Ergodic behavior of lower hybrid decay wave ray trajectories in toroidal geometry

Abstract

In cylindrical geometry, as a result of both magnetic shear and density gradient there exist radially trapped lower hybrid decay waves bouncing back and forth in the low density region of the plasma. If this situation persists in toroidal geometry, it could have a severely destabilizing effect on the nonresonant parametric decay of an externally launched lower hybrid heating wave. However, it is shown here that weak toroidicity causes some of these waves to follow ergodic ray trajectories, and hence to be detrapped and to reach their lower hybrid resonance. The fraction of waves which shows ergodic behavior increases as the aspect ratio R0/a decreases. For realistic aspect ratios, i.e., R0/a≲5, nearly all the rays are ergodic and energy flows toward the resonance. The consequences for lower hybrid wave heating are discussed.