Reflectometry measurements during operation of the dynamic ergodic divertor at TEXTOR

Krämer–Flecken, A.; Soldatov, S.; Busch, C.; Liang, Y.; Hellermann, M. von; Wolf, R. C.; Zimmermann, O.

doi:10.1088/0029-5515/46/9/s09

Cited by 25 publications

(31 citation statements)

References 31 publications

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“…The E r B shear rate is, however, reduced by the DED at all measured radial locations. Similar changes in E r were observed earlier on TEXT [13] and recently on TEXTOR by other diagnostics [14]. A possible explanation is that with ED the electrons move faster than massive ions along the field lines to the wall so that a positive E r must be created to restore the ambipolarity.…”

supporting

confidence: 81%

Influence of the Static Dynamic Ergodic Divertor on Edge Turbulence Properties in TEXTOR

Weynants

Jachmich

et al. 2006

Phys. Rev. Lett.

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Systematic measurements on the edge turbulence and turbulent transport have been made by Langmuir probe arrays on TEXTOR under various static Dynamic Ergodic Divertor (DED) configurations. Common features are observed. With the DED, in the ergodic zone the local turbulent flux reverses sign from radially outwards to inwards. The turbulence properties are profoundly modified by energy redistribution in frequency spectra and suppression of large scale eddies. The fluctuation poloidal phase velocity changes direction from electron to ion diamagnetic drift, consistent with the observed reversal of the E r B flow. In the laminar region, the turbulence is found to react to an observed reduced flow shear. [4] have demonstrated that an ergodized magnetic boundary can be effective to optimize the plasma-wall interaction. Meanwhile, the local effects of the magnetic ergodization on edge turbulence and turbulent cross-field transport have also been studied both experimentally [1,5,6] and theoretically [7,8]. It has been observed on TEXT [1] and Tore Supra [2,5,6] that in the ergodic divertor (ED) configuration the edge density fluctuations are decreased whereas the turbulent cross-field diffusivity is less affected. However, a systematic investigation of the turbulence properties, such as frequency and wave-number spectra and the fluctuation propagations, has not been made or was done for a reduced set of wavenumber values [5]. A distinct description in the ergodic and laminar zones was also not given.Recently, on the tokamak TEXTOR the Dynamic Ergodic Divertor (DED) [4] has been installed at the high-field side of the torus (R=a 1:75=0:47 m), contrary to other machines where the ED coils were mounted at the low-field side [1][2][3]. The DED consists of 16 perturbation coils oriented parallel to the field lines on the magnetic flux surface with a safety factor q 3. With different current distributions in the coils, the base poloidal/toroidal modes, m=n, can be adjusted as 12=4, 6=2, and 3=1. The penetration depth into the plasma depends on m: In 12=4 the influence is restricted to the plasma boundary, while in 3=1 it can reach much deeper. In the outer plasma layer, DED induces stochastization of the magnetic field lines, including an ergodic zone with long and a laminar zone with short connection lengths to the wall [9]. In this Letter, we present the first systematic measurements by Langmuir probe arrays on the edge turbulence properties and fluctuation-driven transport in the presence of various static DED configurations (dc current on the coils).To get effective impacts of the DED at the plasma boundary, the discharge conditions have been optimized as follows: For m=n 12=4, I p 250 kA, B T 1:4 T, R=a 1:73=0:46 m, dc DED current I DED 12 kA; for 6=2, I p 270 kA, B T 1:9 T, R=a 1:73=0:46 m, I DED 6 kA; and for 3=1, I p 250 kA, B T 1:9 T, R=a 1:75=0:48 m, I DED 1 kA. The I DED is applied during the stationary phase of the Ohmic discharge. In all cases, no external tearing modes are excited, the lineaveraged plasma densit...

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supporting

confidence: 81%

Influence of the Static Dynamic Ergodic Divertor on Edge Turbulence Properties in TEXTOR

Weynants

Jachmich

et al. 2006

Phys. Rev. Lett.

