Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session BO5: DIII-D Tokamak |
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Chair: Walter Guttenfelder, PPPL Room: OCC B113-114 |
Monday, November 5, 2018 9:30AM - 9:42AM |
BO5.00001: Overview of Recent DIII-D Experimental Results* Max Fenstermacher Recent DIII-D experiments contributed to the ITER physics basis and to physics understanding for extrapolation to future devices. RMP ELM suppression and QH-mode operation advance toward ITER rotation and shape. GPEC multi-modal plasma response predictions were extended and validated for MHD damping rates and NTV torque control. Multiple locked islands are essential for deterioration of thermal confinement preceding thermal quench. Machine Learning algorithms predict disruptions correctly >90% of the time with <5% false positives. P_L-H found to have previously unidentified dependence on Ip or q95 at mid-density (ne~3e19 m-3). Detachment found at 20–30% lower ne,ped in a closed divertor, due to decreased pedestal fueling and increased SOL dissipation, both from increased neutral trapping in the divertor. Fluid codes factor of 3 under-prediction of radiating volume may contribute to divertor radiation shortfall at detachment. Data shows more than 10x reduction in erosion with slight positive voltage biasing due to reduced incident energy and ion flux. Tungsten sourcing during ELMs by D and low-Z impurities consistent with new ‘free-streaming’ model predictions of scaling with ELM deposited energy density |
Monday, November 5, 2018 9:42AM - 9:54AM |
BO5.00002: Measurement of impurity assimilation into the post-disruption runaway electron plateau in DIII-D and comparison with the plasma vertical loss rate Eric Hollmann, I. Bykov, R.A. Moyer, A. Pigarov, D. Rudakov, D. Shiraki, J. Herfindal, C. Lasnier, N.W. Eidietis, A. Lvovskiy, P. Parks, C. Paz-Soldan Experiments on the DIII-D tokamak studied the assimilation of high-Z impurities into the runaway electron (RE) plateau and the effect on the current channel vertical loss. It is found that the equilibrium impurity assimilation into the RE plateau can be explained by a balance between the neutral gas ionization source term and the impurity ion radial transport out to the wall, with significant (>50%) of the impurities typically residing as neutral gas outside the RE beam. Radial ion transport is found to be significantly (>10x) faster than neoclassical (collisional) transport. A clear saturation in assimilation efficiency is observed, but this saturated assimilated quantity can be increased by increasing power input into the REs or background plasma, consistent with the assimilation being ionization source term-limited. The experiments find that the RE plateau vertical loss rate is not directly proportional to the current dissipation rate, with a slow initial vertical loss and faster current dissipation, suggesting that the evolution of the current profile is playing a role. |
Monday, November 5, 2018 9:54AM - 10:06AM |
BO5.00003: Advances in Wide-Pedestal Quiescent H-mode on DIII-D Towards ITER Conditions Xi Chen, Keith H Burrell, Tom H Osborne, Kshitish Kumar Barada, Darin R Ernst, Colin Chrystal, B.A. Grierson, George Mckee, Tomas Odstrcil, Carlos Alberto Paz-Soldan, C Craig Petty, Terry L Rhodes, J. Chris Rost, Wayne M Solomon, Theresa M Wilks, Zheng Yan, Lei Zeng Significant parameter space expansion of the wide-pedestal quiescent H-mode (QH-mode) towards ITER and reactor conditions has been achieved in DIII-D. This natural ELM-stable QH regime was first discovered when the NBI torque was ramped from strong ctr-Ip towards net-zero in balanced double null standard QH plasma. Recent experiments reveal it can operate at a large range of NBI torque fully covering the scaled ITER-equivalent NBI torque. Stationary wide-pedestal QH-mode with net-zero injected NBI torque throughout the discharge has also been demonstrated; zero torque operation greatly benefits from using NTV from 3D fields to maintain enough plasma rotation to avoid locking. Plasma shapes with wide-pedestal QH operation have been expanded to include LSN with ITER δavg. In the assessment of its compatibility with RF-heated zero-torque operation, confinement improvements with ECH were observed in some conditions and the plasma remains ELM-stable at the power ratio of ECH to NBI up to 3:1. Initial results of impurity transport in wide-pedestal QH will be presented. Work is continuing to lower q95 for higher fusion gain (to date, obtained with 4.7<q95<7.5, transiently 3.8) |
