American Institute of Physics, Physics of Plasmas, 6(13), p. 062503
DOI: 10.1063/1.2206168
Full text: Unavailable
Experiments on the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] have demonstrated the existence of a density limit that appears to be caused not by radiation but by perpendicular heat convection in the scrape-off layer (SOL). The present paper shows that the collisionality dependence of the blob model provides a plausible explanation for this convective density limit under certain conditions. The thermal equilibrium and stability of the SOL are studied in a two-point (midplane, divertor) model including perpendicular heat convection. A general scaling of the perpendicular heat flux q⊥ with temperature is used to derive conditions for the SOL thermal equilibrium to have two roots and a fold catastrophe associated with root merger. For the particular scaling of q⊥ given by a ``disconnected'' blob model, this equilibrium limit can be interpreted as a SOL density limit associated with X-point cooling in which the blob heat transport plays a role analogous to radiation in other theories.