] Y. Ono et al., Plasma Phys. Cont. Fus. 54 124039, (2012); Y. Ono, Phys. Plasmas in press. [Preview Abstract] |
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NP12.00075: Fast magnetic reconnection due to anisotropic electron pressure Paul Cassak, Robert Baylor, Raymond Fermo, Matthew Beidler, Michael Shay, Marc Swisdak, James Drake, Homa Karimabadi A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks. [Preview Abstract] |
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NP12.00076: Effects of in-plane magnetic field on the transport of 2D electron vortices in non-uniform plasmas Justin Angus, Andrew Richardson, Joseph Schumer The formation of electron vortices in current-carrying plasmas is observed in 2D particle-in-cell (PIC) simulations of the plasma-opening switch. In the presence of a background density gradient in Cartesian systems, vortices drift in the direction found by crossing the magnetic field with the background density gradient as a result of the Hall effect. However, most of the 2D simulations where electron vortices are seen and studied only allow for in-plane currents and thus only an out-of-plane magnetic field. Here we present results of numerical simulations of 2D, seeded electron vortices in an inhomogeneous background using the generalized 2D electron-magneto-hydrodynamic model that additionally allows for in-plane components of the magnetic field. By seeding vortices with a varying axial component of the velocity field, so that the vortex becomes a corkscrew, it is found that a pitch angle of around 20 degrees is sufficient to completely prevent the vortex from propagating due to the Hall effect for typical plasma parameters. [Preview Abstract] |
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NP12.00077: Electron Inertia Effects in Hall-Driven Magnetic Field Penetration in Electron-Magnetohydrodynamics Andrew Richardson, Justin Angus, Stephen Swanekamp, Joseph Schumer, Paul Ottinger Magnetic field penetration in electron-magnetohydrodynamics (EMHD) can be driven by density gradients through the Hall term. Here we describe the effect of electron inertia on simplified one- and two- dimensional models of a magnetic front. Nonlinear effects due to inertia cause the 1D model to develop peaked solitary waves, while in 2D a shear-driven Kelvin-Helholtz like instability causes the front to break into a series of vortices which propagate into the plasma. The combination of these two effects means that in 2D, Hall driven magnetic field penetration will typically happen in the form of complex vortex-dominated penetration, rather than as a transversely-smooth shock front. [Preview Abstract] |
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NP12.00078: Localized Electron Heating by Strong Guide-Field Magnetic Reconnection Xuehan Guo, Takumichi Sugawara, Michiaki Inomoto, Kotaro Yamasaki, Yasushi Ono Localized electron heating of magnetic reconnection was studied under strong guide-field (typically $B_t \sim 15B_p$) using two merging spherical tokamak plasmas in Univ. Tokyo Spherical Tokamak (UTST) experiment. Our new slide-type two-dimensional Thomson scattering system documented for the first time the electron heating localized around the X-point. The region of high electron temperature, which is perpendicular to the magnetic field, was found to have a round shape with radius of 2 [cm]. Also, it was localized around the X-point and does not agree with that of energy dissipation term $E_t \cdot j_t$. When we include a guide-field effect term $B_t/(B_p+\alpha B_t)$ for $E_t \cdot j_t$ where $\alpha = \sqrt{(v_{in}^2+v_{out}^2)/{v_{\parallel}^2}}$, the energy dissipation area becomes localized around the X-point, suggesting that the electrons are accelerated by the reconnection electric field parallel to the magnetic field and thermalized around the X-point. [Preview Abstract] |
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NP12.00079: Laboratory Measurement of 3D Magnetic Reconnection of Arched Flux Tubes Magnus Haw, Paul M. Bellan An experiment has been constructed to collide two arched magnetic flux tubes at different angles with fully 3D, non-symmetric geometry. The configuration is designed to mimic sheared solar arcades and evaluate the importance of magnetic reconnection in such systems. Time resolved (1MHz) 3D magnetic measurements are taken with a multi-channel 3D magnetic probe. Preliminary analysis shows good agreement between calculated current density and external current diagnostics. Additional simultaneous diagnostics include voltage probes, fast camera imaging, and a 12-channel spectrometer. The spectrometer measures temperature, density, velocity, while the camera provides a view of global plasma behavior. Fast camera images indicate that the topology of the flux tubes evolves such that two equally sized, overlapping loops reconnect to form a small underlying loop and a large overarching loop. [Preview Abstract] |
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NP12.00080: EUV burst, particle heating and whistler wave emission in fast magnetic reconnection induced by kink-driven Rayleigh-Taylor instability Xiang Zhai, Kil-Byoung Chai, Paul Bellan Fast magnetic reconnection associated with a Rayleigh-Taylor instability in a kinked flux rope is studied in the Caltech jet experiment. As the kinked plasma accelerates laterally away from its equilibrium position, an effective gravity due to the acceleration results in a secondary Rayleigh-Taylor instability. This Rayleigh-Taylor instability erodes the plasma to a scale smaller than the ion skin depth and induces a fast magnetic reconnection. A spatially localized energetic EUV burst is observed at the position of fast magnetic reconnection, indicating strong localized electron heating. A circularly polarized high frequency magnetic field perturbation is simultaneously observed at some distance from the reconnection region indicating that the reconnection emits whistler waves and that Hall dynamics governs the reconnection. Spectroscopic measurement including Stark broadening and Doppler broadening shows simultaneous fast ion heating. It is also observed that the voltage across the source electrodes spikes when there is fast magnetic reconnection resulting from the fact that magnetic reconnection changes the magnetic flux linking the electrode circuit. The electron heating is consistent with Ohmic dissipation while the ion heating is consistent with stochastic heating. [Preview Abstract] |
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NP12.00081: Sausage Instabilities on top of Kinking Lengthening Current-Carrying Magnetic Flux Tubes Jens von der Linden, Setthivoine You Observations indicate that the dynamics of magnetic flux tubes in our cosmos and terrestrial experiments involve fast topological change beyond MHD reconnection. Recent experiments suggest that hierarchies of instabilities coupling disparate plasma scales could be responsible for this fast topological change by accessing two-fluid and kinetic scales. This study will explore the possibility of sausage instabilities developing on top of a kink instability in lengthening current-carrying magnetic flux tubes. Current driven flux tubes evolve over a wide range of aspect ratios $\bar{k}$ and current to magnetic flux ratios $\bar{\lambda}$. An analytical stability criterion and numerical investigations, based on applying Newcomb's variational approach to idealized magnetic flux tubes with core and skin currents, indicate a dependence of the stability boundaries on current profiles and overlapping kink and sausage unstable regions in the $\bar{k}$ - $\bar{\lambda}$ trajectory of the flux tubes. A triple electrode planar plasma gun (Mochi.LabJet) is designed to generate flux tubes with discrete core and skin currents. Measurements from a fast-framing camera and a high resolution magnetic probe are being assembled into stability maps of the $\bar{k}$ - $\bar{\lambda}$ space of flux tubes. [Preview Abstract] |
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NP12.00082: Characteristic Dynamics of a Non-Linear Flux Rope Timothy DeHaas, Walter Gekelman, Bart Van Compernolle A magnetic flux rope is a tube-like, current carrying plasma embedded in an external magnetic field. Commonly observed on the solar surface extending into the solar atmosphere, flux ropes are naturally occurring and have been observed by satellites in the near earth and often recreated in laboratory environments. In a series of experiments, a single flux rope of varying cross-section and length was formed in the cylindrical, magnetized plasma of the Large Plasma Device (LaPD, L $=$ 2200 cm, r $=$ 30 cm, n$_{\mathrm{o}}$ $=$ 10$^{12}$ cm$^{-3}$, T$_{\mathrm{e}} =$ 4 eV, ~He). The flux rope was generated via a DC discharge between a cathode and anode with a fixed-free boundary condition. Upon the initiation of the kink instability (I$_{\mathrm{Kink}}$ \textgreater $\pi $r$^{2}$B$_{\mathrm{z}}$c/2L), the displacement of the flux rope saturates, commencing complex motion. The flux ropes exhibit two types of motion, common to all cases of varying Alfven speeds, injection currents, lengths, and cross-sections. The first motion is characterized by a circular path in the transverse plane, whose displacement depends on the input power and whose frequency varies with injection current. The second motion is characterized by random Lorentzian pulses in the magnetic signals. The polarity of these pulses align with the transverse magnetic field and manifest with greater frequency with increases in magnetic field and injection current. [Preview Abstract] |
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NP12.00083: Pulsating Reconnection in the interaction of two magnetic flux ropes Walter Gekelman, Tim DeHaas, William Daughton, Bart Van Compernolle Two flux ropes (dia $=$ 7 cm, ds$=$ 3 cm, L $=$ 10m, I$_{\mathrm{rope}} = $ 300 A/rope) are generated by using a mask in front of a high emissivity cathode (n $=$ 4X10$^{12}$ cm$^{3}$, T$_{\mathrm{e-rope}}$ $=$ 8.5 eV) in a background magnetoplasma (He, B$_{\mathrm{oz}}=$ 330 G, n$=$1.0X10$^{12}$cm$^{3}$, T$_{\mathrm{e}} = $ 4 eV) in the LAPD device at UCLA. The ropes are kink unstable ( I \textgreater 250 A) but not violently so. All three components of the magnetic field were measured with small (1 mm dia) 3-axis probes sensitive to $\frac{\partial \vec{{B}}}{\partial t}$ and the plasma potential measured with an emissive probe. These were measured at 42,075 locations in the volume containing the ropes and 7000 time steps ($\delta \tau = $ .33 $\mu $s). The total electric field $\vec{{E}}=-\nabla \phi -\frac{\partial \vec{{A}}}{\partial t}$ and parallel resistivity as well as the Quasi Seperatrix layer (QSL) were derived from the data. The flux ropes periodically collide as they kink. Each time this happens a strong QSL (Q\textless 400) forms and the resistivity jumps to over a hundred times the classical value at locations within the QSL and also on the gradient of the rope current. The reconnection rate is directly evaluated by integrating the electric field along field lines as well as the energy deposition $\vec{{J}}\cdot \vec{{E}}$. The data indicate that there is more than one process causing the enhanced resistivity. The reconnection rate cannot be explained by conventional 2D theories. [Preview Abstract] |
