Effect of fast-ion dilution on turbulence

2024 Research Campaign, Thrust: Fast Ions, Turbulence and Alfven Waves Interaction

Purpose of Experiment

As the world fusion program enters the eve of burning plasmas, the traditional balkanization of topical areas into “transport”, “energetic particles” etc is overtaken by events. In a burning plasma, it is no longer meaningful to separate energetic particle confinement and the instabilities, which govern (hereafter AE modes”) from the familiar thermal confinement, drift waves etc. Cross scale coupling occurs between EPs and high-frequency AEs, and thermal particles and low-frequency drift waves. A key channel for this coupling is zonal modes (flow, fields and corrugations). Both of AE interactions and drift wave interactions can drive zonal modes, which in turn feedback on both. Confinement physics must confront and understand this “Brave New World” of feedback loops. This experiment is a necessary first step toward that challenging end. In particular, this experiment seeks to examine the confinement physics of a plasma with a population of EPs, which is sufficient to cause significant dilution, but where AEs are not excited or are of negligible effect. Recent theoretical work has argued that dilution can significantly enhance ZF couplings and production, and therefore can lower turbulence (ITG) levels. The key physical effect is dilution-induced weakening of drift wave dispersion and generation of zonal flow. A good understanding of this state establishes a solid foundation for the further study of nonlinear interaction among fast ions, AEs and turbulence, where AE dynamics enters.