Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session PP8: Poster Session VI: MST and Other Reversed Field Pinches; NSTX and Other Spherical Tori; Magnetic Reconnection |
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Room: Preservation Hall |
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PP8.00001: MST AND OTHER REVERSED FIELD PINCHES |
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PP8.00002: Overview of MST research J.S. Sarff MST progress in advancing the RFP for (1) fusion plasma confinement with ohmic heating and minimal external magnetization, (2) predictive capability in toroidal confinement physics, and (3) basic plasma physics is summarized. Integrated data analysis (IDA) tools are under development to maximize information in MST's advanced diagnostic set. Special emphasis is given to $Z_{eff}$ measurements, for which no single diagnostic appears adequate. Measurements of the plasma resistivity are essential for MHD validation studies, e.g., scaling of magnetic fluctuations with Lundquist number, $S$. At high $S$, the plasma transitions to the quasi-single helicity state with a stellarator-like equilibrium. Applied magnetic perturbations allow controlling the 3D phase for optimal diagnosis of the 3D structure. Density fluctuations that are distinct from tearing instability are observed in improved-confinement, reduced-tearing plasmas. Gyrokinetic simulations indicate several types of drift waves can be unstable for these plasma conditions. Runaway of NBI-generated energetic ions is observed during sawtooth magnetic reconnection, which induces a large inductive electric field that could be important for the observed spontaneous ion energization as well. Supported by U.S. DoE and NSF. [Preview Abstract] |
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PP8.00003: Ion Runaway Due to Magnetic Reconnection in the MST RFP J.A. Reusch, J.K. Anderson, W. Capecchi, S. Eilerman, J. Kim, J.J. Koliner, M.D. Nornberg, Y. Tsidulko Non-collisional heating and energization of ions is a powerful process in reversed-field pinch plasmas and in many astrophysical settings. The energization mechanism is connected to magnetic reconnection, but it is still not well understood. Neutral beam injection in MST reveals ion runaway could be important to this energization process. During reconnection events (i.e. sawteeth), measurements of the fast ion distribution using a neutral particle analyzer show substantial energy gain. This is due to a large, transient inductive electric field that overwhelms friction forces on the ions, allowing unrestricted acceleration during the sawtooth ($\sim$ 200$\mu$s). The energy gain is larger for higher initial ion energy, and deceleration is observed with reversed electric field (counter-current injection). Full orbit test particle tracing in the 3D time evolving \textbf{E} and \textbf{B} fields from visco-resistive MHD simulations of sawteeth in MST shows that beam ions are well confined and experience parallel acceleration during the sawtooth, despite the presence of magnetic stochasticity from a broad spectrum of large-amplitude tearing modes. While parallel runaway energization affects fast ions, it cannot explain the dominantly perpendicular heating observed in the bulk majority and impurity ions. However, transport and pitch-angle scattering are likely important processes that may weakly affect the relatively high-energy NBI ions. This work supported by the US DOE and the NSF. [Preview Abstract] |
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PP8.00004: Energy and Pitch Distribution of Spontaneously-generated High-energy Bulk Ions in the RFP Jungha Kim, Jay Anderson, Joshua Reusch, Scott Eilerman, William Capecchi Magnetic reconnection events in the reversed field pinch (RFP) are known to heat bulk and impurity ions. Runaway due to a parallel electric field has recently been confirmed as an important acceleration mechanism for high energy test ions supplied by a neutral beam. This effect does not, however, explain the change in distribution of nearly Maxwellian bulk ions at a reconnection event. By operating MST near maximum current and low electron density, significant fusion neutron flux can be generated without neutral beam injection. The bulk ion distribution created in these plasmas is well-confined, non-Maxwellian, and can be measured by the Advanced Neutral Particle Analyzer (ANPA) placed at a radial or tangential porthole. Data show a high energy tail up to 25keV with a relatively higher signal in the low energy channels (8-15keV) at the radial port following a reconnection event. Analysis of the energy dependence of trapped orbits sampled by the ANPA at the radial view implies an abundance of lower energy particles in regions of higher neutral density. This mandates a careful deconvolution of the measured ANPA signal to compute the fast ion distribution. [Preview Abstract] |
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PP8.00005: Fast ion beta limit measurements by collimated neutron detection in the MST William Capecchi, Jay Anderson, Scott Eilerman, Jon Koliner, Josh Reusch, Liang Lin, 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 of an NBI-driven mode (presumably driven by a fast ion pressure gradient) 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 of 1-2{\%} (local core value up to 10{\%}). A collimated neutron detector for fusion product profile measurements is in development to determine the energy and spatial distribution of fast ions, the design of which is informed by recent neutron moderation measurements with polyethylene. Characterization of both the local and global fast ion beta will be done for deuterium beam injection into deuterium plasmas for comparison to TRANSP predictions. Work supported by US DOE. [Preview Abstract] |
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PP8.00006: Characterization of an ultra-low-frequency beam-driven instability in the RFP E. Parke, J.K. Anderson, D.J. Den Hartog, S. Eilerman, J.J. Koliner, S. Munaretto, M.D. Nornberg, J.A. Reusch Decoupling of fast ions from the magnetic field in the reversed-field pinch allows neutral beam injection (NBI) on MST to achieve large fast ion populations that drive a rich variety of instabilities. A bursting instability near the plasma rotation frequency has recently been observed under appropriate conditions. Plasma flow measurements show that the instability propagates at $\sim$7 kHz in the plasma reference frame. This mode also participates in energetic particle mode avalanches to drive significant fast ion transport; a neutral particle analyzer measures reduction of signal at the beam energy by almost 30\%. During a burst the tearing mode amplitudes and rotation velocities increase. Bulk-ion heating of approximately 10 eV is also observed, which may be indicative of a reconnection event. Correlated electron temperature fluctuations exhibit a core-peaked structure with an amplitude of 10-15 eV and which depends sensitively on reversal parameter. The correlated electron temperature fluctuations indicate that this mode is electromagnetic in nature. We offer a qualitative comparison to expectations for fishbones, beta-induced Alfv\'{e}n eigenmodes, and reconnection which highlights the need for greater theoretical support for energetic particle effects in the RFP. [Preview Abstract] |
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PP8.00007: Investigation of fast-ion instabilities and tearing-mode reduction during neutral beam injection in a reversed field pinch L. Lin, W.X. Ding, D.L. Brower, J.K. Anderson, W. Capecchi, S. Eilerman, J.J. Koliner, M.D. Nornberg, J. Reusch, J.S. Sarff, D. Liu Neutral beam injection into the MST-RFP is observed to drive instabilities that induce fast-ion transport and quench the fast-ion density below classical predictions. These instabilities are detected for both super- and sub-Alfv\'{e}nic fast ions, indicating that free energy arises from the real space gradient. As plasma current and fast-ion species are changed, the mode number of the dominant instability varies to maintain the wave-particle resonance condition. The dominant instability also exhibits a dependence on fast-ion velocity (v). As v increases, the mode frequency linearly increases and the spatial asymmetry of associated density fluctuations becomes more pronounced. These features link the observed instabilities to continuum modes destabilized by strong drive. In addition to driving instabilities, fast ions are observed to affect intrinsic tearing modes. For certain plasma scenarios, fast ions reduce the core-resonant tearing mode amplitude by 60{\%} while enhancing the kinetic dynamo arising from coherent interactions between density and radial magnetic fluctuations. This implies the potential importance of kinetic dynamo in the tearing mode suppression. Tearing modes can also impact fast-ion redistribution as suggested by edge-resonant tearing mode triggering of a chirping fast-ion mode. [Preview Abstract] |
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PP8.00008: Alfv\'en Modes in the MST Revised Field Pinch Meng Li, Boris Breizman, Linjin Zheng, Liang Lin, Weixing Ding, David Brower This work presents a theoretical and computational analysis of core-localized energetic particle driven modes observed near the magnetic axis in MST[L. Lin, W. X. Ding, D. L. Brower, et al., Phys. Plasmas 20, 030701 (2013)]. Using the measured safety factor and plasma pressure profiles as input, the linear ideal MHD code AEGIS [L. J. Zheng and M. Kotschenreuther, J. Comp. Phys. 211, 748 (2006)] reveals Alfv\'enic modes close to the measured frequencies. The AEGIS results together with a reduced analytical model demonstrate that the modes are essentially cylindrical and dominated by a single poloidal component (m = 1). The calculated modes are localized in the plasma core where the magnetic shear is weak and continuum damping is minimal. Detailed analysis establishes constraints on the safety factor and plasma pressure under which two modes can exist simultaneously as seen in experiment. [Preview Abstract] |
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PP8.00009: Feasibility study of off-axis NBI in the Reversed Field Pinch J.K. Anderson, W. Capecchi, J. Kim, J.J. Koliner, M.D. Nornberg, J.A. Reusch, L. Lin The reversed field pinch is a unique and complementary magnetic configuration for the study of energetic ion driven instabilities and their effects. EP-driven modes (destabilized by fast ion spatial gradients) have been discovered in MST with use of 1 MW tangentially-oriented neutral beam injection (NBI). More widely ranging studies of EP modes would be possible in MST with control of the fast ion density profile: the tangential NBI can only generate a core-localized, high pitch fast ion population. Here we present an initial physics study on the feasibility of off-axis NBI in the RFP. Simple deposition calculations suggest a flexible mounting system on a single large port allows localized placement of the fast ion source over a significant radial range (r/a$\sim0.1 - 0.6$). TRANSP/ NUBEAM calculations are used with a subset of MST equilibria to predict classical behavior of fast ions in these injection geometries. Ion orbit tracing through the tearing-mode-induced turbulent magnetic field is performed with RIO to evaluate the behavior of fast ions at mid-radius. Expected fast ion density profiles and implications on mode stability are presented for a variety of MST discharges. [Preview Abstract] |
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PP8.00010: An Update on the Retarding Potential Grid System for a Neutral Particle Analyzer Ephrem D. Mezonlin, James B. Titus, Jay K. Anderson, Josh A. Reusch The ion energy distribution in a magnetically confined plasma can be inferred from the neutral particles that escape the plasma. On the Madison Symmetric Torus (MST), deuterium neutrals are measured in the energy range 0.34 to 45 keV by the Florida A{\&}M University compact neutral particle analyzer (CNPA) and the MST advanced neutral particle analyzer (ANPA). The CNPA energy range covers the bulk ions from the thermal to the beginning of the fast-ion tail (0.34 to 5.2 keV) with high energy resolution (25 channels) while the ANPA covers the vast majority of the fast-ion tail distribution (up to 45 keV) with low energy resolution (10 channels). MST has gained a wealth of information about the fast-ion distribution, but much more information could be gleaned from a higher energy resolution analyzer over the fast-ion tail. Retarding potential analyzers have been used on spacecraft for decades to measure ion distributions in extraterrestrial atmospheres by slowing down charged particles before they reach the detectors. Recent work has been done to design and implement a retarding potential between the vacuum chamber of MST and the CNPA to be able to use the CNPA's high energy resolution over any 5 keV range between 0 and 35 keV. An update of the construction and implementation of the retarding potential grid system will be presented. [Preview Abstract] |
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PP8.00011: Preliminary MHD Validation Studies on MST C.M. Jacobson, A.F. Almagri, K.J. McCollam, J.A. Reusch, J.P. Sauppe, J.C. Triana Quantitative validation of visco-resistive MHD models using MST takes advantage of its well-diagnosed, standard RFP plasmas. These plasmas are largely governed by MHD relaxation activity, so that a broad range of validation metrics can be evaluated. Nonlinear simulations using the single-fluid MHD code DEBS at Lundquist number $S=4\times10^{6}$ produced equilibrium relaxation cycles in qualitative agreement with experiment, but magnetic fluctuation amplitudes $\tilde{b}$ were at least twice as large as in experiment. The extended-MHD code NIMROD was used previously at $S=8\times10^{4}$, which is below MST's operational lower limit of $S\geq5\times10^{5}$. The predicted $\tilde{b}$ from NIMROD was about half as large for a two-fluid case as for a single-fluid, suggesting that a two-fluid model may be necessary for quantitative agreement with experiment. Comparisons of linear and nonlinear DEBS and NIMROD runs at low $S$ are presented, focusing on how their different numerical algorithms affect their performance. Experimental equilibrium and fluctuation measurements at low $S$ from an insertable magnetic probe are compared with simulation results. Future scaling studies of $\tilde{b}$ as a function of $S$ are planned using NIMROD at low $S$ complemented by DEBS at higher $S$. [Preview Abstract] |
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PP8.00012: Using IDA to Understand Electron Temperature Structures in High Temperature Discharges in the Madison Symmetric Torus L.M. Reusch, M.E. Galante, D.J. Den Hartog, P. Franz, J.R. Johnson, M.B. McGarry, H.D. Stephens The Madison Symmetric Torus (MST) Reversed-Field Pinch is equipped with two independent electron temperature (Te) diagnostics: Thomson scattering (TS) and double-filter soft x-ray (SXR). Both diagnostics are able to measure Te at a rate up to 25 kHz and are in good qualitative agreement in the hot plasma core, where Te $>$ 1 keV. We are able to combine information from both TS and SXR diagnostics along with prior physics knowledge using integrated data analysis techniques (IDA) [R. Fischer and A. Dinklage, Rev. Sci. Instrum. 75, 4237 (2004)] to improve the precision and utility of Te measurements on MST. Using IDA, there is a factor of 4 improvement in the uncertainty of all temperature measurements. We have also implemented a Markov Chain Monte Carlo analysis for analyzing the various temperature structures that MST is capable of sustaining. We have compared emissivity maps and flux surface reconstructions to the electron temperatures from several discharges to characterize the phenomenology of temperature structures in high temperature plasmas in MST. [Preview Abstract] |
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PP8.00013: Determination of $Z_{eff}$ in MST Plasmas through Integrated Data Analysis Matthew Galante, Lisa Reusch, Daniel Den Hartog, Paolo Franz, Jay Johnson, Meghan McGarry, Hillary Stephens On most plasma science experiments, a maximum of knowledge must be gleaned from limited data. Integrated data analysis (IDA) enables combinations of measurements and uncertainties from multiple, distinct diagnostics in a statistical framework to determine the single most probable result for a physical parameter of interest. The method is highly modular, allowing for easy inclusion of many independent diagnostics. Using data from several diagnostics on the MST, a framework is being developed to determine the effective ionic charge ($Z_{eff})$. This parameter cannot be accurately determined by any single diagnostic on MST, but is of key importance to the growing MHD validation effort at MST. Initial results from MST indicate that $Z_{eff}$, as determined from soft x-ray tomography coupled with charge exchange recombination spectroscopy (CHERS) measurements for carbon and aluminum, is approximately 2 in the core of high current, high temperature, improved confinement discharges and has a hollow profile, peaking near mid-radius. The method for determination of $Z_{eff}$ is also being applied to data from RFX-Mod. Work supported by U.S. Dept. of Energy and NSF. [Preview Abstract] |
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PP8.00014: Improvements in SXR and Te Measurements on MST J.A. Goetz, M.B. McGarry, P. Franz, D.J. Den Hartog, J. Johnson A diagnostic that uses soft-x-ray (SXR) emission to provide both tomographically reconstructed x-ray emissivity and double-foil electron temperature from either brightness or emissivity has been in use on MST. Analysis of the data from this system has revealed several effects that were not accounted for in past diagnostics. For example, the purity (\textgreater 99.8{\%}) of the beryllium foils used to block visible light and to select the SXR energy range can produce significant changes in the data by altering the transmission function. In addition, the detailed geometry of the SXR detectors (silicon photodiodes) must be taken into account, including any difference in material composition such as the presence of oxides, front windows and frames, etc., to avoid misinterpretation of the data. All of these effects have been studied and will be presented in this work. Modifications of the diagnostic have been implemented in order to decrease the impact of these features on the measurements and have thus led to improved measurements and a validation of the results from the diagnostic. Time-resolved SXR emissivity and full radial profiles of electron temperature have been analyzed. In particular, high current improved confinement discharges often exhibit enhanced emission from island structures, both rotating and locked. Analysis has been concentrated on the correlation of SXR structures with temperature profiles in locked plasmas. [Preview Abstract] |
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PP8.00015: Statistical Analysis of Thomson Scattering Measurements for High-Frequency Temperature Fluctuations Lucas Morton, Daniel Den Hartog, Eli Parke, James Duff, Liang Lin The MST Thomson Scattering (TS) Diagnostic is used to study electron temperature (Te) fluctuations at frequencies ($\le $30kHz) higher than those of global core-resonant tearing modes (m$=$1, n$=$5-8). Each of the twin TS lasers can fire 4-5 pulses at repetition rate of 12.5 kHz. Adjusting the time delay between the lasers (as low as 1 $\mu $s) allows probing of high-frequency (up to 1 MHz) fluctuations by autocorrelating the resulting Te measurements. This technique's effectiveness is demonstrated by comparing its results to those of tearing-mode-correlation studies. In 400 kA standard MST discharges, the dominant tearing modes have associated Te fluctuations of up to 25 $+$/- 5eV in the core. The TS autocorrelation measures total fluctuations of 42 $+$/- 5eV, indicating that tearing comprises much of the core Te fluctuations. With improved laser alignment, we investigate 400 kA improved confinement (PPCD) plasmas where global tearing activity is reduced and electrostatic turbulence may dominate electron thermal transport and fluctuation power. We also find no significant Te fluctuation (\textless 5eV) correlated with edge-localized density fluctuations seen by the FIR interferometer in 200kA PPCD plasmas. [Preview Abstract] |
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PP8.00016: Probing the limits on beta and density in the RFP B.E. Chapman, K.J. Caspary, J.K. Anderson, W. Capecchi, D.J. Den Hartog, S.T. Limbach, L.A. Morton, S.P. Oliva, E. Parke, J.S. Sarff, W.C. Young, D.L. Brower, W.X. Ding, J.R. Duff, L. Lin, S.K. Combs RFP-record values of total beta and normalized density have been achieved in MST plasmas with inductive current profile control combined with pellet and neutral beam injection. Total beta reaches 28{\%} and appears to be limited by transport. The density has thus far reached 2.0*n$_{\mathrm{Greenwald}}$, limited only by the size of pellet that can be ablated inside the MST plasma. Current profile control is used routinely in MST to reduce current-gradient-driven tearing fluctuations and stochastic transport. Beta is thereby increased but is well below pressure-related stability limits due to a concomitant reduction in P$_{\mathrm{Ohmic}}$. With edge-deposited fueling based on gas puffing, the density is also necessarily low, typically at or below 0.2*n$_{\mathrm{Greenwald}}$, in order to avoid edge-resonant instability. With direct pellet fueling of the plasma core, a substantially larger density is possible, and this leads to a substantially larger P$_{\mathrm{Ohmic}}$ and beta. NBI further augments beta with modest heating and a population of 25 keV fast ions. The beta limit is probed with a three-fold variation in P$_{\mathrm{Ohmic}}$ brought about with a density scan. Magnetic fluctuations increase with density, likely leading to increased transport and the observed saturation of beta. [Preview Abstract] |
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PP8.00017: Ion energy transport in MST Z.A. Xing, D.J. Den Hartog, S. Kumar, M.D. Nornberg, J.S. Sarff, J.B. Titus The mechanisms governing ion energy transport must be identified and quantified in order to further understand non-collisional ion heating in the RFP. In improved confinement (PPCD) plasmas in MST, non-collisional ion heating appears to be small, making these the ideal baseline plasmas in which to investigate ion energy transport. Previous work has demonstrated that impurity ion particle transport is classical in PPCD plasmas. Characterizing both particle and energy transport in PPCD plasmas will serve as a first step in understanding the transition to strong non-collisional ion heating and stochastic transport in standard RFP plasmas. The energy transport model now being developed accounts for collisional equilibration between species, classical convective and conductive energy transport, and energy loss due to charge exchange collisions. This model uses MSTfit to provide equilibrium magnetic geometry, and a modified STRAHL code for particle profile and transport modeling. Previous measurements from sub-optimal PPCD plasmas with residual magnetic fluctuations are being analyzed with this model to examine the possible transition to stochastic transport. [Preview Abstract] |
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PP8.00018: Nonlinear gyrokinetic simulations of improved confinement RFP plasmas Daniel Carmody, M.J. Pueschel, J.K. Anderson, P.W. Terry The reversed field pinch (RFP), a device dominated by global tearing modes in standard modes of operation, has been able to achieve reduced transport and increased energy confinement time through the use of pulsed poloidal current drive (PPCD), a current profile control technique. To evaluate the potential contributions of microinstabilities to transport fluxes in the PPCD regime of the Madison Symmetric Torus, we use the gyrokinetic code \textsc{Gene} and experimental profile data. Linear results from 200 kA and 500 kA PPCD discharges show the dominant instabilities to be an ion temperature gradient mode and a density-gradient-driven trapped electron mode, respectively. Nonlinear simulations of the 500 kA case show strong zonal flow activity that results in a significant Dimits-like shift, with a nonlinear threshold about a factor of three larger than the linear critical value. We find magnetic shear to play an important role in determining the nonlinear saturation levels, with lower shear resulting in the reduction of zonal flow shearing rate and the enhancement of linear growth rates. The nonlinear threshold occurs at roughly the experimental value, suggesting that microturbulent processes may be an important factor in determining experimental transport levels. [Preview Abstract] |
