Experiments in high-performance JET plasmas in preparation of second harmonic ICRF heating of tritium in ITER

The reference ion cyclotron resonance frequency (ICRF) heating schemes for ITER deuterium–tritium (D-T) plasmas at the full magnetic field of 5.3 T are second harmonic heating of T and ^3 He minority heating. The wave-particle resonance location for these schemes coincide and are central at a wave frequency of 53 MHz at 5.3 T. Experiments have been carried out in the second major D-T campaign (DTE2) at JET, and in its prior D campaigns, to integrate these ICRF scenarios in JET high-performance plasmas and to compare their performance with the commonly used hydrogen (H) minority heating. In 50:50 D:T plasmas, up to 35% and 5% larger fusion power and diamagnetic energy content, respectively, were obtained with second harmonic heating of T as compared to H minority heating at comparable total input powers and gas injection rates. The core ion temperature was up to 30% and 20% higher with second harmonic T and ^3 He minority heating, respectively, with respect to H minority heating. These are favourable results for the use of these scenarios in ITER and future fusion reactors. According to modelling, adding ICRF heating to neutral beam injection using D and T beams resulted in a 10%–20% increase of on-axis bulk ion heating in the D-T plasmas due to its localisation in the plasma core. Central power deposition was confirmed with the break-in-slope and fast Fourier transform analysis of ion and electron temperature in response to ICRF modulation. The tail temperature of fast ICRF-accelerated tritons, their enhancement of the fusion yield and time behaviour as measured by an upgraded magnetic proton recoil spectrometer and neutral particle analyser were found in agreement with theoretical predictions. No losses of ICRF-accelerated ions were observed by fast ion detectors, which was as expected given the high plasma density of n _e ≈ 7–8 × 10 ^19 m ^−3 in the main heating phase that limited the formation of ICRF-accelerated fast ion tails. ^3 He was introduced in the machine by ^3 He gas injection, and the ^3 He concentration was measured by a high-resolution optical penning gauge in the sub-divertor region. The DTE2 experiments with ^3 He minority heating were carried with a low ^3 He concentration in the range of 2%–4% given the fact that the highest neutron rates with ^3 He minority heating in D plasmas were obtained at low ^3 He concentrations of ∼2%, which also coincided with the highest plasma diamagnetic energy content. In addition to ^3 He introduced by ^3 He gas injection, an intrinsic concentration of ^3 He of the order of 0.2%–0.4% was measured in D-T plasmas before ^3 He was introduced in the device, which is attributed to the radioactive decay of tritium to ^3 He. According to modelling, even such low intrinsic concentrations of ^3 He lead to significant changes in ICRF power partitioning during second harmonic heating of T due to absorption of up to 30% of the wave power by ^3 He.