To explore the possibility of efficient fast wave current drive in an ignited plasma in the ion cyclotron (IC) range of frequency in spite of competition from absorption by ions, we have added to the full-wave toroidal code TORIC a set of subroutines which evaluate absorption by these particles at IC harmonic resonances, using a realistic 'slowing-down' distribution function, and taking into account that their Larmor radius is comparable or even larger than the fast wave wavelength. The thermalized population of α-particles is not a serious competitor for power absorption as long as their number density is compatible with maintenance of ignition. By contrast, the energetic slowing down fraction, in spite of its even greater dilution, can absorb from the waves a substantial amount of power at the cyclotron resonance and its harmonics. An extensive exploration both in frequency and in toroidal wavenumbers using the parameters of one of the European versions of DEMO shows that three frequency windows exist in which damping is nevertheless predominantly on the electrons. Designing an antenna capable of shaping the launched spectrum to optimize current drive, however, will not be straightforward. Only in a narrow range when the first IC harmonic of tritium is deep inside the plasma on the high-field side of the magnetic axis, and that of deuterium and helium is still outside on the low-field side, it appears possible to achieve a satisfactory current drive efficiency with a conventional multi-strap antenna, preferentially located in the upper part of the vessel. Exploiting the other two windows at quite low and quite high frequencies is either impossible on first principles, or will demand novel ideas in antenna design.
イオンによる吸収との競合にもかかわらず、ion cyclotron (IC) 周波数帯での効率的なfast wave current driveの可能性を探るために、我々はfull-wave toroidal code TORICに、現実的な'slowing-down'分布関数を用いてIC harmonic resonancesでのこれらの粒子による吸収を評価し、それらのLarmor radiusがfast wave wavelengthと同等またはそれ以上であることを考慮に入れた一連のサブルーチンを追加した。熱化されたα-particles集団は、その数密度が点火の維持と両立する限り、power absorptionの深刻な競争相手ではない。対照的に、高エネルギーのslowing down fractionは、その希釈度がさらに高いにもかかわらず、cyclotron resonanceとその高調波でかなりの電力を吸収し得る。欧州DEMOの一バージョンのパラメータを用いて周波数とtoroidal wavenumbersにわたる広範な探索により、減衰が依然として主に電子によるものである3つの周波数窓が存在することが示された。しかし、current driveを最適化するためにlaunched spectrumを整形できるアンテナの設計は容易ではない。tritiumの第1IC harmonicが磁気軸のhigh-field sideに深く位置し、deuteriumとheliumのそれがlow-field sideにまだある狭い範囲でのみ、従来のmulti-strap antennaを容器の上部に配置することで満足のいくcurrent drive efficiencyが達成可能と思われる。他の2つの窓をかなり低い周波数と高い周波数で利用することは、原理的に不可能か、あるいはantenna designにおける新しいアイデアを必要とする。