A SCRAPE-OFF LAYER BASED MODEL FOR HUGILL-GREENWALD-TYPE DENSITY LIMITS

In present tokamaks the density limit (DL) is observed to coincide wit h complete divertor detachment. A scrape-off layer (SOL) based model i s presented for this 'detachment limit'. Emphasis is placed on underst anding 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 target s, is used to describe the underlying mechanism and derive scalings fo r the critical density. Physically, the power independent regime is as sociated with an intermediate transverse neutral collisionality in the divertor. Extensive B2-EIRENE studies were conducted to verify the an alytical considerations. Computational results are compared with exper imental findings on JET. Possible ways to access the power dependent r egime are discussed.