Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session BO4: NSTX and Spherical Torus |
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Chair: Ted Biewer, Oak Ridge National Laboratory Room: Ballroom E |
Monday, November 14, 2011 9:30AM - 9:42AM |
BO4.00001: Overview of Recent Results from the National Spherical Torus Experiment* S.A. Sabbagh Both spherical torus devices for fusion development and ITER require high energy confinement, sustained stability, and manageable first wall heat fluxes. NSTX research targets a predictive understanding of these needs. Low-k microtearing simulations predict lower collisionality, $\nu$, to be nonlinearly stabilizing. Measured ion gyro-scale fluctuations transiently decrease after the H-mode transition, while high-k scattering shows electron gyro-scale fluctuations may increase at lower $\nu$. Other channels for transport such as high frequency Alfvenic modes are examined. Increased RWM stability is expected at lower $\nu$ only if stabilizing precession drift/bounce resonance conditions are maintained. Improved RWM control now includes radial and poloidal field sensors, and state space feedback with a 3D conducting structure model. Non- inductive current fractions of 65-$70\%$ have been sustained. Divertor heat flux width strongly decreases as $I_{p}$ increases but snowflake divertor studies have reduced the heat flux significantly. Beneficial effects due to lithium depend nearly continuously on the amount of pre-discharge Li evaporation. Mo divertor tiles have been installed to determine the impact of Li-coated metallic PFCs at strike point locations. Coaxial helicity injection has produced 0.37MA peak current and yielded a 40\% inductive flux savings for ohmic startup to 1MA plasma current. *Work supported by U.S. DOE contract DE-AC02-09CH11466. [Preview Abstract] |
Monday, November 14, 2011 9:42AM - 9:54AM |
BO4.00002: Measurements of low-wavenumber turbulence in NSTX H-mode plasmas David R. Smith, R.J. Fonck, G.R. McKee, D.S. Thompson, I.U. Uzun-Kaymak New Beam Emission Spectroscopy (BES) measurements provide the first radially and poloidally-resolved measurements of low-k turbulence in a fusion-grade spherical torus plasma in the National Spherical Torus Experiment (NSTX). Measured broadband turbulence with k$_{\bot }\rho _{i}$ $\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}}$ 1 and frequencies up to 100 kHz is qualitatively similar to higher-field tokamak turbulence. Multi-point correlation analysis indicates poloidal and radial correlation lengths are on the order of 10 cm at r/a$\sim $0.85. The correlation lengths are larger than those in higher-field tokamaks, but consistent with a rho-star scaling of turbulence. In stationary H-mode phases, longer poloidal correlation lengths correlate with higher density, density gradient, and electron temperature, suggesting a connection to instability drive mechanisms. Eddy poloidal velocities are about 10 km/s, consistent with equilibrium E$\times $B dominated flow. Poloidal flow fluctuations from time-delay estimation calculations will be discussed. The measurements and calculations motivate the inclusion of low-k turbulence in transport models of spherical torus plasmas. [Preview Abstract] |
Monday, November 14, 2011 9:54AM - 10:06AM |
BO4.00003: Turbulence Characteristics Near the L-H Transition in NSTX Ohmic Discharges S. Kubota, W.A. Peebles, S.J. Zweben, R.E. Bell, S.M. Kaye, B.P. LeBlanc, R. Maqueda, R. Maingi, C.E. Bush, R. Raman Ohmic discharges offer a good template for studying the L-H transition in NSTX. Peaked density profiles provide good accessibility for the reflectometers, there are no fast-ion driven fluctuations to complicate the turbulence measurements, and issues due to external momentum input and hot fueling from neutral beams is avoided. Details of NSTX Ohmic H-modes have been reported previously [C.E. Bush et al., APS-DPP08, NP6.00091]. Here we present additional turbulence measurements near the L-H transition using FM-CW reflectometry (profile, correlation and backscattering) and high-speed gas puff imaging (GPI). Reflectometry shows changes in the turbulence localized to the ETB radius: 1) $\tilde{n}/n$ decreases across a broad spectral range ($k_r$$=$0$-$20 cm$^{-1}$), and 2) the radial correlation length drops from $\sim$1$-$2 cm to less than 0.5 cm. GPI sees coherent zonal flows at $\sim$3$-$4 kHz prior to the L-H transition. The relationship between these observations and quantities such as the mean flow shear and edge gradients, as well as their connection to the ETB formation, will be investigated. [Preview Abstract] |
