Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session CP8: Poster Session II: Astro, Simulations, RFP, Tokamak, Stellarator |
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Room: Plaza ABC |
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CP8.00001: SPACE AND ASTRO |
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CP8.00002: Density Fluctuations in the Dissipation Range of Interstellar Turbulence Steven Spangler The inertial subrange of MHD turbulence terminates in dissipation on spatial scales of order the ion inertial length, or smaller. In the solar wind and solar corona, fluctuations on these scales have properties of kinetic Alfv\'{e}n waves. This is inferred from the enhanced compressibility of those fluctuations, as measured by radioastronomical observations of the scattering of radio waves by the density fluctuations. We discuss similar radioastronomical propagation measurements of turbulence in photoionized interstellar plasmas (HII regions). These measurements show no evidence of enhanced compressibility on the ion inertial scale. Instead, there is evidence for a simple spectral break, consisting of a steepening of the density spectrum on the ion inertial scale. This result suggests a possible difference between MHD turbulence in the corona/solar wind and interstellar turbulence. A possible reason for this difference is the much higher value for $\beta$ (15-30) in the interstellar plasmas. [Preview Abstract] |
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CP8.00003: Radiation production by and transport of non-relativistic particles in plasmas with sub-Larmor-scale magnetic fields B. Keenan, Mikhail Medvedev Plasma turbulence often has sub-Larmor-scale magnetic field fluctuations. Particle diffusion of and radiation production by ultra-relativistic particles has been explored earlier. In particular, it was shown that jitter radiation theory replaces synchrotron radiation one. The spectral characteristics and particle diffusion coefficient were shown to be intimately related. Here we extend these results to non-relativistic particles. In particular, we are interested in spectral characteristic of the emitted radiation. We show that such radiation is drastically different from cyclotron radiation, which is applicable only for nearly homogeneous fields but fails if the field correlation scale is smaller than the particle's Larmor radius. These results can be very valuable for remote diagnostics of laboratory and astrophysical plasmas. [Preview Abstract] |
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CP8.00004: ABSTRACT WITHDRAWN |
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CP8.00005: Progress Towards Creating the Magnetorotational Instability in the Plasma Couette Experiment C. Collins, C. Cooper, P. Bonofiglo, B. Seidlitz, C. Wahl, M. Clark, J. Wallace, C.B. Forest The magnetorotational instability (MRI) is a mechanism of interest for its role in angular momentum transport in astrophysical accretion disks, yet its existence has never been verified in a laboratory plasma. In the Plasma Couette Experiment (PCX), a technique for stirring a sufficiently hot, unmagnetized plasma has been demonstrated, making it possible to access regimes shown to excite the MRI in local linear analysis and global Hall-MHD numerical simulations. In the experiment, plasma is confined in a cylindrical, axisymmetric, multicusp magnetic field. Azimuthal flows (up to 10 km/s) are driven by JxB torque using biased, heated filaments at a single toroidal position in the magnetized edge. Mach probe measurements show that collisional ion viscosity couples momentum from the magnetized edge to the unmagnetized bulk. A laser-induced fluorescence diagnostic has been developed to verify the ion flow, measure the ion temperature, and confirm Braginskii's formulas for viscosity. Efforts are now underway to observe the MRI by driving sheared flow and applying a weak vertical magnetic field to destabilize the plasma. [Preview Abstract] |
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CP8.00006: Instabilities in a Line-tied Screw Pinch with Non-monotonic Current Profiles Matthew Brookhart, Yan Li, Aaron Stemo, Cary Forest Models suggest that voriticity in active regions of the sun may drive non-monotonic current profiles in coronal loops. Instabilities in these loops may be responsible for some solar flare types. The Line-tied Reconnection Experiment, a linear screw pinch with line-tied axial boundaries, uses electrostatic current injectors (washer guns) at both ends of a cylinder to create equilibria with centrally reversed and hollow current profiles. Reversed-current plasmas with no net current are stable at high magnetic field but undergo sawtooth-like events as axial field is decreased. The onset conditions and internal structure of these events are compared to the literature on zero-net current instabilities in coronal loops. Hollow current profiles in the line-tied geometry are unstable to several instabilities at high safety factor (q$>$10), directly contradicting results on ``current holes'' in tokamaks. The growth and structure of these modes is highly dependent on equilibrium profiles. [Preview Abstract] |
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CP8.00007: Theoretical foundation of Madison Plasma Dynamo Experiment Ivan Khalzov, Cary Forest The Madison Plasma Dynamo Experiment (MPDX) has successfully started its operational phase. The primary goal of the MPDX is to investigate the dynamo -- magnetic field self-generation and sustainment in hot, unmagnetized, fast flowing plasmas. This is achieved by combining a multicusp confinement technique with a novel method of driving plasma flows at the vessel boundary. We present a detailed numerical study of plasma confinement by the multicusp field and dynamo generation by boundary driven spherical plasma flows in a model of MPDX. In the dynamo study, we first consider the class of steady, axisymmetric, counter-rotating flows inside an insulating sphere and optimize them to obtain the lowest critical magnetic Reynolds number $Rm_{cr}$ required for dynamo excitation. Second, we investigate the influence of magnetic boundary conditions (resistive and ferritic walls) on the dynamo action for this class of flows. Third, we consider the dynamos generated by three-dimensional steady flows enclosed in a perfectly conducting sphere. Finally, we present results for the so-called ``fast" dynamos, which are obtained with time-periodic axisymmetric boundary-driven flows. Based on these results we describe possible scenarios for experimental demonstration of dynamo action in MPDX. [Preview Abstract] |
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CP8.00008: Investigating Confinement for Dynamo Action in the Madison Plasma Dynamo Experiment Christopher Cooper, Ivan Khalzov, Cary Forest First measurements of plasma temperature, density, and flow have been made on the Madison Plasma Dynamo Experiment (MPDX) that allow the particle and energy confinement as well as the plasma resistivity ($\eta )$ and viscosity ($\nu )$ to be estimated. The MPDX is designed to create hot flowing plasmas with high magnetic Reynolds number \textit{Rm}$=$v$_{\mathrm{p}}$L/$\eta $\textgreater \textgreater 1000, and adjustable fluid Reynolds number 10\textless \textit{Re}$=$v$_{\mathrm{p}}$L/$\nu $\textless 1000, where the kinetic energy of the flow exceeds the magnetic energy (M$_{\mathrm{A}}=$v$_{\mathrm{p}}$/v$_{\mathrm{A}}$\textgreater \textgreater 1). Simulations show these parameter ranges generate large scale ``slow'' dynamos and small scale ``fast'' dynamos to be studied. The 3 m diameter vacuum vessel is lined with rings of alternately oriented \textgreater 3 kG SmCo magnets to create a multicusp magnetic confinement scheme. Lanthanum hexaboride (LaB$_{\mathrm{6}})$ stirring rods and molybdenum anodes inserted into the vessel are biased \textless 500 V at 50 A each, heating and stirring the plasma. A model for particle and energy balance in MPDX is developed to predict $T_{e}$, $T_{i}$, and $n_{e}$ (therefore \textit{Re}, \textit{Rm}, and $M_{A})$ over a scaling of input power and neutral density. This model is compared to data from the initial MPDX plasmas and a PIC code simulating cusp losses to predict the laboratory settings for dynamo onset. Plasma losses through a multicusp field are measured with probes and compared to theory. [Preview Abstract] |
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CP8.00009: Generating counter-rotating two-vortex flows in the Madison Plasma Dynamo Experiment via boundary-driven JxB stirring D. Weisberg, C. Cooper, I. Khalzov, Y. Lee, J. Milhone, J. Wallace, C.B. Forest The Madison Plasma Dynamo Experiment (MPDX) is a plasma device designed to explore magnetic field self-excitation across a range of astrophysical dynamo regimes. Numerical simulations have shown that a laminar counter-rotating two-vortex flow in spherical geometry will produce a large-scale dynamo at certain values of fluid Reynolds number (Re$=$LV/$\nu $\textless 300) and magnetic Reynolds number (Rm$=\mu _{\mathrm{0}}\sigma $LV \textgreater\ 250). These requirements are achieved in large (L$=$1.5m), hot (T$_{\mathrm{e}}$ \textgreater\ 10eV), fast-flowing (Ma$=$V/V$_{\mathrm{A}}$ $\gg$ 1) MPDX plasmas. This poster presents advances toward producing two-vortex flow using eight thermally emissive LaB$_{\mathrm{6}}$ cathodes differentially biased up to 500V with respect to cold Mo anodes and drawing currents of up to 50A, generating a JxB torque and driving flow at the cusp-field magnetized edge of Ar, He, and H plasmas. We present Mach probe measurements showing momentum transport into the unmagnetized center of the plasma via ion-ion viscous coupling, as well as results of flow drive optimization in which a series of cathode biasing schema are tested with the goal of reproducibly generating specific global flow geometries. By changing the location and bias of each cathode/anode pair, we aim to control the pitch angle of the two-vortex flow with the intention of using pitch angle as an optimization parameter for dynamo generation. Flow measurements are compared to simulation results, and the sensitivity of internal flow structure to variations in cathode biasing schema is discussed. [Preview Abstract] |
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CP8.00010: ECH on the Madison Plasma Dynamo Experiment Jason Milhone, Cami Collins, Alf Koehn, Cary Forest The Madison Plasma Dynamo Experiment (MPDX) is a 3 meter diameter spherical vessel lined with 3000 SmCo permanent magnets (B \textgreater 3 kG) that create an axisymmetric multi-cusp ring for confining the plasma. The MPDX is designed to study flow driven MHD instabilities and dynamo action in the regime of high magnetic Reynolds number Rm$=$vL/$\eta $. This will be achieved through electron cyclotron heating of the electrons leading to good electrical conduction and large ionization fraction. The system consists of five 20 kW, CW magnetrons operating at 2.45 GHz. The system will be described in detail, including the power supplies, RF vacuum feedthroughs, and modulator/regulator circuit used to control the magnetrons. The power will be injected at various latitudes and is resonant at the fundamental cyclotron frequency in the multi-cusp edge. Prototype experiments in a smaller version of the device routinely operate in a density regime that is overdense. Experiments and numerical modeling will be described that determine how the power is absorbed in this mode. [Preview Abstract] |
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CP8.00011: Velocity Inversion from Magnetic Field Measurements in a Liquid Sodium Experiment N.Z. Taylor, M.M. Clark, C.B. Forest, M.D. Nornberg, J.P. Wallace In an analogy to helioseismology, the measured induced magnetic field generated by a spherical flow of liquid metal is used to determine the mean velocity profile through an inversion process. In the Madison dynamo experiment (MDE) the two vortex flow, driven by two counter-rotating impellers, is predicted to self-excite at low magnetic Reynolds number in the laminar case. The conductive flow is probed with external magnetic fields and the resulting induced field is measured by external and internal hall sensors. The measurements are compared with a forward model prediction of the induced magnetic field which is adjusted to fit the data. Knowledge of the mean flow can be used to optimize the pitch of the flow using rotatable vanes. A direct measurement of the turbulent EMF confirms that turbulent eddies act as an enhanced resistivity, keeping the experiment below the dynamo threshold. The detrimental large-scale turbulence has been mitigated with the installation of baffles. Although no self-excited dynamo has been observed, the resulting induced field closely matches laminar predictions for flows just below threshold. [Preview Abstract] |
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CP8.00012: Design of a Magnetic Reconnection Experiment in the Collisionless Regime Joseph Olson, Jan Egedal, Cary Forest, John Wallace In collisionless plasmas the electron diffusion region lies in different regimes depending on the pressure anisotropy, which is regulated by the properties of thermal electron orbits. In the presence of a guide magnetic field to magnetize the electrons, large scale current layers form extending to the system size [1]. In geometries with low upstream electron pressure the heating of the electrons becomes significant and relevant to observations in the magnetotail [2]. Utilizing the Madison Plasma Dynamo Experiment (MPDX) facility at UW-Madison a new reconnection experiment is now being implemented to address the role of electron pressure anisotropy in reconnection. This requires an experiment that accesses plasmas with much lower collisionality and lower plasma beta than are available in present experiments. The new experiment will be a major addition to the MPDX facility, including an insert with internal coils to drive reconnection. The insert is designed to supplement the ongoing dynamo experiments and to permit flexible reconfiguration of the magnetic geometry for a range of reconnection studies. \\[4pt] [1] Le A et el., PRL 110, 135004 (2013).\\[0pt] [2] Egedal J et al., Nature Physics, 8, 321 (2012). [Preview Abstract] |
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CP8.00013: Initial Operation of the Madison Plasma Dynamo Experiment John Wallace, Cami Collins, Cary Forest Operation of the Madison Plasma Dynamo Experiment (MPDX) has begun. This facility creates large, un-magnetized, fast flowing, hot plasma for investigating magnetic field self-generation and flow driven MHD instabilities. The experiment is 3 meters in diameter and utilizes a permanent magnet multicusp plasma confinement. Five 20KW magnetrons produce electron cyclotron heating for plasma generation. Eight lanthanum hexaboride (LaB6) stirring rods and molybdenum anodes are inserted into the vessel to produce JxB flows. This poster will describe the operational status of the facility including laboratory infrastructure, stirring electrodes, RF sources, diagnostics, currently produced plasma parameters and future experimental system additions. [Preview Abstract] |
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CP8.00014: Installation of center disk baffle into Madison Dynamo Experiment vessel M.M. Clark, M.D. Nornberg, N.Z. Taylor, J.P. Wallace, C.B. Forest The Madison Dynamo Experiment (MDE) comprises a 1 m diameter spherical chamber~that contains a turbulent flow of liquid sodium driven by two counter rotating impellers. One of the goals of the MDE is to observe a magnetic field grow at the expense of kinetic energy in the liquid sodium flow. It has been found that turbulence in the MDE plays a significant and solely detrimental role in the generation of magnetic fields. The installation of an equatorial baffle and the three rotatable vanes in each hemisphere resulted in a reduction of large scale eddies in the flow and enhanced field generation. However, no self-excited field was observed. This Poster will present recent modifications made to the experiment consisting of installing a copper disk baffle in the center of the spherical vessel. The design and installation of the structure will be illustrated and discussed. Results from before and after the center disk baffle installation will be shown. [Preview Abstract] |
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CP8.00015: Alfven wave tomography for plasma in a sphere Robert Siller, Ivan Khalzov, Cary Forest We present a diagnostic technique to study the properties of plasma in a spherical vessel based on analysis of Alfven spectra -- Alfven wave tomography. The technique is closely related to the helioseismic inversion, but instead of acoustic waves the Alfven waves are used, Alfven waves in plasma are assumed to be excited in the presence of an external axial magnetic field. The Alfven mode frequencies depend on the distribution of plasma parameters, inverting this dependence for a given (experimentally measured) set of Alfven modes, we are able to infer the spatial structure of plasma characteristics. We demonstrate this inversion technique by determining the differential plasma rotation from the splitting of the low-frequency Alfven modes. The developed diagnostic will be used for reconstruction of plasma equilibrium states with flows in the Madison Plasma Dynamo Experiment (MPDX). This is an important step towards realizing the plasma dynamos for the first time in the laboratory. [Preview Abstract] |
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CP8.00016: Study of colliding jets and shock formation for laboratory astrophysical phenomena on a small scale current driver Julio Valenzuela, Gilbert Collins, Derek Mariscal, Farhat Beg In this work we demonstrate the ability of a small linear transformer driver (LTD) yielding a 250 kA current in 150 ns to produce counter-propagating flows. The flows were produced by two vertically opposed conical arrays comprised of 8 wires, separated by a 1cm tall cylindrical insulating spacer. A section of the cylinder was removed from both sides to allow observation of the jets. Different materials were tested in order to vary the radiative cooling parameter as well the mean free path of the flow. With this array configuration we are able to produce counter-propagating jets, with velocities of the order of 1e7cm/s. A shock was observed at the colliding region that remains stationary for an extended period of time. A mean free path larger than the jet size was calculated for aluminum, making it very promising for studying astrophysical collisionless shocks. Investigation of instability formation in the shock region and eventual magnetic field production will be discussed. [Preview Abstract] |
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CP8.00017: 3D MHD simulation of Caltech plasma jet experiment: First results Xiang Zhai, Hui Li, Paul Bellan, Shengtai Li We present a 3D ideal MHD simulation of the Caltech plasma jet experiment using an adaptive mesh refinement code previously developed by H. Li and S. Li for simulating magnetically driven AGN jets. Initially, plasma and a poloidal magnetic field are placed at the simulation domain center. A toroidal magnetic flux is injected into the system continuously near the electrodes. In both the simulation and the experiment the Lorentz force is observed to squeeze the plasma radially and lengthen it axially to form a jet. The Lorentz force also transports magnetic energy and helicity and converts magnetic energy into kinetic energy. The simulation agrees quantitatively with the experiments in numerous aspects, such as magnetic/kinetic energy, current, jet radius, and propagation velocity. Specifically, the simulation shows, in agreement with both the experiment and analytical theory, that the jet is an MHD Bernoulli flow and the jet velocity is proportional to the poloidal current divided by the square root of the jet density. This simulation study provides a new and quantitative method for understanding the nature of plasma jets and also for relating experiments, numerical simulations and astrophysical observations. [Preview Abstract] |
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CP8.00018: Effects of the Violation of Taylor's Hypothesis on Observed Turbulent Power Spectra in the Solar Wind Kristopher Klein, Gregory Howes, Jason TenBarge All frequency measurements of magnetized turbulence in the solar wind are a convolution of temporal and spatial terms. For typical \textit{in situ} measurements near 1 AU, the super-Alfv\'enic solar wind velocity motivates the adoption of Taylor's Hypothesis, in which the temporal contribution to the frequency measurement is neglected. The Solar Orbiter and Solar Probe Plus missions will sample the solar wind in the near-Sun environment where the solar wind velocity is not significantly greater than the Alfv\'en velocity. In this case, Taylor's Hypothesis ceases to be applicable. We make qualitative and quantitative predictions of the changes in measured turbulent power spectra due to the violation of Taylor's Hypothesis. It is shown that critically balanced Alfv\'en waves do not significantly alter the structure of magnetic power spectra while higher frequency fast/whistler waves do. Observed changes in measured power spectra may therefore be useful in distinguishing between competing models of solar wind turbulence. [Preview Abstract] |
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CP8.00019: How Do Alfven Wave Collisions Dominate the Properties of Plasma Turbulence? G.G. Howes, J.M. TenBarge, K.D. Nielson, D.J. Drake, J.W.R. Schroeder, F. Skiff, C.A. Kletzing, T.A. Carter Turbulence plays a key role in the evolution of space and astrophysical plasmas, mediating the transfer of energy from large-scale turbulent motions to small scales where the turbulent energy is ultimately converted to plasma heat. The cascade of energy from large to small scales is mediated by the nonlinear interaction between counterpropagating Alfven waves, or Alfven wave ``collisions,'' the fundamental building block of astrophysical plasma turbulence. These nonlinear interactions are inherently three-dimensional, and the mathematical properties of the nonlinearity govern the nature of the turbulent cascade, dictating the phase relationships between fluctuations at different scales that give rise to structure in the turbulence. Here, we present a synthesis of recent results that illuminates the nature of turbulence in weakly collisional plasmas, such as the solar wind and solar corona. We explore how the nonlinearity controls the development of structure in plasma turbulence. [Preview Abstract] |
