Speaker
Kenneth Hammond
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1011.pdf
Development of a pop-up Langmuir probe array for the W7-X
high-heat-flux divertor
K. C. Hammond1 , J. Biermann1 , M. Endler1 , J. Fellinger1 , S. Freundt1 , F. Gottong1 , S. Klose1 ,
M. Krause1 , J. Kügler1 , L. Rudischhauser1 , J. Wendorf1 , and the W7-X team
1 Max Planck Institute for Plasma Physics, Greifswald, Germany
Divertor target-mounted Langmuir probes are foreseen as a crucial diagnostic during Oper-
ational Phase 2 (OP 2) of the W7-X experiment to aid our understanding of detachment, edge
fueling, strike patterns, and other scrape-off layer phenomena. The high-heat-flux divertor [1],
to be used during this period, will be water-cooled to withstand the sustained power flux from
continuous discharges lasting up to 30 minutes. Since it is not forseen to actively cool the probes,
however, they must periodically withdraw from the plasma to avoid damage from overheating.
Finite-element modeling indicates that, in the worst case, the graphite-tipped probes must stay
retracted for 10 s after 200 ms of plasma exposure.
The system currently under design will consist of poloidal arrays of probes in two of the ten
island divertors. The arrays will span up to 400 mm of their respective horizontal target elements
with 25 to 50 mm of separation between each probe. Both the probes and the mechanisms
driving the reciprocation will be integrated into the targets. The driving mechanism consists
of a rigid, current-carrying loop which moves in response to the Lorentz force from the W7-
X magnetic field. Although most of the instrumentation will be mounted on the back sides of
divertor target modules and therefore not accessible during the operational phase, the interface
between the probes and the drive units will permit the probes to be periodically removed and
replaced from the plasma-facing side. The in-vessel cabling system is designed for compatibility
with high-speed measurements using the “Mirror Langmuir probe” technique [2].
Here we present the current status of the project. Topics to be addressed include the techni-
cal challenges which had to be overcome to realize the system, solutions developed for those
challenges, and the results of simulations and prototype testing to predict and qualify the per-
formance of the probes.
References
[1] A. Peacock et al., 25th IEEE Symposium on Fusion Engineering (SOFE), San Francisco, CA, USA, 2013,
pp. 1-8
[2] B. LaBombard and L. Lyons, Rev. Sci. Instrum. 78, 7 (2007)
