Rational Surface

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The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform

Ju Wang - One of the best experts on this subject based on the ideXlab platform.

Zhiqun Lin - One of the best experts on this subject based on the ideXlab platform.

F Zonca - One of the best experts on this subject based on the ideXlab platform.

  • fine structure zonal flow excitation by beta induced alfven eigenmode
    Nuclear Fusion, 2016
    Co-Authors: Zhiyong Qiu, Liu Chen, F Zonca
    Abstract:

    Author(s): Qiu, Z; Chen, L; Zonca, F | Abstract: © 2016 IAEA, Vienna. Nonlinear excitation of low frequency zonal structure (LFZS) by beta-induced Alfven eigenmode (BAE) is investigated using nonlinear gyrokinetic theory. It is found that electrostatic zonal flow (ZF), rather than zonal current, is preferentially excited by finite amplitude BAE. In addition to the well-known meso-scale radial envelope structure, ZF is also found to exhibit fine radial structure due to the localization of BAE with respect to mode Rational Surfaces. Specifically, the zonal electric field has an even mode structure at the Rational Surface where radial envelope peaks.

  • fine structure zonal flow excitation by beta induced alfven eigenmode
    arXiv: Plasma Physics, 2016
    Co-Authors: Zhiyong Qiu, Liu Chen, F Zonca
    Abstract:

    Nonlinear excitation of low frequency zonal structure (LFZS) by beta-induced Alfven eigenmode (BAE) is investigated using nonlinear gyrokinetic theory. It is found that electrostatic zonal flow (ZF), rather than zonal current, is preferentially excited by finite amplitude BAE. In addition to the well-known meso-scale radial envelope structure, ZF is also found to exhibit fine radial structure due to the localization of BAE with respect to mode Rational Surfaces. Specifically, the zonal electric field has an even mode structure at the Rational Surface where radial envelope peaks.

Jun Wang - One of the best experts on this subject based on the ideXlab platform.

S Inagaki - One of the best experts on this subject based on the ideXlab platform.

  • response of plasma toroidal flow to the transition between nested and stochastic magnetic field in lhd
    Nuclear Fusion, 2017
    Co-Authors: K Ida, S Inagaki, M Yoshinuma, M Yokoyama, H Tsuchiya, T Kobayashi, C Suzuki, A Shimizu, Kenichi Nagaoka, K Itoh
    Abstract:

    Response of the plasma toroidal flow to the forward and backward transition between the nested and the stochastic magnetic field is studied using the charge exchange spectroscopy in the large helical device (LHD). Abrupt damping of toroidal flow associated with a transition from nested magnetic flux Surface to a stochastic magnetic field is observed when the magnetic shear at the Rational Surface decreases to 0.5 after the exchange of the neutral beam injection (NBI) direction from co- to counter-direction in LHD. The stochastization of magnetic field occurs only in a narrow range of magnetic shear near 0.5 and spontaneous back-transition from stochastic to nested magnetic field (healing) is observed in the steady-state phase of magnetic shear. When the NBI direction is changed from counter- to co-direction, the healing of magnetic field occurs associated with the increase of magnetic shear.

  • bifurcation phenomena of a magnetic island at a Rational Surface in a magnetic shear control experiment
    Physical Review Letters, 2008
    Co-Authors: K Ida, S Inagaki, M Yoshinuma, Y Narushima, K Itoh, T Kobuchi, K Y Watanabe, H Funaba, S Sakakibara
    Abstract:

    Three states of a magnetic island are observed when the magnetic shear at the Rational Surface is modified using inductive current associated with the neutral beam current drive in the Large Helical Device. One state is the healed magnetic island with a zero island width. The second state is the saturated magnetic island with partial flattening of the T(e) profile. The third state is characterized by the global flattening of the T(e) profile in the core region. As the plasma assumes each of the three states consecutively through a bifurcation process a clear hysteresis in the relation between the size of the magnetic island and the magnetic shear is observed.

