The space potential in the tokamak text

Yang, Xiaodong; Zhang, B. Z.; Wootton, A. J.; Schoch, P. M.; Richards, B.; Baldwin, D.E.; Brower, D. L.; Castle, G.G.; Hazeltine, R. D.; Heard, J. W.; Hickok, R. L.; Li, W. L.; Lin, Hong; McCool, S. C.; Simcic, V.; Ritz, Ch. P.; Yu, Cheng‐Wei

doi:10.1063/1.859774

Cited by 81 publications

(50 citation statements)

References 25 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: 54%

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: 54%

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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“…The usual explanation for the E r well inversion is the ambipolar plasma response to the opening and/or mixing (stochastization) of the edge magnetic field lines resulting from the MP field penetration [7]. The creation of a stochastic field region in the edge is also indicated by Poincoré plots of the computed vacuum field [24], and by the field line connection length L c (from inner to outer divertor), as shown in figure 5(b).…”

Section: Edge E R Response To Mpmentioning

confidence: 99%

“…Langmuir probes have been widely used to measure plasma potential and density variations (which are seen to be particularly sensitive indicators of the MP effect) in the plasma scrape-off-layer (SOL) and separatrix regions of tokamaks [12,14,15,16], while deeper probing has been achieved with reflectometry [17], CXRS [11,18], BES [9] and HIBP [7] for example. In this work, Doppler reflectometry (described in section 3) is used on AUG to directly measure the radial electric field E r , its fluctuations, as well as the density turbulence δn e behaviour with high spatial resolution from the far SOL to inside the edge gradient region of low collisionality L-mode discharges.…”

Section: Introductionmentioning

confidence: 99%

“…Although the theory and application of small perturbing magnetic fields is not new, cf. TEXT and JFT-2M results from the 1990's [6,7,8], the nonlinear response of the plasma to the MPs is rather complex and there remain many open questions. For example, a recent aspect is the role of the MP on the edge turbulence and zonal flows and their impact on the ELM cycle [9].…”

Section: Introductionmentioning

confidence: 99%

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Impact of magnetic perturbation coils on the edge radial electric field and turbulence in ASDEX Upgrade

Fietz

Müller

Lunt

et al. 2014

Plasma Phys. Control. Fusion

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Abstract. The impact of applied magnetic perturbations (MPs) on tokamak edge parameters has been investigated in ASDEX Upgrade low collisionallity L-mode discharges using a flexible set of in-vessel saddle coils (capable of generating n = 0, 1, 2 and 4 toroidal modes) and an extensive set of high resolution edge diagnostics. Doppler reflectometry is used principally to probe the MP field penetration and structure via the radial electric field E r and density fluctuation δn e behaviour. Different MP response behaviour are observed for the near scrape-off-layer, SOL, (where E r flattens) and in the confinement region (where the negative E r well reverses). The radial structure of E r and δn e are particularly sensitive to the degree of MP resonance with the edge rational field-lines. Specifically, the edge turbulence is enhanced for strongly resonant MPs and reduced when non-resonant. The toroidal structure of the MP response has also been probed for various MP configurations by rotating the MP field toroidally, and is found to be different for the edge and near SOL regions.

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“…In specific discharges dust particles levitating in a plasma can also be used but the electric field measurement relies heavily on good knowledge of the dust charging process [19]. In the plasma core of tokamaks, injection of a heavy singly charged ion beam creates doubly charged ions the measurement of whose energy gives information on the plasma potential at the ionization point, and thus indirect electric field measurement [21]. The effect of the field on the motion of charged particles can also be recorded using laser induced fluorescence techniques and the electric potential in the plasma is indirectly derived from the shift of the measured velocity distribution function [2].…”

Section: Introductionmentioning

confidence: 99%

Lamb-shift and electric field measurements in plasmas

Doveil¹,

Chérigier-Kovacic²,

Ström³

2016

Plasma Phys. Control. Fusion

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The electric field is a quantity of particular relevance in plasma physics. Indeed, its fluctuations are responsible for different macroscopic phenomena such as anomalous transport in fusion plasmas. Answering a long-standing challenge, we offer a new method to locally and non-intrusively measure weak electric fields and their fluctuations in plasmas, by means of a beam of hydrogen ions or atoms. We present measurements of the electric field in vacuum and in a plasma where Debye shielding is measured. For the first time, we have used the Lamb-shift resonance to measure oscillating electric fields around 1 GHz and observed the strong enhancement of the Lyman-α signal. The measurement is both direct and non-intrusive. This method provides sensitivity (mV cm −1 ) and temporal resolution (ns) that are three orders higher compared to current diagnostics. It thus allows measuring fluctuations of the electric field at scales not previously reached experimentally.

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The space potential in the tokamak text

Cited by 81 publications

References 25 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

Impact of magnetic perturbation coils on the edge radial electric field and turbulence in ASDEX Upgrade

Lamb-shift and electric field measurements in plasmas

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