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NP12.00084: Breakdown of adiabatic electron behavior in expanding magnetic fields Emily Lichko, Jan Egedal, William Daughton During magnetic reconnection the incoming magnetic flux tubes expand in the inflow region. If this expansion is sufficiently slow the results are well described by a previously developed adiabatic model [1]. Using kinetic simulations in a simple geometry and applying rapid magnetic perturbations, this study investigates the point at which the adiabatic assumption fails. To this end a 2D VPIC simulation was constructed, where the magnetic field in a uniform plasma is perturbed by externally driven currents. By varying the onset speed of the magnetic perturbation and the electron thermal speed, we found a sharp threshold at which this model breaks down. We believe that this point is determined by the time of the magnetic pumping compared to the electron transit time through the region, i.e.\(\omega \sim \dot{B}/{B} \sim v_{the}/L\). This threshold was also characterized by the launching of Whistler waves and with time domain structures, such as electron holes and double layers, which agree with those seen during magnetic reconnection and may relate to similar structures in the Van Allen Belts [2,3]. \\[1ex][1] J. Egedal, et al, Phys. Plasmas 20, 061201 (2013) \\[0ex][2] J. Egedal, et al, Phys. Plasmas, in press (2015) \\[0ex][3] F.S. Mozer, et al, Phys. Rev. Lett. 113, 035001 (2014) [Preview Abstract] |
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NP12.00085: Quasi-Separatrix Layers and Line-tied Reconnection in Collisionless Plasmas Zachary Billey, Ellen Zweibel, John Finn, William Daughton Many plasmas undergoing magnetic reconnection have boundaries that have constant magnetic flux on the dynamical timescales of the system, such as coronal loops and planetary magnetospheres. Systems where the boundary magnetic flux is constant are called line-tied systems. We conduct collisionless fully 3D particle-in-cell simulations in slab geometry to study how line-tying changes the dynamics relative non-tied systems. We confirm Quasi-Separatrix Layers (QSLs)\footnote{Titov V. S. \textit{et. al.} Geophys. Res. \textbf{107} 1164 (2002)} as a model for predicting potential reconnection sites in 3D systems. Based on this theory, we use line-integrated diagnostics to investigate the collisionless physics relating to the parallel electric field. Here we find non-gyrotopic terms in the pressure tensor are important at the center of the reconnection layer. We investigate the effect of varying the length of the line-tied plasma on the growth rate and reconnection process and compare oblique modes with equivalent periodic systems. We discuss the extension into collisionless regimes of the geometric width vs tearing width theory,\footnote{Y. M. Huang and E. G. Zweibel Phys. Plasmas \textbf{16} 042102 (2009)} developed to explain line-tied suppression of tearing in MHD reconnection. [Preview Abstract] |
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NP12.00086: Cluster observations of electron heating during magnetospheric reconnection Harsha Gurram, Jan Egedal In-situ spacecraft observations have shown that strong kinetic effects are present during the magnetic reconnection in the Earth's magnetosphere. This study investigates the electron distribution functions and electron heating recorded by the Cluster Mission, during the reconnection event on August 21, 2002. For this event the electrons see an increase in their bulk energy by about a factor of 100 from the inflow to the exhaust, rendering this data set particularly interesting. The observed electron distribution functions delineate inflow and exhaust regions by comparing it with distinct morphology of distributions i.e. inflow electrons are colder and with a temperature anisotropy [1]. In addition, beam like features are observed in the distributions which are in good agreement with kinetic simulation results. The observations are consistent with a new model for electron energization in reconnection exhausts [2]. \\[1ex] [1] L. J. Chen, et al., J Geophys. Res., 113, (2008)\\[0ex] [2] J.Egedal, et al., Phys. Plasmas, in press (2015). [Preview Abstract] |
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NP12.00087: Magnetic Flux Array for the Terrestrial Reconnection Experiment (TREX) Samuel Greess, Jan Egedal, Joseph Olson, John Wallace, Michael Clark, Cary Forest The Terrestrial Reconnection Experiment (TREX) at the Wisconsin Plasma Astrophysics Laboratory (WiPAL) studies kinetic reconnection in a variety of regimes. As currently configured, TREX is designed to use Helmholtz coils outside the 3m spherical WiPAL vacuum vessel to create a field that opposes a different field pulsed on by two internal coils. In order to characterize the main properties of the reconnection process in a single shot, we have constructed a nine-layered, 160 channels magnetic flux array. This array allows us to infer the magnetic flux function, $\Psi$, and thus the toroidal component of the vector potential, $A_{\phi}$, as a function of time for each shot. From $A_{\phi}$, we further obtained the magnetic field geometry, current density, and reconnection rate [1]. Preliminary data from this flux array will be presented. \\[1ex][1] Kesich et al., Review of Scientific Instruments 79,063505 (2008). [Preview Abstract] |
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NP12.00088: Accessing the new collisionless reconnection regime in laboratory experiment Joseph Olson, Jan Egedal, Samuel Greess, John Wallace, Michael Clark, Cary Forest The Terrestrial Reconnection Experiment (TREX), the largest dedicated reconnection experiment to date, is currently in operation at the Wisconsin Plasma Astrophysics Laboratory (WiPAL). In its inaugural run, TREX demonstrated its ability to operate in what has traditionally been called the collisionless reconnection regime by observing the out-of-plane magnetic field characteristic of Hall reconnection. Additionally, TREX is projected to access even more collisionless parameters in which electron pressure anisotropy develops, greatly influencing the dynamics of the reconnection process beyond two fluid effects [1]. For example, spacecraft observations [2] and kinetic simulations [3] show that large-scale current layers are driven by this pressure anisotropy. In the last year, TREX has undergone upgrades to its plasma heating, reconnection drive, and diagnostic suite in order to study these features exclusive to truly collisionless reconnection. Preliminary results from the newly optimized experimental runs will be presented.\\[1ex][1] J. Egedal, et al., \textit{Phys. Plasmas} \textbf{20}(6) (2013).\\[1ex][2] K.-J. Hwang, et al., \textit{J. Geophys. Res. Space Physics} \textbf{118} (2013).\\[1ex][3] A. Le, et al., \textit{J. Plasma Phys.} \textbf{81} (2015). [Preview Abstract] |
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NP12.00089: Double layer electric fields aiding the production of superthermal electrons within magnetic reconnection exhausts Jan Egedal, William Daughton, Ari Le Using a kinetic simulation of magnetic reconnection it was recently shown that parallel electric fields ($E_{\parallel}$) can be present over large spatial scales in reconnection exhausts [1]. The largest values of $E_{\parallel}$ are observed within double layers, which form through large parallel streaming of electrons into the reconnection region. The electron confinement, provided in part by the structure in $E_{\parallel}$, allows sustained energization by perpendicular electric fields ($E_{\perp}$). The energization is a consequence of the confined electrons' chaotic orbital motion that includes drifts aligned with the reconnection electric field. The mechanism is effective in an extended region of the reconnection exhaust allowing for the generation of superthermal electrons in reconnection scenarios, including those with only a single $x$-line. The numerical and analytical results agree with detailed spacecraft observations recorded during reconnection events in the Earth's magnetotail [2].\\[4pt] [1] J. Egedal, W. Daughton, and A. Le, Nature Physics \textbf{8}, 321 (2012)\\[0pt] [2] J. Egedal, W. Daughton, A. Le, and A.L. Borg, Phys. Plasmas, in press (2015) [Preview Abstract] |
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NP12.00090: Scaling Laws for Magnetic Reconnection when Electron Pressure Anisotropy is near the Firehose Threshold Obioma Ohia, Jan Egedal, Vyacheslav S. Lukin, William Daughton, Ari Le Magnetic reconnection in weakly-collisional, a process linked to solar flares, coronal mass ejections, and magnetic substorms, has been widely studied through fluid and kinetic simulations. While two-fluid models often reproduce the fast reconnection rate of kinetic simulations, significant differences are observed in the structure of the reconnection regions [1]. Recently, new equations of state that accurately account for the development of anisotropic electron pressure due to the electric and magnetic trapping of electrons have been developed [2]. Guide-field, fluid simulations using these equations of state have been shown to reproduce the detailed reconnection region observed in kinetic simulations [3]. Implementing this two-fluid simulation using the HiFi framework [4], we describe a mechanism for regulation of electron pressure anisotropy as well as study force balance of the electron layers in guide-field reconnection. Scaling laws for the heating observed in these layers based on upstream conditions are derived.\\[4pt] [1] Daughton W et al., Phys. Plasmas 13, 072101 (2006).\\[0pt] [2] Le A et al., Phys. Rev. Lett. 102, 085001 (2009).\\[0pt] [3] Ohia O, et al., Phys. Rev. Lett. In Press (2012).\\[0pt] [4] Lukin VS, Linton MG, Nonlinear Proc. Geoph. 18, 871 (2011) [Preview Abstract] |
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NP12.00091: Kinetic Structure of the Electron Diffusion Region in Guide-Field Magnetic Reconnection Blake Wetherton, Jan Egedal, William Daughton During magnetic reconnection the magnetic field decouples from the electrons' motion inside the electron diffusion region. Because this decoupling is necessary for magnetic reconnection to proceed, the structure of the diffusion region is of special interest. In the case of magnetic reconnection with a guide field, the role of pressure anisotropy and heat conduction in the electron distribution function has been studied through fluid models [1,2], but the detailed kinetic behavior of the electrons in the diffusion region is still not fully understood. By using data from kinetic simulations to track particles backwards in time from the reconnection region, the electron distribution function will be reconstructed in a similar method to that employed by Ng et al.~in the case of anti-parallel magnetic reconnection [3]. The method will thus elucidate the connection between structures in velocity space balancing the reconnection electric field and the details of the electron motion. \\[1ex] [1] M. Hesse, et al., Phys. Plasmas, {\bf 11} (2004). \\[0ex] [2] M. Hesse, et al., Phys. Plasmas, {\bf 13} (2006). \\[0ex] [3] J. Ng, el al., PRL, {\bf 106} (2011). [Preview Abstract] |
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NP12.00092: Fast magnetic reconnection in the strong guide-field regime Adam Stanier, Andrei Simakov, Luis Chacon, William Daughton To explain many magnetised plasma phenomena in nature and the laboratory, it is important to understand how the rates of magnetic reconnection behave in large and weakly collisional systems. Here we show for the large guide field regime, which is pertinent to tokamaks and the solar corona, that reconnection can be fast and independent of both collisional dissipation and system-size regardless of the relative sizes of the ratio of plasma pressure to magnetic pressure and the electron-to-ion mass ratio. We present results from a discrete analysis of the dissipation region, which illustrates how this region adjusts to permit rates independent of the magnitude of dissipation. Finally, we compare a reduced two-field reconnection model with fully kinetic Particle-In-Cell simulations to demonstrate that this model is adequate to reproduce the rates and evolution of the equivalent fully kinetic system in the strong guide-field regime. [Preview Abstract] |
