Dependence of the L-H Transition on X-point Geometry and Divertor Recycling on NSTX
Authors: J.L. Battaglia et. al.
Abstract:
The edge electron (Te ) and ion temperature (Ti ) at the time of the L-H transition increase when the X-point
radius (RX ) is reduced to a high-triangularity shape while maintaining constant edge density. Consequently the L-H
power threshold (PLH ) is larger for the high-triangularity shape. This supports the prediction that a single-particle
loss hole, whose properties are strongly linked to RX and Ti , influences the edge radial electric field (Er ) and Er X B
flow-shearing rate available for turbulence suppression. Simulations using XGC0, a full-f drift-kinetic neoclassical
code, indicate that maintaining a constant Er X B flow-shearing rate does require a larger heat flux and edge Ti as RX
decreases. NSTX also observes a decrease in PLH when the divertor recycling is decreased using lithium coatings.
However, the edge Te and Ti at the L-H transition appear independent of the divertor recycling for a constant shape.
XGC0 calculations demonstrate that more heat flux is needed to maintain the edge Ti and the Er x B flow-shearing
rate as the contribution of divertor recycling to the overall neutral fueling rate increases.
__________________________________________________
Submitted to: Nuclear Fusion (May 2013)
__________________________________________________
Download PPPL-4880 (pdf 275 KB 14 pp)
__________________________________________________