Achieving reduced turbulent transport in stellarators through enhanced nonlinear energy transfer

Advances in stellarator optimization have significantly enhanced the prospects of stellarators as fusion reactors. Configurations with excellent neoclassical transport properties have been demonstrated experimentally, however recent results from the W7-X stellarator show significant ion thermal transport that effectively clamps peak ion temperatures as predicted by strong ion-temperature-gradient turbulence. These results motivate efforts to find turbulence optimized stellarator configurations. A scheme for using three-dimensional shaping to reduce ion-temperature-gradient-driven turbulent transport is formulated that emphasizes turbulent saturation physics. Strong nonlinear energy transfer from unstable to damped eigenmodes results in lower nonlinear fluctuation levels and correspondingly reduced turbulent transport. A fluid model for predicting nonlinear energy transfer is implemented in a new computational optimization framework. Nonlinear energy transfer between modes is mediated by geometry-dependent resonant three-wave interaction lifetimes and coupling coefficients and can be effectively predicted from fast linear eigenvalue calculations, providing a natural optimization metric. Optimization calculations on candidate quasihelically symmetric stellarator configurations show a strong correlation between increases in resonant three-wave coupling lifetimes between unstable and stable modes and reductions in transport as predicted by nonlinear gyrokinetic simulations with the GENE code. Analysis of the dominant interactions in the fluid and gyrokinetic models indicate a key feature for improving nonlinear energy transfer is localization of the coupled eigenmodes to the same location along magnetic field lines. New candidate configurations for turbulence-optimized stellarators obtained through these calculations will be presented.