Monday, November 14, 2011 10:06AM - 10:18AM |
BO4.00004: The continuous improvement of H-mode discharge performance with progressively increasing lithium coatings in NSTX R. Maingi, S.M. Kaye, C.H. Skinner, D.P. Boyle, J.M. Canik Lithium wall coatings have been shown to reduce recycling, improve energy confinement [1,2], and suppress edge localized modes [3,4] in the NSTX. Here we show that these effects depend nearly \textit{continuously} on the amount of pre-discharge lithium evaporation. We observed a nearly monotonic reduction in recycling and a decrease in edge electron transport [5] with increasing lithium. Moreover we see a reduction in the electron temperature and profile peaking factors, as well as an improvement in ELM stability [6] with increasing lithium. These correlations challenge basic expectations, given that even the smallest coatings provided a nominal minimum lithium coating thickness of 30 nm, and an average of 60 nm near the outer divertor strike point; the maximum coating thickness was 8x higher. In comparison, the nominal implantation range, which is the relevant scale length for recycling and pumping, was $<$ 10 nm. *Supported in part by U.S. DoE contracts DE-AC05-00OR22725 and DE-AC02-09CH11466. \\[0pt] [1] Kugel H. W.\textit{, et al.} \textit{PoP} \textbf{15,} 056118 (2008). [2] Bell M.G.\textit{, et al.} \textit{PPCF }\textbf{51,} 124054 (2009). [3] Maingi R.\textit{, et al.} \textit{PRL} \textbf{103,} 075001 (2009). [4] Mansfield D. K.\textit{, et al.} \textit{JNM} \textbf{390-391,} 764 (2009). [5] Canik J. M.\textit{, et al.} \textit{PoP} \textbf{18,} 056118 (2011). [6] Boyle D. P.\textit{, et al.} \textit{PPCF} submitted (2011). [Preview Abstract] |
Monday, November 14, 2011 10:18AM - 10:30AM |
BO4.00005: The relationships between ELM suppression, pedestal profiles, and lithium wall coatings in NSTX D.P. Boyle, R. Maingi, P.B. Snyder, T.H. Osborne, J. Manickam Recently in the National Spherical Torus Experiment (NSTX), increasing lithium wall coatings suppressed edge localized modes (ELMs), gradually but not quite monotonically. While the quantity of Li deposited did not uniquely determine the presence of ELMs, profile analysis demonstrated that Li was correlated to wider density and pressure pedestals with peak gradients farther from the separatrix. Ultimately, ELMs were suppressed \emph{only} when lithium caused the $n_e$ pedestal to widen and shift inward. This supports the theory that ELMs in NSTX are caused by kink/peeling modes, which are stabilized when the edge current and pressure gradient follow the the $n_e$ gradient and shift away from the separatrix. Edge stability analysis using ELITE corroborated this picture, as reconstructed equilibria from ELM-free discharges were generally farther from their kink/peeling stability boundaries than ELMy discharges. We conclude that density profile control provided by Li is the key first step to ELM suppression in NSTX. [Preview Abstract] |
Monday, November 14, 2011 10:30AM - 10:42AM |
BO4.00006: RWM Stabilization to Sustain High Normalized Beta at Low Internal Inductance in NSTX$^{*}$ J.W. Berkery, S.A. Sabbagh, J.M. Bialek, O. Katsuro-Hopkins, R.E. Bell, S.P. Gerhardt, B.P. LeBlanc, K. Tritz Spherical torus fusion applications aim to operate at high normalized beta, $\beta_{N}$, and non-inductive current fraction. These plasmas exhibit broad current profiles and low plasma internal inductance, $\ell_{i}$. In NSTX, such plasmas show a significant reduction of the ideal $n = 1$ no-wall stability limit at $0.4 < \ell_{i} < 0.6$. High $\beta_{N} > 6$ is reached, exceeding the ideal limit by a factor of two. Plasmas below this $\ell_{i}$ range are computed to be near the current- driven ideal kink stability limit, where mode stabilization is required at finite beta. Two active control approaches are used: (i) combined use of radial and poloidal