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NP12.00093: Influence of Line-Tied Boundary Conditions on the Development of Magnetic Reconnection in Force-Free Current Layers Cihan Akcay, William Daughton, Vycheslav Lukin, Yi-Hsin Liu The evolution of plasmas in magnetically dominated low-$\beta$ regimes often leads to the formation of nearly force-free current sheets where magnetic reconnection may be triggered by the tearing instability. In three-dimensional systems, the tearing of a current sheet and ensuing magnetic reconnection can result in the formation and interaction of magnetic flux ropes. In addition, many space and laboratory plasmas feature current sheets of finite extent that are embedded in larger systems with line-tied field boundary conditions. Motivated by these properties, we examine the influence of line-tied boundary conditions on the onset and development of three-dimensional magnetic reconnection in kinetic-scale force-free layers. To better understand the physics, we perform cross-comparisons between fully kinetic VPIC simulations and two-fluid HiFi simulations. We focus on a range of guide fields $B_g=(1-10)B_0$ relevant to both space and laboratory plasmas, and compare the evolution between systems with line-tied and periodic boundary conditions. [Preview Abstract] |
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NP12.00094: Hybrid (Kinetic Ion/Fluid Electron) Simulations of Reconnection Including Electron Pressure Anisotropy Ari Le, William Daughton Fully kinetic simulations have shown that the structure of the thin current sheets that form during collisionless reconnection can fall into a variety of regimes depending on the electron pressure anisotropy [1]. Furthermore, recent two-fluid simulations with anisotropic electron equations of state appropriate for reconnection confirm that the electron pressure anisotropy may drive highly elongated current sheets in the reconnection exhaust [2]. While fully kinetic simulations are useful to model small regions of the Earth's magnetosphere, they are still far too expensive for global modeling. Thus, we have implemented the electron equations of state in the hybrid (kinetic ions and fluid electrons) code H3D [3], and initial 2D hybrid simulations of reconnection agree well with fully kinetic simulations. The updated hybrid code is a first step towards including electron anisotropy and full ion kinetics in global simulations of Earth's magnetosphere and laboratory experiments. \\[4pt] [1] Le et al., Phys. Rev. Lett. 110, 135004 (2013)\\[0pt] [2] Ohia et al., Phys. Rev. Lett. 109, 115004 (2012)\\[0pt] [3] Karimabadi et al., Phys. Plasmas 21, 062308 (2014) [Preview Abstract] |
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NP12.00095: Three-dimensional Dynamics of Magnetic Reconnection in Line-Tied Systems William Daughton, Cihan Akcay, Walter Gekelman, Tim DeHass, Bart Van Compernolle Using both fully kinetic and two-fluid simulations, we consider the evolution of thin current layers with line-tied boundary conditions, and plasma parameters relevant to an experiment on the LAPD device. With initial half-thickness $3 d_e \approx 0.5 \;{\rm cm}$, the current layers are unstable to (1) the tearing instability which gives rise to the formation of magnetic flux ropes, and (2) to a short wavelength instability ($kd_e \sim 3$) that grows on the edge of layer. A simple theory is presented for collisionless tearing in line-tied geometry which roughly agrees with the simulations. The nonlinear evolution of magnetic reconnection is characterized using field-line mapping diagnostics to compute the quasi-potential and the squashing factor. While the number of flux ropes predicted by these simulations is in agreement with the experiment, the subsequent evolution features a variety of differences which will be discussed. Finally, we also consider the dynamics of the short-wavelength instability, which appears to be driven by gradients in the current density on the edge of layer. These modes give rise to fluctuations in the electric field and density, and may be a possible candidate to explain the anomalous resistivity that has been observed within the experiment. [Preview Abstract] |
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NP12.00096: 3-D Particle Simulation of Current Sheet Instabilities Zhenyu Wang, Yu Lin, Xueyi Wang, Kurt Tummel, Liu Chen The electrostatic (ES) and electromagnetic (EM) instabilities of a Harris current sheet are investigated using a 3-D linearized ($\delta{f}$) gyrokinetic (GK) electron and fully kinetic (FK) ion (GeFi) particle simulation code. The equilibrium magnetic field consists of an asymptotic anti-parallel $B_{x0}$ and a guide field $B_G$. The ES simulations show the excitation of lower-hybrid drift instability (LHDI) at the current sheet edge. The growth rate of the 3-D LHDI is scanned through the $ (k_x, k_y)$ space. The most unstable modes are found to be at $k_{\parallel}=0$ for smaller $k_y$. As $k_y$ increases, the growth rate shows two peaks at $k_{\parallel} \not =0$, consistent with analytical GK theory. The eigenmode structure and growth rate of LHDI obtained from the GeFi simulation agree well with those obtained from the FK PIC simulation. Decreasing $B_G$, the asymptotic $\beta_{e0}$, or background density can destabilize the LHDI. In the EM simulation, tearing mode instability is dominant in the cases with $k_{y} < k_{x}$. For $k_{y} > k_{x}$, there exist two unstable modes: a kink-like (LHDI) mode at the current sheet edge and a sausage-like mode at the sheet center. The results are compared with the GK eigenmode theory and the FK simulation. [Preview Abstract] |
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NP12.00097: High Energy Particle Populations Associated with Magnetic Reconnection B. Coppi, B. Basu Magnetic reconnection events associated with a variety of laboratory and astrophysical plasmas have been observed to be related to the production of high energy particles. In this context the theory of weakly collisional or collisionless reconnecting modes has been found to generate relatively large ``temperatures fluctuations'' (e.g. of the electron population) associated with significant reconnection fields. The spatial singularity of the temperature fluctuations can, in fact, be removed by the finiteness of the transverse relevant thermal conductivity. An important requirement for this is that the temperature gradient of the involved particle species be significant within the layer where reconnection takes place. With reconnection depending on a finite inductivity [1] associated with the local current density the characteristic layer over which these modes are localized remains significant (strong reconnection) even when the involved macroscopic distances (e.g. in astrophysics) are very large. This is in contrast with the features of the well known tearing mode that is a case of weak reconnection.\\[4pt] [1] B. Coppi, \textit{Bull. Am. Phys. Soc}. \textbf{45}, (2000) 366. [Preview Abstract] |
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NP12.00098: Role of Inertial and Inductive Modes in Magnetic Reconnection Events P. Buratti, B. Coppi, B. Basu Recently, an accurate analysis of the database of magnetic island rotation performed with the JET machine [1] has revealed that, in the frame of zero radial electric field, the island rotation frequency is about 0.9$\omega_{di}$, where $\omega_{di}$ is the ion diamagnetic frequency. The drift-tearing mode theory of reconnection in low collisionality regimes predicts a phase velocity in the opposite direction [2] and, under strictly collisionless conditions, stability in the presence of electron temperature gradients. To explain the observations, a ``mode inductivity'' $L_{\parallel}\equiv(4\pi/c^{2})S_{L}$ has been introduced [3] whose effects replace those of finite resistivity. This has led to a linear instability [4] with $\omega$ close to $\omega_{di}$. The reconnection layer thickness is proportional to the inductivity [4] and the mode has a dissipative growth rate. When considering plasmas with ultrarelativistic energies, the inertial skin depth becomes significant. Thus the width of the reconnection layer can be considered as relevant to realistic theories.\\[4pt] [1] P. Buratti et al., 41st EPS Conference, ECA 38F, paper P1.014.\\[0pt] [2] B. Coppi, Phys. Fluids 8, (1965) 2273.\\[0pt] [3] B. Coppi, Bull. Am. Phys. Soc. 45, (2000) 366.\\[0pt] [4] B. Coppi, et al., Int. Fus. En. Conf. TH-P7/10. [Preview Abstract] |
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NP12.00099: Study of the energy inventory during asymmetric magnetic reconnection in a laboratory plasma Jongsoo Yoo, Masaaki Yamada, Ben Na, Jonathan Jara-Almonte, Hantao Ji, Will Fox, Abe Chien The energy inventory of a magnetic reconnection layer is studied in a laboratory plasma with a significant density asymmetry across the current sheet. The upstream density ratio of about 7 is generated by the in-plane inductive electric field during the plasma formation in MRX. Compared to the symmetric case where the ion energy gain is about twice more than that of electrons, the ion energy gain becomes smaller and the electron energy gain is larger. The reduction in the ion energy gain is mainly caused by the asymmetric profile of the in-plane electrostatic potential, which is generated to balance the Lorentz force in the electron momentum equation. Since the out-of-plane electron fluid velocity on the high-density side is small due to the high density, the potential drop on the high-density side becomes also small. The potential drop on the low-density side is also smaller than expected from the general scaling ($\sim B^2/n$), since it is suppressed by a large density gradient across the low-density side separatrices. As a result, the ion energy gain from the in-plane electrostatic field is smaller than the symmetric case, thereby decreasing the total ion energy gain. Discussion on the electron energization process during asymmetric reconnection is also presented. [Preview Abstract] |
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NP12.00100: A twenty-moment model for collisionless guide field reconnection Jonathan Ng, Ammar Hakim, Amitava Bhattacharjee The integration of kinetic effects in fluid models is an important problem in global simulations of the Earth's magnetosphere and space weather modelling. Here we introduce a new fluid model and closure for collisionless magnetic reconnection and more general applications. It has recently been shown that electron pressure anisotropy is important in setting the structure of the reconnection region, and a closure based on the drift kinetic equation using a distribution of trapped and passing particles has been derived [1,2,3]. We extend the model and present a general expression for moments of the distribution function. By evolving the heat flux tensor and closing at the fourth velocity moment, we obtain a self-consistent set of fluid equations, which includes the evolution of the off-diagonal elements of the pressure tensor. The model is implemented in a two-fluid code [4] and the results are compared to PIC simulations of guide field reconnection.\\[4pt] [1] Egedal et al. Phys Plasmas (2013)\\[0pt] [2] Le et al. Phys Rev Lett (2013)\\[0pt] [3] Ohia et al. Phys Rev Lett (2012)\\[0pt] [4] Hakim et al. J Comp Phys (2006) [Preview Abstract] |
