Speaker
Lukas Rudischhauser
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1028.pdf
Characterisation of power flux reduction in the Wendelstein 7-X divertor
plasma with Langmuir probes
L. Rudischhauser1, K.C. Hammond1, M. Endler1, H. Niemann1, M. Krychowiak1, T. Barbui2,
B.D. Blackwell3, F. Effenberg2, Y. Feng1, R. König1, M. Jakubowski1 and the W7-X Team1
1
Max-Planck-Institut für Plasmaphysik, Greifswald, Germany
2
University of Wisconsin-Madison, Madison, USA
3
Australian National University, Canberra, Australia
During the first divertor operation phase (OP1.2a) of the stellarator Wendelstein 7-X strong
reductions in heat load by a factor of five or more on the graphite divertor targets were
observed for periods of time within numerous discharges. During these we recorded
substantial drops in temperature by an order of magnitude to below 6 eV and increases in
density with the 40 Langmuir probes embedded in the divertor. The probe results support the
ongoing discussion on this power detachment [1] with measurements over a range of poloidal
distances, sampling the magnetic island divertor field inside the island, in the strike line and
in the private flux region.
Design, capabilities and limitations of the Langmuir diagnostic as well as the data analysis
model are briefly introduced. The Langmuir probe data are compared with observations of
other relevant diagnostics such as the infrared and spectroscopic observation systems, and
the thermal helium beam diagnostic [2, 3, 4]. In addition to discussing the indications for
detachment in detail we characterise the general behaviour of the divertor plasma under
different global plasma conditions and present evidence of up-down asymmetry and active
scrape-off layer cooling by impurity seeding and edge fuelling.
[1] D. Zhang et al., First observation of a stable highly-radiative divertor regime at stellerator
W7-X, EPS 2018, to be published
[2] M.W. Jakubowski et al., Infrared Imaging Systems for wall protection in the W7-X
stellarator, HTPD 2018, to be published
[3] M. Krychowiak et al., Overview of diagnostic performance and results for the first
operation Phase in Wendelstein 7-X, RSI 87, 2016
[4] T. Barbui et al., Feasibility of line-ratio spectroscopy on helium and neon as edge
diagnostic tool for Wendelstein 7-X, RSI 87, 2016
