RETENTION OF NE AND N-2 IN THE CLOSED AND PUMPED TDEV DIVERTOR WITH ATTACHED AND DETACHED PLASMAS

We have studied the retention of a recycling impurity, neon, and a wal l-pumped impurity, nitrogen, in the closed and cryopumped divertor of the TdeV tokamak, under a variety of heating, puffing, pumping and bia sing conditions. Retention times were deduced from either the decay ti me or the rise time of the plasma impurity content, measured spectrosc opically, following either a short puff or a steady injection. For bot h neon and nitrogen, the compression ratio increases rapidly with the main plasma density. Retention is not degraded by plasma detachment, i n density scans, other conditions being kept constant, the compression ratio for neon is found to grow with the divertor neutral pressure as similar to P-D(1.5). However, by puffing, pumping or biasing, retenti on and P-D can be varied quite independently of each other.