Observation of the parametric-modulational instability between the drift-wave fluctuation and azimuthally symmetric sheared radial electric field oscillation in a cylindrical laboratory plasma

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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.

Section: Critical Condition For the Onset Of Transitionmentioning

confidence: 99%

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

Itoh

Fujisawa

2013

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“…The direction of the energy transfer between microscopic fluctuations and the large-scale fluctuation has not been identified yet. However, observation of the Reynolds stress makes it possible to study the energy transfer process [36], which is the objective of future work.…”

Section: Nonlinear Coupling With Microscopic Fluctuationsmentioning

confidence: 99%

Long Range Temperature Fluctuation in LHD

Inagaki

Tokuzawa

Itoh

et al. 2011

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We report a detailed correlation technique to identify the long-range temperature fluctuation in the Large Helical Device. Correlation hunting has successfully realized the observation of electron temperature fluctuations, which are characterized by their correlation length comparable to the plasma minor radius, with low frequency of ∼ 1-3 kHz, ballistic radial propagation (at a speed of ∼1 km/s, of the order of diamagnetic drift velocity), spatial mode number of m/n = 1/1 (or 2/1), and amplitude of ∼2% at the maximum. Bicoherence analysis confirmed their nonlinear coupling with local microscopic turbulent fluctuations. This long-range temperature fluctuation is a possible carrier of fast propagation in transport processes observed so far. We also comment on the theoretical interpretation.

“…Parametric-modulational instability of drift waves can lead to the generation of a low-frequency sideband, or a zonal flow [3]. This mechanism of the zonal flow formation has recently been confirmed and systematically studied in cylindrical plasma [20]. The formation of a zonal flow as a result of modulational instability has also been observed in the H-1 heliac [21] where the growth of the k θ ≈ 0 low frequency structure was found to be correlated with the modulation of a coherent parent wave.…”

Section: Resultsmentioning

confidence: 98%

Spectrally Condensed Fluid Turbulence and L-H Transitions in Plasma

Shats

Xia

2009

Recent experimental and theoretical studies of two-dimensional (2D) turbulence reveal that spectrally condensed turbulence which is a system of coupled large-scale coherent flow and broadband turbulence, is similar to plasma turbulence near the L-H transition threshold. Large condensate vortices fed via the turbulent inverse energy cascade, can control both the level of the broadband turbulence by shear decorrelation, and the energy injected into turbulence at the forcing scale via sweeping of the forcing-scale vortices. The interaction between these ingredients of spectrally condensed fluid turbulence is in many aspects similar to the interactions in the zonal flow-GAMs-turbulence system in plasma. In this paper we overview recent results on condensed 2D turbulence and present evidence of interaction between its three components: condensate structures, turbulence and forcingscale vortices. This is compared with the modifications in the spectra of plasma electrostatic potential during L-H transitions. It is shown that mean zonal flows are spatially and temporally correlated with both the broadband turbulence and with the narrow spectral range identified as the spectral range of the underlying instability.