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“…If the GAM has a radial eigenmode structure then the spacing of adjacent plateaus maybe indicative of the mode half wavelength (cf. [29]). For the circular plasmas the radial wavelength would be λ r ∼ O(5 − 6) cm.…”

Section: Summary and Discussionmentioning

confidence: 99%

Amplitude behaviour of geodesic acoustic modes in the ASDEX Upgrade tokamak

Conway

2008

Plasma Phys. Control. Fusion

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Abstract. The amplitude behaviour of the geodesic acoustic mode (GAM), a low frequency (5 − 25 kHz) coherent plasma turbulence-generated E r × B zonal flow oscillation, is studied in the ASDEX Upgrade tokamak using Doppler reflectometry. Across the tokamak edge the GAM displays one or more maxima a few cm wide, coinciding with radial plateaus in the GAM frequency, suggesting nested zonal flow layers. The GAM peak amplitude ranges between 0.2 − 0.8 km s −1 , corresponding to several tens of percent of the mean E × B flow velocity. In limiter configurations the GAM amplitude is found to decrease inversely with the plasma vertical elongation 1.1 < κ < 1.4, but rises again in X-point divertor configurations at higher κ ∼ 1.6. At low κ the GAM amplitude increases with the safety factor q, consistent with damping effects, but becomes less sensitive to q at high κ. A positive linear dependence on the electron temperature gradient, an indicator of the linear turbulence drive, is also observed.

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“…It was previously suggested that the pedestal, or more specifically the second radial derivatives in the profiles, ∂ 2 /∂r 2 , may act as some form of barrier [11]. It is notable that most shaped devices report GAMs only within a few cm of the edge, while both the TEXT and TEXTOR circular devices have observed GAMs relatively deep in the plasma [16,39]. Unfortunately density profiles were not provided, but parabolic-like profiles are suspected, c.f.…”

Section: Gam Peak Splittingmentioning

confidence: 99%

Frequency scaling and localization of geodesic acoustic modes in ASDEX Upgrade

Conway

Tröster

Scott

et al. 2008

Plasma Phys. Control. Fusion

110

163

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Abstract. The frequency behaviour and localization of the geodesic acoustic mode (GAM), believed to be a coherent plasma turbulence-generated E r × B T zonal flow oscillation, is studied in the ASDEX Upgrade tokamak using Doppler reflectometry. In typical elongated (1.4 < κ < 1.75) plasmas with an X-point divertor configuration the GAM is observed only in the edge density gradient region 0.95 < ρ pol < 1.0 between the density pedestal top and the flux surface boundary. The GAM frequency (5 − 25 kHz) is found to scale linearly as ω = G c s /R o (sound speed over major radius) but with an inverse dependence on the plasma elongation κ and a weak direct dependence on the safety factor q. The lower the GAM frequency the more important it is expected to become in moderating the turbulence via shear decorrelation. A heuristic scaling law for the frequency scale factor G ∼ O(1) involving κ and finite aspect ratio terms has been obtained from dedicated parameter scans. For circular plasmas κ ∼ 1 touching the limiter the density pedestal is weakened and the GAM is seen to reach in radially as far as ρ pol ∼ 0.75, depending on the q profile, with a frequency scale G → √ 2 consistent with theoretical predictions. Radially the GAM frequency is not a smooth function but displays a series of plateaus a few cm wide coinciding with peaks in the GAM amplitude, suggesting several zonal flow layers. At the plateau edges the GAM spectral peak splits into two frequency branches.

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Reflectometry measurements during operation of the dynamic ergodic divertor at TEXTOR

Cited by 25 publications

References 31 publications

Influence of the Static Dynamic Ergodic Divertor on Edge Turbulence Properties in TEXTOR

Influence of the Static Dynamic Ergodic Divertor on Edge Turbulence Properties in TEXTOR

Amplitude behaviour of geodesic acoustic modes in the ASDEX Upgrade tokamak

Frequency scaling and localization of geodesic acoustic modes in ASDEX Upgrade

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