StellDOS: Alfvén-gap density of states (DOS) for stellarators

StellDOS is a modern open-source highly-extensible object-oriented C++ code that duplicates and extends upon the capabilities of Stellgap. Gaps arise in the shear Alfvén continuum because of poloidal and toroidal mode coupling through the magnetic geometry. Alfvén modes that exist in gaps can be driven by energetic particles via inverse Landau damping, but these modes do not experience continuum damping, potentially leading to instabilities and particle transport. Minimizing the width of Alfvén gaps is an additional criterion that can be optimized for in stellarator design. StellDOS was created to understand the basic features of quasisymmetric configurations that lead to gaps and further study Alfvénic instabilities caused by gap eigenmodes. StellDOS can directly calculate the shear Alfvén continuum of general 3-D geometries while accounting for toroidal and poloidal mode coupling. A comparison between an analytical model for the gap eigenmodes and the numerical result from StellDOS is directly made. In the future, StellDOS's extensibility will be further taken advantage of to implement the general Boozer coordinate geometry found in Stellgap and to calculate the density of continuum modes, which can be used to identify gaps.