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PP8.00019: Measurement of high-frequency, small scale density fluctuations in improved confinement RFP plasmas J.R. Duff, B.E. Chapman, J.S. Sarff, D. Carmody, P.W. Terry, D.J. Den Hartog, L.A. Morton, L. Lin, W.X. Ding, D.L. Brower In standard MST RFP plasmas, core transport is governed by magnetic fluctuations associated with global tearing modes. Using pulsed parallel current drive, tearing is significantly reduced and smaller-scale fluctuations are likely important to electron particle and heat transport for these improved confinement plasmas. On MST, an 11-chord FIR laser-based interferometry diagnostic, with $\sim$ 8 cm chord spacing, is used to measure electron density fluctuations with wavenumbers k\textless 1-2 cm$^{-1}$. An upgrade underway will allow resolution up to k $\sim$ 15 cm$^{-1}$. A fast magnetic coil array is employed for magnetic fluctuations. High-frequency (\textgreater 50 kHz) small-scale (n\textgreater 15) density and magnetic fluctuations have been observed in the edge plasma, where density and temperature gradients are largest. These fluctuations are distinct from tearing and have amplitudes that correlate with the density gradient and electron beta. The MST is well suited to explore beta scaling given the large dynamic range (9-26{\%}) found in the device. Correlation of the measured density fluctuations with plasma parameters in high beta plasmas will serve to identify the drive and contribute to validation of gyrokinetic codes. Work supported by DOE and NSF. [Preview Abstract] |
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PP8.00020: Investigation of density and potential fluctuations measured in the interior of improved confinement RFP plasmas P.J. Fimognari, D.R. Demers, D. Carmody, P.W. Terry The Heavy Ion Beam Probe (HIBP) is uniquely capable of simultaneously measuring density and potential fluctuations in the plasma core. Characterizing the amplitudes, wavelengths, and cross phases of these quantities is necessary for validation efforts. During improved confinement (IC) periods in the Madison Symmetric Torus (MST) Reversed Field Pinch (RFP), HIBP measurements indicate density and potential fluctuations are broadband with most power below 100 kHz; the cross phase varies with radius and frequency. Gyrokinetic simulations of MST experimental discharges, focusing on microinstabilities during IC periods, suggest ITG and TEM are the predominant linear instabilities; profiles of various parameters are key to the growth or stability of these modes. Comparison of these simulations to interior fluctuation profiles is made possible by using the HIBP along with equilibrium temperature and density gradients. Measurements of fluctuations have been acquired with the HIBP at multiple radial locations inside the MST reversal surface; analysis of these and other relevant experimentally measured quantities will be presented. [Preview Abstract] |
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PP8.00021: 3D Equilibrium Reconstruction with Internal Measurements on Madison Symmetric Torus J.J. Koliner, B.E. Chapman, J.S. Sarff, J.K. Anderson, S. Munaretto, W. Capecchi, L. Lin, J.D. Hanson, M.R. Cianciosa Plasmas in the MST reversed field pinch (RFP) 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$^{-3})$. In order to understand the physics of confinement and self-organization in SHAx, 3D equilibrium reconstruction is needed. The V3FIT equilibrium reconstruction code is applied using measurements from the 11-chord interferometer-polarimeter, 22-point Thomson scattering system, 4-camera soft x-ray probes, and magnetics. Equilibria have been generated using a fixed plasma boundary with no external currents. Model signals fit well to observed signals, $\chi ^{2} \approx $ 1, and the zero crossing of line-averaged n$_{\mathrm{e}}$B$_{\mathrm{z}}$ from Faraday rotation is matched by the model. External magnetics are shown to be an inadequate equilibrium constraint with the VMEC model, due to possible shear in the poloidal phase of the helical structure, as well as strong contribution to the edge magnetic field from currents in the conducting shell. To address this shortcoming, a filament current model has been created to simulate the conducting shell with many external currents for a free plasma boundary. Axisymmetric equilibria have been reconstructed using the filament model and compared to solutions obtained with the MSTFIT axisymmetric equilibrium reconstruction code. The filament model has been extended to allow reconstruction of helical equilibria. Supported by DoE. [Preview Abstract] |
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PP8.00022: Dynamics of helical states in MST Stefano Munaretto, F. Auriemma, D. Brower, B.E. Chapman, D.J. Den Hartog, W.X. Ding, J. Duff, P. Franz, J.A. Goetz, D. Holly, L. Lin, K.J. McCollam, M. McGarry, L. Morton, M.D. Nornberg, E. Parke, J.S. Sarff The thermal and the magnetic dynamics of quasi-single-helicity (QSH) plasmas evolve independently during the formation and sustainment of the core helical structure. At higher plasma current (and Lundquist number) MST plasmas transition from an axisymmetric multi-helicity state to a QSH state characterized by a strong core helical mode and reduced secondary mode amplitudes. Plasmas in the QSH state tend to wall-lock, often in an orientation that is unfavorable for optimized measurements of the 3D structure using MST's advanced diagnostics. Recently a technique to control the locking position through an applied resonant magnetic perturbation has been developed. Using this technique it is possible to adjust the 3D phase more optimally for specific diagnostics, to study the dynamics of the QSH structure and thermal features. The multi-chord FIR interferometer shows the presence of a density structure for the duration of the QSH state. Measurements of the time evolution of the electron temperature profile using the Thomson Scattering diagnostic reveal that the transition to QSH allows the presence of a 3D thermal structure, but this structure is intermittent. Understanding the mechanism(s) driving these dynamics is the goal of this work. Work supported by the US DOE and NSF. [Preview Abstract] |
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PP8.00023: Diagnosing 50kV hydrogen neutral beam characteristics M.D. Nornberg, J.K. Anderson, D.J. Den Hartog, S. Munaretto, S. Oliva, D. Craig The 50 kV hydrogen diagnostic neutral beam on MST facilitates charge-exchange spectroscopy measurements of impurity ions and core-localized magnetic field measurements through the motional Stark effect. Interpretation of these measurements requires good knowledge of the beam energy components and divergence. The characteristics of this neutral beam are now time dependent and sub-optimal for sufficient signal-to-noise discrimination in spectroscopic measurements. In particular, the 1/3 energy component is comparable to the primary energy component at the beginning of the beam pulse and the beam current is not steady. Design considerations for test stand diagnostics including Doppler-shift spectroscopy to quantify molecular and impurity species, optical beam profile measurements with filtering to address outgassing and impurity sources, and calorimetery are presented. The test stand will facilitate optimization of the beam fueling, current, and voltage to restore the beam operation to specification. Of particular concern are reliable formation of an arc in the ion source, plasma temperature, outgassing, and neutralization. [Preview Abstract] |
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PP8.00024: Electron Bernstein Wave Studies in MST Andrew Seltzman, Jay Anderson, Cary Forest, Paul Nonn, Mark Thomas The overdense condition in an RFP prevents electromagnetic waves from propagating past the edge, however use of the electron Bernstein wave (EBW) has the potential to heat and drive current in the plasma. A 450kw RF source that generates 2ms pulses at 5.55GHz and an antenna system with suitable power handling capability has been constructed. The design and implementation of a suitable launch structure is challenging in the RFP for several reasons. It is necessarily a low-field-side launch due to the magnetic field geometry, the close-fitting conducting shell requires a minimum port hole size, the port hole leads to local magnetic field perturbation that affects the resonance condition, and there is a very small vacuum gap between the shell and plasma leading to substantial antenna-plasma interaction. Testing of an EBW waveguide antenna system for use in heating experiments is underway. A multi-chord soft x-ray camera and spectrum analyzer connected to a receiving probe antenna are used to look for evidence of electron heating and coupling effects. Power handling tests on the antenna are used to determine the maximum capabilities of the system without arcing. Heating with the EBW is attempted and methods to improve coupling in the RFP are examined. [Preview Abstract] |
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PP8.00025: First Assessment of the MST Plasma Wall Interaction Ryan Norval, John Goetz, Daniel Den Hartog, Oliver Schmitz Studies of RFP plasma-wall interaction (PWI) have been rather limited. To rectify this, a new program on studying the plasma wall interaction at MST is being developed. A new endoscope camera setup is used to study neutral recycling and impurity production at the poloidal limiter structure of MST, a known area of PWI due to field errors and protective tiles. Initial measurements show a strong dependence of the plasma wall interaction at MST on the magnetic mode structure and 3D equilibrium. The results show that the main interaction domain is shifted from the inboard limiter to the outboard limiter with increasing plasma current. Also, operation in the single helical axis state results in a very strong localization of the plasma wall interaction. The camera provides a temporal resolution of up to 1 kHz. Light from the 400 nm to 1100 nm can be collected by the camera. Emitted light may then be filtered in order to identify different impurity species. The experimental data will be compared to modeling of the magnetic field structure in order to connect the magnetic topology to the plasma equilibrium features. Work Supported by DOE and NSF. [Preview Abstract] |
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PP8.00026: Studies of X-ray Spectroscopy in Improved Confinement Plasmas 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 are installed on chords ranging from r/a $=$ 0.87 inboard to r/a $=$ 0.84 outboard. X-ray measurements have been made in MST improved confinement plasmas, PPCD, with plasma current of 400 kA, electron density of 0.6x10$^{19}$ m$^{-3}$, and electron temperature of 1200 eV. A simple model for Zeff yields a central value around 4 for these plasma conditions. The measured X-ray spectra are also consistent with the temperature measured by Thomson Scattering. At the end of the improved confinement period we observe the X-ray emission at energy above 6 keV to decay faster than at the lower energies, suggestive of reemerging stochastic transport. A large reconnection event usually terminates PPCD. At this event, the x-ray flux increases at all energies for a few microseconds followed by a rapid decrease. Measured spectra will be used to constrain radial profiles for Zeff and radial diffusion, D$_{\mathrm{r}}$, by comparison with the Bremsstrahlung spectra calculated from CQL3D, a Fokker-Planck solver.$^1$ Presently CQL3D cannot account for the recombination spectra, which are present in the measured x-ray emission. [Preview Abstract] |
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PP8.00027: Investigation of Electron Energization During Magnetic Reconnection Events in MST Ami M. DuBois, J.D. Lee, A.F. Almagri, B. Chapman, D.J. Den Hartog, J.A. Goetz, K. McCollam, M. Nornberg, J.S. Sarff Magnetic reconnection plays an important role in particle transport and energization in space, astrophysical, and laboratory plasmas. Strong ion heating and energetic ion tail formation are observed in MST plasmas at the time of reconnection events, but Thomson scattering measurements indicate a slight drop in the electron temperature. This drop is probably the result of an increase in stochastic thermal transport, which may mask electron energization during reconnection events. Motivated by recent astrophysical results, electron dynamics during reconnection are now the subject of increased experimental attention in MST. Sensitive high-speed measurements of the electron energy distribution are required to uncover reconnection effects. A Fast X-Ray (FXR) detector, capable of measuring photon energies between 1 and 20 keV with a shaping time of 20 ns, has been installed on MST to measure x-ray spectra and calculate electron energy distributions. Distributions at tens of microsecond intervals around reconnection events will be compared to determine if a tail population of electrons is generated. [Preview Abstract] |
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PP8.00028: Burst Electron Heating Measured with High-Repetition-Rate Thomson Scattering on the MST Reversed-Field Pinch William Young, L.A. Morton, E. Parke, D.J. Den Hartog Improved operation of a high-repetition-rate laser allows MST Thomson scattering measurements at rates up to 200 kHz. The new laser will be applied to improving electron temperature measurements during spontaneous periods of improved confinement with RFP plasmas, where ensembled data shows a 4\% increase in core electron temperature during bursts of magnetic activity associated with edge-resonant m=0 modes. This heating is concurrent with a reduction of core parallel current and magnetic energy, then followed by an inward propagating cold pulse consistent with previous SXR data. The Thomson scattering diagnostic uses a 1064 nm high-repetition-rate laser currently employing a Nd:YVO\({}_4\) oscillator, four Nd:YAG amplifier stages, and a final Nd:glass amplifier. This laser can operate under a variety of modes while maintaining 1.5 J pulse energies necessary for Thomson scattering, including 4 ms long bursts at 10 kHz pulsing rate, ten bursts of 15 pulse at 75 kHz, or bursts of 5 pulses at 200 kHz. [Preview Abstract] |
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PP8.00029: Effects of m$=$0 mode suppression on m$=$1 magnetic and velocity fluctuations in the RFP D. Craig, D.J. Den Hartog, D. Martin, M.R. Nornberg, J.A. Reusch, J. Triana Poloidal mode number m$=$0 and m$=$1 fluctuations are strongly coupled in the Reversed Field Pinch (RFP) and hence suppression of one mode can have a significant effect on the other. We present measurements of differences in magnetic and velocity fluctuations in MST when m$=$0 modes are suppressed by removal of their resonant surface from the plasma. The m$=$1 magnetic fluctuation amplitudes are similar without m$=$0 present but the velocity fluctuations are lower and have a different phase with respect to the magnetic field fluctuation. We have reproduced the experiment in nonlinear resistive MHD simulations with the DEBS code. In the code, removal of the reversal surface reduces m$=$0 modes but not as strongly as in the experiment. The m$=$1 magnetic fluctuations in the code are of similar amplitude with and without the reversal surface but the velocity fluctuations are reduced without m$=$0 present. The phase between v and b does not change significantly in the code in contrast to the experiment. Advection of the mean flow profile by the magnetic fluctuations (an effect not included in the code) may be responsible for the difference in phase observed in the experiment. This work has been supported by the USDOE and NSF. [Preview Abstract] |
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PP8.00030: Using Temperature Fluctuation Measurements for Equilibrium Reconstruction and Dynamo Measurement D.J. Den Hartog, E. Parke, J.K. Anderson, C.A. Johnson The high-repetition-rate Thomson scattering system on MST, in combination with advanced Bayesian statistical methods, enables determination of tearing-mode-correlated temperature fluctuations as small as a few percent of the equilibrium temperature. Tearing mode rational surface locations are determined from the characteristic phase flip observed in temperature fluctuation structures, providing a strong constraint for equilibrium reconstruction. Recent experiments in neutral beam heated plasmas indicate an inward shift of the $m=$ 1, $n=$ 6 rational surface of approximately 1 cm relative to non-beam heated plasmas. The measured shift of the rational surface enables diagnosis of current redistribution and safety factor modification due to the fast ion population. Additionally, from the phase of correlated temperature fluctuations, the product \textless $\delta T_{e}\delta b_{r}$\textgreater is determined. This term is part of \textless $\delta p_{e}\delta b_{r}$\textgreater, the divergence of which is often called the kinetic dynamo. The kinetic dynamo emf depends on an imbalance of the radial transport of field-aligned current. Previous measurements of the density fluctuation term \textless $\delta n_{e}\delta b_{r}$\textgreater suggest that the kinetic dynamo plays a role in the RFP dynamo process. These measurements of temperature-fluctuation-driven current transport indicate that both terms are needed for a complete picture of the kinetic dynamo. [Preview Abstract] |
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PP8.00031: Density Fluctuation Induced Kinetic Dynamo and Tearing Mode Nonlinear Saturation in the MST Reversed Field Pinch Weixing Ding, Liang Lin, J.R. Duff, D.L. Brower, J.S. Sarff In the MST reversed field pinch (RFP), the evolution of core tearing mode nonlinear evolution is partially determined by the electron current density profile along with nonlinear interactions among multiple tearing modes. Density fluctuations driven by intrinsic magnetic perturbations are usually large, approximately 1{\%}, in RFP plasmas. These density fluctuations can modify the current density profile via the \textit{kinetic dynamo effect}, defined as the correlated product of parallel electron pressure and radial magnetic field fluctuations, which alters the temporal dynamics of tearing modes in MST. A component of the kinetic dynamo originating from the correlated product of density and radial magnetic fluctuations has been measured using a high-speed, low phase noise polarimetry-interferometry diagnostic. Between sawtooth crashes it is found that the measured kinetic dynamo has finite amplitude that generates an anti-dynamo in the plasma core, which would tend to flatten the current density profile. These measurements suggest that density fluctuations passively driven by magnetic fluctuations can actively alter tearing modes via fluctuation-induced current transport. Work supported by US DOE and NSF. [Preview Abstract] |
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PP8.00032: Extended Measurements of Current-Momentum Relaxation in the Madison Symmetric Torus J.C. Triana, A.F. Almagri, J.S. Sarff, J.P. Sauppe, C.R. Sovinec Direct measurements of the turbulent emf and stresses associated with tearing-induced fluctuations in MST reveal coupled current and momentum relaxation in the RFP. These forces were previously measured in the edge of MST plasmas $(\frac{r}{a}>0.85)$, showing that the Hall $\frac{1}{ne}\langle\tilde{\bf{j}}\times\tilde{\bf{b}}\rangle_{||}$ and MHD $\langle\tilde{\bf{v}}\times\tilde{\bf{b}}\rangle_{||}$ terms are both large but dominate Ohm's law at different radii. A robust deep-insertion probe has been developed to measure the radial profile of the Hall term to $\frac{r}{a}>0.45$ in 200 kA plasmas. The modal composition of the emf/stress is inferred using pseudospectral (cross-correlation) analysis with the spectrum measured with a toroidal magnetic array at the plasma surface. Extended MHD simulations with parameters comparable to the experiment have been performed using NIMROD, revealing intricate behavior of the Hall dynamo profile across the plasma radius. The extended profile measurements using the deep-insertion probe allow more complete comparisons with computational predictions and provide constraints for future simulations. The DEBS (single fluid MHD) code is also used to compare results for different plasma models. [Preview Abstract] |
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PP8.00033: Dissipation Range of Anisotropic Magnetic Fluctuations in MST plasmas James B. Titus, Abdulgader F. Almagri, Paul W. Terry, John S. Sarff, Ephrem D. Mezonlin Previous measurements of broadband magnetic fluctuations in the Madison Symmetric Torus (MST) revealed a turbulent cascade that is anisotropic with respect to the large-scale (equilibrium) magnetic field and characterized by a power spectrum with exponential falloff at scales larger than expected for classical processes. The cascade is supported by tearing instabilities at the global scale that undergo strong nonlinear coupling. The non-classical dissipation feature may be indicative of the powerful non-collisional ion heating observed in MST plasmas. We report new measurements with increased spatial resolution, by decreasing the distance between coils (4 mm) and increasing the number of coils (7) in each direction. Initial analysis shows similar anisotropic behavior for larger values of $k $but with a modest difference in the spectral characteristics. In particular, the exponential falloff appears to weaken at shorter wavelengths, suggesting strong dissipation occurs over an intermediate range of scales somewhat larger than the ion gyroradius. Also, the power spectrum is much steeper at small scales during pulsed poloidal current drive and non-reversed plasmas, where tearing instability and/or non-linear coupling is reduced. [Preview Abstract] |
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PP8.00034: Electromagnetic energy transport in RFP magnetic relaxation K.J. McCollam, D.J. Thuecks, D.R. Stone, J.K. Anderson, D.J. Den Hartog, J. Duff, J. Ko, S. Kumar, E. Parke, L. Lin, D.L. Brower, W.X. Ding In an RFP driven by steady toroidal induction, tearing modes responsible for magnetic relaxation redistribute electromagnetic energy throughout the plasma, generating the net EMF that regulates the equilibrium profile. In MST experiments, insertable edge probes measure local fluctuations in electric and magnetic fields, from which flux-surface-average Poynting flux is derived. This outwardly directed flux is maximum during discrete ``sawtooth'' magnetic relaxation events and is a significant fraction (a few 10s of percent) of the total input inductive power when averaged over time. Spatially, the flux is maximum at the reversal surface and decreases outside, indicating that transported energy is deposited at the plasma edge. These results are similar to expectations from a simple model of an incompressible fluid plasma with a resistive boundary and consistent with estimates of global power balance from time-resolved equilibrium reconstructions. This work was supported by the US DOE and NSF. [Preview Abstract] |
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PP8.00035: NIMROD Extended MHD Simulations of Reversed-Field Pinch Relaxation Dynamics Joshua Sauppe, Carl Sovinec, John Sarff, Joseph Triana The nonlinear evolution and relaxation dynamics of an initially non-reversed two-fluid plasma in cylindrical geometry is investigated using the NIMROD code. The initial relaxation event brings the plasma to the characteristic reversed-field state. There is significant magnetic activity with MHD and Hall dynamos working together to relax the parallel current profile while the fluctuation-induced Lorentz force drives plasma flows. Subsequent events have considerably less magnetic activity and often have opposing MHD and Hall dynamos. The direction of the driven flows in these events differs from the initial event, and is consistent with experimental observations on the MST RFP. The nonlinear mode coupling during relaxation events is investigated, and the presence of the Hall dynamo is found to significantly alter the spectral power flow. Synthetic diagnostics are used to compare simulation results to experimental measurements of Hall dynamo mode structure with laser Faraday rotation and magnetic probes. At modest Lundquist number the time-scales of relaxation and drive are well-separated and the simulations are compared to two-fluid relaxation theories. Generalized two-fluid helicities are well-conserved relative to magnetic energy over the simulated relaxation events. [Preview Abstract] |
