Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session JO4: NSTX Spherical Torus |
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Chair: Houyang Guo, Tri Alpha Energy Room: Regency VI |
Tuesday, November 3, 2009 2:00PM - 2:12PM |
JO4.00001: Overview of Results from the NSTX FY09 Run Roger Raman NSTX research in toroidal magnetic confinement at low aspect ratio has made extensive use this year of both lithium coatings for wall conditioning and external non-axisymmetric field correction to reliably produce high-performance discharges extending to 1.7s in duration. Toroidal beta above 25{\%} has been sustained for up to 0.4s. The error-field correction coils have been used to trigger ELMs for controlled ELM pace-making and have also contributed to an improved understanding of both neoclassical tearing mode and resistive wall mode physics. The supersonic gas injector has been used to fuel long-pulse high-performance discharges without reliance on the high field side fueling previously employed. The edge pedestal of ELM-free H-mode plasmas has been studied using transient edge probes, fast infrared cameras and divertor bolometers. A reduction in the required central solenoid flux has been realized in NSTX when discharges initiated by coaxial helicity injection were ramped in current using induction. Other experiments have been conducted to address research of high priority to the ITPA and ITER. This work supported by U.S. DOE Contracts DE-AC02-09CH11466 and DE-FG02-99ER54519 AM08. [Preview Abstract] |
Tuesday, November 3, 2009 2:12PM - 2:24PM |
JO4.00002: Plasma Response to Lithium-Coated Plasma-Facing Components in NSTX M.G. Bell, H. Kugel, D. Mansfield, R. Kaita, S. Gerhardt, S. Paul, R.E. Bell, R. Maingi, J. Canik Lithium coatings have been routinely applied by evaporation onto the carbon surfaces of the lower divertor and other plasma-facing components in NSTX. In NBI-heated, deuterium H-mode plasmas, the lithium decreased the density in the plasma edge, reduced the inductive flux consumption, and increased the electron temperature, ion temperature and energy confinement time. Reductions in ELM frequency occurred, including complete ELM suppression for periods up to about 1s, apparently as a result of altering the stability of the edge. With ELM suppression, the effective ion charge and radiated power increased as a result of a buildup in carbon and metallic impurities, although lithium itself remained at a low level in the core, $<0.1\%$. The impurity buildup could be inhibited by repetitively triggering ELMs with brief pulses of an n = 3 field perturbation. Lithium powder has also been injected directly into the scrape-off layer where it became ionized and flowed along the magnetic field to the divertor plates. This method of coating produced similar effects to evaporated lithium but at much lower amounts. [Preview Abstract] |
Tuesday, November 3, 2009 2:24PM - 2:36PM |
JO4.00003: Reversed magnetic shear suppression of electron-scale turbulence on NSTX Howard Y. Yuh, F.M. Levinton, R.E. Bell, J.C. Hosea, S.M. Kaye, B.P. LeBlanc, E. Mazzucato, D.R. Smith, C.W. Domier, N.C. Luhmann, H.K. Park Electron thermal internal transport barriers (e-ITBs) are observed in reversed (negative) magnetic shear NSTX discharges$^{1}$. These e-ITBs can be created with either neutral beam heating or High Harmonic Fast Wave (HHFW) RF heating. The e-ITB location occurs at the location of minimum magnetic shear determined by Motional Stark Effect (MSE) constrained equilibria. Statistical studies show a threshold condition in magnetic shear for e-ITB formation. High-k fluctuation measurements at electron turbulence wavenumbers$^{3}$ have been made under several different transport regimes, including a bursty regime that limits temperature gradients at intermediate magnetic shear. The growth rate of fluctuations has been calculated immediately following a change in the local magnetic shear, resulting in electron temperature gradient relaxation. Linear gyrokinetic simulation results for NSTX show that while measured electron temperature gradients exceed critical linear thresholds for ETG instability, growth rates can remain low under reversed shear conditions up to high electron temperatures gradients. $^{1}$H. Yuh, et. al., PoP \textbf{16}, 056120 $^{2}$D.R. Smith, E. Mazzucato et al., RSI \textbf{75}, 3840 $^{3}$E. Mazzucato, D.R. Smith et al., PRL \textbf{101}, 075001 [Preview Abstract] |
