Showing papers by "Kimitaka Itoh published in 2012"

New Thermodynamical Force in Plasma Phase Space that Controls Turbulence and Turbulent Transport

Abstract: Physics of turbulence and turbulent transport has been developed on the central dogma that spatial gradients constitute the controlling parameters, such as Reynolds number and Rayleigh number. Recent experiments with the nonequilibrium plasmas in magnetic confinement devices, however, have shown that the turbulence and transport change much faster than global parameters, after an abrupt change of heating power. Here we propose a theory of turbulence in inhomogeneous magnetized plasmas, showing that the heating power directly influences the turbulence. New mechanism, that an external source couples with plasma fluctuations in phase space so as to affect turbulence, is investigated. A new thermodynamical force in phase-space, i.e., the derivative of heating power by plasma pressure, plays the role of new control parameter, in addition to spatial gradients. Following the change of turbulence, turbulent transport is modified accordingly. The condition under which this new effect can be observed is also evaluated.

Dynamics of particle flux in a cylindrical magnetized plasma

Abstract: Fluctuation-induced cross-field convective particle flux is studied in the cylindrical magnetized plasma assembly for non-linear turbulence analysis (PANTA). The plasma presents a wave turbulence characterized by a limited number of developing modes. To study the link between mode dynamics and particle transport, frequency-dependent time evolution of transport is obtained by applying complex continuous wavelet decomposition on spatio-temporal data from electrostatic probes. The method is shown to recover transport description consistent with other analysis methods for a weakly turbulent state. For a plasma state featuring a large non-sinusoidal structure, the time evolution of transport on the second harmonic of the main fluctuation shows a slow modulation of azimuthal mode number m = 0 with similarities to a zonal perturbation.

On influences of long-range fluctuations on transport in Large Helical Device plasmas

Abstract: The competing effects of the long-range fluctuation, which has been found on the Large Helical Device (Inagaki et al 2011 Phys. Rev. Lett. 107 115001), on the net turbulent transport are discussed. The transport, which is driven directly by this mode, is evaluated. Then, the associated reduction of fluctuations in the range of drift wave turbulence is analyzed, by employing the predator–prey model. The transport driven by microscopic turbulence is reduced by the appearance of long-range fluctuations. Comparing these two competing processes, the condition is derived for the net transport to be reduced by the appearance of this long-range fluctuation. In addition, the experimental observations of microscopic fluctuations are discussed. The reduction of high-frequency fluctuations occurs in conjunction with the long-range fluctuations.

Ballistic propagation of turbulence front in tokamak edge plasmas

Abstract: The flux-driven nonlinear simulation of resistive ballooning mode turbulence with tokamak edge geometry is performed to study the non-steady component in the edge turbulence. The large-scale and dynamical events in transport are investigated in a situation where the mean flow is suppressed. Two types of dynamics are observed. One is the radial propagation of the pulse of pressure gradient, the other is the appearance/disappearance of radially elongated global structure of turbulent heat flux. The ballistic propagation is observed in the pulse of pressure gradient, which is associated with the front of turbulent heat flux. We focus on this ballistic propagation phenomenon. Both of the bump of pressure gradient and the front of heat flux propagate inward and outward direction. It is confirmed that the strong fluctuation propagates with the pulse front. It is observed that the number of pulses going outward is close to those going inward. This ballistic phenomenon does not contradict to the turbulence spreading theory. Statistical characteristics of the ballistic propagation of pulses are evaluated and compared with scaling laws which is given by the turbulence spreading theory. It is found that they give qualitatively good agreement.

Fluctuations with long-distance correlation in quasi-stationary and transient plasmas of LHD

Abstract: Electron temperature fluctuations with long-distance correlation have been discovered in LHD. This paper reports the extended observations recently made on the spatiotemporal structure of the long-range fluctuations both in quasi-stationary and transient plasmas. The detailed characteristics or spatiotemporal characteristics of long-range temperature fluctuations are revealed successfully using correlation analysis. Particularly, the dynamics of the long-range fluctuations is investigated to find that the amplitude of the fluctuations decreased and their radial correlation lengths shortened during the transient phase induced by pellet injection. Temporal changes of radial correlation structure and amplitude of fluctuations at the onset of change in the plasma state are discussed.

Bi-coherence analysis of fluctuations with long distance correlation in toroidal plasmas

Abstract: Macro-scale temperature fluctuations, which have long radial correlation length (of the order of the plasma radius), have been discovered on Large Helical Device (LHD). In this paper, non-linear coupling between the long-range fluctuations and microscopic fluctuations is investigated using cross bi-coherence analysis. The significant cross bi-coherence between long-range fluctuations and micro-fluctuations is observed in the range of frequency f < 100 kHz, where f is the frequency of microscopic fluctuations. It is found that the squared bi-coherence and bi-phase for this three-wave coupling is weakly dependent on the frequency of microscopic fluctuations. These observations demonstrate that the microscopic fluctuations are coherently interacting with long-range fluctuations.