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PP8.00036: Effect of parallel electron heat transport on drift and drift-tearing modes in RFP plasmas V.V. Mirnov, C.C. Hegna, J.P. Sauppe, C.R. Sovinec Linear numerical simulations were performed for plasma slab with cold ions and hot electrons in a doubly periodic box bounded by two perfectly conducting walls. Within this model, configurations with magnetic shear are unstable to current-driven drift-tearing instability. Additionally, there is an unstable pressure-gradient driven mode that is largely electrostatic in nature, suggestive of a resistive-drift type instability. The simultaneous presence of linear drift-like and tearing instabilities was observed using both two fluid extended modeling with NIMROD and analytical methods. The primary motivation for these studies is to understand the electrostatic transport thought to be present in Madison Symmetric Torus RFP experiments. Our previous analytical studies were performed either in the limit of infinitively large parallel electron heat conduction or in the pure adiabatic regime with an isentropic equilibrium. We report now on a general model with arbitrary equilibrium and finite parallel thermal conduction. Drift mode stability is sensitive to the ratio of density and temperature gradient scales and the instability exists even for pure transverse perturbations. Preliminary analytical results confirm some reduction of the drift-tearing mode growth rate caused by finite electron thermal conduction, consistent with previous works. Results of NIMROD simulations for different regimes of the electron thermal conduction are reported as well. [Preview Abstract] |
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PP8.00037: 3D MHD modeling of fusion plasmas with the PIXIE3D and SpeCyl codes 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 and SpeCyl codes are reported. The fundamental mathematical correctness of the two codes was proven by a nonlinear cross-benchmark study [1]. The codes have then been used to model the three main configurations for magnetic confinement, namely tokamak, stellarator and reversed-field pinch (RFP) plasmas. Qualitative agreement with respect to experimental observations in the RFX-mod device operated as both RFP and tokamak has been demonstrated by taking advantage of numerical features such as the possibility of applying 3D external magnetic perturbations [2]. More recently, 3D magnetic perturbations have also been used to obtain stellarator fields. The toroidal coupling between MHD modes (as provided by PIXIE3D in toroidal geometry) affects both the MHD dynamics and the magnetic topology. In addition, PIXIE3D solves the heat transport equation with self-consistent coupling between resistivity (affecting Ohmic heating) and temperature. The use of this feature will be discussed, with particular attention to the RFP case. \\[4pt] [1] D. Bonfiglio \textit{et al.}, Phys. Plasmas \textbf{17}, 082501 (2010)\\[0pt] [2] D. Bonfiglio \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 085002 (2013) [Preview Abstract] |
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PP8.00038: A new technique for effective core fueling and density control in RFX-mod Gianluca De Masi, Fulvio Auriemma, Roberto Cavazzana, Emilio Martines, Gianluca Spizzo High current plasmas in the RFX-mod Reversed Field Pinch device can be presently sustained either operating at low density (ne/nG \textless 0.3, being nG the Greenwald density) or transiently at high density by pellet injection. Discharges at ne/nG \textgreater 0.3 are difficult to sustain due to the high ohmic power required and a confinement properties downgrading. In these regimes, the transport mechanism results in a hollow density profile preventing an effective core fueling. A different behavior is observed in Ultra-low q configuration (q[r$=$a]\textgreater 0), in which the increased particle diffusivity produces flat density profiles and makes easier neutral particle penetration. In this contribution we show the main results of a new method to produce a more effective core fueling based on the previous empirical observations. The idea was to produce during the discharges narrow time windows with q[r$=$a]$\ge $0 values and, during this phase, to apply a strong gas puffing. This experimental condition is found to allow an increased particles core penetration. From the operational point of view, a lower input power was needed to sustain the discharges with similar core density. A deeper analysis through the ASTRA code will highlight the relation between transport properties and magnetic topology. [Preview Abstract] |
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PP8.00039: Neutron and Gamma-ray Detection in Reversed-Field Pinch Deuterium Plasmas in the RFX-mod Device Matteo Zuin, Luca Stevanato, Emilio Martines, Winder Gonzalez, Roberto Cavazzana, Davide Cester, G. Nebbia, Laszlo Sajo-Bohus, Giuseppe Viesti An experimental analysis of neutron and gamma-ray fluxes exiting purely ohmically heated plasmas in reversed-field pinch (RFP) configuration is presented. The diagnostic system, installed in the RFX-mod, is made of 2 scintillators (EJ-301 liquid and NaI(Tl)) coupled to flat-panel photomultipliers, which can be operated under magnetic fields. The production of neutrons and gamma rays in Deuterium plasmas is found to be strongly dependent on the Ohmic input power, with a threshold value of about 1.2MA in terms of plasma current level, below which low levels of gamma rays and almost no neutrons are detected. Neutron and gamma production is characterized by a bursty behavior, correlated to the spontaneous magnetic reconnection events, occurring almost cyclically in the RFP plasmas. The role of ion heating and particle acceleration during such events is discussed. [Preview Abstract] |
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PP8.00040: Magnetic topology change induced by reconnection events in RFP plasmas Barbara Momo, Emilio Martines, Paolo Innocente, Rita Lorenzini, Cristina Rea, Paolo Zanca, Matteo Zuin Magnetic reconnection is a phenomena observed in various plasmas across the Universe, where a conversion of magnetic to kinetic energy of plasma particles is consequent to a change in the global magnetic topology. In laboratory plasmas magnetic reconnections are associated to relaxation processes, like sawtooth crashes in Tokamak dynamics and the so-called dynamo effect in Reversed Filed Pinches (RFPs). In this work we propose the study of magnetic crashes in RFP dynamics, where the recursive transition from a more ordered helical state to a chaotic one is associated with rapid magnetic reconnection events. More into details, we propose to analyse RFX-mod discharges reconstructing the magnetic topology in the whole plasma volume at fixed time snapshots. Times are chosen in a window around the crashes, and the magnetic topology is reconstructed by using the solutions of a Newcomb-type equation, solved consistently with experimental boundary conditions. New boundary conditions are given by internal magnetic measurements coming from the ISIS probe system, in order to detect high frequency dynamics. Poincar\'{e} plots are used as a tool for the visualization of magnetic topology changes. [Preview Abstract] |
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PP8.00041: Fast growing instabilities and non-linear saturated states in hybrid tokamak and RFP plasmas Daniele Brunetti, Jonathan Graves, Wilfred Cooper, David Terranova, Christer Wahlberg The stability of large scale m=1 helical displacements of tokamak and RFP plasmas with reversed shear are investigated using the 3D equilibrium code VMEC/ANIMEC and the non-linear initial value stability code XTOR. The non-linear amplitude of such saturated modes obtained with XTOR is compared both with the helical core structure resulting from VMEC/ANIMEC calculations, and with analytic predictions. For conditions where the magnetic shear is allowed to become small over a large portion of the plasma, resistive sidebands coupled to a core kink-like mode exhibit extremely fast growth. The sensitivity of the dependence of the growth rate upon the Lundquist number to two-fluid effects has been examined analytically and also numerically with the XTOR code. It is found that these additional non-MHD effects tend to moderately reduce the growth rate of resistive modes. A family of modes are obtained, including modes with novel scaling on Lundquist number, some of which rotate in the electron diamagnetic direction, and others in the ion diamagnetic direction. In ideal and resistive numerical simulations, qualitative agreement has been found between XTOR and analytical predictions in absence of non-MHD effec [Preview Abstract] |
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PP8.00042: Evaluation of time evolution of 3-D Structure in RELAX RFP with SXR Imaging Technique Akio Sanpei, Sadao Masamune, Kanae Nishimura, Ryota Ueba, Go Ishii, Ryosuke Kodera, Yosuke Aoki, Haruhiko Himura, Satoshi Ohdachi, Naoki Mizuguchi, Tsuyoshi Akiyama In a low-A RFP machine RELAX (R = 0.51 m/a = 0.25 m (A = 2)), a quasi-periodic transition to quasi-single helicity (QSH) state has been observed. During the QSH state, the fluctuation power concentrates in the dominant m = 1/n = 4 mode, and a (toroidally rotating) 3-D helical structure has been observed with radial array of magnetic probes. We have applied a high-speed (10-microsecond time resolution) dual soft-X ray (SXR) imaging diagnostic system to take SXR images from tangential and vertical directions simultaneously to observe 3-D dynamic structures of the SXR emissivity. The magnetic field topology for the QSH RFP phase in RELAX plasmas are identified with obtained dual SXR images and results of external magnetic measurements. Recently, we have developed a two-dimensional electron temperature diagnostic system for thermal structure studies. The system consists of a SXR camera with two pin-holes for two-kinds of absorber foils, combined with a high-speed camera. We have succeeded in distinguishing Te image in QSH from that in multi-helicity (MH) RFP states. The most recent results using above techniques will be presented, together with discussion on possible reconstruction methods from 3-D imaging. [Preview Abstract] |
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PP8.00043: Reversed Field Pinch Dynamics in Toroidal and Cylindrical Geometries Jorge A. Morales, Wouter J.T. Bos, Kai Schneider, David C. Montgomery The effect of the curvature of the imposed magnetic field on Reversed Field Pinch dynamics is investigated by comparing the flow of a magnetofluid in a torus with aspect ratio 1.83, with the flow in a periodic cylinder. It is found that an axisymmetric toroidal mode is always present in the toroidal, but absent in the cylindrical configuration. In particular, in contrast to the cylinder, the toroidal case presents a double poloidal recirculation cell with a shear localized at the plasma edge. Quasi-single-helicity states are found to be more persistent in toroidal than in periodic cylinder geometry. [Preview Abstract] |
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PP8.00044: NSTX AND OTHER SPHERICAL TORI |
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PP8.00045: Overview of NSTX Facility Upgrades Status and Research Plans Masayuki Ono NSTX is undergoing a major device upgrade as well as an addition of a second more tangential Neutral Beam Injection (NBI) heating and current drive system. NSTX upgrade will double the toroidal field from 0.5 T to 1 T, the plasma current from 1 MA to 2 MA, the NBI heating and current drive power from 7 MW to 14 MW, and increase the peak field plasma pulse length from 1 sec to 7 sec. More tangential NBI system is designed to achieve 100{\%} non-inductive operation needed for a compact FNSF design. Innovative plasma start-up and ramp-up techniques without the central solenoid operation which is needed for a compact FNSF design will be explored. A major physics/technology goal for NSTX-U is to develop an attractive divertor solution for the very high steady-state divertor heat flux expected for FNSF. With doubling of the heat flux and plasma current, the peak divertor heat flux in NSTX-U could quadruple to about 40 MW/m$^2$ compared to up to 10 MW / m$^2$ of NSTX. For divertor heat mitigation, snow-flake divertor configuration and liquid lithium divertor are being considered. The first plasma operation of NSTX-U is planned in January 2015. [Preview Abstract] |
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PP8.00046: Plans for Conditioning Plasma-Facing Components at Initiation of NSTX-U Operations R. Kaita, W. Blanchard, D. Cai, S. Gerhardt, M.A. Jaworski, M. Lucia, S. Rossi, C.H. Skinner, J.-P. Allain, F. Bedoya The conditioning of plasma-facing components (PFCs) has been critical to the achievement of high performance plasmas in fusion devices. The NSTX-U PFCs will initially consist of graphite. Well-established PFC conditioning will be applied, including high temperature bakeout and glow discharge cleaning (GDC). As in NSTX, the center stack (CS) will be electrically-isolated from the outer vacuum vessel in NSTX-U for coaxial helicity injection (CHI), and this also permits high currents to pass through the CS for baking. Other conditioning techniques are required to further reduce the dominant impurities, which are expected to be carbon and oxygen. Boronization will first be performed, where helium glow discharge cleaning (GDC) is followed by GDC with a mixture of 95\% helium and 5\% deuterated trimethyl boron (TMB), and another period of helium GDC. This is to be compared with lithiumization, where lithium vapor is evaporated directly on PFC surfaces. The effectiveness of both conditioning techniques has been inferred from plasma measurements subsequent to their application, but the link between them and actual PFC conditions has not been made. The new Materials Analysis and Particle Probe (MAPP) is intended to do this with in situ analysis of PFC samples exposed to NSTX-U plasmas. [Preview Abstract] |
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PP8.00047: Status and Plans for Transient CHI and MGI Experiments on NSTX-U R. Raman, T.R. Jarboe, B.A. Nelson, D. Mueller, F. Ebrahimi, G. Taylor, S.C. Jardin Results from NSTX Transient Coaxial Helicity Injection (CHI) experiments have demonstrated generation of 300kA start-up currents, and when these discharges were coupled to induction they attained 1MA of plasma current consuming 65{\%} of the inductive flux of standard inductive-only discharges in NSTX. The NSTX-U device will have numerous improvements to enhance transient CHI capability. TSC simulations have been used to guide the choice of NSTX-U coil currents for initial CHI operations in FY14, which show more than a doubling of the CHI current generation potential in NSTX-U. The NIMROD code has been used to understand basic physics trends, which are consistent with scaling relations that have been used to guide the CHI design on NSTX and NSTX-U. In support of ITER disruption mitigation studies, an ITER-type MGI valve has been designed, built and tested, including in the presence of externally imposed magnetic fields. FY14 research on NSTX-U will use three of these valves positioned at different poloidal locations to study the MGI gas assimilation efficiency. This work is supported by U.S. DOE Contracts: DE-AC02-09CH11466, DE-FG02-99ER54519 AM08, and DE-SC0006757. [Preview Abstract] |
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PP8.00048: Plasma injection and evolution in CHI simulations of NSTX E.B. Hooper, C.R. Sovinec, R. Raman Simulations of co-injection of helicity and plasma into a low density plasma in NSTX are compared with experiment, extending previous simulations that assumed helicity injection into a constant density plasma [1, 2]. The background plasma response is minimized by density-dependent artificial radiation. Helicity and plasma flow from the slot at the ExB velocity due to the applied voltage. A simple model of impurity radiation from the injected plasma improves agreement with the temperature during experimental plasma buildup and following flux closure after injection [3]. The simulations also explore the effect of impurity concentration near the bottom plate where impurities are generated at the footprints of the currents associated with the injection. As in previous simulations [4], non-axisymmetric flows and currents are generated during injection but have little impact on the final closed-flux configuration.\\[4pt] [1] E. B. Hooper, et al., Phys. Plasmas \textbf{20}, 092510 (2013).\\[0pt] [2] F. Ebrahimi, et al. Phys. Plasmas \textbf{20}, 090702 (2013); Phys. Plasmas \textbf{21}, 0566109 (2014).\\[0pt] [3] E. B. Hooper, et al., Bull. Am. Phys. Soc. \textbf{59}(16), NP8.23 (2013).\\[0pt] [4] E. B. Hooper, et al., Bull. Am. Phys. Soc. \textbf{56}(12), 255 (2011). [Preview Abstract] |
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PP8.00049: Magnetic Field Measurements in NSTX-U with the MSE-LIF Diagnostic Fred Levinton, Jill Foley, Darrell DiCicco, David Cylinder, Hannah La Fleur, Howard Yuh The motional Stark effect with laser-induced fluorescence diagnostic (MSE-LIF) was installed on NSTX during the 2011 run year. The MSE-LIF will enable radially resolved measurements of the magnetic field pitch angle and magnitude, both of which can be used to constrain plasma equilibrium reconstructions. A diagnostic neutral beam with low axial energy spread, low divergence, and high reliability has been developed. It operates routinely at 35 kV and 40~mA. A laser has been developed with high power ($\sim$10 W) and optimal linewidth matched to the energy spread of the neutral beam ($\sim$6\,GHz). The laser wavelength is near 651 nm for a match to the Doppler-shifted Balmer-alpha transition in the beam neutrals. The unique high-power, moderate linewidth laser system utilizes a 19 emitter diode laser bar and feedback from a volume Bragg grating for line width narrowing. A magnetic shield protects the ion source from the NSTX stray fields. Initial data in a gas-filled torus and low magnetic fields was taken on NSTX. Several improvements have been made to the system during the NSTX upgrade, including adding more spatial channels and several laser improvements. [Preview Abstract] |
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PP8.00050: Preparing Plasma Control and Digital Coil Protection for NSTX-U S.P. Gerhardt, K. Erickson, D.A. Gates, R. Kaita, D. Mueller, S.A. Sabbagh, T. Stevenson, P. Titus, W. Que Compared to NSTX, NSTX-Upgrade will have twice the toroidal field (B$_{\mathrm{T}}$ 0.5T $\to$ 1.0T) and plasma current (I$_{\mathrm{P}}$ 1MA $\to$ 2MA). These increases in capability have mandated a new digital coil protection system (DCPS). This software computes forces, stresses, and coil heating in real time, and brings down the coil power supplies in a controlled manner when the forces, stresses, heating and currents exceed limits; the algorithms and their numerical implementation will be described. The algorithms have been run on the legacy NSTX database of discharges, motivating a reexamination of some limit values and identification of the plasma control behaviors that lead to large forces and stresses. These and other changes in the conversion to NSTX-U have motivated improvements to the plasma control system (PCS) algorithms. The preliminary design of an architecture for a automated discharge termination system will be presented, motivated by the desire to reduce large current transients during disruptions, thereby reducing stresses and avoiding DCPS faults. Other improvements to the plasma control system include the automation of the TF current rampdown, improvements to the gas delivery algorithms, and the addition of many more flux loops and magnetic pickup (Mirnov) sensors for real time equilibrium reconstruction. This work was sponsored by the U.S. Department of Energy. [Preview Abstract] |
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PP8.00051: First-Principles-Driven Model-Based Optimal Control of the Current Profile in NSTX-U Zeki Ilhan, Justin Barton, William Wehner, Eugenio Schuster, David Gates, Stefan Gerhardt, Egemen Kolemen, Jonathan Menard Regulation in time of the toroidal current profile is one of the main challenges toward the realization of the next-step operational goals for NSTX-U. A nonlinear, control-oriented, physics-based model describing the temporal evolution of the current profile is obtained by combining the magnetic diffusion equation with empirical correlations obtained at NSTX-U for the electron density, electron temperature, and non-inductive current drives. In this work, the proposed model is embedded into the control design process to synthesize a time-variant, linear-quadratic-integral, optimal controller capable of regulating the safety factor profile around a desired target profile while rejecting disturbances. Neutral beam injectors and the total plasma current are used as actuators to shape the current profile. The effectiveness of the proposed controller in regulating the safety factor profile in NSTX-U is demonstrated via closed-loop predictive simulations carried out in PTRANSP. [Preview Abstract] |
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PP8.00052: A framework for control simulations using the TRANSP code Mark D. Boyer, Rob Andre, David Gates, Stefan Gerhardt, Imene Goumiri, Jon Menard The high-performance operational goals of present-day and future tokamaks will require development of advanced feedback control algorithms. Though reduced models are often used for initial designs, it is important to study the performance of control schemes with integrated models prior to experimental implementation. To this end, a flexible framework for closed loop simulations within the TRANSP code is being developed. The framework exploits many of the predictive capabilities of TRANSP and provides a means for performing control calculations based on user-supplied data (controller matrices, target waveforms, etc.). These calculations, along with the acquisition of ``real-time'' measurements and manipulation of TRANSP internal variables based on actuator requests, are implemented through a hook that allows custom run-specific code to be inserted into the standard TRANSP source code. As part of the framework, a module has been created to constrain the thermal stored energy in TRANSP using a confinement scaling expression. Progress towards feedback control of the current profile on NSTX-U will be presented to demonstrate the framework. [Preview Abstract] |
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PP8.00053: Modeling and control of plasma rotation for NSTX using Neoclassical Toroidal Viscosity (NTV) and Neutral Beam Injection (NBI) Imene Goumiri, Clarence Rowley, Steven Sabbagh, David Gates, Stefan Gerhardt A model-based system to control plasma rotation in a magnetically confined toroidal fusion device is developed to maintain plasma stability for long pulse operation. This research uses experimental measurements from the National Spherical Torus Experiment (NSTX) and is aimed to control plasma rotation by using momentum from injected neutral beams and viscosity generated by three-dimensional applied magnetic fields as actuators. Based on the data driven model obtained, a feedback controller is designed to theoretically sustain the toroidal momentum of the plasma in a stable fashion and to achieve desired plasma rotation profiles. On going work includes extending this method to NSTX Upgrade which has more complete radial coverage of the neutral beams momentum sources which enable simultaneous control of plasma stored energy (Beta control). [Preview Abstract] |
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PP8.00054: Modeling of fully non-inductive startup and ramp-up towards development of advanced scenarios in NSTX-U Francesca Poli, Robert Andre, Nicola Bertelli, Stefan Gerhardt, Charles Kessel, Roger Raman, Gary Taylor The National Spherical Torus eXperiment Upgrade (NSTX-U) will operate at maximum axial toroidal field of 1T, maximum plasma current Ip of 2MA, and pulse length up to 5 seconds. Three additional neutral beam injection (NBI) sources, with tangency radii of 110-130cm, will provide significant off-axis current drive. Time-dependent free-boundary TRANSP/ISOLVER simulations have self-consistently modelled fully non-inductive (NI) Ip ramp-up. Non-solenoidal startup with Coaxial Helicity Injection (CHI), simulated with TSC, provides the initial plasma condition for the simulations. RF waves are injected to prepare target plasmas where NBI can be used with minimal power/particle losses. 1MW of Electron Cyclotron (EC) power can rapidly heat CHI plasmas up to 1keV, and 4MW of High Harmonics Fast Wave power can drive 350kA. With 10MW of NBI, NI Ip ramp up to 900kA is possible in 2.5s. In this paper we discuss how the time of the NBI sources and the NBI source energy affect the profile evolution and access to advanced target scenarios in NSTX-U. These simulations provide a reference operational space for NI ramp-up experiments during the first two years of operation of NSTX-U, as well as guidance for the EC accessibility and use for optimization in non-solenoidal startup experiments. [Preview Abstract] |
