Study On The Surface Damages Of Tungsten Under High Plasma Heat Fluxes In Tokamak

Tungsten(W)has been selected as the plasma-facing material in the divertor region of the future fusion devices such as ITER and CFETR,because of its low retention with tritium and good radiation resistance.To understand the damage behaviors of tungsten surface under high heat fluxes and search the solution to thermal-induced surface damages,which are important for achieving better plasma quality and improving the lifetime of plasma-facing materials and components in the future.In this thesis,the study on the damage behaviors of tungsten surface under high plasma heat flux in tokamak has been performed,which based on the damage behaviors in high heat flux test facilities and tokamak devices,combined with finite element method(FEM)and supported by heat flux diagnosis such as probe and infrared camera in tokamak.On the respect of heat flux diagnoses on tungsten surface in tokamak,an infrared camera with high temporal-spatial resolution and large temperature range(～3 mm/pixel,1000-3000 Hz,0-2500℃)has been accomplished,which located at H sector in EAST.The long pulse steady-state and periodic transient heat flux on tungsten surface are estimated by infrared temperature data with FEM.The analysis processing has considered the nonuniform toroidal temperature distribution on tungsten surface,the nonuniform toroidal heat flux distribution on divertor target surface as well as the geometry and engineering parameters of monoblock.On the respect of tungsten thermal-structure coupling analysis under high heat flux,the temperature,stress and strain evolution on tungsten surface under transient and the superposition of steady-state and transient heat fluxes during heating and cooling stage were achieved by FEM.The threshold and judgement process of thermal-induced damages are introduced based on the theories of heat transfer and mechanics of materials.It is found that cracking threshold of tungsten surface under transient heat flux is FHF～13.8 MW m-2 s0.5,Tmax～838℃.In addition,the analysis results theoretically identified that the pre-heating by steady-state heat load indeed influenced the transient heat flux induced damages.Typically,no any crack would be generated if the base temperature exceeded the critical temperature(～400℃),and the damage threshold decrease with the increasing of base temperature when base temperature below the critical temperature,which were in good agreement with the experimental results.On the respect of the analysis on in situ tungsten melting phenomenon in EAST,the in situ melting phenomenon of actively cooled ITER-like tungsten divertor components in EAST was observed and reported.It was identified that such tungsten melting was caused by the leading edge induced thermal effect,because all tungsten melting occurred only at the edges of the cassette modules where larger misalignment up to millimeter scale was formed.The melted layer,which was mainly driven by the electromagnetic force,was moved either up or down under different conditions.Such tungsten melting ejected a large number of droplets into the core plasma,and resulted in a sharp increase of tungsten impurity and power radiation and could eventually lead to disruptions,which can be observed by CCD camera.Under current operation conditions,EAST can tolerate such melting to some extent.Several solutions to help avoid this kind of melting have been proposed according to the thermal analysis for future EAST operations.The optimization of the shaping structure(r mm × t mm)is 3.8 mm × 23 mm for current Δr ≤ 3 mm,2.8 mm × 22 mm for Δr ≤ 2 mm)and 1.8 mm × 24 mm for Δr ≤ 1 mm in the future,which can increase the melting threshold heat fluxes and are suggested to apply on inter-cassette leading edge.For CFETR case,the optimization of the fish-scale shaping structure is h=0.55 mm for Δr≤0.5 mm,which can reduce the maximum temperature of monoblock by 66%.On the respect of cracking analysis for W/Cu monoblock,the formation of the central vertical crack in high heat flux test and leading edge induced parallel crack are explained by FEM.The results of fatigue lifetime for EAST monoblock under different heat flux are 12 cycles under 20 MW m-2 and 104 cycles under 10 MW m-2 with high heat flux test case.Besides,for leading edge heat flux distribution,the results of different heat fluxes and misalignment show that the fatigue lifetime is below 103 cycles when the maximum temperature of monoblock exceeds 3000℃.