This work has been carried out within the framework of the

EUROfusion Consortium and has received funding from

the European Union’s Horizon 2020 research and

innovation programme under grant agreement number

633053. The views and opinions expressed herein do not

necessarily reflect those of the European Commission.

Deposition asymmetry on rotating collectors probes in the JET

divertor with carbon wall and metallic ITER-like wall J. Beala,b, A. Widdowsonb, K. Heinolab,c, A. Baron-Wiechecb, K. J. Gibsona, J. P. Coadd, E. Alvese,

B. Lipschultza, A. Kirschnerf, H. G. Esserf, G. F. Matthewsb, S. Brezinsekf and JET Contributors*

EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK.

• Differing chemical properties and temperature dependences of Be and C also contribute to

reversal of deposition asymmetry.

REFERENCES

eInstituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Avenue

Rovisco Pais, 1049-001, Lisboa, Portugal. fForschungszentrum Jülich, Institut für Energie- und Klimaforschung Plasmaphysik, 52425 Jülich,

Germany.

aYork Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, UK. bCulham Centre for Fusion Energy, Abingdon, OX14 3DB, UK. cUniversity of Helsinki, PO Box 43, 00014 University of Helsinki, Finland. dVTT Technical Research Centre of Finland, PO Box 1000, 02044 VTT, Espoo, Finland.

JET divertor:

poloidal cross-

section

Inner

collector

Outer

collector,

QMB

5. CONCLUSIONS

•Erosion/deposition is important for vessel lifetimes and plasma performance.

•For JET-ILW, total Be deposition rate on divertor corner collectors is reduced by order of

magnitude relative to C in JET-C.

•Far larger reduction in inner divertor than outer, partly due to strike point distributions.

1. INTRODUCTION

•Erosion and deposition can limit vessel lifetimes and degrade

plasma performance.

•In 2010, the carbon wall of JET-C was replaced with beryllium

main chamber and tungsten divertor (JET-ILW) [1].

•This has caused an almost complete exchange of C with Be as

the dominant plasma impurity [2].

•The asymmetry in divertor deposition for JET-C and JET-ILW is

investigated using rotating collectors and QMBs.

2. DIAGNOSTICS

•Rotating collectors act as substrates for deposition.

•Rotating the discs allows variations in deposition to be measured.

•Between campaigns, the deposition is quantified using nuclear

reaction analysis.

•Quartz microbalances (QMBs)

measure mass erosion/deposition

via the frequency change of

vibrating quartz crystals.

1

3

4

5

6

7

8

• Reduced main chamber impurity source →

less Be available for sputtering from divertor

tiles [4].

• Lower chemical erosion of Be: no thermally

activated, some chemically assisted

physical sputtering [5].

• Fewer tile 4 strike points → less sputtering

in locations with line of sight to collector.

• For JET-C, inner usually net deposition,

outer net erosion – due to higher heat fluxes

in outer [6].

• Might expect strike points on tile 6 (close to

collector) to cause most of deposition.

• But tile 6 time decreased for JET-ILW.

• Why then is there only a small decrease in

deposition (relative to inner)?

2.1×

decrease in

tile 4 strike

points

3.3×

decrease in

tile 6 strike

points

JET-C

JET-ILW

JET-C

JET-ILW

Deposition

Erosion JET-C:

Net deposition for

tile 5 strike points,

net erosion for tile

6 strike points.

JET-ILW:

Net deposition for

tile 5 and tile 6

strike points.

• Plot cumulative QMB frequency evolution, split into times outer strike point

is on tile 5 or tile 6:

• Tile 6 strike points → high collector temperatures → limits deposition and promotes

chemical/thermal re-erosion of carbon deposits.

• Tile 5 strike points → cooler collector → more deposition/less re-erosion.

• Beryllium is less susceptible to chemical sputtering/erosion → deposition for tile 5

and 6 strike points → relatively high overall deposition.

[3]

Deposition Deposition

C

C

Be Be

[1]

3. INNER DIVERTOR 4. OUTER DIVERTOR

*See the appendix of F Romanelli et al., Proc. 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia.

~30x

reduction in

deposition

rate

~4x

reduction in

deposition

rate

[1] G. F. Matthews et al., Phys. Scr. T145 (2011) 014001.

[2] S. Brezinsek et al., J. Nucl. Mater. 438 (2013) S303.

[3] J. P. Coad et al., Phys. Scr. T138 (2009) 014023.

[4] S. Brezinsek, 21st PSI Conference, Kanazawa, Japan

(2014). Accepted J. Nucl. Mater.

[5] S. Brezinsek et al., Nucl. Fusion 54 (2014) 103001.

[6] J. P. Coad et al., Phys. Scr. 1999 (1999) 7.