Real-Time Control of MHD Activity and Steady-State Current Profile by Non-Inductive Current Drive in Tore Supra

Full text: The Tore Supra tokamak long pulse capability (superconducting toroidal magnets, actively cooled plasma facing components, large non-inductive current drive capability) has allowed a quite unique type of experiment where successive stationary states of the plasma current profile are controlled in real time during the main heating phase. The plasma current profile is controlled by varying the level of Lower Hybrid (LH) power, i.e. replacing part of the ohmic current by a non-inductive source with a different deposition profile. With the LH power available during the experiments, the safety factor profile can thus be varied at will from a sawtoothing monotonic one to a mildly reversed profile with qmin ∼ 3/2. In this range, the q-profile evolves through five distinct states, characterised by specific MHD activity. The q-profiles states are detected by real time analysis of the electron temperature relaxations resulting from the MHD activity, observed on the central channels of the Electron Cyclotron Emission diagnostic (ECE). Several experiments have been carried out on Tore Supra to demonstrate the capability of this control scheme i) to obtain a desired stationary q-profile state and ii) to sustain it in spite of preset variations of other plasma parameters such as plasma density, ICRH power or total current. We achieved i) control of the presence/absence of sawteeth with varying plasma parameters, ii) obtaining and sustaining a 'hot core' plasma regime without MHD activity, iii) recovery from a voluntarily triggered deleterious MHD regime. We report also on another set of experiments featuring real-time control of Electron Cyclotron antenna mirrors for destabilisation of sawteeth in the presence of fast ion tails produced by ICRH, using an improved ECCD positioning algorithm. This experiment shows that achieving and maintaining short sawteeth through real time controlled ECCD seems a viable option for NTM avoidance in plasmas where sawteeth are stabilised by significant fast ion pressure inside the q = 1 surface. This work, supported by the European Communities under the contract of Association between EURATOM and CEA, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. (author)