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CP8.00020: Numerical and experimental investigation of plasma instabilities Kevin Ronald, David Speirs, Karen Gillespie, Martin King, Kathleen Matheson, Sandra McConville, Ross Bryson, Alan Phelps, Colin Whyte, Craig Robertson, Robert Bingham, Mark Koepke, Alan Cairns, Irena Vorgul, Raoul Trines, Barry Kellett Instabilities in energetic electron populations streaming through plasma is important in geophysical and fusion plasmas (both magnetic and fast ignition inertial confinement schemes). This paper will present progress in an experiment aiming to produce scaled reproductions of cyclotron, streaming and anomalous Doppler instabilities important in natural and applied plasma environments. The experiment originally designed as a scaled reproduction of important aspects of X-mode auroral cyclotron wave emissions has supported the proposal that this is driven by free energy in the descending auroral electron flux. The experiment has also observed emissions from energetic electrons in an R-like mode which may be relevant to equatorial cyclotron emissions. The development of the apparatus to reproduce aspects of fusion relevant instabilities in a more benign environment will be presented as will the results of numerical simulations of streaming and anomalous Doppler instabilities supporting the design of these experiments and the interpretation of the measurements. [Preview Abstract] |
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CP8.00021: Fully Kinetic Model of Magnetic Reconnection in a Magnetosphere Vadim Roytershteyn, Homa Karimabadi, Yuri Omelchenko Magnetic reconnection in collisionless plasma is a complex process coupling physical process across multiple scale. It is thought to play a crucial role in the dynamics of planetary magnetospheres, for example by allowing transport of solar wind across magnetospheric boundaries. Here we present and discuss results of large-scale 2D fully kinetic simulations of magnetic reconnection in 2D dipolar magnetospheres driven by interaction with incoming solar wind. Such simulations allow for the first time self-consistent accurate description of magnetic reconnection in this configuration. The focus is on flux rope formation, stability and properties of the magnetopause. The results of fully kinetic simulations are compared and contrasted against hybrid simulations. [Preview Abstract] |
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CP8.00022: 3D PIC investigation of pileup regions in the outflow of magnetic reconnection Giovanni Lapenta, A. Vapirev, S. Markidis, M. Goldman, D. Newman As the magnetic field energy, the electrons and the ions exit a reconnection region eventually they pile up against the larger scale plasma per-existent around the reconnection region. Recent work has focused on uncovering interesting processes developing there. Satellite crossings in the Earth magnetosphere have identified such regions. In the case of an Earthward propagating flow the pile up region becomes a so-called dipolarization front. The name derives from the tendency of these fronts to re-establish a more dipolar-like field closer to the Earth, when compared with the pre-existing stretched configuration of the magnetotail. We have recently conducted a fully 3D PIC study [1] of these regions. Here we focus on new 3D diagnostics designed to identify topological features and investigate the processes of energy exchange and particle dynamics. \\[4pt] [1] Vapirev, A. E., et al. ``Formation of a transient front structure near reconnection point in 3D PIC simulations.'' Journal of Geophysical Research: Space Physics (2013). [Preview Abstract] |
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CP8.00023: Global simulations of dynamo and turbulent magnetic helicity transport in flowing plasmas F. Ebrahimi, A. Bhattacharjee We numerically examine the dynamo effect and turbulent helicity transport for magnetically and flow-driven turbulence. Using direct numerical computations, we calculate the fluctuation-induced dynamo effect, which is shown to have the functional form of a total divergence of a vector field, representing magnetic helicity flux produced by the fluctuations. We calculate the complete form of this flux, and compare it with other approximate fluxes (such as the so-called Vishniac-Cho flux) used in recent astrophysical dynamo studies. We find that for both magnetically driven reconnecting instabilities and flow driven magnetorotational cases, the functional form of this flux is determined by the free-energy source of the instability. If the instabilities are tearing modes (as is often the case for reversed-field pinches), the dynamo effect takes the form of hyper-resistivity which does not amplify flux. However, for the magneto-rotational instability, the form of this dynamo field is qualitatively different, and is shown to generate and support a large-scale toroidal magnetic field. Work supported by DE-FG02-12ER55142. [Preview Abstract] |
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CP8.00024: On magnetospheres of spinning black holes A. Ford, K. Williams, Mikhail Medvedev Spinning black holes in magnetic fields are though to develop force-free magnetospheres. Their structure is a key to the energy extraction via Blandford-Znajek mechanism, which can power extended relativistic jets of active galactic nuclei. The key assumption for the force-free condition is the presence electron-positron plasma with the density being above the Goldreigh-Julian density. Here we explore the conditions under which the pair-cascade can efficiently occur. [Preview Abstract] |
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CP8.00025: Controlled Space Physics Experiments using Laboratory Magnetospheres Michael Mauel, M. Davis, D. Garnier, T.M. Roberts, M. Worstell, J. Kesner Modern society's reliance on space-based platforms for a variety of economic and geopolitical purposes makes understanding the physics of the magnetosphere and ``space weather'' one of the most important applications of plasma science. During the past decade, results from the CTX and LDX laboratory magnetospheres and from the RT-1 device at University of Tokyo, we have developed techniques to explore space physics using controlled experiments in laboratory magnetospheres. This presentation briefly reviews observations from the laboratory magnetospheres at Columbia University and MIT, including adiabatic drift-resonant transport, low-frequency MHD turbulence, and the formation of high-beta plasmas with profiles similar to Earth's inner magnetosphere. First principle validation of ``whole plasma'' space weather models have been completed in relevant magnetic geometry, including the spectrum and dynamics of turbulence successfully modeled with nonlinear bounce-averaged gyrokinetic simulations. Plans to explore Alfv\'enic dynamics and whistler wave trapping are discussed through the achievement of higher-density plasmas using radio-frequency heating. [Preview Abstract] |
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CP8.00026: Flux Tube Dynamics Following Pellet Release Experi- ments in Laboratory Magnetospheres D. Garnier, M. Davis, M. Mauel, M. Roberts, M. Worstell, M. Chilenski, J. Kesner, P. Woskov The rapid release of particles in the magnetosphere has allowed study of a wide range of space plasma dynamics including particle transport, magnetic bubble formation, and rapid flux-tube dynamics.\footnote{Bernhardt, \textit{Phys. Fluids B} \textbf{4}, 2249 (1992).} We report new experiments using the Levitated Dipole Experiment (LDX) (http://www.psfc.mit.edu/ldx/) where we explore the high-speed plasma dynamics following the release of 0.2~mm polystyrene pellets. The pellets are released into high-beta steady-state plasmas containing significant population of quasi-relativistic electrons. Similiar experiments, conducted in a smaller, mechanically-supported, laboratory magnetosphere show pellet ``explosions,'' electron precipitation, ``blob'' formation, and rapid changes of plasma density. A variety of diagnostics are available, including microwave reflectometry, high-speed videography, multi-tip probe arrays, and accurate magnetic reconstruction. Results and analyses will be reported along with plans for futher efforts to increase plasma density and conduct a variety of controlled physics experiments associated with magnetospheric events. [Preview Abstract] |
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CP8.00027: Active Feedback Control of Global Turbulence with Multiple Localized Controllers T. Maximillian Roberts, Michael Mauel, Matthew Worstell, Darren Garnier, Jay Kesner The CTX device is a laboratory magnetosphere confining low density, interchange unstable plasmas which exhibit fully developed turbulent dynamics. A feedback system designed to measure potential fluctuations and apply a variably phase shifted response has been found to amplify or suppress this turbulence. Through multiple azimuthal measurements of floating potential, we see that the influence of the feedback system is localized, amplifying feedback having a larger spatial extend than suppressive. By installing another identical and independent feedback system 180 degrees from the first, we have significantly enhanced global suppression of turbulence. We also discuss the adaptations to previous work on a high performance simulation which evolves the coupled nonlinear PDEs that model (i) the gyro-kinetic electrons, (ii) cold fluid ions and, (iii) the time rate of change of the potential associated with interchange fluctuations. By including a representation of the feedback system in this simulation we hope to see a response similar to that measured experimentally. [Preview Abstract] |
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CP8.00028: Profile Consistency and Turbulent Particle Pinch in Dense Plasma J. Kesner, M. Davis, D. Garnier, M. Mauel In a plasma with central heating and edge fueling a turbulence-driven density pinch is often present. Turbulence tends to drive density inwards while enhancing outwards flow of energy and resulting in predictable, stationary density and pressure profiles. We observe in LDX a strong density pinch and the resulting stationary density profile.\footnote{Boxer, Bergmann, Ellsworth, Garnier et al., Nature-Phys {\bf 6}, (2010) 207.} The pinch is predicted by ``turbulent equipartiton'' theory and also by both MHD and kinetic theory. The effect is particularly strong in a dipole because the magnetic field falls strongly ($B\propto 1/R^3)$ and the turbulent modes are interchange-like. A turbulent pinch is also observed in tokamaks which can result in a density inversion as observed in C-mod during LHCD.\footnote{Kesner, Ernst, Hughes, Mumgaard, Scott et al., PoP {\bf 19}, (2012) 122511.} In a tokamak the stationary density tends to fall as $\sim 1/q$ (i.e. a factor $\sim$3), whereas in a dipole the peak density can rise a factor of $\sim$30 above the edge value. In high density dipole plasmas the density is seen to remain inwardly peaked while the pressure peak can move outwards. Recent experiments with high density plasmas have been carried out in LDX and will be discussed. [Preview Abstract] |
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CP8.00029: Symmetry Breaking and the Inverse Energy Cascade in a Plasma Matthew Worstell, M.S. Davis, D. Garnier, M.E. Mauel, T.M. Roberts, J. Kesner The application of electrostatic bias to high density plasmas with turbulent fluctuations confined by a magnetic dipole are investigated. This research investigates the ap- plication of non-symmetric bias and the influence of broken symmetry on strongly turbulent plasmas. Non-symmetric bias is applied through either point biasing or an equatorial array spanning the device. In both cases, the spatial symmetry of applied bias dramatically effects the plasma fluctuations. With bias applied, the plasma achieves a new equilibrium characterized by amplified low order modes and diminished amplitude of higher order modes. Although the turbulent spectrum changes, the RMS fluctuation level is unchanged by the bias. Bias also causes the turbulent electrostatic fluctuations to coalesce into a quasi- coherent mode and the appearance of increased coherence. The effect of bias configuration is also seen to change the measured levels of non- linear coupling. Non-symmetric biasing increases nonlinear coupling in contrast to symmetric biasing. These results represent the first experi- mental demonstration of symmetry breaking driving the inverse energy cascade in a quasi-two dimensional plasma system. [Preview Abstract] |
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CP8.00030: Interchange turbulence in a dipole-confined plasma Bo Li, Darin Ernst, Jay Kesner The plasma interchange turbulence and convective transport in magnetic dipole and hard-core Z-pinch fields are explored with flux-driven radially global two-fluid simulations, using a new field-aligned turbulence code. We find that the nonlinear evolution of the interchange mode produces coherent structures of electric potential in the bad curvature region. The large-scale convective cells propagate in the azimuthal direction and have long azimuthal wavelengths comparable to the system size. For the Z-pinch field, the pressure profile is broad and the radial transport is large. For the dipole field, the pressure profile becomes much steeper and strong azimuthal shear flows are generated, and the radial transport is reduced even though the system is driven by stronger heat sources. [Preview Abstract] |
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CP8.00031: Extended MHD model for the study of shear flow dynamics in magnetized plasmas S.S. Cerri, P. Henri, F. Califano, D. Del Sarto, M. Faganello, F. Pegoraro We present an ``extended MHD'' model set of equations aimed at studying magnetized plasma regimes where fluctuations arise at scale lengths comparable to the ion Larmor radius (or to the ion skin depth), while the characteristic frequencies remain smaller than the ion cyclotron frequency. This system of equations conserves the total energy explicitly. Our main goal is the investigation of the multi-scale dynamics resulting from the development of the Kelvin-Helmholtz instability driven by of a shear flow as, e.g., is the case of the interaction of the solar wind with the Earth's magnetosphere. Using this model, we have obtained a new set of equilibria that include FLR microscopic effects accounting for the contribution of the pressure tensor that, in such conditions, reacts on the flow itself on a time scale comparable to the ideal time scale. These equilibria are an extension of the standard MHD equilibria and are very well suited for fully kinetic simulations where on the contrary standard mechanical force equilibria generate strong spurious fluctuations and do not relax towards Vlasov equilibria. [Preview Abstract] |
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CP8.00032: Gyrokinetic simulation of nonlinear evolution of the mirror instability Peter Porazik, Jay Johnson Mirror instabilities are typically observed in compressed high beta plasma, associated with shocks in the solar wind and at planetary magnetospheres. Observations suggest that these waves regulate the temperature anisotropy in the solar wind and magnetosheath. Nonlinear structures observed in planetary magnetosheaths and magnetospheres have been associated with the late stage development of mirror instabilities. In order to understand the nonlinear evolution of the mirror instability including regulation of anisotropy and the development of nonlinear structuring known as ``peaks'' and ``dips,'' we have developed a gyrokinetic simulation model. The model is implemented with a noise reducing delta-f, particle-in-cell method, and has been successfully verified against previous studies with a single unstable mode present, showing saturation due to particle trapping. Simulations of a 2D spectrum of unstable modes display formation of a saturated state with peaked magnetic structures. We discuss the physical mechanisms responsible for saturation and nonlinear development of the instability and compare the qualitative features with observations. [Preview Abstract] |
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CP8.00033: Convective Amplification of EMIC Waves from Ring-distribution Protons in the Inner Magnetosphere Manish Mithaiwala, Chris Crabtree, Gurudas Ganguli, Leonid Rudakov, Kunihiro Keika The growth of electromagnetic ion cyclotron waves (EMIC) due to a ring distribution of Hydrogen ions is examined. Though these distributions are more commonly implicated in the generation of equatorial noise, their potential for exciting EMIC waves is considered here. It is shown that since the ring distribution is non-monotonic in perpendicular velocity, the amplification achieved by this instability is greater than bi-Maxwellian distributions for typical anisotropies, because the waves can maintain resonance over a much longer part of its trajectory. For ring speeds (V$_r$) close to the Alfven speed (V$_A$), the growth rate is maximum at parallel propagation but decreases less rapidly towards oblique angles compared with a bi-Maxwellian. Additionally there can be a second peak approximately at (kperp c/wpH)(V$_r$/V$_A$) $\sim$ 2.3 for ring speeds about the parallel thermal speed. Strong wave gain is achieved for moderate ring speeds (V$_r$ $\sim$ V$_A$). The analysis suggests that EMIC wave activity should be closely associated with equatorial noise. [Preview Abstract] |
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CP8.00034: Auroral Arc Lifetimes and the Stationary Inertial Alfv\'{e}n Wave S.H. Nogami, M.E. Koepke, D.M. Gillies, D.J. Knudsen, M. Tornquist, E. Donovan Images from the NORSTAR array of All-Sky-Imagers are used to analyze discrete auroral arcs by observing optical emission of the arcs from formation through breakup. 2-D images of the aurora, collected by All-Sky-Imagers at three sites across Canada, have been analyzed to define the lifetime of a discrete auroral arc. This poster presents a frequency distribution of auroral arc lifetimes and the statistics of other temporal quantities defined in this study (e.g., duty cycle of a given structure). The prevalence of long-lived static arcs in this study is contrasted with the lack of a complete theory to explain these statistics. We suggest that this prevalence is consistent with the laboratory-observed and predicted properties of the Stationary Inertial Alfv\'{e}n Wave, a non-fluctuating, non-travelling, spatially periodic pattern in electromagnetic field and fluid quantities that arises in the simultaneous presence of a magnetic-field-aligned current channel and cross-magnetic field plasma flow [1,2].\\[4pt] [1] D.J. Knudsen, J. Geophys. Res. \textbf{101}, 10761, 1996;\\[0pt] [2] S.M. Finnegan, et. al., Phys. Plasmas. \textbf{15}, 052108, 2008. [Preview Abstract] |
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CP8.00035: Coupling of an exploding plasma to a magnetized ambient plasma measured with LIF Jeffrey Bonde, Stephen Vincena, Walter Gekelman The coupling of plasma jets to ambient media near young stellar objects, Herbig-Haro objects, and supernova remnants is of considerable interest to the astrophysical community. In this work, we study the interaction of a laboratory scale jet formed by a carbon laser-produced plasma (LPP) with the ions of a magnetized argon background plasma ($n_{jet} /n_{Ar} <30$,$\mbox{v}_{\mbox{jet}} \mbox{/c}_{\mbox{s}} =20$,$\mbox{v}_{\mbox{jet}} \mbox{/v}_{\mbox{A}} \le \mbox{1})$ using laser-induced fluorescence (LIF). The excitation light was provided by a planar beam of a pulsed dye laser which, by tuning to the Doppler-broadened 611.5 nm absorption line, sampled the distribution function of metastable Ar-II separating the background from the components of the jet. A fast shutter ($\ge 3$ ns) CCD camera captured the 461 nm fluorescence with 40 ns time and .6 mm$^{2}$ spatial resolutions. The distribution functions obtained from the LIF diagnostic reveal significant density enhancement and a subsonic parallel drift localized at the LPP-ambient interface. Within the jet region, the background ion signal indicates the formation of a density void and suggests a lateral snow-plow effect. To our knowledge, this is the first LIF measurement of a supersonic jet coupling to an ambient plasma. Supplemental Langmuir probe measurements characterize the jet's dimensions and dependence on magnetic field strength and background ion mass up to 6 meters from the LPP source. [Preview Abstract] |
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CP8.00036: Collisional Effects in Simulations of High Altitude Nuclear Explosions Tanim Islam The simulation of the later-time ($> 1$ second) debris dynamics of a high altitude nuclear explosion (HANE) require, at a minimum, an understanding of the interaction of the ionized blast material with the relatively collisional upper ionosphere and lower exosphere ($\le 200$ km). At these altitudes, the collisional mean free path of ionized atmospheric particles may become smaller than the length scale of the diamagnetic bubble. Here we report on the local dynamics about the debris/air interface for Starfish Prime [1] like, and lower energy, HANEs at altitudes in which collisionality becomes important. We model the debris dynamics with the hybrid plasma simulation code \texttt{KIM3D} [2], and use a standard Miller-Combi particle pairing algorithm [3] to model particle collisions. We demonstrate new dynamics associated with finite collisionality in mildly collisional HANEs. \\[4pt] [1] P. Dyal, Journal of Geophysical Research \textbf{111}, A12211 (2006).\\[0pt] [2] S.H. Brecht, in \textit{Space Plasma Simulation}, edited by J.~B\"{u}chner, C.~T.~Dum, and M.~Scholer (2001), p.~70.\\[0pt] [3] R.~H.~Miller and M.~R.~Combi, Geophysical Research Letters \textbf{21}, 1735 (1994). [Preview Abstract] |
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CP8.00037: Effects of strapping field profiles on plasma loop expansion Bao Nguyen Quoc Ha, Paul Bellan Tokamak-like forces may explain fundamental behaviors of solar plasma arches. The hoop force causes arched, current-carrying plasma loops to expand unless additional forces are applied. This expansion was slowed and even inhibited by a magnetic field of proper polarity in previous solar loop experiments at Caltech [1] but there was no attempt to characterize the strapping field's spatial profile. Kliem and Torok [2] predicted an explosive-like transition from slow expansion to fast eruption if the vertical decay rate of the strapping field exceeds an instability threshold. We have constructed a new set of independently powered auxiliary coils designed to be placed inside the vacuum chamber and closer to the plasma source. The resulting strapping field has a sharper decay rate than with our previous coils and is expected to exceed the instability threshold. Progress on the interaction between arched plasma loops and strapping magnetic fields will be presented. \\[4pt] [1] J. F. Hansen and P. M. Bellan, Astrophys. J. Lett. \textbf{563}, L183 (2001)\\[0pt] [2] B. Kliem and T. Torok, Phys. Rev. Lett. \textbf{96}, 255002 (2006) [Preview Abstract] |
