Shear Alfvén continuum and gaps in stellarator equilibria*

In stellarators, gaps arise in the shear Alfvén continuum because of poloidal and toroidal mode coupling through the magnetic geometry. Discrete Alfvén modes can exist in the gaps, which do not exhibit continuum damping. Due to the lack of continuum damping, these discrete Alfvén modes can be driven unstable by energetic particles via inverse Landau damping, potentially leading to instabilities and particle transport. To further understand Alfvénic instabilities and the structure of gaps in stellarators, a broad numerical investigation of the shear Alfvén continuum for several quasi-symmetric (QS) equilibria using Stellgap [1] is presented. Comparisons between analytically and numerically calculated gap widths are made. Additionally, updates on StellDOS, a modern open-source highly-extensible object-oriented C++ code that duplicates and extends upon the capabilities of Stellgap, are presented. StellDOS can directly calculate the shear Alfvén continuum of general 3-D geometries and will be expanded upon to calculate the shear Alfvén continuum in general Boozer geometry and calculate the density of continuum modes.

[1] D. A. Spong, R. Sanchez, A. Weller; Shear Alfvén continua in stellarators. Phys. Plasmas 1 August 2003; 10 (8): 3217–3224.