2016
ACL
|
L.Pentecoste, A.L.Thomann, A.Melhem, A.Caillard, S.Cuynet, T.Lecas, P.Brault, P.Desgardin, M.F.Barthe, 'Low flux and low energy helium ion implantation into tungsten using a dedicated plasma source', Nucl. Instr. Meth. Phys. Res. B 383 38–46 (2016) doi:10.1016/j.nimb.2016.06.011
The aim of this work is to investigate the first stages of
defect formation in tungsten (W) due to the accumulation of helium (He)
atoms inside the crystal lattice. To reach the required implantation
conditions, i.e. low He ion fluxes and kinetic energies below the W atom
displacement threshold (about 500 eV for He+), an ICP source has been
designed and connected to a diffusion chamber. Implantation conditions
have been characterized by means of complementary diagnostics modified
for measurements in this very low density helium plasma. It was shown
that lowest ion fluxes could only be reached for the discharge working in
capacitive mode either in alpha or gamma regime. Special attention was
paid to control the energy gained by the ions by acceleration through the
sheath at the direct current biased substrate. At very low helium
pressure, in alpha regime, a broad ion energy distribution function was
evidenced, whereas a peak centered on the potential difference between
the plasma and the biased substrate was found at higher pressures in the
gamma mode. Polycrystalline tungsten samples were exposed to the helium
plasma in both regimes of the discharge and characterized by positron
annihilation spectroscopy in order to detect the formed vacancy defects.
It was found that W vacancies are able to be formed just by helium
accumulation and that the same final implanted state is reached, whatever
the operating mode of the capacitive discharge.
|