Ion-temperature-gradient stability near the magnetic axis of quasisymmetric stellarators

TL;DR

This study evaluates ion-temperature-gradient mode stability in quasisymmetric stellarators using analytical estimates and gyrokinetic simulations, comparing models and realistic designs to understand stability characteristics across the plasma radius.

Contribution

It introduces a comparison between near-axis expansion models and realistic stellarator designs, revealing limitations and differences in ITG stability predictions.

Findings

Near-axis expansion accurately predicts core stability but overestimates growth rates at larger radii.

Quasi-axisymmetric configurations tend to have higher ITG growth rates than quasi-helically symmetric ones near the axis.

No systematic stability differences are observed between QA and QH configurations away from the axis.

Abstract

The stability of the ion-temperature gradient mode in quasisymmetric stellarators is assessed. This is performed using a set of analytical estimates together with linear gyrokinetic simulations. The peak growth rates, their corresponding real frequencies and wave-vectors are identified. A comparison is made between a first-order near-axis expansion model and eleven realistic designs obtained using numerical optimization methods. It is found that while the near-axis expansion is able to replicate the growth rates, real frequencies and perpendicular wave-vector at the inner core (both using simplified dispersion relations and first-principle gyrokinetic simulations), it leads to an overestimation of the growth rate at larger radii. An approximate analytic solution of the ITG dispersion relation for the non-resonant limit suggests growth rates could be systematically higher in quasi-axisymmetric (QA) configurations compared to quasi-helically (QH) symmetric ones. However except for very close to the axis, linear gyrokinetic simulations do not show systematic differences between QA and QH configurations.