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

Abstract

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.

Scale-selective turbulence reduction in H-mode plasmas in the TJ-II stellarator

“…Comparison of E r from Doppler reflectometry with E r obtained with a heavy ion beam probe at TJ-II showed good agreement [35]. Furthermore, comparisons with E r from spectroscopy in W7-AS [40] and with E r from a Ball pen probe in ASDEX Upgrade [45] have also shown that v ph ≪ v E×B holds. In DIII-D, E r from beam emission spectroscopy and charge exchange recombination spectroscopy measurements also confirm the above statement [9].…”

Comparison of small edge-localized modes on MAST and ASDEX Upgrade

“…. Surprisingly, the net radial efflux of particles during a type II ELMing period is larger than that during type I ELMs (see also reference [19]). Some of the enhanced flux in the type II ELMs is due to the larger J SAT signals in between the filaments but the effect of this is estimated to be less than 10 % of the total flux.…”

Characterization of the Li-BES at ASDEX Upgrade

“…The CPS of the Li-BES at higher frequencies f > 30 kHz remains at the same level, whereas the spectrum of the ion saturation data continuously decreases. The image size of the optics is 6 mm × 12 mm, whereas the probe has a diameter of < 2 mm [34]. Therefore, structures with a perpendicular wavenumber k ⊥ larger than the corresponding wavenumber of the observation volume cannot be resolved, because of ω = v ⊥ k ⊥ depending on k ⊥ and the perpendicular velocity v ⊥ .…”