A scrape-off layer based model for Hugill-Greenwald type density limits

Abstract

In present tokamaks the density limit (DL) is observed to coincide with complete divertor detachment. A scrape-off layer (SOL) based model is presented for this `detachment limit'. Emphasis is placed on understanding the power independent, 1/q-type scaling (Hugill-Greenwald type scaling) observed in many divertor machines. A simple analytical model for the SOL, complemented by dimensional considerations on gas targets, is used to describe the underlying mechanism and derive scalings for the critical density. Physically, the power independent regime is associated with an intermediate transverse neutral collisionality in the divertor. Extensive B2-EIRENE studies were conducted to verify the analytical considerations. Computational results are compared with experimental findings on JET. Possible ways to access the power dependent regime are discussed