Investigation of toroidal flow effects on L-H transition in tokamak plasma based on bifurcation model

Abstract

The impacts of toroidal flow on the L-H transition phenomenon in tokamak plasmas based on bifurcation concept are investigated. A set of pressure and density transport equations with both neoclassical and anomalous effects included is considered. The anomalous transport suppression mechanism considered is only from a flow shear, which is calculated based on a force balance equation with both pressure gradient and toroidal velocity components included. The toroidal velocity can be calculated using four different models. The first model is an empirical model in which the velocity is dependent on a local ion temperature. The second model is based on neoclassical toroidal viscosity theory in which the velocity is driven by ion temperature gradient. In the third model, the velocity is dependent on current density flow in plasma. The fourth model is essentially the third model including effect of an intrinsically generated bootstrap current. It is found that inclusion of toroidal velocity can substantially increase the plasma pressure and density, mainly due to an increase of the pedestal width. It is also found that the pedestal for pressure tends to form first. After the pedestal forms, it expands inwards with the characteristic of super-diffusive nature in initial state and become subdiffusive nature in final state before reaching steady state. The expansion speed depends sensitively on the strength of flow shear effect. It is also found that the time required plasma to reach steady state after the L-H transition is much longer than that for L-H transition.