Organizational Unit:
Fusion Research Center

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Now showing 1 - 10 of 12
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    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.
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    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.
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    Higher Order Approximations of the TEP Method for Neutral Particle Transport in Edge Plasmas
    (Georgia Institute of Technology, 2006) Stacey, Weston M. ; Zhang, Dingkang ; Mandrekas, John
    Higher order approximations, which take into account the effects of angular anisotropy, spatial non-uniformity and energy dependence of the distribution of neutral particles, have been developed and implemented to extend the range of validity of the Transmission and Escape Probabilities (TEP) method for the calculation of neutral particle transport in plasmas. Comparisons with Monte Carlo calculations of model test problems and DIII-D L- and H- mode discharges show that these new extensions significantly improve the accuracy and extend the range of validity of the TEP methodology.
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    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.
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    Nuclear Mission for Fusion (Transmutation, fissle breeding and Pu dispostion)
    (Georgia Institute of Technology, 2006) Stacey, Weston M.
    There are potential applications of fusion neutron sources to 'drive' sub-critical fission reactors to perform one or more possible 'nuclear' missions. Since only a fraction of the neutrons in these applications would be fusion neutrons, the requirements are modest relative to the requirements for pure fusion electrical power (e.g. for the transmutation mission-- fusion power P[subscript fus] ≤ 250 MW, fusion power density β [subscript N] ≤ 2.5, 14 MeV neutron wall load Γ[subscript n] < 1 MW/m² and power amplification Q[subscript p] ≤ 2). A sub-critical, source-driven reactor almost certainly would be more expensive and initially would have lower availability than a conventional critical reactor, because of the additional cost and lower initial availability of the fusion or accelerator neutron source. In order to be competitive with a critical reactor for a given mission, a sub-critical reactor must introduce certain advantages that allow the mission to be carried out more efficiently, and there appear to be such advantages. Making use of ITER physics and technology, using ITER as a prototype, and adopting the reactor and processing technology being developed in the nuclear program could lead to a fusion-driven sub-critical reactor for the transmutation of spent nuclear fuel, fissile breeding or disposition of weapons-grade plutonium being on-line by 2040, as compared to the plans for putting critical and accelerator-driven sub-critical reactors on-line for such missions by 2030. All of the R and D needed to develop the fusion neutron source for such a facility is directly on the path to fusion power (in fact is needed for an electric power DEMO); and the operation of a fusion-driven sub-critical reactor could also serve the purposes envisioned for a ‘volume neutron source’, thus taking the place of such a device in the development path to fusion power.
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    A neoclassical calculation of toroidal rotation profiles and comparison with DIII-D measurements
    (Georgia Institute of Technology, 2006) Stacey, Weston M. ; Johnson, R. W. ; Mandrekas, John
    Momentum and particle balance and neoclassical viscosity were applied to calculate the radial profile of toroidal rotation velocity in several DIII-D [J. Luxon, Nucl. Fusion, 42, 614 (2002)] discharges in a variety of energy confinement regimes (Lowmode, Low-mode with Internal Transport Barrier, High-mode, and High-mode with Quiescent Double Barrier). Calculated toroidal rotation velocities generally were found to (over-) predict measured values to well within a factor of 2.
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    A sub-critical, He-cooled, fast reactor for the transmutation of spent nuclear fuel (DOE Grant ER54350)
    (Georgia Institute of Technology, 2005-10) Stacey, Weston M.
    A design concept and supporting analysis are presented for a He-cooled, fast reactor for the transmutation of spent nuclear fuel. Coated TRU fuel particles in a SiC matrix are used. The reactor operates sub-critical (k ≤ 0.95), with a tokamak D-T fusion neutron source, to achieve > 90% TRU burnup in repeated 5-batch fuel cycles, fissions 1.1 MT/FPY, and produces 700 MWe net electrical power. The reactor design is based on nuclear, fuels, materials and separations technologies being developed in the GEN-IV, NGNP and AFCI Programs and similar international programs, and the fusion neutron source is based on the physics and technology supporting the ITER design.
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    Calculation of toroidal rotation profiles in DIII-D using neoclassical viscosity (DoE Grant ER54538)
    (Georgia Institute of Technology, 2005-10) Stacey, Weston M. ; Johnson, R. W. ; Mandrekas, John
    Momentum and particle balance and neoclassical viscosity were applied to calculate the radial profile of toroidal rotation in several DIII-D [J. Luxon, Nucl. Fusion, 42, 614 (2002)] discharges in a variety of energy confinement regimes (Low-mode, Low-mode with Internal Transport Barrier, Highmode, and High-mode with Quiescent Double Barrier). Calculated toroidal rotation velocities were found to over-predict measured values most in the center—by factors of 1.5 to 3--with the over-prediction generally decreasing with increasing radius, for the L, H and ITB mode shots, but the single impurity species approximation could not properly model the multiple Ni and Cu charge states in the QDB shots.
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    Rotation velocities and radial electric field in the plasma edge
    (Georgia Institute of Technology, 2005-10) Stacey, Weston M.
    The toroidal and poloidal rotation and related radial electric field observed in the edge (and core) of tokamak plasmas are of interest for several reasons, not least of which is what they reveal about radial momentum transport, but also because of their apparent role in the L-H transition and the edge pedestal. It was recently shown that if the heat transport coefficients and rotation velocities are taken from experiment, then the particle, momentum and energy balance equations and the conductive heat conduction relation are sufficient to determine the observed edge pedestal profile structure in the density and temperature profiles in several DIII-D discharges. Thus, it would seem that understanding the edge pedestal structure is a matter of understanding the edge rotation profiles. We present a practical computational model for the rotation and the radial electric field profiles in the plasma edge that is based on momentum and particle balance, includes both convective (including anomalous) and neoclassical gyroviscous momentum transport, and incorporates atomic physics effects associated with recycling neutrals.
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    Investigation of edge pedestal structure in DIII-D (DoE Grant ER54538)
    (Georgia Institute of Technology, 2005-10) Stacey, Weston M. ; Groebner, Rich J.
    A calculation based on the requirements of particle, momentum and energy conservation, conductive heat transport and atomic physics resulting from a recycling and fueling neutral influx was employed to investigate the experimental density, temperature, rotation velocities and radial electric field profiles in the edge of three DIII-D [J. Luxon, Nucl. Fusion, 42, 614 (2002)] high-mode plasmas. The calculation indicated that the cause of the pedestal structure in the density was a momentum balance requirement for a steep negative pressure gradient to balance the forces associated with an edge peaking in the inward pinch velocity (caused by the observed edge peaking in the radial electric field and rotation velocity profiles) and, to a lesser extent, in the outward radial particle flux (caused by the ionization of recycling neutrals). Thermal and angular momentum transport coefficients were inferred from experiment and compared with theoretical predictions, indicating that thermal transport coefficients were of the magnitude predicted by neoclassical and ion-temperature-gradient theories (ions) and electrontemperature- gradient theory (electrons), but that neoclassical gyroviscous theory plus atomic physics effects combined were not sufficient to explain the inferred angular momentum transfer rate throughout the edge region.