Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session CI1b: Transport Barrier Physics |
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Chair: Charles Bush, Oak Ridge National Laboratory Room: Adam's Mark Hotel Plaza Ballroom ABC |
Monday, October 24, 2005 3:00PM - 3:30PM |
CI1b.00001: Structure and Motion of Edge Turbulence in NSTX and Alcator C-Mod Invited Speaker: This talk will describe recent 2-D imaging measurements of edge turbulence in NSTX and present some comparisons with Alcator C-Mod and with theory and simulation. The measurements were made using the visible line emission from localized neutral deuterium or helium gas puffs [1-3]. Ultra-high speed movies of the turbulent structure and motion of this light emission were made in both NSTX and C-Mod with a spatial resolution of 64 pixels (poloidally) x 64 pixels (radially), with 300 time frames at 250,000 frames/sec. In Ohmic and L-mode plasmas the edge is always highly turbulent and often contains localized structures (``blobs'') which move at about 1 km/sec both radially and poloidally. A model has been developed to convert the local light emission to density and temperature for comparisons of the observed blob motion in NSTX with analytic scalings [4] and 2D simulations. Movies of the L-H transition in NSTX will be presented and analyzed; the transition evolves from a turbulent state to a more quiescent state without much apparent change in the local turbulence flows. Additional NSTX phenomena will be shown if time allows (e.g. ELMs, MHD, etc.). The imaging results in NSTX will be compared with those in C-Mod and with turbulence simulations where possible. Opportunities for additional comparisons between experiment and theory will be discussed.\newline [1] S.J. Zweben et al, Nucl. Fus. 44, 134, 2004 \newline [2] R.J. Maqueda et al, Rev. Sci. Inst. 74, 2020, 2003 \newline [3] J.L. Terry et al, Phys. Plasmas 10, 1739, 2003 \newline [4] J.R. Myra and D.A. D'Ippolito, Lodestar Report {\#}LRC-05-105 (2005). \newline \newline In collaboration with: R.J. Maqueda, J.L. Terry, D. D'Ippolito, O. Grulke, J.A. Krommes, T. Munsat, J. Myra, D. Russell, D. Stotler, T. Stoltzfus-Dueck, M. Umansky, A.E. White, K.M. Williams, X. Xu and the NSTX and Alcator C-Mod Teams [Preview Abstract] |
Monday, October 24, 2005 3:30PM - 4:00PM |
CI1b.00002: Advances in measurement and modeling of the H-mode pedestal on Alcator C-Mod Invited Speaker: Edge transport barriers (ETBs) forming pedestals are crucial in determining H-mode plasma confinement. ETB studies on Alcator C-Mod have improved understanding in several areas, including pedestal scalings, edge stability, and radial transport of both plasma and neutrals. Results depend on the H-mode type, two examples of which are typically observed on C-Mod: ELM-free and enhanced D$_\alpha$(EDA). Pedestal profiles from edge Thomson scattering show clear trends with plasma operational parameters, particularly in EDA operation. Notably, a ballooning-like $I_p^2$ dependence is seen in $\nabla p_e$, despite calculated stability to ideal ballooning modes. A similar scaling is seen in near scrape-off-layer probe data for both L- and H-mode discharges, possibly due to electromagnetic fluid drift turbulence setting transport at the pedestal foot. Neutral density diagnosis at the ETB has allowed examination of fueling by atomic D$^0$ in typical H-modes, and has yielded profiles of an effective diffusivity well in the ETB, the depth of which varies between ELM-free and EDA operation. This experimental work is supplemented with a 1D semi-analytic model for neutral transport in the edge, in order to understand how changes in source D$^0$ impact the $n_e$ pedestal. By coupling a fluid analysis to a kinetic computation of neutral distribution functions, the thermal equilibration between ions and neutrals is considered. Rapid equilibration at typical C-Mod densities can explain the invariance of $n_e$ pedestal width and gradient with source rate, a result that contrasts with those of lower-density tokamaks. Incorporating neutral kinetics may resolve these apparently contradictory results, as well as dimensionless comparisons between C-Mod and larger tokamaks such as DIII-D and JET. [Preview Abstract] |
