BALLOONING MODES OR FOURIER MODES IN A TOROIDAL PLASMA?
The relationship between two different descriptions of eigenmodes in a torus is investigated. In one the eigenmodes are similar to Fourier modes in a cylinder and are highly localised near a particular rational surface. In the other they are the so-called ballooning modes which extend over many rational surfaces. Using a model which represents both drift waves and resistive interchanges we investigate the transition from one of these structures to the other. In this simplified model the transition depends on a single parameter which embodies the competition between toroidal coupling of Fourier modes (which enhances ballooning) and variation in frequency of Fourier modes from one rational surface to another (which diminishes ballooning). As the coupling is increased each Fourier mode acquires a sideband on an adjacent rational surface and these sidebands then expand across the radius to form the extended mode described by the conventional ballooning mode approximation. This analysis shows that the ballooning approximation is appropriate for drift waves in a Tokamak but not for resistive interchanges in a pinch. In the latter the conventional ballooning effect is negligible but they may nevertheless show a ballooning feature. This is localised near the same rational surface as the primary Fourier mode and so does not lead to a radially extended structure.
Bibliographic Reference: WRITE TO THE LIBRARIAN, UKAEA, CULHAM LABORATORY, ABINGDON, OXON OX14 3DB (UK), MENTIONING REPORT CLM-P797, 1987
Record Number: 1989126018100 / Last updated on: 1989-03-01
Available languages: en