NUMERICAL MODELING OF EDGE PLASMAS IN TOKAMAKS WITH POLOIDAL LIMITERS

A two-dimensional, two-fluid numerical modeling of tokamak edge plasma s with poloidal limiters is investigated to generate a transport code for an understanding of edge plasma characteristics and behaviors. Pol oidal drift motions, an electrostatic potential, and plasma currents a re solved self-consistently with other edge plasma variables, which ha s seldom been done in other published work. An implicit MacCormack num erical scheme is employed to solve the nonlinear time-dependent transp ort equations describing particle, momentum, and energy conservations for electrons and ions. Transports of recycling neutrals are modeled u sing an analytical method and the validity is confirmed through a comp arison with the Monte-Carlo simulation results. As a result of the pre sent calculations, it is found that the induced sheared radial electri c field in the edge region turns out to change its sign across the Las t Closed Flux Surface (LCFS) due to the radially varying electrostatic potential and that the sheared poloidal rotation, which is believed t o play an important role in suppressing the edge turbulence, is mainly driven by an electric (ExB) drift dominating over a diamagnetic (V(pi )xB) drift and changes its direction across the LCFS from the ion diam agnetic direction in the Scrape-Off Layer (SOL) to the electron one in the Radiating Layer (RL). The calculated results agree relatively wel l with the measured data, which have not been explained satisfactorily by the neoclassical theory. As a practical application of the code, s ome design-related parameters, such as heat fluxes onto and sputtering rates of the limiter target plates, are presented.