A Powerful Remote Source of O Atoms for the Removal of Hydrogenated Carbon Deposits

摘要

We describe a system to deliver a large flux of O atoms for the removal of hydrogenated carbon films from surfaces in remote areas of tokamaks with carbon divertors. The oxygen plasma is generated via electrode-less radiofrequency discharge in a discharge chamber connected to a remote chamber by a 2 m long complex-shaped glass tube 4 cm in diameter. The density of O atoms in the remote chamber was measured with a nickel catalytic probe and its variation with discharge power obtained. The density was close to the detection limit of the probe (around 1 × 10~(19) m~(-3)) as long as the vacuum system was pumped with a rotary pump at a nominal pumping speed of 80 m~3 h~(-1). The density increased well over 10~(20) m~(-3) when a Roots pump was added. The effective pumping speed at the current setup was up to 200 m~3 h~(-1). At such conditions, the maximal O-atom density at 2 m from the source was up to 3 × 10~(20) m~(-3). The density depended on the pressure as well as the discharge power. The behavior of O-atom density far away from the source was explained by gas phase and surface phenomena. The effective pumping speed was found to be of crucial importance. The setup was used for removal of model hydrogenated carbon films. Experiments were performed at sample temperatures up to 600 K and etching rates up to 50 nm/s were obtained. We found that the experimental setup is suitable for removal of hydrogenated atoms on a large scale.