Impurity transport studies at Wendelstein 7-X by means of x-ray imaging spectrometer measurements**Invited paper published as part of the Proceedings of the 45th Conference on Plasma Physics (EPS), Prague, Czech Republic, July, 2018.

A Langenberg ,G Fuchert ,P Drews ,F Warmer ,U Neuner ,H Thomsen ,U Höfel ,Th Krings ,A Dinklage ,B Geiger ,P Traverso ,K P Hollfeld ,J Schilling ,O Grulke ,Thomas Wegner ,E Pasch ,G Satheeswaran ,J Knauer ,C Killer ,K Rahbarnia ,O Marchuk ,Guido Offermanns ,S Bozhenkov ,K J Brunner ,F Kunkel ,B Buttenschön ,B Schweer ,N Pablant ,R Burhenn ,Javier Alonso ,M Hirsch ,R C Wolf

doi:10.1088/1361-6587/aaeb74

A Langenberg , G Fuchert + Show 30 more

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This paper reports on the effect of on- and off-axis heating power deposition on the impurity confinement in purely electron cyclotron resonance heated He plasmas on the stellarator Wendelstein 7-X. Therefore, impurity transport times τI have been determined after Fe impurity injections by laser ablations and monitoring the temporal impurity emissivities by the x-ray imaging spectrometer HR-XIS. A significant increase of τI has been observed when changing the power deposition from on- to off-axis heating with energy confinement times τE being mainly unaffected. In addition, the scaling of impurity transport properties with respect to a variation of heating power PECRH and electron density ne has been investigated by keeping the heating power deposition on-axis. The observed τI scaling compares well to known τI scaling laws observed in other machines. A comparison of τI and τE yields an averaged ratio of τE/τI = 1.3 and transport times in the range of τI = 40–130 ms and τE = 40–190 ms. Comparing those absolute values to neoclassical predictions supports the recently observed nature of anomalous transport in Wendelstein 7-X, given within the up to now investigated operational parameters.

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Due to non axis symmetric 3D magnetic fields, impurity transport in the hot plasma core in stellarators is fundamentally different to tokamaks
In view of reactor-like operation, understanding the impurity transport is a prerequisite for steady-state operation
Spatio-temporal impurity emissivities were measured by the x-ray imaging spectrometers XICS [3] and HR-XIS [4], optimized to detect

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Due to non axis symmetric 3D magnetic fields, impurity transport in the hot plasma core in stellarators is fundamentally different to tokamaks. Burhenn1, R.C. Wolf 1 and the W7-X team 1 Max-Planck-Institut für Plasmaphysik, 17491 Greifswald, Germany 2 Princeton Plasma Physics Laboratory, Princeton, NJ, USA 3 Auburn University, Auburn, Alabama, USA

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