Ion heating and axially evanescent modes in RF experiments on bumpy tori and other toroidal devices

A fluid model including quasilinear heating, collisions, charge exchange, and spatial transport is used to determine the perpendicular wavenumbers and electric fields which can permit appreciable coupling of RF waves to the ions in ELMO Bumpy Torus-Scale (EBT-S) plasmas. The observed significant ion heating at frequencies above the first harmonic ion-cyclotron frequency in EBT-S is consistent with a quasilinear heating model if RF wave amplitudes vary on a spatial scale which is much smaller than the inverse wavenumber for propagating, cold-plasma waves in the WKB approximation. It is shown that axially evanescent modes could account for the necessary, short perpendicular wavelengths. In general, axially evanescent modes could be a powerful means for heating ions at harmonics of the ion-cyclotron frequency. Charge exchange and transport are the dominant loss mechanisms acting on ions and preclude strong collisional coupling of ion tails or minority ion species to the bulk distribution of the majority ion species in EBT-S. This is true even if the applied rf frequency is comparable to the ion-cyclotron frequency of the minority ion species