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

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 gyrotronsis delivered to the tokamak at the second-harmonic frequency (82.7 GHz) inX-mode. The power is transmitted to the plasma by six independent launchers,each equipped with steerable mirrors that allow a wide variety of injectionangles in both the poloidal and toroidal directions. Fully non-inductiveoperation of the tokamak has been achieved in steady state, for the full 2 sgyrotron pulse duration, by co-ECCD with a highest current to date of 210 kAat full power. The experimentally measured ECCD efficiency agrees well withpredictions obtained from linear modelling. We have observed that the highestglobal efficiency attainable at a given power is limited by stabilityconstraints. While the efficiency is maximum on the magnetic axis, adisruptive MHD instability occurs when the width of the deposition profile islower than a minimum value, which increases with total power. Many ECCDdischarges display a high level of electron energy confinement, enhanced by upto a factor of two over the Rebut-Lallia-Watkins (RLW) scaling law, which bycontrast is well satisfied in ohmic conditions. The longest confinement times(up to four times RLW) are observed with central counter-ECCD. Centralelectron heat diffusivities comparable to ohmic levels are obtained in thesescenarios, with electron temperatures in excess of 10 keV.