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B. LaBombard, J.E. Rice, A.E. Hubbard, J.W. Hughes, M. Greenwald,
R.S. Granetz, J.H. Irby, Y. Lin, B. Lipschultz, E.S. Marmar, K. Marr, D. Mossessian,
R. Parker, W. Rowan, N. Smick, J.A. Snipes, J.L. Terry, S.M. Wolfe, S.J.
Wukitch and the Alcator C-Mod Team
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Factor of ~2 higher power thresholds for low-
to high-confinement mode transitions (LH) with unfavorable x-point topologies
in Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] are linked to flow boundary
conditions imposed by the scrape-off layer (SOL). Ballooning-like transport
drives flow along magnetic field lines from low- to high-field regions with
toroidal direction dependent on upper/lower x-point balance; the toroidal
rotation of the confined plasma responds, exhibiting a strong counter-current
rotation when Bx∇B points away from the x-point. Increased auxiliary heating
power (rf, no momentum input) leads to an L-H transition at approximately
twice the edge electron pressure gradient when Bx∇B points away. As gradients
rise prior to the transition, toroidal rotation ramps toward the co-current
direction; the H-mode is seen when the counter-current rotation imposed by
the SOL flow becomes compensated. Remarkably, L-H thresholds in lower-limited
discharges are identical to lower x-point discharges; SOL flows are also found
similar, suggesting a connection. |
