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  • 11
    Electronic Resource
    Electronic Resource
    Transport in gyrokinetic tokamaks (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1217-1228 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A comprehensive study of transport in full-volume gyrokinetic (GK) simulations of ion temperature gradient driven turbulence in core tokamak plasmas is presented. Though this "gyrokinetic tokamak'' is much simpler than experimental tokamaks, such simplicity is an asset, because a dependable nonlinear transport theory for such systems should be more attainable. Toward this end, two related lines of inquiry are pursued. (1) The scalings of GK tokamaks with respect to important system parameters are studied. In contrast to real machines, the scalings of larger GK systems (a/ρs(approximately-greater-than)64) with minor radius, with current, and with a/ρs are roughly consistent with the approximate theoretical expectations for electrostatic turbulent transport which exist as yet. Smaller systems manifest quite different scalings. (2) With the goal of developing a first-principles theory of GK transport, the GK data are used to infer the underlying transport physics. The data indicate that, of the many modes k present in the simulation, only a modest number (Nk∼10) of k dominate the transport, and for each, only a handful (Np∼5) of couplings to other modes p appear to be significant, implying that the essential transport physics may be described by a far simpler system than would have been expected on the basis of earlier nonlinear theory alone. Part of this analysis is the inference of the coupling coefficients Mkpq governing the nonlinear mode interactions, whose measurement from tokamak simulation data are presented here for the first time. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871482
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  • 12
    Electronic Resource
    Electronic Resource
    Anomalous energy exchange in the gBL and quasilinear theories (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 1229-1237 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The rate of turbulence-induced energy exchange W(overdot)0 between species is computed in the framework of the quasilinear and generalized Balescu–Lenard (gBL) transport theories, and the relationship between these two similar theories is thereby elucidated. For both theories, general formal expressions for W(overdot)0 are developed and then applied to the trapped electron mode for illustration. The general expressions for W(overdot)0 in the two theories are formally closely related, but can yield predictions of very different magnitude in concrete applications. The fact that quasilinear theory is not valid for saturated steady-state turbulence gives rise to certain peculiarities in its predictions for this normal experimental situation, such as permitting energy to flow from the cooler to the hotter species, even in the limit of thermal equilibrium, where real-space gradients vanish. The gBL theory may be viewed as a modification of quasilinear theory to be valid for steady-state turbulence, keeping extra terms due to the self-consistent back reaction of particles on the fluctuations, which are just such as to eliminate these peculiarities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860130
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  • 13
    Electronic Resource
    Electronic Resource
    Erratum: "A unified theory of tokamak transport via the generalized Balescu–Lenard collision operator'' [Phys. Fluids B 1, 750 (1989)] (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 1758-1758 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859212
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  • 14
    Electronic Resource
    Electronic Resource
    A unified theory of tokamak transport via the generalized Balescu–Lenard collision operator (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 750-765 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A unified basis from which to study the transport of tokamaks at low collisionality is provided by specializing the "generalized Balescu–Lenard'' collision operator to toroidal geometry. Explicitly evaluating this operator, ripple, turbulent, and neoclassical transport coefficients are obtained, simply by further specializing the single operator to different particular classes of fluctuation wavelength and mode structure. For each class of fluctuations, the operator possesses a diffusive, test-particle contribution D, and in addition a dynamic drag term F, which makes the operator self-consistent, and whose presence is accordingly essential for the resultant fluxes to possess the appropriate conservation laws and symmetries. These properties, well known for axisymmetric transport, are demonstrated for one type of turbulent transport, chosen for definiteness, by explicit evaluation of both the "anomalous diffusion'' term arising from D, as well as the closely related "anomalous pinch'' term coming from F. The latter term is neglected by test-particle calculations, but is shown to have an important impact on the predicted fluxes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859140
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  • 15
    Electronic Resource
    Electronic Resource
