High-power ECH and fully non-inductive operation with ECCD in the TCV tokamak

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Abstract
Experiments with high-power electron cyclotron heating (ECH) and current drive (ECCD) in the TCV tokamak are discussed. Power up to 2.7 MW from six gyrotrons is delivered to the tokamak at the second-harmonic frequency (82.7 GHz) in X-mode. The power is transmitted to the plasma by six independent launchers, each equipped with steerable mirrors that allow a wide variety of injection angles in both the poloidal and toroidal directions. Fully non-inductive operation of the tokamak has been achieved in steady state, for the full 2 s gyrotron pulse duration, by co-ECCD with a highest current to date of 210 kA at full power. The experimentally measured ECCD efficiency agrees well with predictions obtained from linear modelling. We have observed that the highest global efficiency attainable at a given power is limited by stability constraints. While the efficiency is maximum on the magnetic axis, a disruptive MHD instability occurs when the width of the deposition profile is lower than a minimum value, which increases with total power. Many ECCD discharges display a high level of electron energy confinement, enhanced by up to a factor of two over the Rebut-Lallia-Watkins (RLW) scaling law, which by contrast is well satisfied in ohmic conditions. The longest confinement times (up to four times RLW) are observed with central counter-ECCD. Central electron heat diffusivities comparable to ohmic levels are obtained in these scenarios, with electron temperatures in excess of 10 keV.