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22-27 October 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
CONFERENCE MATERIAL NOW AVAILABLE!

Increasing the Density in W7-X: Benefits and Limitations

24 Oct 2018, 15:20
20m
Mahatma Mandir Conference Centre

Mahatma Mandir Conference Centre

Gandhinagar (nearest Airport: Ahmedabad), India
Oral EXC - Magnetic Confinement Experiments: Confinement EX/3 Plasma Performance & Control

Speaker

Dr Golo Fuchert (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany)

Description

As the first comprehensively optimized stellarator, Wendelstein 7-X (W7-X) is an essential experiment to study high density operation in this kind of device. This contribution presents first experiments on the density dependence of the energy confinement in W7-X and limitations of the achievable density. Theoretical predictions and empirical scaling laws for the energy confinement time in stellarators (e.g. the ISS04) predict a positive correlation between the plasma density and the energy confinement time. However, this might not be valid for plasma operation close to operational limits. Hence, the energy confinement time scaling and the presence of operational limits have to be studied as an intertwined system. The experimental exploitation of W7-X has only started, however, the gradual completion of the machine capabilities is an ideal opportunity to map out the configuration space and to identify key issues on the route to high-performance long-pulse operation. In the first two experimental campaigns, featuring a limiter and a test divertor configuration, the energy confinement time has been analyzed. A positive density dependence has been found and the scaling coefficient is close to the expectation from ISS04. During these experiments, however, radiative collapses have been observed. Such a radiative density limit is predicted by simplified analytical models. Such a model has been applied to W7-X in order to estimate a critical density and in purely gas-fueled hydrogen plasmas, no stable plasma operation has been achieved above this density. It has been observed that the critical density also depends on the magnetic configuration, which directly relates this issue to scenario development. Furthermore, first experiments with pellet-fueling showed densities well above the critical density, which indicates the importance of profile and fueling effects. These experiments confirm that an increasing density is indeed beneficial for the energy confinement, at least in the currently accessible density range. It remains to be shown that this trend extrapolates to the high-performance plasmas W7-X was designed for. The experiments have also shown, however, that high-density operation involves a careful scenario development, as fueling issues and radiative instabilities limit the currently accessible operational space of W7-X in its current state of completion.
Country or International Organization Germany
Paper Number EX/3-5

Primary author

Dr Golo Fuchert (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany)

Co-authors

Dr Andreas Dinklage (Max-Planck-Institut für Plasmaphysik) Dr Andreas Langenberg (Max-Planck-Institut für Plasmaphysik,17491 Greifswald, Germany) Dr Craig Beidler (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Daihong Zhang (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Ekkehard Pasch (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Felix Warmer (Max Planck Institute for Plasma Physics) Mr Hannes Damm (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Heinrich Laqua (Max-Planck-Institute for Plasma Physics, Greifswald, Germany) Dr Jens Knauer (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Jürgen Baldzuhn (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Kai-Jakob Brunner (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Kian Rahbarnia (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Matthias Hirsch (Max-Planck-Institut für Plasmaphysik) Dr Novimir Pablant (Princeton Plasma Physics Laboratory) Dr Rainer Burhenn (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Prof. Robert Wolf (Max-Planck-Institute for Plasma Physics) Dr Samuel Lazerson (Princeton Plasma Physics Laboratory) Dr Scott Evan (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Sergey Bozhenkov (Max-Planck Insitute of Plasma Physics) Prof. Thomas Sunn Pedersen (Max Planck Institute for Plasma Physics) Dr Torsten Stange (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany) Dr Yevgen Kazakov (Laboratory for Plasma Physics, LPP-ERM/KMS) Dr Yühe Feng (Max-Planck-Institut für Plasmaphysik, Greifswald, Germany)

Presentation Materials

Paper