Monday, November 5, 2018 10:06AM - 10:18AM |
BO5.00004: Resolving the Rotation Threshold for RMP-ELM Suppression Carlos Alberto Paz-Soldan, Yueqiang Liu, Brendan C Lyons, Thomas Osborne, David Weisberg, Raffi Nazikian, Nikolas C Logan, Andrew Kirk, Wolfgang Suttrop, Matthew T Beidler, James D Callen, George Mckee Varying the neutral beam injection (NBI) mix reveals pedestal-top toroidal rotation thresholds for RMP-ELM suppression. The threshold rotation increases when the pedestal width is reduced. Observations are interpreted by considering zero-crossings in the flow as expected for the RMP penetration mechanism. A critical radius for the ExB rotation (wExB) zero-crossing is found at the exit from ELM suppression. No such constraint is observed for the electron perpendicular flow (w⟂e), challenging linear two-fluid models. The increased threshold in the narrow pedestal is consistent with a more outward critical radius for the wExB zero-crossing. The observed thresholds are not found at constant injected NBI torque, due to variations in the torque deposited locally at the edge. Understanding the rotation threshold is of great importance in predicting the ability of RMPs to suppress ELMs in future tokamaks such as ITER, where the rotation is expected to be small and consequently the zero-crossings far from the pedestal. |
Monday, November 5, 2018 10:18AM - 10:30AM |
BO5.00005: Plasma Response in Single Null and Double Null Plasmas Morgan W Shafer, Carlos Alberto Paz-Soldan, Todd E Evans, Alan Douglas Turnbull, Brendan C Lyons, David B Weisberg, Xiaodi D Du, Satoshi Ohdachi, Yasuhiro Suzuki The 3D plasma response to applied non-axisymmetric magnetic fields decreases significantly as the plasma shape transitions from single null to double null divertor configuration, indicating a role of the separatrix and scrape-off-layer in determining the internal plasma response. This may provide a reason as to why full RMP ELM suppression in double-null divertor configuration plasmas has been elusive—coupling to the high-field side (HFS) plasma response has been previously correlated with ELM suppression. In single null plasmas, the measured plasma response via magnetics shows both a HFS and low-field side (LFS) component consistent in size with previous results. Additionally, internal kink-like perturbations in the steep pedestal gradient region measured by Soft X-Ray (SXR) imaging are observed in the X-point region. Transitioning from a single null to balanced double null configuration reduces or damps the HFS magnetics response by 60% and similarly damps the perturbations measured by SXR imaging on the inner side or HFS of the X-point. Linearized time-independent extended MHD modeling similarly shows a reduction in HFS response in double null configurations. |
Monday, November 5, 2018 10:30AM - 10:42AM |
BO5.00006: High Performance Core-Edge Solutions in Super H-Mode Philip B Snyder, Tom Osborne, Carlos Paz-Soldan, Wayne M Solomon, David Eldon, Todd E Evans, Brian A Grierson, Richard Groebner, Jerry W Hughes, Matthias Knolker, Florian Laggner, Anthony W Leonard, Orso Meneghini, Saskia Mordijck, Thomas Petrie, Huiqian Wang, Jonathan Watkins, Howard R Wilson The Super H-Mode (SH) regime is predicted to enable pedestal height and fusion performance substantially higher than standard H-mode operation. This regime exists due to a bifurcation of the pedestal pressure as a function of density, predicted by the EPED model to occur in strongly shaped plasmas above a critical density. The SH regime can have pedestal pressure twice as high, and collisionality 4x lower, than the standard H-mode at the same density. Because the pedestal in SH mode is limited by current-driven modes, increasing the near separatrix density to enable attractive divertor solutions is predicted to be compatible with high fusion performance in the core (unlike in standard H-modes). DIII-D SH experiments have achieved record levels of fusion gain on a medium scale tokamak, and have sustained high performance using 3D magnetic perturbations. New experiments have employed D2 and N2 gas to improve divertor conditions. High pedestal pressure (>20kPa) and core confinement (τE~.15s) are sustained across a 30x gas scan, and with a strongly radiating divertor with a 3x reduction in divertor Te. We discuss DIII-D results, and further predictions for DIII-D and ITER. |
