Effect of ion ∇B drift direction on density fluctuation poloidal flow and flow shear

Fenzi, C.; McKee, G. R.; Fonck, R. J.; Burrell, K.H.; Carlstrom, T. N.; Groebner, R. J.

doi:10.1063/1.1915349

Cited by 22 publications

(19 citation statements)

References 45 publications

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“…Results from DIII-D also show the existence of such counter-propagating turbulent fluctuations in the region just inside the LCFS [299]. Two poloidally separated BES density fluctuation channels show that fluctuations propagate in both electron and ion directions near the LCFS.…”

Section: Transition Between Edge and Core Turbulencementioning

confidence: 81%

A review of experimental drift turbulence studies

Tynan¹,

Fujisawa²,

McKee³

2009

Plasma Phys. Control. Fusion

180

167

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Experimental drift turbulence and zonal flow studies in magnetically confined plasma experiments are reviewed. The origins of drift waves, transition to drift turbulence and drift turbulence-zonal flow interactions in open field line and toroidal closed flux surface experiments are discussed and the free energy sources, dissipation mechanisms and nonlinear dynamics of drift turbulence in the core, edge and scrape-off layer plasma regions are examined. Evidence that turbulence across these regions is linked and that turbulence-driven zonal flows exist is presented, and evidence that these flows help regulate the turbulent scale lengths, amplitude and fluxes is summarized. Seemingly contradictory reports exist regarding the scale of turbulent transport events; gyro-Bohm behavior of turbulence correlation lengths as well as evidence for long-range transport phenomena both exist. Changes in turbulence during and after transport barrier formation are summarized and compared. The inferred turbulent particle and heat fluxes due to turbulent transport are usually consistent with global confinement, and edge plasma momentum transport appears to be linked to plasma flows at the last-closed flux surface and in the open field line region. However, inconsistencies between observed transport and turbulence have sometimes been reported and are pointed out here. Special attention is given to open issues, and suggestions for future experimental studies are given.

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Section: Transition Between Edge and Core Turbulencementioning

confidence: 81%

A review of experimental drift turbulence studies

Tynan¹,

Fujisawa²,

McKee³

2009

Plasma Phys. Control. Fusion

180

167

View full text Add to dashboard Cite

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“…This could potentially occur as a result of changes in edge magnetic shear, SOL flows, 11 or flows of the edge turbulent density fluctuations. 54 Alternatively, given that E ϫ B shear is more effective at longer wavelengths, the observed I-mode transport could be explained if, in the unfavorable drift direction, the radial edge turbulent energy flux peaked at a higher poloidal wave I-mode radial electric field well depths and E ϫ B shear rates are lower than their H-mode counterparts. number than the particle flux.…”

Section: Comparison Of I-mode To Eda H-modementioning

confidence: 95%

Edge radial electric field structure and its connections to H-mode confinement in Alcator C-Mod plasmas

McDermott¹,

Lipschultz²,

Hughes³

et al. 2009

Physics of Plasmas

160

191

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High-resolution charge-exchange recombination spectroscopic measurements of B5+ ions have enabled the first spatially resolved calculations of the radial electric field (Er) in the Alcator C-Mod pedestal region [E. S. Marmar, Fusion Sci. Technol. 51, 261 (2006)]. These observations offer new challenges for theory and simulation and provide for important comparisons with other devices. Qualitatively, the field structure observed on C-Mod is similar to that on other tokamaks. However, the narrow high-confinement mode (H-mode) Er well widths (5 mm) observed on C-Mod suggest a scaling with machine size, while the observed depths (up to 300 kV/m) are unprecedented. Due to the strong ion-electron thermal coupling in the C-Mod pedestal, it is possible to infer information about the main ion population in this region. The results indicate that in H-mode the main ion pressure gradient is the dominant contributor to the Er well and that the main ions have significant edge flow. C-Mod H-mode data show a clear correlation between deeper Er wells, higher confinement plasmas, and higher electron temperature pedestal heights. However, improved L-mode (I-mode) plasmas exhibit energy confinement equivalent to that observed in similar H-mode discharges, but with significantly shallower Er wells. I-mode plasmas are characterized by H-mode-like energy barriers, but with L-mode-like particle barriers. The decoupling of energy and particle barrier formation makes the I-mode an interesting regime for fusion research and provides for a low collisionality pedestal without edge localized modes.

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“…For the purpose of evaluating the electric field at the edge, we use global parameters for a DIII-D-like discharge, with B tor = 2.1 T, I p = 1 MA, n e = 2.5 × 10 19 m −3 , n e-sep = 10 19 m −3 , T e-sep = 50 eV, R 0 = 1.66 m, a = 0.67 m [32]. With these we have the following: the characteristic Alfvén velocity, v A ≡ (a/R 0 )B tor / √ µ 0 m i n i = 3.7 × 10 6 m s −1 , the Alfvén time, τ A ≡ a/v A = 1.8 × 10 −7 s, the classical resistivity at the separatrix, η sep = 4.37 × 10 −6 m, and the resistive diffusion time, τ R ≡ µ 0 a 2 /η = 1.3 × 10 −1 s, which gives a normalized resistivity ofη ≡ τ A /τ R = 1.4 × 10 −6 .…”

Section: General Properties Of the E-field And E × B Velocitymentioning

confidence: 99%

Pfirsch–Schlüter current-driven edge electric fields and their effect on the L–H transition power threshold

Aydemir¹

2012

Nucl. Fusion

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An important contribution to the magnetohydrodynamic equilibrium at the tokamak edge comes from the Pfirsch-Schlüter current. The parallel electric field that can be associated with these currents is necessarily poloidally asymmetric and makes a similarly nonuniform contribution to the radial electric field on a flux surface. Here the role of the poloidal variation of this radial electric field in the L-H transition power threshold is investigated. Dependence of the resulting electric fields on magnetic topology, geometric factors such as the upper/lower triangularity and elongation, and the relative position of the X-point(s) in the poloidal plane are examined in detail. Starting with the assumption that an initially more negative radial electric field at the edge helps lower the transition power threshold, we find that our results are in agreement with a variety of experimental observations. In particular, for a 'normal' configuration of the plasma current and toroidal field we show the following. (i) The net radial electric field contribution by the Pfirsch-Schlüter currents at the plasma edge is negative for a lower single null and positive for a corresponding upper single null geometry. (ii) It becomes more negative as the X-point height is reduced. (iii) It also becomes more negative as the X-point radius is increased. These observations are consistent with the observed changes in the L-H transition power threshold P LH under similar changes in the experimental conditions. In addition we find that (iv) in USN with an unfavourable ion ∇B drift direction, the net radial electric field contribution is positive but decreases as the X-point radius decreases. This is consistent with the C-Mod observation that an L-I mode transition can be triggered by increasing the upper triangularity in this configuration. (v) Locally the radial electric field is positive above the outer mid-plane and reverses sign with reversal of the toroidal field, consistent with DIII-D observations in low-power L-mode discharges. Thus, taken as a whole, the Pfirsch-Schlüter current-driven fields can explain a number of observations on the L-H or L-I transition and the required power threshold P LH levels not captured by simple scaling laws. They may indeed be an important 'hidden variable'.

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Effect of ion ∇B drift direction on density fluctuation poloidal flow and flow shear

Cited by 22 publications

References 45 publications

A review of experimental drift turbulence studies

A review of experimental drift turbulence studies

Edge radial electric field structure and its connections to H-mode confinement in Alcator C-Mod plasmas

Pfirsch–Schlüter current-driven edge electric fields and their effect on the L–H transition power threshold

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