Zonal flows induced by symmetry breaking with existence of geodesic acoustic modes

Abstract: The nonlinear dynamics of zonal flows (ZFs) is investigated when geodesic acoustic modes (GAMs) have substantial influence on plasma states. Simultaneous existence of multiple GAMs with different radial phase velocities gives additional nonlinear mode couplings, and asymmetry of the turbulence spectrum induces energy exchanges between GAMs and a ZF. A set of model equations is derived to describe the nonlinear dynamics of a ZF, GAMs and ambient turbulence. The model includes the mechanism of ZF generation by a pair of GAMs, and the back interaction from the turbulence is solved self-consistently. Two stationary solutions are obtained; one is the known solution that the GAM propagates as a travelling wave with no ZF excitation, and the other is a new solution that a pair of GAMs forms a standing wave, which induces a ZF. The accessibility to the steady states, and the threshold for the transition between them are obtained. The GAM and ZF formation in the new state affects the background plasmas. The effects on the turbulent heat diffusivity and the ion heating rate are discussed.

Configuration of Flows in a Cylindrical Plasma Device

Abstract: Stella OLDENBÜRGER1), Kazuya URIU2), Tatsuya KOBAYASHI2) , Shigeru INAGAKI1,3), Makoto SASAKI1,3), Yoshihiko NAGASHIMA1,4), Takuma YAMADA1,4), Akihide FUJISAWA1,3), Sanae-I. ITOH1,3) and Kimitaka ITOH1,2,5) 1)Itoh Research Center for Plasma Turbulence, Kyushu University, 6-1 Kasuga-Koen, Kasuga 816-8580, Japan 2)Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-Koen, Kasuga 816-8580, Japan 3)Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-Koen, Kasuga 816-8580, Japan 4)Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8561, Japan 5)National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan

Self-nonlinear coupling of long-range temperature fluctuation in Toroidal plasma

Abstract: This paper proposes a clever usage of bispectral analysis for extracting or reconstructing a quasi-coherent and quasi-periodic structural evolution of plasma fluctuations. The method has been applied on electron temperature fluctuation signals measured with a multi-point electron cyclotron emission (ECE) radiometer in the Large Helical Device (LHD). The method successfully reconstructs the averaged spatiotemporal evolution of fluctuating coherent structure (Inagaki et al 2011 Phys. Rev. Lett. 107 115001), while the structure is buried in (or comparable in power to) background fluctuations. The bicoherence analysis has found the faint fluctuating structure consisting of a fundamental mode and the harmonic modes from the existence of significant couplings between them. The reconstructed spatiotemporal structure with the proposed method is compared with that obtained with the lock-in (conditional) average. Three cases of spatiotemporal evolution of the non-sinusoidal waveform are presented.

Observations of Intermittent Structures in the Periphery of a Cylindrical Linear Plasma in PANTA

Abstract: Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan 1)Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan 2)Itoh Research Center for Plasma Turbulence, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan 3)Japan Atomic Energy Agency, 801-1 Mukoyama, Naka, Ibaraki 311-0193, Japan 4)Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Tokyo 277-8561, Japan 5)Japan Atomic Energy Agency, 2-166 Oaza-obuchi-aza-omotedate, Rokkasho, Kamikita, Aomori 039-3212, Japan 6)National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan

Response of plasma turbulence against externally-controlled perturbations

Abstract: A theory of the dynamic response of the turbulent plasma against the externally-controlled perturbations is reported. Based on Mori's method [Prog. Theor. Phys. 33 423 (1965)], the nonlinear force is assumed to be separated into the memory function and the nonlinear fluctuating force. The former corresponds to the damping term, and the latter is categorized into the noise term. The response of the turbulent plasma against the externally-controlled source is formulated. The response kernel, which connects the externally-controlled source and the response of the turbulent field, is shown to have both the nonlocal property (in space) and the non-Markovian response (in time). A discussion is made on the nonlocal and non-Markovian response, including the case of disparate-scale interactions. A new method is proposed to observe experimentally the nonlocal interaction in the drift wave turbulence via the zonal flows.