Tuesday, November 3, 2009 2:36PM - 2:48PM |
JO4.00004: Advanced Scenario and Plasma Control Development on NSTX David Gates, S.P. Gerhardt, E. Kolemen The NSTX extreme shaping regime ($\kappa \sim 3$, $\delta \sim 0.8$) and $\beta_N \sim 6$, has been extended to include lower q95 plasmas at high $< \beta_t > \sim 25 \%$. These plasmas aim at the operating regime required for future ST reactors such as ARIES-ST. Additionally, record low sustained surface voltages have been sustained at higher q95 $\sim$ 13. The non-inductive current drive fraction is calculated to be $f_{BS} \sim 65 \%$, however actual values may be higher as the sum of the calculated inductive and non-inductive currents is less than the measured plasma currents. Possible causes of this discrepancy will be discussed. These plasmas aim at the regime envisioned for proposed future spherical torus devices, such as NHTX and ST-CTF which plan to operate in this strongly shaped regime but at lower $\beta_t$. These plasmas utilized n=1 RFA/RWM suppression and n=3 error field correction. Lithium evaporation was used to improve plasma confinement. Divertor strike point control experiments in support of the planned liquid lithium divertor upgrade will also be described. [Preview Abstract] |
Tuesday, November 3, 2009 2:48PM - 3:00PM |
JO4.00005: Discharge Evolution Control via 3D Field ELM Pacing in NSTX A.C. Sontag, J.M. Canik, R. Maingi, D.A. Gates, S.P. Gerhardt, V.A. Soukhanovskii ELMs triggered by externally applied 3D magnetic perturbations in conjunction with fueling optimization have been used to prevent the uncontrolled rise in density, radiated power and impurity accumulation observed in otherwise ELM-free NSTX discharges. The frequency of n=3 perturbations was scanned from 10 to 80 Hz with 100{\%} ELM triggering achieved up to 60 Hz. Triggered ELMs arrested the rise in radiated power with only a 10{\%} decrease in the total stored energy. Gas fueling optimization by replacing the typical single-pulse high field side fueling with pulsed supersonic gas injection resulted in elimination of the secular density rise during the discharge. Density remained below $5\times 10^{19}m^{-3}$ for the first time throughout the length of a high-performance discharge. Negative amplitude current pulses in the n=3 coils subsequent to the triggering pulses were successful in minimizing the duration of the n=3 perturbations at the surface of the plasma. This resulted in less rotation braking and delayed the appearance of deleterious MHD. [Preview Abstract] |
Tuesday, November 3, 2009 3:00PM - 3:12PM |
JO4.00006: Fine structure of heat flux deposition for different ELM types in NSTX Joon-Wook Ahn, Rajesh Maingi, Lane Roquemore, Henry Kugel, Vlad Soukhanovskii Investigation of heat flux deposition onto the divertor plates for different ELM types has been carried out in NSTX using a high temporal resolution IR camera, and revealed distinctive features for each ELM type. A single Type-III ELM is found to carry up to $\sim $5{\%} of total plasma energy to the divertor, during which typical peak heat flux (q$_{peak})$ values are 30-60 MW/m$^{2}$ with near SOL e-folding length ($\lambda _{q})$ of 3-4cm. Peak heat flux during the inter-ELM period is typically 3-5MW/m$^{2}$ with $\lambda _{q}$=1-1.5cm. On the other hand, the small, Type-V ELMs exhibit very different heat deposition characteristics. Due to the presence of multiple ELM filaments for a single time slice, q$_{peak}$ becomes significantly lower and $\lambda _{q}$ larger, q$_{peak}$=1-2MW/m$^{2}$ and $\lambda _{q}$=2-3cm. Filaments also make the far SOL $\lambda _{q}$ very long, $\lambda _{q}\ge $10cm. As a result, Type-V ELMs are found to effectively disperse heat flux onto a large area of divertor plates with significantly lower q$_{peak}$. A detailed data analysis for heat flux deposition for different ELM types will be presented and implications for the future machine will be discussed. This work was supported by the US Department of Energy, contract numbers DE-AC05-00OR22725, DE-AC02-09CH11466, and DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, November 3, 2009 3:12PM - 3:24PM |
