Parallel flow in the JET scrape-off layer

Parallel plasma flows with Mach number M≈0.5 have been measured in the JET scrape-off layer (SOL). These flows show a maximum some 10-20 mm mid-plane outside the separatrix, and decrease towards the separatrix. The flow reverses when BT and IP are reversed, but is asymmetric about zero. A number of possible mechanisms for the observed flow are suggested, including ion Pfirsch-Schlüter flow, ballooning transport, momentum transfer, reverse flow (ionization driven flow) and co-current toroidal momentum generated in the SOL by ion ∇ and centrifugal drifts.