Particle and thermal transport in JET Helium and Hydrogen-Helium H-mode plasmas

• Thermal and Helium (He) particle confinement in JET He and H-He H-mode discharges are analysed. • Empirical thermal and He particle Bohm-gyroBohm models validated in D plasmas has been recalibrated for He plasmas. • Helium particle confinement strongly improves with toroidal magnetic field B tor , increasing the risk of the He ash accumulation in future power plants operating at high B tor . • Based on the validated models the He ash accumulation in burning plasmas in the future devices (ARC and EU-DEMO) is estimated. Fusion performance in a tokamak-reactor strongly depends on the confinement of thermalised α-particles (Helium (He) ash) in the core plasma region. Consequently, the development of He particle transport models and their validation in present experiments is an important step towards a more accurate prediction of fusion power production in future devices. In the absence of a computationally fast well-validated theory-based transport models for He, the empirical Bohm-gyroBohm (BgB) model is tested here for the first time to our knowledge in the predictive self-consistent temperature and density simulations of JET H-mode He and Hydrogen (H) - He discharges. The thermal confinement in JET He plasmas is found to be well below the Deuterium (D) BgB model reference – this result is qualitatively consistent with the observation of reduced global thermal confinement in He discharges observed on ASDEX Upgrade, Cmod, DIII-D and EAST tokamaks compared to the confinement of D plasmas. The “Helium” version of the BgB model including the re-calibrated BgB thermal diffusivity and the He particle diffusion coefficient defined as a fixed fraction of the thermal electron diffusivity is proposed here. This model is validated in the JET discharges performed at different toroidal magnetic fields, plasma densities, wall materials (Carbon and ITER-like wall) and main ion compositions. Strong reduction of He particle transport with the increase of magnetic field has been found in JET discharges. However, the simulations of the He ash accumulation in the future high-field tokamak-reactor ARC with the model validated in JET predict a tolerable amount of He content in the burn phase in the investigated parameter space, with a weak impact on the fusion power production. Similar conclusion has been drawn for the H-mode EU-DEMO scenario by extrapolating the JET He particle transport model to this device.