Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session BI2: MFE: ScenariosInvited
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Chair: Saskia Mordijck, College of William and Mary Room: 210 CDGH |
Monday, October 31, 2016 9:30AM - 10:00AM |
BI2.00001: Critical Gradient Threshold for Alfv\'en Eigenmode Induced Fast-Ion Transport* Invited Speaker: W.W. Heidbrink Experiments on the DIII-D tokamak have identified how multiple simultaneous Alfv\'en eigenmodes (AEs) lead to overlapping wave-particle resonances and stochastic fast ion transport in fusion grade plasmas [1]. The behavior results in a sudden increase in fast ion transport at a threshold that is well above the linear stability threshold for Alfvén instability. This causes fast ion transport to become stiff, leading to virtually unchanged equilibrium fast-ion density profiles as beam drive increases further. A novel beam modulation technique [2] in conjunction with an array of fast-ion diagnostics probes the critical gradient by measuring the fast-ion flux in different phase-space volumes. Above a threshold, which occurs when more than four AEs are simultaneously destabilized, the modulated flux suddenly increases. Fast-ion D$\alpha$ (FIDA) spectroscopy indicates the peak of the modulated flux is localized to mid-core radii, corresponding to the radial location of AEs. As distributions and instability behavior are manipulated further through variations in electron cyclotron heating and beam deposition, measured thresholds track the resulting shifts in resonances. Well above threshold, the fast-ion losses often become intermittent and exhibit a bursty behavior. Theoretical analysis confirms that fast-ion orbits become stochastic in the measured modes. This critical gradient transport, wherein the fast-ion pressure gradient destabilizes AEs and the fast ions respond by diffusing in phase space to flatten the pressure profile, suggests that reduced models for fast ion transport in ITER can effectively describe the fusion alpha and beam ion profiles.\par [1] C.S. Collins et al., Phys. Rev. Lett. 116 (2016) 095001\par [2] W.W. Heidbrink et al., Nucl. Fusion 56 (2016) in press\par *Work supported by the US Department of Energy under DE-FC02-04ER54698. [Preview Abstract] |
Monday, October 31, 2016 10:00AM - 10:30AM |
BI2.00002: Validation of the Model for ELM Suppression with 3D Magnetic Fields Using Low Torque ITER Baseline Scenario Discharges in DIII-D Invited Speaker: R.A. Moyer Suppression of Edge Localized Modes (ELMs) is lost in ITER Baseline Scenario discharges when the torque $T_{inj}$ and toroidal rotation $\nu_\phi$ are reduced. This is due to a shift in the tearing response deeper into the plasma. ELM suppression is recovered by reducing the normalized plasma pressure $\beta_N$. In H-mode plasmas, edge turbulence is suppressed in a “pedestal” region, leading to large pressure gradients that trigger MHD instabilities (ELMs), causing rapid heat expulsion. 3D magnetic fields are used to drive resonances that limit the pedestal width preventing the ELM. Reducing $T_{inj}$ changes the $\nu_\phi$ profile such that the drive shifts to a resonance deeper in the plasma, allowing the pedestal to grow again to MHD instability. Reducing $\beta_N$ reduces the edge pressure, which reduces the electron diamagnetic flow and moves the drive to a resonance that is closer to the boundary, recovering suppression. In two-fluid theory, the tearing response occurs at a resonant surface where the electron perpendicular rotation $\omega_({\bot e})\sim0$. Linear two-fluid resistive MHD simulations show that the tearing response shifts to a resonance deeper in the plasma when $T_{inj}$ is reduced. The experimental results confirm that the tearing response occurs for a resonance where $\omega_({\bot e})\sim0$, and suggest that the transport which limits the pedestal width is linked to the tearing response, even if any islands are predicted to be small. Although this model describes the differences between ELMing and ELM suppressed H-modes, it doesn’t address the transition from ELMs to suppression, because $\omega_({\bot e})\sim0$ is initially too deep ($\psi_N\le$0.9, q=3) for the tearing response to limit the pedestal width. Although understanding this transition is important, ITER must apply the RMP in L-mode to avoid the first ELM. These results suggest that manipulation of the edge rotation profile will be important to optimize ELM suppression in future tokamaks. [Preview Abstract] |
