Effects of transient heating events on W PFCs in a steady-state divertor-plasma environment

1. Effects of transient heating events on W PFCs in a steady-state divertor-plasma environment Karl R. Umstadter, R. Doerner, G.R.Tynan PFC Annual Meeting UCLA August 5, 2010

2. Overview • Use of Laser Heat Pulse to Mimic ELMs • Experiment & Apparatus • Enhanced Mass Loss & Bulk Temperature • Comparison with DIII-D Experiments • Cause of Synergistic Effect • Damage Threshold & Fluence Scaling • Conclusion

3. YAG Pulse Mimics ELM Surface Heating Carslaw & Jaeger, “Conduction of Heat in Solids”, Oxford University Press, 1959 Longer duration results in deeper heat transport May effect retention

4. PISCES A & Beam Delivery Q-Switched Nd:YAG Laser 1064nm <850 mJ ~5 nsec <1 mrad Laser Path PISCES A G~1017 – 1019 D+/cm2 s-1 n ~ 1012 D+/cm3 Te ~ 5-10eV Vbias ~ up to 250V Rp ~ 4cm

5. Overview • Use of Laser Heat Pulse to Mimic ELMs • Experiment & Apparatus • Enhanced Mass Loss & Bulk Temperature • Cause of Synergistic Effect • Comparison with DIII-D Experiments • Damage Threshold & Fluence Scaling • Conclusion

6. Erosion of W PFC under Simulated ELM Transients at High Repetition Rate (low fluence between ELMs) F~ 1026 D+/m2 Tsurf ~ 50ºC 3000 Transients with Absorbed Energy Impact ~45 MJ/m2 s1/2

7. Bursted Blisters

8. Hypothetical Process Laser Pulse D+ W+ W W (C) (B) (A) D ion W atom W cluster W ion

9. Near surface location of D in samples exposed cold

10. Near surface D decreases with increasing Temp V.Kh. Alimov et al., JNM 375 (2008) 192–201 QSW Nd:YAG Heated Region

11. No Excess Mass Loss at Elevated Temperature

12. Excess Mass Loss Not Limited to PISCES ASIPP Tungsten ATJ Graphite C B1 D G Osaka A E F • DiMES samples of graphite and tungsten were “loaded” by bombardment with deuterium ions (Eion~125eV) in the PISCES-A • Exposed on DiMES in DIII-D • He Exposures (VPS-W) • Excess mass loss is 2-5x • D exposures (Plansee IG W)Excess mass loss is 3.5-8x • C:ATJ samples still under analysis • DIII-D with D.Rudakov & C.Wong • Presented at PSI-19, San Diego

13. Overview • Use of Laser Heat Pulse to Mimic ELMs • Experiment & Apparatus • Enhanced Mass Loss & Bulk Temperature • Comparison with DIII-D Experiments • Cause of Synergistic Effect • Damage Threshold & Fluence Scaling • Conclusion

14. Threshold for Damage Decreases Tbulk~325K

15. Effects of D Loading on Damage Fluence to surface between heating transients F = 5x1022/m2 F = 5x1023/m2 F = 2x1024/m2 SAMPLE Vbias=125V G=2x1022/m2-sec Te=11eV ne=2x1024/m3

16. Damage ~ Fluence1/3

17. Observations When fluence to PISCES-A targets between transients is increased synergistic effect between thermal transients & plasma exposure leads to enhanced material removal • Change in near surface material properties - TSurf a function of the energy density of deposition and thermal conduction to the bulk during and following the deposition • Operation above DBTT may alleviate excess mass loss mechanism and should prevent fracturing • Damage depends upon D fluence between transients and ion energy - more frequent ELMs may reduce damage by limiting fluence

18. Conclusion • Experiments with D-loaded samples in DIII-D on DiMES have shown that the enhanced erosion effect is not phenomena only witnessed in the laboratory PISCES device. • Important to mimic ELM heat pulses in divertor-like plasma environment with ITER-relevant fluences • Effects will be most apparent in high-performance steady-state tokamaks (EAST & KSTAR) as these experiments explore inter-ELM fluxes greater than operating tokamaks but in line with ITER

19. Extra Slides

20. Arcing during transient heating experiments Early Focus On Avoiding These Phenomena

21. Long Pulse Experiments • 25J – 5msec - 1200 Laser Pulses @1/3 Hz • 5 kW – 50MJ/m2s1/2 • 75V Bias – Total Fluence ~1026 D+/m2 Room Temperature >500ºC