Fuel retention in impurity seeded long discharges in Tore Supra

In next step machines, tritium retention in the carbon walls is a major concern. In Tore Supra, long pulses with impurity seeding were developed to study deuterium retention in stationary conditions at low edge temperatures. A double feedback was implemented, with deuterium injection set on plasma density and impurity injection set on the radiated fraction. Long discharges (∼1 min) were obtained with radiated fractions in the range 60–80%. Neon and argon seeding were tested. In both cases, a small fraction of the injected impurity is transiently trapped in the wall, and released at the end of the shot. The deuterium retention rate decreases significantly with increasing radiated fraction, both for Ne and Ar seeding, in absolute and relative value (when related to D2 injection rate, or Dα recycling on the limiter). Interpretation is still an open question, as different retention mechanisms could be affected (implantation, codeposition, bulk diffusion).