Measurement and calculation of the radial electric field in the stellarator W7-AS

Baldzuhn, J.; Kick, M.; Maaßberg, H.; Team, W As

doi:10.1088/0741-3335/40/6/006

Cited by 69 publications

(99 citation statements)

References 73 publications

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“…The percentage is variable, although anywhere between 2 − 25% is realistic at the plasma edge. The density distribution modified by the plasma turbulence, via the δn e term, is then given by n e (x, y,t) = n e (x, y) t + δn e (x, y,t) (19) where n e (x, y) is the average experimental density profile. Figure 8 illustrates for a 7.5% fluctuation level, the parameters: perturbation (ζ), density fluctuation (δn e ), experimental density (n e ) and the density distribution (n e + δn e ) along the r direction with constant z =1480 grid points.…”

Section: Modelling Turbulent Density Fluctuationsmentioning

confidence: 99%

Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

Schirmer

Conway

Holzhauer

et al. 2007

Plasma Phys. Control. Fusion

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Abstract.The technique of correlation Doppler reflectometry for providing radial correlation length L r measurements is explored in this paper. Experimental L r measurements are obtained using the recently installed dual channel Doppler reflectometer system on ASDEX Upgrade. The experimental measurements agree well with theory and with L r measured on other fusion devices using different diagnostic techniques. A strong link between L r and plasma confinement could be observed. During an L-H transition, an increase in the absolute value of E r shear was detected at the same plasma edge region where a decrease in L r was measured. This observation is in agreement with theoretical models which predict that an increase in the absolute shear suppresses turbulent fluctuations in the plasma, leading to a reduction in L r . Furthermore, L r decreases from plasma core to edge due to the more highly confined plasma pedestal region. Measurements of L r versus plasma triangularity δ were also obtained, showing a decrease of L r with increasing δ. This indicates that plasma confinement is improving with triangularity. In connection with the experimental results, an investigation of the correlation Doppler reflectometer response function using a 2-dimensional finite difference time domain (FDTD) code was performed. The simulation results confirm that Doppler reflectometry provides robust radial correlation lengths of the turbulence with high resolution and suggests that L r is independent of the turbulence wavenumber k ⊥ and its fluctuation level.

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Section: Modelling Turbulent Density Fluctuationsmentioning

confidence: 99%

Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

Schirmer

Conway

Holzhauer

et al. 2007

Plasma Phys. Control. Fusion

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“…This is because the toroidal flow associated with the ambipolar radial electric field is small compared with the flow due to NBI in most L-mode discharges. In the hot electron mode where the electron thermal transport barrier is established, a large electric field ͑.10 kV͞m͒ appears in the plasma core [20][21][22], then toroidal flow associated with the ambipolar radial electric field may overcome the toroidal flow due to NBI. In this paper, the first observation of toroidal flow antiparallel to the ͗E r 3 B u ͘ drift direction (also antiparallel to the neutral beam injection) near the electron transport barrier in the hot electron mode plasmas is described.…”

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confidence: 99%

“…R ax ͒ of the midplane in the vertically elongated cross section, calculated with a finite-beta three-dimension equilibrium code VMEC. The profile of tan 21 ͑ir͞R͒ is plotted in (a) as a reference for magnetic flux averaged a.…”

mentioning

confidence: 99%

Observation of Toroidal Flow Antiparallel to the〈Er×Bθ〉Drift Direction in the Hot Electron Mode Plasmas in the Compact He

et al. 2001

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A toroidal flow antiparallel to the ͗E r 3 B u ͘ drift direction is observed in the hot electron mode plasmas when a large positive electric field and a sharp electron temperature gradient are sustained inside the internal transport barrier in the Compact Helical System. This toroidal flow reaches up to 5 3 10 4 m͞s at the plasma center, and it is large enough to reverse the toroidal flow driven by a tangentially injected neutral beam. These observations clearly show the plasma favors flow in the minimum =B direction at the transport barrier. DOI: 10.1103/PhysRevLett.86.3040 PACS numbers: 52.55.HcA spontaneous poloidal flow has been recognized to be important in the confinement of toroidal plasmas, since poloidal flow velocity and radial electric field shear were found to contribute an improvement of plasma confinement in H-mode plasmas [1,2]. The turbulence in the plasma is predicted to be suppressed by a E 3 B velocity shear through the mechanism of a nonlinear decorrelation [3,4]. It has been confirmed experimentally that the E 3 B shearing rate exceeds the growth rate of the turbulence at the transport barrier [5][6][7]. The toroidal flow has been considered to be determined by the radial transport of momentum driven by neutral beam injection (NBI) with anomalous shear viscosity [8,9]. Although the E 3 B velocity shear due to the toroidal flow can be important in the plasma core [10], there have been few experiments that demonstrate a spontaneous toroidal flow not driven by NBI in tokamaks [11][12][13][14]. It is generally the case in tokamaks that the plasma rotates parallel (antiparallel) to the plasma current for the coinjected (counterinjected) NBI, which corresponds to a positive (negative) radial electric field ͑E r ͒ in L-mode plasmas [15]. However, when a negative E r is produced at the transport barrier, both localized toroidal flow in the counterdirection and localized poloidal flow in the electron diamagnetic direction are observed even for plasmas with coinjected NBI [5,16,17]. These results show the need for detailed measurements of both toroidal and poloidal flow at the transport barrier, where the toroidal flow profile is not determined by the transport of momentum driven by NBI alone. Yet, despite the importance of the coupling between toroidal and poloidal flow, no studies until now have discussed the toroidal flow at the internal transport, where there is a large radial electric field.In a Heliotron/Torsatron device, there is no toroidal symmetry, and the minimum =B trace has a helical structure and helical flow along the minimum =B can be expected. The significant difference between tokamaks and Heliotron devices is the relationship between the poloidal field direction and the direction of dominant symmetry.The pitch angle of dominant symmetry is even larger than that of the averaged poloidal field in a Heliotron device, while it is zero due to the toroidal symmetry in tokamaks. This helical flow should change sign depending on the sign of E r or on the direction of the magnetic field. H...

