The radial electric field and its associated shear in the ASDEX Upgrade tokamak

Schirmer, J.; Conway, G. D.; Zohm, H.; Suttrop, W.

doi:10.1088/0029-5515/46/9/s13

Cited by 82 publications

(90 citation statements)

References 60 publications

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“…Due to the profile of the pressure gradient, v el,dia approaches zero when leaving this radial interval in either direction. It is also known that v E×B will rapidly become negative outside the illustrated radial range [29]. In summary, the region where v SMP,perp is contained within the theoretically predicted interval extends from the separatrix to the pedestal top or slightly further inside.…”

Section: Radial Location Of Smpsmentioning

confidence: 66%

Solitary magnetic perturbations at the ELM onset

et al. 2012

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Tokamak H-mode plasmas frequently exhibit edge-localized modes (ELMs). ELMs allow maintaining sufficient plasma purity and thus enable stationary H-mode. On the other hand in a future device ELMs may cause divertor power flux densities far in excess of tolerable material limits. The size of the energy loss per ELM is determined by saturation effects in the non-linear phase of the ELM, which at present is hardly understood. ASDEX Upgrade is now equipped with a set of fast sampling diagnostics, which is well suited to investigate the chain of events around the ELM crash with appropriate temporal resolution ( 10 µs).Solitary magnetic perturbations (SMPs) are identified as dominant features in the radial magnetic fluctuations below 100 kHz. They are typically observed close (±100 µs) to the onset of pedestal erosion. SMPs are field aligned structures rotating in the electron diamagnetic drift direction with perpendicular velocities of about 10 km s −1 . A comparison of perpendicular velocities suggests that the perturbation evoking SMPs is located at or inside the separatrix. Analysis of very pronounced examples showed that the number of peaks per toroidal turn is 1 or 2, which is clearly lower than the corresponding numbers in linear stability calculations. In combination with strong peaking of the magnetic signals this results in a solitary appearance resembling modes like palm tree modes, edge snakes or outer modes. This behaviour has been quantified as solitariness and correlated with main plasma parameters.SMPs may be considered as a signature of the non-linear ELM phase originating at the separatrix or further inside. Thus they provide a handle to investigate the transition from linear to non-linear ELM phase. By comparison with data from gas puff imaging processes in the non-linear phase at or inside the separatrix and in the scrape-off layer (SOL) can be correlated. A connection between the passing of an SMP and the onset of radial filament propagation has been found. Eventually the findings related to SMPs may contribute to a future quantitative understanding of the non-linear ELM evolution.

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Section: Radial Location Of Smpsmentioning

confidence: 66%

Solitary magnetic perturbations at the ELM onset

et al. 2012

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“…The measurement was performed in an Ohmic heated discharge, so there was no external momentum input. The plasma parameters were , [31]. The negative E r dip is able to overcompensate the ion diamagnetic drift and allows for a positive poloidal plasma rotation as visible in figure 6 for an Ohmic discharge.…”

Section: Turbulence Potentials and Electric Field In Ohmic And L-modmentioning

confidence: 96%

Latest investigations on fluctuations, ELM filaments and turbulent transport in the SOL of ASDEX Upgrade

et al. 2011

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Abstract. This paper presents turbulence investigations in the scrape off layer (SOL) of ASDEX Upgrade in Ohmic, L-mode and H-mode discharges using electrostatic and electromagnetic probes. Detailed studies are performed on small scale turbulence and on ELM filaments. Simultaneous measurements of floating and plasma potential fluctuations revealed significant differences between these quantities. Large errors can occur when the electric field is extracted from floating potential measurements, even in Ohmic discharges. Turbulence studies in Ohmic plasmas show the existence of density holes inside the separatrix and blobs outside. Close to the separatrix a reversal of the poloidal blob propagation velocity occurs. Investigations on the Reynolds stress in the scrape-off layer show its importance for the momentum transport in L-mode while its impact for momentum transport during ELMs in H-mode is rather small. In the far SOL the electron density and temperature were measured during type-I ELMy H-mode at ASDEX Upgrade resolving ELM filaments. Strong density peaks and temperatures of several 10 eV were detected during the ELM events. Additional investigations on the ions in the filaments by a retarding field analyzer indicate ion temperatures of 50-80 eV. ELMs expel also current concentrated in filaments into the scrape off layer. Furthermore discharges with small ELMs were studied. In N 2 seeded discharges the type-I ELM frequency rises and the ELM duration decreases. For discharges with small type-II ELMs the mean turbulent radial particle flux is increased over the mean particle flux in type-I ELM discharges at otherwise similar plasma parameters.

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“…The edge negative Ö well depth scales with the energy confinement. Typically Ö of ¿ to kV m ½ are observed for ohmic and and NBI L-mode conditions, increasing to ½ kV m ½ for improved confinement L-modes, and finally rising from ¾ to ¼kVm ½ for well developed H-modes [22].…”

Section: Aug Doppler Systemsmentioning

confidence: 98%

Interaction of Mean and Oscillating Plasma Flows Across Confinement Mode Transitions

Poli

Happel²,

Team

2010

Plasma and Fusion Research

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The interaction of mean plasma flows (driven by equilibrium ¢ forces) and oscillating plasma flows -such as zonal flows (ZFs) and geodesic acoustic modes (GAMs) -(radially localized ¢ plasma flows generated by non-linear turbulence interactions) is investigated in the ASDEX Upgrade tokamak using Doppler reflectometry. ZFs and GAMs play an important role in magnetic confinement devices by enhancing the velocity shearing of turbulent eddies which reduces turbulent cross-field transport. Although the GAM is universally observed across the plasma edge gradient region of ohmic and additionally heated low confinement L-mode regimes, it is not seen in the high confinement H-mode. Using slow power ramping of the additional ECRH and NBI heating systems the behaviour of the edge GAM, and its interaction with the mean flow shear, i.e. the radial electric field Ö profile, across the L to H-mode transition are studied. As the edge Ö well deepens with improving confinement, the associated negative Ö shear region narrows and the radial extent of the GAM shrinks. The main feature, however, is the movement of spectral power out of the coherent GAM into large amplitude, low frequency random flow perturbations. It is speculated that these flow perturbations may play a role in the triggering of the confinement transition.

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The radial electric field and its associated shear in the ASDEX Upgrade tokamak

Cited by 82 publications

References 60 publications

Solitary magnetic perturbations at the ELM onset

Solitary magnetic perturbations at the ELM onset

Latest investigations on fluctuations, ELM filaments and turbulent transport in the SOL of ASDEX Upgrade

Interaction of Mean and Oscillating Plasma Flows Across Confinement Mode Transitions

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