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NP12.00101: Fluctuation dynamics in reconnecting current sheets Adrian von Stechow, Olaf Grulke, Hantao Ji, Masaaki Yamada, Thomas Klinger During magnetic reconnection, a highly localized current sheet forms at the boundary between opposed magnetic fields. Its steep perpendicular gradients and fast parallel drifts can give rise to a range of instabilities which can contribute to the overall reconnection dynamics. In two complementary laboratory reconnection experiments, MRX (PPPL, Princeton) and VINETA.II (IPP, Greifswald, Germany), magnetic fluctuations are observed within the current sheet. Despite the large differences in geometries (toroidal vs. linear), plasma parameters (high vs. low beta) and magnetic configuration (low vs. high magnetic guide field), similar broadband fluctuation characteristics are observed in both experiments. These are identified as Whistler-like fluctuations in the lower hybrid frequency range that propagate along the current sheet in the electron drift direction. They are intrinsic to the localized current sheet and largely independent of the slower reconnection dynamics. This contribution characterizes these magnetic fluctuations within the wide parameter range accessible by both experiments. Specifically, the fluctuation spectra and wave dispersion are characterized with respect to the magnetic topology and plasma parameters of the reconnecting current sheet. [Preview Abstract] |
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NP12.00102: Status and Plans for the FLARE (Facility for Laboratory Reconnection Experiments) Project H. Ji, A. Bhattacharjee, S. Prager, W. Daughton, S. Bale, T. Carter, N. Crocker, J. Drake, J. Egedal, J. Sarff, J. Wallace, Y. Chen, R. Cutler, W. Fox, P. Heitzenroeder, M. Kalish, J. Jara-Almonte, C. Myers, Y. Ren, M. Yamada, J. Yoo The FLARE device (flare.pppl.gov) is a new intermediate-scale plasma experiment under construction at Princeton to study magnetic reconnection in regimes directly relevant to space, solar, astrophysical, and fusion plasmas. The existing small-scale experiments have been focusing on the single X-line reconnection process either with small effective sizes or at low Lundquist numbers, but both of which are typically very large in natural and fusion plasmas. The design of the FLARE device is motivated to provide experimental access to the new regimes involving multiple X-lines, as guided by a reconnection ``phase diagram'' [Ji \& Daughton, PoP (2011)]. Most of major components of the FLARE device have been designed and are under construction. The device will be assembled and installed in 2016, followed by commissioning and operation in 2017. The planned research on FLARE as a user facility will be discussed. [Preview Abstract] |
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NP12.00103: Abstract Withdrawn
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NP12.00104: Experimental Studies on the 3D Macro- and Microphysics of Magnetic Reconnection Jonathan Jara-Almonte, Hantao Ji, Masaaki Yamada, Jongsoo Yoo, Will Fox, Byungkeun Na 2D magnetic reconnection has been studied for many decades and considerable progress has been made, yet in real plasmas reconnection is fundamentally 3D in nature. Only recently has it become possible to simulate 3D reconnection, and some results have suggested that 3D does not strongly affect the basic properties of reconnection. In contrast, previous experiments have implied that 3D effects could be important even in a quasi-2D system. Here both the (1) macro- and (2) microphysics of 3D reconnection are experimentally studied in order to test the importance of 3D effects. Using fully simultaneous 3D measurements, it is shown that during highly driven reconnection the macroscopic structure of the current sheet can become strongly 3D despite a nearly 2D upstream. Results from new experiments with diagnostics designed to estimate the 3D reconnection rate will be discussed. With regards to (2), the 3D microphysics, new diagnostics capable of measuring fluctuations at frequencies up to the electron cyclotron frequency (300 MHz) have been developed and have identified the presence of very high frequency waves (100 MHz) during asymmetric reconnection, localized to the low-density side. The detailed properties of these waves including the measured dispersion relation will be discussed. [Preview Abstract] |
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NP12.00105: Experiments on the effects of global force balance and local reconnection physics on magnetic reconnection with a guide field W. Fox, F. Sciortino, J. Yoo, J. Jara-Almonte, B. Na, H. Ji, M. Yamada In many plasma environments ranging from astrophysics to fusion, magnetic reconnection occurs with a finite guide field ranging from a fraction to many times the upstream reconnecting component. Theory and simulation yields a range of predictions of scaling of the rate of reconnection with guide field. Recent experiments on the Magnetic Reconnection Experiment observed a systematic decrease in reconnection rates with increasing guide field [1]. Here we present a new set of experimental results on MRX with a controlled applied guide magnetic field ranging from 0 to approximately 3 times the upstream reconnection field, where we observe both global and local processes which affect the reconnection rate in the guide field regime. First, we observe and quantify the effects of global force balance, in particular global back pressure due to pileup of magnetic field in the downstream, which decreases the outflow of plasma from the current sheet and hence the reconnection rate. Second, we study the role of electron pressure in the generalized Ohm's law in the guide field regime and its role in setting the reconnection rate.\\[4pt] [1] T.D. Tharp, M. Yamada, H. Ji, et al, Phys. Rev. Lett. 109, 165002 (2012) [Preview Abstract] |
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NP12.00106: Experimental Observation of Energetic Electrons during Magnetic Island Merging Byungkeun Na, Jongsoo Yoo, Jonathan Jara-Almonte, Will Fox, Masaaki Yamada, Hantao Ji Non-thermal particles have been observed in space as a consequence of magnetic reconnection, but the exact acceleration mechanisms are not well understood. The energization of electrons during magnetic island merging is studied in the Magnetic Reconnection Experiment (MRX). A double-sided electron energy analyzer is developed to simultaneously measure the electron energy distribution in two directions, parallel and anti-parallel to the electron flow. The bias of the selector grid is swept from -30 to 0 V with respect to the floating potential within 1 $\mu s$, comparable to the Alfv\'{e}n time of the typical MRX plasma. Energetic electrons are found inside the magnetic island after island merging is completed. The measured electron tail distribution is well modeled by a high temperature Maxwellian. In the parallel direction, the tail temperature ($\sim $ 28 eV) is found to be up to four times higher than the bulk temperature ($\sim $ 7 eV). In the anti-parallel direction, a negligible tail population is observed. The measured electron energy distribution is discussed in connection with possible electron acceleration mechanisms. [Preview Abstract] |
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NP12.00107: Effects of Guide Field on Plasmoid-Instability-Mediated Turbulent Reconnection Yi-Min Huang, A. Bhattacharjee It has been established that the Sweet-Parker current layer in high Lundquist number reconnection is unstable to the super-Alfvenic plasmoid instability. Past two-dimensional magnetohydrodynamic simulations have demonstrated that the plasmoid instability leads to a new regime where the averaged reconnection rate becomes nearly independent of the Lundquist number. Three-dimensional simulations show that the plasmoid instability can mediate self-generated turbulent reconnection, with energy fluctuations forming elongated eddies along the direction of local magnetic field. In this work we examine the effects of the guide field strength on the self-generated turbulent state and reconnection of the large scale magnetic field. The presence of a guide field provides magnetic shear across the reconnection layer and coupling between different reconnecting planes, which cause elongated turbulent eddies to change direction across the reconnection layer and significantly influence the energy cascade process. In contrast, a complete lack of guide field and coupling allows energy cascade to smaller scales along the out-of-plane direction. While the characteristics of turbulence vary, reconnection rate remains a robust feature which is not significantly affected by the guide field strength. [Preview Abstract] |
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NP12.00108: Decoupling of double-tearing resonant layers by sheared flows Stephen Abbott, Kai Germaschewski Double-tearing modes consist of two resonant, reconnecting layers of the same mode number coupled together by an ideal MHD outer region. Linearly this interaction can result in faster growth as the two layers drive each other. Nonlinearly it may lead to explosive releases of energy, and is a possible driver for off-axis sawtooth crashes in advanced tokamaks. Recent work has shown that differential rotation effects, such as equilibrium sheared flows or diamagnetic drifts, can decouple the DTM layers leaving two drifting, single tearing modes. These isolated tearing layers are slower growing and easier to stabilize. Understanding and producing this decoupling is thus an important element of preventing disruptive DTM activity. In this work we present progress on developing an analytic theory of DTM decoupling. We show that the application of equilibrium sheared flows mixes the symmetric and antisymmetric DTM eigenmode solutions, reducing the growth rate. This representation predicts a linear relationship between the growth rate and the amplitude of differential sheared flow needed to decouple the layers, which we confirm with linear MHD simulations. Through numerical scaling studies we examine the relationship between mode decoupling and the slab-kink mode underlying DTM growth. [Preview Abstract] |
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NP12.00109: Experimental and theoretical developments in the Mochi project Setthivoine You, Jens von der Linden, Keon Vereen, Eric Sander Lavine, Evan Carroll, Alexander Card, Manuel Azuara-Rosales, Morgan Quinley, Gunsu Yun The Mochi project investigates the interaction between magnetic fields and plasma flows in cylindrical and toroidal geometries. The configuration is designed to tailor the radial electric field profile with three annular electrodes and allow for shear helical flows in magnetized plasma jets or merging spheromaks. First plasma has been achieved and characterization is in progress with images, magnetic probes, an energy analyzer, an interferometer, a fast ion gauge, and optical and RF spectroscopy. Vector tomography of ion Doppler spectroscopy is progressing with the design of the custom fiber bundle and implementation of the numerical code. The first experiments are investigating the coupling of sausage and kink instabilities, comparing measurements to a new stability criterion and a numerical stability code. A new canonical field theory has been developed to help interpret the dynamics of plasma self-organization. The theory augments the Lagrangian of general dynamical systems to rigourously demonstrate that canonical helicity transport is valid across single particle, kinetic and fluid regimes, that dynamical equations can be re-formulated as a form of Maxwell's equations, and that helicity is conserved only when density gradients are shallow. [Preview Abstract] |
