Full steady-state operation in Tore Supra

In order to produce fully non-inductive, lower hybrid (LH) driven discharges in a systematic and reproducible manner, new operation modes have been studied on the superconducting Tore Supra tokamak. To cope with some uncertainties in the LH current drive efficiency (e.g. profile dependences), the plasma current is not imposed a priori, but evolves freely until the equilibrium (which depends on the LH power level) is reached. The voltage applied on the primary circuit no longer controls the plasma current. In an 'open loop' scenario in which this voltage is preset and constant, the timescale required to attain the equilibrium is the longest characteristic time of the coupled plasma-poloidal field coils system (approximate to 60 s). In order to obtain a stationary state faster, a new feedback scheme has been implemented in which the primary circuit voltage is controlled in such a way that the Bur consumption vanishes. It is shown that this operation mode allows full steady-state to be reached within a characteristic time of a few seconds. The underlying physics is described and a detailed analysis of the experiments is made. It is shown, in particular, that this operation scenario generates stable stationary plasmas with improved confinement, so that the so-called 'LHEP' regime can be extrapolated to continuous operation.