Speaker
Description
Energetic particle (EP)-driven Alfvénic instabilities have been shown to play an important role in stellarators, as demonstrated by neutral beam fast ions in the W7-AS [1] and TJ-II [2] experiments. Alfvénic instabilities may be similarly important in fusion power plants driven by fusion-born alpha particles. However, a comprehensive numerical study of the shear Alfvén continuum in modern stellarator fusion power plant equilibria has not been previously performed. To address this void, we use Stellgap [3] to calculate the shear Alfvén continuum across several quasi-isodynamic and quasi-symmetric configurations — including quasi-helically symmetric and quasi-axisymmetric equilibria. We describe the considerations for selecting poloidal and toroidal modes, along with several newly developed tools, including a method for automatically labeling continuum gaps. Our analysis focuses on frequency ranges resonant with fusion-born alphas or neutral beam fast ions, and we detail how these ranges were determined. The continuum structure of stellarators can differ substantially from that of tokamaks; in particular, we find that stellarator continuum gaps are dominated by HAE gaps with small TAE gaps. We examine the aspects of magnetic geometry that likely drive this gap structure [4]. Finally, we explore possible methods to optimize stellarator equilibria for reduced continuum gap widths in EP-resonant frequency ranges.
[1] A. Weller, et al. Phys. Rev. Lett. 72, 1220–1223 (1994).
[2] R. Jiménez-Gómez, et al. Nucl. Fusion. 51 (3), 033001 (2011).
[3] D. A. Spong, et al. Phys. Plasmas. 10 (8): 3217–3224 (2003).
[4] E. Paul, et al. Journal of Plasma Physics. 91 (4), E101 (2025).