A numerical study of density limit mechanisms in tokamaks

A core particle and power balance model with radiative cooling, a ballooning-mode-limit pedestal model, a “two-point” plasma divertor model with radiative and atomic recycling cooling, and a two-dimensional (2-D) neutral transport model have been combined to model the coupled plasma core-edge physics in a tokamak. This model has been applied to examine the physical mechanisms which limit the attainable core density. The first limitation is fueling rate and core penetrability. For fueling that is able to penetrate the core, the core density increases with a fueling rate up to a limit set by one of two other mechanisms. When a sufficient impurity concentration is present, the mechanism that limits the maximum density attainable in the core seems to be an impurity-driven thermal instability that produces a thermal collapse of the core plasma. At lower impurity concentrations, the buildup of plasma density in the divertor and the associated increase in atomic recycling and ionization cooling causes a collapse of the divertor plasma temperature, leading to a thermal collapse of the core plasma. An increase in core heating input power or in plasma confinement extends the point of thermal collapse to higher density.