JO4.00007: Edge current profile measurements of peeling-like modes at high $\left \langle j_{edge} \bigl/ B \right \rangle$ in {\sc Pegasus} M.W. Bongard, R.J. Fonck, E.T. Hinson, B.T. Lewicki, A.J. Redd Large-scale, coherent, high-$m$ filamentary edge instabilities are routinely observed under conditions of high $\left \langle j_{edge}\bigl/B\right \rangle$ in {\sc Pegasus}. These ELM-like filaments are characterized with high-speed imaging, as well as scanning magnetic and Langmuir probes. Their properties include: low- to intermediate-$n$; a coherent electromagnetic signature; large poloidal coherence lengths; rotation with the bulk plasma; and explosive detachment from the edge with outboard radial propagation. Stability is sensitive to $j_{edge}$, with mode drive or suppression dependent on the sign of $\dot{I}_{p}$. The extremely low $\mathbf{B}$ $\left(B_{t,0}\leq0.1\mbox{ T}\right)$ and high $ j_{edge} \approx 0.1 \mbox{ MA/m}^2$ in {\sc Pegasus} lead to high peeling instability drive, proportional to $\left \langle j_{edge}/ B\right \rangle$, comparable to that achieved in H-mode on larger experiments. However, in {\sc Pegasus} $j_{edge}$ is driven by large $\dot{I}_{p}$ ($\leq$ 50 MA/s) and associated skin currents as opposed to a localized region of high bootstrap current in an H-mode pedestal. A new radial array of Hall-effect sensors measures internal $B_{\theta,edge}(R)$ directly with high spatial and temporal resolution to provide strong experimental constraint on $j_{edge}(\psi)$ in equilibrium reconstructions. Such equilibria may be used to uniquely test predictions of peeling-ballooning stability theory. [Preview Abstract] |
Tuesday, November 3, 2009 3:24PM - 3:36PM |
JO4.00008: Deuterium retention in NSTX with lithium conditioning C.H. Skinner, H.W. Kugel, L. Roquemore, J.P. Allain, C.N. Taylor, V. Soukhanovskii Fuel retention is an important constraint in the selection of plasma facing materials for next-step tokamaks. Gas balance measurements of retention in NSTX have been performed before- and with lithiumization of the vessel. The gas retained in ohmic discharges was measured by comparing the vessel pressure rise after a discharge to that of a gas-only pulse with the pumping valves closed. For neutral beam heated discharges the gas input and gas pumped by the NB cryopanels was tracked. Preliminary results show high ($\sim$ 90\%) prompt retention both with- and without lithiumization. Outgassing of deuterium follows, initially at a high rate that then slowed over the following 24 hours to become comparable to the baseline vessel presure rate of rise and reduced the retention to the $\sim$ 50\% level. Four material samples were exposed to the plasma and analysed in-vacuo the same evening in order to investigate the fundamental processes governing particle balance with lithium coatings. [Preview Abstract] |
Tuesday, November 3, 2009 3:36PM - 3:48PM |
JO4.00009: Error Field Physics and Correction at High Beta in NSTX Stefan Gerhardt, Jon Menard, Jong-Kyu Park, Ron Bell, Dave Gates, Benoit LeBlanc, Steve Sabbagh, Howard Yuh NSTX experiments over the past two years have made progress in understanding the physics and correction of both resonant and non-resonant error fields (EFs). By varying the amplitude and phase of an applied n=3 field, it has been demonstrated that there is an intrinsic n=3 EF. The EF source has been isolated to the main vertical field coil by experimentally determining the optimal correction for various values of plasma current, toroidal field, and elongation. This coil is measured to be slightly out of round, producing a dominantly n=3 EF; both vacuum field and neoclassical toroidal viscosity torque calculations indicate a correcting field similar in magnitude and phase to the experimental correction. Measurements of n=1 EF penetration at high-beta show that the critical error field for driving an island is substantially smaller than the standard density scaling would predict. The critical role of the high-beta plasma response in determining the penetration threshold will be discussed, using results from the IPEC code. This research is funded by the US Department of Energy contract {\#} DE-AC02-09CH11466. [Preview Abstract] |
Tuesday, November 3, 2009 3:48PM - 4:00PM |