Monday, October 24, 2005 4:00PM - 4:30PM |
CI1b.00003: Physics and Operational Space of the Small ELM regime in NSTX Invited Speaker: The search for high performance regimes with small or no periodic heat pulses, due e.g. to edge-localized modes (ELMs), has been a focus of international research, owing to possible erosion damage to plasma facing components during large, Type I ELMs as projected...\footnote{A. Loarte, et. al., 2003 \textit{J. Nucl. Materials} \textbf{313-316} 962.} for ITER. A high-performance, small ELM regime is now routinely observed in the National Spherical Torus Experiment (NSTX). The ELMS (called \footnote{R. Maingi, et. al., 2005 \textit{Nuclear Fusion} \textbf{45} 264.} ``type V'') are consistent with high bootstrap current operation with line average density approaching Greenwald scaling. Each individual ELM has no measurable impact on stored energy. The ELM perturbation is observed via ultra-soft X-rays to typically originate near the lower divertor region and propagate with a poloidal component toward the outer midplane. A pre-cursor with n=1 is observed on an outboard near-midplane toroidal Mirnov array. The filament-like structure propagates in the counter plasma current direction and persists for \underline {$<$} 1-2 toroidal transit times. After crossing the separatrix to the open field lines, the ELM flux is observed to reach the outer strike point up to 400 $\mu $sec before the inboard side, consistent with leakage of ions from the pedestal top $\sim $ 200 eV across the separatrix on the low field side near the lower X-point, and sound speed propagation on the open field lines. In contrast, a large Type I ELM is observed to reach the outer strike point only up to 200 $\mu $sec before the inboard side, consistent with a ballooning-type perturbation near the outboard midplane. To date, the Type V ELM regime without intervening Type I ELMs has been obtained with $\beta _{N}$ \underline {$<$ }5, $\nu _{e,ped}^{\ast } \quad >$ 1, $\beta _{pol}$ \underline {$>$} 0.5 and $\beta _{ped}$ \underline {$<$ }5{\%}. The differences between these Type V ELMs and conventional Types I and III ELMs, which also observed in NSTX, will be presented. [Preview Abstract] |
Monday, October 24, 2005 4:30PM - 5:00PM |
CI1b.00004: Core Barrier Formation Near Integer q Surfaces in DIII-D Invited Speaker: Recent \hbox{DIII-D} experiments have significantly improved our understanding of internal transport barriers (ITBs) that are triggered close to the time when an integer value of the minimum in q is crossed. While this phenomenon has been observed on many tokamaks, the extensive transport and fluctuation diagnostic set on DIII-D has allowed us to study in detail the generation mechanisms of q-triggered ITBs as pertaining to turbulence suppression dynamics, shear flows, and energetic particle modes. In these discharges, the evolution of the q profile is determined by MSE polarimetry and the integer $q_{min}$ crossings are further pinpointed in time by the observation of Alfven cascades. High time resolution measurements of the ion and electron temperatures and the toroidal rotation show that the start of improved confinement is simultaneous in all 3 channels, and that this event precedes the traversal of integer $q_{min}$ by 5-20~ms. There is no significant low-frequency MHD activity prior to or just after the crossing of integer $q_{min}$. A drop in core ion temperature and rotation that often occurs just before the ITB onset is coincident with a high frequency core-localized TAE mode that begins at the crest of the preceding Alfven chirping modes and is a possible cause of the core ion loss via expulsion of fast ions. A reduction in local turbulent fluctuations is observed at the start of the temperature rise, and concurrently, an increase in turbulence poloidal flow velocity and flow shear is measured with the beam emission spectroscopy diagnostic. For the case of a transition to a steady internal barrier the fluctuation level remains at a reduced amplitude. The timing and nature of the temperature and rotation changes leading to internal barriers suggests transport improvement due to increased shear flow arising from fast ion loss. [Preview Abstract] |
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