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CP8.00038: Probing the Earth's magnetosphere with an electron gun Gian Luca Delzanno, Enrico Camporeale, Erik Hogan, J. David Moulton, Joseph Borovsky, Elizabeth MacDonald, Michelle Thomsen The ability to unambiguously connect different parts of magnetosphere and ionosphere through magnetic field line tracing is critical to the understanding of the coupling between these two systems. A possible way to achieve this goal could use a magnetospheric spacecraft to emit an energetic electron beam along the local magnetic field and detect the emission optically at the magnetic foot-point in the ionosphere. In this idea it is critical to keep the spacecraft charging under control by emitting a contactor plasma before firing the beam. We present an overview of our effort to tackle this complex problem. We will focus on: (1) the further development of the Particle-In-Cell (PIC) code CPIC used for this study. CPIC couples the standard PIC algorithm with the generation and adaptation of the computational grid; (2) the widely-used static modeling of the contactor plasma and its inadequacy in some parameter regimes; (3) the PIC modeling of the contactor plasma injected across a static magnetic field and the possible development of instabilities at the edges of the contactor cloud, complemented by a simplified linear stability analysis to highlight the physics of these instabilities. [Preview Abstract] |
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CP8.00039: On the Application of a Hybrid Monte Carlo Technique to Radiation Transfer in the Post-Explosion Phase of Type IA Supernovae Ryan Wollaeger, Daniel van Rossum, Carlo Graziani, Sean Couch, George Jordan, Donald Lamb, Gregory Moses We apply Implicit Monte Carlo (IMC) and Discrete Diffusion Monte Carlo (DDMC) to Nomoto's W7 model of Type Ia Supernovae (SNe Ia). IMC is a stochastic method for solving the nonlinear radiation transport equations. DDMC is a stochastic radiation diffusion method that is generally used to accelerate IMC for Monte Carlo (MC) particle histories in optically thick regions of space. The hybrid IMC-DDMC method has recently been extended to account for multifrequency and velocity effects. SNe Ia are thermonuclear explosions of white dwarf stars that produce characteristic light curves and spectra sourced by radioactive decay of $^{56}$Ni. We exhibit the advantages of the hybrid MC approach relative to pure IMC for the W7 model. These results shed light on the viability of IMC-DDMC in more sophisticated, multi-dimensional simulations of SNe Ia. This work was supported in part by the University of Chicago and the National Science Foundation under grant AST-0909132. [Preview Abstract] |
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CP8.00040: Skeleton Particle-in-Cell Codes on Emerging Computer Architectures Viktor Decyk, Tajendra Singh, Warren Mori The UCLA Plasma Simulation Group has long been active in developing Particle-in-Cell (PIC) codes for parallel computers. In recent years, High Performance Computer (HPC) architectures are being increasingly complex, with up to 4 different layers of parallelism, each of which may require different programming styles. To help the plasma physics community cope with this challenge, we are providing documented, open source, parallel skeleton codes at the UCLA IDRE web site: https://idre.ucla.edu/hpc/parallel-plasma-pic-codes. These skeleton codes are deliberately simple, yet contain all the crucial pieces needed in a production code: deposit, push, reordering and a field solver. They illustrate a variety of parallel architectures for both electrostatic and electromagnetic PIC codes. [Preview Abstract] |
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CP8.00041: Modeling Laboratory Astrophysics Experiments using the CRASH code Matthew Trantham, R.P. Drake, Michael Grosskopf, Matthew Bauerle, Carolyn Kruanz, Paul Keiter, Guy Malamud The understanding of high energy density systems can be advanced by laboratory astrophysics experiments. Computer simulations can assist in the design and analysis of these experiments. The Center for Radiative Shock Hydrodynamics (CRASH) at the University of Michigan developed a code that has been used to design and analyze high-energy-density experiments on OMEGA, NIF, and other large laser facilities. This Eulerian code uses block-adaptive mesh refinement (AMR) with implicit multigroup radiation transport and electron heat conduction. This poster/talk will demonstrate some of the experiments the CRASH code has helped design or analyze including: Radiative shocks experiments, Kelvin-Helmholtz experiments, Rayleigh-Taylor experiments, plasma sheet, and interacting jets experiments.\\[4pt] This work is funded by the Predictive Sciences Academic Alliances Program in NNSA-ASC via grant DEFC52- 08NA28616, by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-FG52-09NA29548, and by the National Laser User Facility Program, grant number DE-NA0000850. [Preview Abstract] |
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CP8.00042: Constrained-transport Hall-MHD simulations using CWENO reconstruction with libMRC Liwei Lin, Kai Germaschewski, Stephen Abbott, Kris Maynard, Jimmy Raeder We present a new CWENO (Centrally-Weighted Essentially Non-Oscillatory) reconstruction based extended MHD (XMHD) solver that has been built for libMRC. libMRC is a library for creating efficient parallel PDE solvers on structured grids, which is used in the MRC (Magnetic Reconnection Code), OpenGGCM (Open Global Geospace Circulation Model) and PSC (Plasma Simulation Code) codes. The use of libMRC gives us access to its core functionality of providing an automated code generation framework which takes a user provided PDE right hand side in symbolic form to generate an efficient, computer-architecture specific, parallel code. libMRC also supports block-structured adaptive mesh refinement, and implicit-time stepping through integration with the PETSc library. We demonstrate validation of the new CWENO MHD solver against existing solvers both in standard test problems as well as in 3D global magnetosphere simulations. [Preview Abstract] |
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CP8.00043: COMPUTER SIMULATION METHODS |
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CP8.00044: Guiding Center Codes of High Accuracy Roscoe White Guiding center simulations are an important means of predicting the effect of resistive and ideal magnetohydrodynamic instabilities on particle distributions in toroidal magnetically confined thermonuclear fusion research devices. Because saturated instabilities typically have amplitudes of $\delta B/B$ of a few times $10^{-4}$ numerical accuracy is of concern in discovering the effect of mode particle resonances. We develop a means of following guiding center orbits which is greatly superior to the methods currently in use. In the presence of ripple or time dependent magnetic perturbations both energy and canonical momentum are conserved in a time step to better than one part in $10^{14}$, an improvement of nine orders of magnitude, and the relation between changes in canonical momentum and energy is also conserved to very high order. [Preview Abstract] |
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CP8.00045: Time-spectral modelling of drift wave turbulence Jan Scheffel Time spectral methods for initial-value partial differential equations avoid the time stepping being characteristic for temporal finite difference schemes. Large gains in efficiency should then be within reach for problems in plasma physics with widely separated time scales. In the recently developed Generalized Weighted Residual Method GWRM [1], temporal, spatial and parameter domains are all handled using a Chebyshev polynomial solution ansatz. The coefficients of the ansatz are determined using a generalized weighted residual method, for which a new efficient equation system solver has been applied [2]. In addition, subdomain methods for the temporal and spatial domains have been developed [3] and employed successfully in a number of test problems. We will here also present a related method, being based on least square minimization of the residual rather than on the Galerkin method. Both methods are applied to problems in drift wave turbulence from which results will be presented.\\[4pt] [1] Scheffel J, Partial Differential Equations: Theory, Analysis and Applications (Nova Science Publishers) p 1-49, 2011.\\[0pt] [2] Scheffel J and H{\aa}kansson C, Appl. Numer. Math. 59(2009)2430.\\[0pt] [3] Scheffel J and Mirza A, Am. J. of Comp. Math. 2(2012)72. [Preview Abstract] |
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CP8.00046: Physics-based preconditioners for two-fluid electrostatic and electromagnetic models with charge separation C. Leibs, L. Chacon, D.A. Knoll Recently, fluid acceleration of a fully implicit kinetic particle-in-cell (PIC) simulation has been successfully demonstrated.\footnote{Taitano et al., SISC, 2013}$^,$\footnote{Chen et al., JCP, submitted.} Central to these algorithms is robust preconditioning of the fluid system. In the context of kinetic simulations, the fluid system features conservation equations for both ions and electrons, plus field evolution equations, and must allow for charge separation effects. In this work, we concern ourselves with electrostatic and electromagnetic two-fluid models in multiple dimensions. Electromagnetic fields are prescribed via the Darwin approximation to project out spurious light-wave time scales.\footnote{Nielson and Lewis, Meth. Comput. Phys., 1976.} Disparate time scales remain among the abundance of supported plasma waves. The resulting nonlinear, stiff hyperbolic PDE systems are effectively preconditioned using physics-based preconditioning ideas,\footnote{Knoll and Keyes., JCP 2004.} whereby their linearized form is transformed into parabolic PDEs that target the fast wave behavior. These elliptic systems can be efficiently inverted by multigrid methods.\footnote{Brandt., Math. Comp., 1977} We will demonstrate the effectiveness of the approach via numerical experiments. [Preview Abstract] |
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CP8.00047: Plasma Physics Simulations on Next Generation Platforms Alice Koniges The current high-performance computing revolution provides opportunity for major increases in computational power over the next several years, if it can be harnessed. This transition from simply increasing the single-processor and network performance to a different architectural paradigms forces application programmers to rethink the basic models of parallel programming from both the language and problem division standpoints. One of the major computing facilities available to researchers in fusion energy is the National Energy Research Scientific Computing Center. As the mission computing center for DOE, Office of Science, NERSC is tasked with helping users to overcome the challenges of this revolution both through the use of new parallel constructs and languages and also by enabling a broader user community to take advantage of multi-core performance. We discuss the programming model challenges facing researchers in fusion and plasma physics in for a variety of simulations ranging from particle-in-cell to fluid-gyrokinetic and MHD models. [Preview Abstract] |
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CP8.00048: Efficient simulation of 2+2-D multi-species plasmas waves using an Eulerian Vlasov code Jeffrey Banks, Richard Berger, Thomas Chapman, Jeffrey Hittinger, Stephan Bruner We discuss multi-species aspects of the Eulerian-based kinetic code LOKI that evolves the Vlasov-Poisson system in 2+2-dimensional phase space (Banks et al., Phys. Plasmas 18, 052102 (2011)). In order to control the inherent cost associated with phase-space simulation, our approach uses a minimally diffuse, fourth-order-accurate finite-volume discretization (Banks and Hittinger, IEEE T. Plasma Sci. 39, 2198--2207). The scheme is discretely conservative and controls unphysical oscillations. The details of the numerical scheme will be presented, and the implementation on modern highly concurrent parallel computers will be discussed. We will present results of 2D simulations of propagating ion acoustic waves (IAWs) created using an external driving potential. The evolution of the plasma wave field and associated self-consistent distribution of trapped electrons and ions is studied after the external drive is turned off. \it{This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Laboratory Research and Development Program at LLNL under project tracking code 12-ERD-061.} [Preview Abstract] |
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CP8.00049: Discontinuous Galerkin version of PERSEUS for Studying HED plasma Xuan Zhao, Charles Seyler, John Greenly, Yang Yang The Discontinuous Galerkin(DG) version of Cornell PERSEUS [1,2] code is developed in an effort to reduce the numerical diffusivity of the existing Finite Volume(FV) version of PERSEUS code. A Positivity- Preserving limiter [3] is used instead of TVD limiter to maintain a second order accuracy at smooth extrema. A Structure-Preserving limiter [4] is used when $\nabla\cdot B=0$ needs to be enforced. The code is tested on several MHD benchmark problems with both resistive MHD and extended-MHD models. The simulation results of compressible magnetic reconnection problem are presented here as an example. Since DG-PERSEUS is better for simulating shocks with less numerical diffusivity, we applied the code for studying the difference between the structures of a shock formed by a super-Alfvenic-supersonic flow and that formed by a sub-Alfvenic-supersonic flow, so that one can tell whether the outflow is super-Alfvenic in a magnetic reconnection experiment. The code is parallelized with MPI, we expect a higher parallel efficiency, since the only information needed to be passed between computation units is the flux through boundary, which is less than the passed information in the FV case.\\[4pt] [1] M.R. Martin, PhD. Thesis\\[0pt] [2] C.E. Seyler, M.R. Martin, 2011\\[0pt] [3] X. Zhang, PhD. Thesis\\[0pt] [4] F. Li, C. Shu, 2006 [Preview Abstract] |
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CP8.00050: The Multi Level Multi Domain (MLMD) method: a semi-implicit adaptive algorithm for Particle In Cell plasma simulations Maria Elena Innocenti, Arnaud Beck, Stefano Markidis, Giovanni Lapenta Particle in Cell (PIC) simulations of plasmas are not bound anymore by the stability constraints of explicit algorithms. Semi implicit and fully implicit methods allow to use larger grid spacings and time steps. Adaptive Mesh Refinement (AMR) techniques permit to locally change the simulation resolution. The code proposed in Innocenti et al., 2013 and Beck et al., 2013 is however the first to combine the advantages of both. The use of the Implicit Moment Method allows to taylor the resolution used in each level to the physical scales of interest and to use high Refinement Factors (RF) between the levels. The Multi Level Multi Domain (MLMD) structure, where all levels are simulated as complete domains, conjugates algorithmic and practical advantages. The different levels evolve according to the local dynamics and achieve optimal level interlocking. Also, the capabilities of the Object Oriented programming model are fully exploited. The MLMD algorithm is demonstrated with magnetic reconnection and collisionless shocks simulations with very high RFs between the levels. Notable computational gains are achieved with respect to simulations performed on the entire domain with the higher resolution. Beck A. et al. (2013). submitted. Innocenti M.E. et al. (2013). JCP, 238(0):115-140 [Preview Abstract] |
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CP8.00051: Benchmarking of the LAPS dust-plasma code Maximilian Forstner, Gian Luca Delzanno, Zehua Guo, Bhuvana Srinivasan, Xianzhu Tang Los Alamos Plasma Simulation (LAPS) is an integrated modeling code based on a common-data framework for multi-physics simulation of both magnetic and inertial confinement fusion plasmas. As part of the larger LAPS framework, we developed a fluid code to simulate the interaction between electron, ions and dust. In the model we solve continuity equations for two of the three species (quasi-neutrality provides the density of the third species), momentum equations for ion and dust velocities, Faraday's and Ampere's laws, an equation for the current density and Ohm's law where we assume massless electrons. To solve the system of equations we use second order accurate finite difference schemes on a structured mesh and second order accurate implicit time stepping. We implemented the code in C++ in parallel by making extensive use of the Portable Extensible Toolkit for Scientific Computing (PETSc) to handle both vectors and non-linear solvers. We will present some benchmark studies of the code against standard waves and instabilities tests. We will also present preliminary studies of the interaction of a dust cloud with a streaming, magnetized plasma under different plasma and magnetic field configurations. [Preview Abstract] |
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CP8.00052: Codesign approach towards an Exascale scalable plasma simulation code J. Amaya, J. Deca, M.E. Innocenti, A. Johnson, G. Lapenta, S. Markidis, V. Olshevsky, A. Vapirev Particle in cell simulations represent an excellent paradigm for codesign efforts. PIC codes are simple and flexible with many variants addressing different physics applications (e.g. explicit, implicit, hybrid, gyrokinetic, fluid) and different architecture (e.g. vector, parallel, GPU). It is relatively easy to consider radical changes and test them in a short time. For this reason, the project DEEP funded by the European Commission (www.deep-project.eu) and the Intel Exascience Lab (www.exascience.com) have used PIC as one of their target application for a codesign approach aiming at developing PIC methods for future exascale comupters [1]. The starting point is the iPic3D implicit PIC approach [2]. Here we report on the analysis of code performance, on the use of GPUs and the new MICs (Intel Xeon processors). We describe how the method can be rethinked for hybrid architectures composed of MICs and CPUs (as in the new Deep Supercomputer in Juelich, as well as in others). The focus is on a codesign approach where computer science issue motivate modifications of the algorithms used while physics constraints what should be eventually achieved.\\[4pt] [1] G. Lapenta et al, doi.ieeecomputersociety.org/10.1109/MCSE.2012.86\\[0pt] [2] S. Markidis et al, Math. Comput. Simul. 80.7 (2010): 1509-1519 [Preview Abstract] |
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CP8.00053: Variational Formulation of Particle Algorithms for Kinetic E{\&}M Plasma Simulations Alexander Stamm, Bradley Shadwick, Evstati Evstatiev A rigorous variational method was used to derive a self-consistent set of equations of motion from a discretized Lagrangian to study kinetic plasmas. Discretization of the Lagrangian was performed by reduction of the phase-space distribution function to a collection of finite-sized macro-particles of arbitrary shape and discretization of field quantities onto a spatial grid. The equations of motion were then obtained by demanding the action be stationary [1]. This approach may be used in both lab frame and moving window coordinates, which improve computational efficiency when modeling laser-plasma interactions. The primary advantage of the variational approach is preservation of Lagrangian symmetries, which in our case leads to energy conservation and avoids difficulties with grid heating. Additionally, this approach decouples particle size from grid spacing and relaxes restrictions on particle shape, leading to a decrease in numerical noise. The variational approach also guarantees consistent ordering and is amiable to higher order methods in both space and time. Simulations conducted with the new equations of motion demonstrate the desired energy conservation and a decrease in numerical noise. \\[4pt] [1] E. G. Evstatiev and B. A. Shadwick J. Comput. Phys 245, 376 (2013). [Preview Abstract] |
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CP8.00054: Coupling the BATS-R-US global MHD code with the implicit particle-in-cell code iPIC3D L.K.S. Daldorff, G Toth, I. Sololov, T. Gombosi, J. Amaya, G. Lapenta, J. Brackbill, S. Markidis, V. Olshevsky Magnetohydrodynamic (MHD) codes achieved considerable success in modeling space physics systems, such as the solar corona or the magnetosphere. Unfortunately, fluid models cannot describe the magnetic reconnection physics accurately, which plays an important role in determining the dynamics of the whole system. Particle-in-cell (PIC) codes can model the reconnection process accurately, but they are much more expensive than fluid models. Coupling the global fluid code with a regional PIC code can provide a physically accurate yet economic global model. Implicit PIC codes are especially suitable to be coupled with fluid codes, as they can employ much larger time steps and grid spacing than explicit PIC codes, and allow the fluid and PIC discretizations to employ comparable spatial and temporal resolutions. Adaptive mesh refinement in the fluid code can also facilitate bridging the scales between the two codes. We describe our initial progress towards coupling the adaptive mesh based BATS-R-US MHD code and the implicit PIC code iPIC3D. [Preview Abstract] |
