Modelling of D majority ICRH at JET: impact of absorption at the Doppler-shifted resonance

Recent experiments in JET have provided new information on majority ICRF heating schemes in large tokamak plasmas. Adopting a wide range of available diagnostics, the plasma behaviour in a fundamental D majority ICRF heating scenario was investigated. The main results of the experiments are that, due to the modest RF power available at the frequencies necessary for central D absorption in JET associated with the reduced RF electric field amplitude near the cold ion-cyclotron resonance layer of the D majority ions, ICRH alone is barely capable of heating the plasma. On the other hand, when preheating the plasma using neutral beam injection, the wave-plasma coupling is improved and considerable plasma heating followed by increased neutron yield was observed in several discharges. The enhancement of the ICRH efficiency in the presence of NBI is attributed not only to the lower collisionality of the pre-heated plasma but also to the Doppler-shifted ion-cyclotron resonance absorption of the fast beam ions. This effect has been observed with several diagnostics and was confirmed by numerical simulations. The numerical modelling was done adopting a coupled full wave/quasi-linear Fokker-Planck (QLFP) code that takes into account the non-Maxwellian distributions of the injected beam ions in the wave equation, and the actual local RF electric field structure obtained by the full-wave computations in the Fokker-Planck description. By studying the response of the plasma to sudden changes in the ICRF power level, the experimental RF power deposition profiles were estimated and compared with the theoretical predictions. Finally, a qualitative link between the RF-heated beam distributions obtained in the QLFP simulations and some of the experimental results is also presented.