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Fusion Research Center

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Now showing 1 - 10 of 12
  • Item
    Edge Pedestal Structure
    (Georgia Institute of Technology, 2004-12) Stacey, Weston M.
    The hypothesis is advanced and investigated that, in between or in the absence of ELMs (edge-localized-modes), the structure of the edge pedestal is determined by the transport requirements of plasma particle, momentum and energy balance and recycling neutral atoms. "Pedestal equations" following from this hypothesis are presented and applied to calculate the edge density, temperature, rotation velocity and radial electric field profiles in a DIII-D H (high)-mode plasma. A distinct pedestal structure in the density and temperature profiles and sharp negative peaks in the radial electric field and poloidal velocity just inside the separatrix are predicted, in qualitative and quantitative agreement with measured values. Details of the calculation are discussed.
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    Structure of the edge density pedestal in tokamaks
    (Georgia Institute of Technology, 2004-09) Stacey, Weston M.
    A 'first-principles' model for the structure of the edge density pedestal in tokamaks between or in the absence of edge localized magnetohyrodynamic instabilities is derived from ion momentum and particle conservation and from the transport theory of recycling neutral atoms. A calculation for (high) H-mode tokamak discharge parameters indicates that the equations have a self-consistent solution which has an edge pedestal in the ion density profile and sharp negative spikes in the poloidal velocity and radial electric field profiles in the edge pedestal, features characteristic of H-mode edge profiles. These sharp negative spikes in radial electric field and poloidal rotation produce a peak in the inward ion pinch velocity in the sharp gradient (pedestal) region which produces an edge particle transport barrier. The calculated magnitude of the density at the top of the pedestal and the density gradient scale length and radial electric field in the pedestal region are comparable to measured values.
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    A neoclassical model for toroidal rotation and the radial electric field in the edge pedestal
    (Georgia Institute of Technology, 2004-06) Stacey, Weston M.
    A model for the calculation of toroidal rotation velocities and the radial electric field in the edge pedestal of tokamaks is described. The model is based on particle and momentum balance and the use of the neoclassical gyroviscous expression for the toroidal viscous force. Predicted toroidal rotation velocities in the edge pedestal are found to agree with measured values to within about a factor of 2 or less, for a range of DIII-D [Luxon, Nucl. Fusion, 42, 614, 2002] edge pedestal conditions.
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    Investigation of transport in the DIII-D edge pedestal
    (Georgia Institute of Technology, 2004-04) Stacey, Weston M.
    A comparison of various heat conduction theories with data from several DIII-D [Luxon, Nucl. Fusion, 42, 614, 2002] shots indicates: 1) that neoclassical theory is in somewhat better agreement with experiment than is ion temperature gradient mode theory for the ion thermal conductivity in the edge pedestal, although both are in reasonable agreement with experiment for most discharges; and 2) that electron temperature gradient theory (k┴cs ≤ ωpe) is in much better agreement with experiment than is electron drift wave theory (k┴cs ≤ Ωi) for the electron thermal conductivity. New theoretical expressions derived from momentum balance are presented for: 1) a ‘diffusive-pinch’ particle flux, 2) an experimental determination of the momentum transfer frequency, and 3) the density gradient scale length. Neither atomic physics nor convection can account for the measured momentum transfer frequencies, but neoclassical gyroviscosity predictions are of the correct magnitude.
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    Extensions of the TEP Neutral Transport Methodology
    (Georgia Institute of Technology, 2004-04) Stacey, Weston M. ; Zhang, Dingkang ; Mandrekas, John
    Recent extensions of the Transmission and Escape Probability methodology and its implementation in the 2-D neutral transport code GTNEUT are presented. These extensions address the issues of anisotropy of the neutral distribution function at the interfaces and the non-uniformity of the first collision source in short mean free path regions. Comparisons with Monte Carlo for a number of model problems are discussed.
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    Particle Transport and Density Gradient Scale Lengths in the Edge Pedestal
    (Georgia Institute of Technology, 2004-04) Stacey, Weston M.
    A new flux-gradient relation for the ion particle flux in the edge pedestal is derived from continuity and momentum balance, taking into account atomic physics, and cast in the form of a generalized ‘diffusion-pinch’ transport relation. This flux-gradient relation is used to derive a new expression for a first-principles calculation of the ion density gradient scale length.
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    Testing of an edge thermal instability stabilization model for the low-to-high mode power threshold
    (Georgia Institute of Technology, 2004-02) Stacey, Weston M.
    A test of a new model for the low-to-high (L-H) mode power threshold, based on the stabilization of edge thermal instabilities, is made by comparison with a set of DIII-D [J. L. Luxon, Nucl. Fusion, 42, 614, 2002] discharges at times just prior to a L-H transition. Agreement is found between the measured power crossing the separatrix just prior to the L-H transition and the predicted power threshold for the stabilization of transport enhancing thermal instabilities.
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    A Superconducting Tokamak Fusion Transmutation of Waste Reactor
    (Georgia Institute of Technology, 2004-01) Stacey, Weston M. ; Mauer, A. N. ; Mandrekas, John ; Hoffman, E. A. (Elisha Albright)
    We are developing a Fusion Transmutation of Waste Reactor (FTWR) concept—a sub-critical, metal fuel, liquid metal cooled fast reactor driven by a tokamak DT fusion neutron source. An emphasis is placed on using nuclear, separation/processing and fusion technologies that either exist or are at an advanced state of development and on using plasma physics parameters that are supported by the existing database. We have previously discussed the general capabilities of DT tokamak neutron sources for driving transmutation reactors [1] and developed a design concept for a FTWR [2] based on normal conducting magnets. The concept has been further developed in papers dealing with nuclear design and safety [3] and with the evaluation of the potential impact on radioactive waste management [4]. The purpose of this paper is to examine how the FTWR design concept would change if superconducting magnets were used.
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    Compartative fuel cycle analysis of critical and sub-critical fast reactor transmutation systems
    (Georgia Institute of Technology, 2003-10) Stacey, Weston M. ; Hoffman, E. A. (Elisha Albright)
    Fuel cycle analyses are performed to evaluate the impacts of further transmutation of spent nuclear fuel on high-level and low-level waste mass flows into repositories, on the composition and toxicity of the high-level waste, on the capacity of high-level waste repositories, and on the proliferation-resistance of the high-level waste. Storage intact of LWR spent nuclear fuel, a single recycle in a LWR of the plutonium as MOX fuel, and the repeated recycle of the transuranics in critical and sub-critical fast reactors are compared with the focus on the waste management performance of these systems. Other consideration such as cost and technological challenges were beyond the scope of this study. The overall conclusion of the studies is that repeated recycling of the transuranics from spent nuclear fuel would significantly increase the capacity of high-level waste repositories per unit of nuclear energy produced, significantly increase the nuclear energy production per unit mass of uranium ore mined, significantly reduce the radio-toxicity of the waste streams per unit of nuclear energy produced, and significantly enhance the proliferation-resistance of the material stored in high-level waste repositories.
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    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.