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CP8.00055: An energy- and charge-conserving, nonlinearly implicit, electromagnetic particle-in-cell algorithm Guangye Chen, Luis Chacon, Dana Knoll, William Daughton A recent proof-of-principle study proposes a nonlinear electrostatic implicit particle-in-cell (PIC) algorithm in one dimension.\footnote{Chen, Chac\'on, Barnes, {\em J. Comput. Phys.} {\bf 230}, 7018 (2011)} The algorithm employs a kinetically enslaved Jacobian-free Newton-Krylov (JFNK) method, and conserves energy and charge to numerical round-off. In this study, we generalize the method to electromagnetic simulations in 1D using the Darwin approximation of Maxwell's equations. An implicit, orbit-averaged central finite difference scheme is applied to both the Darwin field equations and the particle orbit equations to produce a discrete system that remains exactly charge-and energy-conserving. Furthermore, the canonical momentum in any ignorable direction is exactly conserved per particle by appropriate interpolations of the magnetic field. A fluid preconditioner targeting the stiffest electron waves has been developed to accelerate the linear GMRES solver of JFNK. We present 1D numerical experiments (e.g. the Weibel instability, kinetic Alfven wave ion-ion streaming instability, etc.) to demonstrate the accuracy and efficiency of the implicit Darwin PIC algorithm, and the performance of the fluid preconditioner. [Preview Abstract] |
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CP8.00056: Design and development of a multi-architecture, fully implicit, charge and energy conserving particle-in-cell framework Joshua Payne, Dana Knoll, Allen McPherson, William Taitano, Luis Chacon, Guangye Chen, Scott Pakin As computer architectures become increasingly heterogeneous the need for algorithms and applications that can utilize these new architectures grows more pressing. CoCoPIC is a fully implicit charge and energy conserving particle-in-cell framework developed as part of the Computational Co-Design for Multi-Scale Applications in the Natural Sciences (CoCoMANS) project at Los Alamos National Laboratory. CoCoMANS is a multi-disciplinary computational co-design effort with the goal of developing new algorithms for emerging architectures using multi-scale applications. This poster will present the co-design process evolved within CoCoMANS, and details regarding the design and development of multi-architecture framework for a plasma application. This framework utilizes multiple abstraction layers in order to maximize code reuse between architectures, while providing low level abstractions to incorporate architecture specific operation optimizations such as vectorizations or hardware fused multiply-add. CoCoPIC's target problems include 1D3V slow shocks, and 2D3V magnetic island coalescence. Results of the multi-core development and optimization process will be presented. [Preview Abstract] |
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CP8.00057: Extended generalized Lagrangian multipliers for magnetohydrodynamics using adaptive multiresolution methods Margarete O. Domingues, Anna Karina F. Gomes, Odim Mendes, Kai Schneider We present a new adaptive multiresoltion method for the numerical simulation of ideal magnetohydrodynamics. The governing equations, i.e., the compressible Euler equations coupled with the Maxwell equations are discretized using a finite volume scheme on a two-dimensional Cartesian mesh. Adaptivity in space is obtained via multiresolution analysis, which allows the reliable introduction of a locally refined mesh while controlling the error. The explicit time discretization uses a compact Runge-Kutta method for local time stepping and an embedded Runge-Kutta scheme for automatic time step control. An extended generalized Lagrangian multiplier approach with the mixed hyperbolic-parabolic correction type is used to control the incompressibility of the magnetic field. Applications to a two-dimensional problem illustrate the properties of the method. Memory savings and numerical divergences of the magnetic field are reported and the accuracy of the adaptive computations is assessed by comparing with the available exact solution. [Preview Abstract] |
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CP8.00058: Energetic Particle and Varying Aspect Ratio Effects on Resistive and Ideal MHD Modes Michael Halfmoon, D.P. Brennan, J.M. Finn The effect of energetic particles on MHD mode stability is strongly dependent on the aspect ratio of the equilibrium configuration in the high beta and Lundquist number regimes. Advances in computational analysis have made it possible to simulate toroidal MHD equilibrium and stability with the high degree of precision necessary to study these effects in detail. We first obtain a full MHD stability analysis in beta vs. aspect ratio of a circular cross section configuration, with monotonic q \textgreater\ 1 and monotonic pressure, using PEST-III and NIMROD. We then include energetic particle effects using the $\delta $f hybrid kinetic-MHD particle in cell model in the NIMROD code. By mapping out the stability results and comparing with reduced analyses we form a basis for understanding the particle damping or driving effects and the underlying physics of particle-mode interactions. [Preview Abstract] |
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CP8.00059: Time-discretized action principle variational formulation of finite-size particle simulation methods for electrostatic plasmas Bradley Shadwick, Evstati Evstatiev We formulate finite-size particle plasma simulation methods from a time-discretized action principle viewpoint. Using Low's Lagrangian as a starting point, we first discretized in spatially the continuous fields and formulate a time-continuous action for the self-consistent system of particles and fields in the electrostatic (ES) plasma approximation. We then utilize a technique due to Lew et. al. [1] to formulate time-discrete action and its variation to obtain a particular time integrator. Such time integrators are symplectic (provided there is symmetry with respect to time inversion); they do not conserve energy exactly but the energy variation is bounded and its magnitude depends on the time step. These general time integrators can be of any order of accuracy, however as a rule, beyond second order they are implicit. Time-implicit schemes are easy to formulate for the general cases of electromagnetic and magnetized plasmas. We provide numerical examples of both explicit and implicit time integrators and discuss their advantages and disadvantages. \\[4pt] [1] A. Lew, J. E. Marsden, M. Ortiz, and M. West. International Journal for Numerical Methods in Engineering 60(1), 153-212, 2004. [Preview Abstract] |
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CP8.00060: Finite-size particle simulations in the drift-kinetic approximation Evstati Evstatiev, Andy Spencer, Jin-Soo Kim, Bradley Shadwick We extend previous variational formulations of finite-size particle plasma simulation methods to the drift-kinetic approximation. Such approximation is applicable to strongly magnetized plasmas, e.g., in tokamacs and magnetic mirrors. In our numerical examples we apply the drift-kinetic approximation to the electron population of the plasma in an electron cyclotron heating ion source (ECRIS) device. The electrons in an ECRIS device are strongly non-Maxwellian (due to the radio-frequency heating) and require kinetic treatment. The drift-kinetic approximation has allowed us to reduce the computational load associated with resolving the electron motion by about two orders of magnitude and to extend the simulation time to hundreds of microseconds. Details of the algorithms and some numerical results will be presented. Simulations are done with FAR-TECH's SIMulation of PLasmas code, SIMPL. [Preview Abstract] |
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CP8.00061: Fourth-Order Continuum Methods for Simulating Plasma Kinetics in Phase Space G.V. Vogman, P. Colella Continuum methods for solving the Maxwell-Boltzmann equation system offer a high-fidelity means of simulating plasma kinetics. These methods are advantageous because they can be cast in conservation-law form, are not susceptible to sampling noise, and can be implemented using high-order numerical methods. The continuum approach evolves a distribution function in position-velocity phase space by numerically solving a hyperbolic advection equation in up to six dimensions. A fourth-order accurate method has been developed to solve the continuum kinetic Vlasov-Poisson system in one spatial and one velocity dimension. This method has been extended to two velocity dimensions to model magnetized plasmas. Adaptive mesh refinement is implemented to reduce computational cost and to allow for the extension of the model into more dimensions. The governing equation is solved in its conservation-law form using a fourth order finite volume discretization. The model demonstrates conservation of mass, momentum, and energy, and is benchmarked against theoretical predictions for Landau damping, two-stream instability, and the Dory-Guest-Harris instability. Features of the continuum model as well as its extension into additional phase space dimensions are discussed. [Preview Abstract] |
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CP8.00062: Solving the Vlasov-Poisson Equation and Preserving the Positivity: Comparisons between the Operator Splitting and Flux Limiting Method Michael Carrie, Bradley Shadwick When solving the Vlasov-Poisson equation on a phase-space grid, one has to deal with preserving positivity of the distribution. Filamentation, a consequence of the entropy conserving property of the Vlasov equation, ultimately leads to gradients of the distribution function at the grid size level. To overcome this issue, numerical methods using flux limiters to enforce the positivity can be employed. The time-implicit numerical algorithm we developed is no exception to this positivity preserving issue. To assess if in the context of Vlasov-Poisson equation these negative values are of importance on the system dynamics, we present two versions of the algorithm, one based on the operator splitting method and one using a positivity preserving method to solve the unsplit system. It is shown numerically that the flux limiter method is causing more damaged in terms of Casimir invariants and phase-space volume rearrangements compared to the operator splitting scheme even with negative values. Moreover, the operator splitting method is faster, more efficient, and easier to implement compared to the flux limiting method which requires the use of iterative solvers (Newton-Krylov method for instance) for highly nonlinear (if and else statements), nonsymmetric, large systems. [Preview Abstract] |
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CP8.00063: Minimization of the numerical phase velocity error in Particle-In-Cell simulations for relativistic charged particle systems Michael Meyers, Chengkun Huang, B.J. Albright The microbunching instability arises when GeV electrons interact with their coherent synchrotron radiation (CSR). Accurate particle-in-cell (PIC) modeling of this instability requires a method where the numerical phase velocity of light is very close to its physical value. This is also advantageous for mitigating the effects of Numerical Cherenkov Radiation (NCR), arising when simulating highly relativistic particles in astrophysical and high energy density laboratory settings. It has been shown that the use of a weighted stencil when calculating fields from the Ampere and Faraday laws affords a solver with a tunable phase velocity [1]. A numerical dispersion relation appropriate to the PIC algorithm with the 3D FV24 scheme has been derived. Stencil weights that minimize the phase velocity error for the CSR and NCR problems will be presented along with simulations demonstrating the comparative advantages of this approach.\\[4pt] [1] Mohamed F. Hadi, A Finite Volume-Based 3-D Low Dispersion FDTD Algorithm, IEEE Transactions on Antennas and Propagation, Vol. 55, No. 8, (2007) [Preview Abstract] |
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CP8.00064: Fourier-Hermite spectral method for the Vlasov equation Enrico Camporeale, Gian Luca Delzanno, Benjamin Bergen, J. David Moulton We present a spectral method to solve the Vlasov equation for collisionless plasma, by means of an expansion of the distribution function into a Fourier-Hermite basis. The proof-of-principle results are obtained for the 1D-1V Vlasov-Poisson equation, and the focus of this work is to compare the performance of this approach with a standard Particle-in-Cell (PIC) method. With a fully-implicit time integrator, the Fourier-Hermite method conserves charge, momentum, and energy exactly. Currently no PIC code is able to conserve these three quantities simultaneously. We show results for several cases routinely used as benchmarks in computational plasma physics: Langmuir wave, Landau damping, two-stream instability, and ion-acoustic wave. It is shown that the Fourier-Hermite method can achieve a much more accurate solution in a tiny fraction of the time relative to PIC. [Preview Abstract] |
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CP8.00065: Modeling of Laser wakefield accelerator in Lorentz boosted frame using EM-PIC code with spectral solver Peicheng Yu, Xinlu Xu, Viktor Decyk, Weiming An, Jorge Vieira, Frank Tsung, Ricardo Fonseca, Wei Lu, Luis Silva, Warren Mori Simulating laser wakefield acceleration (LWFA) in a Lorentz boosted frame can reduce the computational time over existing fully explicit methods tremendously. In these simulations the relativistic drifting plasma inevitably induces a high frequency numerical instability that contaminates the interested physics, which we mitigate by solve Maxwell equations in Fourier space (a spectral solver) plus using a low pass or ring filter in Fourier space. We describe the development of UPIC-EMMA that uses a spectral solver and that includes the ability to launch a laser using a moving antenna. We show that using UPIC-EMMA LWFA simulations in boosted frames with arbitrary $\gamma_b$ can be conducted without any evidence on the numerical instability. We also benchmark the results with lab frame simulations using OSIRIS. These simulations include the modeling cases where there are no self-trapped electrons, and modeling the self-trapped regime. Detailed comparison among Lorentz boost ed frame results and lab frame results obtained from OSIRIS shows the feasibility of using UPIC-EMMA to conduct LWFA simulation at high $\gamma_b$. [Preview Abstract] |
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CP8.00066: REVERSED-FIELD PINCH |
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CP8.00067: Overview of MST Research J.S. Sarff MST progress in advancing the RFP for (1) fusion plasma confinement with minimal external magnetization, (2) predictive capability in toroidal confinement physics, and (3) basic plasma physics is summarized. Investigation of energetic ion confinement and stability continues using a 1 MW, 25 keV neutral beam injector. Fast ion confinement is near classical, although the onset of bursty energetic particle modes limits the fast ion density. For plasmas with a 3D helical equilibrium (quasi-single-helicity regime) the fast ion confinement is reduced. Full orbit calculations are underway to investigate various classes of orbits, some different from the tokamak. The onset of the 3D equilibrium correlates with Lundquist number, and progress is made developing 3D equilibrium reconstructions using V3FIT. Several new diagnostics are in use. An advanced neutral particle analyzer measures both the NBI-generated and spontaneous energetic ion tail created by magnetic reconnection. A 2-color SXR tomography system measures the electron temperature profile and 3D thermal structures. The FIR interferometer-polarimeter is being upgraded for higher spatial resolution to measure $k_{\bot}\rho_s\sim 1$ fluctuations. Also, MST's frozen pellet injector has been upgraded for injecting larger pellets. [Preview Abstract] |
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CP8.00068: MHD validation studies for RFPs K.J. McCollam, D.J. Den Hartog, J.A. Reusch, J.S. Sarff, J.P. Sauppe, D.D. Schnack, C.R. Sovinec, S. Masamune The reversed-field pinch (RFP) provides a nonlinear dynamical system suitable for validation studies of extended MHD, wherein laboratory measurements and resistive-MHD code results are compared so as to quantitatively evaluate the physical applicability of the mathematical model expressed in the code. We present plans for validation of the DEBS and NIMROD codes using the MST and RELAX RFP experiments, along with updates of results to date. MST operates in the full range of Lundquist number $S$ accessible to the codes, and we identify metrics to be used for comparisons of sawtooth relaxation cycles at different $S$ values. Previous work with the cylindrical, single-fluid DEBS code showed some quantitative agreement between code and experiment but disagreement for magnetic-fluctuation amplitudes (Reusch {\it et al.}, PRL, 2011). Cylindrical NIMROD simulations with two-fluid effects gave closer agreement for the amplitudes (King {\it et al.}, POP, 2012), but the two code studies were done using different fluid viscosities, which may also contribute to the different results. RELAX has a low aspect ratio of 2, which motivates comparisons to NIMROD runs in both cylindrical and toroidal geometry. [Preview Abstract] |
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CP8.00069: Improving electron temperature measurement on the MST RFP using integrated data analysis D.J. Den Hartog, J.R. Johnson, K.J. McCollam, M.B. McGarry, L.M. Reusch, H.D. Stephens Two completely independent electron temperature ($T_{e})$ diagnostics are in use on the MST RFP: Thomson scattering and double-filter soft x-ray (SXR). Both diagnostics are able to measure $T_{e}$ at a rate up to 25 kHz and are in good qualitative agreement in the hot plasma core, where $T_{e}$ \textgreater\ 1 keV. Thomson scattering measures a radial profile of $T_{e}$, whereas SXR measurements can be either 1D or 2D. 3D information can be extracted from both diagnostics by exploiting the rotation of helical plasma structures past the measurement points. We are beginning an effort to improve the scope, accuracy, and utility of $T_{e}$ measurement on MST by combining information from both diagnostics and prior physics knowledge using the techniques of integrated data analysis (IDA) [R. Fischer and A. Dinklage, \textit{Rev. Sci. Instrum.} \textbf{75}, 4237 (2004)]. Bayesian probability theory provides the most natural framework for this type of analysis, and will be the basis of the IDA toolkit to be developed. The overall goal of IDA is to combine data from heterogeneous and complementary diagnostics, considering all dependencies within and between diagnostics, in order to obtain the most reliable results in a transparent and standardized way. IDA exploits the redundancy of complementary diagnostics to resolve measurement inconsistencies, and will maximize the value of experimental measurements to the Validation process. [Preview Abstract] |
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CP8.00070: Interaction of Tearing Modes and Fast Ions in the MST RFP J.A. Reusch, J.K. Anderson, S. Eilerman, J. Falk, J.J. Koliner, M.D. Nornberg, J. Waksman, L. Lin, D. Liu, Y. Tsidulko Energetic ions sourced by a 1 MW, 25 keV, tangential neutral-beam injector (NBI) are well confined in RFP discharges in MST. In beam blip experiments, classical slowing and charge exchange loss can often account for the measured neutron flux decay. While these experiments give a sense of the global fast ion confinement, there are many important details that are lost in such an analysis. To gain insight into the effects of tearing modes on the fast ion distribution, a full orbit particle tracing code (RIO) has been used. RIO is capable of taking as input the 3D time varying electric and magnetic field output from the nonlinear resistive MHD code D\textsc{ebs}. While the tearing modes present in MST do not appear to cause significant direct loss of the highest energy ions due to drift orbit averaging, the ions do begin to interact with the tearing modes as they slow down, leading to a flattening of the ion density profile and an enhancement in the fast ion loss rate. While RIO allows the study of the effect of tearing modes on the fast ions we have also observed, in a separate set of long pulse NBI experiments, that the fast ions affect the tearing modes. Specifically, the core-most tearing mode amplitude is suppressed during NBI with the degree of suppression tracking directly with neutral particle analyzer measurements of the core localized circulating fast ions. The interaction of fast ions with the tearing modes in both beam blip and long pulse experiments will be presented. This work supported by the US DOE and NSF. [Preview Abstract] |
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CP8.00071: Electron Temperature and Density Fluctuations in Neutral Beam Heated Plasmas E. Parke, D.J. Den Hartog, L. Lin Neutral beam injection on the MST RFP suppresses core-resonant tearing modes and excites higher-frequency energetic particle (EP) modes. The Thomson scattering diagnostic on MST is capable of high effective repetition rates for observation of electron dynamics on short time scales, with recent upgrades increasing the maximum frequency from 25 kHz to 50 kHz. With these new capabilities, we present the results of Thomson scattering measurements of EP modes and tearing modes. We compare measurements of electron density fluctuations during an EP mode burst to interferometric measurements, which both show core-localization of the fluctuations. Temperature fluctuations correlated with the EP modes further constrain the mode structure. Measurements correlated with tearing modes show $\sim$50\% reduction in temperature fluctuation amplitude during NBI, similar to the reduction in mode magnetic field amplitude at the wall. From the fluctuation structure, we infer the island stability factor and compare to theoretical predictions. The fluctuation structure and phase also allow measurement of the Shafranov shift and improved constraints on the equilibrium q-profile. [Preview Abstract] |
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CP8.00072: Investigation of the fast ion beta limit in MST William Capecchi, Scott Eilerman, Joshua Reusch, Jonathan Koliner, Jay Anderson, Liang Lin, Jerry Clark, Deyong Liu Fast ion orbits in the reversed field pinch (RFP) magnetic configuration are well ordered and have low orbit loss, even considering the stochasticity of the magnetic field generated by multiple tearing modes. Purely classical TRANSP modeling of a 1MW tangentially injected hydrogen neutral beam in MST deuterium plasmas predicts a core-localized fast ion density that can be up to 25{\%} of the electron density and a fast ion beta of many times the local thermal beta. However, neutral particle analysis (NPA) of an NBI-driven mode (presumably driven by a fast ion pressure gradient) clearly shows transport of core-localized fast ions and a saturated fast ion density. The TRANSP modeling is presumed valid until the onset of the beam driven mode and gives an initial estimate of the volume-averaged fast ion beta in the range of 1-2{\%} (local core value up to 10{\%}). Distinguishing between an experimental fast ion number limit or fast ion beta limit is performed by scanning both the magnetic field strength and the NBI energy while observing conditions at the onset of the beam driven mode. Upcoming experiments will further investigate the empirical fast ion beta limit through the use of a deuterium beam into deuterium plasma which will allow for the NPA and neutron flux signals to provide a local and global fast ion beta measurement respectively. [Preview Abstract] |
