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

Inagaki, S.; Tamura, N.; Tokuzawa, T.; Ida, K.; Kobayashi, Tatsuya; Shimozuma, Τ.; Kubo, S.; Takahashi, Hayato; Nagayama, Y.; Kawahata, K.; Sudo, S.; Fujisawa, A.; Itoh, K.; Itoh, Sanae–I.

doi:10.1088/0029-5515/52/2/023022

Cited by 13 publications

(16 citation statements)

References 32 publications

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“…The fast change of core transport at the onset of H-mode transition has been known for more than 20 years [44,45]. Recently, fluctuations that could induce rapid transmission of change of transport have been searched for [55][56][57][58][59][60][61][62]. The response of LCO provides a good opportunity to observe the response of core plasma against the change at the edge.…”

Section: Propagation Of the Lco Dynamics Into The Corementioning

confidence: 99%

An Assessment of Limit Cycle Oscillation Dynamics Prior to L-H Transition

Itoh

Fujisawa

2013

Plasma and Fusion Research

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In this article, experimental observations of limit cycle oscillations (LCO) that precede L-to-H transition are discussed. Issues are: (1) the existence of zonal flows, (2) spatio-temporal evolutions of turbulence intensity, and (3) periodic generations/decays of mean radial electric field and density. The role of Reynolds stress to accelerate the LCO flow is also addressed. The propagation of changes of the density gradient and turbulence amplitude into the core is commented. Varieties in experimental reports on these issues are explained, and possible origins of different interpretations are discussed. Problem definitions for the future research for resolution are presented.

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Section: Propagation Of the Lco Dynamics Into The Corementioning

confidence: 99%

An Assessment of Limit Cycle Oscillation Dynamics Prior to L-H Transition

Itoh

Fujisawa

2013

Plasma and Fusion Research

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“…The possible impact by the enhanced ECRH efficiency on the core T e rise has been eliminated as the core plasma density is not dipped after the SMBI. This transient, long-distance and reversed polarity response where the core T e rise on response to edge cold pulses is the so-called “nonlocal” heat transport, a phenomenon widely observed in many fusion devices1234567. However, the physical mechanisms responsible for triggering the nonlocal transport remain unclear.…”

Section: Resultsmentioning

confidence: 99%

On the interplay between neoclassical tearing modes and nonlocal transport in toroidal plasmas

Hidalgo³

et al. 2016

Sci Rep

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This Letter presents the first observation on the interplay between nonlocal transport and neoclassical tearing modes (NTMs) during transient nonlocal heat transport events in the HL-2A tokamak. The nonlocality is triggered by edge cooling and large-scale, inward propagating avalanches. These lead to a locally enhanced pressure gradient at the q = 3/2 (or 2/1) rational surface and hence the onset of the NTM in relatively low β plasmas (β N < 1). The NTM, in return, regulates the nonlocal transport by truncation of avalanches by local sheared toroidal flows which develop near the magnetic island. These findings have direct implications for understanding the dynamic interaction between turbulence and large-scale mode structures in fusion plasmas.Over the past years, a number of experiments which study transient transport events in magnetically confined plasmas have revealed a provocative phenomenon [1][2][3][4][5][6][7] . A rapid change of local thermal transport in response to a change of plasma parameters occurs at a location distant from the perturbation, i. e., the so-called "nonlocal" transport, which cannot be explained just by the standard diffusive local transport paradigm. Although several theoretical models [8][9][10][11][12] have been proposed which try to interpret this nonlocal effect, the underlying physical mechanisms remain unclear, due to limited experimental data.On the other hand, to achieve economic viability for a future fusion reactor, a crucial task is to operate the plasma at relatively high β values, where β is the ratio of plasma pressure p to the magnetic field (B) pressure defined as β = p/(B 2 /2μ 0 ). However, the maximum achievable β can be limited by resistive MHD instabilities, in particular, the neoclassical tearing mode (NTM) [13][14][15][16][17] . The NTM is driven by a loss of a large bootstrap current related to a large plasma pressure gradient within a magnetic island. The occurrence of NTMs may considerably increase parallel heat transport across the width of the island, degrade energy confinement and even cause plasma disruptions. Thus, the NTM will prevent plasmas moving into a high-β state if it appears in the low-β case. As a consequence, understanding the physical processes for the onset of NTMs, as well as their prevention, is also one of major challenges for fusion.In this Letter, we present the first experimental observation of self-regulation of nonlocal transport events by NTMs generated during transient nonlocal transport events. The nonlocal effect is excited by edge cooling and propagates inward by avalanche events. These cause a local increase of the pressure gradient at the inversion surface, and thus the onset of the NTM in relatively low β plasmas (β N < 1). The presence of the NTM, results in the development of sheared flows at the magnetic island. The dual role of low-order rationals as both damping 18 and drive mechanism of steady-state 19 and fluctuating 20 E r × B flows has been identified, thus the magnetic topology is an important regulator of radial ...

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“…The spatiotemporal features ofĨ are determined by a two-point two-time correlation 23 ofĨ in the range of 1 kHz (a low pass filter is applied) along the flux contours. The correlation function between the intensities at two different points in space and time is defined as…”

Section: Spatio-temporal Characteristics Of Fluctuationsmentioning

confidence: 99%

Observation of an edge coherent mode and poloidal flow in the electron cyclotron wave induced high βp plasma in QUEST

et al. 2016

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Fluctuations are measured in the edge and scrape-off layer (SOL) of QUEST using fast visible imaging diagnostic. Electron cyclotron wave injection in the Ohmic plasma features excitation of low frequency coherent fluctuations near the separatrix and enhanced cross-field transport. Plasma shifts from initial high field side limiter bound (inboard limited, IL) towards inboard poloidal null (IPN) configuration with steepening of the density profile at the edge. This may have facilitated the increased edge and SOL fluctuation activities. Observation of the coherent mode, associated plasma flow, and particle out-flux, for the first time in the IPN plasma configuration in a spherical tokamak may provide further impetus to the edge and SOL turbulence studies in tokamaks.

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

Cited by 13 publications

References 32 publications

An Assessment of Limit Cycle Oscillation Dynamics Prior to L-H Transition

An Assessment of Limit Cycle Oscillation Dynamics Prior to L-H Transition

On the interplay between neoclassical tearing modes and nonlocal transport in toroidal plasmas

Observation of an edge coherent mode and poloidal flow in the electron cyclotron wave induced high βp plasma in QUEST

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