OKMC Simulations On The Manipulation Of Near-Surface Helium Bubbles

From the aspects of energy sustainability and economy,nuclear fusion energy will be the most ideal new energy for sustainable human development in the future.Facing the development of nuclear fusion energy,the selection of fusion reactor materials is one of the key challengs..In fusion reactor devices,tungsten is considered to be the best candidate material for PFMs.During service,tungsten will be irradiated with low-energy and high-flux helium plasma.,causing tungsten surface damage,especially forming a"villi"nanowire structure（often called a fuzz structure）,which causes seriously effects on the life of tungsten materials and the stable operation of the plasma.Therefore,suppressing the formation of the fuzz structure in the material tungsten is an important issue urgently.There have been many studies at abroad and internally,but the results are not suitable for us This paper proposes a new method to control fuzz by introducing vacancy traps near the surface of tungsten material to effectively regulate the behavior of helium bubbles,thereby suppressing the formation of fuzz.In this paper,the Kinetic Monte Carlo simulation method is used to explore the effectiveness of this control method,find the best control parameters.The main research contents and conclusions are as follows:（1）The effects of the preexisting vacancy density,temperature,helium implantation flux and other factors on the retention and the depth distribution of helium have been studied.The results show that high preexisting vacancy density,low temperature,or high helium implantation rate will cause high helium retention and lead to the overall distribution of helium close to the surface.The size and number distribution of helium bubbles is related to the density of preexisting vacancies and the nucleation mechanism of helium bubbles.When helium self-trapping dominates,the number of helium bubbles decreases significantly,and the size of helium bubbles becomes larger.When vacancy trapping dominates,the number of helium bubbles is similar to the initial distribution of preexisting vacancies,and the size of helium bubbles becomes relatively smaller.（2）The influences of the two nucleation mechanisms of helium self-trapping and vacancy-trapping to helium retention has been studied.For high helium implantation rates(approximately 10²⁶m^-2s^-1),helium self-trapping completely dominates and it is independent of temperature and number density of preexisting vacancies.For the medium helium fluence rate(approximately 10²⁴m^-2s^-1),at low temperatures,the helium self-trapping dominates and it is not sensitive to the density of preexisting vacancies;at middle temperatures,The amount of retention is highly correlated with the density of preexisting vacancies;at high temperatures,vacancy trapping dominates.For low helium implantation rates(approximately 10²² m^-2s^-1),vacancy trapping dominates.（3）The effects of the depth distribution to the preexisting vacancies on the retention of helium,the depth distribution of helium,and the size distribution of helium has been studied.The lower the incident energy of tungsten in non-uniform vacancies,the more helium retention,the faster the accumulation rate of helium,the closer the overall distribution of helium to the surface,vice versa.For low incident energy（50 ke V）,vacancy trapping dominates.For medium incident energy（100 ke V）,it is related to the number density of preexisting vacancies,and for low vacancy density,helium self-trapping dominates,and for medium vacancy density,helium self-trapping and vacancy trapping each account for half.For high incident energy（400 ke V）,vacancy trapping dominates.In general,non-uniform vacancies have more helium retention than uniform vacancies,and non-uniform vacancies are easier to fix helium near the surface than uniform vacancies.Through this study,we explored the most effective parameters of the vacancy in the material tungsten to control the evolution of helium bubble behavior:at the temperature of933K and the helium implantation rates of 10²⁴m^-2s^-1,the more the density of preexisting vacancies,the lower the incident energy of tungsten ions,the more conducive to capture helium.