The Experts below are selected from a list of 1824 Experts worldwide ranked by ideXlab platform

K.e. Thome - One of the best experts on this subject based on the ideXlab platform.

D J Schlossberg - One of the best experts on this subject based on the ideXlab platform.

  • high Confinement Mode and edge localized Mode characteristics in a near unity aspect ratio tokamak
    Physical Review Letters, 2016
    Co-Authors: K.e. Thome, R.j. Fonck, Michael W. Bongard, J.l. Barr, G.m. Bodner, Marcus G. Burke, D.m. Kriete, J.m. Perry, D J Schlossberg
    Abstract:

    Tokamak experiments at near-unity aspect ratio A ≲ 1.2 offer new insights into the self-organized H-Mode plasma Confinement regime. In contrast to conventional A ~ 3 plasmas, the L–H power threshold PLH is ~15× higher than scaling predictions, and it is insensitive to magnetic topology, consistent with Modeling. Edge localized Mode (ELM) instabilities shift to lower toroidal Mode numbers as A decreases. Furthermore, these ultralow-A operations enable heretofore inaccessible Jedge(R,t) measurements through an ELM that show a complex multimodal collapse and the ejection of a current-carrying filament.

P H Diamond - One of the best experts on this subject based on the ideXlab platform.

  • turbulence elasticity a key concept to a unified paradigm of l i h transition
    Nuclear Fusion, 2015
    Co-Authors: P H Diamond, Yusuke Kosuga, Ö D Gürcan
    Abstract:

    We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow (ZF) shears. It is shown that when |?V??ZF|/??k???1, with |?V??ZF| the ZF shearing rate and ??k the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DW?ZF turbulence, we propose a unified paradigm of low-Confinement-Mode to intermediate-Confinement-Mode to high-Confinement-Mode (L???I???H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-Mode. The transition from an unstable L-Mode to I-Mode is predicted to occur when ??k? ??V??cr(?V?E?B is mean E???B shear flow driven by edge radial electrostatic field), the I-Mode will terminate and spiral into the H-Mode.

  • Turbulence elasticity: a key concept to a unified paradigm of L → I → H transition
    Nuclear Fusion, 2015
    Co-Authors: P H Diamond, Yusuke Kosuga, Ö D Gürcan
    Abstract:

    We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow (ZF) shears. It is shown that when |?V??ZF|/??k???1, with |?V??ZF| the ZF shearing rate and ??k the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DW?ZF turbulence, we propose a unified paradigm of low-Confinement-Mode to intermediate-Confinement-Mode to high-Confinement-Mode (L???I???H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-Mode. The transition from an unstable L-Mode to I-Mode is predicted to occur when ??k? ??V??cr(?V?E?B is mean E???B shear flow driven by edge radial electrostatic field), the I-Mode will terminate and spiral into the H-Mode.

  • Phase dynamics criterion for fast relaxation of high-Confinement-Mode plasmas
    Physical Review Letters, 2014
    Co-Authors: Pw Xi, X.q. Xu, P H Diamond
    Abstract:

    We derive a new nonlinear criterion for the occurrence of fast relaxation (crash) events at the edge of high-Confinement-Mode plasmas. These fast relaxation events called ELMs (edge-localized Modes) evolve from ideal magnetohydrodynamics (MHD) instabilities, but the crash is not due only to linear physics. We show that for an ELM crash to occur, the coherence time of the relative phase between potential and pressure perturbations must be long enough to allow growth to large amplitude. This phase coherence time is determined by both linear and nonlinear dynamics. An ELM crash requires that the instability growth rate exceed a critical value, i.e., γ>γc, where γc is set by 1/τc and τc is the phase coherence time. For 0

  • phase dynamics criterion for fast relaxation of high Confinement Mode plasmas
    Physical Review Letters, 2014
    Co-Authors: X.q. Xu, Pw Xi, P H Diamond
    Abstract:

