Operational domain for the new 3MW/1000s ECRH System on WEST

The ECRH system formerly used in Tore Supra is being upgraded to start on WEST in 2023, at a power level of 1MW and frequency of 105 GHz. Its ultimate 3MW/1000s capability is expected to enlarge the WEST operational domain by increasing margins with respect to H-mode access, and by providing additional flexibility in terms of achievable scenarios using impurity and/or MHD control. This flexibility is made possible using an antenna based on three steerable mirrors for controlled power injection. In order to determine an appropriate range of EC wave injection angles for WEST scenarios, the fast and reliable ray-tracing code REMA has been interfaced with the WEST IMAS database. This allows the EC power damping rate to be quickly assessed, as well as deposition profiles to be predicted in realistic plasma conditions. Based on a typical WEST discharge at central magnetic field B0~3.6 T, central line-averaged electron density nl~4 × 1019 m−3 and central electron temperature Te0~3keV, ray-tracing calculations have been performed. Comprehensive poloidal and toroidal angle scans, as well as variations of Bt, nl and Te0 with respect to the reference parameters have allowed an adequate range of injection angles to be determined for efficient use of ECRH and/or ECCD in typical WEST scenarios, and compared with the mechanical limits set by the antenna mechanical characteristics. In order to further characterize the effect of this new power source in WEST scenarios, EC wave deposition and current profiles from ray-tracing calculations have been included in integrated simulation codes. It has been shown that this additional power source could allow central electron heating to be achieved, potentially alleviating the issue of radiative collapse caused by impurities observed in some situations.

Demonstration of effective control of fast-ion-stabilized sawteeth by electron-cyclotron current drive

In a tokamak plasma, sawtooth oscillations in the central temperature, caused by a magnetohydrodynamic instability, can be partially stabilized by fast ions. The resulting less frequent sawtooth crashes can trigger unwanted magnetohydrodynamic activity. This Letter reports on experiments showing that modest electron-cyclotron current drive power, with the deposition positioned by feedback control of the injection angle, can reliably shorten the sawtooth period in the presence of ions with energies >or=0.5 MeV. Certain surprising elements of the results are evaluated qualitatively in terms of existing theory.

Synergy of electron-cyclotron and lower-hybrid current drive in steady-state plasmas

Improvement (up to a factor of approximately 4) of the electron-cyclotron (EC) current drive efficiency in plasmas sustained by lower-hybrid (LH) current drive has been demonstrated in stationary conditions on the Tore Supra tokamak. This was made possible by feedback controlled discharges at zero loop voltage, constant plasma current, and constant density. This effect, predicted by kinetic theory, results from a favorable interplay of the velocity space diffusions induced by the two waves: the EC wave pulling low-energy electrons out of the Maxwellian bulk, and the LH wave driving them to high parallel velocities.