Impurity transport studies at Wendelstein 7-X by means of x-ray imaging spectrometer measurements

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

*Corresponding author for this work

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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 τ I 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 τ I has been observed when changing the power deposition from on- to off-axis heating with energy confinement times τ E being mainly unaffected. In addition, the scaling of impurity transport properties with respect to a variation of heating power P ECRH and electron density n e has been investigated by keeping the heating power deposition on-axis. The observed τ I scaling compares well to known τ I scaling laws observed in other machines. A comparison of τ I and τ E yields an averaged ratio of τ E /τ I  = 1.3 and transport times in the range of τ I  = 40–130 ms and τ E  = 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.
Original languageEnglish
Article number014030
JournalPlasma Physics and Controlled Fusion
Issue number1
Number of pages8
Publication statusPublished - 2019

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  • Impurity transport
  • Impurity confinement
  • Energy confinement
  • Imaging spectrometer
  • Laser blow-off
  • Plasma physics

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