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 YI2: Tokamak Advanced Scenarios, Postdeadline Invited |
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Chair: Mickey Wade, General Atomics Room: Ballroom BD |
Friday, November 18, 2011 9:30AM - 10:00AM |
YI2.00001: Neoclassical Toroidal Viscosity from Non-Axisymmetric Magnetic Fields Allows ELM-free, Quiescent H-mode Operation in DIII-D under Reactor-relevant Conditions Invited Speaker: Application of static, non-axisymmetric magnetic fields (NAMFs) to high $\beta$ DIII-D plasmas allows sustained operation with an ELM-free, quiescent H-mode (QH-mode) edge under reactor-relevant conditions with torque from neutral beam injection (NBI) in the co-current direction. QH-mode is an ideal plasma for next step devices, exhibiting H-mode confinement levels while operating without ELMs at constant density and radiated power. The key to QH-mode operation is the presence of an edge-localized, electromagnetic mode, the edge harmonic oscillation (EHO), which increases the edge particle transport, allowing the plasma to reach a transport steady state at edge parameters just below the peeling-ballooning, ELM stability limit. Peeling-ballooning theory suggests and previous studies confirm that QH-mode operation requires sufficient radial shear in the toroidal rotation near the plasma edge. In most past experiments, this rotation shear was produced by torque from counter-directed NBI. In recent experiments, co-NBI torque was overcome by the counter torque due to neoclassical toroidal viscosity (NTV) produced by the applied NAMFs. Theoretical predictions of this NTV torque show promising levels of agreement with experimental measurements. The application of the NAMFs does not degrade the global energy confinement of the plasma; indeed, at the lower rotation achieved with balanced torque, the energy confinement times actually improve. By utilizing the physics of NTV from NAMFs, these new results open a path for QH-mode utilization in self-heated, burning plasmas, where toroidal momentum input from NBI will be small or absent. [Preview Abstract] |
Friday, November 18, 2011 10:00AM - 10:30AM |
YI2.00002: Developing the Core Physics Scenarios For Next Step STs Invited Speaker: Steady-state spherical torus devices for nuclear component testing, or possibly fusion power generation, will need high-beta, excellent confinement and non-inductive current drive. In pursuit of these goals, the National Spherical Torus Experiment (NSTX) has, in recent years, developed lithium conditioning of the plasma facing components, improved control of strongly shaped equilibria, and implemented routine feedback control of n=1 error fields and resistive wall modes. The synergistic combination of these techniques has produced non-inductive current fractions of 65-70{\%}, sustained for multiple current relaxation times, as well as the highest sustained values of toroidal beta, poloidal beta, and stored energy in NSTX. The confinement scaling with plasma current in these discharges is somewhat stronger than the previous NSTX scaling, and the toroidal field dependence is nearly absent. Intrinsic n=1 stability is improved by strong shaping and a broad pressure profile. Active n=1 control further improves reliability at high beta, although core n=1 kink/tearing instabilities often end the high-performance phase. Assessments of stability with the PEST-1, NOVA-K, M3D, and M3D-C1 codes show that these modes can be avoided by maintaining an elevated central q. The relaxed current profile in MHD-quiescent phases is well modeled by TRANSP with (neo)classical physics only. The effect of large TAE avalanches in elevating the central q has been simulated by including a transient fast ion diffusion; this produces a good match to the observed neutron emission drops and toroidal current profile. Using this overall physics basis, simulations with free-boundary TRANSP and DCON show that the higher toroidal field and off-axis beam current drive in NSTX-Upgrade can support a wide range of ideally stable non-inductive scenarios, and partially inductive operation with central safety factor $>$1 and toroidal beta$>$25{\%}. [Preview Abstract] |
Friday, November 18, 2011 10:30AM - 11:00AM |
YI2.00003: Pedestal and Transport Properties of Steady-state I-mode Plasmas over Expanded Operational Space in Alcator C-Mod Invited Speaker: I-mode operation on Alcator C-Mod combines a strong edge thermal transport barrier with L-mode levels of particle and impurity transport, allowing access to very high performance discharges with low pedestal collisionality and central temperatures up to 8 keV, and without large ELMs or other intermittent edge instabilities. In recent campaigns, C-Mod I-modes have been extended to quasi-steady-state, with access in both favorable and unfavorable ion drift directions and typical normalized energy confinement quality factor H$_{98} \quad \sim $1.0 to 1.2. Adding ICRF mode-conversion flow-drive enhances toroidal flow shear near the plasma edge and confinement is further enhanced. I-mode has been maintained with input power up to nearly 2x the I-mode threshold power, with the largest accessible range in closed divertor geometry at modest triangularity. Simple extrapolations at fixed field imply that ITER in unfavorable drift could access I-mode with available power, and stay in I-mode with alpha-dominant heating. Detailed pedestal fluctuation measurements reveal changes in the turbulence, with decreases in the power at some frequencies and size scales, and growth of a weakly coherent mode (WCM) (k$_{\theta }\sim $1.5 cm-1, $\delta $f/f $\sim $ .3) which propagates in the electron diamagnetic direction in the plasma frame. The WCM, which has density, temperature and magnetic signatures, appears to play a key role in pedestal density and impurity regulation, and detailed experimental results and associated modeling are presented. The distribution of divertor exhaust power depends on ion drift direction; new measurements of I-mode heat flux footprints on the outer divertor are compared with those in H-mode. Pedestal stability analyses will be shown for I-modes, including some which exhibited small ELMs. [Preview Abstract] |
