First divertor physics studies in Wendelstein 7-X
Thomas Sunn Pedersen1,2, on behalf of the W7-X team, in particular:

R. König1, M. Jakubowski1, Y. Feng1, A. Ali1, G. Anda6, J. Baldzuhn1, T. Barbui5, C. Biedermann1, B. Blackwell3, H.-S. Bosch1,S. Bozhenkov1, R. Brakel1, S. Brezinsek4, J. Cai4, J.W. Coenen4, J. Cosfeld4, A. Dinklage1, T. Dittmar4, P. Drewelow1, P.
Drews4, D. Dunai6, F. Effenberg5, M. Endler1, J. Fellinger1, O. Ford1, H. Frerichs5, G. Fuchert1, J. Geiger1, Y. Gao4, A.
Goriaev13,14, M. Henkel4 , K. Hammond1, J. Harris9, D. Hathiramani1, M. Henkel4, H. Hölbe1, Y. Kazakov13, C. Killer1, A.
Kirschner4, A. Knieps4, M. Kobayashi11; P. Kornejew1, M. Krychowiak1, G. Kocsis6, S. Lazerzon7, C. Li4, Y. Li4, Y. Liang4, S.
Liu4, J. Lore9, S. Masuzaki11, V. Moncada15 , O. Neubauer4, T. T. Ngo16, H. Niemann1, J. Oelmann4, M. Otte1, V. Perseo1, F.
Pisano8, A. Puig-Sitjes1, M. Rack4, M. Rasinski4, F. Reimold1, J. Romazanov4, L. Rudischhauser1, J.C. Schmitt7, G. Schlisio1,O. Schmitz5, B. Schweer4, S. Sereda4, T. Szepesi6,Y. Suzuki11, E. Wang4, Y. Wei4, U. Wenzel1, S. Wiesen4, V. Winters5, T.
Wauters13, G. A. Wurden10, D. Zhang1, S. Zoletnik6
1Max-Planck-Institut für Plasmaphysik 2University of Greifswald, 3Australian National University, Canberra, Australia 4Forschungszentrum Jülich5

University of Wisconsin, Madison, 6Wigner Institute, 7Princeton Plasma Physics Laboratory, 8University of Cagliari, 9Oak Ridge NationalLaboratory, 10Los Alamos National Laboratory,11National Institute for Fusion Science , 12Auburn University 13 Laboratory for Plasma Physics,ERM/KMS Brussels14 Ghent University
15100 Impasse des Houllières, ZA Le Pontet, Meyreuil,France. 16 CEA, IRFM, Saint Paul-lez-Durance, FranceThis work was partially funded by the U.S. Department of Energy under grant DE-SC0014210
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme
2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
02.11.18

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Overview•

The path forward for W7-X in terms of upgrades to the PFCs•

The W7-X island divertor concept•

Heat load patterns during attached operation:•

Spatial patterns•

Scaling of heat fluxes•

Detachment at lower power, before boronization•

Effects of boronization•

Detachment at higher power, after boronization• Summary

02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

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Successive upgrades to plasma-facing components

Limiter

OP 1.1: 2015 - 2016
Graphite limiter configuration
P < 4.3 MW achieved∫ "#$£ 4 MJ achieved

02.11.18

OP 1.2: 2017 – 2018
Uncooled graphite divertor
P £ 7 MW achieved∫ "#$£ 200 MJ achieved

OP 2: 2021 …
Actively cooled CFC divertor
Pcw ~ 10 MW (30 mins)
Divertor G£ 10 MW/m2
ò P dt £ 18000 MJ

Divertor

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

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Introducing the W7-X island divertor

02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

4Introducing the W7-X island divertor

02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

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Divertor heat load patterns for attached plasmas

High-iota configuration (iota=5/4)

Standard configuration (iota=1)

Heat load patterns generally asexpected,
See also M Jakubowski et al.,this conference
For strike line motion
and general issues withthose: see J. D. Lore etal., this conference

02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

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A look onto a divertor unit halfway through OP1.2
Baff le sGraphitetargets divertor

Staiste nlessel panelsPum
p in gGraph
ite divertor gap
targets
Graphiteheatshields••

02.11.18

View into torus after OP1.2a: Visible signs of plasma-divertor contact
The OP1.2 test divertors are uncooled, robust against overload ⇒ Ideal testbed for later operation
with the water-cooled divertor which has a 10 MW/m2 heat flux design specification
T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

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Attached divertor operation: heat loads are a function of density

Higher ne

02.11.18• Standard island chain (5/5): scaling to P=10 MW looks good (Lc~240 m)[1]• High iota island chain (5/4): Scaling presents a challenge at low density(Lc~130 m)[1]• Higher density features more desirable scaling• Primarily due to increased radiation but also indicative of large wetted area (up to 1.5 m 2)
[1] P. Sinha et al., Nucl. Fusion 58, 016027 (2017)
T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India8

Full heat-flux detachment at high ne, low P~3 MWComplete detachment
for t> 2 s on all
10 divertors (one shown)

Heat flux essentiallydisappears from target
for t>2 s, persisting until
end of plasma heating at
t=4 s
tE~100 ms rather
constant for over 1 s

Heat flux
derived from IR
camera data

Increased radiation near the LCFS
02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

9But what about the particle exhaust (P~3 MW)?

