Microscopic damage of tungsten exposed to deuterium–helium mixture plasma in PISCES and its impacts on retention property

Abstract Microscopic damage and D retention in tungsten have been investigated for low-energy (∼60–120 eV), high flux (∼1022 m−2 s−1), high fluence (∼5 × 1025 m−2) ion bombardment at moderate temperature (∼573–773 K) in mixed species D + He plasmas in the linear divertor plasma simulators PISCES-A and B. A significant reduction in D retention and the formation of nanometer-sized He bubbles occur in W due to seeding of He into the D plasma. The volume fraction of He bubbles, estimated with TEM observations and ellipsometric measurements, exceeds the percolation threshold. The desorption mechanism that injected D atoms diffuse back to the surface through the percolating bubbles is suggested. The seeding of Be into D + He mixture plasma eliminates this He effect on the reduction in D retention.