Monday, November 5, 2018 10:42AM - 10:54AM |
BO5.00007: Some important issues for radiating mantle and divertor operation in AT plasmas* T. W. Petrie, T. Osborne, C. Petty, T. Turco, B. Grierson We report on recent experiments that directly relate to the feasibility of applying radiating mantle or radiating divertor approaches to high performance single-null (SN) and double-null (DN) plasmas: H98 ≈ 1.3–1.7, betaN ≈ 3-4, and PIN ≤ 15 MW. We explore the interplay of magnetic balance and divertor closure on controlling argon seed and deuterium inventory. Our results indicate that inner divertor pumping was far more important in controlling argon and deuterium inventories in SNs than it was for DNs. Detachment from the upper closed divertor led to strong reductions in divertor heat flux but also resulted in a rapid buildup in both carbon (intrinsic) and argon (injected) in the main plasma. Regardless of whether neon (low-Z) or argon (medium-Z) seeds were used, MHD activity evolved in essentially the same sequence, i.e., 4/2, 5/2, 3/1, and each mode added to the cumulative toll on tE. Lowering the seed injection rates did not suppress these modes but only delayed their onset. |
Monday, November 5, 2018 10:54AM - 11:06AM |
BO5.00008: Target Concavity as a Design Parameter for Closed Divertors Facilitating Detachment Brent Covele, L. Casali, Huiqian Wang, Morgan Shafer, Auna Louise Moser, Adam McLean, Cameron M Samuell, John Canik, Dan M Thomas, Houyang Guo Exploration of different divertor targets with SOLPS has suggested target concavity as being potentially important for closed slot divertors to facilitate detachment at lower nsep/ndiv via neutral trapping. Concave targets in SOLPS better position the neutral distribution with respect to the ion/electron distribution, increasing ndiv at the strike point by a factor of 2, and similarly reducing the heat flux, compared to similar slots with vertical or horizontal targets, with upstream Q|| = 450 MW/m2. Assume a divertor target, spanning 0 < ψn < 2λq of the SOL heat footprint, is divided into two conjoined segments oriented at angles θ1 and θ2 with respect to the separatrix normal. Concavity is approximated here simply as the difference θ1-θ2 < 0, where more negative values concentrate the neutral population more sharply. SOLPS modeling is presented for a divertor target which utilizes concavity to (1) maximize divertor volumetric dissipation, and (2) better confine neutrals to the divertor volume at detachment. Target probe Jsat and Te profiles from recent DIII-D experiments with the Small-Angle Slot (SAS) divertor qualitatively support the value of target concavity. |
Monday, November 5, 2018 11:06AM - 11:18AM |
BO5.00009: Deuterium Atomic and Molecular Radiation in DIII-D high-recycling divertor plasmas Mathias Groth, Steven L Allen, Petra Boerner, Sebastijan Brezinsek, Igor Bykov, Gerard Corrigan, Max E Fenstermacher, Aaro E Jaervinen, Derek Harting, Eric M Hollmann, Charles J Lasnier, Anthony W Leonard, Bartosz Lomanowski, Adam Mclean, Detlev Reiter, Cameron M Samuell, Morgan Shafer, Huiqian Wang, Jonathan Watkins, Sven Wiesen, Robert Wilcox EIRENE predictions of emission from the deuterium Lyman series and molecular bands in DIII-D high-recycling divertor plasmas show that Lyman-a emission is approximately an order of magnitude stronger than the Lyman-Werner molecular bands, indicating that emission from molecules plays a secondary role in the divertor radiation power balance. The underlying deuterium-carbon plasma, calculated by EDGE2D-EIRENE and serving as the basis for this assessment, electron temperatures of 1 eV at the low-field target plate are predicted as measured. The simulations, however, under-predict the measured electron density by a factor of 2 to 3, and thus the atomic and molecular densities. The impact of the magnitudes and spatial distributions of the plasma profiles on divertor deuterium emission is discussed. |
Monday, November 5, 2018 11:18AM - 11:30AM |