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PP8.00055: Experimental Observation of High-k Turbulence Evolution across L-H Transition in NSTX Y. Ren, R.E. Bell, D.R. Smith, S.J. Zweben, W. Guttenfelder, S.M. Kaye, B.P. LeBlanc, E. Mazzucato, K.C. Lee, C.W. Domier, L. Shao, H. Yuh It is well accepted that L-H transition is due to suppression of edge turbulence and thus forming Edge Transport Barrier(ETB). While suppression of low-k (ion-scale) turbulence across L-H transition was widely reported, here we report high-k turbulence evolution across L-H transition in NSTX for the first time. The high-k turbulence at r/a$\approx$0.7-0.8 was measured using a microwave scattering system with the ETB located at r/a $\agt$ 0.9. An intermittent phase for the high-k turbulence is observed after the L-H transition with gradual decrease in the overall turbulence spectral power and intermittent periods (about 0.5-1 ms) of minimum high-k turbulence. A phase of minimum high-k turbulence is observed following the intermittent phase, and the high-k turbulence suppression is found to occur only at lower wavenumbers, namely $k_{\perp}\rho_s \alt$ 9-10. The suppression is found to be consistent with the decrease in maximum ETG linear growth rate from gyrokinetic stability analysis. However, the observed intermittency cannot be explained by the linear analysis. Low-k turbulence, measured by Beam Emission Spectroscopy and Gas Puff Imaging, shows similar temporal behavior as the measured high-k turbulence. [Preview Abstract] |
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PP8.00056: Studying high-k turbulence with microwave scattering in NSTX J. Ruiz, Y. Ren, A.E. White, W. Guttenfelder, S.M. Kaye, B.P. LeBlanc, E. Mazzucato, K.C. Lee, C.W. Domier, D.R. Smith, H. Yuh Understanding electron thermal transport is important for achieving predictive capability for the performance of future fusion devices such as ITER. In NSTX, electron thermal transport is found to dominate energy loss. Numerical simulations and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, $k_{\perp}\rho_{e}\leq1$, can be responsible for anomalous electron thermal transport in NSTX. Electron scale (high-k) turbulence with $k_{\perp}\rho_{e}\leq0.6$ was measured with a high-k microwave scattering system in NSTX. Enhanced high-k fluctuations have been previously observed when electron ETG exceeds critical gradient, and are affected by ExB shear flows, reverse magnetic shear, electron density gradient and electron collisionality. A description of the NSTX high-k scattering diagnostic will be presented, as well as the effect of magnetic field curvature and turbulence anisotropy on the spatial localization and k-resolution. Analysis of high-k turbulence measurements during plasma current ramp-down in a set of NSTX H-mode plasmas will also be presented. [Preview Abstract] |
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PP8.00057: Core electron thermal transport in NSTX due to orbit stochastization by high frequency Alfv\'{e}n eigenmodes N.A. Crocker, E. Belova, E.D. Fredrickson, N.N. Gorelenkov, K. Tritz, W.A. Peebles, S. Kubota, R.E. Bell, A. Diallo, B.P. Leblanc, J.E. Menard, R.B. White, H. Yuh Progress is reported in understanding the role of high frequency Alfv\'{e}n eigenmodes (AE) in anomalously high electron thermal transport in the core of high performance, beam-heated NSTX plasmas. Compressional (CAE) and global (GAE) AEs have been hypothesized to cause the transport by stochastization of electron guiding-center drift orbits. Results reported here, arrived at via new measurements of CAE and GAE $\delta n_{e}$ coupled with the guiding-center code ORBIT, support this. The measurements are also compared to eigenmodes from the initial value code HYM---which simulates an MHD plasma coupled to fully kinetic fast-ions. Reflectometer measurements are inverted using a synthetic diagnostic to obtain $\delta n_{e}$. The measurements show that the CAEs peak in the core, while the GAEs peak in the edge. Simulation also shows edge peaking for the GAEs, but with notable differences in structure. The differences will be examined with HYM to elucidate the importance of effects such as coupling of shear and compressional Alfv\'{e}n waves. The measured modes are used in ORBIT simulations to model the effects on electron orbits and the resulting transport is compared with that inferred from the experimental power balance calculated with TRANSP. [Preview Abstract] |
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PP8.00058: Reduced model prediction of electron temperature profiles in microtearing-dominated NSTX plasmas S.M. Kaye, W. Guttenfelder, R. Bell, S. Gerhardt, B. LeBlanc, R. Maingi A representative H-mode discharge from the National Spherical Torus Experiment (NSTX) is studied in detail as a basis for a time-evolving prediction of the electron temperature profile using an appropriate reduced transport model. The time evolution of characteristic plasma variables such as $\beta_{\mathrm{e}}$, $\nu_{\mathrm{e}}^{\mathrm{\ast }}$, the MHD $\alpha $ parameter and the gradient scale lengths of T$_{\mathrm{e}}$, T$_{\mathrm{i\thinspace }}$and n$_{\mathrm{e\thinspace }}$were examined prior to performing linear gyrokinetic calculations to determine the fastest growing microinstability at various times and locations throughout the discharge. The inferences from the parameter evolutions and the linear stability calculations were consistent. Early in the discharge, when $\beta _{\mathrm{e\thinspace }}$and $\nu_{\mathrm{e}}^{\mathrm{\ast \thinspace }}$were relatively low, ballooning parity modes were dominant. As both $\beta_{\mathrm{e\thinspace }}$and $\nu _{\mathrm{e}}^{\mathrm{\ast \thinspace }}$increased with time, microtearing became the dominant low-k$_{\mathrm{\theta \thinspace }}$mode, especially in the outer half of the plasma. There are instances in time and radius where other modes, at higher-k$_{\mathrm{\theta }}$, may be important for driving electron transport. The Rebut-Lallia-Watkins (RLW) electron thermal diffusivity model, which is based on microtearing-induced transport, was used to predict the time-evolving electron temperature across most of the profile. The results indicate that RLW does a good job of predicting T$_{\mathrm{e\thinspace }}$for times and locations where microtearing was determined to be important, but not as well when microtearing was predicted to be stable or subdominant. [Preview Abstract] |
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PP8.00059: Identification of new turbulence contributions to plasma transport in NSTX W.X. Wang, S. Ethier, Y. Yen, S. Kaye, J. Chen, Z. Lu, E. Startsev Nonlinear global gyrokinetic simulations critical for addressing highly distinct turbulence and transport physics in spherical tokamaks (ST) such as NSTX have led to new insights. The drift wave Kelvin-Helmholtz instability, driven by strong shear flow, was found to be unstable in NSTX L-mode plasmas. The mode is characterized by finite k$_{//}$ and broader k$_{\theta }$ than for the ITG mode. While the strong rotation shear leads to a reduction in the lowest-k turbulence, the remaining low-k fluctuations can produce a significant ion thermal transport comparable to experimental levels in the outer core region. Low-k fluctuations in L-mode also produce significant toroidal momentum flux, including a large anti-gradient residual stress mainly due to zonal flow shear induced symmetry breaking. Another new, important turbulence source identified in NSTX is the dissipative trapped electron mode (DTEM), which is believed to play little role in the core of conventional tokamaks. Due to the high trapped electron fraction in NSTX, long wavelength DTEMs peaking around k$_{\theta }\rho _{s}$ $\sim$ 0.1 are destabilized by collisions with steep electron temperature and density gradients achieved there. The DTEM survives the strong ExB shear associated with the toroidal rotation. The grad-n driven DTEM is found to produce significant particle, ion energy and toroidal momentum transport in agreement with experimental levels in NSTX H-modes. The grad-Te driven DTEM may potentially produce a large heat flux contributing to the highly anomalous electron thermal transport, and possibly contribute to the strong increase of confinement in NSTX with decreasing collisionality. This work was supported by U.S. DOE Contract DE-AC02-09CH11466. [Preview Abstract] |
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PP8.00060: Variations in Edge and SOL Turbulence in NSTX S.J. Zweben, W.M. Davis, R.M. Bell, B.P. LeBlanc, S.M. Kaye, T. Munsat, J.R. Myra, Y. Sechrest Variations in edge and SOL turbulence were studied using a 140 shot database from the NSTX gas puff imaging (GPI) diagnostic. Analyses of the turbulence structure and motion was done using both cross-correlation and blob-tracking techniques. The relative fluctuation levels and blob formation rates were lower near and inside the separatrix for H-mode plasmas compared to Ohmic and L-mode plasmas, but similar in the far SOL. Poloidal correlation and blob lengths were roughly the same as radial turbulence correlation and blob lengths, and roughly independent of confinement regime. The radial turbulence velocity was outward in all cases, but the poloidal velocity reversed direction inside the separatrix for Ohmic and low-power L-mode plasmas. Variations of the turbulence quantities with global and local edge parameters will be described in detail. An attempt will be made to identify the basic mechanisms of the turbulence based on these variations and to evaluate the scaling of turbulent transport in the edge and SOL. [Preview Abstract] |
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PP8.00061: Direct Comparison of GPI and BES measurements of Edge Fluctuations in NSTX Y. Sechrest, T. Munsat, D. Smith, S.J. Zweben We compare GPI and BES fluctuation measurements of NSTX edge plasmas, and examine the local effects of the GPI neutral deuterium puff on BES during MHD-quiescent H-mode plasmas without large ELMs. The GPI and BES views on NSTX provide partially overlapping coverage of the edge and SOL regions above the outboard midplane. The separation in the toroidal direction is $16^\circ$, and field lines passing through diagnostic views are separated by $\sim 14$ cm in the direction perpendicular to the magnetic field. Strong cross-correlation is observed, and strong cross-coherence is seen for frequencies between 5-15 kHz. Also, probability distribution functions of fluctuations measured $\sim 3$ cm inside the separatrix exhibit only minor deviations from a normal distribution for both diagnostics, and good agreement between correlation length estimates is found. While the two instruments agree closely in many respects, some discrepancies are observed. Most notably, GPI normalized fluctuation levels exceed BES levels by a factor of 5 or more. BES mean intensity is found to be sensitive to the GPI neutral gas discharge, and BES normalized fluctuation levels for frequencies between 1-10 kHz are observed to increase during the GPI puff. [Preview Abstract] |
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PP8.00062: Experimental Study of Kink-like Modes in NSTX Plasmas Ge Dong, Mario Podesta Internal kink modes destabilized by energetic trapped particles can cause particle losses and deteriorate plasma performance in toroidal fusion devices . In this study, we characterized the main properties of kink-link instabilities in National Spherical Torus Experiment (NSTX) plasmas, including the wave number spectrum, effective mode growth rate and real frequency, as a function of the thermal plasma, fast ion and magnetic field parameters, which is re-constructed using LRDfit and TRANSP, utilizing experimental data from motional Stark effect(MSE) diagnostic for direct measurements of the q profiles. Results indicate that the bursting fishbone modes are unstable at preferentially higher fast ion beta regime, while the long-lived non-resonant kink (NRK) modes are unstable at lower and higher fast ion beta values. Both the fishbones and the NRK tend to be stable with q-min above around 1.5. [Preview Abstract] |
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PP8.00063: Equilibrium reconstruction including kinetic effects and impact on MHD stability interpretation Jonathan Menard, Zhirui Wang, Yueqiang Liu Non-ideal plasma equilibrium effects such as toroidal rotation and the presence of fast-ions from neutral beam heating can play an important role in MHD stability for both ideal-wall-mode and resistive-wall-mode instabilities. Systematic comparisons between measured and predicted ideal-wall-mode instability characteristics (such as marginal stability threshold and mode real frequency) have been carried out and highlight the sensitivity of the results to the rotation profile and fast-ion density and pressure profiles. A key uncertainty is the potential redistribution of fast-ions by higher frequency Alfvenic instabilities. Analysis indicates that utilizing reconstructed total pressure and rotation profiles as opposed to using modeled/predicted fast-ion pressure and angular momentum profiles from TRANSP in the limit of zero anomalous fast-ion diffusion can yield better agreement between measured and predicted stability characteristics -- consistent with apparent redistribution of fast-ions. Reconstruction methodologies and associated stability implications will be discussed. [Preview Abstract] |
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PP8.00064: Kinetic resistive wall mode stability evaluation and physics insight application in NSTX J. Berkery, S. Sabbagh, R. Betti, R. Bell, A. Diallo, S. Gerhardt, B. LeBlanc, J. Menard, M. Podesta Research on the National Spherical Torus Experiment (NSTX) has studied the stability of resistive wall modes (RWMs) in high-beta fusion plasmas for disruption avoidance. Stabilizing mechanisms for RWMs have been identified as the transfer of energy from the mode to thermal particles through rotational resonances and the effect of energetic particles to resist distortion of the magnetic field lines. These kinetic effects have been implemented in the \verb"MISK" code and results have compared favorably with NSTX experiments including prediction of the marginal stability point and agreement with the trends of low-frequency MHD spectroscopy experiments. Further improvement of the already close agreement of \verb"MISK" to experimental results is being pursued by comparison to a large database of NSTX discharges and implementation of additional physics in the code, including rotational effects on the fluid stability and the effect of pressure anisotropy. An ITPA MHD Stability Group joint analysis task verified several kinetic RWM codes, including \verb"MISK", and generally good agreement between the codes was achieved. A new disruption avoidance algorithm in NSTX-Upgrade will utilize the knowledge gained by kinetic stability physics insight, calculation, and comparison with experiment. [Preview Abstract] |
(Author Not Attending)
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PP8.00065: Effect of Externally Applied Perturbation Fields on Alfv\'{e}nic MHD Activity in the NSTX Tokamak Alessandro Bortolon Observations from NSTX demonstrate that externally applied magnetic perturbations (MP) can alter the dynamic of beam driven Alfv\'{e}n modes. Bursting Global Alfv\'{e}n Eigenmodes (GAE, n$=$7-9, 400-700 kHz) respond to pulses of static n$=$3 fields ($\delta $B/B $\sim$ 0.01 at the plasma edge) reducing mode amplitude, bursting period and frequency sweep by a factor of 2-3 [Bortolon et al., Phys. Rev. Letters, Vol. 110 (2013) 265008]. Similar MP attenuate the amplitude of continuous Toroidal Alfv\'{e}n Eigenmodes (TAE, n$=$2-3, 50-90 kHz). Calculations of the perturbed beam-ion distribution function, considering MP from ideal or resistive plasma response, confirm an enhanced fast-ion transport consistent with a reduced drive for the GAE. At the same time, MP can also affect the Alfv\'{e}n stability by altering the structure of Alfv\'{e}n continua through modification of the kinetic profiles or introducing toroidal coupling as result of the broken axisymmetry. Computations of the n$=$2 Alfv\'{e}n continuum for NSTX equilibria with n$=$3 MP show strong modification of the TAE continuum near the plasma edge, where coupling between n$=$2 and n$=$5 continuum modes reduces the gap, providing an additional damping for TAE modes extending in this region. [Preview Abstract] |
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PP8.00066: Time-series classification of ELMs on NSTX with machine learning analysis David Smith, R. Fonck, G. McKee The linear peeling-ballooning model can describe onset conditions for edge-localized modes (ELMs), but understanding saturation mechanisms and transport dynamics requires nonlinear models and experimental validation. Here, we examine time-series data of ELM bursts on NSTX to identify representative ELM groups with similar evolution characteristics. We apply hierarchical cluster analysis, an unsupervised machine learning technique popular in genomics, to beam emission spectroscopy (BES) measurements of ELM bursts. The application of cluster analysis to time-series data requires metrics to quantify the similarity or dissimilarity among time-series data. We find that both correlation and dynamic time warping similarity metrics reveal 2 groups of ELM events: fast, non-oscillatory ELMs ($\sim$ 200 micro-s) and slow, oscillatory ELMs ($\sim$ 1 ms). We also report on progress with wavelet-based similarity metrics. The identification of representative ELM groups with cluster analysis can establish validation scenarios for simulations, facilitate the automatic identification of ELMs in data, prepare ELM measurements for subsequent supervised machine learning analysis, and, more broadly, demonstrate the applicability of machine learning analysis to fusion plasma data. [Preview Abstract] |
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PP8.00067: Pedestal Structure Evolution between ELMs for Different Lithium Wall Conditioning on NSTX Qian Teng, Ahmed Diallo, Travis Gray, Rajesh Maingi, Benoit LeBlanc Understanding the evolution of the pedestal structure is important to develop predictive capabilities of fusion power for future devices. A detailed analysis of electron pressure, density and temperature profiles measured on H-mode discharges on NSTX is performed to investigate the dynamics of pedestal parameters between ELMs. [Discharges with lithium wall conditioning of 50mg, 150mg and 300mg.] This work is an extension from previous pedestal structure investigations [1]. The result shows that almost all the pedestal parameters, the width and height of pressure, density and temperature monotonously increase between ELMs while density height increases and sometimes saturates at the last 30{\%} of an ELM cycle, and the density width decreases monotonously. The dependence of pedestal parameters on plasma current and neutral beam injection power is investigated. The pedestal width scaling with poloidal beta will also be performed and compared with previous works. The impact of lithium wall conditioning on pedestal height and width dynamics will be reported. [Preview Abstract] |
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PP8.00068: The 3D Structure of Flux Tubes That Admit Flute Instability in the Scrape-Off-Layer (SOL) of Tokamaks Hironori Takahashi A severe reduction in size down to an ion gyro-radius scale, commonly known as ``squeezing,'' in a lateral dimension of the cross section of a flux tube is traditionally thought to inhibit the occurrence of the flute instability in the Scrape-off-Layer of a diverted tokamak by isolating the main volume of the flux tube from its ends at electrically conducting target plates. A study reported here in the 3D flux tube structure reveals the absence of squeezing for a flux tube that is sufficiently large in its toroidal extent (small toroidal harmonic number n) and located in a layer of low field-line shear around the ``sweet spot'' (about mid-way between the primary and secondary separatrices). The low-shear layer does not hence inhibit the flute instability through the squeezing mechanism, and may thus restore the flute instability, among the most virulent in the magnetized plasma, to the ranks of candidate electrostatic instabilities thought to underlie the turbulence in the SOL in tokamaks. Variations along the flux tube of geometrical characteristics including the cross section will be calculated to develop criteria for the absence of squeezing. [Preview Abstract] |
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PP8.00069: Characterization of inter-ELM impurity transport following 3D-field-triggered ELMs in NSTX Filippo Scotti, V.A. Soukhanovskii, R.E. Bell, J.M. Canik, A. Diallo, B.P. LeBlanc, M. Podesta' The response of electron temperature and density profiles to edge localized modes (ELMs) triggered by 3D fields and the inter-ELM impurity transport are studied in NSTX H-mode discharges. 3D magnetic perturbations ($n=$3) were used in lithium-conditioned ELM-free H-mode discharges to trigger ELMs (f$_{\mathrm{ELM}}=$10-62.5 Hz) and mitigate core impurity buildup [Canik PRL 2010]. For 10 Hz triggering, impurity flushing was observed for $\psi _{N}$\textgreater 0.5 following the ELM crash, with up to a 30{\%} drop in carbon density at the pedestal top and comparable effects on the T$_{\mathrm{e}}$, n$_{\mathrm{e}}$, T$_{\mathrm{i}}$, v$_{\mathrm{\Phi }}$ profiles. The increase in the triggered ELM frequency progressively reduced the total carbon inventory, affecting profiles at inner radii. Carbon transport during and following the ELM crash is modeled using the impurity transport code MIST. Transient perturbations to the steady state particle diffusivity $D$ and convective velocity $v$ are applied to simulate the effect of the ELM on the carbon density evolution. An inward (convective) transport perturbation for $\psi_{N}$ \textless 0.5 and an outward (diffusive and/or convective) transport perturbation for $\psi_{N}$ \textgreater 0.5 are needed to reproduce the carbon density profile temporal evolution. While the equilibrium $v/D$ ratio is inferred from the steady state carbon density profiles, the profile recovery following the ELM crash is used to estimate absolute values of the transport coefficients. [Preview Abstract] |
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PP8.00070: Midplane Neutral Density Profiles in NSTX D.P. Stotler, F. Scotti, R.E. Bell, B.P. LeBlanc, R. Raman A procedure for using the DEGAS~2 Monte Carlo neutral transport code to infer neutral density profiles from the Balmer-$\beta$ emission data recorded by a tangential camera was proposed previously and is examined in more detail here. The simulations track the penetration of an ad hoc neutral gas source at the vacuum vessel wall; the associated light emission seen by the camera is obtained via a synthetic diagnostic. The resulting radial emission profiles compare well with the measured ones, with the ratio of the profile peaks providing scaling factors for the neutral source strength and all output quantities. The procedure yields absolute radial profiles of deuterium atoms and molecules at the NSTX midplane. We will first show that the modeled camera image and density profiles are insensitive to variations in the spatial distribution of the neutral source. The procedure will then be applied to data from a variety of different NSTX operating regimes. A detailed uncertainty and error analysis will also be presented. [Preview Abstract] |
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PP8.00071: Effects of TAE Avalanches on the neutral beam driven current profile and fast ion loss in NSTX and NSTX-U plasmas Douglass Darrow, Alessandro Bortolon, Neal Crocker, Eric Fredrickson, Nikolai Gorelenkov, Marina Gorelenkova, Gerrit Kramer, Shigeyuki Kubota, Mario Podesta, Roscoe White Strong bursts of TAEs with multiple n numbers present, termed TAE avalanches, are observed in NSTX plasmas, including early in the discharge, when the plasma current is being ramped up. These avalanches cause radial redistribution, slowing down, and loss of beam ions from the plasma. Losses are often particularly pronounced during these events. All of these changes in the beam ion distribution can affect the beam driven current profile. Measurements and modeling of the beam ion population and driven currents in NSTX, using the ORBIT and SPIRAL codes will be compared, including the effects of avalanches and associated MHD activity during the current ramp up phase. In addition, since NSTX-U will utilize additional neutral beams with different orientations from the set on NSTX, modeling of the effects of avalanches on the distribution of beam ions and associated beam driven current from the new beam lines will also be discussed. [Preview Abstract] |
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PP8.00072: The Effect of Different Fast-ion Instabilities on the Fast-ion Profile E. Ruskov, W. Heidbrink, D. Liu, E. Fredrickson, A. Bortolon Fast-ion driven instabilities in NSTX take many forms, including steady, bursting, and avalanching toroidal Alfven eigenmodes (TAE), avalanching global AEs, energetic particle modes (EPM), long-lived modes (LLM) and abrupt large-amplitude events (ALE). The occurrence or absence of these modes on Mirnov signals correlates with the ratio of fast-ion to Alfven speed and the ratio of fast-ion to thermal pressure [1]. The drop in neutron rate at these events correlates differently with mode amplitude for the different types of events [1]. In this study, we expand this database to investigate the correlation of vertical fast-ion D-alpha (FIDA) data with the different types of MHD. The measured profiles are compared with classically-predicted profiles. \\[4pt] [1] E. Fredrickson et al., ``Parametric dependence of fast-ion transport events on the NSTX,'' Nucl. Fusion 54 (2014) in press. [Preview Abstract] |
