Hydrogen pumping and release by graphite under high-flux plasma bombardment

Abstract

Inert gas (helium or argon) plasma bombardment has been found to increase the surface gas adsorptivity of isotropic graphite (POCO-graphite), which can then getter residual gases in a high-vacuum system. The inert gas plasma bombardment was carried out at a flux ≊1×1018 ions s−1 cm−2 to a fluence of the order of 1021 ions/cm2 and at temperatures around 800 °C. The plasma bombarding energy was varied between 100 and 200 eV. The gettering speed of the activated graphite surface is estimated to be as large as 25 l s−1 cm−2 at total pressures between 10−6 and 10−7 Torr. The gettering capacity estimated is 0.025 Torr l/cm2 at room temperature. The gettering capability of graphite can be easily recovered by repeating inert gas plasma bombardment. The activated graphite surface exhibits a smooth, spongelike morphology with significantly increased pore openings, which correlates with the observed increase in the surface gas adsorptivity. The activated graphite surface has been observed to pump hydrogen plasma particles as well. From calibrated H-alpha measurements, the dynamic hydrogen retention capacity is evaluated to be as large as 2×1018 H/cm2 at temperatures below 100 °C and at a plasma bombarding energy of 300 eV. The graphite temperature was varied between 15 and 480 °C. Due to the plasma particle pumping capability, hydrogen recycling from the activated graphite surface is significantly reduced, relative to that from a presaturated surface. A presaturated surface was also observed to reproducibly pump a hydrogen plasma to a concentration of 9.5×1017 H/cm2. The hydrogen retention capacity of graphite is found to decrease with increasing temperature. At temperatures between 450 and 480 °C, the hydrogen retention estimated for the activated surface and presaturated surface is 7.9×1017 and 2.7×1017 H/cm2, respectively. A transient pumping mechanism associated with the spongelike surface morphology is conjectured to explain the large hydrogen retention capacity. Hydrogen release behavior under helium and argon plasma bombardment was also investigated, and the result indicated the possibility of some in-pore retrapping effect. The effective plasma impact desorption cross sections are evaluated in the plasma bombarding energy range from 15 to 300 eV. A significant fraction (70%–80%) of preimplanted hydrogen was observed to release under inert gas plasma bombardment to a fluence of 6×1019 ions/cm2. The graphite surface, once saturated, can be reactivated via plasma-impact hydrogen desorption, regardless of the helium and argon bombarding energy in the range examined here.