Effects of fast-ion-orbit width on Alfven instabilities in stellarators: a general theory and its application to a W7-AS experiment

摘要

A general expression for the growth rate of Alfven instabilities driven by circulating and semi-trapped energetic ions in stellarators is derived, which generalizes that obtained in a recent work (Kolesnichenko Ya.I. et al 2004 Phys. Plasmas 11 158) by taking into account the finite orbit width of the energetic ions. It is found that the finite orbits typically reduce the growth rate, but in some cases they enhance instabilities, leading to additional resonances. The developed theory is applied to a particular shot (shot #34723) in Wendelstein 7-AS, where Alfvenic activity had a bursting character, being strongest at the end of each burst. Although the presented analysis of this shot is not complete, it is sufficient to conclude that the finite orbit width triggers a weak instability at the initial stage of each instability burst, but it weakens a strong instability at the burst end. The latter effect makes the perturbative approach used in the paper applicable. An analysis of destabilized Alfven eigenmodes in W7-AS precedes the stability analysis. In addition, an invariant of the particle motion in the stellarator field is derived, and the mode structure in weak-shear systems is determined.