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 July 19-20, 2011 Hefei, ASIPP

Outline 1.Background and objectives 2.Plasma exposure of W/Cu FGM and W-La 2 O 3 in HT-7 tokamak - W/Cu FGM - W-1%La 2 O 3 3.Summary & future plane 2

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 Solutions Grain boundary weakening ◆ Engineer requirement ◆ Thermal mismatch Macro-brush Mono-block W/Cu FGM 3

Smooth the composition transition One step processing (fabrication and jointing) Materials processing W or W-La 2 O 3 layer W-20%Cu layer W-40%Cu layer W-60%Cu layer W-80%Cu layer pure Cu Graded distribution 4

Investigation on the performance of W/Cu FGMs in edge plasma exposure - failure behavior - failure mechanism Evaluation of the W-1% wt. La 2 O 3 under plasma exposure - Influence of La 2 O 3 on thermal resistance Objectives 5

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

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

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

9 (a) 3D and (b) Surface distribution of von mises stress under typical transient flux 0.2 MJ/m 2 (2ms) Under typical heat flux ~0.2 MJ/m 2 (~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 and extend by quasi-stationary heat load

(a)W-1%La 2 O 3 composite; (b) pure W La 2 O 3 caused the severe damages (severe large macro cracks) ； Degradation of thermal conductivity may be the reason. W-1%La 2 O 3 under plasma exposure Macro cracks Exfoliation 10

 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.  La 2 O 3 dispersoid caused the severe damages. - W-1%La 2 O 3 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%La 2 O 3 for fusion application. Summary 11

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

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