Zhu Dahuan Liu yang Chen Junling

1. July 19-20, 2011 Hefei, ASIPP Performance of W/Cu FGM in edge plasma of HT-7 tokamak Zhu Dahuan Liu yang Chen Junling Institute of plasma physics, Chinese Academic of Science, China

2. Outline • Background and objectives • Plasma exposure of W/Cu FGM and W-La2O3 in HT-7 tokamak - W/Cu FGM - W-1%La2O3 • Summary & future plane

3. W as PFM Advantages • High melting point • Low sputtering yield • Promising thermal conductivity • Low tritium inventory Fusion applications Disadvantages • Low temperature embrittlement • Recrystallization embrittlement • Radiation embrittlement • Mismatch with Copper based heat sink materials Grain boundary weakening Solutions ◆ Engineer requirement ◆ Thermal mismatch W/Cu FGM Macro-brush Mono-block

4. W/Cu FGM W/Cu FGM Graded distribution • Smooth the composition transition • One step processing (fabrication and jointing) W or W-La2O3 layer Materials processing W-20%Cu layer W-40%Cu layer W-60%Cu layer W-80%Cu layer pure Cu

5. Objectives • Investigation on the performance of W/Cu FGMs in edge plasma exposure - failure behavior - failure mechanism • Evaluation of the W-1%wt.La2O3 under plasma exposure - Influence of La2O3 on thermal resistance

6. Exposure of W/Cu FGM in HT-7 tokamak Schematic of device W/Cu FGM block （10 X 10 X 12mm） Sample support ◆ Main parameters: aL ~280mm, Ip~150KA, PLHCD~350kW, Total shots ~210 cycles, (#111377~#111587 ) Average duration time ~1s.

7. Failure behavior of W/Cu FGM under plasma exposure • Surface components analysis(XRF) • Beside the initial compositions of W (99.92%) and Fe (0.07%), only little Ti, Cr and Mn (<0.1%) • Surface morphology Macro crack Micro crack • Cross-section morphology Center cracks and exfoliation Macro crack Termination propagation along interface Edge exfoliation Exfoliation (b) ※ No failure was found at interface, but surface was damaged. ※ cracks originated from the edge and propagated to the inside and interface.

8. Failure mechanism • Under quasi-stationary heat load ~2MW/m2(~1s) maximum surface temperature is 107oC → embrittlement behavior maximum stress at the interface is 57 Mpa • But the damages occur at surface • The damage cause by another reason. (a) Temperature and (b) Stress distribution under 2MW/m2 (1s)

9. (a) 3D and (b) Surface distribution of von mises stress under typical transient flux 0.2 MJ/m2 (2ms) • Under typical heat flux ~0.2 MJ/m2(~2ms) maximum stress at the surface is 993 Mpa • Stress singularity is basically accordance with the loop-like crack • The damage at surface is likely to be created by transient flux andextendby quasi-stationary heat load

10. W-1%La2O3under plasma exposure Macro cracks • La2O3 caused the severe damages (severe large macro cracks)； • Degradation of thermal conductivity may be the reason. Exfoliation (a)W-1%La2O3 composite; (b) pure W

11. Summary • W/Cu FGMs show good performance of interface to withstand such thermal fatigue in general; • W surface was destroyed by the typical high transient flux； • - Cracks were initially created at surface edge, then extended deeply into the interface and would cause the failure of interface. • - Formation of columnar crystal structure at surface may be advantageous for thermal resistance . • La2O3 dispersoid caused the severe damages. • - W-1%La2O3 may be not fit as the PFM in currently tokamak device. • - Optimization of the fabrication process and enhancing the thermal conductivity are critical issues for W- 1%La2O3 for fusion application.

12. Future plan • Use a electron generator to simulate the 2MW/m2(~1s) on W/Cu FGMs surface • Built some diagnostic technologies • -Infrared camera → Surface temperature • -Thermal couple measurement → Body temperature

13. Thanks for your attention!