Impurity study to assess N = 1D ICRF heating scenario in T-rich plasmas with D-beams during JET-ILW DT experimental campaigns

A JET D-T scenario with optimised non-thermal fusion has been developed and successfully demonstrated during the second JET deuterium–tritium campaign (DTE2) at JET-ILW [1]. By using fundamental deuterium (D) minority ICRF heating scenario (N = 1 D) and pure D-NBI heating in T-rich plasmas, an energy world record for the most fusion energy produced in a single fusion shot has been achieved, by generating 69 MJ of heat from fusion reactions. During scenario optimisation, a systematic study of impurity behaviour has been performed. The impurities were analysed using various diagnostic techniques, e.g. VUV, VIS spectroscopy, and bolometry. The influence of different factors, including the plasma parameters and the heating methods has been investigated. The high-power T-rich hybrid pulses with different D gas puff rates in the main heating phase were compared to determine the influence of the D/T ratio on impurity behaviour. It was found that the intrinsic impurity content and the radiated power increase with the D/T ratio. Additionally, discharges in T-rich plasmas were compared with hybrid-like pulses with 50/50 D/T plasma mixtures and different ICRF heating schemes. It was found that T-rich plasmas, in which the highest fusion power was achieved are characterised by the lowest impurity content and highest ELM frequency (f_ELM ∼ 80–100 Hz), which increases with the fuelling rate. In some cases, the duration of the high-performance phase in T-rich plasmas was limited by impurity accumulation in the plasma core. Experimental and theoretical investigation showed that the high-Z impurity accumulation is related to the impurity transport inside the plasma and not related to the impurity source.