Retention of Ne and N2 in the closed and pumped TdeV divertor with attached and detached plasmas

We have studied the retention of a recycling impurity, neon, and a wall-pumped impurity, nitrogen, in the closed and cryopumped divertor of the TdeV tokamak, under a variety of heating, puffing, pumping and biasing conditions. Retention times were deduced from either the decay time or the rise time of the plasma impurity content, measured spectroscopically, following either a short puff or a steady injection. For both neon and nitrogen, the compression ratio increases rapidly with the main plasma density. Retention is not degraded by plasma detachment. In density scans, other conditions being kept constant, the compression ratio for neon is found to grow with the divertor neutral pressure as ∝PD1.5. However, by puffing, pumping or biasing, retention and PD can be varied quite independently of each other. (orig.)