Investigation Of Helium Bombardment Of Tungsten Fuzz Using BCA Method

The working conditions of plasma facing materials(PFMs)in fusion devices are extremely tough,including high temperature,high heat flux,14 MeV neutron,escaping particle bombardment and etc.Due to its excellent properties such as high melting point,high thermal conduction,high density,low thermal expansion,low vapor pressure,low retention of tritium,low sputtering yields,high sputtering threshold and so on,tungsten(W)has been chosen as the most promising candidate for PFM and will be used in the ITER diverter.Experimental studies have revealed that tungsten surfaces can be damaged under bombardment by low-energy helium ion below sputtering threshold.Helium bubbles have been found to form in subsurfaces and nanostructures,usually referred to as “fuzz” taking the form of thin tendrils,grow out of the bulk material.Such damages not only affect the quality and service life of PFMs,but also harm to tritium self-sufficiency and plasma stability.Studies have found that fuzz can grow to several hundred nanometers or even a few micrometers in depth.Helium ions have been found to be the key driving force to account for the growth of fuzz structures.However,low-energy helium ions(under 100 eV)has a limit penetration power in tungsten.The depth distribution of implanted ions can only reach to a dozen nanometers.So the outstanding question is how helium ions transport through thick fuzz to reach the tungsten bulk layers.Besides,the formation of fuzz leads to an increase of sputtering yield.If the sputtered atoms go to the plasma,the plasma core in Tokamak will be polluted and the stability of plasma could be influenced.Thus,the fuzz formation on tungsten surfaces needs to be studied.In the present work,various fuzz models with different characteristics are generated and the recently developed code IM3 D is employed to simulate the interactions between incident ions and W fuzz structures.IM3 D is a Monte Carlo code based on the binary collision approximation(BCA)method for rapidly simulating the transportation of ions in 3D complex nanostructures.In this work,helium penetration in W fuzz layers and sputtering properties of fuzz surfaces under argon ion bombardment are studied.The results show that the size of building blocks,the fuzz texture and the fuzz density all have significant effects on the helium penetrative power.The fraction of open channels in fuzz structures is the key factor that influences the penetration power.The half depth of helium ion ranges from 153 nm to 338 nm depending on the fuzz structures,which means that helium ions can penetrate to W surface through the fuzz.The sputtering yields of fuzz surfaces bombarded by argon are also studied.It is found that the sputtering yield of a fuzz surface is much lower than that of a flat surface.As the porosity of fuzz increases,the sputtering yield decreases.Meanwhile,the sputtering yield increases gradually with increasing incident ion energy.The reduction of sputtering yield is mainly due to the porous nanostructures.All these results shed more light on the penetrative and sputtering properties of the W fuzz,including the mechanism of the fuzz formation.