Properties of Intermittent Bursts in Edge Plasma of LHD

Takahashi, Hayato; Morisaki, T.; Budaev, V. P.; Komori, A.; Yamada, H.

doi:10.1585/pfr.5.s2078

Cited by 7 publications

(5 citation statements)

References 18 publications

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“…The blob can create a perpendicular convective flux near the Xpoint with velocities of 0.1À1 km/s in LHD, which are clearly smaller than those of the blob in tokamaks, e.g., 2.5 km/s. 34 In addition, the direction of E Â B force is frequently changeable in the stochastic magnetic field layer of LHD because the gradients of magnetic field strength and magnetic field curvature are considerably different due to the presence of a helical ripple. Since the perpendicular CIV profile is symmetric as shown in Fig.…”

Section: -12mentioning

confidence: 99%

Vertical profiles and two-dimensional distributions of carbon line emissions from C2+−C5+ ions in attached and RMP-assisted detached plasmas of large helical device

Zhang

Morita

Dai³

et al. 2017

Physics of Plasmas

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In Large Helical Device (LHD), the detached plasma is obtained without external impurity gas feed by supplying an m/n = 1/1 resonant magnetic perturbation (RMP) field to a plasma with an outwardly shifted plasma axis position of Rax = 3.90 m where the magnetic resonance exists in the stochastic magnetic field layer outside the last closed flux surface. The plasma detachment is triggered by the appearance of an m/n = 1/1 island when the density, increased using hydrogen gas feed, exceeds a threshold density. The behavior of intrinsically existing impurities, in particular, carbon originating in the graphite divertor plates, is one of the important key issues to clarify the characteristic features of the RMP-assisted plasma detachment although the particle flux still remains on some divertor plates even in the detachment phase of the discharge. For this purpose, vertical profiles and two-dimensional (2-D) distributions of edge carbon emissions of CIII to CVI have been measured at extreme ultraviolet wavelength range, and the results are compared between attached and RMP-assisted detached plasmas. It is found that the CIII and CIV emissions located in the stochastic magnetic field layer are drastically increased near the m/n = 1/1 island O-point and in the vicinity of both inboard and outboard edge separatrix X-points during the RMP-assisted detachment, while those emissions are only enhanced in the vicinity of the outboard edge X-point in attached plasmas without RMP. The result clearly indicates a change in the magnetic field lines connecting to the divertor plates, which is caused by the growth of the m/n = 1/1 edge magnetic island. In contrast, the intensity of CVI emitted radially inside the magnetic island significantly decreases during the detachment, suggesting an enhancement of the edge impurity screening. The measured carbon distribution is analyzed with a three-dimensional edge plasma transport simulation code, EMC3-EIRENE, for the attached plasmas without RMP. It is found that the narrow strip-shaped impurity trace emitted along the edge X-point and its width are sensitive to the cross-field impurity diffusion coefficient, DZ⊥. As a result, the value of DZ⊥ of C3+ ions is evaluated to be 20 times larger than that of the bulk ions in the Rax = 3.90 m configuration, while the reason is unclear at present. The measured 2-D carbon distribution is also discussed and compared to the structure of the m/n = 1/1 magnetic island, which quickly expanded during the appearance of the plasma detachment.

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Section: -12mentioning

confidence: 99%

Vertical profiles and two-dimensional distributions of carbon line emissions from C2+−C5+ ions in attached and RMP-assisted detached plasmas of large helical device

Zhang

Morita

Dai³

et al. 2017

Physics of Plasmas

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“…Experiments [7][8][9][10][11][12] and numerical simulations [13][14][15][16][17] have often shown the existence of non-local heat avalanches in magnetically confined plasmas, in addition to local diffusive transport. In particular, a recent gyrokinetic simulation study highlights one self-organization process, which can determine transport: the prevalence of self-organized, avalanche-dominated transport on mesoscales (with a Lorentzian kernel), and the persistence of static E Â B staircases, the spacing of which defines the outer scale of the avalanche distribution.…”

Section: Introductionmentioning

confidence: 99%

A statistical analysis of avalanching heat transport in stationary enhanced core confinement regimes

et al. 2012

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We present a statistical analysis of heat transport in stationary enhanced confinement regimes obtained from flux-driven gyrofluid simulations. The probability density functions of heat flux in improved confinement regimes, characterized by the Nusselt number, show significant deviation from Gaussian, with a markedly fat tail, implying the existence of heat avalanches. Two types of avalanching transport are found to be relevant to stationary states, depending on the degree of turbulence suppression. In the weakly suppressed regime, heat avalanches occur in the form of quasiperiodic (QP) heat pulses. Collisional relaxation of zonal flow is likely to be the origin of these QP heat pulses. This phenomenon is similar to transient limit cycle oscillations observed prior to edge pedestal formation in recent experiments. On the other hand, a spectral analysis of heat flux in the strongly suppressed regime shows the emergence of a 1/f (f is the frequency) band, suggesting the presence of self-organized criticality (SOC)-like episodic heat avalanches. This episodic 1/f heat avalanches have a long temporal correlation and constitute the dominant transport process in this regime. V

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“…Presently, there is significant interest [1][2][3][4][5][6][7][8][9][10][11] in understanding the properties of plasma fluctuations at the edge of magnetically confined devices because of their important role in particle and energy transport in a fusion environment. Large density structures exhibiting temporal intermittency 12 and propagating through the scrape-off layer (SOL) have been observed in the DIII-D, 13 HL-2A, 14 and TEXTOR 15 tokamaks to result in approximately 50% of the particle transport.…”

Section: Introductionmentioning

confidence: 99%

Observation of exponential spectra and Lorentzian pulses in the TJ-K stellarator

et al. 2011

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An experimental investigation of the low-frequency density fluctuations in the plasma edge region of the TJ-K stellarator [N. Krause et al., Rev. Sci. Inst. 73, 3474 (2002)] finds that the ensembleaveraged frequency spectra exhibit a near exponential frequency dependence whose origin can be traced to individual pulses having a Lorentzian temporal shape. Similar features have been previously observed [D. C. Pace et al., Phys. Plasmas 15, 122304 (2008)] in a linear magnetized device under conditions in which cross-field pressure gradients are present. The reported observation of such features within the turbulent environment of a toroidal confinement device provides support for the conjecture that the underlying processes are a general feature of pressure gradients. Also presented is the magnetic field strength dependence of the pulse widths and the waiting time distribution between pulses.

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Properties of Intermittent Bursts in Edge Plasma of LHD

Cited by 7 publications

References 18 publications

Vertical profiles and two-dimensional distributions of carbon line emissions from C2+−C5+ ions in attached and RMP-assisted detached plasmas of large helical device

Vertical profiles and two-dimensional distributions of carbon line emissions from C2+−C5+ ions in attached and RMP-assisted detached plasmas of large helical device

A statistical analysis of avalanching heat transport in stationary enhanced core confinement regimes

Observation of exponential spectra and Lorentzian pulses in the TJ-K stellarator

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