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NP12.00110: Reduced Extended MHD P.J. Morrison, H.M. Abdelhamid, D. Grasso, R.D. Hazeltine, M. Lingam, E. Tassi Over the years various reduced fluid models have been obtained for modeling plasmas, with the goal of capturing important physics while maintaining computability. Such models have included the physics contained in various generalizations of Ohm's law, including Hall drift and electron inertia. In a recent publication [1] it was shown that full 3D extended MHD is a Hamiltonian system by finding its noncanonical Poisson bracket. Subsequently, this bracket was shown to be derivable from that for Hall MHD by a series of remarkable transformations [2], which greatly simplifies the proof of the Jacobi identity and allows one to immediately obtain generalizations of the helicity and cross helicity. In this poster we use this structure to obtain exact reduced fluid models with the effects of full two-fluid theory. Results of numerical computations of collisionless reconnection using an exact reduced 4-field model will be presented and analytical comparisons of mode structure of previous reduced models will be made. \\[4pt][1] H.M.~Abdelhamid, Y.~Kawazura \& Z.~Yoshida, J.~ Phys.~A {\bf48}, 235502 (2015) \\ \noindent [2] M.~Lingam, P.J.~Morrison \& G.~Miloshevich, Phys. Plasmas {\bf22}, 072111 (2015) [Preview Abstract] |
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NP12.00111: Viscous dissipation and radiative transport in magnetic reconnection of collisionless magnetized plasma Gunsu Yun, Jeong-young Ji, Shekar Thatipamula Viscous dissipation rate of magnetic field energy due to wave-like fluctuations in collisionless magnetized plasma is obtained analytically using the exact integral closure for electron fluid viscosity [Ji, Phys. Plasmas 21 (2014)]. For typical high-temperature tokamak plasma, the viscous resistivity is several orders larger than the Spitzer (collisional) resistivity. For magnetic reconnection, it is also found that the radiative transport (i.e. Poynting flux) of the field energy of Alfven waves [Bellan, Phys. Plasmas 5, 3081 (1998)] is comparable to the viscous dissipation. The viscous dissipation is more effective for shorter wavelength fluctuation. The importance of viscous dissipation is supported by broadband emission and chirping-down phenomena observed in the ion cyclotron harmonic frequency range at the crash onset of edge-localized mode on the KSTAR tokamak. [Preview Abstract] |
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NP12.00112: ASTROPHYSICAL AND SPACE PLASMAS |
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NP12.00113: Three-dimensional magnetized and rotating hot plasma equilibrium in a gravitational field Peter J. Catto, Sergei I Krasheninnikov, Istvan Pusztai We present analytic and numerical solutions for three-dimensional magnetized axisymmetric equilibria confining rotating hot plasma in a gravitational field. Our solution to the full Shafranov-Grad equation can exhibit strong equatorial plane localization of the plasma density and current, resulting in disk equilibria for the plasma density. We find that a toriodal magnetic field is sometimes necessary to find a equilibrium in the presence of gravity for many cases of interest. [Preview Abstract] |
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NP12.00114: Particle acceleration in magnetically dominated reconnection in the presence of an external guide field Fan Guo, Hui Li, William Daughton, Xiaocan Li, Yi-Hsin Liu Recent studies have shown that magnetically dominated reconnection leads to strong energy release and nonthermal particle acceleration. Here we report results from 2D fully kinetic simulations of magnetic reconnection in guide field geometry, within the magnetically dominated regime. While the Fermi-like process driven by particle curvature drift motion is the main acceleration mechanism for the nearly anti-parallel geometry, acceleration by parallel electric field dominates particle energization in the strong guide field regime.~ Although the mechanism of particle acceleration is strongly modified, the simulations reveal that nonthermal power-law distributions still develop for a range of magnetic shear angles. This implies that power-law distributions are a robust feature of particle acceleration in a magnetically dominated reconnection layer. We also present a quantitative model to explain the observed spectral index. [Preview Abstract] |
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NP12.00115: Measuring Magnetic Fields in Photoionized Interstellar Plasmas (HII Regions) Steven Spangler, Allison Costa Hot luminous stars photoionize the interstellar gas around them, creating plasmas with a very high ionization fraction. In astronomical terminology, these are called HII regions. They are dynamic plasmas, expanding due to overpressure with respect to the interstellar medium. We are making diagnostic measurements to determine the strength and structure of magnetic fields in these objects. This paper presents our results on the Rosette Nebula. We diagnose the magnetic field in the Rosette by measurements of Faraday rotation on lines of sight passing through the nebula. These measurements are made with the Very Large Array radio telescope of the National Radio Astronomy Observatory. We have measurements of the rotation measure for 18 lines of sight. Values of the mean, line of sight component of the magnetic field range from about 3 to 5 microGauss. We will discuss comparison of these measurements with models for modification of the interstellar magnetic field by an HII region. [Preview Abstract] |
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NP12.00116: Regimes of transport (T-conduction) in IGM plasmas Mikhail Medvedev Galaxy clusters are the largest gravitationally bound systems of the universe. They contain large amounts (about $10\%$ by mass, whereas $90\%$ is dark matter) of thermal, $T\sim$~few keV, plasmas. Thermal conduction in this intergalactic medium (IGM) is thought to play crucial role in plasma dynamics but its value is debated. Unlike collisional plasmas, energy transport in magnetized collisionless or weakly collisional plasmas of the IGM exhibit various regimes with vastly different values of the thermal conduction coefficient. Here we discuss these regimes and their implication for galaxy cluster physics. [Preview Abstract] |
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NP12.00117: Modeling electron-positron plasma production around supermassive BHs in AGNs Alex Ford, Brett Keenan, Mikhail Medvedev We develop a numerical code for modeling the electron-positron plasma production in magnetic fields around supermassive spinning black holes (BH) in order to explore its role in launching relativistic jets in active galactic nuclei (AGN). We demonstrate that plasma production is sensitive to the spectrum of the ambient photon field, e.g., from an accretion disc. Therefore, we use observed photon spectra to make realistic models of specific astrophysical systems, including the Galactic Center BH (Sagittarius A*). We discuss the results and make observational predictions. [Preview Abstract] |
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NP12.00118: Influence of plasma instabilities on interpenetrating plasma clouds as a test for electromagnetic dark matter self-interactions N. Shukla, B. Feldstein, J. Mardon, K. Schoeffler, J. Vieira, L. Silva Dark matter (DM) could be charged under its own ``dark electromagnetism'' (DEM), behaving like a cold collisionless plasma of self-interacting DM~particles. Under this hypothesis, DM could exhibit plasma-like instabilities [1] with observational consequences. We investigate this via PIC simulations [2], exploring the instabilities driven by the interpenetration of two e-e$+$ plasma clouds that mimic the ``dark plasma.'' We show that the clouds slow down mostly due to~Weibel generated magnetic fields, which deflect the particle trajectories, such that particles acquire transverse momentum, thus leading to an isotropic velocity~distribution. This process causes the flow velocity to decrease approximately by 1/2 in a time interval $\Delta $t~$=$ 1/$\surd \alpha \Delta $v(c/v), where $\alpha $ is the equipartition parameter, v~the initial flow speed and c the light speed, comparable with the plasma instability growth time. We show that if the typical DM slab length is much longer than v$\Delta $t, DM particles are always expected to slow~down by a factor of about 1/2. Comparison with astronomical observations may yield new constraints on DEM. \\[4pt] [1] L. Ackerman et al~~Phys.Rev. D79 023519 (2009)(2006).\\[0pt] [2] R. A. Fonseca et al., LNCS 2331, 342 (2002). [Preview Abstract] |
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NP12.00119: Tridimensional Burning Structures Associated with Anisotropic Thermal Conductivities in Magnetically Confined and Pulsar Plasmas A. Cardinali, B. Coppi, G. Sonnino A surprising result of the most recent theory [1] of the thermonuclear instability, which can take place in D-T plasmas close to ignition, is that it can develop with tridimensional structures emerging from an axisymmetric toroidal confinement configurations. These structures are helical filaments (``snakes'') that are localized radially around a given rational magnetic surface [2]. Until now well known analyses of fusion burning processes in magnetically confined plasmas, that include the thermonuclear instability, have been carried out by 1+1/2 D transport codes [3] and, consequently, the onset of tri-dimensional structures has not been investigated. The importance of the electron thermal conductivities anisotropy is pointed out also for the inhomogeneous thermonuclear burning of plasmas on the surface of pulsars and for the formation of the observed bright spots on some of them.\\[4pt] [1] COPPI B., Presentation at the Sherwood Theory Meeting, San Diego, CA, April 2014.\\[0pt] [2] COPPI B., \textit{et al}., \textit{Nucl. Fus}. \textbf{55}, (2015) 053011.\\[0pt] [3] AIROLDI, A., CENACCHI G., \textit{Nucl. Fus}. \textbf{37}, (1997) 1117. [Preview Abstract] |
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NP12.00120: Faraday rotation of plasmas in the vicinity of a Schwarzschild black hole Felipe Asenjo, Chinmoy Bhattacharjee, Swadesh Mahajan The propagation of an electromagnetic wave in a multi-specie plasmas (ion-electron and ion-electron-positron), embedded in the gravitational field of a Schwarzschild black hole, is investigated with particular emphasis on studying the Faraday rotation (rotation of the phase angle of the right and left-handed components of wave). In order to appropriately deal with the strong gravitational field (affecting the plasma in the proximity of the black hole horizon), we employ Rindler coordinates in the 3$+$1 decomposition of general relativity. The rather complex dispersion relation for high-frequency electromagnetic waves reveals the dependence of Faraday rotation on the number density of different constituents of the multi-specie plasma, the background magnetic field, and the mass of the black hole. Amongst other things, the expression for the Faraday rotation allows us to determine the black hole mass if the number density and magnetic field strength are estimated, and the rotation of the phase angle is measured. It is also shown how Faraday rotation could be harnessed to infer black hole features in a more complete theory that pertains, for example, to Kerr black holes. Different astrophysical implications are pointed out. [Preview Abstract] |