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PP8.00073: TRANSP modeling of NB current drive including MHD efects M. Podest\`{a}, M. Gorelenkova, R.B. White Simulations using a newly developed physics-based fast ion transport model are used to understand and quantify MHD effects on neutral beam (NB) current drive efficiency. NSTX results confirm that toroidal Alfv\'{e}n eigenmodes (TAEs) and kink-like instabilities can cause substantial decrease in the central NB-driven current and modify its radial profile. Quantitative analysis is performed through a new model, developed for the TRANSP code, which computes EP transport in phase space to account for resonant wave-particle interactions. Simulations show that so-called TAE avalanches can cause decrements of up to 30\% in the core NB-driven current, and even larger (relative) changes toward the plasma edge. Perturbations of the current profile persist over a considerable fraction of the slowing down time, with a recovery rate set by the NB injection rate. In contrast to ad-hoc diffusive models previously available in TRANSP, the new model captures the feature that TAEs mainly affect fast ions with large parallel velocity, i.e. the most effective in driving current, leaving other portions of the fast ion distribution nearly unperturbed. [Preview Abstract] |
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PP8.00074: Validation of critical gradient model for fast ion relaxation in NSTX Jeff Lestz, Nikolai Gorelenkov, Mario Podest\`{a} Confinement of energetic particles (EP) is essential to optimize performance of present day tokamaks and future fusion devices. With superalfv\'{e}nic velocities, EP interact resonantly with Toroidal Alfv\'{e}n Eigenmodes (TAEs), resulting in enhanced radial transport and particle loss. A reduced quasilinear ``critical gradient model'' (CGM) has been developed by Ghantous to account for this interaction, and was previously validated against DIII-D demonstrating surprising agreement.\footnote{K. Ghantous et al., Phys. Plasmas 19, 092511 (2012)} The CGM uses linear instability theory to calculate the fast ion pressure gradient corresponding to marginal TAE stability. Integration of this gradient determines the relaxed fast ion profile and losses. This work focuses on applying the CGM to an NSTX plasma, using neutral beam injection for its validation. Thorough confinement studies were done with the help of TRANSP code in order to infer the EP radial diffusion rates consistent with the plasma performance in experiments. The analysis found typical radial diffusion of $\sim 1$ m$^2$/s, with a transient peak value of $\sim 8$ m$^2$/s. As a relatively fast code, the CGM can be used for predictive modelling of EP profiles in future devices such as ITER. [Preview Abstract] |
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PP8.00075: 3D halo neutral simulations in TRANSP code and application to NPA diagnostic on NSTX M.V. Gorelenkova, S.S. Medley The TRANSP-based NPA simulations made up to 2014 were not accurate since they did not handle halo neutrals properly. The halo neutrals were volume averaged both poloidally and toroidally. However estimates show that halo neutrals remain in the vicinity of the neutral beam footprint and because of multi-generations they have comparable density as primary beam neutrals. To address this inconsistency a 3D halo neutral module has been developed and implemented for the analysis in TRANSP code. The 3D halo neutral module uses a ``beam-in-a-box'' model that encompasses both injected beam neutrals and resulting halo neutrals. Upon the deposition by charge exchange, subsets of the full, one-half and one-third beam energy components produce thermal halo neutrals that are tracked through successive halo neutral generations until an ionization event occurs or a descendant halo exits the box. The Neutral Particle Analyzer (NPA) simulator in TRANSP is applied to NSTX discharges to study the effect of 3D halo neutrals on temporal evolution of NPA flux and the shape of energy spectra of fast particles. [Preview Abstract] |
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PP8.00076: Benchmark of 3D halo neutral simulation in TRANSP and FIDASIM and application to projected neutral-beam-heated NSTX-U plasmas D. Liu, S.S. Medley, M.V. Gorelenkova, W.W. Heidbrink, L. Stagner A cloud of halo neutrals is created in the vicinity of beam footprint during the neutral beam injection and the halo neutral density can be comparable with beam neutral density. Proper modeling of halo neutrals is critical to correctly interpret neutral particle analyzers (NPA) and fast ion D-alpha (FIDA) signals since these signals strongly depend on local beam and halo neutral density. A 3D halo neutral model has been recently developed and implemented inside TRANSP code. The 3D halo neutral code uses a ``beam-in-a-box'' model that encompasses both injected beam neutrals and resulting halo neutrals. Upon deposition by charge exchange, a subset of the full, one-half and one-third beam energy components produce thermal halo neutrals that are tracked through successive halo neutral generations until an ionization event occurs or a descendant halo exits the box. A benchmark between 3D halo neural model in TRANSP and in FIDA/NPA synthetic diagnostic code FIDASIM is carried out. Detailed comparison of halo neutral density profiles from two codes will be shown. The NPA and FIDA simulations with and without 3D halos are applied to projections of plasma performance for the National Spherical Tours eXperiment-Upgrade (NSTX-U) and the effects of halo neutral density on NPA and FIDA signal amplitude and profile will be presented. *\textit{Work supported by US DOE} [Preview Abstract] |
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PP8.00077: Upgrades to the NSTX SOL reflectometer to study plasma-antenna coupling and RF-edge interactions Cornwall Lau, John B. Wilgen, John B. Caughman, Greg R. Hanson, Joel Hosea, Rory Perkins, Phil Ryan, Gary Taylor The goal of the Oak Ridge National Laboratory (ORNL) scrape-off-layer (SOL) reflectometer is to measure the density profiles and fluctuations in front of the HHFW antenna on NSTX-U to help understand plasma-antenna coupling and RF-edge interactions, such as profile modifications due to field-aligned power losses and/or parametric decay instabilities. Originally designed for NSTX conditions, the reflectometer is being upgraded to operate at the increased magnetic fields of NSTX-U. General upgrades will be discussed. Most importantly, due to the doubling of the magnetic field for NSTX-U, the use of the current 6-27 GHz X-mode R cutoff on NSTX needs to be reconsidered. If only the X-mode R-cutoff is used, the operating frequencies will need to be modified, requiring significant hardware modifications to both the electronics and reflectometer launchers. It will be shown that the frequencies will not need to be modified for NSTX-U operation if both X-mode L and R cutoffs are measured. The measured SOL density profiles are intended to be used as inputs into RF simulation codes, and one such simulation using COMSOL multiphysics is being developed to understand the electric fields in front of the antenna for cold plasma conditions. Progress on the COMSOL simulation will be reported. [Preview Abstract] |
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PP8.00078: Benchmark of ICRF codes in mid and high harmonic regimes in view of NSTX-U operation Nicola Bertelli, C.K. Phillips, E.J. Valeo, R. Bilato, M. Brambilla, E.F. Jaeger, P.T. Bonoli NSTX-Upgrade (NSTX-U) is presently scheduled to operate at the beginning of 2015 with toroidal magnetic fields ($B_{T}$) up to 1 T, nearly twice the value used in the experiments on NSTX, and with NBI power up to 10 MW. The doubling of $B_{T}$ while retaining the 30 MHz rf source frequency moves the heating regime from the high harmonic fast wave (HHFW) regime (up to 10th harmonic) used in NSTX to the mid harmonic fast wave (MHFW) regime (up to 5th harmonic). Both the MHFW regime and the doubling of the NBI power can strongly affect the power absorption partitioning. In fact, the thermal and fast ions absorption can significantly increase [Bertelli et al AIP Conf. Proc. {\bf 1580}, (2014) 310]. Thus, it is crucial to have an accurate evaluation of the power absorbed by fast and thermal ions for experimental analysis. Detailed benchmarking comparisons between the full wave codes TORIC v.5 [Brambilla, PPCF {\bf 44}, (2002) 2423], TORIC v.6 [Bilato et al, NF {\bf 51}, (2011) 103034], AORSA [Jaeger et al, PoP {\bf 8}, 1573 (2001)], and the ray tracing code GENRAY [Smirnov et al, Bull. Am. Phys. Soc. {\bf 39}, 1626 (1994)] have been performed. Finally, numerical predictions on NSTX-U are discussed for different scenarios. [Preview Abstract] |
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PP8.00079: Comparison of NSTX FIDA, Charge Exchange, and Neutron Fluxes with Calculated Signals Based on CQL3D-FOW Distribution Functions R.W. Harvey, Yu.V. Petrov, J.E. Kinsey, D. Liu, W.W. Heidbrink, G. Taylor, P.T. Bonoli Ion distribution function calculations with CQL3D [1] have been substantially advanced through implementation of guiding-center-orbit-based Fokker-Planck Coefficients [2]. The resulting finite-orbit-width (FOW) calculations are carried out with a fast CQL3D-Hybrid-FOW option, and in a slower but neoclassically complete (except no Er yet) CQL3D-FOW option. Good comparison between time-dependent Fast Ion Diagnostic FIDA [3], NPA, and neutron signals resulting from neutral beaminjection(NBI) and high harmonic fast wave (HHFW) power injected into the NSTX spherical tokamak have been simulated with the CQL3D-Hybrid-FOW, using only the FOW effects on QL diffusion, and particle losses, direct and CX. Comparisons are also made with recent CQL3D-FOW results [2], as well as between the original FIDA calculation code [3,4] and a recent fortran version [5]. \newline [1] R.W. Harvey and M. McCoy, ``The CQL3D Fokker Planck Code,'' http://www.compxco.com/cql3d.html. [2] Yu.V. Petrov and R.W. Harvey, this meeting (2014). [3] W. W. Heidbrink, et al. Plasma Phys. Controlled Fusion, 1855 (2004).Comm. Comp. Phys., 716 (2011). [4] D. Liu, W.W. Heidbrink, et al., Pl. Phys. Contr. Fusion, 025006 (2010). [5] Geiger, Benedikt, See https://github.com/D3DEnergetic/FIDASIM. [Preview Abstract] |
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PP8.00080: Numerical optimization of perturbative coils for tokamaks Samuel Lazerson, Jong-Kyu Park, Nikolas Logan, Allen Boozer Numerical optimization of coils which apply three dimensional (3D) perturbative fields to tokamaks is presented. The application of perturbative 3D magnetic fields in tokamaks is now commonplace for control of error fields, resistive wall modes, resonant field drive, and neoclassical toroidal viscosity (NTV) torques. The design of such systems has focused on control of toroidal mode number, with coil shapes based on simple window-pane designs. In this work, a numerical optimization suite based on the STELLOPT 3D equilibrium optimization code is presented. The new code, IPECOPT, replaces the VMEC equilibrium code with the IPEC perturbed equilibrium code, and targets NTV torque by coupling to the PENT code. Fixed boundary optimizations of the 3D fields for the NSTX-U experiment are underway. Initial results suggest NTV torques can be driven by normal field spectrums which are not pitch-resonant with the magnetic field lines. Work has focused on driving core torque with $n=1$ and edge torques with $n=3$ fields. Optimizations of the coil currents for the planned NSTX-U NCC coils highlight the code's free boundary capability. [Preview Abstract] |
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PP8.00081: Suppressed gross erosion of high-temperature lithium films under high-flux deuterium bombardment Tyler Abrams, M.A. Jaworski, R. Kaita, A.M. Capece, J.H. Nichols, D.P. Stotler, J.P. Roszell, G. De Temmerman, M.A. van den Berg, H.J. van der Meiden, T.W. Morgan Liquid lithium is an attractive plasma facing component (PFC) for a fusion reactor because it improves confinement and protects the underlying substrate from high heat fluxes. However some previous studies have implied the maximum Li temperature permitted on such devices may be unacceptably low. Recently thin (\textless 1 $\mu$m) and thick ($\sim$ 500 $\mu$m) Li films on TZM molybdenum substrates were studied in the Magnum-PSI linear plasma device with ion fluxes \textgreater 10$^{24}$ m$^{-2}$ s$^{1}$ and Li surface temperatures $\le $ 800 $^{\circ}$C. Measured Li erosion yields under neon plasma bombardment were similar to previous studies on low-flux devices, but erosion under deuterium bombardment was significantly suppressed. This motivated development of a mixed-material Li-D surface model incorporating D diffusion in Li, preferential sputtering, and LiD chemistry. This model is coupled to the adatom-evaporation equation for thermally-enhanced sputtering and the Langmuir law evaporation equation to obtain realistic predictions of temperature-dependent erosion rates. This model is found to predict the correct functional dependence of the mixed-material Li-D erosion rate vs. temperature in these discharges. Further investigations via molecular dynamics (MD) simulations and surface science experiments will also be presented. [Preview Abstract] |
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PP8.00082: Advances in global mixed-material surface evolution modeling for NSTX-U J.H. Nichols, M.A. Jaworski, R. Kaita, T. Abrams, D.P. Stotler, K. Schmid NSTX-U will initially operate with graphite walls, periodically coated with thin lithium films to improve plasma performance. Prior experiments with Li evaporation on NSTX suggest that poloidally inhomogenous mixed-material C/Li surfaces will evolve over the course of the campaign due to wall material migration during plasma operation. Understanding the depletion and accumulation of Li in different parts of the machine is a key component of optimizing the Li conditioning process. To that end, the WallDYN global mixed-material surface evolution model [K. Schmid et al., J. Nucl. Mater. 415, S284-S288 (2011)] has been applied to the NSTX-U geometry. The WallDYN model couples local erosion and deposition processes with plasma impurity transport in a non-iterative, self-consistent manner that maintains overall material balance. For this work, a simplified C/Li mixed-material erosion model is generated by parameterizing sputter and reflection yield calculations from TRIM-like codes. The sensitivity of global lithium migration rates to various model parameters will be examined. A qualitative comparison will be made between the WallDYN model and post-campaign Li coverage measurements from NSTX. [Preview Abstract] |
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PP8.00083: Overview of results from the Lithium Tokamak eXperiment (LTX) R. Majeski, R. Bell, D. Boyle, A. Capece, A. Diallo, E. Granstedt, C.M. Jacobson, R. Kaita, B. Koel, T. Kozub, B. LeBlanc, M. Lucia, R. Maingi, E. Merino, J. Schmitt, D. Stotler, G. Tchilingurian, T.M. Biewer, T.K. Gray, S. Kubota, W.A. Peebles, P. Beiersdorfer, K. Tritz, J.P. Allain, F. Bedoya LTX is a low aspect ratio tokamak with a heated liner or shell, which covers 80 percent of the plasma surface area (4 square meters). In 2014, a new approach to wall coatings was developed. The shells are now preheated to 300 C, and previously applied lithium coatings are allowed to oxidize. An electron beam system is then used to evaporate lithium from a pool of liquid at the bottom of the lower shell. The e-beam system produces a 50 -- 100 nm coating of liquefied lithium in \textless 5 minutes. Compared to previous results with helium-dispersed coatings, discharges using the new approach have strongly reduced impurities, especially oxygen. Magnetic analysis indicates that confinement in LTX Ohmic discharges is now improved by 10x, compared to best previous results. Confinement times exceed H-mode scalings by 4-5x. This is the first experimental evidence that high performance tokamak discharges are compatible with large-area liquid lithium walls. An overview of LTX, especially the lithium systems, will be presented, as well as a summary of results. [Preview Abstract] |
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PP8.00084: Diagnostic Overview of the Lithium Tokamak Experiment (LTX) T.K. Gray, T.M. Biewer, J.M. Canik, R.E. Bell, D.P. Boyle, A. Diallo, E. Grandstedt, C.M. Jaocbson, R. Kaita, T. Kozub, B. LeBlanc, M. Lucia, R. Maingi, E. Merino, R. Majeski, J.C. Schmitt, S. Kubota, W.A. Peebles, P. Beiersdorfer, J.H.T. Clementson, A.G. McLean, K. Widmann, K. Tritz, J.P. Allain, F. Bedoya The Lithium Tokamak Experiment (LTX) is a low aspect ratio tokamak with a conformal low recycling first wall. The first wall is comprised of four stainless steel-lined copper shells, heatable to 300C, onto which lithium is evaporated. The magnetic diagnostic suite has recently been upgraded to be more compatible with high temperatures and the lithium environment. A Thomson scattering system with new edge channels measures radial profiles of ne and Te. While Doppler spectroscopy is used to measure the ion temperature and speed of carbon and lithium impurities. Two 20 AXUV-diode arrays, 1 filtered for Ly-alpha and the other for bolometry, provide full radial coverage at the toroidal midplane, while a XUV spectrometer provides measurements of core impurities. The Materials Analysis and Particle Probe (MAPP) provided crucial information about the surface conditions of the plasma-facing wall between shots. Measurements of the edge plasma are accomplished with filterscopes, visible spectrometers, Langmuir probes and a fast framing, filtered tangential camera. [Preview Abstract] |
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PP8.00085: Enhanced Alignment Techniques for the Thomson Scattering Diagnostic on the Lithium Tokamak eXperiment (LTX) Enrique Merino, Tom Kozub, Dennis Boyle, Matthew Lucia, Richard Majeski, Robert Kaita, John C. Schmitt, Benoit Leblanc, Ahmed Diallo, C.M. Jacobson The Thomson Scattering (TS) System in LTX is used to measure electron temperature and density profiles of core and edge plasmas. In view of TS measurements showing low signal-to-noise and high stray light, numerous improvements were performed in recent months. These will allow for better measurements. Due to the nature of LTX's lithium coated walls, a particular challenge was presented by alignment procedures which required insertion and precise positioning of equipment in the vacuum vessel without breaking vacuum. To overcome these difficulties, the laser flight tubes were removed and an alignment probe setup placed along the beam line on a differentially pumped assembly. The probe was then driven into the vacuum vessel and~back-illumination of the viewing optics on it allowed for alignment and spatial calibration. Other upgrades included better bracing of flight tubes and viewing optics as well as a redesigned beam dump. An overview of these improvements will be presented. [Preview Abstract] |
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PP8.00086: Estimates of the Electron Density Profile on LTX Using FMCW Reflectometry and mm-Wave Interferometry W.A. Peebles, S. Kubota, X.V. Nguyen, T. Holoman, R. Kaita, T. Kozub, D. Labrie, J.C. Schmitt, R. Majeski An FMCW (frequency-modulated continuous-wave) reflectometer has been installed on the Lithium Tokamak Experiment (LTX) for electron density profile and fluctuation measurements. This diagnostic consists of two channels using bistatic antennas with a combined frequency coverage of 13.5$-$33 GHz, which corresponds to electron density measurements in the range of $0.2$$-$$1.3$$\times$$10^{13}$ cm$^{-3}$ (in O-mode). Initial measurements will utilize O-mode polarization, which will require modeling of the plasma edge. Reflections from the center stack (delayometry above the peak cutoff frequency), as well as line density measurements from a 296 GHz interferometer (single-chord, radial midplane), will provide constraints for the profile reconstruction/estimate. Typical chord-averaged line densities on LTX range from $2$$-$$6$$\times$$10^{12}$ cm$^{-3}$, which correspond to peak densities of $0.6$$-$$1.8$$\times$$10^{13}$ cm$^{-3}$ assuming a parabolic shape. If available, EFIT/LRDFIT results will provide additional constraints, as well as the possibility of utilizing data from measurements with X-mode or dual-mode (simultaneous O- and X-mode) polarization. [Preview Abstract] |
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PP8.00087: Impurities in the Lithium Tokamak Experiment D.P. Boyle, R.E. Bell, R. Kaita, R. Majeski, T.M. Biewer, T.K. Gray, K. Tritz, K. Widmann The Lithium Tokamak Experiment (LTX) is designed to study the low-recycling regime through the use of close-fitting, lithium-coated, heatable shell quadrants surrounding the plasma volume. Lithium coatings can getter and bury impurities, but they can also become covered by impurity compounds. Liquefied coatings can both dissolve impurity compounds and bring them to the surface, while sputtering and evaporation rates increase strongly with temperature. Here, we use spectroscopic measurements to assess the effects of varying wall conditions on plasma impurities, mainly Li, C, and O. A passive Doppler spectroscopy system measures toroidal and poloidal impurity profiles using fixed-wavelength and variable-wavelength visible spectrometers. In addition, survey and high-resolution extreme ultraviolet spectrometers detect emission from higher charge states. Preliminary results show that fresh Li coatings generally reduced C and O emission. C emission decreased sharply following the first solid Li coatings. Inverted toroidal profiles in a discharge with solid Li coatings show peaked Li III emissivity and temperature profiles. Recently, experiments with fresh liquid coatings led to especially strong O reduction. Results from these and additional experiments will be presented. [Preview Abstract] |
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PP8.00088: Examining the temperature behavior of stainless steel surfaces exposed to hydrogen plasmas in the Lithium Tokamak eXperiment (LTX) Felipe Bedoya, Jean Paul Allain, Robert Kaita, Matthew Lucia, Denis St-Onge, Robert Ellis, Richard Majeski The Materials Analysis Particle Probe (MAPP) is an \textit{in-situ} diagnostic designed to characterize plasma-facing components (PFCs) in tokamak devices. MAPP is installed in LTX at Princeton Plasma Physics Laboratory. MAPP's capabilities include remotely operated XPS acquisition and temperature control of four samples. The recent addition of a focused ion beam allows XPS depth profiling analysis. Recent published results show an apparent correlation between hydrogen retention and temperature of Li coated stainless steel (SS) PFCs exposed to plasmas like those of LTX. According to XPS data, the retention of hydrogen by the coated surfaces decreases at above 180 $^{\circ}$C. In the present study MAPP will be used to study the oxidation of Li coatings as a function of time and temperature of the walls when Li coatings are applied. Experiments in the ion-surface interaction experiment (IIAX) varying the hydrogen fluence on the SS samples will be also performed. Conclusions resulting from this study will be key to explain the PFC~temperature-dependent variation of plasma performance observed in LTX. [Preview Abstract] |
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PP8.00089: Dependence of LTX plasma performance on surface conditions as investigated by the Materials Analysis and Particle Probe M. Lucia, R. Kaita, R. Majeski, D.P. Boyle, M.A. Jaworski, J.C. Schmitt, D.A. St. Onge, F. Bedoya, J.P. Allain The Lithium Tokamak Experiment (LTX) is a spherical torus magnetic confinement device designed to accommodate solid or liquid lithium as the primary plasma-facing component (PFC). Results are presented from the implementation on LTX of the Materials Analysis and Particle Probe (MAPP), a compact \textit{in vacuo} surface science diagnostic. With MAPP's \textit{in situ} analysis techniques of x-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS), evolution of the PFC surface chemistry in LTX is studied as a function of varied hydrogen plasma exposure, surface temperature, and lithium coating. Performance of LTX plasma discharges depends on the composition and temperature of the PFCs in a strong and complex fashion. This work attempts to relate LTX plasma performance to the surface conditions as determined by XPS and TDS with MAPP. As proxies for the LTX PFCs, MAPP samples are exposed to both lithium evaporations and plasma discharges inside LTX. Metrics of LTX plasma performance include energy confinement time, plasma temperature and density profiles, and state of impurity species. Single Langmuir probes on MAPP and triple Langmuir probes throughout LTX are also used to relate LTX edge plasma parameters to MAPP results. [Preview Abstract] |