field resistive wall mode (RWM) sensors with $n = 1$ proportional gain feedback and (ii) an RWM state-space controller including an unstable RWM eigenfunction model and currents induced in nearby 3D conducting structure. Long-pulse plasmas reached $\beta_{N}/ \ell_{i} > 13$. Disruption probability was significantly reduced in plasmas at high $\beta_{N}/\ell_{i} > 11$ with more disruptions seen at lower $\beta_{N}/\ell_{i}$ consistent with theory showing decreased passive RWM stabilization at intermediate plasma rotation levels. New independent control of the six actuator coils will allow the RWM state space controller to produce mode control field spectra with $n > 1$. $^{*}$Work supported by U.S. DOE Contracts DE-FG02-99ER54524 and DE-AC02-09CH11466. [Preview Abstract] |
Monday, November 14, 2011 10:42AM - 10:54AM |
BO4.00007: Low frequency multiple harmonic oscillations in NSTX King-Lap Wong Low frequency (100kHz) MHD activities are very common in NSTX. They can appear in many plasma conditions with neutral beam and/or high harmonic fast wave heating. Their presence is usually associated with a rotating magnetic island identified by a flat region in the toroidal plasma rotation profile, and their frequencies equal to the multiple harmonics of the island toroidal rotation frequency. They can produce stochastic magnetic field and enhance plasma transport. The island location may vary from the plasma core to the plasma edge. These oscillations are usually detected by Mirnov coils. Oscillations at the same frequencies can also appear in the high-k scattering signal if the scattering volume is at the right location. Data from various plasmas will be presented, and their effects on plasma transport will be discussed. [Preview Abstract] |
Monday, November 14, 2011 10:54AM - 11:06AM |
BO4.00008: HHFW Edge Heating Properties for H-mode Plasmas in NSTX R.J. Perkins, R.E. Bell, A. Diallo, S. Gerhardt, J.C. Hosea, M. Jaworski, B.P. LeBlanc, G.J. Kramer, C.K. Phillips, L. Roquemore, G. Taylor, J.R. Wilson, S. Zweben, J.-W. Ahn, T.K. Gray, D.L. Green, A. McLean, R. Maingi, P.M. Ryan, J. Wilgen, K. Tritz RF wave interaction with edge plasma in toroidal confinement devices can reduce core heating, and a significant decrease in core heating along with increased edge heating can occur in the NB- and HHFW-driven ELMy H-mode regimes of NSTX [1]. This effect may be primarily due to the edge density exceeding the onset density for perpendicular wave propagation, leading to enhanced wave propagation in the SOL. To explore this, we present measurements of RF heating of and interactions with the divertor, including IR cameras, RF probes, and Langmuir probes. Direct observation of the divertor-edge RF-deposition zone with the fast IR camera is planned to quantify the effect of the ELMs there. Langmuir probe characteristics, sensitive to the location of the RF ``hot'' zone, will be related to the wave/particle properties of the launched waves in the SOL.\\[4pt] [1] J. Hosea et al., RF Conference (Newport, 2011). [Preview Abstract] |
Monday, November 14, 2011 11:06AM - 11:18AM |
BO4.00009: Comparison of Diverted Plasmas Incident on Liquid Lithium and Lithiated Molybdenum, and Graphite Surfaces* H.W. Kugel, M.G. Bell, R. Bell, A. Diallo, S. Gerhardt, M. Jaworski, R. Kaita, J. Kallman, B. Leblanc, D. Mansfield, D. Mueller, S. Paul, A.L. Roquemore, F. Scotti, C.H. Skinner, J. Timberlake, L. Zakharov, J.P. Allain, B. Heim, C. Taylor, A. McLean, R. Maingi, R. Raman, S. Sabbagh, V. Soukhanovskii NSTX 2010 experiments were conducted using a molybdenum Liquid Lithium Divertor (LLD) surface installed on the outer part of the lower divertor. This tested the effectiveness of maintaining the D retention properties of a static liquid lithium surface when refreshed by lithium to approximate a surface replenished with flowing liquid lithium. Lithium evaporators were used to deposit lithium on the LLD surface. Noteworthy improvements in plasma edge conditions were obtained, and work is in progress to determine the role of lithium surface impurities in affecting these and the previous lithiated graphite results. A row of molybdenum tiles was installed inboard of the LLD for 2011 experiments with both inner and outer strike points on lithiated molybdenum to allow investigation of lithium plasma facing issues encountered in the first testing of the LLD, and a comparison of diverted plasmas incident on lithiated Mo and graphite surfaces. *Work supported by USDOE Contract DE-AC02-09CH11466. [Preview Abstract] |