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NP12.00121: Concomitant generation of outflows and magnetic fields via Hall magnetohydrodynamics Swadesh M. Mahajan, Manasvi Lingam Conventional large-scale dynamo models rely on ideal magnetohydrodynamics (MHD) and generate magnetic fields. We use Hall MHD as our physical model, enabling us to treat flows and fields concomitantly. We show that this results in a mixture of large-scale magnetic and kinetic energies produced simultaneously, and the latter is manifest as outflows/jets. The large-scale Alfven Mach number, an observable quantity, is intimately linked to its small-scale (turbulent) counterpart. In the limiting case where this quantity is much greater than unity, outflows are preferentially generated from a predominantly magnetic reservoir and vice-versa. We suggest that this phenomenon, the Dynamo-Reverse Dynamo mechanism, may explain the existence of astrophysical outflows with high values of observed large-scale Alfven Mach numbers.\\[4pt] [1] M. Lingam \& S.M. Mahajan, MNRAS, {\bf 449}, L36 (2015) \\[0pt] [2] S.M. Mahajan, N.L. Shatashvili, S.V. Mikeladze \& K.I. Sigua, ApJ, {\bf 634}, 419 (2005) [Preview Abstract] |
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NP12.00122: Beltrami state in black-hole accretion disk: A Magnetofluid approach Chinmoy Bhattacharjee, Rupam Das, David J. Stark, S.M. Mahajan We examine electron-ion Beltrami states in a black hole accretion disk (both Schwarzchild and Kerr) using magnetofluid unification in order to delineate the types of plasma behavior accessible in these geometries. Following the same assumptions as Hall MHD, we find both that the curvature in the spacetime radically alters the magnetic/velocity decay rate and that the predicted geodesic velocity profiles for particles solely constrained by gravity also deviate substantially from our solutions. These departures suggest that these systems are governed by the rich interplay of plasma dynamics and general relativity. Furthermore, the relationship between the helicities of each species introduces a new oscillatory length scale into the system that is strongly influenced by relativistic and thermal effects. [Preview Abstract] |
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NP12.00123: New Mexico Dynamo Experiment: an Experiment to Demonstrate $\alpha\omega$-dynamos in Accretion Disks Jiahe Si, Richard Sonnenfeld, Author Colgate, Joe Martinic, Mark Nornberg, Hui Li The New Mexico Liquid Metal $\alpha\omega$-dynamo experiment uses two coaxial cylinders to generate Couette flows to simulate the differential rotation of accretion disks. By varying the speed ratio of both cylinders, the flow can be made stable or unstable. Experimental research in both regions have made contributions to our knowledge of the dynamo mechanism. In the stable region, we have demonstrated an 8-fold $\omega$-amplification by minimizing turbulence. In unstable region, we used two methods to study the effect of the turbulence. The first method is to measure the omega-effect over a range of Magnetic Reynolds numbers ($17 < Rm < 86$) by varying the cylinder rotation rate. The second method is to measure the decay time of the axial field Bz over the same range of Rm's. By using both methods, we have demonstrated that at very high Reynolds number ($>5\times10^6$) rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations. The experimental apparatus is being upgraded. The upgrade is intended to demonstrate a complete $\alpha\omega$-dynamo, as well as obtain more details to understand the dynamo in highly turbulent Couette flow. [Preview Abstract] |
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NP12.00124: Distinguishing the Magnetorotational Instability (MRI) from Magnetized Ekman Flows in the PPPL MRI Experiment Erik Gilson, Kyle Caspary, Jeremy Goodman, Hantao Ji, Ethan Schartman, Xing Wei Results are presented from initial experiments on the upgraded Magnetorotational Instability (MRI) experiment that uses GaInSn as the working fluid and now operates with conductive end caps to improve the coupling of angular momentum to the fluid to increase the saturation amplitude of the MRI signal. Measurements of the fluid velocity field and perturbed magnetic field over a range of magnetic Reynolds numbers, $Rm$, and Lundquist numbers, $S$, are compared with results from the SFEMaNS code in order to separate the effects of MRI on the system from effects such as Ekman flows and Shercliff layer instabilities. The MRI can be identified by observing its growth rate, noting the relative magnitudes and spatial distributions of the perturbed radial flow velocity $u_r$ and radial magnetic field $B_r$, and measuring the scaling of $u_r$ and $B_r$ with $Rm$. The clear identification of the onset of MRI in the apparatus is complicated by the geometry and boundary conditions creating an imperfect supercritical pitchfork bifurcation. Nevertheless, a stability diagram can be created that shows that MRI is a weak-field instability that occurs only below a certain value of the normalized magnetic field $S/Rm$ but above a threshold where viscous effects damps the growth of the instability. [Preview Abstract] |
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NP12.00125: Flow Optimization in the Princeton MRI Experiment and Zonal Flow Generation in HTX Kyle Caspary, Michael Burin, Erik Gilson, Jeremy Goodman, Hantao Ji, Michael McNulty, Ethan Schartman, Peter Sloboda, Xing Wei The Princeton Magneto-Rotational Instability (MRI) experiment and the Hydrodynamic Turbulence Experiment (HTX) are a pair of modified Taylor-Couette devices which explore rotating magnetohydrodynamic and hydrodynamic flows. The Princeton MRI experiment uses a GaInSn working fluid and was designed to study the MRI, which is believed to be the mechanism responsible for the rapid accretion rate observed in some magnetized accretion disks. The experiment utilizes ultrasound Doppler velocimetry to measure velocity profiles and a newly installed suite of hall sensors on the inner and outer cylinders to characterize the magnetic field. Results are presented from experiments which seek to optimize the flow by varying the inner ring speed for a given magnetic field strength. In HTX, we explore the generation of zonal flows from turbulence by flow jets with water as the working fluid. Laser Doppler velocimetry is used to measure the mean and fluctuating velocity. The generation of anisotropic mean flow by means of beta plane turbulence is investigated through the use of a sloped end-cap. The impact of varying the end cap slope, fluid height and jet flow rate will be discussed. [Preview Abstract] |
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NP12.00126: Generation of large-scale magnetic fields by small-scale dynamo in shear flows Jonathan Squire, Amitava Bhattacharjee A new mechanism for turbulent mean-field dynamo is proposed, in which the magnetic fluctuations resulting from a small-scale dynamo drive the generation of large-scale magnetic fields. This is in stark contrast to the common idea that small-scale magnetic fields should be harmful to large-scale dynamo action. These dynamos occur in the presence of large-scale velocity shear and do not require net helicity, resulting from off-diagonal components of the turbulent resistivity tensor as the magnetic analogue of the ``shear-current'' effect. The dynamo is studied using a variety of computational and analytic techniques, both when the magnetic fluctuations arise self-consistently through the small-scale dynamo and in lower Reynolds number regimes. Given the inevitable existence of non-helical small-scale magnetic fields in turbulent plasmas, as well as the generic nature of velocity shear, the suggested mechanism may help to explain generation of large-scale magnetic fields across a wide range of astrophysical objects. [Preview Abstract] |
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NP12.00127: Status of the Wisconsin Plasma Astrophysics Laboratory John Wallace, Mike Clark, Doug Endrizzi, Ken Flanagan, Jason Milhone, Ethan Peterson, Joseph Olson, Aaron Stemo, Dave Weisberg, Jan Egedal, Cary Forest The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a facility that now encompasses a collection of novel plasma astrophysics experimental configurations. In the MPDX configuration large, un-magnetized, fast flowing, hot plasma is being used to investigate a variety of flow driven MHD instabilities. The experiment is 3 meters in diameter and utilizes a permanent magnet multicusp plasma confinement. Five 20KW, 2.45 GHz, CW magnetrons produce electron cyclotron heating for plasma generation. Ten lanthanum hexaboride (LaB6) stirring rods and molybdenum anodes are inserted into the vessel to produce JxB flows. The chamber has a variety of multiuse ports, and is able to split open to allow experimental apparatus to be inserted. This poster will describe recent improvements to the laboratory. We will also provide an overview of existing and future experimental configurations including: reconnection (TREX); acoustic and Alfven wave propagation in connection with helioseismology; pulsar and stellar wind launching from a rotating dipolar magnetosphere; jet formation and propagation into background plasma; and small-scale, high power helicity injection. [Preview Abstract] |
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NP12.00128: Astrophysically Relevant Dipole Studies at WiPAL Douglass Endrizzi, Cary Forest, John Wallace A novel terrella experiment is being developed to immerse a dipole magnetic field in the large, unmagnetized, and fully ionized background plasma of WiPAL (Wisconsin Plasma Astrophysics Lab). This allows for a series of related experiments motivated by astrophysical processes, including (1) inward transport of plasma into a magnetosphere with focus on development of Kelvin-Helmholtz instabilities from boundary shear flow; (2) helicity injection and simulation of solar eruptive events via electrical breakdown along dipole field lines; (3) interaction of Coronal Mass Ejection-like flows with a target magnetosphere and dependence on background plasma pressure; (4) production of a centrifugally driven wind to study how dipolar magnetic topology changes as closed field lines open. A prototype has been developed and preliminary results will be presented. An overview of the final design and construction progress will be given. [Preview Abstract] |
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NP12.00129: Optical Emission Spectroscopy in an Unmagnetized Plasma Jason Milhone, Christopher Cooper, Victor Desangles, Mark Nornberg, Blair Seidlitz, Cary Forest An optical emission spectroscopic analysis has been developed to measure electron temperature, neutral burnout, and Zeff in Ar and He plasmas in the Wisconsin plasma astrophysics laboratory (WiPAL). The WiPAL vacuum chamber is a 3 meter diameter spherical vessel lined with 3000 SmCo permanent magnets (B $>$ 3 kG) that create an axisymmetric multi-cusp ring for confining the plasma. WiPAL is designed to study unmagnetized plasmas that are hot ($T_e > 10$ eV), dense ($n_e > 10^{18}$), and with high ionization fraction. Electron temperature and density can be measured via Langmuir probes. However, probes can disturb the plasma, be difficult to interpret, and become damaged by large heat loads from the plasma. A low cost non-invasive spectroscopy system capable of scanning the plasma via a linear stage has been installed to study plasma properties. From the neutral particle emission, the neutral burnout and estimated neutral temperature can be inferred. A modified coronal model with metastable states is being implemented to determine Te for Ar plasmas. [Preview Abstract] |
