Sustained high fusion power production with the hybrid scenario in the recent JET D-T campaign

In the recent JET D-T experiments, ‘hybrid’ scenario plasmas with I_p=2.3MA and a current overshoot, B_t=3.45T, β_p>1, q₀≥1 and n_D~n_T fuel mixture delivered sustained high D-T fusion power, with a time average over 5sec up to ~8.3MW. The fusion power and gain achieved with the ‘hybrid’ scenario, for the first time operated in D-T, are higher with respect to those obtained in steady ELMy H-mode plasmas in the previous D-T campaign in 1997, and now with the metal ITER-like wall. Very high fusion power >10MW could be maintained for periods longer than 3 α-particle slowing-down times.

To prepare for the D-T experiments, the ‘hybrid’ scenario at JET was further developed in D in recent years, culminating in the successful optimization of the scenario for high performance. Notably, a reduction in the W content due to temperature gradient screening was achieved, as high T_i and v_φ at the pedestal top led to enhanced outward neoclassical convection in the low collisionality regime of these plasmas. Deleterious MHD activity was avoided. The follow-up experiments in T provided experience in navigating the isotope effects and their impact on plasma performance. As example, both in T and D-T, the current ramp-up phase density had to be tailored to counteract the increased core impurity radiation and to recover the desired q-profile. The access to H-mode had to be re-optimised, by fine-tuning the heating and fuelling trajectories.

Throughout, extensive modelling of these isotope and impurity transport effects, both interpretative and predictive, accompanied the experiments and contributed to the successful results. In turn, the high performance and confinement D-T pulses provide valuable data for model validation and extrapolation to future devices.

In this contribution, the scenario challenges and the strategies used to deal with the implications of increasing fuel mass, both beneficial and adverse, will be reviewed, focussing on the lessons learned for future D-T operation.