BO5.00010: Assessing erosion and retention of silicon carbide based materials in DIII-D divertor* Stefan Bringuier, Tyler W Abrams, Ezekial Unterberg, Dmitry Rudakov, Gokul Vasudevamurthy, Sean Gonderman, Robert Wilcox, Dean Alan Buchenauer, William Raymond Wampler, Guinevere Shaw, Brian Wirth, Dan Thomas, Leo Holland A series of experiments performed on DIII-D have exposed SiC based materials to a variety of plasma conditions using the DiMES apparatus in order to better characterize local gross erosion and hydrogenic retention. Our most recent results suggest that SiC is less susceptible to chemical sputtering as compared to graphite and is not solely due to the stoichiometric reduction in carbon. Furthermore, within the silicon physical sputtering regime, inclusion of silicon surface enrichment of SiC is necessary to account for differences between predictions and experiments. An extended effort to characterize the hydrogen retention of exposed samples and the influence of SiC microstructure are obtained using TDS, NRA, and LIMS-LIBS along with SEM and EBSD, as this is historically, a primary concern with SiC. The combination of techniques enables association of grain boundaries and orientation with aerial and depth profiles of deuterium implanted during DIII-D experiments. These results provide a needed dataset for SiC and further motivate in identifying plasma-facing materials that exhibit limited erosion and chemical reactivity but are thermally conductive, thermomechanically tough, and low-Z by nature. |
Monday, November 5, 2018 11:30AM - 11:42AM |
BO5.00011: Measurements and neoclassical modeling of deuterium and impurity toroidal rotation and temperature profiles in the H-mode pedestal and steep gradient region Shaun Haskey, Devon J Battaglia, Brian A Grierson, Arash Ashourvan, Emily A Belli, Colin Chrystal Advances in main-ion charge exchange recombination spectroscopy (MICER) have enabled first measurements of the simultaneous main-ion (D+) and impurity ion (C6+) properties from the core to the scrape off layer in a tokamak. This capability has revealed clear differences between the deuterium and impurity temperature and toroidal rotation near the separatrix. These differences are compared with neoclassical models including analytic (Kim-Diamond-Groebner), local drift-kinetic (NEO), and global drift-kinetic including the effect of neutrals (XGC0). A low temperature H-mode is studied where kinetic and non-local effects are expected to be small. Initial analysis shows the strong effect that an edge neutral population can have in reducing both the main ion toroidal rotation and temperatures, and their importance in bringing predictions into better agreement with experimental measurements. |
Monday, November 5, 2018 11:42AM - 11:54AM |
BO5.00012: Density profile peaking on DIII-D and JET – fuelling versus transport Antti Salmi, Tuomas Tala, Saskia Mordijck, Hans Nordman, Jerry W Hughes Dimensionless collisionality (ν*) scans with fixed density in H-mode plasmas on DIII-D and JET together with modelling have shown that the NBI fuelling plays a more important role in density peaking than found in earlier multi-machine datasets where the increased peaking at low collisionality has been primarily ascribed to turbulent effects. Perturbative gas puff modulations are performed to disentangle the fuelling and the transport effects. On DIII-D, we find that, at the lowest ν*, the pinch is directed outward and the diffusion coefficients are larger compared to higher ν* cases. With increasing ν*, the pinch becomes inward outside the mid-radius, while remaining outward inside the mid-radius. At the same time, a strong increase in ion-scale density fluctuations with reduced ν* is observed with BES and predicted with TGLF. On JET, perturbative transport analysis, predictive integrated modelling with JINTRAC+TGLF and non-linear and linear GENE runs all show that NBI fuelling provides an important contribution to density peaking. These results indicate that the density profiles on ITER, where NBI fuelling is very small, may be less peaked than previously thought. |
Monday, November 5, 2018 11:54AM - 12:06PM |
