Theory of mode conversion in helicon plasmas

Abstract

Abstract The helicon wave can be converted into the Trivelpiece–Gould (TG) mode and vice versa in nonuniform helicon plasmas. A theory is presented to study this phenomena using a full wave analysis with the stratified model and spectral collocation methods. The properties of the power flux are analyzed, and the reflection and conversion coefficients are obtained by constructing appropriate solutions representing the incident helicon or TG mode propagating toward the mode conversion layer (MCL). It is found that these coefficients are independent of the incident mode type in the collisionless limit, and that conversion of power from the helicon to the TG mode is the same as that from the TG to the helicon mode, regardless of collisions. It is also found that the radiation resistance peaks approximately where the conversion peaks when there is an MCL, or the reflection reaches its local minimum when there is no MCL. Finally, the absorbed power of the waves excited by an antenna is decomposed into those of the incident helicon and TG modes, which shows that their partitions are affected by the existence of mode conversion as well as the collision frequency.