Monday, October 31, 2016 10:30AM - 11:00AM |
BI2.00003: Comparative investigation of ELM control based on toroidal modelling of plasma response to RMP fields Invited Speaker: Yueqiang Liu The type-I edge localized mode (ELM), bursting at low frequency and with large amplitude, can channel a substantial amount of the plasma thermal energy into the surrounding plasma-facing components in tokamak devices operating at the high-confinement mode, potentially causing severe material damages. Learning effective ways of controlling this instability is thus an urgent issue in fusion research, in particular in view of the next generation large devices such as ITER and DEMO. Among other means, externally applied, three-dimensional resonant magnetic perturbation (RMP) fields have been experimentally demonstrated to be successful in mitigating or suppressing the type-I ELM, in multiple existing devices. In this work, we shall report results of a comparative study of ELM control using RMPs. Comparison is made between the modelled plasma response to the 3D external fields and the observed change of the ELM behaviour on multiple devices, including MAST, ASDEX Upgrade, EAST, DIII-D, JET, and KSTAR. We show that toroidal modelling of the plasma response, based on linear and quasi-linear magnetohydrodynamic (MHD) models, provides essential insights that are useful in interpreting and guiding the ELM control experiments. In particular, linear toroidal modelling results, using the MARS-F code, reveal the crucial role of the edge localized peeling-tearing mode response during ELM mitigation/suppression on all these devices. Such response often leads to strong peaking of the plasma surface displacement near the region of weak equilibrium poloidal field (e.g. the X-point), and this provides an alternative practical criterion for ELM control, as opposed to the vacuum field based Chirikov criteria. Quasi-linear modelling using MARS-Q provides quantitative interpretation of the side effects due to the ELM control coils, on the plasma toroidal momentum and particle confinements. The particular role of the momentum and particle fluxes, associated with the neoclassical toroidal viscosity, will also be reported. Finally, predictive simulations are carried out for ITER and DEMO plasmas, for the purpose of the RMP configuration optimization, in terms of both the coil geometry and the coil currents. [Preview Abstract] |
Monday, October 31, 2016 11:00AM - 11:30AM |
BI2.00004: Confinement improvement in the high poloidal beta regime on DIII-D and application to steady-state H-mode on EAST Invited Speaker: S. Ding Systematic experimental and modeling investigations on DIII-D and EAST show attractive transport properties of fully non-inductive high $\beta_P$ plasmas. The improved understanding is used to develop steady state scenarios for ITER and CFETR. Experiments on DIII-D show that the large-radius internal transport barrier (ITB), a key feature providing improved performance in the high $\beta_P$ regime, is maintained when the scenario is extended from $q_{95}{\sim10}$ to 7 and from rapid to near-zero plasma rotation. The robustness of confinement versus rotation was predicted by gyrofluid modeling showing dominant neoclassical ion energy transport even without E$\times$B shear effect on turbulence suppression. Measured electron turbulent transport is large when ion turbulent transport is low, consistent with recent multi-scale simulations. With decreasing $q_{95}$, dominant turbulent transport shifts from electrons to ions, which exceeds the neoclassical ion transport level, and may set a $q_{95}$ limit for the large-radius ITB regime. Experiments also show that the ITB is lost below $\beta_N\sim1.5$, when long wavelength turbulence increases in agreement with predictions of turbulence suppression by Shafranov shift. In DIII-D, a broad current profile enabling large radius ITB is accessed via early heating and sustained with high bootstrap current fraction. Experiments on EAST show that a broad current profile can be accessed and sustained exploiting a large fraction of lower hybrid wave current drive (LHCD). Results show that as the electron density is increased, the fully non-inductive current profile broadens on EAST. Overall, these results provide encouragement that high performance high $\beta_P$ regimes can be extended to lower safety factor and very low rotation, providing potential paths to steady state in ITER and CFETR. [Preview Abstract] |
