Synergy between Electron Cyclotron and Lower Hybrid current drive on Tore Supra

An improvement of the Current Drive (CD) efficiency of Electron Cyclotron (EC) waves in the presence Lower Hybrid (LH) waves has been predicted by kinetic calculations since the '80s, but not yet observed in stationary conditions. This effect is potentially important because a LH+EC system could combine the much higher efficiency of LHCD with the good localisation of ECCD, to control the profile of the non-inductive current. In order to provide the first experimental demonstration of this synergy effect, dedicated experiments have been performed on Tore Supra. A multiple feedback strategy has been used to realise discharges with a 20 s phase at zero loop voltage, constant plasma current and density. During this phase, a 10 s ECCD pulse has been applied. The additional current driven by the EC waves is determined from the drop of the LH power. This technique has allowed a clear demonstration of the synergy effect in steady state conditions: for instance, 500 kW of LHCD have been replaced by 700 kW of ECCD only. ECCD efficiency improvements of up to a factor of 4 have been measured, in good agreement with kinetic calculations performed by means of a 3-D Fokker-Planck code.