Analysis of ion cyclotron heating and current drive at ω≈2ωcH for sawtooth control in JET plasmas*

Ion cyclotron heating and current drive at ω≈2ωcH in JET deuterium plasmas with a hydrogen concentration nH/(nD+nH) in the range of 5–15% are analysed, comparing results of numerical computer modelling with experiments. Second harmonic hydrogen damping is found to be maximized by placing the resonance on the low-field side (LFS) of the torus, which minimizes competing direct electron damping and parasitic high-harmonic D damping in the presence of D beams. The shape of the calculated current perturbation and the radial localization of the heating power density for the LFS resonance are consistent with the experimentally observed evolution of the sawtooth period when the resonance layer moves near the q = 1 surface. Since the calculated driven current is dominated by a current of diamagnetic type caused by finite orbit widths of trapped resonating ions, it is not too sensitive to the ICRF phasing. Control of sawteeth with ion cyclotron current drive using the LFS ω≈2ωcH resonance in the present experimental conditions can thus be best obtained by varying the resonance location rather than the ICRF phasing. Due to differences in fast ion orbits, collisional electron heating and fast ion pressure profiles are significantly more peaked for a LFS resonance than for a high-field side (HFS) resonance. For the HFS ω≈2ωcH resonance, an enhanced neutron rate is observed in the presence of D beam ions, which is consistent with parasitic D damping at the ω≈5ωcD resonance in the plasma centre.