    We derive a new nonlinear criterion for the occurrence of fast relaxation (crash) events at the edge of high-Confinement-Mode plasmas. These fast relaxation events called ELMs (edge-localized Modes) evolve from ideal magnetohydrodynamics (MHD) instabilities, but the crash is not due only to linear physics. We show that for an ELM crash to occur, the coherence time of the relative phase between potential and pressure perturbations must be long enough to allow growth to large amplitude. This phase coherence time is determined by both linear and nonlinear dynamics. An ELM crash requires that the instability growth rate exceed a critical value, i.e., γ>γc, where γc is set by 1/τc and τc is the phase coherence time. For 0<γ<γc, MHD turbulence develops and drives enhanced turbulent transport. The results indicate that the shape of the growth rate spectrum γ(n) is important to whether the result is a crash or turbulence. We demonstrate that ELMs can be mitigated by reducing the phase coherence time without changing linear instability. These findings also offer an explanation of the occurrence of ELM-free H-Mode regimes. © 2014 American Physical Society.

Ö D Gürcan - One of the best experts on this subject based on the ideXlab platform.

  • turbulence elasticity a key concept to a unified paradigm of l i h transition
    Nuclear Fusion, 2015
    Co-Authors: P H Diamond, Yusuke Kosuga, Ö D Gürcan
    Abstract:

    We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow (ZF) shears. It is shown that when |?V??ZF|/??k???1, with |?V??ZF| the ZF shearing rate and ??k the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DW?ZF turbulence, we propose a unified paradigm of low-Confinement-Mode to intermediate-Confinement-Mode to high-Confinement-Mode (L???I???H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-Mode. The transition from an unstable L-Mode to I-Mode is predicted to occur when ??k? ??V??cr(?V?E?B is mean E???B shear flow driven by edge radial electrostatic field), the I-Mode will terminate and spiral into the H-Mode.

  • Turbulence elasticity: a key concept to a unified paradigm of L → I → H transition
    Nuclear Fusion, 2015
    Co-Authors: P H Diamond, Yusuke Kosuga, Ö D Gürcan
    Abstract:

    We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow (ZF) shears. It is shown that when |?V??ZF|/??k???1, with |?V??ZF| the ZF shearing rate and ??k the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DW?ZF turbulence, we propose a unified paradigm of low-Confinement-Mode to intermediate-Confinement-Mode to high-Confinement-Mode (L???I???H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-Mode. The transition from an unstable L-Mode to I-Mode is predicted to occur when ??k? ??V??cr(?V?E?B is mean E???B shear flow driven by edge radial electrostatic field), the I-Mode will terminate and spiral into the H-Mode.

R. J. Groebner - One of the best experts on this subject based on the ideXlab platform.

  • Transition to Self-Organized High Confinement States in Tokamak Plasmas
    Nonequilibrium Phenomena in Plasmas, 2020
    Co-Authors: Parvez N. Guzdar, R. J. Groebner, R. G. Kleva, P. Gohil
    Abstract:

    Shear flow stabilization of edge turbulence leads to self-organized high (H) Confinement Modes in tokamak plasmas. Thus understanding the mechanisms for generation of shear/zonal flow and fields in finite β plasmas is an important area of research. A brief review of various mechanisms for shear flow generation and discussion of our recent theory which yields a criterion for bifurcation from low to high (L-H) Confinement Mode is presented. The predicted threshold based on this parameter shows good agreement with edge measurements on discharges undergoing L-H transitions in DIII-D with ▽B both towards and away from the X-point, as well as for pellet induced H-Modes.

