Alfvén stability characteristics of stellarator reactors in the high-density regime

A unique feature of stellarators is access to higher plasma density regimes than for tokamaks. As described previously (D. Spong, APS-DPP 2023), above a certain density (around 3 x 10²⁰ m^-3) this can suppress alpha-driven instabilities by lowering the energetic particle drive (through decreased slowing-down times). However, this effect must be balanced against the effects of higher aspect ratio (leads to shorter gradient scale lengths), lower shear rotational transform profiles (results in a more global mode structure) and a greater variety of orbit classes present in 3D systems. The earlier study is continued, updating to the latest versions of the FAR3d gyrofluid and GTC gyrokinetic models, and considering both the Wistell-D QH configuration as well as the recent CIEMAT QI reactor model (J. A. Alonso, et al., Nucl. Fusion 2022). Using profiles and parameters from a 0.5D model with consistent equilibria, stability is examined along contours of constant fusion power output. Several toroidal mode number families (n = 1, 2, 3, 4 for a 4-field period device) are examined. The goals are to map out the regions of Alfvén instability on the plasma operations (POPCON) plot and ultimately to perform nonlinear simulations in the unstable regions to evaluate alpha particle transport rates and their compatibility with burning plasma operation.