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“…However, there has been no experimental study to test these issues on the neoclassical ion transport in stellarator plasmas. This is because the transition of the radial electric field from small negative (ion root) to large positive (electron root) was observed only in plasmas with the assistance of electron cyclotron heating (ECH), where electron heating is dominant [7][8][9][10][11]. The ion temperature is much lower than the electron temperature because ions are heated only by the energy exchange between ions and electrons.…”

mentioning

confidence: 99%

“…In previous experiments [7][8][9][10][11], the transition from ion root to electron root was observed only in plasmas with ECH, therefore it has been also open to question as to whether the nonthermal electrons driven by ECH are required to achieve the transition. The transition of radial electric field from ion root to electron root clearly demonstrates that nonthermal electrons are not necessary for the transition, because there are no nonthermal electrons in NBI heated plasma.…”

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confidence: 99%

Reduction of Ion Thermal Diffusivity Associated with the Transition of the Radial Electric Field in Neutral-Beam-Heated Plasmas in the Large Helical Device

Ida¹,

Funaba²,

Kado³

et al. 2001

Phys. Rev. Lett.

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Recent large helical device experiments revealed that the transition from ion root to electron root occurred for the first time in neutral-beam-heated discharges, where no nonthermal electrons exist. The measured values of the radial electric field were found to be in qualitative agreement with those estimated by neoclassical theory. A clear reduction of ion thermal diffusivity was observed after the mode transition from ion root to electron root as predicted by neoclassical theory when the neoclassical ion loss is more dominant than the anomalous ion loss. Neoclassical ion transport is important in stellarator plasmas, because the helical ripple losses are comparable to or sometimes even higher than the anomalous losses in contrast to those in tokamaks. The crucial issues of neoclassical ion transport are (1) the reduction of ion thermal diffusivity due to a large positive radial electric field in the electron root [1][2][3], and (2) the reduction of ion thermal diffusivity due to the optimization of the magnetic field structure (s optimization) [3][4][5][6]. However, there has been no experimental study to test these issues on the neoclassical ion transport in stellarator plasmas. This is because the transition of the radial electric field from small negative (ion root) to large positive (electron root) was observed only in plasmas with the assistance of electron cyclotron heating (ECH), where electron heating is dominant [7][8][9][10][11]. The ion temperature is much lower than the electron temperature because ions are heated only by the energy exchange between ions and electrons. In these experiments, the significant increase of electron temperature and a clear reduction of electron thermal diffusivity were observed in the plasma core in the electron root. However, no reduction of ion thermal diffusivity was observed because of the lack of direct ion heating. There have been no experimental results to show the improvement of ion transport in the electron root, although a significant improvement of ion transport (rather than the electron transport) is predicted by the neoclassical theory [6]. This paper describes the experimental results of the neoclassical feature of ion transport for the first time, the reduction of ion thermal diffusivity due to the transition to the large positive electric field (electron root), and/or the optimization of the magnetic field structure (s optimization).The large helical device (LHD) [12] is a Heliotron device (poloidal period number L 2, and toroidal period number M 10) with a major radius of R ax 3.5 4.1 m, an average minor radius of 0.6 m, and magnetic field up to 3 T. The radial electric field ͑E r ͒ is derived from the poloidal and toroidal rotation velocity and pressure gradient of neon impurity measured with charge exchange spectroscopy [13] at the midplane in LHD (vertically elongated cross section) using a radial force balance. The radial force balance equation can be expressed as E r ͑en I Z I ͒ 21 ͑≠p I ͞≠r͒ 2 ͑y u B f 2 y f B u ͒, where B f and B u are toroidal a...

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Measurement and calculation of the radial electric field in the stellarator W7-AS

Cited by 69 publications

References 73 publications

Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

Observation of Toroidal Flow Antiparallel to the〈Er×Bθ〉Drift Direction in the Hot Electron Mode Plasmas in the Compact He

Reduction of Ion Thermal Diffusivity Associated with the Transition of the Radial Electric Field in Neutral-Beam-Heated Plasmas in the Large Helical Device

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