Modelling of charge-exchange induced NBI losses in the COMPASS Upgrade tokamak

The COMPASS Upgrade tokamak [1] will be a tokamak of major radius R0 = 0.894m with the possibility to reach high field (Bt ~ 5 T) and high current (Ip ~ 2 MA). The machine should see its first plasma in 2023 and H-mode plasma will be obtained from 2025. The main auxiliary heating system used to access H-mode will be 4MW of Neutral Beam Injection (NBI) power. The NBI will have a nominal injection energy of 80 keV, a maximum injection radius Rtan = 0.65m and will create a population of well-confined energetic D ions. In this contribution, our modelling studies the NBI deposition and losses when a significant edge background density of neutrals is assumed. We follow the fast particles in the 3D field generated by the 16 toroidal field (TF) coils using the upgraded EBdyna orbit solver. We have implemented a Coulomb collision operator similar to that of NUBEAM and a charge-exchange operator that follows neutrals and allows for multiple re-ionizations. Detailed integrated modelling with the METIS code yields the pressure and current profiles for various sets of achievable engineering parameters. The FIESTA code calculates the equilibrium and a Biot-Savart solver is used to calculate the intensity of the perturbation induced by the TF coils. Initial distributions of the NBI born fast ions are obtained from the newly developed NUR code, based on [S. Suzuki et al. 1998 Plasma Phys. Control. Fusion 40 2097]. We evolve the NBI ions during the complete thermalization process and we calculate the amount of NBI ions loss in the edge region due to neutralizations. Results indicate the NBI losses for various injection geometries, various engineering parameters and various assumptions on the magnitude of the background neutral density. [1] R. Panek et al. Fusion Engineering and Design 123 (2017) 11–1