On The Formation of Large-Scale Quasi-Coherent Structures in the Helically-Symmetric eXperiment

The optimization of stellarators for reduced turbulent transport is an essential consideration when designing a stellarator reactor, and detailed verification and validation studies are needed to support the theories and numerical frameworks employed. In previous publications, it has been shown that trapped-electron mode (TEM) growth rates can be reduced in the Helically-Symmetric eXperiment (HSX) stellarator by modifying individual coil currents. While quasilinear (QL) estimates of the heat flux show a corresponding reduction in transport, nonlinear simulations in Gene show that these QL estimates fail to predict changes in transport due to changes in flux-surface geometry. The discrepancy between nonlinear simulations and QL estimates is due to the self-organization of the turbulent plasma into large-scale quasi-coherent density and potential structures at k_yρ_s=0.1. These structures are shown to be driven by a set of density-gradient-driven tearing-parity TEMs that nonlinearly couple into the zonal flow with a possible ion-diamagnetic mode acting as a mediator. This triplet leads to modifications in the radial structure of the zonal flow while driving a significant amount of transport. The physical mechanism of this process is investigated and prospects for mitigating the impact of these structures by modifying the flux-surface geometry in the experiment are discussed.