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NP12.00130: Plasma MRI Experiments at UW-Madison K. Flanagan, M. Clark, V. Desangles, R. Siller, J. Wallace, D. Weisberg, C.B. Forest Experiments for driving Keplerian-like flow profiles on both the Plasma Couette Experiment Upgrade (PCX-U) and the Wisconsin Plasma Astrophysics Laboratory (WiPAL) user facility are described. Instead of driving flow at the boundaries, as is typical in many liquid metal Couette experiments, a global drive is implemented. A large radial current is drawn across a small axial field generating torque across the whole profile. This global electrically driven flow is capable of producing profiles similar to Keplerian flow. PCX-U has been purposely constructed for MRI experiments, while similar experiments on the WiPAL device show the versatility of the user facility and provide a larger plasma volume. Numerical calculations show the predicted parameter spaces for exciting the MRI in these plasmas and the equilibrium flow profiles expected. In both devices, relevant MRI parameters appear to be within reach of typical operating characteristics. [Preview Abstract] |
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NP12.00131: Magnetic Diagnostics at the Wisconsin Plasma Astrophysics Laboratory Ethan Peterson, Michael Clark, Jan Egedal, John Wallace, David Weisberg, Cary Forest A flexible suite of magnetic diagnostics is being developed to measure low and high frequency magnetic fields, the 3-D magnetic field structure throughout the plasma volume, and the 2-D structure (polar and azimuthal fields) on the surface of the sphere. The internal 3-D structure is ascertained by scanning insertion probes with high sensitivity, high bandwidth, 3-axis hall effect sensors. Careful engineering of these insertion probes is required to effectively remove the heat load while simultaneously maintaining high performance (hot, dense, steady state) plasmas. A surface array of 3-axis hall-effect sensors and 2-axis flux loops will provide 3-D, low frequency magnetic field measurements as well as high frequency fluctuations in the polar and azimuthal directions due to plasma waves. This surface array can be used to observe the spatial structure of global modes such as spherical ion acoustic waves and can provide insight into the structure and magnitude of internal plasma flows. The engineering and capabilities of these diagnostics is the focus of this poster. [Preview Abstract] |
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NP12.00132: Electrically driving large magnetic Reynolds number flows on the Madison plasma dynamo experiment David Weisberg, John Wallace, Ethan Peterson, Douglass Endrezzi, Cary B. Forest, Victor Desangles Electrically-driven plasma flows, predicted to excite a large-scale dynamo instability, have been generated in the Madison plasma dynamo experiment (MPDX), at the Wisconsin Plasma Astrophysics Laboratory. Numerical simulations show that certain topologies of these simply-connected flows may be optimal for creating a plasma dynamo and predict critical thresholds as low as $Rm_{crit}=\mu_0\sigma LV=250$. MPDX plasmas are shown to exceed this critical $Rm$, generating large ($L=1.4$\,m), warm ($T_e>10$\,eV), unmagnetized ($M_A>1$) plasmas where $Rm<600$. Plasma flow is driven using ten thermally emissive LaB$_6$ cathodes which generate a $J\times B$ torque in Helium plasmas. Detailed Mach probe measurements of plasma velocity for two flow topologies will be presented: edge-localized drive using the multi-cusp boundary field, and volumetric drive using an axial Helmholtz field. Radial velocity profiles show that edge-driven flow is established via ion viscosity but is limited by a volumetric neutral drag force ($\chi \sim 1/(\nu\tau_{in})$), and measurements of velocity shear compare favorably to Braginskii transport theory. Volumetric flow drive is shown to produce stronger velocity shear, and is characterized by the radial potential gradient as determined by global charge balance. [Preview Abstract] |
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NP12.00133: Construction of a 100kW Electron Cyclotron Resonant Heating (ECRH) system on the Madison Plasma Dynamo Experiment (MPDX) M.M. Clark, J. Milhone, P. Nonn, J.P. Wallace, C.B. Forest A system of five 20 kW magnetrons is being installed for the Madison Plasma Dynamo Experiment (MPDX) to produce and heat the plasma with RF energy. Each magnetron will receive 2.5A of 14kV DC power. The source of the DC power is from a 240V three phase line which is transformed to high voltage, rectified, and processed through a series modulator regulator circuit. The RF is transmitted to the vessel via WR284 waveguide. The actions taken to develop the DC power source will be discussed and illustrated. The vessel of MPDX is a 3 meter diameter sphere comprised of two nearly identical hemispherical shells of 1.25'' thick cast aluminum. 36 Rings of SmCo magnets attached to the inner vessel surface create a cusp field to contain the plasma and provide a resonance surface for the RF. [Preview Abstract] |
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NP12.00134: Characterization of High Beta Boundary Driven Spherical Flows Robert Siller, Vladimir Mirnov, Cary Forest The Madison Plasma Dynamo Experiment (MPDX) has started to investigate a new regime of plasma characteristics with the application of a variable strength Helmholtz field with the same axis as the rotation profile. We examine the theoretical fluid response of the fully compressible, isothermal plasma in a wide range of parameters applicable to MPDX in a full spectral code, assuming axisymmetric flows. To model the experiment, we look at both a specified velocity boundary condition, and a specified current distribution to model the experimental flows. We look at the dependence on plasma beta of the omega effect and the suppression of driven poloidal circulation. We look at a generalization of Ferraro's Theorem of iso-rotation and its applicability to non-ideal spherical flows, and the motion towards validity in the limit of strong applied field. [Preview Abstract] |
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NP12.00135: A Model of the Heliosphere with Jets James Drake, Marc Swisdak, Merav Opher The conventional picture of the heliosphere is that of a comet-shaped structure with an extended tail produced by the relative motion of the sun through the local interstellar medium. Recent magnetohydrodynamic (MHD) simulations of the heliosphere have revealed that the heliosphere drives magnetized jets to the north and south similar to those driven by the Crab Nebula. That the sun's magnetic field can drive such jets when $\beta=8\pi P/B^2\gg 1$ in the outer heliosphere is a major surprise. An analytic model of the heliosheath (HS) is developed in the limit in which the interstellar flow and magnetic field are neglected. The heliosphere in this limit is axi-symmetric and the overall structure of the HS is controlled by the solar magnetic field even for very high $\beta$. The tension of the solar magnetic field produces a drop in the total pressure between the termination shock and the HP. This same pressure drop accelerates the plasma flow into the north and south directions to form two collimated jets. MHD simulations of the global heliosphere embedded in a stationary interstellar medium match well with the analytic model. Evidence from the distribution of energetic neutral atoms from the outer heliosphere from IBEX and CASSINI supports the picture of a heliosphere with jets. [Preview Abstract] |
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NP12.00136: Stochastic Heating and Diffusion in the Near-Sun Solar Wind Kristopher Klein, Benjamin Chandran, Sofiane Bourouaine A plethora of mechanisms have been proposed to explain the observed temperature profiles of solar wind plasma in the near-Sun environment. In this work, we study the efficacy of one mechanism, stochastic ion heating, in reproducing these observations. We find, through evaluation of the gyroaveraged kinetic equation with an additional stochastic diffusion term using a fast solar wind model for magnetic and velocity magnitudes, that stochastic heating is capable of producing realistic perpendicular temperature profiles. Additionally, we calculate the evolution of the proton velocity distribution and show that it significantly deviates from a Maxwellian. In its core, the distribution is significantly flattened, while the tails are significantly steeper when compared to a Maxwellian fit to the distribution. We can test for the presence of stochastic heating by looking for these signatures in the Helios data set as well as in upcoming observations from Solar Probe Plus and Solar Orbiter. [Preview Abstract] |
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NP12.00137: Towards a global model of equilibrium solar behavior: the tachocline as a trans-Alfvenic feature Lee Gunderson, Amitava Bhattacharjee, Luca Guazzotto Observations and simulations suggest that thermal wind balance holds to lowest order in the solar convection zone (SCZ), and an analytic model of solar rotation has been developed by further assuming a functional relationship between the entropy and rotation. Below the SCZ, analytic arguments and simulations have implicated the necessity of a large poloidal field in the deep solar interior for the thin structure of the tachocline. We seek to unify these pictures and find a global equilibrium base state of solar behavior by starting from the exact solution to axisymmetric ideal MHD, the generalized Grad-Shafranov equation, thereby including magnetic field and poloidal flow from the onset. We find that a tachocline-like structure naturally arises as an equilibrium feature if the poloidal Alfv\'{e}nic Mach number approaches unity near the bottom of the SCZ. FLOW, a code developed to analyze tokamak equilibrium with arbitrary flow, has been adapted for use in the solar regime and used to examine effects of magnetic field and poloidal flow on thermal wind balance. The next steps include: including the Alfv\'{e}nic transition, establishing linear stability, and considering the effects of stresses and dissipation. [Preview Abstract] |
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NP12.00138: Cosmic Ray Transport with Magnetic Focusing and the ``Telegraph'' model Roald Sagdeev, Mikhail Malkov Cosmic rays (CR), scattered by MHD waves, must propagate diffusively. However, because some of the particles diffuse unrealistically fast, an alternative CR transport model based on the ``telegraph'' equation was put forward. Though, its derivations often lack rigor and transparency leading to inconsistent results. We apply the Chapman-Enskog method to the CR transport. No ``telegraph'' $\partial^2f/\partial t^2$ term emerges in a proper $t\gg1$ asymptotic expansion. Nevertheless, this term may be \emph{converted} from the $\partial^4f/\partial z^4$ term of that expansion. However, both the telegraph and hyperdiffusive terms are important only for a short relaxation period associated with the initial CR anisotropy/inhomogeneity. Then, the system evolves diffusively in both cases. The term conversion is possible only \emph{after} this relaxation period. \emph{During} this period, the telegraph solution is argued to be unphysical. Unlike the hyperdiffusion correction, it is not uniformly valid and introduces implausible \emph{singular} components to the solution. These dominate the solution during the relaxation period. Because they are shown not to be inherent in the underlying scattering problem, the telegraph term is involuntarily acquired in an asymptotic reduction. [Preview Abstract] |