  • Dependence of the low to high Confinement Mode transition power threshold and turbulence flow shear on injected torque
    Physics of Plasmas, 2009
    Co-Authors: David J. Schlossberg, K. H. Burrell, R. J. Groebner, W.m. Solomon, G.r. Mckee, R.j. Fonck, P. Gohil, Morgan Shafer, G. Wang
    Abstract:

    The power required to induce a bifurcation from a low-Confinement Mode to a high-Confinement Mode in DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] plasmas is found to depend sensitively on the injected neutral beam torque and consequent toroidal rotation. Plasmas exhibit a factor of 2–4 reduction in this power threshold, dependent on ion ∇B drift direction. Correlated with this change, turbulence velocity measurements near 0.9

  • Low to high Confinement transition theory of finite-beta drift-wave driven shear flow and its comparison with data from DIII-D
    Physics of Plasmas, 2004
    Co-Authors: Parvez N. Guzdar, R. J. Groebner, R. G. Kleva, P. Gohil
    Abstract:

    Shear flow stabilization of edge turbulence in tokamaks has been the accepted paradigm for the improvement in Confinement observed in high (H) Confinement Mode plasmas. Results on the generation of zonal flow and fields in finite β plasmas are presented. This theory yields a criterion for bifurcation from low to high (L–H) Confinement Mode, proportional to Te/Ln, where Te is the electron temperature and Ln is the density scale-length at the steepest part of the density gradient. When this parameter exceeds a critical value (mostly determined by the strength of the toroidal magnetic field), the transition occurs. The predicted threshold based on this parameter shows good agreement with edge measurements on discharges undergoing L–H transitions in DIII-D [J. L. Luxon, R. Anderson, F. Batty et al., in Proceedings of the 11th Conference on Plasma Physics and Controlled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. The observed differences in the transitions with the reversal of the toroidal magnetic field are reconciled in terms of this critical parameter due to the differences in the density gradient scale-lengths in the edge. The theory also provides a possible explanation for lowered threshold power, pellet injection H Modes in DIII-D, thereby providing a unified picture of the varied observations on the L–H transition.Shear flow stabilization of edge turbulence in tokamaks has been the accepted paradigm for the improvement in Confinement observed in high (H) Confinement Mode plasmas. Results on the generation of zonal flow and fields in finite β plasmas are presented. This theory yields a criterion for bifurcation from low to high (L–H) Confinement Mode, proportional to Te/Ln, where Te is the electron temperature and Ln is the density scale-length at the steepest part of the density gradient. When this parameter exceeds a critical value (mostly determined by the strength of the toroidal magnetic field), the transition occurs. The predicted threshold based on this parameter shows good agreement with edge measurements on discharges undergoing L–H transitions in DIII-D [J. L. Luxon, R. Anderson, F. Batty et al., in Proceedings of the 11th Conference on Plasma Physics and Controlled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. The observed differences in the transitions with th...

  • quiescent double barrier high Confinement Mode plasmas in the diii d tokamak
    Physics of Plasmas, 2001
    Co-Authors: K. H. Burrell, C. M. Greenfield, P. Gohil, E J Doyle, M E Austin, D P Brennan, J C Deboo, C Fenzi, C Fuchs, R. J. Groebner
    Abstract:

    High-Confinement (H-Mode) operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge localized Modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the beta limit and reduced core transport regions needed for advanced tokamak operation. Experimental results from DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] this year have demonstrated a new operating regime, the quiescent H-Mode regime, which solves these problems. We have achieved quiescent H-Mode operation that is ELM-free and yet has good density and impurity control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-Mode edge barrier for long periods of time (>3.5 s or >25 en...

  • Improved High-Confinement Mode with Neon Injection in the DIII-D Tokamak
    Physical Review Letters, 1999
    Co-Authors: G.m. Staebler, G.l. Jackson, W.p. West, S.l. Allen, R. J. Groebner, M.j. Schaffer, Dennis Whyte
    Abstract:

    The first observation of a high-Confinement Mode with reduced energy transport in both the center and the edge induced by the injection of neon impurities is reported in this paper. This improved high Mode develops from an improved low-Confinement Mode. Linear growth rate calculations indicate that a new theoretical mechanism, the stabilization of high wave number drift waves by impurities, is at work, combined with {bold E}{times}{bold B} velocity shear suppression of low wave number instabilities. {copyright} {ital 1999} {ital The American Physical Society}