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PP8.00090: Thermal Desorption Spectroscopy of plasma-facing components with the Materials Analysis and Particle Probe D.A. St-Onge, R. Kaita, M. Lucia, R. Ellis, J.P. Allain, F. Bedoya The Material Analysis and Particle Probe (MAPP) is an \textit{in vacuo} diagnostic device for studying surface-plasma interactions on plasma facing components (PFC) in fusion devices. This diagnostic allows four samples to be exposed simultaneously and analyzed individually \textit{in situ}. The supercenter is the location at which an X-ray source, an ion source, a residual gas analyser and an electron-energy analyzer all focus. Samples in this position are interchanged by rotation of the probe head using an eight-position Geneva drive. Currently the remote automation of the probe head is being developed using LabVIEW instruments, which shall monitor and control the rotational position of the samples. One technique to analyse samples used here is through thermal desorption spectroscopy (TDS), where a sample is heated sufficiently to release adsorbed molecules from the surface. To ensure samples are analysed individually, they must be thermally isolated. We will study the thermal isolation between samples using the Lithium Tokamak Experiment (LTX), which is a spherical torus designed to study lithium-based PFCs. Preliminary TDS experiments on stainless-steel samples with a range from 20$^{\circ}$C to 800$^{\circ}$C and a ramp of 2$^{\circ}$C/s will be performed on LTX plasmas. [Preview Abstract] |
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PP8.00091: Initiatives in Non-Solenoidal Startup and H-mode Physics at Near-Unity $A$ M.W. Bongard, J.L. Barr, M.G. Burke, R.J. Fonck, E.T. Hinson, B.T. Lewicki, J.M. Perry, A.J. Redd, D.J. Schlossberg, K.E. Thome, G.R. Winz Research on the $A\sim 1$ Pegasus ST is advancing the physics of non-solenoidal tokamak startup and the H-mode confinement regime. Local helicity injection (LHI) uses current sources in the plasma edge to initiate and drive $I_{p} $ via DC helicity injection, subject to constraints from helicity conservation and Taylor relaxation. To date, $I_{p} \sim 0.18$ MA has been initiated with $I_{inj} \sim 6$ kA. A predictive 0-D power balance model of LHI $I_{p} (t)$ evolution matches present discharges with strong PF induction. It projects $I_{p} \sim 0.3$ MA operation in Pegasus will achieve the LHI-dominated physics regime expected for 1 MA NSTX-U startup. Ohmic H-mode plasmas are routinely attained, due to the low $P_{th} $ at the low $B_{T} $ of $A\to 1$ plasmas. However, both limited and favorable $\nabla B$ SN plasmas have $P_{th} \sim 11$ times higher than expected from high-$A$ scalings. They have improved $\tau_{e} $ ($H_{98} \sim 1)$ and a quiescent $J_{edge} $ pedestal between edge localized modes (ELMs). Unique $J_{edge} (t)$ measurements through a single Type I ELM show a complex, multimodal pedestal collapse and filament ejection. A proposed Pegasus-U initiative will upgrade the centerstack assembly and LHI injector systems, increasing $B_{T} $ to 1 T, Ohmic V-s by $\times 6$, and pulse length to 100 ms at $A=1.2$. This allows the physics and technology of LHI to be validated at NSTX-U relevant parameters, supports studies of nonlinear ELM dynamics, and will test high-$\beta_{T} $ tokamak stability. [Preview Abstract] |
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PP8.00092: Magnetics and Power System Upgrades for the Pegasus-U Experiment R.C. Preston, M.W. Bongard, R.J. Fonck, B.T. Lewicki To support the missions of developing local helicity injection startup and exploiting advanced tokamak physics studies at near unity aspect ratio, the proposed Pegasus-U will include expanded magnetic systems and associated power supplies. A new centerstack increases the toroidal field seven times to 1 T and the volt-seconds by a factor of six while maintaining operation at an aspect ratio of 1.2. The poloidal field magnet system is expanded to support improved shape control and robust double or single null divertor operation at the full plasma current of 0.3 MA. An integrated digital control system based on Field Programmable Gate Arrays (FPGAs) provides active feedback control of all magnet currents. Implementation of the FPGAs is achieved with modular noise reducing electronics. The digital feedback controllers replace the existing analog systems and switch multiplexing technology. This will reduce noise sensitivity and allow the operational Ohmic power supply voltage to increase from 2100 V to its maximum capacity of 2400 V. The feedback controller replacement also allows frequency control for ``freewheeling''---stopping the switching for a short interval and allowing the current to coast. The FPGAs assist in optimizing pulse length by having programmable switching events to minimize energy losses. They also allow for more efficient switching topologies that provide improved stored energy utilization, and support increasing the pulse length from 25 ms to 50--100 ms. [Preview Abstract] |
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PP8.00093: Predictions for Non-Solenoidal Startup in Pegasus with Lower Divertor Helicity Injectors J.M. Perry, J.L. Barr, M.W. Bongard, R.J. Fonck, B.T. Lewicki Non-solenoidal startup in Pegasus has focused on using arrays of local helicity injectors situated on the outboard midplane to leverage PF induction. In contrast, injector assemblies located in the lower divertor region can provide improved performance. Higher toroidal field at the injector increases the helicity injection rate, providing a higher effective loop voltage. Poloidal flux expansion in the divertor region will increase the Taylor relaxation current limit. Radial position control requirements are lessened, as plasma expansion naturally couples to injectors in the divertor region. Advances in cathode design and plasma-facing guard rings allow operation at bias voltages over 1.5 kV, three times higher than previously available. This results in increased effective loop voltage and reduced impurity generation. Operation of helicity injectors in the high field side elevates the current requirements for relaxation to a tokamak-like state, but these are met through the improved injector design and increased control over the poloidal field structure via the addition of new coil sets. These advances, combined with the relocation of the injectors to the divertor region, will allow access to the operational regime where helicity injection current drive, rather the poloidal induction, dominates the discharge---a prerequisite for scaling to larger devices. Initial estimates indicate that plasma currents of 0.25--0.30 MA are attainable at full toroidal field with 4 injectors of 2 cm$^{2}$ each and 8 kA total injected current. [Preview Abstract] |
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PP8.00094: Physics of Intense Electron Current Sources for Helicity Injection E.T. Hinson, J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, B.T. Lewicki, J.M. Perry, A.J. Redd, G.R. Winz DC helicity injection (HI) for non-solenoidal ST startup requires sources of current at the tokamak edge. Since the rate of HI scales with injection voltage, understanding of the physics setting injector impedance is necessary for a predictive model of the HI rate and subsequent growth of $I_{p} $. In Pegasus, arc plasma sources are used for current injection. They operate immersed in tokamak edge plasma, and are biased at $\sim $1--2 kV with respect to the vessel to draw current densities $J\sim 1$ kA/cm$^{2}$ from an arc plasma cathode. Prior to tokamak formation, impedance data manifests two regimes, one at low current ($<1$ kA) with $I\sim V^{3/2}$, and a higher current mode where $I\sim V^{1/2}$ holds. The impedance in the $I\sim V^{3/2}$ regime is consistent with an electrostatic double layer. Current in the $I\sim V^{1/2}$ regime is linear in arc gas fueling rate, suggesting a space-charge limit set by $n_{edge} $. In the presence of tokamak plasmas, voltage oscillations of the order 100s of volts are measured during MHD relaxation activity. These fluctuations occur at the characteristic frequencies of the $n=1$ and $n=0$ MHD activity observed on magnetic probes, and are suggestive of dynamic activity found in LHI simulations in NIMROD. Advanced injector design techniques have allowed higher voltage operation. These include staged shielding to prevent external arcing, and shaped cathodes, which minimize the onset and material damage due to cathode spot formation. [Preview Abstract] |
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PP8.00095: Anomalous Ion Heating, Intrinsic and Induced Rotation in the Pegasus Toroidal Experiment M.G. Burke, J.L. Barr, M.W. Bongard, R.J. Fonck, E.T. Hinson, J.M. Perry, A.J. Redd, K.E. Thome Pegasus plasmas are initiated through either standard, MHD stable, inductive current drive or non-solenoidal local helicity injection (LHI) current drive with strong reconnection activity, providing a rich environment to study ion dynamics. During LHI discharges, a large amount of anomalous impurity ion heating has been observed, with $T_{i} \sim 800$ eV but $T_{e} <100$ eV. The ion heating is hypothesized to be a result of large-scale magnetic reconnection activity, as the amount of heating scales with increasing fluctuation amplitude of the dominant, edge localized, $n=1$ MHD mode. Chordal $T_{i} $ spatial profiles indicate centrally peaked temperatures, suggesting a region of good confinement near the plasma core surrounded by a stochastic region. LHI plasmas are observed to rotate, perhaps due to an inward radial current generated by the stochastization of the plasma edge by the injected current streams. H-mode plasmas are initiated using a combination of high-field side fueling and Ohmic current drive. This regime shows a significant increase in rotation shear compared to L-mode plasmas. In addition, these plasmas have been observed to rotate in the counter-$I_{p} $ direction without any external momentum sources. The intrinsic rotation direction is consistent with predictions from the saturated Ohmic confinement regime. [Preview Abstract] |
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PP8.00096: Initial Thomson Scattering Survey of Local Helicity Injection and Ohmic Plasmas at the Pegasus Toroidal Experiment D.J. Schlossberg, G.M. Bodner, M.W. Bongard, R.J. Fonck, G.R. Winz A multipoint Thomson scattering diagnostic has recently been installed on the Pegasus ST. The system utilizes a frequency-doubled Nd:YAG laser ($\lambda_{0} \sim 532$ nm), spectrometers with volume phase holographic gratings, and a gated, intensified CCD camera. It provides measurements of $T_{e} $ and $n_{e} $ at 8 spatial locations for each spectrometer once per discharge. A new multiple aperture and beam dump system has been implemented to mitigate interference from stray light. This system has provided initial measurements in the core region of plasmas initiated by local helicity injection (LHI), as well as conventional Ohmic L- and H-mode discharges. Multi-shot averages of low-density ($n_{e} \sim 3\times 10^{18}$ m$^{-3})$, $I_{p} \sim 0.1$ MA LHI discharges show central $T_{e} \sim 75$ eV at the end of the helicity injection phase. $I_{p} \sim 0.13$ MA Ohmic plasmas at moderate densities ($n_{e} \sim 2\times 10^{19}$ m$^{-3})$ have core $T_{e} \sim 150$ eV in L-mode. Generally, these plasmas do not reach transport equilibrium in the short 25 ms pulse length available. After an L-H transition, strong spectral broadening indicates increasing $T_{e}$, to values above the range of the present spectrometer system with a high-dispersion VPH grating. Near-term system upgrades will focus on deploying a second spectrometer, with a lower-dispersion grating capable of measuring the 0.1--1.0 keV range. The second spectrometer system will also increase the available number of spatial channels, enabling study of H-mode pedestal structure. [Preview Abstract] |
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PP8.00097: Probe Measurements in the H-mode Pedestal Region in the Pegasus Toroidal Experiment G.M. Bodner, M.W. Bongard, R.J. Fonck, K.E. Thome, D.S. Thompson In near-unity aspect ratio Pegasus discharges, Ohmic heating and high-field-side fueling together trigger an L-H mode transition in both limited and diverted configurations. H-mode plasmas are predicted to exhibit pedestals in both the pressure and current density profiles. Operation at $A\sim 1$ allows for the use of local magnetic and Langmuir probes in the pedestal region. A current pedestal is routinely observed in Pegasus H-mode plasmas, but not in L-mode plasmas or during ELMs. Conventionally, edge pedestal measurements are observed in the edge pressure profile. A triple Langmuir probe has recently been installed in order to investigate the structure of the edge pressure pedestal in Pegasus H-mode discharges and complement the current density profile measurements. Local density and temperature measurements will be collected using the triple Langmuir probe at varying spatial locations to identify edge pressure profiles. These pressure profiles will be measured in both the L-mode and H-mode regimes. The triple probe will additionally be used to observe the turbulence levels before, during, and after the L-H mode transition. Complete density and temperature profiles including the pedestal will be obtained using a combination of Langmuir probe and Thomson scattering measurements. [Preview Abstract] |
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PP8.00098: Progress Toward a Technique for Measuring Electric Field Fluctuations in Tokamak Core Plasmas D.S. Thompson, M.R. Bakken, M.G. Burke, H.P. Couto, R.J. Fonck, B.T. Lewicki, G.R. Winz Measurements of electric field fluctuations in magnetic confinement experiments are desired for validating turbulence and transport models. A new diagnostic to measure $E_{z}(r$,$t)$ fluctuations is in development on the Pegasus Toroidal Experiment. The approach is based on neutral beam emission spectroscopy using a high-throughput, high-resolution spectrometer to resolve fluctuations in wavelength separation between components of the motional Stark effect spectrum. Fluctuations at mid-minor-radius, normalized to an estimated MSE field, are estimated to be $\delta E$/$E_{MSE}\approx $ 10$^{-3}$. In order to resolve fluctuations at turbulent time scales ($f_{Ny}\approx $ 500 kHz), beam and spectrometer designs minimize broadening and maximize signal-to-noise ratio. The diagnostic employs a Fabry-P\'{e}rot spectrometer with \'{e}tendue-matched collection optics and low noise detectors. The interferometer spacing is varied across the face of the etalon to mitigate geometric Doppler broadening. An 80 keV $H^{0}$ beam from PBX-M with a divergence $\Omega $\textless 0.5 degrees is being refurbished for this project. The beam includes a new ion source to maximize full energy species fraction and is designed to provide $\approx $ 2 cm spatial resolution and 50 ms of 6 mA/cm$^{2\, }$current density at the focal plane. Successful development and demonstration on Pegasus will guide future deployment on larger fusion facilities. [Preview Abstract] |
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PP8.00099: Error field minimization strategies towards MAST Upgrade operation Lidia Piron, I. Chapman, G. Cunningham, G. Fishpool, R. Gowland, I. Katramados, A. Kirk, G. Naylon, R. Martin In fusion devices, the presence of magnetic error fields (EF) leads to toroidal asymmetries in the magnetic field. Even small EFs of the order of B$_{\mathrm{r}}$/B$_{\mathrm{t}}$ of about 10$^{-4}$ can have detrimental effect on plasma operations, since they can induce locked mode formation and thus plasma termination. In the MAST spherical tokamak, intrinsic EFs have been identified as limiting low density experiments and have been compensated using error field correction coils. In building MAST Upgrade device, a careful design, manufacture and installation of axisymmetric coils has been adopted to reduce the EF amplitude to the lowest possible value. In the present work, passive and active control strategies for EF correction are presented. The passive control concerns the optimization of the fine-scale coil alignment in order to minimize the n$=$1 EF amplitude. Such studies require high accuracy magnetic measurements and associated 3D modelling. A model-based optimization approach has been adopted to identify the right shift and tilt in the coil position which allow for the n$=$1 EF correction. However, inevitably residual EFs will be present and active control algorithms will need to be implemented in the plasma control system to compensate them. Such control schemes will be discussed. [Preview Abstract] |
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PP8.00100: Heat flux and plasma flow in the scrape off layer on the spherical tokamak QUEST with inboard poloidal field null configuration Takumi Onchi, Hideki Zushi, Kishore Mishra, Kazuaki Hanada, Hiroshi Idei, Kazuo Nakamura, Akihide Fujisawa, Yoshihiko Nagashima, Makoto Hasegawa, Arseny Kuzmin, Kenichi Nagaoka Heat flux and plasma flow in the scrape off layer (SOL) are examined in the inboard poloidal null (IPN) configuration on the spherical tokamak (ST) QUEST. In the ST, trapped energetic electrons on the low field side are widely excursed from the last closed flux surface to SOL so that significant heat loss occurs. Interestingly, plasma flows in the core and the SOL are also observed in IPN though no inductive force like ohmic heating is applied. High heat flux (\textgreater 1 MW/m$^{2})$ and sonic flow ($M$ \textgreater 1) in far-SOL arise in current ramp-up phase. In quasi-steady state, sawtooth-like oscillation of plasma current with 20 Hz has been observed. Heat flux and subsonic plasma flow in far-SOL are well correlated to plasma current oscillation. The toroidal Mach number largely increases from $M_{\mathrm{\varphi }} \approx $ 0.1 to $\approx $ 0.5 and drops although the amplitude of plasma current is about 10 {\%} of that. Note that such flow modification occurs before plasma current crash, there may be some possibility that phenomena in the SOL or the edge trigger reactions in the core plasma. [Preview Abstract] |
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PP8.00101: Experimental and numerical studies of dynamic divertor with plasmoid Shizuo Inoue, Yasushi Ono, Yasuhiro Kaminou, Ritoku Horiuchi Novel dynamic divertor operation with controlling plasmoid motion has been studied in TS-4 ST experiment and in MHD /PIC simulation. In the TS-4, the plasmoid ejection from the core ST plasmas, which is a standard operation of dynamic divertor, was demonstrated for the first time.\footnote{S. Inoue et al., in Proceedings of the 24th IAEA Fusion Energy Conference, PD/P8-17 (IAEA, San Diego, 2012).} Using this experimental data as background condition, the confinement of the heat flux from the core plasma in the plasmoid is indicated though orbit calculation. In addition, physical pictures of the heat flux transport from the SOL region to the plasmoid will be studied in detail by 2-1/2 dimensional collisionless particle simulations in the rectangular coordinate.\footnote{H. Ohtani and R. Horiuchi, Plasma Fusion Res, 4, 024 (2009).} Since the strong guide field exists the region where the plasmoid is formed and transported, field-aligned motion is important for understanding the heat flux transport from the SOL region to the plasmoid both for ions and electrons. [Preview Abstract] |
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PP8.00102: Conceptual Design of a Small Aspect Ratio Tokamak of Variable Configuration Julio Herrera-Velazquez, Ismael Arroyo-Diaz, Domenica Corona-Rivera, Esteban Chavez-Alarc\'on We show the preliminary work being done in order to propose a mid-term project for a Mexican nuclear fusion programme, with the necessary flexibility to produce original results. The purpose is to study the feasibility of a medium size, low aspect ratio tokamak, with the capability of actively controlling the shape and position of the plasma column. Its objective would be to explore the necessary operational conditions for high $\beta $ and high bootstrap currents. The 3D-MAPTOR code is used in order to estimate the magnetic field surfaces behaviour. The TEMEX tokamak would consist in a stainless-steel toroidal vacuum chamber with semi-rectangular cross section, with external toroidal and poloidail field coils. The central post would include the central solenoid, as well as inner control coils. The toroidal magnetic field is produced by 10 rectangular coils, made out of 40 turns of water cooled copper conductor. Six poloidal field coils have been included, distributed in two groups of three, one on the upper, and another one on the lower side of the torus. [Preview Abstract] |
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PP8.00103: MAGNETIC RECONNECTION |
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PP8.00104: Dual Species Trapping by RF Field Structure Nathaniel Hicks A computational particle-in-cell study is performed in which the effects of RF trapping of a light species and consequent effects on accompanying heavier species in bulk plasma are explored. Electrode structures such as the RF quadrupole are simulated in 2-D, and parameters such as relative charge-to-mass ratios of the light and heavy species are investigated. Scaling to electron trapping and trapping dependences on species temperatures and RF parameters are investigated as well. The RF plasma sheath for this configuration is a topic of particular interest. [Preview Abstract] |
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PP8.00105: Anomalous resistivity due to high-frequency waves at the X-line during magnetic reconnection J. Jara-Almonte, H. Ji, W. Daughton, V. Roytershteyn, M. Yamada, J. Yoo, W. Fox One major consequence of magnetic reconnection is the efficient transfer of energy from magnetic fields to plasma particles. During collisionless reconnection, the decoupling of the field from the plasma is known to occur only within the localized ion and electron diffusion regions, however predictions from fully kinetic simulations do not agree with experimental observations on the size of the electron diffusion region, implying a different reconnection mechanism. Using 3D, fully explicit kinetic simulations with a realistic and unprecedentedly large separation between the Debye length and the electron skin depth, we show that high frequency electrostatic waves ($\omega \gg \omega_{LH}$) can exist within the electron diffusion region. These waves generate small-scale turbulence within the electron diffusion region which acts to broaden the layer. Anomalous resistivity is also generated by the turbulence and significantly modifies the force balance. In addition to simulation results, initial attempts to measure high frequency fluctuations (f $\leq$ 1 GHz) in the Magnetic Reconnection Experiment (MRX) will be presented. [Preview Abstract] |
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PP8.00106: Studies of electron energization during magnetic reconnection in a laboratory plasma Jongsoo Yoo, Masaaki Yamada, Hantao Ji, Jonathan Jara-Almonte, Byungkeun Na, Clayton E. Myers, William Fox Bulk electron heating and energetic electron generation during magnetic reconnection are studied in the Magnetic Reconnection Experiment (MRX). Analysis of the measured 2-D electron temperature profile shows that electrons are heated non-classically near the electron diffusion region. Electron heating is observed over the broad downstream region during anti-parallel reconnection without a significant density asymmetry across the current sheet. Classical Ohmic heating accounts for about 20\% of the required heating power. When there is a strong density asymmetry across the current sheet, the electron temperature profile changes such that the electron temperature is highest near the low-density-side separatrix where strong density gradients and electromagnetic fluctuations in the lower hybrid frequency range are also observed. These laboratory measurements agree with space observations at the dayside magnetopause. Possible mechanisms for bulk electron heating are discussed. Recent diagnostic developments, including 2-D EUV imaging and electron energy analyzer, for measurements of energetic electron generation are presented. [Preview Abstract] |
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PP8.00107: Laboratory evidence that line-tied tension forces can suppress loss-of-equilibrium flux rope eruptions in the solar corona C.E. Myers, M. Yamada, E. Belova, H. Ji, J. Yoo, W. Fox, J. Jara-Almonte, L. Gao Loss-of-equilibrium mechanisms such as the ideal torus instability [Kliem \& T\"or\"ok, \textit{Phys. Rev. Lett.} \textbf{96}, 255002 (2006)] are predicted to drive arched flux ropes in the solar corona to erupt. In recent line-tied flux rope experiments conducted in the Magnetic Reconnection Experiment (MRX), however, we find that quasi-statically driven flux ropes remain confined well beyond the predicted torus instability threshold. In order to understand this behavior, \textit{in situ} measurements from a 300 channel 2D magnetic probe array are used to comprehensively analyze the force balance between the external (vacuum) and internal (plasma-generated) magnetic fields. We find that the line-tied tension force---a force that is not included in the basic torus instability theory---plays a major role in preventing eruptions. The dependence of this tension force on various vacuum field and flux rope parameters will be discussed. [Preview Abstract] |
