New Insights into Tokamak Density Limit
Experiments on the Alcator C-Mod tokamak have shed new light on the mechanism for the density limit, one of the fundamental operational boundaries in magnetic confinement experiments. As the density is raised toward the limit, strong fluctuations move from outer regions into the main plasma, cooling the plasma edge which disturbs the basic plasma equilibrium. For reactor designs, performance depends critically on density and is often optimized near the limit. In the tokamak, the limit manifests itself first as a drop in edge temperature followed by a major disruption, a macroscopic MHD event which terminates the discharge. Based purely on empirical observations, the density limit can be described by a simple and robust empirical scaling equation. The physical mechanism which leads to this scaling is the subject of active research. On C-Mod, measurements of plasma fluctuations in the plasma edge with a reciprocating probe show a dramatic change in the character of the turbulence as the density is raised to the limit. At densities well below the limit, the plasma near the boundary (separatrix) is characterized by moderate-frequency turbulence (on the order of a few hundred kHz with auto-correlation times of a few microseconds). In this regime, heat transport is dominated by parallel conductivity (along magnetic field lines). As the density is increased, a regime of fluctuations characterized by large amplitudes and long-correlation times moves inward from the outer scrape-off region, crossing the separatrix and intruding into the main plasma as the limit was reached. In this regime, transport is dominated by turbulent convection which could account for the edge cooling which precedes density limiting phenomena. (Paper BI1.006 - contact Brian LaBombard, 617 253-6942, Martin Greenwald, 617 253-6053)