Improvement of density control by feedback on Langmuir probe signals in Tore Supra ergodic divertor experiments

Real-time control of deuterium or helium gas injection by feedback on Langm uir probe signals is implemented in Tore Supra ergodic divertor discharges. The feedback schemes are based on the robust experimental observation that the density limit coincides with the edge temperature T-e similar to 10 eV . Three control algorithms are used: (1) proportional feedback on the centr al line-averaged density with real-time attenuation of the system gain and security cut-off of the gas injection if the edge temperature becomes too l ow; (2) proportional feedback on the central line-averaged density with sec urity cut-off controlled by the degree of detachment (DoD); (3) proportiona l feedback on edge temperature with security cut-off on the DoD. The DoD is defined for deuterium discharges, but not for helium since those do not detach. All three feedback modes permit operation close to the dens ity limit and have been successfully applied for plasma currents 0.4 < I-p < 1.8 MA, and at I-p = 1.4 MA with up to 4 MW of ICRH power.