Large transport-induced operation limits of tokamak plasmas

Abstract

The two-dimensional phase space of tokamak edge plasmas identified in the numerical simulations by B. Rogers et al. [Phys. Rev. Lett. 81, 4396 (1998)] provides a unique prescription for the various regimes of operation of tokamak plasmas. Recent observations on Alcator C-Mod of these regimes, identified in terms of the above-mentioned phase-space parameters, is found to be in very good agreement with simulation results of Rogers et al. In this phase space, they identified a boundary at high collisionality that defines a region that is operationally inaccessible owing to very large transport in the edge region of the tokamaks. A second boundary at moderate to low collisionality is also indicated and associated with the transition between the low-confinement mode and the high-confinement mode. The high collisionality boundary is of particular interest since it appears to be fundamentally related to the empirical "density limit" that is observed in tokamaks. In this Letter, we provide a theory that determines the conditions necessary for very high transport and hence the origin of the inaccessible "density limit" in the two-dimensional phase space.