Turbulent transport in rotating tokamak plasmas
Casson, F. J. (Francis James)
arxiv: Physics::Plasma Physics | Physics::Fluid Dynamics
Small scale turbulence in a magnetically confined fusion plasma drives energy\ud and particle transport which determine the confinement. The plasma in a tokamak\ud experiment has a toroidal rotation which may be driven externally, but can also\ud arise spontaneously from turbulent momentum transport. This thesis investigates\ud the interaction between turbulence and rotation via nonlinear numerical simulations,\ud which use the gyrokinetic description in the frame that corotates with the plasma.\ud A local gyrokinetic code is extended to include both the centrifugal force, and the\ud stabilising effect of sheared equilibrium flow.\ud Sheared flow perpendicular to the magnetic field suppresses the turbulence,\ud and also breaks a symmetry of the local model. The resulting asymmetry creates\ud a turbulent residual stress which can counteract diffusive momentum transport and\ud contribute to spontaneous rotation. The competition between symmetry breaking\ud and turbulence suppression results in a maximum in the nondiffusive momentum flux\ud at intermediate shearing rates. Whilst this component of the momentum transport\ud is driven by the sheared flow, it is also found to be suppressed by the shearing more\ud strongly than the thermal transport. The direction of the residual stress reverses\ud for negative magnetic shear, but also persists at zero magnetic shear.\ud The parallel component of the centrifugal force traps particles on the outboard\ud side of the plasma, which destabilises trapped particle driven modes. The\ud perpendicular component of the centrifugal force appears as a centrifugal drift which\ud modifies the phase relation between density and electric field perturbations, and is\ud stabilising for both electron and ion driven instabilities. For ion temperature gradient\ud dominated turbulence, an increased fraction of slow trapped electrons enhances\ud the convective particle pinch, suggesting increased density peaking for strongly rotating\ud plasmas. Heavy impurities feel the centrifugal force more strongly, therefore\ud the effects of rotation are significant for impurities even when the bulk ion Mach\ud number is low. For ion driven modes, rotation results in a strong impurity convection\ud inward, whilst a more moderate convection outward is found for electron driven\ud modes.