Damage of niobium by pulse flows of helium ions and helium plasma

Abstract

The damageability of niobium by pulsed fluxes of helium ions (HI) and helium plasma (HP) in the Plasma Focus (PF) setup was studied at a flux power density qi ~ 108 W/cm2 and qp ~ 107 W/cm2, respectively, and pulse duration τi ≈ 30 – 50 ns and τp ≈ 100 ns. In the implemented irradiation conditions the erosion of the material is observed associated with the evaporation of the surface layer (SL), which occurs somewhat more intensively in the central part of the irradiation zone under the action of the most high-energy fluxes of HI and HP. The typical features of the damageability of the niobium SL under the considered irradiation conditions are revealed. These include: melting of SL with the formation of a wavy surface relief and a large number of blisters of two types — gas-filled and with destroyed shells, as well as the presence of microcracks. The appearance of blisters was associated with the formation of complexes based on the combination of implanted helium atoms with vacancies and interstitial impurity atoms (C, O, N, etc.) and their subsequent growth and coagulation in the liquid phase under pulsed action of energy fluxes on the irradiated Nb surface. Some of the microcracks formed in the SL under the action of thermal stresses coincide with the sliding lines of the material that arise under the action of high-speed plastic deformation. A network of such microcracks creates a block structure on the Nb surface. In the irradiated surface layer of niobium, zones of columnar crystals and a cellular microstructure of the surface are found, in which the average cell size is ~ 100 nm. It was shown by the method of numerical simulation that in the indicated zones the process of solidification of the SL proceeded through directional crystallization at a high speed, which reached ~ 35 m/s near the irradiated surface.