JO4.00010: Search for Multiple Resistive Wall Modes at High Normalized Beta in NSTX S.A. Sabbagh, J.W. Berkery, J.M. Bialek, L. Delgado-Aparicio, K. Tritz, R.E. Bell, S.P. Gerhardt, B. LeBlanc Resistive wall mode (RWM) research has focused on the behavior and control of the least stable mode. However, at sufficiently high normalized beta, multi-mode theory suggests that stable modes may significantly influence the plasma dynamics. [1] This may affect active RWM control reliability in high beta plasmas. [2] Experiments at very high normalized beta have been conducted in NSTX to search for direct evidence of multi-mode RWM dynamics. Discharges were created with normalized beta reaching 7.4 in conditions with the ideal MHD no-wall limit near 4. Evidence of stable RWMs is examined by both kinetic and magnetic means. Characteristics are consistent with RWM behavior. At sufficiently high normalized beta, stable RWM-level low frequency activity is observed. X-ray data amplitude depends on beta, shows the activity to be global, and propagates in the co-NBI direction at frequencies near the expected peak in resonant magnetic field amplification. Observed growing RWMs appear to be separate from the stable mode activity, with different radial extent, supporting the hypothesis of multiple modes. [1] A.H. Boozer, Phys. Plasmas \textbf{10} (2003) 1458. [2] S.A. Sabbagh, et al., Phys. Rev. Lett. \textbf{97} (2006) 045004. [Preview Abstract] |
Tuesday, November 3, 2009 4:00PM - 4:12PM |
JO4.00011: On the Analysis of HHFW Heated Plasmas in NSTX B.P. LeBlanc, R.E. Bell, L.A. Berry, P. Bonoli, D.L. Green, R.W. Harvey, J.C. Hosea, E. Mazzucato, C.K. Phillips, A.L. Roquemore, P.M. Ryan, G. Taylor, J.R. Wilson, J. Wright, H. Yuh Recent developments in the HHFW research program include a better understanding of edge effects that has enabled improved heating, leading to Te $\sim $ 5 KeV, and installation of an upgraded antenna that should nearly double power delivery. While the time-dependent analysis of a HHFW-only discharge can be simulated using the TORIC module in TRANSP, a self consistent treatment of the fast ion evolution during combined HHFW and NBI heating is not yet possible because the computed power deposited into the fast ions is not transferred into the fast particles via Fokker-Planck calculations. To remedy this problem we intend to use CQL3D with wave fields from the GENRAY, AORSA or TORIC codes to evolve the fast-ion distribution at a given time-slice. Combining these results with TRANSP should improve understanding of the behavior of the fast ions during HHFW heating. [Preview Abstract] |
Tuesday, November 3, 2009 4:12PM - 4:24PM |
JO4.00012: Point-source helicity injection for ST plasma startup in {\sc Pegasus} A.J. Redd, D.J. Battaglia, M.W. Bongard, R.J. Fonck, D.J. Schlossberg Plasma current guns are used as point-source DC helicity injectors for forming non-solenoidal tokamak plasmas in the {\sc Pegasus} Toroidal Experiment. Discharges driven by this injection scheme have achieved $I_p \ge$ 100 kA using $I_{\rm inj} \le$ 4 kA. They form at the outboard midplane, transition to a tokamak-like equilibrium, and continue to grow inward as $I_p$ increases due to helicity injection and outer- PF induction. The maximum $I_p$ is determined by helicity balance (injection rate {\em vs} resistive dissipation) and a Taylor relaxation limit, in which $I_p \propto \sqrt{I_{\rm TF} I_{\rm inj}/w}$, where $w$ is the radial thickness of the gun-driven edge. Preliminary experiments tentatively confirm these scalings with $I_{\rm TF}$, $I_{\rm inj}$, and $w$, increasing confidence in this simple relaxation model. Adding solenoidal inductive drive during helicity injection can push $I_p$ up to, but not beyond, the predicted relaxation limit, demonstrating that this is a hard performance limit. Present experiments are focused on increasing the injection voltage ({\em i.e.}, helicity injection rate) and reducing $w$. Near-term goals are to further test scalings predicted by the simple relaxation model and to study in detail the observed bursty $n$=1 activity correlated with rapid increases in $I_p$. [Preview Abstract] |
Tuesday, November 3, 2009 4:24PM - 4:36PM |
JO4.00013: Absorber arc mitigation during CHI on NSTX D. Mueller, M.G. Bell, A.L. Roquemore, R. Raman, B.A. Nelson, T.R. Jarboe A method of non-inductive startup, referred to as transient coaxial helicity injection (CHI), was successfully developed on the Helicity Injected Torus (HIT-II) experiment and employed on the National Spherical Torus Experiment (NSTX). This technique has produced 160 kA of plasma current on closed flux surfaces. Over 100 kA of the CHI current has been coupled to inductively driven current ramp-up. In transient CHI, a voltage is applied across the insulating gap separating the inner and outer vacuum vessel and gas is introduced at the lower gap (the injector). The resulting current in the injector follows the helical magnetic field connecting the electrodes, forms a toroidal current and expands into the vacuum vessel. At higher CHI current, the poloidal field due to the plasma can connect the inner and outer vessels at the insulating gap at the top (called the absorber) of NSTX and lower the impedance there. This results in arcs in the absorber which are a source of impurities and which reduce the desired current in the injector. Two coils installed in the absorber will be used to reduce the magnetic field across the absorber gap and mitigate the absorber arcs. [Preview Abstract] |
