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Fusion Research Center

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George W. Woodruff School of Mechanical Engineering

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Now showing 1 - 10 of 10

Non-Diffusive Transport in the Tokamak Edge Pedestal

(Georgia Institute of Technology, 2012) Stacey, Weston M. ; Groebner, Rich J. ; Evans, T. E.

There are (at least) two classical mechanisms for non-diffusive transport in the edge plasma: i) particle “pinch” velocities due to forces such as VxB, and Er; and ii) outward drifts due to ion-orbit loss and X-transport. A theoretical development for the treatment of these non-diffusive transport mechanisms within the context of fluid theory is assembled and applied to several DIII-D discharges in order to investigate the importance of these non-diffusive transport mechanisms in the edge pedestal. Several interesting insights emerge from this investigation.
Force Balance and Ion Particle Transport Differences in High and Low Confinement Tokamak Edge Pedestals

(Georgia Institute of Technology, 2010-11-22) Stacey, Weston M. ; Groebner, Rich J.

The various terms in the radial force balance in the edge plasma are evaluated using experimental data from the low (L) and high (H) confinement phase of a DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] discharge in order to investigate the differences in the radial force balance among the several electromagnetic and pressure gradient forces in L-mode and H-mode. The roles of cross-field toroidal momentum transport and of a radial pinch velocity in determining different radial particle fluxes in L-mode and H-mode are elucidated
Evolution of the H-mode edge pedestal between ELMs

(Georgia Institute of Technology, 2010-08) Stacey, Weston M. ; Groebner, Rich J.

The evolution of edge pedestal parameters between edge-localized modes (ELMs) is analyzed for an H-mode DIII-D [J Luxon, Nucl. Fusion 42, 612 (2002)] discharge. Experimental data are averaged over the same sub-intervals between successive ELMs to develop data that characterize the evolution of density, temperature, rotation velocities, etc. over the interval between ELMs. These data are interpreted within the context of the constraints imposed by particle, momentum and energy balance, in particular in terms of the pinch-diffusion relation for radial particle flux that is required by momentum balance. It is found that in the edge pedestal there is an increase of both inward (pinch) electromagnetic and outward (diffusive) pressure gradient forces over the inter-ELM interval.
Interpretation of particle pinches and diffusion coefficients in the edge pedestal of DIII-D H-mode plasmas

(Georgia Institute of Technology, 2009-10-15) Stacey, Weston M. ; Groebner, Rich J.

A procedure is described for evaluating particle pinches to be used in interpreting particle diffusion coefficients from measured density and temperature profiles in the edge pedestal of tokamak plasmas. Application to the interpretation of two DIII-D [ J. Luxon, Nucl. Fusion 42, 614 (2002) ]. discharges yields new information about particle pinches and particle diffusion coefficient profiles in the edge pedestal.
Interpretation of edge pedestal rotation measurements in DIII-D

(Georgia Institute of Technology, 2008-01-25) Stacey, Weston M. ; Groebner, Rich J.

A novel methodology for inferring experimental toroidal angular momentum transfer rates from measured toroidal rotation velocities and other measured quantities has been developed and applied to analyze rotation measurements in the DIII-D J. Luxon, Nucl. Fusion 42, 6149 2002 edge pedestal. The experimentally inferred values have been compared with predictions based on atomic physics processes and on neoclassical toroidal viscosity. The poloidal rotation velocities have been calculated from poloidal momentum balance using neoclassical parallel viscosity and a novel retention of all terms in the poloidal momentum balance, and compared with measured values in the DIII-D edge pedestal.
Experimentally inferred thermal diffusivities in the edge pedestal between edge-localized modes in DIII-D

(Georgia Institute of Technology, 2007-12-11) Stacey, Weston M. ; Groebner, Rich J.

Using temperature and density profiles averaged over the same subinterval of several successive inter-edge-localized-mode (ELM) periods, the ion and electron thermal diffusivities in the edge pedestal were inferred between ELMs for two DIII-D [ J. Luxon, Nucl. Fusion 42, 614 (2002) ] discharges. The inference procedure took into account the effects of plasma reheating and density buildup between ELMs, radiation and atomic physics cooling, neutral beam heating and ion-electron equilibration, and recycling neutral and beam ionization particle sources in determining the conductive heat flux profiles used to infer the thermal diffusivities in the edge pedestal. Comparison of the inferred thermal diffusivities with theoretical formulas based on various transport mechanisms was inconclusive insofar as identifying likely transport mechanisms.
Thermal transport analysis of the edge region in the low and high confinement stages of a DIII-D discharge

(Georgia Institute of Technology, 2007-01-04) Stacey, Weston M. ; Groebner, Rich J.

The ion and electron thermal diffusivities have been inferred from measured density and temperature profiles in the edge of a DIII-D [ J. Luxon, Nucl. Fusion 42, 614 (2002) ] discharge with a low confinement (L-mode) stage followed by a high confinement (H-mode) stage free of edge localized modes. Conductive heat flux profiles used to construct the inferred thermal diffusivities were calculated taking into account heat convection, radiation, atomic physics effects of recycling neutrals, ion-electron equilibration, and neutral beam heating. The inferred thermal diffusivities were compared with theoretical predictions.
Thermal Transport in the DIII-D Edge Pedestal

(Georgia Institute of Technology, 2006) Stacey, Weston M. ; Groebner, Rich J.

A new procedure for inferring χ[subscript i,e] in the plasma edge from experimental data and integrated modeling code calculations has been developed which takes into account atomic physics and radiation effects and convective as well as conductive heat flux profiles. Application to DIII-D shots indicates that the sharp temperature gradient pedestal region may be caused as much, if not more, by an increase (with radius) of the conductive heat flux as by a decrease of the thermal transport coefficient. Inferred χ[subscript i,e][superscript exp] are compared with theoretical predictions.
Application of a particle, momentum and energy balance model to calculate the structure of the edge pedestal in DIII-D

(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.
A framework for the development and testing of an edge pedestal model: formulation and initial comparison with DIII-D data

(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.