Parallel flow driven instability due to toroidal return flow in high-confinement mode plasmas

Sasaki, M.; Itoh, K.; Kosuga, Y.; Dong, J. Q.; Inagaki, S.; Kobayashi, Tatsuya; Cheng, J.; Zhao, K.J.; Itoh, Sanae I.

doi:10.1088/1741-4326/ab1292

Cited by 11 publications

(8 citation statements)

References 31 publications

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“…A potential candidate for the exciting of QCM is the parallel velocity gradient (PVG) mode [29], which is possibly driven by the strong shear flow in pedestal region of steepest gradient [30]. In addition, the recent theoretical simulation indicates that the D' Angelo mode due to toroidal return flow is unstable in transport barriers of steep gradients and it could drive the influx particle as well [26], but the relevant experimental data are not available for this experiment and will be our future focus. Therefore, the possibility for QCM being generated by the PVG mode or toroidal return flow cannot be completely excluded.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…It is noted that the toroidal return flow might have some contributions to the poloidal motion of the mode due to the barber pole effect. The parallel flow [26] is estimated as V // = 2q cosθ V p = 2 × 3.5 ×(4.5 ~5.2) km s -1 31.5 ~36.4 km s −1 , where q = 3.5 is edge safety factor and θ ≈ 0 is the poloidal angle at the middle plane. Nevertheless, the measured V p already includes the barber pole contribution from the toroidal return flow to the poloidal rotation in the lab frame, that is, V p is the total poloidal rotation in the laboratory frame.…”

Section: Characteristics and The Nonlinear Coupling Of The Pedestal Qcmmentioning

confidence: 99%

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Pedestal dynamics prior to type-III ELM onset on HL-2A tokamak*

et al. 2020

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A dramatic increase of density gradient and a slight decrease of electron temperature gradient were observed in the pedestal just prior to each onset in a series of ELM bursts. An inward particle flux induced by a quasi-coherent mode was found to be responsible for such changes. The characteristics of the quasi-coherent mode, including frequency, wave vector, propagation direction and velocity were identified. The nonlinear three-wave coupling is a plausible mechanism for the generation of the mode. Decoupling of the mode induced density transport from energy transport was discovered. The gradient of density was observed always much higher than that of temperature for electrons at the starting point of the ELM burst and, therefore, the dominant role of density gradient over electron temperature gradient for ELM onset is demonstrated. Consistence and contrast of the results with confinement and transport in I-mode discharges are discussed.

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Section: Summary and Discussionmentioning

confidence: 99%

Section: Characteristics and The Nonlinear Coupling Of The Pedestal Qcmmentioning

confidence: 99%

Pedestal dynamics prior to type-III ELM onset on HL-2A tokamak*

et al. 2020

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“…The typical flow speed of order of 10 4 m s −1 within the channel width of several centimetres around the separatrix provides a shear flow of several hundreds kHz, which is large enough to suppress the turbulence. Such a mechanism due to parallel shear flow is also discussed in a theoretical model [20,21]. More careful analysis is left for future work.…”

Section: Core and Divertor Plasmas With And Without An Rmp Fieldmentioning

confidence: 99%

Confinement improvement during detached phase with RMP application in deuterium plasmas of LHD

et al. 2022

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In order to explore compatibility of good core plasma performance with divertor heat load mitigation, interaction between cold edge plasma and core plasma transport including edge transport barrier (ETB) has been analysed in the divertor detachment discharges of deuterium plasmas in LHD with RMP (resonant magnetic perturbation) field application. The RMP application introduces widened edge stochastic layer and sharp boundary in the magnetic field structure between the confinement region and the edge stochastic layer. The widened edge stochastic layer enhances impurity radiation and provides stable detachment operation as compared with the case without RMP. It is found that ETB is formed at the confinement boundary at the onset of detachment transition. However, as the detachment deepens resistive pressure gradient driven MHD mode is excited, which degrade the ETB. At the same time, however, the core transport decreases to keep global plasma stored energy (Wp) unchanged, showing clearly core-edge coupling. After gradual increase of density fluctuation during the MHD activity, spontaneous increase of Wp and recovery of ETB are observed while the detachment is maintained. Then the coherent MHD mode ceases and ELM like bursts appear. In the improved mode, the impurity decontamination occurs, and the divertor heat load increase slightly. Key controlling physics in the interplay between core and cold edge plasma is discussed. Comparison between deuterium and hydrogen plasmas show that the hydrogen plasmas exhibit similar features as the deuterium ones in terms of density and magnetic fluctuations, impurity decontamination toward higher confinement etc. But most of the features are modest in the hydrogen plasmas and thus no clear confinement mode transition with clear ETB formation is defined. Better global confinement is obtained in the deuterium plasmas than the hydrogen ones at higher radiation level.

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“…PVG mode has been found to drive the inward particle flux theoretically and experimentally [13,14,15]. Recently, it has been pointed out that PVG mode in the H-mode plasmas can be precursor of the type-III ELM [16,17].…”

Section: Introductionmentioning

confidence: 96%

Neutral particle drag on parallel flow shear driven instability

et al. 2020

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Neutral drag effect on the parallel velocity gradient driven instability (PVG) in the presence of density inhomogeneity is theoretically investigated. The dispersion relation of PVG mode with the effect of density gradient and neutral particle drag is derived, and its solution is analytically obtained. The neutral particle drag gives rise to the phase shift between parallel flow and electrostatic potential fluctuations, and modifies the parallel compression. As a result, the stability of the PVG mode changes. It is found that the neutral particle drag does not only reduces but also enhances the instability. Especially, near the marginal condition, the neutral particle effect suppresses the density gradient effect, and the parameter region where the PVG mode is linearly unstable significantly expands.

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Parallel flow driven instability due to toroidal return flow in high-confinement mode plasmas

Cited by 11 publications

References 31 publications

Pedestal dynamics prior to type-III ELM onset on HL-2A tokamak*

Pedestal dynamics prior to type-III ELM onset on HL-2A tokamak*

Confinement improvement during detached phase with RMP application in deuterium plasmas of LHD

Neutral particle drag on parallel flow shear driven instability

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