Tuesday, November 3, 2009 4:36PM - 4:48PM |
JO4.00014: Overview of recent results from MAST Simon Pinches The MAST programme continues to address key issues for ITER and beyond, whilst undertaking research and development to establish the physics basis for a CTF based upon a spherical tokamak. The programme is benefiting from a range of technical enhancements which include: two long-pulse neutral beam PINIs; a twelve coil array of internal coils for ELM control; a multi-chord MSE diagnostic with 35 channels, 2.5 cm spatial resolution and down to 0.5ms time resolution, a new long pulse 28 GHz gyrotron (on loan from ORNL) allowing EBW start-up studies at higher power; an upgraded Thompson scattering system (240Hz, 120 core channels) together with a ``smart'' triggering system; a divertor science facility; and a disruption mitigation valve (on loan from FZJ). Recent studies include: ELM control using RMPs; transport and confinement including the effects of collisionality; off-axis current drive including the impact of fast particle instabilities; a campaign of counter injected neutral beam; and NTV studies in collaboration with the NSTX team. This presentation will provide an overview of recent results and future plans. Funded by EPSRC and EURATOM. [Preview Abstract] |
Tuesday, November 3, 2009 4:48PM - 5:00PM |
JO4.00015: Exploitation of high resolution beam spectroscopy diagnostics on MAST Clive Michael, Maarten Debock, Neil Conway, Rob Akers, Lynton Appel, Anthony Field, Mike Walsh, Marco Wisse Recent developments in beam spectroscopy on MAST, including CXRS, MSE and a pilot FIDA system have revealed new information about phenomena such as ITBs, MHD instabilities, transport and fast particle physics. For example, ITBs in the ion temperature and toroidal rotation have been observed with the 64ch CXRS system, while reverse-shear q profiles have been observed with the recently commissioned 35ch MSE system. Thus, the synergy of these diagnostics helps us to understand, among other things, the role of magnetic and rotational shear on ITBs. MSE measurements have also helped to understand MHD phenomena such as locked modes (characterized by changes in toroidal momentum, revealed by CXRS), sawteeth, and internal reconnection events. Finally, the temporal/spatial resolution and SNR of the MSE system have been exploited. Interesting results include the detection of low frequency ($\sim $2kHz) magnetic field fluctuations, characterization of the radial structure of higher frequency ($<$100kHz) broadband and coherent density (BES) fluctuations, and the identification of short scale length features ($\sim $1.8cm) in the current profile near the edge pedestal. [Preview Abstract] |
Tuesday, November 3, 2009 5:00PM - 5:12PM |
JO4.00016: Modeling of Optimization and Control of EBW Heating and Current Drive Jakub Urban, Joan Decker, Yves Peysson, Josef Preinhaelter, Gary Taylor, Linda Vahala, George Vahala We present a modeling of Electron Bernstein waves (EBWs) by recently coupled AMR (Antenna---Mode-conversion---Ray-tracing) and LUKE (3D Fokker-Planck) codes. The electrostatic EBW is a promising candidate for localized heating and current drive in high-$\beta$ plasmas, where the standard electron cyclotron O- and X-waves are cutoff. EBW heating and current drive is simulated here in spherical tokamak conditions, particularly in typical NSTX and MAST equilibria and also in equilibria predicted by transport modeling. The EBW injection parameters are varied in order to find optimized scenarios and a possible way to control the deposition location and the driven current. This task is rather challenging because EBW ray trajectories and $N_{\parallel}$ spectra are strongly dependent on the plasma parameters. [Preview Abstract] |
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