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NP12.00139: The Role of Nonlinear Wave Processes in Saturating Instabilities with Frequencies between the Cyclotron Frequencies Chris Crabtree, Gurudas Ganguli, Erik Tejero, Leonid Rudakov Induced nonlinear scattering by particles and nonlinear 3-wave resonant processes redistributes (or scatters) wave action in frequency and wave-vector space. Since the rate of scattering increases with wave amplitude, there exists a point in time when the waves are scattered at the same rate that they are generated by the linear instability. Then saturation of the linear instability may occur, which cannot be described by quasi-linear theory. Failure to account for this nonlinear effect in the evolution of the instability leads to larger amplitude waves and rapid diffusion of the particles. We investigate this effect through particle-in-cell codes, analytical theory, wave kinetic simulations, analysis of \textit{in-situ} spacecraft data, and dedicated laboratory experiments. We focus on two phenomena important to the dynamics of the Earth's magnetosphere: 1) the velocity ring instability (VRI) which generates lower-hybrid waves, and 2) chorus generated whistler-mode waves with a chirping frequency. We show that consideration of NL scattering plays a vital role in explaining the saturated state. We focus on the role of wave-particle trapping, wave-propagation effects, and energetic particle modification of the real part of the dispersion relation. [Preview Abstract] |
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NP12.00140: Particle energization and current sheets in Alfvenic plasma turbulence Kirit Makwana, Hui Li, Fan Guo, William Daughton, Fausto Cattaneo Plasma turbulence is driven by injecting energy at large scales through stirring or instabilities. This energy cascades forward to smaller scales by nonlinear interactions, described by magnetohydrodynamics (MHD) at scales larger than the ion gyroradius. At smaller scales, the fluid description of MHD breaks down and kinetic mechanisms convert turbulent energy into particle energy. We investigate this entire process by simulating the cascade of strongly interacting Alfven waves using MHD and particle-in-cell (PIC) simulations. The plasma beta is varied and particle heating is analyzed. Anisotropic heating of particles is observed. We calculate the fraction of injected energy converted into non-thermal energy. At low beta we obtain a significant non-thermal component to the particle energy distribution function. We investigate the mechanisms behind this acceleration. The velocity distribution function is correlated with the sites of turbulent current sheets. The different dissipative terms due to curvature drift, gradB drift, polarization drifts, and parallel current density are also calculated. This has applications for understanding particle energization in turbulent space plasmas. [Preview Abstract] |
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NP12.00141: Nonthermally Dominated Electron Acceleration during Magnetic Reconnection in a Low-$\beta$ Proton-Electron Plasma Xiaocan Li, Fan Guo, Hui Li, Gang Li By means of fully kinetic simulations, we investigate electron acceleration during magnetic reconnection in a nonrelativistic proton-electron plasma with conditions similar to solar corona and flares. We demonstrate that reconnection leads to a nonthermally dominated electron acceleration with a power-law energy distribution in the nonrelativistic low-$\beta$ regime but not in the high-$\beta$ regime. A guiding-center current description is used to reveal the role of electron drift motions during the bulk nonthermal energization. We find that the main acceleration mechanism is a Fermi-type acceleration accomplished by the particle curvature drift motion along the electric field induced by the reconnection outflows. Although the acceleration mechanism is similar for different plasma $\beta$ regime, the reconnection in the low-$\beta$ regime drives much faster electron energization because of the faster Alfv\'enic outflows. The nonthermally dominated acceleration resulting from magnetic reconnection in the low-$\beta$ regime may have strong implications to the highly efficient electron acceleration in solar flares and other astrophysical systems. [Preview Abstract] |
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NP12.00142: Double-reconnected magnetic structures driven by Kelvin-Helmholtz vortices at the Earth's magnetosphere Matteo Faganello, Dario Borgogno, Francesco Califano, Francesco Pegoraro In an almost collisionless MagnetoHydrodynamic plasma in a relatively strong magnetic field, stresses can be conveyed far from the region where they are exerted e.g., through the propagation of Alfv\`en waves. The forced dynamics of line-tied magnetic structures in solar and stellar coronae is a paradigmatic case. We investigate how this action at a distance develops from the equatorial region of the Kelvin-Helmholtz unstable flanks of the Earth's magnetosphere leading to the onset, at mid latitude in both hemispheres, of correlated double magnetic field line reconnection events that can allow the solar wind plasma to enter the Earth's magnetosphere. This {\it mid-latitude double reconnection process}, first investigated in [1], has been confirmed here by following a large set of individual field lines using a method similar to a Poincar\`e map.\\[4pt] [1] M. Faganello, {et al.}, {\it Europhys. Letters}, {\bf 100}, 69001 (2012)\\[0pt] [2] D. Borgogno, {et al.}, {\it Phys. Plasmas}, {\bf 22}, 032301 (2015) [Preview Abstract] |
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NP12.00143: Parametric Excitation of Very Low Frequency (VLF) Electromagnetic Whistler Waves by Transformation of Lower Oblique Resonance Waves on Density Perturbations in the Vicinity of a Loop VLF Antenna T. Kim, V. Sotnikov, D. Main, E. Mishin, N. Gershenzon Concept of a parametric antenna in the ionospheric plasma is analyzed. Such antennas are capable of exciting electromagnetic radiation fields, specifically the creation of whistler waves generated at the very low frequency (VLF) range, which are also capable of propagating large distances away from the source region. The mechanism of whistler wave generation is considered a parametric interaction of quasi-electrostatic low oblique resonance (LOR) oscillations excited by conventional loop antenna. The transformation of LOR waves on quasi-neutral density perturbations generated by a dipole antenna gives rise to electromagnetic whistler waves on combination frequencies. In this approach extended plasma volume around a loop antenna represents a parametric antenna. Simulation to demonstrate excitation and spatial structure of VLF waves excited by a loop antenna using a PIC code LSP will be presented as well. Possible applications including the wave-particle interactions to mitigate performance anomalies of Low Earth Orbit (LEO) satellites, active space experiments, communication via VLF waves, and modification experiments in the ionosphere will be discussed. [Preview Abstract] |
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NP12.00144: ABSTRACT WITHDRAWN |
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NP12.00145: A New Facility at AFRL for Studying VLF Waves in A Magnetized Plasma Mark Hopkins, Nate Zechar, Daniel Main, James Caplinger, Tony Kim, Vladimir Sotnikov An experimental facility was designed and built in the Plasma Physics Sensors Laboratory (PPSL) at AFRL for the study of very low frequency (VLF) wave generation and propagation in magnetized plasmas. Using a 6kW magnetron and electromagnets to produce fields up to 350 Gauss, a magnetized plasma will be produced inside a 0.5-m-diameter x 1.0-m-long cylindrical chamber. Langmuir probes will be used to characterize plasma parameters and static electric fields. Electromagnetic and electrostatic field strengths will be measured independently using dipole antennas and B-dot probes to characterize the radiation patterns of novel antenna designs. One application of VLF wave-plasma interaction is radiation remediation for protection of space assets. The in-situ generation of electromagnetic whistler waves in the ionosphere is a promising approach for radiation remediation via enhanced pitch angle diffusion of high-energy electrons. The majority of the radiation generated by conventional VLF antennas is quasi-electrostatic and does not propagate large distances from the source. Antenna designs using parametric wave interaction to generate whistlers may increase the percentage of power radiated into the electromagnetic part of the VLF wave spectrum. [Preview Abstract] |
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NP12.00146: Generation of Shear Alfv\'{e}n Waves by Repetitive High Power Microwave Pulses Near the Electron Plasma Frequency -- A laboratory study of a ``Virtual Antenna'' Yuhou Wang, Walter Gekelman, Patrick Pribyl, Bart Van Compernolle, Konstantinos Papadopoulos ELF / ULF waves are important in terrestrial radio communications but difficult to launch using ground-based structures due to their enormous wavelengths. In spite of this generation of such waves by field-aligned ionospheric heating modulation was first demonstrated using the HAARP facility [1]. In the future heaters near the equator will be constructed and laboratory experiments on cross-field wave propagation could be key to the program's success. Here we report a detailed laboratory study conducted on the Large Plasma Device (LaPD) at UCLA. In this experiment, ten rapid pulses of high power microwaves (250 kW X-band) near the plasma frequency were launched transverse to the background field, and were modulated at a variable fraction (0.1-1.0) of f$_{\mathrm{ci}}$. Along with bulk electron heating and density modification, the microwave pulses generated a population of fast electrons. The field-aligned current carried by the fast electrons acted as an antenna that radiated shear Alfv\'{e}n waves. It was demonstrated that a controllable arbitrary frequency (f \textless f$_{\mathrm{ci}})$ shear Alfv\'{e}n wave can be generated by this method. The radiation pattern, frequency variation and power dependence of the virtual antenna is also presented. [1] K. Papadopoulos, et al, Geophys. Res. Lett., 38, L20107, (2011) [Preview Abstract] |
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NP12.00147: Studies of Plasma Instabilities using Discontinuous Galerkin Method with Artificial Viscosity, Limiters and Filters Yang Song, Bhuvana Srinivasan, Ammar Hakim The discontinuous Galerkin (DG) method is employed in this work to study plasma instabilities using high-order accuracy. The DG method has the advantage of resolving shocks and sharp gradients that occur in neutral fluids and plasmas. Artificial viscosity, limiters and filters are explored along with the DG method to mitigate numerical instabilities in the region of discontinuities. Artificial viscosity works in a simultaneous sense by adding a viscous term to the system to damp higher modes. Limiters are expected to reduce the numerical order one by one in regions of sharp gradients so that smooth solutions can be obtained while a fairly good numerical accuracy is maintained. Filters work physically the same as artificial viscosity but mathematically in a sequential way which will only filter the solution at the end of each time-step or at intermediate stages of a time-step. Computational tests are performed in one and two dimensions. Results are presented for Kelvin-Helmholtz and Rayleigh-Taylor unstable plasmas using the code Gkeyll. [Preview Abstract] |
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