SOL Thermal Instability due to Radial Blob Convection

C-Mod data\footnote{M. Greenwald, Plasma Phys. Contr. Fusion {\bf 44}, R27 (2002).} suggests a density limit when rapid perpendicular convection dominates SOL heat transport. This is supported by a recent analysis\footnote{D.A. Russell et al., Phys. Rev. Lett. {\bf 93}, 265001 (2004).} of BOUT code turbulence simulations, which shows that rapid outwards convection of plasma by turbulent blobs is enhanced when the X-point collisionality is large, resulting in a synergistic effect between blob convection and X-point cooling. This work motivates the present analysis of SOL thermal equilibrium and instability including an RX-regime model\footnote{J.R. Myra and D.A. D’Ippolito, Lodestar Report LRC-05-105 (2005).} of blob particle and heat transport. Two-point (midplane, X-point) SOL thermal equilibrium and stability models are considered including both two-field (T) and four-field (n,T) treatments. The conditions under which loss of thermal equilibrium or thermal instabilities occur are established, and relations to the C-Mod data are described.