Monday, November 14, 2011 11:18AM - 11:30AM |
BO4.00010: NSTX operation with reduced gas fueling Dennis Mueller The National Spherical Torus Experiment, NSTX, has successfully produced shaped plasmas that transition to H-Mode early and which have long plasma current, I$_{p}$, flattop durations. These plasmas have low internal inductance, l$_{i}$, and the density rises throughout the discharge. Presently, the successful discharges in NSTX have used gas puffing or a high-recycling wall to obtain good, stable, plasmas free from deleterious MHD. For the NSTX upgrade, it is important to have lower-collisionality plasmas in order to maximize the non-inductive current drive. This talk will describe the efforts to achieve stable, lower-density, high-performance plasmas by reducing the high gas fueling rate that has, so far, been required to routinely produce high-performance plasmas in NSTX. A concern is that locked modes are more common in low-density plasmas and will limit low-density operation. A range of plasma start-up and ramp-up scenarios will be explored using minimal gas fueling. These scenarios include low-voltage ($<$1.5 V/turn) start-up, slow I$_{p}$ ramp-up rate, higher-voltage ($>$4 V/turn) initiation at full aperture, and the use of low or zero solenoid pre-charge. The latter will help clarify if motion of the toroidal-field bundle inside the solenoid winding during the pre-charge is responsible for error-field induced mode locking. [Preview Abstract] |
Monday, November 14, 2011 11:30AM - 11:42AM |
BO4.00011: Snowflake divertor configuration studies for NSTX-Upgrade V.A. Soukhanovskii, S.P. Gerhardt, R. Kaita, E. Kolemen, H.W. Kugel, J.E. Menard, R. Maingi, A. McLean The snowflake divertor configuration is being considered as the divertor solution for high-power density operation in NSTX-Upgrade, where peak divertor heat loads are predicted to be $q_{peak} = 20-30$ MW~m$^{-2}$. Recent experiments in NSTX demonstrated the potential for the snowflake divertor configuration not only to reduce steady-state divertor peak heat flux from 4-8 to 0.5-1 MW m$^{-2}$ (with 4 MW NBI heating), but also to reduce impulsive heat loads due to Type I ELMs to acceptable levels simultaneously with overall good H-mode confinement properties. The reduction of heat flux is attributed to the snowflake geometry effects that led to a reduced $q_{\parallel}$ due to increased radiative losses and radial heat diffusion, and a reduced $q_{div}$ due to the increased plasma-wetted area. Experiments are being planned to study the snowflake divertor power accountability, pumping with solid lithium coatings, impurity production, effects of applied resonant magnetic perturbations and additional gas puffing, plasma shaping and magnetic control to enable the assessment of its operating space and projections to NSTX-Upgrade. Supported by the U.S. DOE under Contracts DE-AC52-07NA27344, DE-AC02-09CH11466, DE-AC05-00OR22725. [Preview Abstract] |
Monday, November 14, 2011 11:42AM - 11:54AM |
BO4.00012: Surface chemistry analysis of NSTX private flux region and upper vessel tiles C.N. Taylor, K.E. Luitjohan, B. Heim, J.P. Allain, C.H. Skinner, H.W. Kugel, R. Kaita Lithium wall conditioning in NSTX reduces plasma impurities and deuterium recycling, thus resulting in enhanced plasma performance. X-ray photoelectron spectroscopy (XPS) is used to investigate the mechanisms of deuterium retention and has identified Li-O-D and Li-C-D functionalities on lithium conditioned NSTX graphite tiles. Methods to remove the passivation layer that develops during transport to Purdue University and recover the underlying surface will be presented. XPS analysis has revealed distinct surface chemistry for different regions in the vessel. Lower center stack and inner divertor tiles show unusual XPS spectra that strongly coincide with the campaign-averaged private flux region. XPS spectra from upper vessel tiles, that experience a wide range of lithium deposition, motivate laboratory tests to identify the minimum lithium threshold capable of sustaining lithium aided deuterium retention. A minimum lithium threshold is highly sensitive to surface morphology and is found between 50-500 nm. [Preview Abstract] |
