Optimizing current drive by electron cyclotron waves in the presence of trapped particles

Current drive by electron cyclotron (EC) waves is a possible candidate for operating a steady-tokamak reactor. In this method, a net toroidal current is generated by heating process which is essentially perpendicular to the toroidal direction. In the limit of a homogeneous magnetic field configuration, this process is in principle almost as efficient as a direct parllel heating, but in a toroidal system electron trapping imposes a stringent limit to its efficiency. This effect is particulary important in the case of wave absorption at downshifted frequency (ω < ω-c, ω and ω-c being the wave and the EC frequency, respectively), and at high wave power levels. In this work, two methods for minimizing this deleterious effect are investigaded: i) moving the absorption region to the inboard side of the magnetic surfaces; ii) combinning EC and lower hybrid (LH) waves. In order to evaluate the current drive efficiency for realistic wave power levels, the time-dependent bounce-averaged quasilinear/Fokker-Planck equation is soved by means of a 2-D Fokker-Planck code. The case of the extraordinary (x) mode absorbed at ω < ω-c is inconsidered in detail, both for low and high wave power. It is shown that, in this case, moving the absorption region is rather ineffective en enhancing the current drive efficiency, because of the interplay of quasilnear and toroidal effects for high wave power. On the other hand, combination with LH waves can be quite efficient minimizing the negative impact of electron trapping