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

Adelle M. Wright, Benjamin J. Faber

TL;DR

This paper investigates the accessibility of a reactor-relevant high-field, low-$\beta$ operating point in a quasi-axisymmetric stellarator. It uses high-fidelity macro (M3D-C1) and micro (Gene) simulations of a reactor-scale equilibrium with $\beta=1\%$ and a flattened core pressure profile to perform linear and nonlinear analyses across $\beta$ values up to $2\%$. The results reveal a favorable macroscopic stability but a dramatic microturbulence transition: ITG-dominated transport at very low local $\beta$ giving way to KBM-dominated behavior around $\beta\in[0.67\%,0.85\%]$, with nonlinear coupling driving explosive KBM transport well below linear thresholds. The findings imply that KBMs and subdominant instabilities must be included in stellarator optimization and reduced transport models to reliably predict confinement for reactor-scale devices.

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.

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

TL;DR

This paper investigates the accessibility of a reactor-relevant high-field, low- operating point in a quasi-axisymmetric stellarator. It uses high-fidelity macro (M3D-C1) and micro (Gene) simulations of a reactor-scale equilibrium with and a flattened core pressure profile to perform linear and nonlinear analyses across values up to . The results reveal a favorable macroscopic stability but a dramatic microturbulence transition: ITG-dominated transport at very low local giving way to KBM-dominated behavior around , with nonlinear coupling driving explosive KBM transport well below linear thresholds. The findings imply that KBMs and subdominant instabilities must be included in stellarator optimization and reduced transport models to reliably predict confinement for reactor-scale devices.

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.
Paper Structure (4 sections, 6 figures)

This paper contains 4 sections, 6 figures.

Table of Contents

  1. Introduction
  2. Method
  3. Results and discussion
  4. Conclusions

Figures (6)

  • Figure 1: Pressure (left, orange) and rotational transform (right, green) profiles for the quasi-axisymmetric equilibrium under consideration.
  • Figure 2: Mercier criterion as computed by VMEC (left, orange) and ballooning growth rate (right, green) for the quasi-axisymmetric equilibrium under consideration.
  • Figure 3: Most unstable linear mode growth rates and frequencies from linear Gene simulations as a function of $\beta$.
  • Figure 4: Eigenfunctions of $A_\parallel$ at $k_y = 0.05$ from linear Gene calculations for $\beta = 0.85\%$ (top) and $\beta=0.68\%$ (bottom).
  • Figure 5: Electron electrostatic and electromagnetic heat fluxes as a function of time from nonlinear Gene simulations at $\beta = \{0.34\%,0.51\%,0.68\%\}$.
  • ...and 1 more figures