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Title: H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod

Abstract

This paper reports on studies of the edge transport barrier and transition threshold of the high confinement (H) mode of operation on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)], over a wide range of toroidal field (2.6-7.86 T) and plasma current (0.4-1.7 MA). The H-mode power threshold and edge temperature at the transition increase with field. Barrier widths, pressure limits, and confinement are nearly independent of field at constant current, but the operational space at high B shifts toward higher temperature and lower density and collisionality. Experiments with reversed field and current show that scrape-off-layer flows in the high-field side depend primarily on configuration. In configurations with the Bx{nabla}B drift away from the active X-point, these flows lead to more countercurrent core rotation, which apparently contributes to higher H-mode thresholds. In the unfavorable case, edge temperature thresholds are higher, and slow evolution of profiles indicates a reduction in thermal transport prior to the transition in particle confinement. Pedestal temperatures in this case are also higher than in the favorable configuration. Both high-field and reversed-field results suggest that parameters at the L-H transition are influencing the evolution and parameters of the H-mode pedestal.

Authors:
; ; ; ; ; ; ; ; ; ;  [1]; ;  [2];  [3]
  1. Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02129 (United States)
  2. Fusion Research Center, University of Texas at Austin, Austin, Texas 78712 (United States)
  3. Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831 (United States)
Publication Date:
OSTI Identifier:
20975048
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2714297; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ALCATOR DEVICE; COLLISIONS; CONFIGURATION; ELECTRIC CURRENTS; ELECTRON TEMPERATURE; H-MODE PLASMA CONFINEMENT; ION TEMPERATURE; MAGNETOHYDRODYNAMICS; PLASMA; PLASMA DENSITY; PLASMA SCRAPE-OFF LAYER

