ISSN:
1089-7674
Source:
AIP Digital Archive
Topics:
Physics
Notes:
A quadratic dispersion relation is derived which governs the feedback-modified stability of the resistive shell mode in a large-aspect ratio, low-β tokamak plasma. The effectiveness of a given feedback scheme is determined by a single parameter, α0, which measures the coupling of different poloidal harmonics due to the nonsinusoidal nature of the feedback currents. Feedback fails when this parameter becomes either too positive or too negative. Feedback schemes can be classified into three groups, depending on the relative values of the poloidal mode number, m0, of the intrinsically unstable resistive shell mode, and the number, M, of feedback coils in the poloidal direction. Group I corresponds to M≤2m0 and M≠m0; group II corresponds to M=m0; finally, group III corresponds to M〉2m0. The optimal group I feedback scheme is characterized by extremely narrow detector loops placed as close as possible to the plasma, i.e., well inside the resistive shell. Of course, such a scheme would be somewhat impractical. The optimal group II feedback scheme is characterized by large, nonoverlapping detector loops, and moderately large, nonoverlapping feedback coils. Such a scheme is 100% effective (i.e., it makes the resistive shell appear superconducting) when the detector loops are located just outside the shell. Unfortunately, the scheme only works efficiently for resistive shell modes possessing one particular poloidal mode number. The optimal group III feedback scheme is characterized by slightly overlapping detector loops, and strongly overlapping feedback coils. Such a scheme is 100% effective when the detector loops are located just outside the shell. In addition, the scheme works efficiently for resistive shell modes with a range of different poloidal mode numbers. © 2001 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1342783
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