Deuterium retention in NSTX with lithium conditioning

High (≈90%) deuterium retention was observed in NSTX gas balance measurements both with- and without lithiumization of the carbon plasma-facing components. The gas retained in ohmic discharges was measured by comparing the vessel pressure rise after a discharge to that of a gas-only pulse with the pumping valves closed. For neutral beam heated discharges the gas input and gas pumped by the NB cryopanels were tracked. The discharges were followed by outgassing of deuterium that reduced the retention. The relationship between retention and surface chemistry was explored with a new plasma–material interface probe connected to an in vacuo surface science station that exposed four material samples to the plasma. XPS and TDS analysis demonstrated that binding of D atoms in graphite is fundamentally changed by lithium – in particular atoms are weakly bonded in regions near lithium atoms bound to either oxygen or the carbon matrix. This is in contrast to the strong ionic bonding that occurs between D and pure Li.