He self-pumping by tokamak pump limiter materials: Al, V, Ni, and Ni/Al alloys

An ECR (electron cyclotron resonance) plasma and Colutron ion gun were used to study He self-pumping by several possible pump-limiter materials: Ni, V, Al, and Ni/Al multilayers. Ni and V exhibited similar pumping capacities (6×10¹⁵ He/cm², 200 eV), whereas Al showed a reduced capacity (6×10¹⁴ He/cm², 200 eV) due to increased sputtering. An He retention model based on ion implantation ranges and sputtering rates agreed with the experimental data. The pumping efficiency increased significantly with ion energy. A novel multilayer/bilayer pumping concept showed improved pumping above that for single-element films. D/He trapping site competition is more important in V than in Ni. However, D/He site competition in V was shown to be less important above 400°C where hydride decomposition is enhanced