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PP8.00108: Development of phosphor imaging diagnostics for particle energization and field line mapping studies in MRX W. Fox, S.J. Zweben, J. Yoo, J. Jara-Almonte, C. Myers, M. Yamada, H. Ji The energization of particles by magnetic reconnection is one of its most important roles in space and astrophysical plasmas. We present results from phosphor-screen imaging diagnostics for the Magnetic Reconnection Experiment, developed to measure the location and timing of particle energization by magnetic reconnection and to map field lines. Phosphor-based imaging diagnostics have previously been to study plasma dynamics in in non-neutral plasmas and low-temperature linear machines [1]. In MRX, movable, phosphor-coated probes are scanned across the current sheet, and phosphor emission is imaged on a fast camera acquiring at typically 500k frames/sec. Optical filters isolate the phosphor emission from line emission in the plasma. The energy sensitivity of the probe is determined by the characteristics of the phosphor and bias of the probe with respect to the plasma. We also present the development of an e-beam diagnostic to directly map the magnetic field line structure and possibly to measure the parallel electric field and/or cross-field electron transport. A modulated electron beam from a hot tungsten filament will be detected downstream by Langmuir probes and the phosphor imager.\\[4pt] [1] A. Liebscher, et al, Rev. Sci. Instrum. \textbf{72}, 953 (2001). [Preview Abstract] |
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PP8.00109: FLARE (Facility for Laboratory Reconnection Experiments): A Major Next-Step for Laboratory Studies of Magnetic Reconnection H. Ji, A. Bhattacharjee, S. Prager, S. Bale, T. Carter, N. Crocker, J. Drake, J. Egedal, J. Wallace, E. Belova, R. Ellis, W. Fox, P. Heitzenroeder, M. Kalish, J. Jara-Almonte, C. Myers, W. Que, Y. Ren, P. Titus, M. Yamada, J. Yoo, W. Daughton A new intermediate-scale plasma experiment, called the Facility for Laboratory Reconnection Experiments or FLARE, is under construction at Princeton as a joint project by five universities and two national labs to study magnetic reconnection in regimes directly relevant to space, solar, astrophysical, and fusion plasmas. The currently existing small-scale experiments have been focusing on the single X-line reconnection process in plasmas either with small effective sizes or at low Lundquist numbers, but both of which are typically very large in natural and fusion plasmas. The design of the FLARE device is motivated to provide experimental access to the new regimes involving multiple X-lines at large effective sizes and high Lundquist numbers. The motivating major physics questions, the construction status, and the planned collaborative research will be discussed. [Preview Abstract] |
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PP8.00110: Experimental Study of a Linear/Non-Linear Flux Rope Timothy DeHaas, Walter Gekelman, Bart Van Compernolle Flux Ropes are magnetic structures of helical field lines, accompanied by spiraling currents. Commonly observed on the solar surface extending into the solar atmosphere, flux ropes are naturally occurring and have been observed by satellites in the near earth and laboratory environments. In this experiment, a single flux rope (r $=$ 2.5 cm, dz $=$ 1100 cm) was formed in the cylindrical, magnetized plasma of the Large Plasma Device (LaPD, L $=$ 2200 cm, r $=$ 30 cm, n$_{o} = $ 10$^{12}$ cm$^{-3}$, T$_{e} = $ 4 eV, ~He). The flux rope was generated via a DC discharge between a cathode and anode. This fixes the rope at its source while allowing it to freely move about the anode. At large currents (I \textgreater B$_{o}\pi $r$^{2}$c/2dz), the flux rope becomes helical in structure and oscillates about a central axis. Under varying Alfven speeds and injection current, the transition of the flux rope from stable to kink-unstable was examined. As it becomes non-linear, oscillations in the magnetic field shift from sinusoidal to Sawtooth-like. The frequency, mode structure, and MHD quantities such as cross-helicity were examined during this transition from linear to non-linear behavior. [Preview Abstract] |
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PP8.00111: Experiments on the tearing of a current sheet into a bundle of interacting flux ropes Walter Gekelman, Tim DeHaas, Bart Van Compernolle, Alex Latshaw, William Daughton A narrow ( $\frac{\delta }{L}\approx .05, \delta \simeq 3-10\frac{c}{\omega_{pe} },\delta \mbox{=1cm})$ current sheet is established in a magnetized (B$_{\mathrm{0z}} = $ 200G, He, Len $=$ 17 m, Dia $=$ 60 cm) plasma column. The current sheet is observed to tear into multiple magnetic islands in several Alfv\'{e}n transit times. Volumetric magnetic field data is acquired at 16,500 spatial locations and 16,000 time steps ($\delta $t $=$ .34 $\mu $s). The flux ropes appear as multiple ``O'' and ``X'' points when viewed in a plane perpendicular to the local current but, in fact are three-dimensional. The kink unstable ropes writhe, and twist about each other as the ensemble of ropes spin about the background axial magnetic field. Fast framing camera images ($\tau_{\exp } = 1\mu s$, 34,000 fps) clearly show the motion but differ from shot to shot. The movies are analyzed using correlation techniques. The rope dynamics becomes chaotic therefore correlation techniques using a fixed magnetic probe as well a He II line ($\lambda =303A)$ are used to generate 3D images of the ropes. An emissive probe is used to measure the plasma potential and the total electric field, $\vec{{E}}=-\nabla \phi -\frac{\partial \vec{{A}}}{\partial t}$, and plasma resistivity are evaluated. The perpendicular electric fields are two orders of magnitude larger than the parallel ones. The entropy and complexity of the flux ropes are evaluated. [Preview Abstract] |
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PP8.00112: Formation and Interaction of Magnetic Flux Ropes in Kinetic-Scale Current Layers William Daughton, Cihan Akcay, John Finn, Walter Gekelman, Zach Billey, Ellen Zweibel In space plasmas, current sheets arise spontaneously from the interaction of flows or magnetic structures. As these current layers approach kinetic scales, they may become unstable to the collisionless tearing instability, resulting in the formation and interaction of magnetic flux ropes. While theoretical treatments of the tearing instability have largely focused on 1D equilibria with periodic boundary conditions, current sheets in nature have a finite spatial extent and are embedded within a larger open system. In many applications, the field boundary conditions are line-tied as in the case of flux ropes on the dayside magnetopause where the ionosphere acts as a conducting surface. To assess the applicability of existing linear tearing theory to these more realistic configurations, we consider a series of 3D kinetic simulations of force-free current layers with line-tied boundary conditions for the fields and open boundaries for the particles. The geometry and plasma parameters are motivated by a new laboratory experiment on the Large Plasma Device at UCLA. For sufficiently long systems, we demonstrate that key aspects of the theory remain valid. New diagnostics are employed to characterize the nonlinear reconnection rate and the structure of the magnetic field. [Preview Abstract] |
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PP8.00113: Blasting current sheets into flux ropes using laser plasmas S. Vincena, W. Gekelman, J. Bonde In plasmas, current sheets have been shown to break apart into three-dimensional flux ropes via turbulent magnetic reconnection[1]. Such events occur naturally anywhere from the sun to planetary magnetospheres. In nature, the onset of the breakup is random, but in this experiment it is seeded at a chosen time and location--allowing study of the dynamics in a repeatable laboratory experiment. In the Large Plasma Device (LAPD), current sheets are created using a LaB$_{6}$ cathode, with $T_{e}=20$eV, $n\approx 4\times 10^{12}$cm$^{-3}$, yielding cross-field dimensions $h=0.9c/\omega_{pi}$ and $w=3.8c/\omega_{pe}$ for a H plasma, and a Lundquist number $S=8\times 10^{3}$. The synchronization of the breakup from current sheet to flux ropes is achieved by the cross-field injection of a laser ablation plasma, whose lifetime is less than an ion cyclotron period. The system dynamics are explored using magnetic and emissive probes (providing total E) along with spectral measurements before and after the breakup. \\[4pt] [1] Daughton, W., et al., Nature Physics, v7 pp539-542 (2011). [Preview Abstract] |
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PP8.00114: Nonlinear analysis of explosive growth of collisionless magnetic reconnection in the presence of the effect of finite electron temperature Makoto Hirota, Yuji Hattori Explosive behavior of collisionless magnetic reconnection is investigated by analyzing a two-fluid model that includes the effects of the electron inertia and the electron temperature (or compressibility). By micrifying both the electron skin depth $d_e$ and the ion-sound gyroradius $\rho_s$ such that $\rho_s = d_e < 0.01L$ (where $L$ is the system size), a direct numerical simulation is performed to enlarge strongly nonlinear regime of a collisionless tearing instability. The nonlinear evolution is shown to be explosive when the inverse of the tearing index $1/\Delta'$ is smaller than $\rho_s = d_e$, whereas the maximum reconnection speed at the fully reconnected state does not significantly depend on the size of $\rho_s = d_e$. The singular current-vortex sheets are generated in the form of the X shape [Cafaro et al. Phys. Rev. Lett 80, 4430 (1998)]. In the explosive phase, the expansion of this X shape as well as the magnetic island occurs locally near the reconnection point. By taking an approach similar to the asymptotic matching, the dynamics of the current-vortex sheets is analyzed and the explosive reconnection speed is estimated theoretically. [Preview Abstract] |
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PP8.00115: Impulsive Magnetic Reconnection by Acceleration of Ejecting Plasmoid Motion Y. Ono, C.Z. Cheng, Y. Hayashi, A. Kuwahata, H. Tanabe, K. Kadowaki We show for the first time in laboratory merging experiments, the ejection of plasmoid from current sheet as a major driving mechanism for impulsively fast magnetic reconnection. The high inflow of plasma (and thus magnetic flux) and low current-sheet resistivity cause flux pile-up initially, which is followed by rapid growth of plasmoid in the current sheet. When the flux pileup exceeds a critical level, the plasmoid is ejected from the squeezed current sheet area. The reconnection rate is slow during the flux pileup stage, but becomes drastically fast during the ejection, indicating impulsive reconnection. The reconnection electric field and effective resistivity in the current sheet reach their peak values when the acceleration rate of plasmoid motion peaks. This clear relationship indicates that the acceleration of plasamoid ejection is a major fast reconnection mechanism. The present experimental result explains the simultaneous occurrence of plasmoid acceleration rate and enhanced magnetic reconnection observed during the impulsive phase of solar flares. [Preview Abstract] |
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PP8.00116: Electron Heating Characteristics of Magnetic Reconnection in UTST Merging Tokamak Experiment Xuehan Guo, Takamichi Sugawara, Michiaki Inomoto, Yasushi Ono Localized electron heating from 10eV to 30eV was documented around the X-point during strong guide field (typically $B_t \sim 15B_p$) magnetic reconnection in the UTST tokamak merging experiment. We developed a novel two-dimensional Thomson scattering measurement system by sliding radially the whole 1D system that can measure an axial profile of electron temperature and density in a single discharge. The high electron temperature area was found to have a round shape with radius of 2cm, in sharp contrast with high current density area. This scale length 2cm is close to the orbit amplitude of an ion meandering motion 1.5-2cm but 3 times longer than the ion gyroradius 0.6cm.The electron heating power is about 12MW/m$^3$ which is an order of magnitude larger than heating power calculated from the Splitzer resistivity. The increment in electron thermal energy is about 2.2 J, which is about 15\% of the dissipated magnetic energy of 14 J measured by 2D magnetic probe array. This conversion ratio in the strong guide field magnetic reconnection is higher than that in the weak guide field (typically $B_t \sim 5B_p$) experiment in MAST and TS-3 devices, suggesting that the electrons are accelerated toroidally toroidally by reconnection electric field and thermalized around X-point. [Preview Abstract] |
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PP8.00117: Floating Potential Measurement for High Guide Field Magnetic Reconnection in the UTST Tokamak Merging Experiment Kotaro Yamasaki, Shizuo Inoue, Shuji Kamio, Takenori Watanabe, Anqi Wang, Xuehan Guo, Hiroki Ishikawa, Tomohiko Ushiki, Hiroki Nakamata, Takumichi Sugawara, Keita Matsuyama, Naoto Kawakami, Takuma Yamada, Michiaki Inomoto, Frank Cheng, Yasushi Ono Langmuir probe measurements revealed a clear quadrapole structure of floating potential around the X-point in the UTST tokamak merging experiment with high toroidal (guide) magnetic field (typically Bt/Brec \textgreater 10). The 2D profile of floating potential starts forming quadrapole structure and it is most emphasized when the reconnection speed reaches the maximum. Also using magnetic field measurements, we measured the electric field component parallel to the magnetic field (E\textbar \textbar) around the X-point. It was observed that E\textbar \textbar grows up inside the sheet with 4cm thickness only during the fast reconnection. This result indicates that the thickness of diffusion region is about half of the ion skin depth under the strong guide field. Those measurements were made under three different guide field and the fixed value of reconnecting magnetic field. With increasing the guide field, the length of magnetic field line inside the diffusion region increases, causing localized heating of electrons at the X-point. This result agrees well with the recent reconnection experiment at MAST and also with the recent PIC simulation. [Preview Abstract] |
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PP8.00118: Impulsive magnetic reconnection induced by electromagnetic fluctuations during guide field magnetic reconnection experiment Akihiro Kuwahata, Ryoma Yanai, Michiaki Inomoto, Hiroshi Tanabe, Yasushi Ono Impulsive reconnection was observed subsequently to the occurrence of large-amplitude monochromatic electromagnetic fluctuations, which were generated inside the diffusion region during magnetic reconnection in the presence of a guide field ($B_g/B_r > 1$, where $B_g$ and $B_r$ are the guide field and reconnection field, respectively), in TS-3 plasma merging experiment. The fluctuations satisfy the dispersion relations of kinetic Alfv\'en wave (KAW). We found a positive correlation between the amplitude of fluctuations and the enhancement of reconnection rate. Ion heating was also observed after the waves propagated to the downstream region, but the wave's Poynting flux was not large enough to balance with the observed ion heating power. The integrated scenario in guide field reconnection for impulsive fast reconnection is: (1) energetic electrons are produced by parallel acceleration during initial slow reconnection phase, (2) the fast electrons drive instabilities to excite KAW-like fluctuations inside the diffusion region, and then (3) these waves modify the local current sheet structure to trigger the impulsive fast reconnection. Although the waves do not account for the energy gain itself, they play roles to rapidly release magnetic energy. [Preview Abstract] |
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PP8.00119: Ion Heating Characteristics of Merging/Reconnection Startup in MAST Spherical Tokamak Experiment Hiroshi Tanabe, Takuma Yamada, Takenori Watanabe, Keii Gi, Akihiro Kuwahata, Kazutake Kadowaki, Yasuhiro Kaminou, Hideya Koike, Kento Nishida, Suguru Imanaka, Haruki Yamanaka, Setthivoine You, Brendan Crowley, Neil Conway, Rory Scannel, Mikhail Gryaznevich, Michiaki Inomoto, Yasushi Ono The high power reconnection heating has been investigated in MAST ST (spherical tokamak) merging startup experiment using a new 32-chords ion Doppler tomography diagnostics with optimized spatial resolution for the current sheet. The magnetic reconnection is observed to heat mostly ions in the downstream by the outflow and to heat electrons inside the current sheet. However, the high $T_e$ area is highly localized at the X-point probably due to the X line acceleration of electrons. Finally both of $T_i$ and $T_e$ profiles are observed to form triple-peak structures after the ion-electron energy relaxation time $\tau^E_{ie}$. The guide field does not affect ion heating under high guide field condition $B_t > 3B_{rec}$, while it increases significantly the localized electron heating at the X point. The increment of ion temperature reaches $\sim$200eV in TS-3 and over $\sim$1keV in MAST, as predicted in the $B_{rec}^2$ (reconnecting field energy) scaling of reconnection heating. This scaling suggests the promising high $ B_{rec}$ merging startup scenario for spherical tokamak. [Preview Abstract] |
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PP8.00120: A new coronal mass ejection experiment by use of PF and CS coils in TS-4 ST/CT device Kazutake Kadowaki, Akinori Sato, Michiaki Inomoto, Yasushi Ono A novel simulation experiment of coronal mass ejection (CME) has been performed using two PF coils, two fluxcores and a center solenoid coil in TS-4 merging device. Its operation is described as follows. A fluxcore is used to form a spheromak that has a certain amount of linked flux with the center solenoid coil. Then, we turns on the out PF coil current parallel to that of spheromak to let the spheromak expand in radial direction like a rising plasmoid/filament. This motion causes the linked flux of spheromak to reconnect, transforming the linked flux into the private flux. The reconnection is observed to accelerate the plasmoid to 10$\sim$20 km/s about 20$\%$ of the Alfven velocity. The size of current sheet (L$\sim$10cm, $\delta$$\sim$4cm) and time scale (30$\mu$s) of reconnection are almost equal to those of the conventional tokamak merging experiment. This operation similar to the CME model is useful to study the CME mechanism under varied linked flux, acceleration and boundary conditions. It is also noted that this experiment with free plasmoid ejection has no density/flux pileup unlike the conventional non-steady reconnection/merging. [Preview Abstract] |
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PP8.00121: Three-Dimensional Turbulent Reconnection Induced by the Plasmoid Instability Yi-Min Huang, Amitava Bhattacharjee It has been established that the Sweet-Parker current layer in high Lundquist number ($S$) reconnection is unstable to the super-Alfvenic plasmoid instability. Past two-dimensional (2D) magnetohydrodynamic simulations have demonstrated that the plasmoid instability leads to a new regime where the Sweet-Parker current layer changes into a chain of plasmoids connected by secondary current sheets, and the averaged reconnection rate becomes nearly independent of $S$. In a three-dimensional (3D) configuration with a guide field, the additional degree of freedom allows plasmoid instabilities to grow at oblique angles [Baalrud et al. Phys. Plasmas \textbf{19}, 022101 (2012)] and develop complex dynamics of flux ropes, which may be viewed as a self-generated turbulent state. In our 3D simulations, kinematic and magnetic energy fluctuations are observed to form cigar-shaped eddies elongated along the direction of local magnetic field, which is a signature of anisotropic MHD turbulence. Additionally, the energy fluctuation spectra are found to satisfy power laws in the inertial range. The characteristics of this self-generated turbulent reconnection are compared with corresponding 2D simulations of the same configuration, as well as turbulent reconnection driven by an external forcing. [Preview Abstract] |
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PP8.00122: Scaling of Hall Reconnection in the Coalescence Problem in Large Systems Jonathan Ng, Amitava Bhattacharjee, Yi-Min Huang As collisionless reconnection modeling of most space plasmas with realistic parameters is beyond the capability of today's simulations, due to the separation between global and kinetic length scales, it is important to establish scaling relations in model problems so as to extrapolate to realistic scales. Large scale particle-in-cell (PIC) simulations of island coalescence have shown that the time averaged reconnection rate scales like $\sqrt{d_i/\lambda}$, where $\lambda$ is the system size, while the maximum rate remains constant [1]. This differs from an earlier claim that reconnection rate is independent of system size [2] and predictions of fluid simulations and theory [3,4] that the maximum rate scales like $\sqrt{d_i/\lambda}$. We perform Hall MHD simulations with the same geometry to study this discrepancy. We find that when the scale separation between the current sheet width and ion skin depth is large enough, the maximum reconnection rate is constant and the average rate decreases weakly as system size increases, in contrast to the PIC results. The differences between PIC and fluid results are discussed.\\[4pt] [1] H. Karimabadi et al. PRL 107, 025002 (2011)\\[0pt] [2] M. Shay et al. GRL, 26, 2163 (1999)\\[0pt] [3] J. Dorelli, PoP 10, 3309 (2003)\\[0pt] [4] X. Wang et al. PRL 87, 265003 (2001) [Preview Abstract] |
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PP8.00123: Superthermal electron energization in magnetic reconnection exhausts Jan Egedal, William Daughton Using a kinetic simulation of magnetic reconnection it was recently shown that magnetic-field-aligned electric fields ($E_{\parallel}$) can be present over large spatial scales in reconnection exhausts [1]. Here we document how the electron confinement provided in part by the structure in $E_{\parallel}$ allows sustained energization by perpendicular electric fields ($E_{\perp}$). The energization is a consequence of the confined electrons' bounce motion, that includes so-called curvature and gradient-B drifts aligned with the reconnection electric field. The level of energizaition is proportional to the initial particle energy and is therefore enhanced by the initial energy boost of the acceleration potential $e\Phi t=e\int_x^{\infty} E_{\parallel} dl$, acquired by electrons entering the region. The mechanism is effective in an extended region of the reconnection exhaust allowing for the generation of super-thermal electrons in reconnection scenarios including only a single x-line. An expression for the phase-space distribution of the super-thermal electrons is derived, providing an accurate match to the kinetic simulation results.\\[1ex] [1] J. Egedal, W. Daughton, and A. Le, Nature Physics, vol. 8, 2012. [Preview Abstract] |
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PP8.00124: Demonstration of anisotropic fluid closure capturing the kinetic structure of magnetic reconnection Obioma Ohia, Jan Egedal, Vyacheslav Lukin, William Daughton Weakly-collisional magnetic reconnection, a process linked to solar flares, coronal mass ejections, and magnetic substorms, has been widely studied through fluid and kinetic simulations. While two-fluid models often reproduce the fast reconnection rate of kinetic simulations, significant differences are observed in the structure of the reconnection regions [1]. Recently, new equations of state that accurately account for the development of anisotropic electron pressure due to the electric and magnetic trapping of electrons have been developed [2]. Using these equations of state, guide-field fluid simulations have been shown to reproduce the detailed reconnection region observed in kinetic simulations [3]. Implementing this two-fluid simulation using the HiFi framework [4], we study the force balance of the electron layers in guide-field reconnection and derive scaling laws for the heating observed in these layers.\\[1ex] [1] Daughton W et al., Phys. Plasmas 13, 072101 (2006).\\[0ex] [2] Le A et al., Phys. Rev. Lett. 102, 085001 (2009).\\[0ex] [3] Ohia O, et al., Phys. Rev. Lett. In Press (2012).\\[0ex] [4] Lukin VS, Linton MG, Nonlinear Proc. Geoph. 18, 871 (2011) [Preview Abstract] |