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CP8.00073: Fast Ion Orbit Topology in the Reversed-Field Pinch J.L. Clark, W. Capecchi, J. Egedal, J.K. Anderson Fast ion orbit tracing through the RFP's equilibrium magnetic field (ignoring the substantial turbulent contribution) can lend insight into the behavior of fast particles. Ion orbit topology, as studied in tokamak and spherical torus geometries by considering the motion of the guiding center with a conserved angular momentum p$_{\mathrm{\phi }}$ and magnetic moment $\mu $, reveals several of the well known fast ion orbits in the RFP as well, such as kidney, D, pinch, banana, cusp, stagnation, and passing. While most orbits are similar to those found in conventional tokamaks, additional orbit types are possible in a RFP, as contours of constant $\mu $ in the RFP resemble mirrored parabolas with the foci leading to the magnetic axis, as opposed to a single set of parabolas with foci leading to the high field side. The tangential orientation of the NBI on MST corresponds to a path in the R,$\chi $ topology plane, where R is the major radius and $\chi $ is the cosine of the pitch angle at the Z$=$0 midplane, with co-passing, D, and banana orbits. A majority of the NBI-born ions are on co-passing orbits with a gyro-radius of order 10{\%} of the minor radius. Following the complete ion orbit reveals a slight non-conservation of magnetic moment but this has a negligible effect on orbits crossing topological borders from confined orbits to lost orbits. Consideration of orbits in a very high beta (26{\%}) RFP equilibrium with a deeply reversed toroidal magnetic field leads to the creation of two new orbit topology boundaries in the R,$\chi $ plane. [Preview Abstract] |
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CP8.00074: Helical Equilibrium Reconstruction using V3FIT on MST J.J. Koliner, B.E. Chapman, J.S. Sarff, J.K. Anderson, W. Capecchi, S. Eilerman, J.A. Reusch, J.D. Hanson, M.R. Cianciosa, D. Terranova Plasmas in the MST reversed field pinch bifurcate to a helical equilibrium, forming a Single Helical Axis (SHAx) at high plasma current (I$_{\mathrm{p}}\approx $500 kA) and low density (n$_{\mathrm{e}}\approx $0.5 x 10$^{19}$ m$^{-3})$. Modeling of these plasmas requires an equilibrium solver that does not assume axisymmetry. The V3FIT 3D equilibrium reconstruction code is applied to helical equilibria with diagnostic measurements as constraints. The 11-chord interferometer-polarimeter, 22-point Thomson scattering system, and 4-camera soft X-ray probes have been included in addition to external magnetics. Inputs have been adapted for MST's close-fitting conducting shell. Investigations into the role of shell eddy currents have been made, including comparison to eigenfunctions generated from the Newcomb equation. At the plasma boundary, $\approx $60{\%} of the static $n=$5 toroidal field B$_{\mathrm{T}}$ seen by magnetic probes is generated by currents in the shell. The generated VMEC equilibrium serves as the input for applications relevant to the 1 MW, 25 keV neutral beam injector. During beam injection, fast ion confinement is reduced in periods with a SHAx compared to axisymmetric plasmas. A single particle orbit code has been applied to calculate particle trajectories in the 3D case, confirming a strong influence of SHAx equilibria on fast ion orbits. EPM magnetic bursts terminate at the transition to SHAx. Alfv\'{e}n continua have been generated to study this phenomenon with the reduced-MHD code STELLGAP. Work Supported by USDoE and NSF. [Preview Abstract] |
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CP8.00075: Energetic particle confinement and effects in 3D RFP plasmas J.K. Anderson, W. Capecchi, S. Eilerman, J.J. Koliner, M.D. Nornberg, J.A. Reusch, L. Lin Fast ions are well confined in the stochastic magnetic field of the multiple-helicity (MH) RFP, with fast ion confinement of neutral-beam-injected ions routinely a factor of 5 to 10 higher than thermal confinement time. As the plasma current is increased in discharges with a weakly reversed edge toroidal magnetic field, the equilibrium tends to transition from nearly axisymmetric to a strongly helical three-dimensional state. In lower current discharges, where the onset of the helical state is uncertain, high power NBI tends to suppress the transition to the single helicity state. In high current discharges ($\sim$ 0.5MA), where the onset of n$=$5 single helicity is quite robust, a short blip of NBI is used to probe the confinement of fast ions with minimal perturbation to the 3D equilibrium. The fast ion confinement time is measured to be substantially lower than fast ions in comparable MH RFP states, and there is a strong dependence on the spectral index. Full ion orbit calculations through the 3D equilibrium field reveal a substantial deviation of the ion trajectory from the helical flux surfaces and degraded confinement. The helical mode is stationary in the laboratory frame but locks at variable phase with respect to the MST vessel. Hence, experiments where the tangential neutral beam injector is effectively moved relative to the helical structure are performed. The total fast particle content and confinement time with respect to this angle are explored. [Preview Abstract] |
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CP8.00076: Temperature structures in MST helical core plasmas Stefano Munaretto, Mark D. Nornberg, Brett E. Chapman, Eli Parke, Daniel J. Den Hartog The RFP configuration is dominated by several MHD tearing modes with poloidal periodicity $m=$\textit{1}. These modes are resonant at different radii within the plasma core, giving rise to stochastization of the magnetic field. Increasing the plasma current (and consequently the Lundquist number $S)$ tends to channel the perturbation energy to the innermost resonant tearing mode, leaving the other modes with negligible amplitude (QSH state). A further topological transition takes place when the main magnetic axis and island X point vanish leaving the O-point as the new helical magnetic axis (SHAx state). Previous work on RFXmod has shown the formation of a thermal structure that becomes wider during the transition between QSH to SHAx states. In MST we have performed experiments at similar $S $to RFXmod plasmas. Ion Doppler spectroscopy and Thomson scattering measurements reveal the presence of in-outboard asymmetries in the impurity ion temperature radial profiles and the formation of a hot electron region. The changes in both the temperature radial profiles are associated with the transitions to non-axisymmetric equilibria. Further analysis will be performed to characterize the thermal structures also through charge-exchange spectroscopy measurements of ion temperature. [Preview Abstract] |
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CP8.00077: Time-dependent behavior of the quasi-single-helicity state in the RFP G.G. Whelan, P.W. Terry A recent theory treats the quasi-single-helicity (QSH) state of the reversed field pinch as a core fluctuation structure tied to a tearing mode of the same helicity and shows that strong magnetic and velocity shears in the structure suppress the nonlinear interaction with other fluctuations.\footnote{J.-H. Kim and P.W. Terry, Phys. Plasmas {\bf 19}, 122304 (2012).} The theory has temporally evolving, nonlinearly coupled equations for the core fluctuation and the fluctuations of other helicities. The QSH core becomes long-lived and equilibrium-like when plasma current is large, as observed in experiment. By summing the multiple helicity fluctuation energies over toroidal wavenumber, we reduce the theory to a predator-prey model. The suppression of the nonlinear interaction is goverened by the single helicity energy, which, for fixed radial structure controls the magnetic and velocity shearing rates. It is also controlled by plasma current, which in the theory, sets the shearing threshold for suppression. The model shows a limit cycle oscillation in which the system toggles between quasi-single helicity and multiple helicity states, with the single helicity phase becoming increasingly long-lived relative to the multiple helicity phase as plasma current increases. [Preview Abstract] |
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CP8.00078: Progress in nonlinear 3D MHD modeling of fusion plasmas with the PIXIE3D code D. Bonfiglio, S. Cappello, M. Veranda, L. Chac\'on, D.F. Escande Recent advancements in nonlinear 3D MHD modeling of fusion plasmas with the PIXIE3D code [1] are reported. After the nonlinear cross-benchmark with SpeCyl proved the fundamental mathematical correctness of both codes [2], PIXIE3D has been used to model both tokamak and reversed-field pinch (RFP) plasmas. Qualitative agreement with respect to experimental observations has been demonstrated by taking advantage of numerical features such as toroidal geometry and the possibility of applying external magnetic perturbations [3]. In particular, the toroidal coupling between (either spontaneous or externally stimulated) MHD modes affects both the MHD dynamics and the magnetic topology. More recently, the inclusion of a momentum source as well as a self-consistent coupling with heat transport have been considered. Preliminary simulations of helical RFP states with self-consistent temperature evolution will be presented. The effect on the final helical equilibrium will be discussed and compared with the reference case in which the temperature equation is not taken into account and a fixed resistivity profile is used.\\[4pt] [1] L. Chac\'{o}n, Phys. Plasmas \textbf{15}, 056103 (2008)\\[0pt] [2] D. Bonfiglio \textit{et al.}, Phys. Plasmas \textbf{17}, 082501 (2010)\\[0pt] [3] D. Bonfiglio\textit{ et al.}, Bull. Am. Phys. Soc. \textbf{57}, 82 (2012) [Preview Abstract] |
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CP8.00079: Electron Temperature Profile Evolution in MST Improved Confinement Discharges L.A. Morton, B.E. Chapman, E. Parke, W.C. Young, D.J. Den Hartog We investigate the temporal evolution of the electron temperature profile in MST RFP during Pulsed Parallel Current Drive (PPCD), to study profile stiffness and dependence on PPCD programming. PPCD discharges are found to exhibit profile stiffness: the core temperature remains flat, with a gradient region outside r/a $=$ 0.5. The profile shape is fixed even as the core temperature rises by a factor of 3-4 during a discharge. We use the recently-upgraded Thomson Scattering system to accurately measure electron temperatures above 2 keV at a 2 kHz repetition rate. In ``crash-heated'' PPCD, large magnetic reconnection events (sawtooth crashes) heat the ions to \textgreater~1 keV prior to the onset of improved confinement. Crash-heated discharges achieve Te of 2 keV in the core. In ``non-crash-heated'' PPCD, large crashes are suppressed by lowered toroidal field reversal, preventing strong ion heating. The core electron temperature is 200-400 eV lower at a given density (0.5-1.2 x 10$^{13}$ cm$^{-3})$ than in crash-heated discharges. We compare the two types of discharges to understand the mechanisms behind this difference. [Preview Abstract] |
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CP8.00080: Gyrokinetic characterization of PPCD plasmas D. Carmody, M.J. Pueschel, J.K. Anderson, P.W. Terry A series of linear and nonlinear gyrokinetic simulations were performed with the {\sc Gene} code to model experimental discharges in the MST reversed field pinch and identify the dominant instabilities. These studies focus on the characteristics of microinstabilities in pulsed poloidal current drive (PPCD), a current profile control technique that results in reduced large scale tearing activity and improved confinement. The equilibria are modeled using a modified version of {\sc Gene}'s circular equilibrium model and experimental measurements of density, temperature, and magnetic field. A variety of PPCD discharges are studied encompassing different plasma currents and relative strengths of density and temperature gradients, and the dominant linear instabilities are found to be ITG and TEM. The critical gradients for these modes are found at different radial locations and compared with the experimental gradients. The dependence of these instabilities on various parameters such as plasma $\beta$ and collisionality is also investigated. Nonlinearly, a strong upshift of the critical gradients is found and the nonlinear mechanisms responsible are discussed. Work supported by US DOE Grant No. DE-FG02-85ER53212. [Preview Abstract] |
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CP8.00081: Momentum and Current Transport in the MST Reversed Field Pinch W.X. Ding, Liang Lin, D.L. Brower, A.F. Almagri, B.E. Chapman, D.J. Den Hartog, J. Duff, J.S. Sarff Self-generated flows and current (dynamo effects) are routinely observed in the MST RFP where both parallel flow and electric field reverse sign compared to the edge. In the absence of external torque and applied poloidal electric field, both the flow and electric field may arise from kinetic effects. Kinetic effects, defined as the correlated product of parallel pressure and radial magnetic field fluctuations, have been measured by using a high-speed polarimetry-interferometry diagnostic (for combined radial magnetic field and density fluctuation measurement). Between sawtooth crashes it is found that the measured kinetic effects associated with density fluctuations (a component of parallel pressure fluctuation) has a finite amplitude that may account for the observed flow in the core. In addition, the same fluctuations also influence electron dynamics via the kinetic dynamo. These results suggest kinetic effects may play an important role in coupling between momentum transport and current transport. Work supported by US DOE and NSF. [Preview Abstract] |
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CP8.00082: Laser-Based Faraday-Effect Measurement of Magnetic Fluctuations and Fluctuation-Induced Transport L. Lin, D.L. Brower, W.X. Ding, J.S. Sarff A multichord far-infrared laser-based Faraday-effect polarimetry diagnostic has been well developed on MST. Combined polarimetry-interferometry capability permits simultaneous measurement of internal structure of density and magnetic field with fast time response ($\sim 4\mu$s) and low phase noise ($<0.01^{\circ})$. With this diagnostic, the impact on toroidal current profile from a tangentially injected neutral beam is directly measured, allowing evaluation of non-inductive current drive. In addition, $0.05^{\circ}$ Faraday-effect fluctuations associated with global tearing modes are resolved with an uncertainty below $0.01^{\circ}$. For physics investigations, these Faraday-effect fluctuations are complicated by contributions from both density and magnetic fluctuations. In our analysis, the local density fluctuations are obtained by inverting the line-integrated interferometry data after resolving the mode helicity through correlation techniques. The local magnetic fluctuations are then reconstructed using a parameterized fit of the polarimetry data, accounting for both the density and magnetic contributions. For the same mode, density and radial magnetic fluctuations exhibit very different spatial structure. In this process, their relative phase is also determined, thereby allowing the determination of magnetic-fluctuation-induced transport. [Preview Abstract] |
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CP8.00083: Measurement of high-frequency density fluctuations in improved confinement RFP plasmas J.R. Duff, B.E. Chapman, J.K. Anderson, J.S. Sarff, L. Lin, W.X. Ding, D.L. Brower In standard RFP plasmas, transport is dominated by global magnetic tearing modes. For improved-confinement plasmas using inductive current profile control (PPCD), smaller-scale fluctuations at higher frequencies (\textgreater 50 kHz) may become more important as the global tearing modes are significantly reduced. In particular, drift-wave-like instabilities are theoretically unstable to the higher temperature and density gradients achieved during PPCD discharges. On the MST, an eleven chord Far-Infrared (FIR) laser-based diagnostic system with $\sim$ 8 cm spacing is used to measure electron density fluctuations by interferometry and far-forward collective scattering. The existing diagnostic measures line-integrated density fluctuations within the divergence of the probe beam covering a wavenumber range k$_{\mathrm{-}}$\textless 1.3 cm$^{-1}$, corresponding to k$_{\mathrm{-}}\rho _{\mathrm{s}}$ \textless 1.3 ($\rho_{\mathrm{s}}$ is the ion-sound Larmor radius). Experimentally, in PPCD plasmas, global tearing modes are reduced while high frequency coherent modes (50 \textless\ f \textless\ 140 kHz) emerge among broadband fluctuations. Correlations of these modes with sources of free energy, such as temperature and density gradients, will be investigated. Additionally, effects of increased plasma flow from a 1MW tangential NBI on high frequency density fluctuations will also be explored. [Preview Abstract] |
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CP8.00084: Studies of Millimeter Scale Magnetic Turbulence in Madison Symmetric Torus RFP Plasmas James B. Titus, Ephrem D. Mezonlin, Abdulgader F. Almagri, Paul W. Terry, John S. Sarff Work has been done to understand the cascade of turbulent magnetic field fluctuations and how the dissipation process is connected to particle heating and energization. Madison Symmetric Torus (MST) reversed-field pinch plasmas experience a quasiperiodic sawtooth relaxation cycle, where fluctuation levels increase due to tearing-mode driven magnetic reconnection bursts. These fluctuations that are anisotropic with respect to the equilibrium field may be related to powerful non-collisional ion heating, where the inferred scale for onset of strong dissipation is larger than classical dissipation. Previous measurements were done with a magnetic probe with 5 mm coil separation between 2 coils in each direction, but could only resolve k-spectra out to $+$/- 1.5 cm$^{-1}$. Measurements have recently been done with five times the spatial resolution, by decreasing the distance between coils (1 mm) and increasing the amount of coils (7) in each direction. These upgrades provide more resolution to areas of the spectrum that were previously too noisy to make any significant analysis. Initial analysis shows similar anisotropic behavior for larger values of k without deviation from the modeled spectrum, though it is unclear if the spectrum is exactly the same for k greater than 1.5 cm$^{-1}$. The dissipation range models well as a produced of a power law and exponential falloff. [Preview Abstract] |
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CP8.00085: Turbulent cascade of magnetic and kinetic energy in RFP plasmas A.F. Almagri, D.J. Thuecks, J.S. Sarff, P.W. Terry The dominant magnetic fluctuations in the reversed field pinch arise from large-scale tearing instabilities, but a broadband spectrum is also observed. Recent measurements in MST suggest that short wavelength fluctuations (spatially resolved to the ion gyro-radius scale, $\sim$1cm in our experiment) in both magnetic and electric fields arise via a nonlinear cascade driven by the tearing modes, but the mechanism responsible for energy transfer across scales remains poorly understood. Magnetic fluctuations dominate the power spectrum in the plasma edge at low frequencies ($f\leq 80$~kHz) but electric field fluctuations become dominant at high frequencies. This observation, paired with measurements of the coherence between fluctuations in magnetic and electric fields, may indicate different mechanisms are responsible for transferring energy across scales in the two spectra. Coherences between electric and magnetic fluctuations peak near the frequency where the fluctuation powers are found to be in equipartition. In addition to measuring coherences in frequency-wavenumber space, we will also examine radial coherence lengths and radial phases of electrostatic fluctuations to assist in identifying the mechanisms at work. [Preview Abstract] |
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CP8.00086: Outward Poynting flux due to electromagnetic fluctuations in an RFP D.J. Thuecks, K.J. McCollam, D.R. Stone In a reversed-field pinch (RFP) driven by a toroidal electric field, tearing modes not only generate the net EMF that sustains the equilibrium profile but are also expected to produce an outward flow of electromagnetic energy, or Poynting flux, to be dissipated at the plasma edge. In MST experiments, insertable edge probes measure both electrostatic $\tilde{E}$ and magnetic $\tilde{B}$ fluctuations, which are used to reconstruct the flux-surface average Poynting flux $< \tilde{E}\times\tilde{B} >$ as it varies with minor radius, time, and equilibrium parameters. Our initial results indicate that this outward flux is a significant fraction of the total input power on time average and increases to large values during the brief periods surrounding discrete magnetic relaxation events, or sawtooth crashes. The flux decreases with radius outside of the reversal surface, suggesting that the electromagnetic energy is deposited there and dissipated into the plasma. These results are qualitatively similar to expectation from a simple model of an incompressible fluid plasma with a solid, resistive boundary. [Preview Abstract] |
