Hydrogenic Fuel Recovery and Retention with Metallic Plasma-FacingWalls in the Alcator C-Mod Tokamak

The retention and recovery of hydrogen (H) and deuterium (D) fuel in the Alcator C-Mod tokamak are studied. C-Mod solely employs high-Z molybdenum (Mo) as its plasma-facing armor material, with intermittent application of thin boron (B) films. The C-Mod wall materials are found to retain large fractions, ~20-40%, of the D2 gas fuelled per shot, regardless if the Mo surfaces are bare or partially covered by B films. The retention appears to be linearly proportional to ion flux to the wall, with ~0.5% D ions incident to the wall retained, and with no indication of the retention rate saturating over 30 s of plasma exposure. The retention is inconsistent with D in Mo retention from laboratory ion beam studies, which would predict rapid saturation of the fuel inventory in the wall a few discharges. Neither is the retention consistent with co-deposition of the B since it occurs with cleaned Mo walls. Overall these results raise concerns for use of high-Z materials in long-pulse D-T devices like ITER. Planned disruptions are exploited to recover H and D from the wall in C-Mod. The disruptions produce rapid heating of the wall surfaces, releasing the H/D from the wall for recovery by the pumps; a technique proposed for tritium recovery in ITER. It is shown that disruptions are effective for removing hydrogen absorbed into the wall during water exposure during vents. Overall control of the H/D wall inventory is demonstrated using frequent, planned disruption, with the result that the C-Mod wall can be net depleted of fuel over a run day.