The influence of a general static external magnetic perturbation on the stability of resistive modes in a tokamak plasma is examined. There are three main parts to this investigation. First, the vacuum perturbation is expanded as a set of well‐behaved toroidal ring functions, and is, thereafter, specified by the coefficients of this expansion. Second, a dispersion relation is derived for resistive plasma instabilities in the presence of a general external perturbation, and finally, this dispersion relation is solved for the amplitudes of the tearing and twisting modes driven in the plasma by a specific perturbation. It is found that the amplitudes of driven tearing and twisting modes are negligible until a certain critical perturbation strength is exceeded. Only tearing modes are driven in low‐β plasmas with εβp≪1. However, twisting modes may also be driven if εβp≳1. For error‐field perturbations made up of a large number of different poloidal and toroidal harmonics the critical strength to drive locked modes has a ‘‘staircase’’ variation with edge‐q, characterized by strong discontinuities as coupled rational surfaces enter or leave the plasma. For single harmonic perturbations, the variation with edge‐q is far smoother. Both types of behavior have been observed experimentally. The critical perturbation strength is found to decrease strongly close to an ideal external kink stability boundary. This is also in agreement with experimental observations.
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October 1994
Research Article|
October 01 1994
Effect of a static external magnetic perturbation on resistive mode stability in tokamaks
R. Fitzpatrick;
R. Fitzpatrick
Institute for Fusion Studies, The University of Texas at Austin, Austin, Texas 78712
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T. C. Hender
T. C. Hender
Institute for Fusion Studies, The University of Texas at Austin, Austin, Texas 78712
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Phys. Plasmas 1, 3337–3355 (1994)
Article history
Received:
April 01 1994
Accepted:
June 17 1994
Citation
R. Fitzpatrick, T. C. Hender; Effect of a static external magnetic perturbation on resistive mode stability in tokamaks. Phys. Plasmas 1 October 1994; 1 (10): 3337–3355. https://doi.org/10.1063/1.870918
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