Citation Formats

Hubbard, A. E., Hughes, J. W., Cziegler, I., LaBombard, B., Lin, Y., McDermott, R., Rice, J. E., Snipes, J. A., Terry, J. L., Wolfe, S. M., Wukitch, S., Bespamyatnov, I. O., Rowan, W. L., and Biewer, T. H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod. United States: N. p., 2007. Web. doi:10.1063/1.2714297.
Hubbard, A. E., Hughes, J. W., Cziegler, I., LaBombard, B., Lin, Y., McDermott, R., Rice, J. E., Snipes, J. A., Terry, J. L., Wolfe, S. M., Wukitch, S., Bespamyatnov, I. O., Rowan, W. L., & Biewer, T. H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod. United States. doi:10.1063/1.2714297.
Hubbard, A. E., Hughes, J. W., Cziegler, I., LaBombard, B., Lin, Y., McDermott, R., Rice, J. E., Snipes, J. A., Terry, J. L., Wolfe, S. M., Wukitch, S., Bespamyatnov, I. O., Rowan, W. L., and Biewer, T. Tue . "H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod". United States. doi:10.1063/1.2714297.
@article{osti_20975048,
title = {H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod},
author = {Hubbard, A. E. and Hughes, J. W. and Cziegler, I. and LaBombard, B. and Lin, Y. and McDermott, R. and Rice, J. E. and Snipes, J. A. and Terry, J. L. and Wolfe, S. M. and Wukitch, S. and Bespamyatnov, I. O. and Rowan, W. L. and Biewer, T.},
abstractNote = {This paper reports on studies of the edge transport barrier and transition threshold of the high confinement (H) mode of operation on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)], over a wide range of toroidal field (2.6-7.86 T) and plasma current (0.4-1.7 MA). The H-mode power threshold and edge temperature at the transition increase with field. Barrier widths, pressure limits, and confinement are nearly independent of field at constant current, but the operational space at high B shifts toward higher temperature and lower density and collisionality. Experiments with reversed field and current show that scrape-off-layer flows in the high-field side depend primarily on configuration. In configurations with the Bx{nabla}B drift away from the active X-point, these flows lead to more countercurrent core rotation, which apparently contributes to higher H-mode thresholds. In the unfavorable case, edge temperature thresholds are higher, and slow evolution of profiles indicates a reduction in thermal transport prior to the transition in particle confinement. Pedestal temperatures in this case are also higher than in the favorable configuration. Both high-field and reversed-field results suggest that parameters at the L-H transition are influencing the evolution and parameters of the H-mode pedestal.},
doi = {10.1063/1.2714297},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Measurements of the steep transport barrier at the edge of the Alcator C-Mod tokamak [I. H. Hutchinson {ital et al.}, Phys. Plasmas {bold 1}, 1511 (1994)] are presented. The parameters at the top of this barrier are in the range T{sub e}=150{endash}750 eV and n{sub e}=0.5{minus}3.3{times}10{sup 20} m{sup {minus}3}, depending on the confinement regime. Type III edge localized modes (ELMs) have an upper temperature limit. T{sub e} pedestal profiles show a barrier width {Delta}{sub T}{approx_equal}8 mm. Soft x-ray emissivity profiles are narrower, with {Delta}=2{endash}4 mm. Edge currents are calculated to alter the ideal stability boundary favorably, leading to ideally stablemore » pedestal profiles. High frequency, broadband, edge density fluctuations are sometimes observed in H-mode (high-confinement mode) and are associated with enhanced particle transport. Coherent magnetic fluctuations localized near the pedestal are also seen. {copyright} {ital 1998 American Institute of Physics.}« less
  • Edge transport barrier (ETB) studies on the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] investigate pedestal scalings and the radial transport of plasma and neutrals. Pedestal profiles show trends with plasma operational parameters such as total current I{sub P}. A ballooning-like I{sub P}{sup 2} dependence is seen in the pressure gradient, despite calculated stability to ideal ballooning modes. A similar scaling is seen in the near scrape-off layer for both low-confinement (L-mode) and H-mode discharges, possibly due to electromagnetic fluid drift turbulence setting transport near the separatrix. Neutral density diagnosis allows an examination of D{sup 0} fueling in H-modes,more » yielding profiles of effective particle diffusivity in the ETB, which vary as I{sub P} is changed. Edge neutral transport is studied using a one-dimensional kinetic treatment. In both experiment and modeling, the C-Mod density pedestal exhibits a weakly increasing pedestal density and a nearly invariant density pedestal width as the D{sup 0} source rate increases. Identical modeling performed on pedestal profiles typical of DIII-D [Nucl. Fusion 42, 614 (2002)] reveal differences in pedestal scalings qualitatively similar to experimental results.« less
  • Recent advances in gyrokinetic simulation of core turbulence and associated transport requires an intensified experimental effort to validate these codes using state of the art synthetic diagnostics to compare simulations with experimental data. A phase contrast imaging (PCI) diagnostic [M. Porkolab, J. C. Rost, N. Basse et al., IEEE Trans. Plasma Sci. 34, 229 (2006)] is used to study H-mode plasmas in Alcator C-Mod [M. Greenwald, D. Andelin, N. Basse et al., Nucl. Fusion 45, S109 (2005)]. The PCI system is capable of measuring density fluctuations with high temporal (2 kHz-5 MHz) and wavenumber (0.5-55 cm{sup -1}) resolution. Recent upgradesmore » have enabled PCI to localize the short wavelength turbulence in the electron temperature gradient range and resolve the direction of propagation (i.e., electron versus ion diamagnetic direction) of the longer wavelength turbulence in the ion temperature gradient (ITG) and trapped electron mode range. The studies focus on plasmas before and during internal transport barrier formation in an enhanced D{sub {alpha}} H-mode plasma assisted with ion cyclotron resonance frequency heating. Nonlinear GYRO simulations have also been performed [J. Candy and R. E. Waltz, Phys. Rev. Lett. 91, 045001 (2003)] and the predicted fluctuation is compared against experimental measurements through a synthetic PCI diagnostic method. The simulated fluctuations from GYRO agree with experimental measurements in the ITG regime. GYRO also shows good agreement in transport predictions with experimental measurements after reducing the ion temperature gradient ({approx}15%) and adding ExB shear suppression, all within the experimental uncertainty.« less