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Title: Measurement of edge currents in DIII-D and their implication for pedestal stability

Abstract

The present performance limits of tokamak discharges are strongly coupled to the stability and transport properties of the edge plasma. Both experimental and modeling efforts have shown a clear connection between the edge pressure pedestal height and core plasma confinement. The key to understanding the stability and performance limits of the pedestal revolves around an accurate knowledge of the plasma current in this region. Using the Zeeman effect in an injected 30 keV lithium beam, we have measured the currents in the edge of the DIII-D [J. L. Luxon, Nucl. Fusion 42, 6114 (2002)] tokamak for various confinement modes. This method of determining j(r) is insensitive to the large electric fields which coexist in the pedestal region and which complicate motional Stark effect measurements. For the high confinement cases, where substantial pedestal pressures exist, we find large ({approx}MA/m{sup 2}), localized ({delta}R{approx}1-2 cm) currents in the pedestal region, located near the maximum in the pressure gradient. These values are consistent with calculations of edge bootstrap current using the neoclassical NCLASS [W. A. Houlberg, K. C. Shaing, S. P. Hirshman, and M. C. Zarnstorff, Phys. Plasmas 4, 3230 (1997)] and Sauter [O. Sauter, C. Angioni, and Y. R. Lin-Lin, Phys. Plasmas 6,more » 2834 (1999)] models and the measured pedestal density and temperature profiles. The apparent consistency of the measured j{sub EDGE} with neoclassical predictions occurs despite the violation of one of the fundamental tenets of the theory, namely, {epsilon}={rho}{sub i}/L{sub P}<<1, where {rho}{sub i} is the ion poloidal gyroradius and L{sub p} is the pressure gradient scale length. The measured j{sub EDGE} has also been used to generate self-consistent reconstructions using the free boundary equilibrium solvers CORSICA [T. A. Casper, T. B. Kaiser, R. A. Jong, L. L. LoDestro, J. Moller, and L. D. Pearlstein, Plasma Phys. Controlled Fusion 45, 1193 (2003)] and EFIT [L. L. Lao, H. E. St. John, R. D. Stambough, A. G. Kellman, and W. Pfeiffer, Nucl. Fusion 25, 1611 (1985)]. These equilibria allow us, in conjunction with the edge localized instabilities in tokamak experiments [P. B. Snyder, H. R. Wilson, J. R. Ferron, L. L. Lao, A. W. Leonard, T. H. Osborne, A. D. Turnbull, D. Mossessian, M. Murakami, and X. Q. Xu, Phys. Plasmas 9, 2037 (2002); H. R. Wilson, P. B. Snyder, G. T. A. Huysmans, and R. L. Miller, Phys. Plasmas 9, 1277 (2002)] magnetohydrodynamic stability code, to assess the linear stability of the edge to peeling/ballooning modes. These results are then compared to the measured edge localized mode onset conditions and again good agreement is found between the experimental and model limits on the maximum permissible j{sub EDGE}.« less

Authors:
; ; ; ; ; ;  [1]
  1. General Atomics, P.O. Box 85608, San Diego, California 92186-5608 (United States)
Publication Date:
OSTI Identifier:
20736588
Resource Type:
Journal Article
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 12; Journal Issue: 5; Other Information: DOI: 10.1063/1.1879992; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1070-664X
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BALLOONING INSTABILITY; BOOTSTRAP CURRENT; DOUBLET-3 DEVICE; MAGNETOHYDRODYNAMICS; NEOCLASSICAL TRANSPORT THEORY; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA DIAGNOSTICS; PLASMA PRESSURE; PRESSURE GRADIENTS; ZEEMAN EFFECT

