Dr. Benjamin Faber, Department of Engineering Physics, University of Wisconsin-Madison

"Modelling nonlinear turbulence saturation dynamics in stellarators and its application to optimization"

Abstract: 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. A fluid model for ITG-driven ion-scale fluctuations using the EDQMN closure predicts that increasing nonlinear energy transfer from unstable to stable eigenmodes results in lower nonlinear fluctuation levels and correspondingly reduced turbulent transport. Nonlinear energy transfer between modes is mediated by geometry-dependent resonant three-wave interaction lifetimes and complex coupling coefficient. These interactions can be effectively predicted from linear eigenvalue calculations, and provides a novel optimization metric. This model has been implemented in a new scalable optimization framework written in the Julia language. Analysis of the dominant interactions indicate a key feature for improving nonlinear energy transfer is localization of coupled eigenmodes to the same location along magnetic field lines. Optimization calculations demonstrate both the effectiveness and shortcomings of this model and will be presented with an outlook on future directions.