MIT PSFC

Abstract: PSFC/JA-04-17

Transport-Driven Scrape-off Layer Flows and the Boundary Conditions Imposed at the Magnetic Separatrix in a Tokamak Plasma

B. LaBombard, J.E. Rice, A.E. Hubbard, J.W. Hughes, M. Greenwald, J. Irby, Y. Lin, B. Lipschultz, E.S. Marmar, C.S. Pitcher1, N. Smick, S.M. Wolfe, S.J. Wukitch


1presently at 132 Boxwood Ave., Toronto, Canada M4N1Y5.

Plasma profiles and flows in the low- and high-field side scrape-off layer (SOL) regions in Alcator C-Mod are found to be remarkably sensitive to magnetic separatrix topologies (upper-, lower-, and double-null) and to impose topology-dependent flow boundary conditions on the confined plasma. Near-sonic plasma flows along magnetic field lines are observed in the high-field SOL with magnitude and direction clearly dependent on x-point location. The principal drive mechanism for the flows is a strong ballooning-like poloidal transport asymmetry: parallel flows arise so as to re-symmetrize the resulting poloidal pressure variation in the SOL. Additionally, the decrease in cross-sectional area of a magnetic flux tube connecting from low to high-field regions appears to act as a ‘nozzle’, increasing flow velocities in the high-field SOL. Secondary flows involving a combination of toroidal rotation and Pfirsch-Schlüter ion currents are also evident. As a result of the transport-driven parallel flows, the SOL exhibits a net co-current (counter-current) volume-averaged toroidal momentum when BxGradB is toward (away from) the x-point. Depending on discharge conditions, flow momentum can couple across the separatrix and affect the toroidal rotation of the confined plasma. This mechanism accounts for a positive (negative) increment in central plasma co-rotation seen in L-mode discharges when BxGradB is toward (away from) the x-point. Experiments suggest that topology-dependent flow boundary conditions may also play a role in the sensitivity of L-H power threshold to x-point location: in a set of otherwise similar discharges, the L-H transition is seen to be coincident with central rotation achieving roughly the same value, independent of magnetic topology. For discharges with BxGradB pointing away from the x-point (i.e., with the SOL flow boundary condition impeding co-current rotation), the same characteristic rotation can only be achieved with higher input power.

5/12/04 Full text: PDF (55  pages,  778kB)