Simulation Studies Of Material Erosion And Deposition In The Magnetic Shadowed Area Of The First-wall In Tokamak

In tokamaks,there are magnetic shadowed areas on some plasma-facing components(PFCs)that are inaccessible to charged particles due to their special structures.Neutrals generated by charge-exchange reactions are not constrained by magnetic field lines,which is the main reason of erosion for PFCs in magnetic shadowed areas.The energy,fluxes,and angular distributions of neutrals on materials in the magnetic shadowed area of the first-wall are still unclear,so it is hard to make accurate predictions for material lifetime and fuel retention in future fusion reactors such as ITER.To understand the effect of interaction between neutrals and materials,the 3D-GAPS code was mainly used to simulate material erosion and re-deposition in the magnetic shadowed area of the first-wall in EAST and ITER.For EAST,the erosion experiments of the first-wall material by neutral particles located at the material and plasma evaluation system(MAPES)was simulated,with the neutral energy spectrums measured by the low energy neutral particle analyzer(LENPA).The modelled aluminum(Al)erosion rates with isotropic angular distributions for incident deuterium(D)neutrals are consistent with the experimental results.The Al erosion is dominated by the bombardment of neutrals with energy in the range from 20 to 500 eV.The effects of neutral reflection coefficients and the heights of molybdenum(Mo)cylindrical sleeve on erosion are also studied.The Al erosion is dominated by the first incidence of neutrals.The Al erosion rates decrease significantly for higher ratios of height to radius of Mo cylindrical sleeve.In addition,the simulated beryllium(Be)erosion rates under the same irradiation conditions are slightly lower than for the Al sample.In this work,3D-GAPS code was firstly used in EAST.The measured material erosion rate by neutrals in experiments verified by the modeling can help to understand the effects of neutrals on the first-wall erosion.For ITER,the erosion and re-deposition of recessed area at the first-wall was predicted,with the neutral energy spectrums simulated by the SOLPS.The erosion and re-deposition rates of probe surface are modelled.The effects of probe recessed heights,material reflection coefficients and concentrations of background impurities on Be erosion are studied.The Be erosion of probe surface is dominated by D flux,while redeposition is dominated by the first incidence of sputtered Be particles.The redeposited rate of probe surface reaches peak value with its recessed height of 15 mm.Generally,for higher recessed height,the re-deposited rate of probe surface decreases with increasing reflection coefficient.For background plasmas with neutral impurities sputtered from other parts of the device such as Be,it does not cause net deposition on the probe surface with the concentration lower than 0.8%.In this work,the simulated results could provide a reference for the design of the material probe in ITER.