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Numerical analysis of electron cyclotron current drive with suppressed Doppler broadening

An electron cyclotron current drive by the fundamental O-mode is numerically analysed in the off-axis region of a reactor-grade tokamak. By scanning the launching direction in both the toroidal and poloidal directions, the optimum direction of beam injection is obtained to drive a current with a maximum value of I/ρw at the desired magnetic surface, where I is the total driven current and ρw is the radial width of the driven current. When the current is driven at the position where a ray trajectory becomes tangent to the magnetic surface (referred to as the `tangential resonance'), Doppler broadening of the current profile is significantly reduced and I/ρw reaches a maximum value. In the case of tangential resonance, the driven current width is mainly determined by the effects of the beam divergence and finite spatial width of the beam at the launching position. The width of the driven current is kept in a range of 2-5% of the minor radius under the assumption of a beam divergence with a full angle of 2°. The dependence of the driven current on both the location of beam injection and the wave frequency is also examined.