Neutral beam injection for DEMO alternative scenarios

Within the current EU pre-conceptual design activities, the reference demonstrative fusion power plant concept relies on ITER-like, pulsed H-mode plasmas ("DEMO1"). Simultaneously, alternative design options are being considered. An example is "Flexi-DEMO", which is designed for steady-state plasma discharges, but can also be run pulsed at the same fusion power with the same auxiliary heating systems if the plasma confinement time turns out to be not sufficient for non-inductive operations. Preliminary investigations have started also for ELM-free regimes, such as I-mode or QH-mode, due to the issues encountered in ELMs active control at reactor scales. Neutral beam injection (NBI) is one of the candidates for a DEMO heating and current-drive system. Depending on the system design, NBI can be optimized for e.g. central heating (as for DEMO1), current-drive (as for Flexi-DEMO) or even to provide torque to the plasma (which may favour the access to ELM-free QH-mode regime). In the present contribution we describe the main NBI features in terms of design parameters (NBI energy, injection geometry) that would optimize these alternative DEMO plasma scenarios, in particular regarding Flexi-DEMO and QH-mode regime. Wide-range parametric scans through METIS numerical simulations of DEMO plasmas indicate the optimal design windows for NBI system in each plasma configuration. These results need however further validations against technical constraints, available technology limits and detailed plasma transport simulations.