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CP8.00087: Magnetic Relaxation with Oscillating Field Current Drive on MST D.R. Stone, A.F. Almagri, K.J. McCollam, J.S. Sarff In oscillating field current drive (OFCD) poloidal and toroidal ac magnetic fields are inductively applied to the plasma to drive dc plasma current through magnetic relaxation. In OFCD experiments on the MST RFP up to 10{\%} additional current is added. Measurements of Ohm's Law terms including the dynamo mechanisms associated with magnetic relaxation are conducted during OFCD to better understand the relaxation dynamics and to possibly aid in optimizing OFCD performance. The fluctuation-induced dynamo ${\left\langle {\tilde{{E}}\cdot \tilde{{B}}} \right\rangle } \mathord{\left/ {\vphantom {{\left\langle {\tilde{{E}}\cdot \tilde{{B}}} \right\rangle } B}} \right. \kern-\nulldelimiterspace} B\cong {\left\langle {\tilde{{V}}\times \tilde{{B}}} \right\rangle -\left\langle {\tilde{{J}}\times \tilde{{B}}} \right\rangle } \mathord{\left/ {\vphantom {{\left\langle {\tilde{{V}}\times \tilde{{B}}} \right\rangle -\left\langle {\tilde{{J}}\times \tilde{{B}}} \right\rangle } {ne}}} \right. \kern-\nulldelimiterspace} {ne}$ and its constituent Hall dynamo ${\left\langle {\tilde{{J}}\times \tilde{{B}}} \right\rangle } \mathord{\left/ {\vphantom {{\left\langle {\tilde{{J}}\times \tilde{{B}}} \right\rangle } {ne}}} \right. \kern-\nulldelimiterspace} {ne}$ are measured in the edge ($r \mathord{\left/ {\vphantom {r {a=.9}}} \right. \kern-\nulldelimiterspace} {a=.9})$ using inserted probes, and compared to $\eta J-E$. A passive secondary-emission capacitive probe was developed to measure the electric field fluctuations (accurate when the plasma temperature \textgreater 20 eV). Ohm's Law balance is observed during OFCD. During sawtooth relaxation events with OFCD the edge dynamo is enhanced compared to events without OFCD. Between events the edge dynamo is a few V/m and opposes the edge current. This opposing dynamo is required to balance Ohm's Law during OFCD since the oscillating electric field adds excess current at the plasma edge and the dynamo tends to flatten the current profile. Supported by the US DOE and the NSF. [Preview Abstract] |
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CP8.00088: Coupled Current-Momentum Relaxation in Reversed Field Pinch Plasmas Joseph Triana, A.F. Almagri, J.S. Sarff, J.P. Sauppe, C.R. Sovinec Magnetic fluctuation induced emf and stresses have been shown to be critical in the self-organization process in RFP physics. These forces were previously measured in the edge of MST, $(r/a>0.85)$, revealing that the MHD dynamo, $<\tilde{v}x\tilde{B}>$, and the Hall dynamo, $<\tilde{j}x\tilde{B}>/en$, terms are both large but dominate Ohm's law at different radial locations. The term $<\tilde{j}x\tilde{B}>$ is the Maxwell stress, which appears in momentum balance for flow parallel to B, coupling the dynamo electric field to plasma momentum. NIMROD simulations that include the Hall term in Ohm's law and the gyro-viscosity in the momentum balance reproduce behavior akin to previous measurements in MST and predict a rich radial structure of the Hall term. We have developed a probe designed to measure the Hall term up to depths of $r/a>0.5$ in plasmas with parameters suitable for NIMROD simulations, making direct comparison straightfoward. Using a toroidal array located at the plasma's edge and correlating with localized measurements of $\tilde{B}$, we can infer the radial structure of the individual tearing modes via pseudospectral analysis. These radial profiles will be compared with both NIMROD and DEBS (single fluid MHD) code predictions. [Preview Abstract] |
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CP8.00089: Acceleration of fast test ions during magnetic reconnection in the MST RFP S. Eilerman, J.K. Anderson, J.A. Reusch, M.D. Nornberg, J. Kim, D.J. Den Hartog, L. Lin, J. Titus, D. Liu, G. Fiksel, S. Polosatkin, V. Belykh Although the mechanism behind anomalous ion heating during periodic magnetic reconnection events in the RFP is not yet fully understood, many features of the energization process or processes have been identified. Recent neutral particle analyzer (NPA) measurements of the acceleration of NBI-born fast ions on MST provide new information about at least one of these energization processes. Modeling shows that the tangentially-viewing NPA primarily measures the parallel component of high-energy, core-localized beam ions. The fast test ions are injected at energies between 8-25 keV and gain between 3-7 keV during a reconnection event. Higher initial energies correspond to higher energy gains, which can be explained by a runaway process in which a parallel electric field is inductively generated by changes in the equilibrium magnetic field. The magnitude of the measured ion acceleration is consistent with a 30 V/m, 100 $\mu $s core electric field calculated from equilibrium reconstructions. Ion deceleration of a comparable magnitude is observed during counter-current beam injection in which the inductive electric field opposes the fast ion motion. The NPA will be moved to a radial viewport so that the perpendicular ion velocity can be sampled to gain further insight into the mechanism(s) at work. This work is supported by the US DOE and NSF. [Preview Abstract] |
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CP8.00090: Statistical analysis of variations in ion heating at individual reconnection events in the MST RFP D. Craig, M.S. Cartolano, D.J. Den Hartog, M.D. Nornberg, S.T.A. Kumar Ion heating in the reversed field pinch (RFP) far exceeds collisional energy transfer from electrons and the mechanism for this heating remains unknown. The connection between ion heating and other physical processes in the plasma is evaluated by studying variations in the amount of ion heating at individual reconnection events in the Madison Symmetric Torus (MST). Ion temperature is measured spectroscopically by Doppler broadening of C V and C VI emission lines. Correlation of the change in ion temperature with the change in magnetic energy confirms that the magnetic energy is the source for the heating. Correlation of the change in ion temperature with individual tearing mode amplitudes indicates that the edge-resonant modes are better predictors for the amount of global ion heating then the core-resonant modes. There is also a strong correlation between ion heating and dynamo activity. Simultaneous measurements of the ion temperature at different toroidal locations reveal a toroidal asymmetry to the ion heating in MST. These results present challenges for existing heating theories and suggest a stronger connection between edge-resonant tearing modes, dynamo activity, and ion heating than has been previously thought. This work was supported by the U.S.D.O.E. and the N.S.F. [Preview Abstract] |
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CP8.00091: Probing RFP Density Limits and the Interaction of Pellet Fueling and NBI Heating on MST K.J. Caspary, B.E. Chapman, J.K. Anderson, S.T. Limbach, S.P. Oliva, J.S. Sarff, J. Waksman, S.K. Combs, C.R. Foust Pellet fueling on MST has previously achieved Greenwald fractions of up to 1.5 in 200kA improved confinement discharges. Additionally, pellet fueling to densities above the Greenwald limit in 200 kA standard discharges resulted in early termination of the plasma, but pellet size was insufficient to exceed the limit for higher current discharges. To this end, the pellet injector on MST has been upgraded to increase the maximum fueling capability by increasing the size of the pellet guide tubes, which constrain the lateral motion of the pellet in flight, to accommodate pellets of up to 4.0 mm in diameter. These 4.0 mm pellets are capable of triggering density limit terminations for MST's peak current of 600 kA. An unexpected improvement in the pellet speed and mass control was also observed compared to the smaller diameter pellets. Exploring the effect of increased density on NBI particle and heat deposition shows that for MST's 1 MW tangential NBI, core deposition of 25 keV neutrals is optimized for densities of 2 - 3 x 10$^{19}$ m$^{-3}$. This is key for beta limit studies in pellet fueled discharges with improved confinement where maximum NBI heating is desired. An observed toroidal deflection of pellets injected into NBI heated discharges is consistent with asymmetric ablation due to the fast ion population. In 200 kA improved confinement plasmas with NBI heating, pellet fueling has achieved a Greenwald fraction of 2.0. Work supported by US DoE. [Preview Abstract] |
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CP8.00092: Investigating fluctuations that influence transport with heavy ion beam probe measurements in the MST RFP P.J. Fimognari, D.R. Demers Drift-wave-like modes are an important instability impacting transport in confined toroidal plasmas. They are the major driver in the tokamak and also likely govern it in optimized stellarators and select RFP regimes. The HIBP can characterize this turbulence and related phenomena by measuring fluctuations of density and potential in the plasma interior. It helps quantify amplitudes, wavelengths, cross phases, and other characteristics necessary for validation of gyrokinetic codes. The unique ability of the HIBP to acquire these quantities from the plasma core, in multiple magnetic configurations, will advance understanding of transport. Properties of low and high confinement regimes in many magnetic configurations, including the RFP, differ. Transport in standard confinement RFP plasmas is governed by magnetic fluctuations arising from tearing instabilities; these fluctuations are reduced with current profile control and the residual transport is likely electrostatic. An HIBP on MST is measuring radial profiles of fluctuations (up to 500 kHz) in the plasma interior. Temperature and density gradients in these plasmas are strongest in the region r/a $\sim$ 0.5-0.8, accessible to the HIBP. These measurements, made with the first and only HIBP operating on an RFP, will shed light on dynamics influencing transport. [Preview Abstract] |
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CP8.00093: Constraining $Z_{\mathrm{eff}}$ and Particle Diffusion using X-ray Spectroscopy on MST J.D. Lee, A.F. Almagri, J.K. Anderson, B.E. Chapman, J.S. Sarff, R.W. Harvey The X-ray spectroscopy diagnostic on MST consists of six SXR detectors and six HXR detectors capable of measuring photons in the energy range $3-25$\ keV and $10-60$\ keV, respectively. The detectors can be installed on any of 17 ports viewing a poloidal cross-section, with tangency radii from $r/a=0.87$ inboard to $r/a=0.84$ outboard. Measurements have been made in enhanced confinement plasmas with plasma current of $\sim400$\ kA, electron density of $\sim0.6\times10^{19}$\ m$^{-3}$, and electron temperature of $\sim1200$\ eV . Measured spectra are used to constrain radial profiles of $Z_{\mathrm{eff}}$ and $D_{\mathrm{r}}$ by comparison with spectra calculated from CQL3D, a Fokker-Planck solver [R.W. Harvey and M.G. McCoy, ``The CQL3D Fokker-Planck Code,'' General Atomics (2011)]. The plasma equilibria required for CQL3D are produced by the reconstruction code MSTfit. Minimization is performed using a custom parallel simplex algorithm on a 248 core cluster. [Preview Abstract] |
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CP8.00094: SXR Double-Foil Measurements of Electron Temperature and Impurity Structures on MST M.B. McGarry, P. Franz, D.J. Den Hartog, J.A. Goetz, J. Johnson A new diagnostic has been developed that uses time-resolved soft x-ray (SXR) emission to measure tomographically reconstructed x-ray emissivity and double-foil electron temperature ($T_{e})$ either directly from brightness or from emissivity mapped to flux surfaces on the MST reversed field pinch. Full radial profiles of double-foil electron temperature have been compared with Thomson scattering $T_{e}$ measurements, confirming that electron temperatures typically reach $\sim$ 1.2-1.8keV during high-current improved-confinement discharges. The diagnostic has also identified enhanced SXR emission from island structures whose poloidal locations are consistent with external magnetic measurements in both rotating and locked discharges. Studies of locked SXR emissivity structures sometimes indicate corresponding $T_{e}$ structures that are correlated to the magnetic island structure and have amplitudes of less than 20{\%} of the core $T_{e}$. Additionally, other discharges exhibit a ring of SXR emission resulting from an enhancement in local effective ionic charge, indicating a hollow impurity density profile. This profile results from classical transport of impurity ions out of the core via the ion temperature screening mechanism. [Preview Abstract] |
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CP8.00095: SXR Diagnosis of Electron Temperature Dynamics in Reversed-Field Pinches P. Franz, M. Gobbin, L. Marrelli, A. Ruzzon, F. Bonomo, A. Fassina, E. Martines, M.B. McGarry, D.J. Den Hartog, J.A. Goetz, J. Johnson Sophisticated multi-chord diagnostics have been developed on RFX-Mod and MST for soft x-ray (SXR) emissivity and electron temperature (Te) profile measurements. Quasi Single Helicity (QSH) plasmas, where a single m$=$1 tearing mode dominates the MHD magnetic spectrum, have been analyzed. To improve the SXR and Te profile reconstructions in the presence of QSH, algorithms that reproduce the magnetic topology have been developed to map the measurements on flux surfaces. In RFX-Mod the analysis shows a strong correlation between magnetic QSH and the appearance of a SXR structure in the emissivity distribution. The SXR structure corresponds to a thermal structure in the Te profile. Differences in the temperature gradient between the rising phase and the flattop phase of QSH have been analyzed. In the rising phase the gradient increases and the thermal structure grows in size. During the flattop phase, where the dominant mode amplitude is constant, the structure is intermittent, with several crashes that make the Te profile flatter. Similar measurements at MST confirm the correspondence between QSH and SXR structure. The analysis shows that this structure can be correlated with a thermal structure but that also impurity could play an important role. [Preview Abstract] |
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CP8.00096: High-Repetition-Rate Laser for Thomson Scattering on the MST Reversed-Field Pinch William C. Young, L.A. Morton, E. Parke, D.J. Den Hartog The MST Thomson scattering diagnostic has operated with a new, high-repetition-rate laser system, demonstrating 2 J pulses at repetition rates up to 50 kHz. The pulse repetition rate can maintain 2 J pulses for bursts of 5 kHz (sustained for 5 ms), to 50 kHz (for 10 bursts of 240 \(\mu\)s each). The 1064 nm laser currently employs a \textit{q}-switched, diode pumped Nd:YVO\({}_4\) master oscillator, four Nd:YAG amplifier stages, and a Nd:glass amplifier. The future implementation of the full laser as designed, including a second Nd:glass amplifier, is expected to produce bursts of 2 J pulses at a repetition rate of at least 250 kHz. The new laser integrates with the same collection optics and detectors as used by the present MST Thomson scattering system: 21 spatial points across the MST minor radius with sensitivity over a 10 eV - 5 keV range. Initial results will be presented from application of this diagnostic to parametric scans of MST plasmas, evolution of energy confinement during spontaneous enhanced confinement periods, and non-Maxwellian electron distributions. [Preview Abstract] |
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CP8.00097: MST's Programmable Power Supplies: Bt Update, Bp Prototype D.J. Holly, B.E. Chapman, K.J. McCollam, J.C. Morin, M.A. Thomas MST's toroidal field programmable power supply (Bt PPS) has now been in operation for several years and has provided important new capabilities. One of the primary goals for the Bt PPS is the partial optimization of inductive current profile control, involving control of the poloidal electric field. The Bt PPS has achieved fluctuation reduction over MST's entire range of Ip. At the largest Ip, the Bt PPS achieves fluctuation reduction with a smaller poloidal electric field than the previous passive system, implying that substantially longer periods of current profile control may be possible. The Bt PPS has also been used to produce Ohmic tokamak plasmas in MST. With an applied toroidal field of 0.135 T, and q(a) \textgreater~2, the estimated energy confinement time is roughly consistent with neo-Alcator scaling. Driving q(a) \textless~2 with larger Ip, the confinement time degrades, but the discharge duration does not terminate prematurely. To fully optimize current profile control and to test MST operational limits, a PPS is also needed for the Bp circuit. Currently in prototype stage, the Bp PPS will feature a number of innovations to increase its flexibility and performance. Isolated charging, control, and monitor systems will eliminate charging relays, reduce coupling between modules, and minimize capacitor heating. Seven-level pulse width modulation will reduce output ripple and switching losses. Solid state shorting bars will eliminate shorting relays and minimize wiring. A balanced switching algorithm will minimize capacitive noise generation. [Preview Abstract] |
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CP8.00098: Electron Bernstein Wave Studies in MST Andrew Seltzman, Jay Anderson, Cary Forest, Paul Nonn, Mark Thomas, Joshua Reusch, Eric Hendries The overdense condition in a RFP prevents electromagnetic waves from propagating past the extreme edge. However use of the electron Bernstein wave (EBW) has the potential to heat and drive current in the plasma. MHD simulations have demonstrated that resistive tearing mode stability is very sensitive to the gradient in the edge current density profile, allowing EBW current drive to influence and potentially stabilize tearing mode activity. Coupling between the X-mode and Bernstein waves is strongly dependent on the edge density gradient. The effects on coupling of plasma density, magnetic field strength, antenna radial position and launch polarization have been examined. Coupling as high as 90{\%} has been observed. Construction of a 450kw RF source is complete and initial experimental results will be reported. The power and energy of this auxiliary system should be sufficient for several scientific purposes, including verifying mode conversion, EBW propagation and absorption in high beta plasmas. Target plasmas in the 300-400kA range will be heated near the reversal surface, potentially allowing mode control, while target plasmas in the 250kA range will allow heating near the core, allowing better observation of heating effects. Heating and heat pulse propagation experiments are planned, as well as probing the stability of parametric decay during mode conversion, at moderate injected power. [Preview Abstract] |
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CP8.00099: MHD Simulation of RF Current Drive in MST J.A. Goetz, E.R. Hendries, J.K. Anderson, C.B. Forest, J.A. Reusch, A.H. Seltzman, C.R. Sovinec, S. Diem, R.W. Harvey Auxiliary current drive using the electron Bernstein wave (EBW) may advance the performance of the RFP. In prior computations, a hypothetical edge-localized current is shown to suppress tearing activity that governs transport in the RFP. Ideal conditions for tearing stabilization include reduced toroidal induction, and precise width and radial position of the current drive. To support MST EBW studies, an integrated modeling scheme incorporates ray tracing and Fokker-Plank predictions of auxiliary current into single fluid MHD. Simulations at low Lundquist number (S $\approx $ 10$^{4})$ agree with the previous work but at MST-like S (S $\approx $ 3x10$^{6})$ show unexpected results. The effect on the current profile by the rf-driven force decreases in magnitude and widens considerably as S increases. Simulations reproduce the experimentally observed periodic current profile relaxation events (sawteeth). With rf drive, reduction of tearing mode amplitudes is seen, but is limited to periods between each sawtooth, which persist with up to 10 MW of rf. Prolonged low tearing amplitudes are predicted with the combination of current drive and reduced toroidal loop voltage, consistent with previous conclusions. Finally, these simulations show that the resistivity profile has a strong effect on the optimal current drive profile for mode stabilization. [Preview Abstract] |
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CP8.00100: Overview of the RFX fusion science program Piero Martin, Maria Ester Puiatti RFX-mod is a toroidal device for fusion plasma magnetic confinement, which can be operated both as a reversed field pinch and as tokamak. In the former configuration it can reach plasma current up to 2 MA, while as a tokamak current is limited to 0.15 MA, due to the toroidal field available. This paper provides an overview of the 2013 RFX physics, both from an experimental and theoretical/numerical point of view. Highlights of the program concern MHD stability feedback control studies - both on physics and engineering -- applied to the RFP and tokamak configurations, edge physics, in particular as far the RFP density behavior is concerned, 3D physics, progress in understanding transport mechanisms, and results of advanced numerical simulations. Initial results on the exposure of various material samples to the RFX plasmas will also be presented. Results on the physics of low qedge tokamak (qedge less than 2) will be discussed. [Preview Abstract] |
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CP8.00101: Multi configuration axysimmetric plasma shaping control on RFX-mod R. Cavazzana, C. Finotti, G. Marchiori, G. Manduchi, L. Zanotto, O. Kudlacek, M. Zuin, P. Franz, P. Zanca, L. Marrelli, T.C. Luce, G.L. Jackson RFX-mod is a flexible experiment, equipped with a full coverage MHD control system, composed by 192 (48 toroidal x 4 poloidal) coils. Being built as a high current RFP (a/R = 0.46m/2.0m; Ip max 2 MA) it has recently operated also as a low current circular tokamak ( Bt = 0.45 T; Ip 85 kA @ q(a) $\sim$ 3 ; Ip 150 kA @ q(a) $\sim$ 2), achieving the full stabilization of m=2, n=1 mode at q(a) $\sim$ 2. In order to extend the significance of MHD control experiments, there arose the need of creating non circular shaped discharges, exploiting the flexibility of the 16 shaping coils of the machine. Plasma of with moderate elliptical and triangular shape can be obtained both in tokamak and RFP configuration. Moreover tokamak D-shaped plasmas with double X-point have been obtained by proper reconfiguration of the power supply. The design structure and the experimental performance of the new shape reconstruction, plasma position and shape real-time control algorithms, tested in both RFP and tokamak configuration, are presented and discussed, along with some preliminary results of the MHD mode interaction and control behavior with the modified plasma shapes. [Preview Abstract] |
