The shear Alfvén continuum of optimized stellarators

Abstract

The shear Alfvén continuum has been studied extensively in tokamaks, but its gap structure in modern optimized stellarators has received less attention, despite its importance for energetic-particle confinement in fusion pilot plants. We compute and compare the continua of five reactor-scale stellarator equilibria: two quasi-axisymmetric (QA), two quasi-helically symmetric (QH), and one quasi-isodynamic (QI). We present selective Fourier mode schemes that provide a sufficient resolution of eigenfrequencies within the frequency range of interest. In all five configurations, the widest gap is the HAE(2,$N_{\text{fp}}$) gap, which is driven by rotating ellipticity. This gap lies in the frequency range of strong resonance with prompt fusion-born alphas in the QA configurations but not in the QH configurations, suggesting that QH configurations may be easier to optimize for alpha confinement. Near-axis theory predicts the HAE(2,$N_{\text{fp}}$) and EAE gap widths well but overestimates the MAE gap widths and the gap widths of the QI configuration. These discrepancies likely arise from interactions between gaps, which depend on the rotational transform profile and are not captured by near-axis theory.