Density profile and turbulence evolution during L-H transition studied with the ultra-fast swept reflectometer on ASDEX Upgrade

Medvedeva, A.; Bottereau, C.; Clairet, F.; Hennequin, Pascale; Stroth, U.; Birkenmeier, G.; Cavedon, M.; Conway, G. D.; Happel, T.; Heuraux, S.; Molina, Diego Alonso Restrepo; Silva, A; Willensdorfer, M.

doi:10.1088/1361-6587/aa9251

Cited by 18 publications

(27 citation statements)

References 39 publications

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“…This behaviour explains the less deep well. In addition, the dynamics observed both in the velocity and in the amplitude of density fluctuations has strong similitude with observations made in DSB signal on ASDEX Upgrade during I-phase [Med+17]. Note that however, in WEST, no sign of this I-phase dynamics has been detected so far in other signals (such as Dalpha or fast sweep reflectometry).…”

Section: L-h Transition Observed In Usn Configurationsupporting

confidence: 78%

Formation of the radial electric field profile in the WEST tokamak

Vermare¹,

Hennequin²,

Honoré³

et al. 2021

Nucl. Fusion

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Sheared flows are known to reduce turbulent transport by decreasing the correlation length and/or intensity of turbulent structures. The transport barrier that takes place at the edge during improved regimes such as H mode, corresponds to the establishment of a large shear of the radial electric field. In this context, the radial shape of the radial electric field or more exactly of the perpendicular $E\times B$ velocity appears as a key element in accessing improved confinement regimes. In this paper, we present the radial profile of the perpendicular velocity measured using Doppler back-scattering system at the edge of the plasma, dominated by the $E\times B$ velocity, during the first campaigns of the WEST tokamak. It is found that the radial velocity profile is clearly more sheared in LSN than in USN configuration for ohmic and low current plasmas ($B=3.7T$ and $q_{95}=4.7$), consistently with the expectation comparing respectively “favourable” versus “unfavourable” configuration. Interestingly, this tendency is sensitive to the plasma current and to the amount of additional heating power leading to plasma conditions in which the $E\times B$ velocity exhibits a deeper well in USN configuration. For example, while the velocity profile exhibits a clear and deep well just inside the separatrix concomitant with the formation of a density pedestal during L-H transitions observed in LSN configuration, deeper $E_r$ wells are observed in USN configuration during similar transitions with less pronounced density pedestal.

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Section: L-h Transition Observed In Usn Configurationsupporting

confidence: 78%

Formation of the radial electric field profile in the WEST tokamak

Vermare¹,

Hennequin²,

Honoré³

et al. 2021

Nucl. Fusion

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“…Different edge coherent modes (ECMs) have been detected in the reflectometer signals during the experiments dedicated to L-H transition studies on AUG [22]. First appearing as sequences of pulsations in the raw reflectometer phase signal the modes could later be visible in the frequency spectra.…”

Section: Observation Of Modes With the Ufsrmentioning

confidence: 99%

High frequency edge coherent modes studied with the ultra-fast swept reflectometer on ASDEX Upgrade

Medvedeva

Bottereau

Clairet

et al. 2019

Plasma Phys. Control. Fusion

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The results on high frequency coherent modes developing at the plasma edge after L-H transitions are presented. These edge coherent modes (ECMs) observed in the I-phase, during ELM-free and ELMy H-mode phases have common characteristics, such as their frequency of 50-200 kHz, radial position and scaling of the frequency with the pressure gradient. The ultra-fast swept reflectometer with the sweep time of 1 µs has provided measurements of the mode dynamics and localisation. The ECMs propagate in the electron diamagnetic direction in the laboratory frame and are localized in the region between the pedestal top and the steepest pressure gradient.

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“…To measure the radial displacement, we use the high resolution profile diagnostics around the LFS midplane. Figure 2(a) shows the set of used diagnostics consisting of profile ECE [31,26], lithium beam (LIB) [32,33], edge charge exchange recombination spectroscopy (CXRS) [34] and X-mode reflectometer (REF-X) [35]. As some diagnostic data were not available for all discharges, the last column of Table 1 lists the availability of the various profile diagnostics.…”

Section: Displacement Measurements Around the Midplanementioning

confidence: 99%

Three dimensional boundary displacement due to stable ideal kink modes excited by external n = 2 magnetic perturbations

et al. 2017

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Abstract.In low-collisionality (ν ) scenarios exhibiting mitigation of edge localized modes (ELMs), stable ideal kink modes at the edge are excited by externally applied magnetic perturbation (MP)-fields. At ASDEX Upgrade these modes can cause three-dimensional (3D) boundary displacements up to the centimeter range. These displacements have been measured using toroidally localized high resolution diagnostics and rigidly rotating n = 2 MP-fields with various applied poloidal mode spectra. These measurements are compared to non-linear 3D ideal magnetohydrodynamics (MHD) equilibria calculated by VMEC. Comprehensive comparisons have been conducted, which consider for instance plasma movements due to the position control system, attenuation due to internal conductors and changes in the edge pressure profiles.VMEC accurately reproduces the amplitude of the displacement and its dependencies on the applied poloidal mode spectra. Quantitative agreement is found around the low field side (LFS) midplane. The response at the plasma top is qualitatively compared. The measured and predicted displacements at the plasma top maximize when the applied spectra is optimized for ELM-mitigation. The predictions from the vacuum modeling generally fails to describe the displacement at the LFS midplane as well as at the plasma top. When the applied mode spectra is set to maximize the displacement, VMEC and the measurements clearly surpass the ‡ matthias.willensdorfer@ipp.mpg.de arXiv:1702.03177v2 [physics.plasm-ph] 24 Jul 2017 3D boundary displacement due to stable ideal kink modes excited by external n=2 MPs 2 predictions from the vacuum modeling by a factor of four. Minor disagreements between VMEC and the measurements are discussed. This study underlines the importance of the stable ideal kink modes at the edge for the 3D boundary displacement in scenarios relevant for ELM-mitigation.

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Density profile and turbulence evolution during L-H transition studied with the ultra-fast swept reflectometer on ASDEX Upgrade

Cited by 18 publications

References 39 publications

Formation of the radial electric field profile in the WEST tokamak

Formation of the radial electric field profile in the WEST tokamak

High frequency edge coherent modes studied with the ultra-fast swept reflectometer on ASDEX Upgrade

Three dimensional boundary displacement due to stable ideal kink modes excited by external n = 2 magnetic perturbations

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