BO5.00013: Internal transport barrier dynamics at high poloidal beta on DIII-D and extensions towards high confinement scenarios for a fusion reactor Juan Huang, Xianzu Gong, Jinping Qian, Siye Ding, Wenfeng Guo, Chengkang Pan, Muquan Wu, Guoqiang Li, Qilong Ren, Andrea MV Garofalo, David Weisberg, Alan W Hyatt, John Roderick Ferron, Edward J Strait, Robert I Pinsker, Gary M Staebler, Lang Li Lao, Joseph McClenaghan, Christopher T Holcomb, George R McKee, Zheng Yan, Terry L Rhodes Towards the ITER steady-state goal of Q=5, recent DIII-D/EAST joint experiments on DIII-D have demonstrated sustainment of excellent energy confinement quality (H98y2>1.5) via a large radius ITB at high normalized beta (βN~3.5) and reactor-relevant q95~6.0. At high βP (>2), the large Shafranov shift can stabilize turbulence and create a bifurcation in kinetic ballooning mode (KBM) transport at large minor radius, leading to an improved confinement state. Large transient perturbations such as type-I ELMs can trigger this transition from the H-mode confinement state with a high edge pedestal to the higher confinement state with a lower pedestal and an ITB. At lower βp (≤2), negative magnetic shear in the plasma core contributes to turbulence suppression and can compensate for reduced Shafranov shift to still produce a large radius ITB and excellent confinement with low plasma rotation, consistent with results of gyrofluid transport simulations. However, resistive wall modes can be a limitation at simultaneous high βN, low internal inductance, and low rotation. Additional off-axis external current drive will further provide a more stable path towards operation at reduced q95. |
Monday, November 5, 2018 12:06PM - 12:18PM |
BO5.00014: Sustaining DIII-D High Beta Discharges Without the Sawteeth That Can Lead to Disruptions Nicholas Zane Taylor, Robert John La Haye, C. Craig Petty, Tim Luce, Paolo Piovesan We discuss three methods for avoiding or minimizing sawteeth (ST) in DIII-D. Two methods rely on relatively benign internal 3D perturbations to redistribute poloidal flux, causing the minimum safety factor (qmin) to stay above unity and thus avoiding sawteeth. The hybrid scenario on DIII-D requires the presence of an internal tearing mode (usually 3/2) for the “flux pumping”, i.e. anomalous broadening of the current profile. During stationary intervals in hybrid discharges, the rate of poloidal magnetic energy consumption is more than the rate of energy flow from the poloidal field coils. DIII-D discharges with a helical core induced by imposed rotating n=1 3D fields have a similar anomalous consumption of poloidal flux. A possible origin of the behavior in the hybrid scenario is that the 3/2 tearing mode and its 2/2 side band produce helical perturbations in the core velocity similar to that from the imposed n=1 magnetic field. These perturbations in each case produce a dynamo EMF that drives the current redistribution. The third approach is to apply electron cyclotron current drive just inside q=1 to reduce ST amplitude and increase ST frequency, keeping q on axis closer to one and thereby providing smaller seeds for NTMs. |
Monday, November 5, 2018 12:18PM - 12:30PM |
BO5.00015: Using Alfvén eigenmode control tools to improve steady state performance in DIII-D Cami S Collins, Michael A Van Zeeland, Christopher T Holcomb, William Walter Heidbrink, Mario L. Podesta, Francesca M Poli, Daisuke Shiraki, Markus Weiland Alfvén Eigenmodes (AEs) can significantly reduce performance in DIII-D steady state scenarios with negative central shear and qmin>2, resulting in measured neutron rates that are typically half of the classically expected values. In recent experiments, AE activity was altered by transiently increasing the density, broadening the density profile with injected deuterium pellets, radially scanning electron cyclotron current drive or heating, reducing neutral beam voltage, or applying n=2 resonant magnetic perturbations. In all cases, AEs remained strongly driven and improvement to performance was minimal. Experiment data are compared to AE simulations using the NOVA code and fast ion transport simulations using the kick model, which includes the impact of AEs on beam slowing-down and scattering calculations in TRANSP. Modeling will be used to understand and predict whether upgrades that are currently underway to increase DIII-D’s off-axis current drive will significantly affect AE stability and improve performance in this scenario. |
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