Friday, November 18, 2011 11:00AM - 11:30AM |
YI2.00004: Sensitivity of Transport and Stability to the Current Profile in Steady-state Scenario Plasmas in DIII-D Invited Speaker: Recent experiments on DIII-D have provided the first systematic data on the impact of the current profile on the transport and stability properties of high-performance, steady-state scenario discharges. To achieve 100\% noninductive conditions, the current profile $J$ must be sustained by a large fraction of bootstrap current $J_{bs}$, which is nonlinearly coupled with the kinetic profiles. Systematic scans of $q_{min}$ and $q_{95}$ were performed to determine empirically the best alignment of the noninductive currents with $J$ and the variation of the transport properties with $q$. Transport analysis indicates that $\chi_e$ and $\chi_i$ are very sensitive to the details of $J$, in a way that makes the pressure profile peaking and $J_{bs}$ scale nonlinearly with both $q$ and $\beta$ in the experiment. Drift wave stability analysis using TGLF indicates the trends in the calculated linear growth rates do not reproduce experimental trends in $\chi$ with $q_{min}$ and $q_{95}$. At high $\beta$, necessary to maximize $f_{bs}$, the discharge duration is often limited by $n=1$ tearing modes, whose stability also depends on the $J$ profile. Broadly deposited EC current at mid-radius was found to supply part of the required noninductive current and to positively affect the tearing stability. The modes appear when $J_{ec}$ is turned off for stable cases, and always appear when the EC deposition is shifted outwards. The variation in the EC scan results is consistent with PEST3 calculations, showing that the tearing stability becomes extremely sensitive to small perturbations of the equilibrium in wall-stabilized discharges run close to the ideal MHD limit. These modeling results are being used to design new experiments with higher ideal and tearing limits. The new off-axis neutral beam injection system will be used to explore higher $q_{min}$ scenarios and different current alignments. [Preview Abstract] |
Friday, November 18, 2011 11:30AM - 12:00PM |
YI2.00005: Two-dimensional imaging of edge-localized filaments in KSTAR \textit{H}-mode plasmas Invited Speaker: Gunsu S. Yun The temporal evolution of edge-localized modes (ELM) has been studied using a 2-D electron cyclotron emission imaging (ECEI) system in KSTAR.\footnote{G. S. Yun \emph{et al.}, Phys. Rev. Lett. \textbf{107}, 045004 (2011).} The ELMs are observed to evolve in three distinctive stages: initial linear growth of multiple filamentary structures having a net poloidal velocity, interim saturated state, and final crash. The crash phase, typically consisting of multiple bursts of a single filament, involves a complex dynamics; an abrupt change in the poloidal mode number, poloidal elongation of the bursting filament, development of a fingerlike bulge, and fast localized burst through the finger. A significant alteration of the ELM dynamics, such as mode number, poloidal flows, and crash time scale, has been revealed during a recent investigation of external magnetic perturbations ($n=1$) on ELMs. [Preview Abstract] |
Friday, November 18, 2011 12:00PM - 12:30PM |
YI2.00006: Lower hybrid current drive at high density in the multi-pass regime Invited Speaker: Gregory M. Wallace Assessing the performance of lower hybrid current drive (LHCD) at high n$_{e}$ is critical for developing non-inductive current drive systems on ITER and future steady-state experiments. Excellent LHCD efficiency has been observed during fully non-inductive operation at moderate n$_{e}$ ($\eta $ = 2.0-2.5x10$^{19}$ AW$^{-1}$m$^{-2}$ at n$_{e}$ = 0.5x10$^{20}$ m$^{-3})$ on Alcator C-Mod under conditions (n$_{e}$, magnetic field and topology, LHCD frequency) relevant to ITER. To extend these results to advanced tokamak regimes with higher bootstrap current fractions on C-Mod, it is necessary to increase n$_{e}$ to 1.0-1.5x10$^{20}$ m$^{-3}$. However, the number of current-carrying, non-thermal electrons generated by LHCD drops sharply in diverted configurations at densities that are well below the density limit previously observed on limited tokamaks. In these cases, increased SOL currents and changes in edge ionization and density profiles are observed during LHCD, indicating that significant power is transferred from the LH waves to the SOL. Fokker-Planck simulations of these discharges utilizing ray tracing and full-wave propagation codes indicate that LH waves in the high n$_{e}$, multi-pass absorption regime pass through the SOL several times, where parasitic losses can play a significant role in the loss of current drive efficiency. Modeling predicts that LHCD efficiency increases with stronger single-pass absorption, independent of the loss mechanism in the edge and SOL. Experimental data show that increasing T$_{e}$ in high n$_{e}$ LH discharges results in higher non-thermal electron emission, as predicted by the models. Recent developments in simulation codes have expanded our understanding of the mechanisms involved in reducing current drive efficiency at high density, thereby enhancing our capability to predict the performance of LHCD for future fusion facilities. [Preview Abstract] |
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