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Title: Maintenance of a stable current profile in a reversed field pinch

Abstract

Externally produced magnetic fields could be used to maintain the reversed field pinch (RFP) plasma confinement configuration in a stable current, j{sub parallel}/B, profile. This would be accomplished by the creation of stochastic magnetic field regions in the vicinity of rational surfaces on which the safety factor q is the reciprocal of an integer N. External coils simultaneously excite the poloidal mode number unity and toroidal mode number N and N-1 helical fields from N=N{sub max} on down in a peristaltic manner, where N{sub max} is given by the maximal value of q in the RFP plasma. The time scale for turning off and on the N and N-1 pairs of coils is given by {tau}{sub w}{identical_to}({mu}{sub o}/{eta})w{sup 2}, where w=a/2N{sub max} is a typical island half-width required for island overlap, which implies local magnetic stochasticity, and a is the minor radius of the plasma. The total time scale for a complete current spreading cycle is N{sub max}{tau}{sub w}=({mu}{sub 0}/{eta})a{sup 2}/4N{sub max}, which is a factor of N{sub max}{approx_equal}10 faster than the time scale for the overall relaxation of the j{sub parallel}/B profile.

Authors:
 [1]
  1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027 (United States)
Publication Date:
OSTI Identifier:
20974953
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 4; Other Information: DOI: 10.1063/1.2719628; (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; CONFIGURATION; CURRENTS; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; MAINTENANCE; MODE RATIONAL SURFACES; PLASMA; PLASMA CONFINEMENT; RELAXATION; REVERSE-FIELD PINCH; REVERSED-FIELD PINCH DEVICES; STOCHASTIC PROCESSES

Citation Formats

Boozer, Allen H. Maintenance of a stable current profile in a reversed field pinch. United States: N. p., 2007. Web. doi:10.1063/1.2719628.
Boozer, Allen H. Maintenance of a stable current profile in a reversed field pinch. United States. doi:10.1063/1.2719628.
Boozer, Allen H. Sun . "Maintenance of a stable current profile in a reversed field pinch". United States. doi:10.1063/1.2719628.
@article{osti_20974953,
title = {Maintenance of a stable current profile in a reversed field pinch},
author = {Boozer, Allen H.},
abstractNote = {Externally produced magnetic fields could be used to maintain the reversed field pinch (RFP) plasma confinement configuration in a stable current, j{sub parallel}/B, profile. This would be accomplished by the creation of stochastic magnetic field regions in the vicinity of rational surfaces on which the safety factor q is the reciprocal of an integer N. External coils simultaneously excite the poloidal mode number unity and toroidal mode number N and N-1 helical fields from N=N{sub max} on down in a peristaltic manner, where N{sub max} is given by the maximal value of q in the RFP plasma. The time scale for turning off and on the N and N-1 pairs of coils is given by {tau}{sub w}{identical_to}({mu}{sub o}/{eta})w{sup 2}, where w=a/2N{sub max} is a typical island half-width required for island overlap, which implies local magnetic stochasticity, and a is the minor radius of the plasma. The total time scale for a complete current spreading cycle is N{sub max}{tau}{sub w}=({mu}{sub 0}/{eta})a{sup 2}/4N{sub max}, which is a factor of N{sub max}{approx_equal}10 faster than the time scale for the overall relaxation of the j{sub parallel}/B profile.},
doi = {10.1063/1.2719628},
journal = {Physics of Plasmas},
number = 4,
volume = 14,
place = {United States},
year = {Sun Apr 15 00:00:00 EDT 2007},
month = {Sun Apr 15 00:00:00 EDT 2007}
}
  • Auxiliary edge current drive is routinely applied in the Madison Symmetric Torus [R.N. Dexter, D. W. Kerst, T.W. Lovell et.al., Fusion Technol. 19, 131 (1991)] with the goal of modifying the parallel current profile to reduce current- driven magnetic fluctuations and the associated particle and energy transport. Provided by an inductive electric field, the current drive successfully reduces energy transport. First-time measurements of the modified edge current profile reveal that, relative to discharges without auxiliary current drive, the edge current density decreases. This decrease is explicable in terms of newly measured reductions in the dynamo (fluctuation-based) electric field and themore » electrical conductivity. Induced by the current drive, these two changes to the edge plasma play as much of a role in determining the resultant edge current profile as does the current drive itself.« less
  • Auxiliary edge current drive is routinely applied in the Madison Symmetric Torus [R. N. Dexter, D. W. Kerst, T. W. Lovell et al., Fusion Technol. 19, 131 (1991)] with the goal of modifying the parallel current profile to reduce current-driven magnetic fluctuations and the associated particle and energy transport. Provided by an inductive electric field, the current drive successfully reduces fluctuations and transport. First-time measurements of the modified edge current profile reveal that, relative to discharges without auxiliary current drive, the edge current density decreases. This decrease is explicable in terms of newly measured reductions in the dynamo (fluctuation-based) electricmore » field and the electrical conductivity. Induced by the current drive, these two changes to the edge plasma play as much of a role in determining the resultant edge current profile as does the current drive itself. (c) 2000 American Institute of Physics.« less
  • An auxiliary poloidal inductive electric field applied to a reversed-field pinch (RFP) plasma reduces the current density gradient, slows the growth of {ital m}=1 tearing fluctuations, suppresses their associated sawteeth, and doubles the energy confinement time. This experiment attacks the dominant RFP plasma loss mechanism of parallel streaming in a stochastic magnetic field. The auxiliary electric field flattens the current profile and reduces the magnetic fluctuation level. Since a toroidal flux change linking the plasma is required to generate the inductive poloidal electric field, the current drive is transient to avoid excessive perturbation of the equilibrium. To sustain and enhancemore » the improved state, noninductive current drivers are being developed. A novel electrostatic current drive scheme uses a plasma source for electron injection, and the lower-hybrid wave is a good candidate for radio-frequency current drive.« less
  • Neutral beam injection in reversed-field pinch (RFP) plasmas on the Madison Symmetric Torus [Dexter et al., Fusion Sci. Technol. 19, 131 (1991)] drives current redistribution with increased on-axis current density but negligible net current drive. Internal fluctuations correlated with tearing modes are observed on multiple diagnostics; the behavior of tearing mode correlated structures is consistent with flattening of the safety factor profile. The first application of a parametrized model for island flattening to temperature fluctuations in an RFP allows inferrence of rational surface locations for multiple tearing modes. The m = 1, n = 6 mode is observed to shift inward by 1.1 ± 0.6 cm withmore » neutral beam injection. Tearing mode rational surface measurements provide a strong constraint for equilibrium reconstruction, with an estimated reduction of q{sub 0} by 5% and an increase in on-axis current density of 8% ± 5%. The inferred on-axis current drive is consistent with estimates of fast ion density using TRANSP [Goldston et al., J. Comput. Phys. 43, 61 (1981)].« less