In ASDEX Upgrade, experimental efforts aim to establish pace making and mitigation of type-I edge localized modes (ELMs) in high confinement mode (H-mode) discharges. Injection of small size cryogenic deuterium pellets (~(1.4?mm)2 ? 0.2?mm ? 2.5 ? 1019?D) at rates up to 83?Hz imposed persisting ELM control without significant fuelling, enabling for investigations well inside the type-I ELM regime. The approach turned out to meet all required operational features. ELM pace making was realized with the driving frequency ranging from 1 to 2.8 times the intrinsic ELM frequency, the upper boundary set by hardware limits. ELM frequency enhancement by pellet pace making causes much less confinement reduction than by engineering means like heating, gas bleeding or plasma shaping. Confinement reduction is observed in contrast to the typical for engineering parameters. Matched discharges showed triggered ELMs ameliorated with respect to intrinsic counterparts while their frequency was increased. No significant differences were found in the ELM dynamics with the available spatial and temporal resolution. By breaking the close correlation of ELM frequency and plasma parameters, pace making allows the establishment of fELM as a free parameter giving enhanced operational headroom for tailoring H-mode scenarios with acceptable ELMs. Use was made of the pellet pace making tool in several successful applications in different scenarios. It seems that further reduction of the pellet mass could be possible, eventually resulting in less confinement reduction as well.

https://iopscience.iop.org/article/10.1088/0029-5515/44/5/010/pdf

ELM pace making and mitigation by pellet injection in ASDEX Upgrade

The installation of a new poloidal charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) has enabled the determination of the radial electric field, Er, using the radial force balance of impurity ions. Er has been derived from charge exchange (CX) spectra measured on different impurity species, such as He2+, B5+, C6+ and Ne10+. The resulting Er profiles are found to be identical within the uncertainties regardless of the impurity species used, thus, demonstrating the validity of the diagnostic technique. The Er profile has been compared to the main ion pressure gradient term, which is found to be the dominant contribution at the plasma edge, thus, supporting that the Er well is created by the main ion species. The Er profile has been measured in different confinement regimes including L-, I- and H-mode. The depth of the Er well and the magnitude of the Er shear are correlated with the ion pressure at the pedestal top. The temporal evolution of the measured CX profiles and the resulting Er have been studied during an edge-localized mode (ELM) cycle. At the ELM crash, the Er minimum is less deep resulting in a reduction of the E???B shear. Within 2?ms after the ELM crash, the edge kinetic profiles have nearly recovered and the Er well is observed to recover simultaneously. In high density type-I ELM mitigated H-mode plasmas, obtained via externally applied magnetic perturbations (MPs) with toroidal mode number n?=?2, no clear effect on Er due to the MPs has been observed.

/pdf/high-accuracy-characterization-of-the-edge-radial-electric-4qt1wm58cv.pdf

High-accuracy characterization of the edge radial electric field at ASDEX Upgrade

This article draws on data generated through interviews with Years 7 to 10 pupils in New Zealand to propose that pupils experience assessment for learning as embedded in, and accomplished through, interactions with peers and teachers. Pupil commentary indicated they perceived assessment for learning as having cognitive, social relationship and affective purposes and consequences. Pupils used a range of criteria to assess their learning. Pupils with learning goals viewed assessment for learning as a joint teacher–pupil responsibility. They expressed a preference for teacher feedback in the form of suggestions because these maintained an active role for them in making sense of ideas. Pupils with performance goals intimated they viewed assessment as a teacher's sole responsibility. They preferred feedback on how to complete tasks. A lack of mutual trust and respect was said to lead pupils to limit the disclosure of their ideas. Overall, pupils intimated that they experience assessment for learning as a compl...

Pupil commentary on assessment for learning

The elements of transport into and across the scrape-off layer in the poloidal divertor tokamak ASDEX Upgrade are analysed for different operational regimes with emphasis on enhanced confinement regimes with an edge barrier. Utilizing the existing set of edge diagnostics, especially the high-resolution multi-pulse edge Thomson scattering system, in combination with long discharge plateaus, radial sweeps and advanced averaging techniques, detailed radial mid-plane profiles of diverted plasmas are obtained. Profiles are smooth across the separatrix, indicating strong radial correlation, and there is no remarkable variation across the second separatrix either. Together with measured input, recycling, pumping and bypass fluxes, a corrected separatrix position is determined and transport characteristics are derived in the different radial zones generally identified in the profile structure. Transport in the steep gradient region inside and across the separatrix shows typical ballooning-type critical electron pressure gradient scaling and, in parallel, even a clear correlation between radial electron density and temperature decay lengths (e.g. η e = d(ln T)/d(ln n) ∼ 2 for type-I ELMy H-modes). These findings indicate the importance of stiff profiles in this region, while diffusion coefficients are secondary parameters, determined essentially by the source distribution. The outer scrape-off layer wing exhibits a more filamentary structure with preferential outward drift especially in high-performance discharges, with formal diffusion coefficients far above the Bohm value in agreement with results on the old ASDEX experiment. A basic mechanism involved there seems to be partial loss of equilibrium and fast curvature-driven outward acceleration, in principle well known from theory, investigated decades ago in pinch experiments and utilized recently in high-field-side pellet fuelling.

Transport into and across the scrape-off layer in the ASDEX Upgrade divertor tokamak

Two families of exact analytical solutions of the Grad–Shafranov equation are presented by specifying the highest polynomial dependence of the plasma current density on the flux function Ψ in such a way that the Grad–Shafranov equation becomes a linear inhomogeneous differential equation. Both the pressure profile and the poloidal current profile each have two free parameters. X-points can be represented by superposition of solutions. Examples of the exact equilibrium solution are given for both a D-shaped plasma and a toroidally diverted plasma.

Analytical solutions to the Grad–Shafranov equation

ASDEX upgrade MHD equilibria reconstruction on distributed workstations

The spatial distribution of density in a fusion experiment is of significant importance as it enters in numerous analyses and contributes to the fusion performance. The reconstruction of the density profile is therefore commonly done in offline data analysis. In this paper, we present an algorithm which allows for density profile reconstruction from the data of the submillimetre interferometer and the magnetic equilibrium in real-time. We compare the obtained results to the profiles yielded by a numerically more complex offline algorithm. Furthermore, we present recent ASDEX Upgrade experiments in which we used the real-time density profile for active feedback control of the shape of the density profile.

/pdf/real-time-feedback-control-of-the-plasma-density-profile-on-4b5bzb2c1j.pdf

R. Merkel

Papers

ASDEX upgrade MHD equilibria reconstruction on distributed workstations

Real-time feedback control of the plasma density profile on ASDEX Upgrade

Review of the ASDEX Upgrade Data Acquisition Environment - Present Operation and Future Requirements

Recent Developments in the ASDEX Upgrade Data Acquisition Environment

Real-time standard diagnostic for ASDEX Upgrade