Feedback control on edge plasma parameters with ergodic divertor in Tore Supra

In order to control highly radiative plasmas with the ergodic divertor, feedback procedures based on real time Langmuir probes measurements have been implemented in Tore Supra. The prediction of the detachment density threshold, which leads to disruption, is a major experimental difficulty. On the other hand, it has been observed that the edge electron temperature limit, measured by probes, is quite reproducible in many different operating scenarios. Therefore, feedback procedures on the edge electron temperature have been developed to control the gas injection. When the plasma reaches detachment, the fuelling efficiency abruptly increases and the feedback loop can become unstable. A detachment criterion, defined as the ratio of the saturation current of the probes scaled from the high recycling regime phase to its current value, has been used to manage detached plasmas. These techniques have been successfully applied in Tore Supra discharges with ergodic divertor for the study of edge density regimes and plasma detachment in ohmic, ICRH heating, and impurity seeded experiments.