Monday, November 14, 2011 11:54AM - 12:06PM |
BO4.00013: Synthetic Aperture Microwave Imaging on MAST Simon Freethy, Billy Huang, Vladimir Shevchenko, Roddy Vann A novel microwave imaging device, the Synthetic Aperture Microwave Imaging (SAMI) radiometer has been designed and built to obtain the first fully 3D images of microwave mode conversion in the edge of tokamak plasmas to facilitate high time resolution measurements of the edge current density. The imaging technique has been adapted from radio astronomy and Earth remote sensing, but with high time resolution ($\sim 5\mu$s) due to the exceptionally bright source. This diagnostic has no optical components, instead using frequency down converting electronics and a digitiser. All image reconstructions are then done in post-processing. First data for SAMI has been obtained at the time of abstract submission and is currently under analysis. Preliminary results will be presented and discussed. [Preview Abstract] |
Monday, November 14, 2011 12:06PM - 12:18PM |
BO4.00014: Recent Results from the Pegasus Toroidal Experiment A.J. Redd, J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, E.T. Hinson, D.J. Schlossberg, N.L. Schoenbeck, K.E. Thome Pegasus is an ultra-low aspect ratio spherical tokamak (A=1.15), used to study the physics of low-A plasmas and develop non-solenoidal tokamak startup techniques. A combination of point-source magnetic helicity injection and poloidal field induction produces non-solenoidal tokamak plasmas with $I_p \le 0.17$ MA using $~4$ kA injected current $I_{inj}$, consistent with the heilicity injection rate and a Taylor relaxation limit. A strong double layer sheath at the injectors describes the impedance of the injection circuit, implying the helicity injection rate for a given $I_{inj}$ is a strong function of the local plasma density. Impurity ion spectroscopy indicates strong heating ($T_i$ $\sim$ 0.5 keV) during helicity drive. Passive gas-fueled electrodes can be used as helicity injectors, complementing active current sources and affording a means to optimize the Taylor and helicity injection limits. Ohmic plasmas in Pegasus are often unstable to peeling modes, an instability underlying edge localized modes (ELMs) in larger tokamaks. Time-resolved current profile measurements show that ELM-like, current-carrying filaments form at the plasma edge, which then detach and propagate outward. [Preview Abstract] |
Monday, November 14, 2011 12:18PM - 12:30PM |
BO4.00015: Verification of bootstrap current models Alexei Pankin, Scott Kruger, Emily Belli, James Callen, Arnold Kritz, Tariq Rafiq The FACETS, PTRANSP, and XGC0 codes are used to verify neoclassical models for the bootstrap current and neoclassical resistivity. The most widely used reduced models for the bootstrap current such as Hazeltine-Hinton, NCLASS, NEO, and Sauter models agree reasonably well in a range of plasma parameters that most modern conventional tokamaks operate. However, the models start to disagree when they are applied to high-beta and low aspect ratio plasmas such as NSTX and MAST. There are also concerns about applicability of asymptotic limits used in some analytical formulations of bootstrap current models in the H-mode pedestal regions where the plasma gradient scale lengths are comparable or smaller than the finite-orbit (banana) widths. In order to discriminate between different reduced models and to refine their ranges of applicability, the models are benchmarked against more first-principle computations available through the XGC0 and NEO codes. The direct verification of the models are facilitated by recent developments in the FACETS framework that support simultaneous access to a selection of neoclassical models including NCLASS, Sauter, Hazeltine- Hinton, and NEO. Model verification results will be presented. A special consideration will be given to the plasma parameter regimes where the models diverge. [Preview Abstract] |
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