DAMAGE OF NIOBIUM SURFACES CAUSED BY BOMBARDMENT WITH HE-4(+) IONS OF DIFFERENT ENERGIES TYPICAL FOR T-20

The surface damage of cold worked and annealed polycrystalline Nb irradiated at room temperature with He+ ions sequentially at different energies over the range from 0.5 keV to 1.8 MeV has been investigated. The individual energy and the dose of the He+ ion was chosen to match the theoretically calculated He+ ion spectrum expected in the Tokamak T-20. In one set of irradiations, targets were irradiated at Kurchatov Institute starting with 0.5 keV 4He+ ions and extending up to 90 keV in eleven steps.Subsequently, the same area was irradiated at ANL starting at 150 keV and increased in eight steps up to 1.8 MeV. The irradiations were carried out for a total dose of 5.0 C/cm2. In another set of irradiations the sequence was reversed. Scanning electron microscopy results show formation of blisters and exfoliation. For the same dose the broad energy implant (due to sequential irradiation) appears to decrease the blister diameter and density as compared to irradiation with monoenergetic He+ ions at a given energy (in the energy range considered). Some estimates of surface erosion yields due to blistering are given. © 1979.