The SOL as a Turbulence-Driven Boundary Layer: Implications for Heat Load Scalings

Present day SOL scalings can be explained by a simple model linking the SOL width to magnetic drifts. The question, then, is if this scaling trend will persist. To this end, it is natural to model the SOL as a \textit{lossy thermal boundary layer} (BL) which is driven by core heat flux, and which balances drift transport, turbulent transport and parallel losses. A puzzle here is that the SOL is turbulent but also remarkably stable --- with FLR, line-tying and drift excursion (analogous to finite banana width) acting to weaken or quench the usual suspects for turbulence generation. The logical deduction is that SOL turbulence originates \textit{inside} the separatrix and subsequently enters the SOL by ``turbulence spreading''. Thus, the SOL BL is seen to be driven by \textit{both} a flux of turbulence intensity as well as heat, both emanating from the core. Note that these are in principle \textit{independent}, thus constituting two separate control parameters for the SOL. Also, since the turbulence flux is determined by pedestal dynamics and turbulence, it can introduce the appearance of ``nonlocality'' to SOL transport. Spreading effects on the SOL width are under study and will be discussed.