(Georgia Institute of Technology, 2005-04)
Stacey, Weston M.; Groebner, Rich J.
A calculation of edge density and temperature profiles based on "classical"
physics - particle, momentum and energy balance, heat conduction closure relations,
neutral particle transport - yielded a pedestal structure that is qualitatively and
quantitatively similar to that found experimentally in five DIII-D [J. Luxon, Nucl. Fusion,42, 614 (2002)] discharges, when experimental radial electric field and rotation profiles
and experimentally inferred heat transport coefficients were used. The principal cause of
the density pedestal was a peaking of the inward pinch velocity just inside the separatrix
caused by the negative well in the experimental electric field, and the secondary cause
was a peaking of the radial particle flux caused by the ionization of incoming neutrals.
There is some evidence that this peaking of the radial particle flux just inside the
separatrix may also be responsible in part for the negative electric field in that location.
(Georgia Institute of Technology, 2003-06)
Stacey, Weston M.; Groebner, Rich J.
A framework has been formulated for the further development and testing of a predictive edge pedestal model. This framework combines models for the interaction of the various physical phenomena acting in the edge pedestal—transport, neutral fueling penetration, atomic physics cooling, MHD (magnetohydrodynamic) stability limit, edge density limit—to determine the pedestal widths and gradient scale lengths. Predictive models for some of these specific phenomena have been compared with DIII-D [ J. L. Luxon, Nucl. Fusion, 42, 614, 2002] measurements. It was found that a neutral penetration model for the density width and a MHD model for the maximum pedestal pressure for stability against ideal pressure-driven surface modes were roughly consistent with experimental observation, but that in both cases some refinements are needed. The major impediments to implementation of a predictive edge pedestal model within the framework of this paper are the lack of knowledge of transport coefficients in the pedestal and the unavailability of an usable characterization of the state-of-the-art MHD stability-limit surface in the space of edge parameters. Efforts to remedy these and other deficiencies and to establish a predictive model for the calculation of density, temperature and pressure widths and gradients in the edge pedestal are suggested.