Magneto-acoustic Modes near X-lines in Toroidal Geometry
Abstract
Magnetic X-lines can be produced in the periphery of a toroidal plasma by external coils (e.g. in a divertor geometry). Assuming uniform plasma density, the two dimensional non uniformity of the magnetic field around a X-line can trap magneto-acoustic perturbations, giving rise to toroidal eigenmodes. Their structure is investigated, in the frame-work of ideal MHD, for a low β, currentless plasma, using a semiclassical (WKB) approach. The cases of a single X-line, and two very close X-lines (X-X) are considered. Ray trajectories in the closest neighborhood of a single X-line are regular: in this case, the eigenfrequencies are determined by means of the semiclassical quantization rules, and the structure of the eigenmodes is determined solving the relevant wave equation. Its separability is explicitly found in some significant cases. Transition to stochasticity is found in the X-X case, when ray trajectories encircle both singular points.
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- November 1996
- Bibcode:
- 1996APS..DPP..9S38F