    Transport of energetic ions by low-n magnetic perturbations (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1471-1481 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The stochastic transport of MeV ions induced by low-n magnetic perturbations is studied, focusing chiefly on the stochastic mechanism operative for passing particles in low-frequency perturbations. Beginning with a single-harmonic form for the perturbing field, it is first shown numerically and analytically that the stochastic threshold of energetic particles can be substantially lower than that of the magnetic field, contrary to earlier expectations, so that magnetic perturbations could cause appreciable loss of energetic ions without destroying the bulk confinement. The analytic theory is then extended in a number of directions, to clarify the relation of the present stochastic mechanism to instances already found, to allow for more complex perturbations, and to consider the more general relationship between the stochasticity of magnetic fields and that of particles of differing energies (and pitch angles) moving in those fields. It is shown that the stochastic threshold is, in general, a nonmonotonic function of energy, whose form can to some extent be tailored to achieve desired goals (e.g., burn control or ash removal) by a judicious choice of the perturbation. Illustrative perturbations are exhibited that are stochastic for low- but not for high-energy ions, for high- but not for low-energy ions, and for intermediate-energy ions, but not for low or high energy. The second possibility is the behavior needed for burn control; the third provides a possible mechanism for ash removal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860886
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  • 16
    Electronic Resource
    Electronic Resource
    MeV ion confinement in the TFTR tokamak (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1411-1414 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper the confinement of the MeV ions that are created by D–D reactions in the TFTR tokamak [in Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol. 1, p. 27] is described. The ions that escape from the plasma are measured by a new type of detector located just outside the plasma edge. Most measurements made with this detector are consistent with the first-orbit loss of these ions. Exceptions are correlated with strong magnetohydrodynamic activity, and a preliminary explanation is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859565
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  • 17
    Electronic Resource
    Electronic Resource
    Alpha particle confinement in tokamaks (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 980-982 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An assessment of diffusive tokamak transport mechanisms of concern for alpha particles indicates that the "stochastic regime'' is the only one that appears to pose a real danger for adequate alpha confinement. This fact, in conjunction with the threshold character of that mechanism, allows one to decide whether an alpha born at a given location will be lost or confined, according to a very simple criterion. Implementing this criterion numerically results in a new code for the assessment of alpha confinement, which is orders of magnitude faster than earlier codes used for this purpose.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858986
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  • 18
    Electronic Resource
    Electronic Resource
    Stochastic transport of MeV ions by low-n magnetic perturbationsf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2460-2467 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent progress toward understanding two transport phenomena of importance to fusion reactors is described, in the framework of a theory of the stochastic transport of energetic ions in low-frequency, low-n perturbations. These are the observed enhancement of fusion product loss by magnetohydrodynamic (MHD) perturbations, and a new possible mechanism for ash removal and burn control. Significant features of the theory bearing on both problems are the variety of transport scalings with energy possible from perturbations of differing structure, and the contribution to the transport from perturbing "grad-B'' drifts, neglected in earlier studies, but that can be dominant for MeV ions. Numerical evidence is presented substantiating these analytic expectations. The transition from above to below the global stochastic threshold, and some remaining disparities between experimental observations and numerical/theoretical results, are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860731
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  • 19
    Electronic Resource
    Electronic Resource
    Quasilinear diffusion in stochastic magnetic fields: Reconciliation of drift-orbit modification calculations (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1160-1163 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An apparent difference between two calculations of drift-orbit modifications to the spatial diffusion rate of particles in stochastic magnetic fields is addressed [H. E. Mynick and J. A. Krommes, Phys. Rev. Lett. 43, 1506 (1979); J. R. Myra and P. J. Catto, Phys. Fluids B 4, 176 (1992)]. The calculations are reconciled by noting the relevance of an inequality which was not discussed in these studies. It is shown, both analytically and by Monte Carlo simulation, that the diffusion coefficient can be sensitive to the spectral width of the magnetic turbulence relative to a finite Larmor radius parameter.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860906
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  • 20
    Electronic Resource
    Electronic Resource
    Cooling the greenhouse with bioenergy (1991)
    [s.l.] : Nature Publishing Group
    Nature 353 (1991), S. 11-12 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] MOST proposals for reducing global warming have focused on the need to plant more trees in forest reserves, the idea being that carbon dioxide absorption would continue until the trees mature, say for 40-100 years. Although it is recognized that this is not a permanent solution, this 'carbon ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/353011a0
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