Density limit and impurity transport investigations in DITE tokamak

The processes leading to disruption with increasing density have been studied. The discharge remains in thermal equilibrium but there is a slow profile contraction as the density is raised because of a deterioration in edge confinement relative to the centre. This is only partly explained by radiation losses. The central confinement time of impurities injected by laser ablation has been measured. At high density in ohmically heated discharges, it can be varied by more than a factor of five by changing the gas feed rate. The largest values occur for refuelling by recycling only. Substantial differences also occur with co- and counter-neutral beam injection, which are not sensitive to the gas feed rate in this case. These differences cannot be mainly ascribed to plasma rotation because they are not removed by switching on the bundle divertor, which reduces the rotation speed by an order of magnitude. An accumulation of intrinsic impurities is observed in discharges with long impurity confinement times. The effects described are much reduced or absent at low density. (author)