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

The electron cyclotron resonance region in a tokamak, ω=ωc(r), is not accessible by the extraordinary wave launched 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 polarisation. Therefore it can already at the cutoff layer interact efficiently with electrons via the Doppler shifted resonance. The driven current can be substantially higher than that driven by the second harmonic X-mode. The scheme is limited to low values of ωp2/ωc2, and may be used for off-axis current drive.