REVIEW OF VACUUM VESSEL CONDITIONING PROCEDURES AT JET AND THEIR IMPACT ON PLASMA OPERATION

The experience at JET and in other tokamaks shows that the achievement of reproducible plasma conditions and the improvement on overall plas ma performance depend crucially on vessel conditioning. The new pumped divertor, with its coils internal to the vacuum vessel, introduces ne w elements in the problem of JET wall conditioning the presence inside the vessel of hidden surfaces not fully exposed to the glow discharge and of non bakable elements, such as the divertor coils case and heat shields. The new cleaning and conditioning techniques, adapted to the changes of the in-vessel configuration, are described in this paper, and the results of the vessel glow cleaning with an improved GDC syste m presented. For the first time at JET, the glow discharge cleaning ha s been extensively diagnosed and the glow current distribution, edge p otential, density and temperature measured in conjunction with systema tic scan of the glow gas species current and pressure. A statistical s urvey of a large number of plasma pulses shows that the ''base level'' impurity content of the plasma is low, and comparable to the 1991 lev els.