Theoretical investigation of GAMs in stellarators

The possibility of observing GAMs in quasisymmetric (QS) stellarators such as HSX is investigated. GAMs in axisymmetric tokamaks can be excited because, theoretically, the parameter $|\vec{B}\cdot\nabla \vec{B}| / | \vec{B}\times \nabla B|$, which is a measure of the bounce frequency compared to the GAM frequency $(\omega_{bounce} \approx c_s/qR << \omega_{GAM} \approx c_s/R)$, is small. In QS stellarators, however, this ratio may not be small enough, leading to Landau damping. We study a QS system, using a model which spans a broad range in this parameter, and apply the results to HSX in the QS mode. HSX can also operate in a Mirror Mode. Due to the intrinsic non-ambipolarity of the Mirror mode, a zonal flow oscillation with frequency lower than that of the standard GAM may be excited, as shown by Helander et al.[1] Trapped particles contribute significantly to this oscillation. We study this case and assess whether GAMs could be observed on HSX in the mirror mode.\\[4pt] [1] Helander P et al. 2011 Plasma Physics and Controlled Fusion \textbf{53} 054006