Electron cyclotron current drive at ω approx. = ωc with X-mode launched from the low field side

The electron cyclotron resonance layer in a tokamak, ω=ωc(r), is not accessible by the extraordinary wave from the low field side, because it is shielded by a cutoff layer. However, a X-mode launched with a nonzero toroidal angle propagates at the cutoff parallel to the magnetic field and has a circular polarization. Therefore it can already at the cutoff layer interact efficiency with electrons via the Doppler shifted resonance. The driven current can be substantially higher than that driven by the second harmonic X-mode. The applicability of this current drive scheme is limited to rather low values of ωp2/ωc2, but may be of interest for high magnetic field devices. (author)