The Giga-joule Discharges

One of the main missions of the Tore Supra tokamak was to open the route toward long-pulse plasma discharges in order to investigate phenomena that are involved in steady-state plasma control. In 1992, a 1-min flattop 1-MA discharge was performed with 2.5 MW of lower hybrid current drive (LHCD) power, the main limitation being the available flux. In 1996, at 0.8 MA, the duration was extended to 120 s (290 MJ injected energy), limited by in-vessel uncontrolled outgassing of inertial parts (away from the last closed flux surface) slowly heated by the plasma radiation. At the same time, fully noninductive operation was sustained at 0.6 MA for more than 1 min using two feedback loops: the control of the loop voltage (kept at zero) with the primary and the control of the plasma current with the LHCD power. Following these results, a major upgrade of the plasma-facing components was undertaken (Composants Internes et Limiteur project) and fully implemented in 2002. The vacuum vessel is now practically fully covered with actively cooled plasma-facing components monitored by a set of infrared endoscopes. In 2003, 1 GJ of injected/extracted energy was achieved in a 6-min, 0.5-MA discharge. All the plasma parameters were kept constant during the whole discharge, the plasma current being fully non inductively driven by 3 MW of LHCD. The pulse length limitation came from the aging klystron, originally designed for 30-s operation. Experimental results and analysis of the physics involved in these long-pulse discharges are reported and discussed. (author)