Ion-Cyclotron Waves in Elmo Bumpy Torus: Perturbation Analysis of the Effects of Wall Bumpiness

The analysis of ion-cyclotron wave propagation and related plasma heating in the Elmo Bumpy Torus (EBT) magnetic confinement experiment is made difficult by the complex geometry of that device. Characteristic wavelengths are of the order of the device size, and the approximation of geometrical optics, used so successfully in studies of electron-cyclotron heating, is not valid. An analysis of ion-cyclotron wave propagation is presented here which is based on the assumption that EBT may be approximated by a weakly bumpy cylinder. Specifically, a perturbation methodology specially developed to analyze the propagation of waves through media with corrugated boundaries is utilized. It is found that even modest amounts of wall modulation can give rise to striking modifications of the cylindrical eigenmodes. Furthermore, cylindrical eigenmodes may resonate with each other and the wall modulations, giving rise to the occurrence of stopband and passband phenomena and periodic exchanges of energy between propagating modes. The resonant exchange of energy between eigenmodes with sharp axial structure and weak radial structure (readily excited by localized antennae) and eigenmodes with weak axial structure and sharp radial structure (conducive to plasma heating) suggests new ways in which antennae and wall structures may be configured to achieve desirable wave-heating properties.