Irregular wave fields in nearly circular cross sections

The fast magnetosonic wave field in nearly circular large aspect ratio toroidal plasmas is analysed in the limit of weak damping using a full wave code. For bounded regions in which the solution to the wave equation is separable the solution can be described by quantum numbers. As the symmetry of a cylindrical equilibrium with circular cross section is broken e.g. by toroidicity, the solutions are in general characterised by broad poloidal Fourier spectra, much broader than obtained by Fourier decomposing the current in the structure launching the wave. The transition to such solutions take place when the deviation from a circular cylinder exceeds a threshold analogous to the Chirikov criterion. (author)