  • transition phenomena and thermal transport properties in lhd plasmas with an electron internal transport barrier
    Nuclear Fusion, 2005
    Co-Authors: T Shimozuma, K Ida, S Inagaki, K Y Watanabe, N Tamura, T Morisaki, S Kubo, H Idei, T Tokuzawa, I Yamada
    Abstract:

    Two types of improved core confinement were observed during centrally focused electron cyclotron heating (ECH) into plasmas sustained by counter (CNTR) and Co neutral beam injections (NBI) in the Large Helical Device. The CNTR NBI plasma displayed transition phenomena to the high-electron-temperature state and had a clear electron internal transport barrier, while the Co NBI plasma did not show a clear transition or an ECH power threshold but showed broad high temperature profiles with moderate temperature gradient. This indicated that the Co NBI plasma with additional ECH also had an improved core confinement. The electron heat transport characteristics of these plasmas were directly investigated using heat pulse propagation excited by modulated ECH. These effects appear to be related to the m/n = 2/1 Rational Surface or the island induced by NBI beam-driven current.

  • radial electric field and transport near the Rational Surface and the magnetic island in lhd
    Nuclear Fusion, 2004
    Co-Authors: K Ida, S Inagaki, K Y Watanabe, N Tamura, T Morisaki, N Ohyabu, K Khlopenkov, S Sudo, M Yokoyama, T Shimozuma
    Abstract:

    The structure of the radial electric field and heat transport at the magnetic island in the large helical device (LHD) are investigated by measuring the radial profile of the poloidal flow with charge exchange spectroscopy and measuring the time evolution of the electron temperature with ECE. A vortex-like plasma flow along the magnetic flux Surface inside the magnetic island is observed when the n/m = 1/1 external perturbation field becomes large enough to increase the magnetic island width above a critical range (15–20% of minor radius) in LHD. This convective poloidal flow results in a non-flat space potential inside the magnetic island. The sign of the curvature of the space potential (∂2Φ/∂r2, where Φ is the space potential) depends on the radial electric field at the boundary of the magnetic island. The heat transport inside the magnetic island is studied with a cold pulse propagation technique. The experimental results show the existence of radial electric field shear at the boundary of the magnetic island and a reduction in heat transport at the boundary and inside the magnetic island.

  • mhd instabilities and their effects on plasma confinement in large helical device plasmas
    Nuclear Fusion, 2004
    Co-Authors: K Toi, S Inagaki, Y Narushima, S Sakakibara, S Ohdachi, S Yamamoto, N Nakajima, Kiyomasa Watanabe, Y Nagayama, Hiroshi Yamada
    Abstract:

    Characteristics of MHD instabilities and their impacts on plasma confinement are studied in current free plasmas of the Large Helical Device. Spontaneous L?H transition is often observed in high beta plasmas close to 2% at low toroidal fields (Bt ? 0.75?T). The stored energy starts to rise rapidly just after the transition accompanying the clear rise in the electron density but quickly saturates due to the growth of the m = 2/n = 3 mode (m and n: poloidal and toroidal mode numbers), the Rational Surface of which is located in the edge barrier region, and edge localized mode (ELM) like activities having fairly small amplitude but high repetition frequency. Even in low beta plasmas without L?H transitions, ELM-like activities are sometimes induced in high performance plasmas with a steep edge pressure gradient and transiently reduce the stored energy up to 10%. Energetic ion driven MHD modes such as Alfv?n eigenmodes (AEs) are studied in a very wide range of characteristic parameters (the averaged beta of energetic ions, ?b?, and the ratio of energetic ion velocity to the Alfv?n velocity, Vb?/VA), of which range includes all tokamak data. In addition to the observation of toroidicity induced AEs (TAEs), coherent magnetic fluctuations of helicity induced AEs (HAEs) have been detected for the first time in NBI heated plasmas. The transition of a core-localized TAE to a global AE (GAE) is also observed in a discharge with temporal evolution of the rotational transform profile, having a similarity to the phenomenon observed in a reversed shear tokamak. At low magnetic fields, bursting TAEs transiently induce a significant loss of energetic ions, up to 40% of injected beams, but on the other hand play an important role in triggering the formation of transport barriers in the core and edge regions.

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