Intricacies of helium effects on displacement cascades in tungsten

MD simulations were performed to study the effect of He on displacement cascades in W. The ability of He clusters to displace W atoms (kick-out mechanism) was studied with thermalization of interstitial He for 200 ps. The minimum cluster size to initiate kick-out decreases from 7 (1025 K) to 5 (2050 K). No kick-out was found at 300 K even at 4600 appm. Effects on damage were studied with 75 keV PKA for 55 ps with no initial He clustering. At 1025 K, data with 100{\%} interstitial He shows the number of surviving SIAs remains unchanged: 81 (pure), 82 (2300 appm), while vacancy count drops to \textless 50 at 2300 appm due to He filling the vacancies. Further study was done for 460 appm at several T. For 100{\%} interstitial, the number of He-filled vacancies is 7 (300 K), 8 (1025 K) and 15 (2050 K) due to increased mobility. For 50{\%} substitutional, the number of vacancies is minimum at 1025 K (65) compared to 87 (300 K) and 73 (2050 K) indicating there is an interplay between increased He mobility to find a vacancy vs decreased He$+$vacancy stability. The 100{\%} substitutional data confirms the observation. The seemingly decreased stability of He$+$vacancy cluster may be due to increased mobility of SIAs and overtake vacancies from those clusters. More studies are planned to elucidate the competition.