Heating and Current Drive in STEP: Neutral Beam Injection
STEP (Spherical Tokamak for Energy Production) is the UK’s reactor design programme aiming to demonstrate net electrical output from a spherical tokamak. STEP requires completely non-inductive current drive for the flat-top and the majority of the ramp-up/down phases. The majority of the current (~80%) is generated by a high bootstrap fraction with the remainder provided by the heating and current drive (HCD) system. The capabilities and limitations of Neutral Beam Injection (NBI) for current drive in relevant STEP scenarios are presented, as well as the additional integration, technology readiness level, maintainability, and financial considerations that determine the optimal HCD system for a reactor class tokamak. It is demonstrated that, in isolation, NBI has excellent current drive efficiency capable of actuating over the entire plasma cross section. However, once considered in an integrated design, the poor wall-plug efficiency, large size and consequent high cost makes NBI undesirable in STEP compared to microwave based HCD.