Diffusion of sputtered ground-state tungsten atoms in krypton

The work is dedicated to investigations of the diffusive transport of sputtered ground-state tungsten atoms in buffer gas krypton at ambient temperatures. For sputtering of the metal, a pulse-periodic longitudinal hollow cathode gas discharge in cylindrical geometry is used. The cathode represents a 20 cm in length and 2.6 cm in diameter hollow copper cylinder, the inner surface of which is coated with a thin tungsten layer. During the discharge W atoms are sputtered from that layer into the cathode volume, while in the afterglow phase their density decreases due to diffusive losses back to the surface. The diffusive loss rates are measured by using a time-resolved double-channel optical absorption technique. By relating the experimental loss rates with a diffusion model based on the experimental geometry and a boundary condition of the third kind, the following value for the diffusion coefficient of W atoms in krypton is obtained: D-0 =130(-30)(+40) cm(2)s(-1) at 133 Pa buffer gas pressure and 315 K temperature. The reflection coefficient of these atoms at the cathode surface is obtained within the range 0 <= rho <= 0.8. The experimental results for the diffusion coefficient are compared with theoretical calculations based on model interaction potentials.