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

Experiments with high-power electron cyclotron heating (ECH) and current dr ive (ECCD) in the TCV tokamak are discussed. Power up to 2.7 MW from six gy rotrons 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 l aunchers, each equipped with steerable mirrors that allow a wide variety of injection angles in both the poloidal and toroidal directions. Fully non-i nductive operation of the tokamak has been achieved in steady state, for th e full 2 s gyrotron pulse duration, by co-ECCD with a highest current to da te of 210 kA at full power. The experimentally measured ECCD efficiency agr ees well with predictions obtained from linear modelling. We have observed that the highest global efficiency attainable at a given power is limited b y stability constraints. While the efficiency is maximum bn the magnetic ax is, a disruptive MHD instability occurs when the width of the deposition pr ofile is lower than a minimum value, which increases with total power. Many ECCD discharges display a high level of electron energy confinement, enhan ced by up to a factor of two over the Rebut-Lallia-Watkins (RLW) scaling la w, which by contrast is well satisfied in ohmic conditions. The longest con finement times (up to four times RLW) are observed with central counter-ECC D. Central electron heat diffusivities comparable to ohmic levels are obtai ned in these scenarios, with electron temperatures in excess of 10 keV.