51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009;
Atlanta, Georgia
Session BI3: Advances in Tokamaks: Core and Edge Physics
9:30 AM–12:30 PM,
Monday, November 2, 2009
Room: Centennial II
Chair: Robert Granetz, Massachusetts Institute of Technology
Abstract ID: BAPS.2009.DPP.BI3.4
Abstract: BI3.00004 : Pedestal regulation techniques for enhanced confinement regimes on Alcator C Mod*
11:00 AM–11:30 AM
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Abstract
Author:
Jerry Hughes
(MIT PSFC)
Recent research on Alcator C-Mod has achieved greater leverage
on global confinement through both optimization and active
modification of the edge pedestal. Pedestal scalings that are
quite robust in typical H-mode operation can be broken, and
particle and thermal transport in the edge barrier can be
decoupled substantially. In H-mode, pedestal parameters show a
striking sensitivity to the ion $B\times \nabla B$ drift
direction, relative to the active x-point position, with
considerable variability observed when the distance between
separatrices is on the order of the pedestal width
($\approx$5mm) or less, $i.e.$ very near double null (DN). Near
DN H-modes can have improved confinement factors ($H_{98}>1$) as
a result of elevated pedestal temperature ($T_{ped}$), with the
edge regulated by benign small ELMs or continuous modes, regimes
desirable for ITER and other future devices. Operating with a
single null and with $\nabla B$ drift away from the x-point
allows the formation of discharges with L-mode-like particle
confinement, yet with excellent energy confinement. This
enhanced confinement regime has demonstrated $H_{98}\approx 1$,
$T_{ped}\approx $1keV, and can be maintained steady-state with
no ELMs by operating with high current and strong shaping, while
holding input power below the L-H threshold to suppress particle
barrier formation. Additional pedestal modification has also
been obtained in H-modes by application of lower hybrid (LH)
waves. Strong relaxation of the density pedestal is observed and
accompanied by increases in $T_{ped}$, providing a substantial
reduction in overall collisionality and somewhat improved
confinement. Direct interaction of the LHRF with the edge plasma
appears to play a role in enhancing the pedestal particle
transport, which conveniently relaxes the discharge to a less
dense and hotter state, one more conducive to core LH
penetration and damping. In all cases, strongly modified
pedestals affect core properties, often including surprising
effects on core rotation.
*Supported by USDoE award DE-FC02-99-ER54512.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.DPP.BI3.4