Kinetic Effects in the Pedestal of a Tokamak

Abstract

Kinetic effects in the pedestal of a tokamakpedestal, transport, residual, flow 52.25. Fi, 52.25.Dg, 52.55.Fa We consider the effects of finite ExB on the residual and neoclassical ion heat transport and ion flow in the pedestal. For a tokamak pedestal with a density scale of the poloidal ion gyroradius the finite ExB drift modfies the ion orbits, moving the trapped-passing boundary towards the tail of the ion distribution function. IntroductionIn many tokamak plasmas the width of the pedestal is observed to be on the order of a poloidal ion gyroradius.Consequently, finite drift orbit effects take on a special significance. We employ a special version of gyrokinetics that replaces the standard radial gyrokinetic variable by canonical angular momentum [1] to investigate kinetic phenomena in an axisymmetric tokamak pedestal in the banana regime. An entropy production argument for a pedestal in which the density drops substantially on the scale of the ion poloidal gyroradius, finds that in the banana regime the ion temperature is not allowed to vary substantially on such a short scale if the poloidal magnetic field is small compared to the toroidal field [1]. When the toroidal ion flow in the pedestal is subsonic this requires the ExB and ion diamagnetic flows to cancel to lowest order, making the flow of the electrons sonic there. These results mean that in a subsonic banana regime pedestal of poloidal ion gyroradius width, the ions are electrostatically confined and the electrons are confined magnetically [1]. Using this observation we then calculate the collisionless zonal flow residual in the pedestal to demonstrate that it is enhanced over its core value [2,3]. We find that in the pedestal, finite orbit effects due to ExB result in a lower level of anomalous transport.Unlike the core, there is a finite ExB reduction in the trapped particle population. Our result allows us to suggest a model for the pedestal formation. We next proceed by retaining collisions in the pedestal to find that finite ExB drift orbit effects reduce the neoclassical banana regime ion heat conductivity below its core value, and substantially modify the poloidal ion flow. Both the thermal diffusivity reduction and the enhancement in the residual are caused by the trapped ion region being moved towards the tail of the ion distribution function once the ExB drift begins to compete with the poloidal projection of parallel streaming. These calculations ignore ExB shear in the pedestal for simplicity, but work in progress will address this more realistic situation. Pedestal backgroundIn the alternate gyrokinetic technique that has been developed to analyze the tokamak pedestal canonical angularis taken as the gyrokinetic radial variable [1] rather than the radial guiding center location Ψ ≡ ψtreatments. This approach allows strong radial plasma gradients to be treated when considering zonal flow, neoclassical effects, and the turbulent behavior. The nonlinear gyrokinetic equation retains large poloidaldrift and orbit squeezing effects on zo...

Summary of the magnetic confinement theory and modelling activity presented at the 23rd IAEA Fusion Energy Conference

“…Not surprisingly the accuracy of numerical simulations has been debated. Parra and Catto have questioned the ability of gyrokinetic codes to predict momentum transport with the required level of precision [78,79]. Pustovitov has produced a general formulation of momentum transport [80], while Miyato built a set of gyrokinetic equations, which is valid for high E × B drift velocity [81].…”

“…Catto presents the effects of a strong pedestal radial electric field. He finds that it enhances the residual ZF regulation of turbulence, decreases the ion heat diffusivity, acts to reverse the poloidal flow and enhances the bootstrap current [79]. Finally, Jenko finds that the residual electron transport can be attributed to ETG turbulence when using the parameters of ASDEX-Upgrade [51].…”