Evaluating nonlinear turbulence saturation in quasi-helically symmetric stellarator geometries

A primary goal of stellarator optimization is to produce new configurations with improved

confinement properties. A new direction in stellarator optimization is the generation of stel-

larator designs with reduced turbulent transport. New quasi-helically symmetric stellarator

configurations with improved collisionless fast particle confinement have been discovered by

applying new metrics for fast particle transport. Among the class of configurations gener-

ated by this technique, some demonstrate substantially improved turbulent transport from

ion temperature gradient (ITG) instabilities as predicted by the Gene gyrokinetics code.

Assessing configuration differences solely through linear instability calculations is deceiving,

however, as the turbulence-improved configuration shows larger growth rates and broader

instability range as compared to the base case. This discrepancy between linear and non-

linear physics has been seen previously in quasisymmetric stellarators and is investigated

here via the theory of turbulence saturation through stable modes by examining three-wave

interaction times and complex coupling coefficients computed from a reduced fluid model

for ITG turbulence in fully 3D geometry. This metric is employed in a new framework for

stellerator optimization written in the Julia language to obtain new stellerator configurations

with reduced turbulent transport.