Factor 7.5
5x10-5 mbar

• For low-power detachment, the subdivertor neutral pressure reaches about 5x10-5 mbar
• In OP1.2: 2x104 l/s pumping rate (divertor turbopumps)• The exhaust rate is therefore about 1 mbar-liter/s~2x1019 particles/s• Compression ratio: About 7.5• Expected/hoped: compression ratios of more than 30, subdivertor pressure 5-10x10-4 mbar
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T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

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After boronization: Reduced edge radiation, higher density

Oxygenx10Carbonx7•

Edge impurity concentrations (relative to hydrogendensity):• Oxygen reduced by a factor of 10 (or a lotmore, in some cases)• Carbon reduced by about a factor of 702.11.18•••

Achieved hydrogen densities increased by factor >3:• avoidance of MARFE-like phenomena
Achieved confinement times increased by ~60%
• consistent with the increased density (!",$%%&' ∝ )&.+'), so:
Boronization was not the direct cause of increased confinement•
Plasma core generally clean before and after boronization

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India

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Post-boronization detachment: Efficient exhaust

High-density, higher-power (5-6 MW) detachment achieved after boronization
Strong heat flux reduction
Triggered in narrow density range in both upper and lower divertors
Strong neutral compression now (x30):• pn =5 to 8x10-4 mbar now (pn =0.5x10-4 mbar for low-power detachment)• All divertors detach but a somewhat stronger neutral compression (30%) is seen in the upper divertors.
• The particle removal rate is ~13 mbar-liter/s~2.6x1020 particles/s – projects well to OP2 with divertor
cryo-pumps and a 4x higher pumping rate:• Should result in 1021 particles/s removed – about the amount expected to be needed••••

02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India12

Why the difference in neutral compression?
Low-power detachment:
Factor 7 neutral compr.

High-power detachment:
Factor 30 neutral compr.

•Hypothesis (illustrated with cartoons):• At low power, the plasma “runs out of energy” near the edge and radiates its energy away before it arrives at the islandand the divertor• Oxygen and carbon act as “radiating mantle”• Therefore, the plasma does not “plug” the hole – neutrals can escape divertor region• See also Florian Effenberg’s talk directly following this one – using neon to trigger the same physics• At high power, after boronization, the plasma radiation cooling and condensation occurs further out, in the divertor• Therefore the plasma effectively “plugs the hole” – neutrals cannot escape divertor region (as designed…!)•The surprising thing is that also the low-power detachment can be stable without feedback stabilization.
02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India13

W7-X divertor works: Efficient particle exhaust, stable detachment
32 second discharge from last week, detached for the last 28 seconds
T=1 sec
Attached

T=10 sec
Detached

PtotW
9dianeTe,TS

T=20 sec
Detached

T=30 sec
Detached

Pdiv

Z• Pulse terminates aspreprogrammed –could have been
extended• Energy confinement
time ~120 msconstant• Efficient exhaust• Divertor neutralpressure ~ 67x10-4 mbar
• Low impurity contenteffT. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India
02.11.18

14W7-X in general has very good confinement:
28-second detached discharge had H-L mode confinement• W7-X discharges lie with the same rangethat regular tokamak H-mode discharges do.
• The 28 sec detached discharge has
confinement between H- and L-mode• The triple product record shot (labeled“transient” here) lies above the H-mode
scaling, and had reduced turbulent
fluctuations (re. Th. Klinger overview talk)

02.11.18

T. Sunn Pedersen et al., IAEA-FEC Meeting, Ahmedabad, India15Summary• First results with the W7-X island divertor were very successful• The divertor heat load patterns were generally as expected• In attached operation, we observe large wetted areas and acceptable heatfluxes:• Projects well to future operation with water-cooled divertor• An indication of the benefits of long connection lengths• Stable detachment was achieved (two varieties)• Low-power, volumetric, limited particle exhaust• High-power, with high neutral compression, efficient particle exhaust•

All divertors detached stably for 28 seconds @ 5MW – could have lasted much longer• Boronization was key; it enabled:• Strong reduction of oxygen and carbon in SOL• Stable high hydrogen density operation• Access to detachment with efficient exhaust
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