On the accessibility of stable reactor operating regimes in quasi-symmetric stellarators

TL;DR

This paper investigates the feasibility of operating quasi-symmetric stellarators at low plasma pressure to avoid instabilities, revealing a sudden transition to poor confinement regimes through advanced simulations.

Contribution

It provides a detailed analysis of the transport regimes in low-beta quasi-symmetric stellarators using high-fidelity simulations, highlighting challenges for reactor design.

Findings

Abrupt transition to deleterious transport at low plasma beta

Configuration with flattened core pressure profile shows favorable properties

Implications for stellarator optimization and design strategies

Abstract

Maximising particle and energy confinement is crucial for achieving the sustained burning plasma conditions necessary to realise fusion energy. For stellarator reactors, one proposed strategy for avoiding destructive instabilities is to operate at high-field but low(er) plasma pressure. In this work, we investigate the accessibility of such a reactor-relevant low-beta regime in a reactor-scale quasi-axisymmetric stellarator using state-of-the-art high-fidelity macro- and microscopic simulation tools. We consider a configuration with a flattened core pressure profile and favourable properties from the macroscopic and neoclassical perspectives. By contrast, linear and nonlinear calculations with the GENE code show an abrupt transition to a regime of highly deleterious transport at low (local) plasma beta. We describe the characterisation of these transport regimes as well as the confinement transition. We discuss the implications broadly for stellarator optimisation and highlight the impact on quasi-symmetric stellarator design strategies.