A numerical investigation of collisionality and turbulent transport

Abstract

An investigation of collisionality's role in turbulent transport in magnetized plasma using the GS2 gyrokinetic simulation software is presented. The investigation consists of three parts, conducted by way of numerical modeling: 1) input calibration using the conditions and results of a reference investigation of a different parameter's influence on turbulence, 2) direct variation of electron-electron and ion-ion collisionality parameters, and 3) comparison between results calculated with the inclusion and exclusion of an additional collisional heating term. The calibration exercise demonstrates reliable agreement between results obtained in the present investigation and those obtained in other studies, the variation of collisionality parameters suggests a stronger dependence of ITG-driven turbulence on electron-electron collisionality than on ion-ion collisionality, and the evaluation of the collisional heating diagnostic shows a diminished influence of collisional heat drive on turbulent transport as this parameter increases. Several significant changes in some steady-state turbulent fluxes are observed at certain "threshold" values of electron-electron or ion-ion collisionality (e.g. time-averaged particle flux changing sign twice as the normalized electron-electron collisionality parameter varies between 0 and 2.5) as well as a lack of correspondence between steady-state heat, momentum and particle flux changes. These seemingly unrelated sensitivities to different ranges of collisionality parameters suggest different drives for these different transport quantities, implying a complex relationship between collisionality and turbulent heat, momentum, and particle transport of which a deeper understanding is fundamental to the design and performance of magnetic fusion projects.