Citation Formats

Thomas, D M, Leonard, A W, Groebner, R J, Osborne, T H, Casper, T A, Snyder, P B, Lao, L L, Lawrence Livermore National Laboratory, Livermore, California 94550-9234, and General Atomics, P.O. Box 85608, San Diego, California 92186-5608. Measurement of edge currents in DIII-D and their implication for pedestal stability. United States: N. p., 2005. Web. doi:10.1063/1.1879992.
Thomas, D M, Leonard, A W, Groebner, R J, Osborne, T H, Casper, T A, Snyder, P B, Lao, L L, Lawrence Livermore National Laboratory, Livermore, California 94550-9234, & General Atomics, P.O. Box 85608, San Diego, California 92186-5608. Measurement of edge currents in DIII-D and their implication for pedestal stability. United States. doi:10.1063/1.1879992.
Thomas, D M, Leonard, A W, Groebner, R J, Osborne, T H, Casper, T A, Snyder, P B, Lao, L L, Lawrence Livermore National Laboratory, Livermore, California 94550-9234, and General Atomics, P.O. Box 85608, San Diego, California 92186-5608. Sun . "Measurement of edge currents in DIII-D and their implication for pedestal stability". United States. doi:10.1063/1.1879992.
@article{osti_20736588,
title = {Measurement of edge currents in DIII-D and their implication for pedestal stability},
author = {Thomas, D M and Leonard, A W and Groebner, R J and Osborne, T H and Casper, T A and Snyder, P B and Lao, L L and Lawrence Livermore National Laboratory, Livermore, California 94550-9234 and General Atomics, P.O. Box 85608, San Diego, California 92186-5608},
abstractNote = {The present performance limits of tokamak discharges are strongly coupled to the stability and transport properties of the edge plasma. Both experimental and modeling efforts have shown a clear connection between the edge pressure pedestal height and core plasma confinement. The key to understanding the stability and performance limits of the pedestal revolves around an accurate knowledge of the plasma current in this region. Using the Zeeman effect in an injected 30 keV lithium beam, we have measured the currents in the edge of the DIII-D [J. L. Luxon, Nucl. Fusion 42, 6114 (2002)] tokamak for various confinement modes. This method of determining j(r) is insensitive to the large electric fields which coexist in the pedestal region and which complicate motional Stark effect measurements. For the high confinement cases, where substantial pedestal pressures exist, we find large ({approx}MA/m{sup 2}), localized ({delta}R{approx}1-2 cm) currents in the pedestal region, located near the maximum in the pressure gradient. These values are consistent with calculations of edge bootstrap current using the neoclassical NCLASS [W. A. Houlberg, K. C. Shaing, S. P. Hirshman, and M. C. Zarnstorff, Phys. Plasmas 4, 3230 (1997)] and Sauter [O. Sauter, C. Angioni, and Y. R. Lin-Lin, Phys. Plasmas 6, 2834 (1999)] models and the measured pedestal density and temperature profiles. The apparent consistency of the measured j{sub EDGE} with neoclassical predictions occurs despite the violation of one of the fundamental tenets of the theory, namely, {epsilon}={rho}{sub i}/L{sub P}<<1, where {rho}{sub i} is the ion poloidal gyroradius and L{sub p} is the pressure gradient scale length. The measured j{sub EDGE} has also been used to generate self-consistent reconstructions using the free boundary equilibrium solvers CORSICA [T. A. Casper, T. B. Kaiser, R. A. Jong, L. L. LoDestro, J. Moller, and L. D. Pearlstein, Plasma Phys. Controlled Fusion 45, 1193 (2003)] and EFIT [L. L. Lao, H. E. St. John, R. D. Stambough, A. G. Kellman, and W. Pfeiffer, Nucl. Fusion 25, 1611 (1985)]. These equilibria allow us, in conjunction with the edge localized instabilities in tokamak experiments [P. B. Snyder, H. R. Wilson, J. R. Ferron, L. L. Lao, A. W. Leonard, T. H. Osborne, A. D. Turnbull, D. Mossessian, M. Murakami, and X. Q. Xu, Phys. Plasmas 9, 2037 (2002); H. R. Wilson, P. B. Snyder, G. T. A. Huysmans, and R. L. Miller, Phys. Plasmas 9, 1277 (2002)] magnetohydrodynamic stability code, to assess the linear stability of the edge to peeling/ballooning modes. These results are then compared to the measured edge localized mode onset conditions and again good agreement is found between the experimental and model limits on the maximum permissible j{sub EDGE}.},
doi = {10.1063/1.1879992},
journal = {Physics of Plasmas},
issn = {1070-664X},
number = 5,
volume = 12,
place = {United States},
year = {2005},
month = {5}
}