Towards an Emerging Understanding of Non-local Transport

Abstract: In this overview, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is overviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality and spontaneously as well as externally triggered non-local transport phenomena will be described in both L-mode and improved-mode plasmas. We will report on ongoing evaluation of ”fast front” and ”intrinsically nonlocal” models, and their success in comparisons with experimental data. PACS numbers: 52.55.Hc, 52.55.Fa, 52.50.Sw, 52.50.Gj Towards an Emerging Understanding of Non-local Transport 2

Bohm-like Dependence of Transport in Scrape-off Layer Plasmas

Abstract: Nondiffusive radial transport in scrape-off layer (SoL) is investigated using the two-dimensional SoL interchange turbulence simulation code, which reproduces plasma blobs. From nonlinear simulations, it is found that a mesoscale convective cell emerges from interchange turbulence and propagates in the radial direction. Blob structures are formed autonomously and their ballistic propagation dominates SoL radial transport. A high-density tail in the radial profile is reproduced. This indicates the existence of a 2nd SoL. The effective transport coefficient is analyzed and a Bohm-like dependence is found. It is concluded that the flux-driven turbulence in SoL induces Bohm-like “nondiffusive” transport. The PDF of the turbulence-driven flux is also investigated. It is found that the PDF distribution follows a stretched Gaussian distribution.

Bifurcation and Phase Diagram of Turbulence Constituted from Three Different Scale-Length Modes

Abstract: Cases where three kinds of fluctuations having the different typical scale lengths coexist are analysed, and the statistical theory of strong turbulence in inhomogeneous plasmas is developed. Statistical nonlinear interactions between fluctuations are kept in the analysis as the renormalized drag, statistical noise and the averaged drive. The nonlinear interplay through them induces a quenching or suppressing effect, even if all the modes are unstable when they are analysed independently. Variety in mode appearance takes place: one mode quenches the other two modes, or one mode is quenched by the other two modes, etc. The bifurcation of turbulence is analysed and a phase diagram is drawn. Phase diagrams with cusp-type catastrophe and butterfly-type catastrophe are obtained. The subcritical bifurcation is possible to occur through the nonlinear interplay, even though each one is supercritical turbulence when analysed independently. Analysis reveals that the nonlinear stability boundary (marginal point) and the amplitude of each mode may substantially shift from the conventional results of independent analyses.

Time Evolution of Power Spectrum Density in Spontaneous Transition in Cylindrical Magnetized Plasma

Abstract: Preliminary observation results are reported for a new discharge regime in the Plasma Assembly for Nonlinear Turbulent Analysis (PANTA), where spontaneous transitions in the equilibrium profile and fluctuation spectra occur. Two different states are defined by using the mean density value. Axial and radial profiles are observed for the two states, and large profile changes are found. The spatiotemporal evolution of the transition front is measured. Changes in the fluctuation spectrum are evaluated using conditional average and lock-in average. c

Three-dimensional Turbulence Simulation of Edge Transport and Impact of Plasma Rotation

Abstract: Global simulations of edge plasma turbulence are studied. The gradient-flux relation is fitted to a model with two parameters: stiffness and intercept. The slope of the grad- p vs flux curve is clearly lower in the case of spontaneous rotation than in the case of artificially suppressed rotation. The causes of this difference are studied by imposing a specified velocity shear. The slopes are similar between cases of controlled rotation with different amplitudes. That is, the stiffness does not change significantly, while the intercept changes. Therefore, the change in the slope observed in the case of spontaneous rotation has to be attributed to the fact that the velocity shear amplitude itself is increasing with the pressure source. The increase in the velocity shear is conjectured to cause a change in the intercept in the case of spontaneous rotation, but not a change in the stiffness.

Dual Maxwell Construction and Transport Barrier of Helical Plasmas

Abstract: The interface of transport in toroidal plasmas is studied. The case, in which the gradient-flux relation is a multiple-valued function, is analyzed by use of the statistical theory of confined plasmas. An analytic formula for the determination of the interface of transport is given, taking an example of the transport in toroidal helical plasmas. The formula is given in a form of a dual Maxwell construction (rule of equal area). The location of the interface in stationary state and the access to the stationary state are discussed.

The Effect of Non-Axisymmetry of Magnetic Configurations on Radial Electric Field Transition Properties in the LHD

Abstract: Transition property of the radial electric field (Er) in LHD have been theoretically investigated and also applied to explain experimental results. Especially, effects of the helicity of the magnetic configuration on the condition to realize the electron root are examined. Larger helicity makes the threshold collisionality higher. This is attributed to the nonlinear dependence of Γe(Er) in a low collisional regime. This interesting feature predicts that the threshold temperature becomes higher for a case of smaller helicity. The variation of the threshold density anticipated from the analysis for cases with different magnetic axis position is qualitatively verified in the density scan experiment.