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CP8.00102: KTX circuit model and discharge waveform prediction Wei Bai, T. Lan, W.Z. Mao, W. You, H. Li, A.D. Liu, J.L. Xie, S.D. Wan, W.D. Liu, L. Yang, P. Fu, C.J. Xiao, W.X. Ding The Keda Torus eXperiment (KTX) is a constructing reversed field pinch (RFP) device in University of Science and Technology of China. The KTX power supply system includes the Ohmic heating, field shaping and toroidal power supply systems, which produce the Ohmic field, equilibrium field and toroidal field, respectively. The detailed circuit model will be introduced in this poster. Another purpose is to predict its discharge waveforms using the modified Bessel function mode (MBFM), which describes the evolution of plasma current and magnetic flux in RFP base on Taylor theory. Furthermore, the power supply requirements of external field shaping winding are also predicted in the model, which will be very helpful for the design of plasma equilibrium controlling system. [Preview Abstract] |
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CP8.00103: TOKAMAK HBT-EP |
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CP8.00104: HBT-EP Program: Active MHD Mode Dynamics {\&} Control G.A. Navratil, S. Angelini, J. Bialek, A.H. Boozer, P. Byrne, A.J. Cole, B. DeBono, P. Hughes, J.P. Levesque, M.E. Mauel, Q. Peng, N. Rath, D. Rhodes, C. Stoafer The HBT- EP active mode control research program aims to (i) quantify external kink dynamics and multimode response to applied magnetic perturbations, (ii) understand the relationship between control coil configuration, conducting and ferritic wall effects, and active feedback control effectiveness, and (iii) explore advanced feedback algorithms. Biorthogonal decomposition is used to observe multiple simultaneous resistive wall modes (RWM) with poloidal mode numbers up to m$=$9 and toroidal mode numbers up to n$=$4. Transitions between dominant poloidal mode numbers were observed for m$=$4/n$=$1 to m$=$3/n$=$1 accompanied by a simultaneous m$=$7/n$=$2 to m$=$6/n$=$2 transition. Improved visualization of MHD kink mode structure was achieved using a toroidal viewing fast camera recording of visible light emission that augment magnetic probe array data. A partial ferritic wall has been installed onto the resistive HBT-EP wall elements to study the ferritic RWM onset. A new Thomson scattering diagnostic has been installed on HBT-EP aiming at 10 spatial point measurements of density and temperature. A 512 core GPU based low latency (\textless 14 microsec) MIMO control has been implemented with 96 inputs and 64 parallel outputs and used to demonstrate the first observation of Adaptive Control of the RWM. [Preview Abstract] |
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CP8.00105: High Speed Videography on HBT-EP Sarah Angelini, J.P. Levesque, M.E. Mauel, G.A. Navratil A Phantom 7.3 fast camera has been installed on HBT-EP to study visible light fluctuations. The light originates from interactions between the plasma and neutrals, and the fluctuations correlate with plasma perturbations. Using Abel inversion and biorthogonal decomposition techniques, a picture of the plasma's response and instabilities as a result of applied magnetic perturbations can be reconstructed. This poster will compare the structures recorded from both the fast camera and the magnetic diagnostics, and demonstrate a forward modelling method for predicting the camera's data from a simulated mode. [Preview Abstract] |
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CP8.00106: Initial Ferritic Wall Mode studies on HBT-EP Paul Hughes, J. Bialek, A. Boozer, 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], the FWM was not observed in JFT-2M [3], probably due to eddy current stabilization. Using its high-resolution magnetic diagnostics and positionable walls, HBT-EP has begun exploring the dynamics and stability of plasma interacting with high-permeability ferritic materials tiled to reduce eddy currents. We summarize a simple model for plasma-wall interaction in the presence of ferromagnetic material, describe the design of a recently-installed set of ferritic shell segments, and report initial results. \\[4pt] [1] Kurtz, R.J., et. al. 2009 \textit{J Nucl Mater } 386-388\\[0pt] [2] Bergerson, W., et. al. 2008 \textit{Phys Rev Lett } \textbf{101}\\[0pt] [3] Tsuzuki, K., et. al. 2006 \textit{Nucl Fus } \textbf{46} [Preview Abstract] |
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CP8.00107: Advanced feedback control of resistive wall modes with high speed GPU on HBT-EP Qian Peng, Nikolaus Rath, Jeffrey Levesque, Dov Rhodes, Rebecca Arbacher, Patrick Byrne, Michael Mauel, Gerald 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. Up to 80 feedback sensors are used to detect the rotating kink mode, which is used by the feedback algorithm to calculate the desired current on the 40 control coils. We are able to suppress the fast rotating mode with feedback using poloidal sensors, but the performance with high gain is limited by the excitation of a low frequency rotating wall mode [1]. We report a study aimed at improving the performance through several ways, including: (a) adapting an algorithm that uses both radial and poloidal sensors, and (b) tracking the state of the modes using a Kalman filter.\\[4pt] [1] N. Rath, et al, Plasma Phys. Control. Fusion 55 (2013) 084003 (9pp). [Preview Abstract] |
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CP8.00108: Non-Ideal Plasma Equilibrium Model for RWM Simulations Dov Rhodes, J. Bialek, A.H. Boozer, A.J. Cole, M.E. Mauel, G.A. Navratil, Q. Peng The resistive wall mode (RWM) in tokamaks has been successfully simulated with the VALEN code [1], which computes the coupling between the plasma and conducting surfaces in the device. Presently, VALEN uses an ideal plasma equilibrium computed from DCON [2], neglecting non-ideal torques which play a central role in RWM stabilization. We are developing a numerical tool to include these non-ideal magnetic torques in future VALEN simulations of the RWM. As a first step, we include the effects of resonant torques on mode-rational surfaces, as formulated in a simplified non-ideal plasma response model [3]. The next phase of this research will include additional magnetic torques from neoclassical toroidal viscosity. Preliminary simulation results will be presented and compared with resonant perturbation experiments on HBT-EP. \\[4pt] [1] J. Bialek, Phys. Plasmas 8, 2170 (2001)\\[0pt] [2] A.H. Glasser, Los Alamos Technical Report No. LA-UR-95-528, 1995\\[0pt] [3] R. Fitzpatrick, Phys. Plasmas 17, 112502 (2010) [Preview Abstract] |
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CP8.00109: Fabrication, Installation and First Results of the HBT-EP Shaping Coil P. Byrne, J.P. Levesque, D. Rhodes, Q. Peng, G.A. Navratil, M.E. Mauel A low-mutual-inductance, zero-net-turn coil and its capacitive power supply have been fabricated and installed on the HBT-EP Tokamak. The coil is used to locally shape the HBT-EP circular cross section, up to and including the creation of a poloidal field null above the inboard midplane. This will enable HBT-EP's first investigation of the effects of shaping on the MHD multimode spectrum. Post-installation tests have affirmatively proven the ability of the coil to impose a continuum of shaping, from circular to fully diverted. Results of initial experiments with are also provided and compared with simulations, and show a clear effect of shaping on HBT-EP's MHD mode spectrum. [Preview Abstract] |
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CP8.00110: Multi-Point Thomson Scattering Upgrade and Measurements on HBT-EP C.C. Stoafer, J.P. Levesque, M.E. Mauel, G.A. Navratil, H.S. McLean The Thomson scattering (TS) system from SSPX [1] has been successfully installed on HBT-EP. When fully operational, the TS system will provide ten spatial point measurements and significantly enhance our single point system. We report our first results, using a single spatial point, and measurements of the $T_e$ and $n_e$ evolution through typical HBT-EP discharges. In addition to the SSPX system, we have installed a new viewing dump. As a result, stray light has been reduced by over an order of magnitude, giving a high signal to noise ratio. A new collection lens and fiber bundle system are being manufactured to allow measurement of all ten spatial points. The multipoint 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. \\[4pt] [1] H.S.~McLean, {\it et. al.\/}, Rev.~Sci. Instr. {\bf 72}, 577, (2001). [Preview Abstract] |
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CP8.00111: Soft X-ray Analysis of HBT-EP Kink and Tearing Modes J.P. Levesque, M.E. Mauel, G.A. Navratil, C.C. Stoafer, L. Delgado-Aparicio, T.B. Dohrn Measurement of soft x-ray (SXR) emission from plasmas is a useful diagnostic for studying internal plasma dynamics including MHD mode structures and equilibrium evolution. We present analysis of tearing and kink mode activity in the HBT-EP tokamak using a combination of SXR and external magnetic measurements. Soft x-ray emission is measured using a 16-channel diode array viewing the poloidal cross-section. Emission characteristics of naturally-occurring $m/n=2/1$ and 3/1 tearing and kink modes are compared with expectations from a synthetic diagnostic. Core 1/1 modes are observed through localized emission enhancement. The internal plasma response to external magnetic perturbations is investigated, and compared with magnetic response measurements. An upgrade to the HBT-EP SXR diagnostic is proposed, including multiple filters and viewing angles to enable tomographic reconstruction of emissivity and temperature profiles. [Preview Abstract] |
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CP8.00112: Tokamak Halo Currents Allen Boozer A halo current flows for part of its path through the plasma edge and for part through the chamber walls and can be tenths of the plasma current. The primary interest in halo currents is the large force that they can exert on machine components. Two discordant constraints are central to the theory: (1) Halo currents must produce the magnetic field distribution required to maintain plasma force balance---a distribution that depends on the two angular coordinates of a torus. (2) Halo currents must flow along the magnetic field lines in the plasma, which implies a dependence on a linear combination of the two angular coordinates---only one angular coordinate is free. The physics basis of these two constraints is explained as is their application to the calculation of the properties of halo currents, such as their broad toroidal spectrum. Existing codes could be used to (1) provide detailed comparisons with experiments to validate that the critical elements of physics are adequately included, (2) allow more complete predictions for future machines such as ITER, and (3) design shunts and resistive elements to ensure halo currents follow paths that are the least damaging to the machine. [Preview Abstract] |
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CP8.00113: CTH, HSX AND OTHER STELLARATORS |
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CP8.00114: Recent Progress in Compact Toroidal Hybrid Research D.A. Maurer, M. Cianciosa, J.D. Hanson, G.J. Hartwell, J.D. Hebert, J.L. Herfindal, S.F. Knowlton, M.C. ArchMiller, P. Traverso, M. Pandya, X. Ma The Compact Toroidal Hybrid (CTH) experiment is investigating the passive avoidance of disruptions with the addition of a small amount of vacuum transform provided by external coils. In ohmically-driven stellarator plasmas, disruption suppression depends upon the particular disruption scenario. Recent progress on the suppression of low edge q, density limit, and vertically unstable plasma disruptions is overviewed. Interpretation of these results makes use of 3D equilibrium reconstructions using the V3FIT code [1]. Several new diagnostic tools, including new magnetic sensors for MHD fluctuation studies, a multipoint Thomson scattering system, and a 2D soft x-ray two-color camera system are under development to further enable our understanding of CTH disruption dynamics. Future research directions, including plans for an island divertor, will be discussed. \\[4pt] [1] J. D. Hanson, et al., (2009) Nucl. Fusion, 49, 075031 [Preview Abstract] |
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CP8.00115: 3D Equilibrium Reconstruction with Improved Magnetic Diagnostics on the Compact Toroidal Hybrid Xinxing Ma, James D. Hanson, Gregory J. Hartwell, Stephen F. Knowlton, David A. Maurer \newcommand{\io}{\raisebox{-3pt}{$\mathchar'26\mkern-14mu$} $\iota_{vac}$} Equilibrium reconstructions using the three-dimensional V3FIT code $[1]$ have been performed for low density (low $\beta$) current carrying plasmas on the Compact Toroidal Hybrid (CTH), a torsatron in which the magnetic configuration can be strongly modified by an ohmically-driven plasma current. These reconstructions use 50 external magnetic diagnostic measurements, including segmented and full Rogowski coils, saddle loops, poloidal and radial magnetic pickup coils. Time dependent 3D reconstructions document the evolution of the plasma cross section and current profile, with calculated value of $\beta_{\theta}$ much less than the plasma internal inductance, $l_i$. Reconstructions typically show the plasma to move outward in major radius and become less elongated in poloidal cross section with increasing plasma current. In plasmas with low vacuum transform (\io$\approx 0.04$), the plasma current profile peaks with $l_i$ above $0.84$ when the total transform is $1/2$, which typically leads disruption. With values of \io $\geq 0.1$, $l_i$ remains low ($\approx 0.7$), and the discharges no longer disrupt.\\[4pt] [1] J. D. Hanson et al., Nucl. Fusion $\mathbf{49}$, 075031 (2009) [Preview Abstract] |
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CP8.00116: MHD mode analysis of magnetic fluctuations on the Compact Toroidal Hybrid M.D. Pandya, G.J. Hartwell, S.F. Knowlton, D.A. Maurer \newcommand{\g}{\raisebox{-3pt}{$\mathchar'26\mkern-14mu$} $\iota$} Strong MHD activity is typically observed in the Compact Toroidal Hybrid (CTH) when the value of the edge rotational transform, \g(a), is near rational values. A unique feature of the CTH torsatron-tokamak hybrid is that it operates with a variable level of vacuum rotational transform, \g$_{vac}$, provided by the external torsatron coils. To measure MHD activity, CTH is equipped with poloidal and toroidal pick-up coil arrays. A new poloidal array with 36 pick-up coils to measure poloidal magnetic field and 18 to measure radial field, has been recently installed. During the current rise when \g$(a)\sim 1/2$ or $1/3$, rotating m/n=2/1 or 3/1 modes respectively, are typically present. Fluctuation analysis using Singular Value Decomposition (SVD) [1,2] leads to identification of dominant spatial and temporal modes present in the plasma. Prior to density-limit disruptions, a 2/1 mode is observed to grow and lock in the laboratory frame for discharges with low vac. transform \g$_{vac}(a)\le 0.08$. However, for \g$_{vac}(a)>0.1$ the 2/1 mode is saturated, and a relatively weak 3/2 mode is also present prior to disruption.\\[4pt] [1] Dudok de Wit et al., Phys. Plasmas 1 (1994),3288 \\[0pt] [2] Kim et al., PPCF 41 (1999),1399 [Preview Abstract] |
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CP8.00117: NIMROD Modeling of Plasma Discharges in the Compact Toroidal Hybrid J. Hebert, J.D. Hanson, D.A. Maurer, M.G. Schlutt, C.R. Sovinec The 3D extended MHD code NIMROD\,[1] has been modified to model plasma discharges in the Compact Toroidal Hybrid (CTH) torsatron. NIMROD has been used to reproduce CTH vacuum fields and has shown the formation of magnetic islands when a constant loop voltage is applied in a zero $\beta$ simulation using constant conductivity\,[2]. To more accurately model CTH discharges, the NIMROD plasma model has been extended to include self-consistent ohmic heating using temperature dependent resistivity. Approximating the initial conditions for CTH discharges (flat temperature and density profiles with $T_e\sim30$eV and $n_e\sim1\times10^{18}$m$^{-3}$) in NIMROD is done through the following process: (1) CTH vacuum fields are loaded into the NIMROD domain. (2) A uniform temperature of $30\,\mbox{eV}$ is set across the domain. (3) NIMROD evolves the temperature with rapid parallel diffusion and a cold wall boundary condition to remove heat on open field lines. (4) The density is set as a multiple of the temperature. Simulations using this initial condition and the extensions to the plasma model with an experimental loop voltage reproduce the CTH current ramp rate.\\[4pt] [1] C.R. Sovinec et al. 2004 J. Comp. Phys., 195, 355 (2004).\\[0pt] [2] M.G. Schlutt et al. 2012 Nucl. Fusion 52 103023 [Preview Abstract] |
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CP8.00118: Two-dimensional Electron Temperature Measurements Using Soft X-Ray Emission on the Compact Toroidal Hybrid J.L. Herfindal, G.J. Hartwell, S.F. Knowlton, D.A. Maurer Electron temperature measurements are important in characterizing the equilibrium as well as MHD fluctuations in fusion plasmas. The Compact Toroidal Hybrid (CTH; $R_0 = 0.75\,$m, $a_p\sim 0.2\,$m, B$\leq 0.7\,$T, $n_e \leq 5~$x$~10^{19}\,$m$^{-3}$, $I_p \leq 80\,$kA, $T_e \leq 200~\,$eV) torsatron has an extensive collection of emissivity diagnostics: four pinhole type cameras, a bolometer, a spectrometer, and a prototype two-color camera. Improvements have been made on the current prototype two-color camera to reduce the electronic noise and ensure a light-tight camera housing. Amplifiers with a bandwidth of approximately $40\,$kHz have been implemented for study of both equilibrium and electron temperature fluctuations due to low frequency MHD activity. Electron temperature estimates from the SXR spectrometer, two-color camera, and conductivity measurements are compared. Two additional two-color cameras are currently under construction and will enable the tomographic reconstruction of the electron temperature. [Preview Abstract] |
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CP8.00119: Design and implementation of a Thomson scattering diagnostic for the Compact Toroidal Hybrid P.J. Traverso, D.A. Maurer, G.J. Hartwell, S.F. Knowlton, M.C. ArchMiller, M.M. Goforth The Compact Toroidal Hybrid (CTH) experiment is investigating the avoidance of disruptions in ohmically driven torsaton plasmas as the ratio of vacuum transform to the total transform is changed. To better characterize these plasmas under this wide range of magnetic configurations, a new Thomson scattering diagnostic is being implemented to measure electron temperature and density profiles. These important internal profile measurements will be incorporated into the V3FIT code [1] to enable better 3D equilibrium reconstruction. The Thomson scattering system uses a frequency doubled Continuum PL DLS 2 J Nd:YaG laser [2]. The incident beam is passed vertically through an entrance Brewster window and a baffle system to minimize stray laser light. The beam exits through another Brewster window to an external beam dump. Polarization optics are planned to maximize the scattered light directed to the collection system for the specific scattering geometry of CTH.\\[4pt] [1] J. D. Hanson, et al., Nucl. Fusion, 49 (2009) 075031\\[0pt] [2] D. J. Schlossberg, et al., Rev. Sci. Instr. 82, 10, 10E335 [Preview Abstract] |
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CP8.00120: Development of a Millimeter Wave Polarimeter-Interferometer Diagnostic for the Compact Toroidal Hybrid D.A. Ennis, M.C. ArchMiller, M.R. Cianciosa, J.D. Hanson, G.J. Hartwell, S.F. Knowlton, D.A. Maurer A new millimeter wave polarimeter-interferometer system is being designed to provide direct measurements of the internal magnetic field structure and electron density in the Compact Toroidal Hybrid (CTH). Results from this diagnostic will aid in characterizing the stability of CTH plasmas confined by both externally generated magnetic fields and internal plasma currents. The polarimeter-interferometer measurements will be incorporated directly into a 3D equilibrium reconstruction model (V3FIT) to more accurately map the non-axisymmetric CTH geometry. The diagnostic will be comprised of three microwave beams---two circularly polarized probing beams and one local oscillator beam---to avoid the use of mechanical components or dependence on amplitude modulations of the sources [1]. Microwave sources in the range of 250 to 300 GHz have been identified to minimize refractive losses while maintaining an acceptable signal to noise ratio in CTH plasmas. A total of five double-pass chords will allow for inversions using V3FIT to provide core rotational transform, and density profiles. Further details of the diagnostic design will be presented.\\[4pt] [1] D. L. Brower et al., Rev. Sci. Instrum., \textbf{74}, 1534 (2003). [Preview Abstract] |
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CP8.00121: V3FIT Reconstructions using Soft X-Ray and Interferometric Diagnostics on the Compact Toroidal Hybrid G.J. Hartwell, M. Cianciosa, J.D. Hanson, M.C. ArchMiller, J. Herfindal, S.F. Knowlton, X. Ma, D.A. Maurer Reconstruction of three-dimensional (3D) equilibrium is important for improving the operation and understanding the physics of non-axisymmetric magnetic confinement devices. We present V3FIT[1] 3D reconstruction results from the Compact Toroidal Hybrid (CTH) experiment ($R_o = 0.75$\,m, $a_p \approx 0.2$\,m, $B \leq 0.7$\,T, $\overline{n}_e \leq 5 \times 10^{19}$\,m$^{-3}$,\, $T_e \leq 300$\,eV, $I_p \leq 80$\,kA) where we have used internal diagnostic signals as inputs to the V3FIT code. The two diagnostics, a three-channel, 1\,{mm} interferometer, and a 54-channel Soft X-ray (SXR) array, are chordal type measurements that generate signals based on the line-integrated values of the plasma density and/or temperature sampled from the interior of the plasma. We use two parameterization models for the electron density and SXR emissivity profiles: a two-power fit model capable of producing flat to centrally peaked profiles, and a line-segment model that can produce hollow profiles. Results show that SXR diagnostics are sensitive to plasma elongations caused by plasma current profile changes not detected by magnetic diagnostics.\\[4pt] [1] J.D. Hanson, S.P. Hirshman, S.F. Knowlton, L.L. Lao, E.A. Lazarus, J.M. Shields, Nucl. Fusion, \textbf{49},(2009)\,075031. [Preview Abstract] |