Monday, October 31, 2016 11:30AM - 12:00PM |
BI2.00005: A plasma rotation control scheme for NSTX and NSTX-U Invited Speaker: Imene Goumiri Plasma rotation has been proven to play a key role in stabilizing large scale instabilities and improving plasma confinement by suppressing micro-turbulence. A model-based feedback system which controls the plasma rotation profile on the National Spherical Torus Experiment (NSTX) and its upgrade (NSTX-U) is presented. The first part of this work uses experimental measurements from NSTX as a starting point and models the control of plasma rotation using two different types of actuation: momentum from injected neutral beams and neoclassical toroidal viscosity generated by three-dimensional applied magnetic fields. Whether based on the data-driven model for NSTX or purely predictive modeling for NSTX-U, a reduced order model based feedback controller was designed. Predictive simulations using the TRANSP plasma transport code with the actuator input determined by the controller (controller-in-the-loop) show that the controller drives the plasma's rotation to the desired profiles in less than 100 ms given practical constraints on the actuators and the available real-time rotation measurements. This is the first time that TRANSP has been used as a plasma in simulator in a closed feedback loop test. Another approach to control simultaneously the toroidal rotation profile as well as $\beta_N$ is then shown for NSTX-U. For this case, the neutral beams (actuators) have been augmented in the modeling to match the upgrade version which spread the injection throughout the edge of the plasma. Control robustness in stability and performance has then been tested and used to predict the limits of the resulting controllers when the energy confinement time ($\tau_E$) and the momentum diffusivity coefficient ($\chi_{\phi}$) vary. [Preview Abstract] |
Monday, October 31, 2016 12:00PM - 12:30PM |
BI2.00006: C-2-Upgrade Field Reversed Configuration Experiment. Invited Speaker: Artem Smirnov In the C-2 field-reversed configuration (FRC) experiment, tangential neutral beam injection (20 - 40 keV hydrogen, 4 MW total), coupled with electrically-biased plasma guns at the plasma ends, magnetic end plugs, and advanced surface conditioning, led to dramatic reductions in turbulence-driven losses and greatly improved plasma stability [1,2]. Under such conditions, highly reproducible FRCs with a significant fast-ion population and total plasma temperature of about 1 keV were achieved [3]. The FRC's were macroscopically stable and decayed on characteristic transport time scales of a few milliseconds. In order to sustain an FRC configuration, the C-2 device was upgraded with a new neutral beam injection (NBI) system, which can deliver a total of 10$+$ MW of hydrogen beam power, by far the largest ever used in a compact toroid plasma experiment. Compared to C-2, the beam energy was lowered to 15 keV and angled injection geometry was adopted to provide better beam coupling to the FRC. The upgraded neutral beams produce a dominant fast ion population that makes a dramatic beneficial impact on the overall plasma performance [4]. Specifically: (1) high-performance, advanced beam-driven FRCs were produced and sustained for times significantly longer (5$+$ ms) than all characteristic plasma decay times without the beams, (2) the sustainment is fully correlated with neutral beam injection, (3) confinement of fast ions is close to the classical limit, and (4) new, benign collective fast ion effects were observed. Collectively, these accomplishments represent a dramatic advance towards the scientific validation of the FRC-based approach to fusion. This talk will provide a comprehensive overview of the C-2U device and recent experimental advances. [1] M. Tuszewski et. al, Phys. Rev. Lett 108, 255008 (2012). [2] H.Y. Guo et al., Nature Comm 6, 6897 (2015). [3] M.W. Binderbauer et al., Phys. Plasmas 22, 056110 (2015). [4] M.W. Binderbauer et al., AIP Conf. Proc. 1721, 030003 (2016). [Preview Abstract] |
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