Blister formation in multiple energy He+ implanted stainless steel

A series of multiple energy He+ implantations has been conducted on type 316 stainless steel at 575 and 775 K. Pre-implantations with 3, 10 and 15 keV He+ were used to create a series of uniform helium concentrations to a depth of approximately 700 A. The effects of these pre-implantation helium concentrations on subsequent 20 keV bombardment were studied by gas re-emission and scanning electron microscopy. If was found that serious blistering and exfoliation would occur unless the pre-implantation helium concentration was approximately greater than 0.3 atom fraction from the surface to nearly the depth of the 20 keV implant. At this pre-implant level, an interconnected pathway to the surface developed, allowing helium release without surface blistering. Evidence for rapid helium mobility along the interconnection was found at temperatures as low as 80K. These results indicate that multiple energy implantation will still produce extensive surface deformation if the helium accumulates to a local critical concentration before an interconnected pathway develops. Until detailed first wall particle fluxes are available, He+ induced surface exfoliation cannot be ruled out as a potentially serious source of wall erosion and plasma contamination in a MFE reactor