1. Surface Analysis of Graphite Limiter and W-coating Testing on HT-7
H.Lin, X. Gong
Institute of Plasma Physics,Chinese Academy of Science
Hefei, , China
December 31, 2004

2. Outline ☆Introduction ☆Surface Analysis
☆X-ray Photoelectron Spectroscopy (XPS)
☆X-ray Fluorescence Spectroscopy (XRF)
☆Conclusion
☆Future work

3. Introduction
☆ The new up-down symmetric toroidal belt limiters were mounted on HT-7 in the spring of 2004
☆ Bottom and top limiters are 29 segments
length:from 230mm to1.1m,width:150 mm
☆ Doped graphite tiles: 223, total area: about 1.7 m2
☆ Doped graphite :1%B, 2.5%Si, 7.5%Ti ,
Thickness of SiC coating:100um

4. Introduction N S East of the top limiter (T-E);
☆ Five graphite tiles from different areas of the toroidal limiters:
East of the top limiter (T-E);
Northwest of the top limiter (T-NW); Northwest of the bottom limiter (B-NW);
Southwest of the top limiter (T-SW); Southwest of the bottom limiter (B-SW).

5. Doped graphite tiles were analyzed by means of XPS
Zoning every tile into four areas along vertical direction. The tile picture shows the corresponding numbers.
The erosion distribution of SiC coating
Survey(XPS)
Obvious damages were found on the surface of graphite tiles

6. Comparing the carbon AT% on the surface of the five tiles
The higher carbon AT% was attributed to
SiC coating eroded and carbon re-deposition

7. SiC Coating have good oxygen gettering ability
The oxygen AT% is higher( mainly >20% ) on the tile’s surface,
Which own to the contribution of silicon. Although the residual silicon on the surface are very small.

9. Abundant impurities (includes metal impurities) were induced by sputtering

10. The metal impurities are found on the graphite tiles
The AT% of metal impurities : 5%~10%
The more metal deposits were found
on the tiles in the neighborhood of antenna

11. More metal impurities are found on the top limiters
The metal deposits maybe come from the antennae and the stainless steel liner.

12. The residual SiC coating measured by XRF
Thickness of layers (μm )
XRF analysis also indicated:
The mostly SiC coating were eroded during the last experimental campaign. The residual coating only few um.

13. The components on the tiles before and after
plasma discharges during the last experimental campaign
The surface of all tiles was covered by higher concentration metal impurities such as Ti, Ni、Fe、Cr etc. On the other hand, the re-deposition of impurities will compensate partly the sputtering erosion, and even offset the sputtering erosion completely in some cases.

14. Conclusion
☆ The mostly SiC coating were eroded during the last experimental campaign. The residual coating only few um. Erosion is attributed to physical sputtering by thermal neutrals and ions, chemical sputtering, evaporation by localized heat deposition due to runaway electrons and disruptions, and so forth.
☆ Silicon is a good oxygen getter, which can enhance the gettering efficiency of oxygen impurity.
☆ The re-deposition behaviors were found on the surface, which also attributed to chemical sputtering. The metal impurities maybe come from the antennae and the stainless steel liner.
☆ Higher carbon erosion yield and co-deposition become unacceptable for PFCs in the future fusion device.

15. Requirements for the next fusion device
ITER
Beryllium
Be, C and W be considered as candidate PFM in ITER

16. The main characters of Be, C and W
Advantages
Drawbacks Be
Good O2 gettering capability
~ tokamak practice (mainly JET);
low Z;
repairability by plasma-spraying;
established joining technology;
low tritium inventory.
Inadequate for the divertor (Tm<1300˚C).
Be dust on ‘hot’ surfaces -> H production due to reactivity with steam during accidents.
Compatibility with Type I ELM loads disruptions or ‘mitigated’ disruptions. C
Good power handling, thermal shock and thermal fatigue resistance;
does not melt (but sublimes);
low radiative power losses with influx to plasma due to low Z;
well established joining technology;
broad tokamak operation experience.
Chemical erosion --> erosion lifetime (~few thousand pulses).
Tritium codeposition --> needs adequate recovery methods. W
low physical sputtering yield and high sputtering threshold energy;
no chemical sputtering in H-plasma;
low tritium inventory;
well established joining technology.
Plasma compatibility-->favourable operation experience from ASDEX-Upgrade, dust radiological hazard.
Near strike points, its operational lifetime, has still uncertainties due to melt layer loss during disruptions and ‘large’ ELMs.

17. W-coating testing on HT-7
Future work
W-coating testing on HT-7
☆ Advantage of HT-7 Superconducting Tokamak
Quasi steady-state plasma operation
☆ Samples
B4C coated on copper , SiC coated on copper and W/Cu materials
☆ Surface Analysis Methods
SEM , XPS , XRF and Ion Beam Analysis

18. The global distribution of tungsten coating was obtained by XRF
Global analysis
The global distribution of tungsten coating was obtained by XRF

19. Pointing analysis
Local distribution was got by Pointing analysis.
The intensity is in direct proportion to the element mass fraction .
Comparing the distribution and thickness of coatings before and after plasma discharges, we will get the net erosion information of all samples. 1 2 5 3 4

20. Original layer thickness
PW2
BC3
PW1
3um tungsten coating
1um tungsten coating
35um B4C coating

21. Original layer component
PW2
BC3
PW1
3um tungsten coating
1um tungsten coating
35um B4C coating

22. Future work ☆ Investigations will be concentrated on:
· Performance of coatings before and after plasma discharges
· Interconnection of W content and discharge conditions / plasma parameters
· Net erosion yield and re-deposition behaviours of tungsten
· Recycling and retention in tungsten-coating materials
provide information for next step fusion device
(EAST and ITER)