Optimising the use of ICRF waves in JET hybrid plasmas for high fusion yield

In the recent JET experimental campaign, good progress was made in the development of high-performance plasma scenarios compatible with the ITER-like wall (ILW). This paper reports on the optimisation of the use of ion cyclotron resonance frequency (ICRF) waves for the hybrid scenario in combination with neutral beam injection. The hybrid scenario is a candidate for ITER long-pulse operation. The combined NBI+ICRF power was increased to 33 MW and the record JET ILW fusion yield, averaged over 100 ms, from 2.3x1016 neutrons/s to 2.9x1016 neutrons/s with respect to the previous 2014 JET ILW fusion record. Impurity control with ICRF waves was one of the key means for extending the duration of the high-performance phase. The ICRF waves were tuned to the fundamental cyclotron frequency of minority hydrogen (H) ions which coincides with the 2nd harmonic cyclotron frequency of D ions in the plasma centre. The H minority concentration and the ICRF resonance location were systematically varied in order to study their effects on the plasma performance. The best fusion performance was obtained with a central ICRF resonance and with a low H concentration. A central resonance was effective in lengthening the high-performance phase by avoiding impurity accumulation while a low H concentration, which favours the damping of ICRF waves on deuterium beam ions, enhanced the D-D fusion reactivity via the ICRF-accelerated D ions. Results from ICRF+NBI modelling with the PION code are presented and show a good overall agreement with the experimental results. We find that the ICRF enhancement of the neutron yield ranged from about 5 to 20% in these plasmas. Finally, we extrapolate the results to D-T and find that the best performing discharges correspond to an equivalent fusion power of ~8.0 MW in D-T.