Numerical analysis of optimum ECCD for localized driven current profile in a tokamak plasma

The optimum launching condition of electron cyclotron wave for a localized driven current profile at a targeted magnetic surface is obtained by numerical analysis, where the fundamental O-mode wave is launched in the low field side. The wave launching condition is determined by the direction and position of beam injection and the wave frequency. First the optimum direction of the wave launched at the midplane is analyzed by scanning the launching direction in both the toroidal and poloidal directions. Doppler broadening of the current profile is significantly suppressed, when the current is driven at the position where a ray trajectory becomes tangent to the magnetic surface (referred to as the 'tangential resonance'). Second the dependence of the current driven through the tangential resonance on both the location of beam injection and the wave frequency is also examined. It is found that the optimum position is at 60 ∼ 70deg above the midplane and the optimum frequency is in the range of 170 ∼ 190 GHz for a toroidal magnetic field of 5.5T. It is also shown that the dependence of the tangential resonance condition on the electron temperature is weak. (author)