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CP8.00122: Eddy Currents in CTH Equilibrium Reconstructions N.A. Roberds, J.D. Hanson, M. Cianciosa, J. Hebert Devices utilizing an ohmic heating transformer induce eddy currents in nearby objects that are electrically conductive. These eddy currents generate external magnetic fields which must be included in accurate free-boundary equilibrium reconstructions. The Compact Toroidal Hybrid (CTH) has, in addition to the metallic vacuum vessel, ten aluminum helical coil frames which surround the vacuum vessel and support the helical coil winding. Princeton Plasma Physics Laboratory has provided us with a model [1] for the spatial distribution of eddy currents within these assemblies. We compare the model computed responses of CTH magnetic diagnostics during a vacuum shot to the measured signals. \\[4pt] [1] A. Zolfaghari, A. Brooks, A. Michaels, J. Hanson, G. Hartwell, PPPL Tech. Report 4814, (2012) [Preview Abstract] |
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CP8.00123: Extending V3FIT with New Equilibrium Solvers M.R. Cianciosa, J.D. Hanson, N. Roberds Due to the non-axisymmetric magnetic field geometry of stellarators, symmetry breaking modes in reverse field pinches and tokamaks with radial magnetic perturbation (RMP) fields applied for edge localized mode (ELM) control, a fully three dimensional equilibrium reconstruction is an important tool for understanding plasma response. V3FIT [1] is a three dimensional equilibrium reconstruction code that uses the VMEC [2] equilibrium solver. While VMEC can model a wide array of devices, it is fundamentally limited to the assumption of closed, nested flux surfaces, thus unable to resolve magnetic islands. Through the use of object oriented programming techniques, the equilibrium model of V3FIT has been abstracted into a generic equilibrium interface. Using this interface, new equilibrium solvers may be added in addition to VMEC, thus extending the regimes V3FIT can reconstruct. This work will discuss the process by which a new equilibrium solver can be included.\\[4pt] [1] J. D. Hanson, S. P. Hirshman, S. F. Knowlton, L. L. Lao, E. A. Lazarus, J. M. Shields, Nucl. Fusion, \textbf{49} (2009) 075031.\\[0pt] [2] S. P. Hirshman and Whitson J. C., Phys. Fluids, \textbf{26} (1983) 3553 [Preview Abstract] |
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CP8.00124: Overview and New Directions for the HSX Program David Anderson Large intrinsic flows have been measured by CHERS in the direction of quasisymmetry. A new 80-coil internal array has been designed and installed for improved equilibrium reconstruction and resolution of the bootstrap current. A motional Stark effect diagnostic is being implemented to directly measure radial electric field profiles in the core, with currents inferred from magnetic field pitch measurements. Edge probe measurements show the Reynolds stress may play an important role in the edge poloidal momentum balance. Impurity transport experiments have begun using laser blow-off. Edge and fuelling studies are carried out with comparisons between experimental measurements and modeling using the DEGAS and EMC3-EIRENE codes. These codes guide new experimental studies of the divertor structure in HSX. A second ECRH system with beam steering and modulation and an upgraded ECE system are used for heat pulse propagation studies, with results compared to gyrokinetic calculations. Efforts are underway to determine and experimentally test elements in the magnetic structure which can be optimized to reduce anomalous transport. [Preview Abstract] |
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CP8.00125: MSE diagnostic in HSX for simultaneous measurement of radial electric field and bootstrap current S.T.A. Kumar, C. Ruiz, F.S.B. Anderson, D.T. Anderson Understanding of the bootstrap current and its temporal evolution is crucial in stellarators as it can significantly affect the magnetic topology and confinement. One of the parameters which could determine the evolution of the bootstrap is the radial electric field (E\textunderscore r). Capability of the Motional Stark Effect (MSE) diagnostic for doing simultaneous E\textunderscore r and bootstrap current measurement is being investigated in the HSX stellarator. Both spectral analysis and polarimetry are being pursued to get unambiguous E\textunderscore r and bootstrap current information. A 30 keV, 5 Amp, 3 ms hydrogen diagnostic neutral beam is injected into vacuum and plasma configurations of HSX. Doppler shifted and Stark-split H-alpha and H-beta emissions are simultaneously measured using two spectrometers. Stark spectra from various plasma configurations are compared with those from vacuum configurations with same applied magnetic field, to study the effect of radial electric field and bootstrap current on the Stark spectra. A single channel, dual PEM polarimetry system has also been designed for Stark polarization measurements. Initial results are presented. [Preview Abstract] |
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CP8.00126: 3-D Plasma Equilibrium Reconstruction at the HSX Stellarator E. Chlechowitz, F.S.B. Anderson, D.T. Anderson A new diagnostic array of 80 magnetic coils has been built and installed inside the vacuum vessel to improve the performance of plasma equilibrium reconstructions at HSX. The location and orientation of the coils were optimized with respect to the effectiveness of the signal to reconstruct specific parameters which describe the plasma current and pressure profiles [1,2]. The use of subsets of the diagnostic array in the reconstruction process, which is performed by the V3FIT code [1], allows one to benchmark the theories used to calculate the effectiveness of each coil and their contribution to the overall reconstruction performance. In addition to the newly installed array, a Thomson Scattering diagnostic and further magnetic diagnostics have been used to put further constraints on the reconstruction and to validate the results, if the new array had been solely used. Different magnetic configurations have been investigated for these studies at HSX. \\[4pt] [1] J.D. Hanson et al, Nucl. Fusion 49 075031 (2009) \newline [2] N. Pomphrey et al, Phys. Plasmas 14, 056103 (2007) [Preview Abstract] |
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CP8.00127: Perturbative electron heat transport experiments in a quasi-helically symmetric stellarator G.M. Weir, K.M. Likin, B.J. Faber, J.N. Talmadge, F.S.B. Anderson, D.T. Anderson Results from perturbative heat transport experiments on the Helically Symmetric eXperiment (HSX) will be presented and compared to linear gyrokinetic predictions from the GENE code made in collaboration with the PPPL. A gyrotron capable of modulating 200 kW at frequencies up to 6 kHZ was installed to perform these experiments. The electron temperature response to 6\% ECRH modulation is monitored with a 16 channel ECE system. The measured stiffness in the electron heat flux, $1\le\chi_e^{HP}/\chi_e^{PB}\le4$, is higher than the gyrokinetic prediction for the quasi-helically symmetric configuration of HSX. The measured stiffness decreases and comes into better agreement with gyrokinetic results with increasing ECRH power per particle. This reduction of stiffness is accompanied by decreased broadband density fluctuations measured through reflectometry. These results will be compared to perturbative heat transport experiments in which the quasi-helical symmetry is intentionally degraded to test the effect of neoclassical transport on stiffness in the electron heat flux. [Preview Abstract] |
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CP8.00128: Characterizing the edge properties of the HSX stellarator with a Langmuir probe and comparisons with EMC3-EIRENE A. Akerson, A. Bader, F.S.B. Anderson, C.C. Hegna, D.T. Anderson In this poster, temperature, density, and floating potential profiles in the HSX stellarator edge are presented. The presence of edge magnetic islands are generic to all stellarator configurations and play a prominent role in all aspects of edge physics. Measurements are conducted with a Langmuir probe equipped with a flapping mechanism, allowing for a 2 dimensional map of plasma parameters in an edge region that includes magnetic islands. Comparisons of the edge measurements with the predictions from the coupled edge code EMC3-EIRENE are made. Qualitatively measurements and predictions of the temperature profile are consistent. However, discrepancies exist with respect to the density profile. Floating potential measurements suggest the presence of an electric field in the island, a feature that is not present in the EMC3-EIRENE modeling. [Preview Abstract] |
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CP8.00129: Impurity Transport Measurements in the HSX Stellarator C. Clark, D.T. Anderson, F.S.B. Anderson, K.M. Likin, J.N. Talmadge Predictive models of impurity transport are required to ensure the successful operation of future magnetic confinement fusion devices. As a step towards the creation and validation of such models for stellarator devices, experiments are under way to measure the impurity transport diffusivity and convective velocity in HSX, the first quasisymmetric stellarator. A laser blow-off impurity injection system is used to rapidly deposit a small, controlled, quantity of aluminum into the confinement volume. Seven AXUV photodiode arrays, some of which are equipped with filters that block the visible portion of the spectrum, are used to take time-resolved measurements of the impurity radiation. One-dimensional emissivity profiles are recovered from those measurement using an inversion process that accounts for the fully three-dimensional detector views. The temporal evolution of the emissivity profiles during impurity injection will be presented for a scan of the line-averaged densities and input powers available to HSX. [Preview Abstract] |
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CP8.00130: Targeted Physics Optimization in the HSX Stellarator J.N. Talmadge, H.E. Mynick, V.V. Nemov To plan out future experiments in HSX, we have developed a code to vary the currents in the auxiliary coils and optimize specific target physics functions. One such function is related to the bounce-averaged grad-B drift velocity of trapped particles such as alphas in a fusion reactor. For HSX, decreasing energetic trapped particle losses by increasing the number of main coils (which decreases the modular ripple) leads to an increase in the effective ripple. Thus, minimizing effective ripple by itself is not a sufficient figure of merit for energetic particle confinement. Of particular interest for optimization is the exploration of configurations in HSX which can lower turbulent transport. Recent optimization studies by Mynick using proxy functions, and subsequent nonlinear GENE calculations, indicate that the level of turbulent transport in HSX is sensitive to the excursion of the~magnetic axis. Applying the simple criterion of reducing the axis excursion in HSX with the auxiliary coils shows that a configuration can be achieved in which the calculated saturated turbulent heat flux is reduced by a factor of 2 from the standard QHS configuration for a given temperature gradient scale length. Initial experimental results will be presented. [Preview Abstract] |
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CP8.00131: Measurements of the contribution of Reynolds stress to momentum balance in HSX Robert Wilcox, D.T. Anderson, J.N. Talmadge, F.S.B. Anderson It has been predicted that for a sufficiently quasi-symmetric stellarator, the neoclassical viscosity can be small enough that other terms, such as the Reynolds stress drive produced by plasma turbulence, can compete with it in the momentum balance to determine the rotation and radial electric field [1]. In this case, the experimental flows may deviate from values calculated using the ambipolarity constraint by purely neoclassical codes such as DKES and PENTA that are commonly used for stellarators. Using multi-tipped Langmuir probes in the edge of the HSX stellarator, the radial electric field is found to deviate from the values calculated by PENTA, and this deviation corresponds qualitatively to a Reynolds stress flow drive measured via fluctuating floating potential signals. Measurements made at two different locations on the device in regions of high and low magnetic field strength indicate that the local flow drive can change directions depending on the local magnetic geometry. Experiments have been run in both the optimized QHS configuration and a configuration with the symmetry intentionally broken to explore the relationship between the neoclassical viscosity and the measured deviation of the flows from the calculated neoclassical value. \\[4pt] [1] Helander P et al 2008 Phys. Rev. Lett. 101, 145003 [Preview Abstract] |
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CP8.00132: Core Density Turbulence in the HSX Stellarator C.B. Deng, D.L. Brower, D.T. Anderson, F.S.B. Anderson, A. Briesemeister, S. Kumar, K.M. Likin, J.N. Talmadge Density fluctuation measurements on the HSX stellarator reveal broadband turbulence that correlates with plasma density gradient and flow. For quasi-helically symmetric plasmas, significant increases in the turbulent density fluctuations are observed when plasma heating location is moved from on-axis to inboard high-field side. Measurements show that the plasma flow velocity also decreases significantly for off-axis heating. In addition, as the electron-cyclotron-resonance-heating power is decreased, core density fluctuations rise while the plasma parallel flow is reduced. When HSX is operated without quasi-helical symmetry, both plasma flow and turbulence characteristics are little changed. No sensitivity to electron temperature gradient is observed. Increased fluctuation amplitude correlates with both increasing density gradient and reduced flow, suggesting a causal relation. In addition to improved neoclassical confinement, quasi-helical symmetry can also lead to increased flow (and flow shear) in the direction of symmetry along with reduced fluctuations and anomalous transport. [Preview Abstract] |
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CP8.00133: Linear Stability and Quasilinear Particle and Heat Fluxes in Ion-ITB Plasmas in LHD D.R. Mikkelsen, K. Tanaka, M. Nunami, T.-H. Watanabe, H. Sugama, Y. Yoshinuma, K. Ida, Y. Suzuki, M. Goto, W. Bernd, I. Yamada, R. Yasuhara, T. Tokuzawa, T. Akiyama The linear stability of ion-scale microinstabilities in an LHD ion-ITB plasma is studied using the GS2 gyrokinetic turbulence code. The ion-ITB phase is preceded by carbon pellet injection, so the carbon density varies considerably during this period. The carbon density develops an ``impurity hole'' that is typical of ion-ITB plasmas in LHD. Quasilinear carbon and helium particle fluxes as well as the influence of the carbon density and its gradient on the quasilinear heat fluxes are discussed. The calculations are based on experimentally measured profiles of electron and ion temperature, as well as electron and carbon density. The measured Zeff and the edge ratio of hydrogen and helium influxes are used to constrain the density profiles of these species. All three ions and the electrons are treated kinetically in the calculations, and the finite electron collision rate is included (the results are not significantly affected by ion collisions so they are usually not included). [Preview Abstract] |
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CP8.00134: Stellarator Research at Columbia University F.A. Volpe, C. Caliri, A.W. Clark, A. Febre, K.C. Hammond, S.D. Massidda, R.M. Sweeney, T.S. Pedersen, X. Sarasola, D.A. Spong, Y. Kornbluth Neutral plasmas were formed and heated by Electron Cyclotron and Electron Bernstein Waves at 2.45 GHz in the Columbia Nonneutral Torus (CNT) and were characterized with Langmuir probe and fast camera measurements. Future research will take advantage of the low aspect ratio (A=2.3-2.7), high fraction of trapped particles and large vessel of CNT. The first plasma was obtained in a prototype circular coil tokamak-stellarator hybrid (Proto-CIRCUS). As a result of the toroidal-field coils being tilted and interlinked with each other, the device can be operated at lower plasma-current than a tokamak of comparable size and field, with implications for disruptions and steady state. Additionally, the toroidal magnetic ripple is less pronounced. Comparisons between field-line calculations and experimental mapping is expected to confirm the generation of rotational transform and its dependence on the radial location and tilt of the coils, both of which can be varied. Finally we propose a small EC-heated classical stellarator to improve the production-rate and charge-state of ions in EC-resonant ion sources (ECRIS) over the conventional magnetic-mirror design, and discuss how ions would be extracted, for injection in research and medical accelerators. [Preview Abstract] |
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CP8.00135: Existence of Low Shear MHD Stellarator Equilibria? Harold Weitzner D. Lortz showed that closed line, zero rotational transform MHD equilibria exist in non-symmetric geometries. H. Grad set up a formalism for such equilibria, whose existence does not contradict his general arguments against existence of non-symmetric equilibria,. Equilibria are regularly computed with some reasonable accuracy, despite these problems. Low shear systems appear to be excluded in these arguments, as they are also omitted in the Kolmogorov, Moser, Arnold theorems on destruction of magnetic surfaces. Such states are of some interest for W7-X and for the inner regions of LHD, In this work Grad formulation is extended and equilibria are ddescribed An alternate representation of equilibria is given, and is applied to a topological torus. An expansion about a simple equilibrium state is given and resonances, which destroy equilibria appear explicitly. It is shown that there are classes of states which exclude such resonances. The work applies to low shear systems. While proof of convergence is not possible, the typical hallmarks of asymptotic expansions do not appear. The work support the conjecture that low shear MHD non-symmetric equilibria may exist. Applications to true tori will also be given. [Preview Abstract] |
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CP8.00136: Plans for first plasma operation and overview of diagnostic development on W7-X Thomas Sunn Pedersen We will give a status report on the construction of the Wendelstein 7-X (W7-X) stellarator and the plans for first physics operation. All five modules have been placed on the machine base and have been welded together. The installation of ports is completed. The present challenges lie to a large degree in the assembly of the in-vessel components, and the completion of the peripheral components. In response to slower than expected progress on the installation of the in-vessel components, a new plan for commissioning and first plasma operation has been developed. This plan calls for a first plasma operation phase entirely without a divertor. Instead, short-pulse plasmas in a limiter configuration will be created. These will primarily serve to commission the diagnostics and the ECRH heating system, but important results can be achieved in a few cases. We will present the expected plasma parameters for this very first phase, and give examples of physics questions that can be addressed already in this early phase. We will also give a few highlights on diagnostic progress with a focus on diagnostics of particular importance to the first operation phase. [Preview Abstract] |
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CP8.00137: The QUASAR facility David Gates The QUAsi-Axisymmetric Research (QUASAR) stellarator is a new facility which can solve two critical problems for fusion, disruptions and steady-state, and which provides new insights into the role of magnetic symmetry in plasma confinement. If constructed it will be the only quasi-axisymmetric stellarator in the world. The innovative principle of quasi-axisymmetry (QA) will be used in QUASAR to study how ``tokamak-like'' systems can be made: 1) Disruption-free, 2) Steady-state with low recirculating power, while preserving or improving upon features of axisymmetric tokamaks, such as 1) Stable at high pressure simultaneous with 2) High confinement (similar to tokamaks), and 3) Scalable to a compact reactor Stellarator research is critical to fusion research in order to establish the physics basis for a magnetic confinement device that can operate efficiently in steady-state, without disruptions at reactor-relevant parameters. The two large stellarator experiments -- LHD in Japan and W7-X under construction in Germany are pioneering facilities capable of developing 3D physics understanding at large scale and for very long pulses. The QUASAR design is unique in being QA and optimized for confinement, stability, and moderate aspect ratio (4.5). It projects to a reactor with a major radius of $\sim$8m similar to advanced tokamak concepts. It is striking that (a) the EU DEMO is a pulsed ($\sim$2.5 hour) tokamak with major R $\sim$ 9 m and (b) the ITER physics scenarios do not presume steady-state behavior. Accordingly, QUASAR fills a critical gap in the world stellarator program. [Preview Abstract] |
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CP8.00138: Ion Extraction from a Toroidal Electron Cyclotron Resonance Ion Source: a Numerical Feasibility Study Claudia Caliri, Francesco Volpe, Santo Gammino, David Mascali Electron Cyclotron Resonance Ion Sources (ECRIS) are magnetic mirror plasmas of microwave-heated electrons and cold multi-charged ions. The ions are extracted from one end of the mirror and injected in accelerators for nuclear and particle physics studies, hadrontherapy, or neutral beam injection in fusion plasmas. ECRIS devices progressed to higher and higher ion currents and charge states by adopting stronger magnetic fields (beneficial for confinement) and proportionally higher ECR frequencies. Further improvements would require the attainment of ``triple products'' comparable with major fusion experiments. For this, we propose a new, toroidal rather than linear, ECRIS geometry, which would at the same time improve confinement and make better use of the magnetic field. Ion extraction is more complicated than from a linear device, but feasible, as our modeling indicates. Possible techniques involve charge-dependent drifts, divertors, specially designed magnetic fields and associated loss-cones, electrostatic and/or magnetic deflectors, or techniques used in accelerators to transfer particles from one storage ring or accelerator to the next. Here we present single-particle tracings assessing and comparing these extraction techniques. [Preview Abstract] |
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