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PP8.00125: In situ evidence of new magnetotail reconnection regime with embedded current layers and anisotropic exhaust electrons John Boguski, Jan Egedal, William Daughton Using kinetic simulations a new regime of magnetic reconnection has been discovered in parameter ranges similar to those found in the Earth's magnetotail [1]. The regime is closely related to the dynamics of magnetized trapped electrons causing strong electron pressure anisotropy which reaches the firehose condition and has the ability to drive current layers in the reconnection exhaust [2]. As part of an endeavor to conclusively verify the existence of the new reconnection regime in Earth's magnetotail, a systematic analysis of the 2002-08-28 reconnection event first identified by A.L. Borg [3] is performed. In particular we assess the pressure anisotropy, the satisfaction of the firehose condition, and the agreement of pressure measurements to those predicted from theory. The analysis of this event and comparison to simulation results document the importance of the new regime to reconnection in the Earth magnetosphere. \\[1ex] [1] A. Le et al., PRL 110, 135004, 2013. \\[0ex] [2] A. Le et al., PRL 102, 085001, 2009. \\[0ex] [3] A.L. Borg et al., Annales Geophysicae, 30(1):109-117, 2012. [Preview Abstract] |
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PP8.00126: Asymmetric Reconnection in the Terrestrial Reconnection EXperiment Joseph Olson, Jan Egedal, Cary Forest, John Wallace The Terrestrial Reconnection EXperiment (TREX) is a new and versatile addition to the Wisconsin Plasma Astrophysics Laboratory (WiPAL) at the University of Wisconsin-Madison. TREX is optimized for the study of kinetic reconnection in various regimes and to provide the first laboratory evidence in support of a new model describing the dynamics of trapped electrons and correlating pressure anisotropy. The initial configuration implemented in TREX is specially designed to study asymmetric reconnection scenarios. These are particularly relevant to the dayside magnetopause in which the plasma beta of the solar wind and of the magnetosphere can differ by factors of 100-1000. The configuration utilizes the Helmholtz coils to produce a static, uniform magnetic field up to 275 G through the 3 m spherical vacuum vessel. Plasma is produced on a 10 s rep rate while two internal coils are pulsed, creating an opposing magnetic field to induce reconnection with asymmetric high and low beta inflows. A Langmuir and Bdot probe array is swept in between pulses to build up the magnetic profiles in the reconnection region. Preliminary data from these initial runs will be presented. [Preview Abstract] |
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PP8.00127: Quasi-separatrix layer diagnostics and electron force balance in line-tied systems Zachary Billey, Ellen Zweibel, John Finn, William Daughton Magnetic reconnection plays a key role in processes such as coronal mass ejections, solar/stellar dynamics, planetary magnetospheres and accretion disk flares. Magnetic reconnection may be influenced by the line-tied boundary conditions in these systems. For example, magnetic field lines that enter and exit a stellar surface are fixed to the surface at the timescales which coronal magnetic reconnection events take place. In some systems, temperatures may be high enough and densities low enough that collisionless effects play the dominant role. Motivated by this, we investigate collisionless magnetic reconnection in line tied geometry with a series of fully kinetic particle-in-cell simulations of varying lengths. To understand the reconnection physics, we employ field line integrated reconnection diagnostics\footnote{Finn et. al., \textit{Plasma Physics and Controlled Fusion} \textbf{56}, 064013, 2014} to examine the formation of quasi-separatrix layers\footnote{E. R. Priest and P. Demoulin,\textit{J Geophys Res}, 100:23443, 1995} and their association with the integrated parallel electric field. In addition, we examine the electron force balance along field lines to identify the nature of non-ideal behavior in the reconnection region and its connection to magnetic topology. [Preview Abstract] |
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PP8.00128: A fluid theory of fast low-beta magnetic reconnection Adam Stanier, Andrei Simakov, Luis Chacon, William Daughton Low-beta reconnection, where the magnetic field is dominated by a strong out-of-plane component, is important in laboratory magnetic confinement devices, the solar corona, and other magnetically dominated astrophysical environments. Despite the importance of these applications, reconnection in this regime remains poorly understood. It has been suggested that fast dispersive waves are responsible for the fast timescales of reconnection in both the high-beta and low-beta regimes. However, recent kinetic simulations have demonstrated that reconnection rates remain fast, even without such waves. Here we show from fluid simulations that rates are fast in cases with and without fast dispersive waves. We present a fluid theory of the dissipation region, which predicts the functional form of the dissipation region thickness, length, upstream magnetic field, and reconnection rate in both cases. These results are benchmarked against fluid simulations with strict control of dissipation, and comparisons are made with kinetic simulations. [Preview Abstract] |
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PP8.00129: Influence of Line-Tied Boundary Conditions on the Development of Magnetic Reconnection in Force-Free Current Layers Cihan Akcay, William Daughton, Adam Stanier, Vyacheslav Lukin The evolution of plasmas in magnetically dominated low-$\beta$ regimes often leads to the formation of nearly force-free current sheets where magnetic reconnection may be triggered by the tearing instability. In this study, we examine the influence of line-tied boundary conditions on the onset and development of three-dimensional magnetic reconnection in kinetic-scale force-free layers. To better understand the physics, we perform cross-comparisons between fully kinetic VPIC simulations and two-fluid HiFi simulations. We focus on a range of guide fields $B_g=(1-10)B_0$ relevant to both space and laboratory plasmas, and compare the evolution between systems with line-tied and periodic boundary conditions. [Preview Abstract] |
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PP8.00130: Developments of Electromagnetic Particle Simulation Code for Magnetic Reconnection Researches in Open System PASMO and Visualization Library VISMO H. Ohtani, R. Horiuchi, M. Nunami, S. Usami, N. Ohno As the capabilities of computers are improved, the sizes of simulations become greater and greater. In this situation, we have some big issues. One of them is how to develop an efficient simulation code, and another is how to visualize the large data by the simulation. In order to investigate magnetic reconnection from the microscopic viewpoint, we develop a three-dimensional electromagnetic PIC code in an open system (PASMO) [1]. For performing the code on a distributed memory and multi-processor computer system with a distributed parallel algorithm, we decompose three-dimensionally the simulation domain, and introduce the charge conservation scheme to exclude the global calculation, such as Poisson solver with FFT. In the visualization of the simulation data, we develop an in-situ visualization library VISMO [2] for the PIC simulation to carry out the visualization in tandem with the simulation on the same computers. The simulation code with VISMO generates image files instead of raw data. We will discuss the performance of the new PASMO and the simulation results visualized by VISMO on the magnetic reconnection.\\[4pt] [1] H.Ohtani and R. Horiuchi: Plasma and Fusion Research, Vol.4, 024 (2009). [2] N. Ohno and H. Ohtani: Plasma and Fusion Research, Vol.9, 3401071 (2014). [Preview Abstract] |
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PP8.00131: Magnetic islands and energy conversion process in collisionless driven reconnection Ritoku Horiuchi, Shunsuke Usami, Hiroaki Ohtani Relationship between magnetic islands and energy conversion process in collisionless driven reconnection is investigated by means of electromagnetic particle simulation in an open system. By controlling an external driving field imposed at the upstream boundary, we realized two different kinds of solutions in the time evolution of a reconnection system starting from the same initial condition, i.e., the first is a solution with no magnetic islands in the current sheet, and the second is a solution with many islands in it. The energy conversion rate in the second solution is found to be much higher than that in the first solution. This suggests that magnetic islands play a key role in the energy conversion process of collisionless reconnection. The energy conversion rate from the EM field to plasmas decreases as a guide field is intensified. However, its rate to ions is always twice of that to electrons, regardless of whether a guide magnetic field and/or magnetic islands exist It is also found that the energy conversion from the EM field to ions is dominantly caused by the in-plane electric field, while the conversion to electrons is by the out-of-plane electric field. [Preview Abstract] |
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PP8.00132: ``Complex Reconnecting'' Modes and Critical Direction of their Propagation* P. Buratti, B. Coppi, G. Pucella Experiments in weakly collisional plasma regimes (e.g. neutral beam heated plasmas in the H-regime [1]), measuring the Doppler shift associated with the plasma local rotation [1], have shown that the direction of the propagation of the observed reconnected structures in the frame with $E_{r}=0$ is in the direction of the ion diamagnetic velocity. This is contrary to the direction of the phase velocity of the ``drift-tearing'' modes found originally in the Ref. [2]. Consequently, the theory of ``Complex Reconnecting'' Modes has been developed that include the effects finite ion gyro radius, finite ion transverse viscosity, finite longitudinal electron pressure and magnetic diffusion coefficient. Thus modes are found with very low values of the growth rate, a phase velocity about equal the ion diamagnetic velocity that is consistent with recent experimental observations, and widths of the reconnection layer larger than the ion gyro radius. Another option involves assuming the presence of a plasma inductivity [3] in the electron momentum conservation equation. *Sponsored in part by the US Department of Energy.\\[4pt] [1] P. Buratti {\it et al.}, Nucl. Fusion {\bf 52}, 023006 (2012).\\[0pt] [2] B. Coppi, Phys. Fluids {\bf 8}, 2273 (1965).\\[0pt] [3] B. Coppi, Bull. APS {\bf 45}, 366 (2000). [Preview Abstract] |
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PP8.00133: Electron Temperature Fluctuations Associated with High Temperature Reconnecting Modes* B. Basu, B. Coppi, P. Montag Experimental observations [1] and theoretical considerations [2] indicate that, in high temperature plasmas, the value of the perpendicular electron thermal conductivity is relatively large and its effects can compete with those of the parallel thermal conductivity in the theory of reconnecting modes. These effects are taken into account in the relevant expression for the electron thermal energy balance equation. Then the equations for high temperature reconnecting modes are derived and solved showing how limited the applicability of the linearized theory of drift tearing modes [3] is to explain current experimental observations. Electrostatic modes that can be excited in the presence of resistivity gradients [4] are also investigated. When the electron drift wave frequency becomes important, no radially localized unstable mode of this kind can be found with or without the combined effects of trapped electrons and weak collisionality. *Sponsored in part by U.S. DOE.\\[4pt] [1] A. Jacchia, F. De Luca, S. Cirant, et al. \textit{Nucl. Fus}. \textbf{42}, 1116 (2002). [2] B. Coppi, in Phys. of Plas. Close to Therm. Cond., \textbf{2}, 479 (1979).\\[0pt] [3] B. Coppi, \textit{Phys. Fluids} \textbf{8}, 2273 (1965).\\[0pt] [4] B. Coppi, \textit{Phys. Lett}. \textbf{8}, 169 (1964). [Preview Abstract] |
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PP8.00134: Plasma Relaxation Dynamics Moderated by Current Sheets Robert Dewar, Amitava Bhattacharjee, Zensho Yoshida Ideal magnetohydrodynamics (IMHD) is strongly constrained by an infinite number of microscopic constraints expressing mass, entropy and magnetic flux conservation in each infinitesimal fluid element, the latter preventing magnetic reconnection. By contrast, in the Taylor-relaxed equilibrium model all these constraints are relaxed save for global magnetic flux and helicity. A Lagrangian is presented that leads to a new variational formulation of magnetized fluid dynamics, \emph{relaxed MHD} (RxMHD), all static solutions of which are Taylor equilibrium states. By postulating that some long-lived macroscopic current sheets can act as barriers to relaxation [1], separating the plasma into multiple relaxation regions, a further generalization, \emph{multi-relaxed MHD} (MRxMHD), is developed. These concepts are illustrated using a simple two-region slab model similar to that proposed by Hahm and Kulsrud---the formation of an initial shielding current sheet after perturbation by boundary rippling is calculated using MRxMHD and the final island state, after the current sheet has relaxed through a reconnection sequence [2], is calculated using RxMHD.\\[4pt] [1] Z Yoshida \& RL Dewar, J Phys A {\bf 45} 365502 (2012);\\[0pt] [2] RL Dewar, A Bhattacharjee, etal, Phys Plasmas {\bf 20}, 082103 (2013) [Preview Abstract] |
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PP8.00135: Kinetic Alfv\'{e}n waves in three-dimensional magnetic reconnection Ji Liang, Yu Lin, Xueyi Wang Ion kinetic structure of magnetic reconnection in a current sheet is investigated with a 3-D hybrid code for cases with various X-line lengths and guide fields. It is found that kinetic Alfv\'{e}n waves (KAWs) are generated in the reconnection. In the cases in which the $X$-line is so long to extend through the entire domain, quasi 2-D configurations are present. For a current sheet with a zero guide field, the KAWs are found near the separatrices, whereas under a finite guide field $B_G$, they are also seen at the reconnection bulge. In the cases in which the $X$-line has a finite length $\xi$, with $\xi \sim 10d_i$ and $d_i$ being the ion inertial length, the wave perturbations are of a highly 3-D nature. Waves with a dominant $k_\perp \rho_i \sim 1$ are found propagating outward along magnetic field lines from the reconnection region with a slightly super-Alfv\'{e}nic velocity. The structure, polarization relations, and damping of KAWs are examined. The dependence of wave propagation on $B_G/B_{x0}$ is also investigated, where $B_{x0}$ is the antiparallel magnetic field component. The critical length of $X$-line for the generation of 3-D like structures is found to be $\xi_c \alt 30d_i$. [Preview Abstract] |
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PP8.00136: Magnetic island evolution in sheared flows for stellarator fields. Julio Martinell, Daniel Lopez-Bruna It has been evidenced that the magnetic islands associated to low-order rational magnetic surfaces in stellarators play an important role in the dynamics and transport properties. In particular, in the TJ-II heliac there is clear evidence of the appearance of transport barriers near the position of rational surfaces that may lead to an L-H transition or to an oscillatory behavior. Low-frequency magnetic activity has also been detected, that indicates variations in the properties of the magnetic islands. In order to explain the observations, we study the evolution of magnetic islands in the 3D geometry of stellarators starting from the vacuum islands due to error fields. The nonlinear equation for the island width is considered with the inclusion of the polarization current which depends on the EXB velocity profiles around the island; this has a destabilizing contribution. Additionally, the electromagnetic torque acting on the islands produced by the currents in the rational surface are computed, which turns out to be proportional to the island width. The results indicate that shear flow produces island growth and when a given width is exceeded the torque gives rise to island rotation in the frame where E$=$0. This leads to reduction of the flow shear and transport barrier vanishing. [Preview Abstract] |
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PP8.00137: On the energy conversion and particle acceleration during magnetic reconnection Xiaocan Li, Fan Guo, Hui Li, Bill Daughton, Yi-Hsin Liu, Gang Li Using two-dimensional particle-in-cell (PIC) kinetic simulations and magnetohydrodynamic (MHD) simulations, we study energy conversion and particle energization during magnetic reconnection. In the PIC simulations that solve collisionless Vlasov-Maxwell equations, pressure anisotropy develops naturally, and it gets stronger with a stronger guide field. The particle acceleration and energy conversion through particle curvature drift, gradient drift, magnetization current, and parallel electric field are compared. We find that their contributions change with guide field. The results are explained in a drift kinetic approximation, where electric current is associated with pressure anisotropy. This explanation is further verified using high-Lundquist-number MHD simulations of magnetic reconnection, where plasma pressure is assumed to be isotropic. This work demonstrates the importance of considering anisotropic velocity distribution in particle acceleration during magnetic reconnection. [Preview Abstract] |
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PP8.00138: Nonlinear Diamagnetic Stabilization of Double Tearing Modes in Cylindrical MHD Simulations Stephen Abbott, Kai Germaschewski Double tearing modes (DTMs) may occur in reversed-shear tokamak configurations if two nearby rational surfaces couple and begin reconnecting. During the DTM's nonlinear evolution it can enter an ``explosive'' growth phase leading to complete reconnection, making it a possible driver for off-axis sawtooth crashes. Motivated by similarities between this behavior and that of the $m=1$ kink-tearing mode in conventional tokamaks we investigate diamagnetic drifts as a possible DTM stabilization mechanism. We extend our previous linear studies of an $m=2$, $n=1$ DTM in cylindrical geometry to the fully nonlinear regime using the MHD code MRC-3D. A pressure gradient similar to observed ITB profiles is used, together with Hall physics, to introduce $\omega_{*}$ effects. We find the diamagnetic drifts can have a stabilizing effect on the nonlinear DTM through a combination of large scale differential rotation and mechanisms local to the reconnection layer. MRC-3D is an extended MHD code based on the libMRC computational framework. It supports nonuniform grids in curvilinear coordinates with parallel implicit and explicit time integration. [Preview Abstract] |
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PP8.00139: Crossed Flux Tube Experiment (CroFT) Magnus Haw, Paul Bellan Magnetic flux tubes are a fundamental feature of solar coronal loops and astrophysical jets, as well as fusion devices, such as tokamaks and spheromaks. These flux tubes are subject to magnetohydrodynamic forces, expanding, undergoing kink instabilities, and magnetically reconnecting. The CroFT experiment arranges for two flux tubes to cross over each other in a variety of geometries (rotating candelabra) with separate control/power supplies for each flux tube. The experiment aims to study the dynamics and interaction of these arched, plasma-filled flux tubes, specifically the magnetic reconnection that occurs at the crossover point and how this is affected by loop geometry. Initial observations indicate these flux tubes magnetically reconnect with each other as predicted: two equally sized loops reconnect to form a small loop and a large loop if the currents and magnetic fields of both tubes are parallel. If the flux tubes are formed adjacent, rather than crossing over each other, they do not merge. It has also been observed that flux tubes of different species generate protrusions/jets not seen in single-species pairs. Additional non-optical diagnostics (voltage, current) are still being built. [Preview Abstract] |
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PP8.00140: Circularly polarized Magnetic Field of Whistler Wave during Fast Magnetic Reconnection Xiang Zhai, Pakorn Wongwaitayakornkul, Paul Bellan Obliquely propagating whistler waves are expected to have circularly polarized magnetic components [1] and to be associated with fast magnetic reconnection. In the Caltech plasma jet experiment, a current-carrying collimated jet is created from the merging of eight plasma-filled flux ropes. Fast magnetic reconnection occurs during the merging process. When the current- carrying jet undergoes fast kink instability, a lateral Rayleigh-Taylor instability occurs on the jet surface and induces another fast magnetic reconnection event [2]. A capacitive coupling probe placed near the jet has measured fast electric field fluctuations at 15MHz which is in the whistler regime for this plasma. A 3D fast Bdot probe with good electrostatic rejection has been specifically designed to measure the 3D magnetic components of the whistler wave. Preliminary results have revealed a 3D 15 MHz magnetic fluctuation. Work is underway to increase the sensitivity of the induction probe and also to reduce electrostatic pickup. With the improved probe, the polarization property of the magnetic component of the whistler wave is expected to be resolved if it exists. \\[4pt] [1] P. M. Bellan, 2013, Phys. Plasmas 20, 082113\\[0pt] [2] A. L. Moser, {\&} P. M. Bellan, 2012, Nature, 482, 379. [Preview Abstract] |
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PP8.00141: 3-D Gyrokinetic Electron and Fully Kinetic Ion Simulation of Current Sheet Instabilities Zhenyu Wang, Yu Lin, Xueyi Wang, Kurt Tummel, Liu Chen Instability of a Harris current sheet is investigated using a 3-D linearized ($\delta{f}$) electromagnetic gyrokinetic electron and fully kinetic ion (GeFi) particle simulation code. The equilibrium magnetic field consists of an asymptotic anti-parallel component $B_{x0}$ and a guide field $B_G$, with the current sheet normal in the $z$ direction. The simulation is performed for cases with a broad range of $B_G$. The eigenmode structure, real frequency, and the growth rate of instabilities are calculated as a function of wave numbers $k_{x}$ and $k_{y}$. In the cases with a small $k_{y}\rho_{e}$, tearing mode is found to dominate, with peak growth rate at $k_{x}L = 0.4$-0.5, where $L$ is the half-width of the current sheet. On the other hand, in the cases with a small $k_{x}\rho_{e}\leq 0.1$, there exist two unstable modes: a quasi-electrostatic mode at the current sheet edge with wave number $0.3\leq k_{y}\rho_{e}\leq 0.6$ and frequency around the lower-hybrid frequency $\omega_{LH}$ and an electromagnetic mode with $k_{y}\rho_{e}\leq 0.2$ at the sheet center under a guide field $B_G/B_{x0}=0.1$. The transition from the tearing-like instability to the $k_y$-dominant instabilities is presented by scanning through the $(k_x, k_y)$ space. The complete 3-D profile of instabilitie [Preview Abstract] |
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PP8.00142: 3D Gyrokinetic Theory of Electromagnetic Lower-Hybrid Drift Instabilities in a Harris Current Sheet with Guide Field Kurt Tummel, Liu Chen, Zhenyu Wang Electromagnetic fluctuations in the lower-hybrid drift(LHD) frequency range have been observed in current sheets in the magnetosphere, magnetopause, and laboratory plasmas. In theory and simulations, the lower hybrid drift instabilities typically dominate the linear phase of current sheet dynamics, appearing as quasi-electrostatic, edge-localized, electron-scale waves that propagate nearly perpendicular to the magnetic field. In current sheets with strong drift velocities, fluctuations in the LHD frequency range are observed near the current sheet center, with longer wavelengths and stronger magnetic fluctuations than conventional LHDIs. Analyses of these modes have used fluid theory or the local approximation to handle the electron response. We have adopted a gyrokinetic-electron, unmagnetized kinetic-ion model to study the electromagnetic effects in a 3D nonlocal analysis of an ion-scale Harris current sheet with a guide field. Both numerical and analytical results will be presented and compared with those of direct simulations. [Preview Abstract] |
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