Damage of the Tungsten Surface Layer under Irradiation by Steady-State Ion and Pulsed Beam-Plasma Helium Fluxes

Abstract

Comparative studies of tungsten surface damageability under irradiation with steady-state helium ion fluxes and pulsed helium ion and helium plasma fluxes in the ion-beam accelerator (ILU) and in the Vikhr Plasma Focus (PF) installation have been carried out. The parameters of irradiation with steady-state He⁺ ion fluxes in the ILU: helium ion energy of 30 keV; fluences of 1.0 × 10¹⁸ and 2.0 × 10¹⁸ cm^–2; and target temperature during irradiation not exceeding ~500 K. For irradiation with pulsed fluxes of helium ions and helium plasma in the Vikhr PF installation, the samples were placed in the cathode zone of the Vikhr PF chamber at a distance of 2, 4, and 6 cm from the anode. Parameters of irradiation of samples in the Vikhr PF: duration of ion and plasma exposure of 10–30 and 50–200 ns, respectively; power density of plasma in the range of ∼10⁷–10⁹ W/cm² and ions in the range of ~10⁹–10¹⁰ W/cm²; number of pulses N = 5, 15, and 30; helium ion energy of ~100 keV; and plasma temperature of ~1 keV. Under irradiation with steady-state He⁺ ion fluxes with a fluence of 10¹⁸ cm^–2, formation of blisters with peripheral rupture of caps characteristic of brittle materials is observed. An increase in the irradiation fluence by a factor of two leads to a change in the mechanism of surface failure; i.e., exfoliation of layers (flaking) is observed. During irradiation of samples in the Vikhr PF installation, melting of the surface layer occurs, resulting in a wavy surface relief with the appearance of cracks and craters, i.e., traces of helium gas release and unopened blisters. The sizes of craters are ∼1–2 µm, which is comparable to the dimensions of blisters observed after He⁺ ion implantation in the ILU accelerator. Numerical modeling of the impact of a fast helium ion beam on tungsten in the Vikhr PF installation has been performed. Using methods of X-ray diffraction analysis, the following effects are observed: a decrease in lattice parameters under all modes of ion and plasma flux exposure; varying degrees of changes in the size of the coherent scattering region (CSR) and the magnitude of lattice microstrain and texture. A decrease in the microhardness of tungsten samples after treatment with helium plasma and He⁺ ions in the Vikhr PF installation is detected, which may result from the influence of two competing factors: annealing of defects under intense thermal loads (reduces H_μ ) and thermal stresses arising during the crystallization and cooling of the melted surface layer (increases H_μ ). The mechanisms of the observed phenomena are discussed.