Studies on Alternative Schemes for the MMC-Based Acceleration Grid Power Supply of DEMO Neutral Beam Injector

The European DEMO will make use of a significant additional heating power, which could be partly provided by neutral beam injectors (NBIs), which accelerate negative ions by means of grids placed at increasing potentials. These grids will be fed by the acceleration grid power supply (AGPS), divided in a number of stages, which has to provide an overall dc voltage down to -1 MV and currents in the order of tens of Amperes. The AGPS will have to satisfy a set of static and dynamic requirements, mainly in terms of ripple, accuracy, and rise time. In addition, during the NBI operation, frequent losses of insulation between the grids, called breakdowns (BDs), are expected. The AGPS will be able to handle such events by nullifying the output currents as fast as possible in order to limit the energy discharged onto the grids. Adopting the modular multilevel converter (MMC) technology for the AGPS of DEMO and future tokamaks seems promising, due to its intrinsic properties of modularity, high efficiency, fast dynamic response and small energy transferred to the arc in case of BD. Since the converter will be air-insulated, one of the main drawbacks is the large volume occupied. This can be partly reduced by adopting alternative MMC schemes, to minimize the number of components and optimize the counter-voltage applied by the converter at BD. In this article, alternative topologies for the MMC submodules (SMs) or combinations of different schemes [full-bridge (FB), half-bridge (HB)] are investigated. After a preliminary design of the converter, the results of numerical simulations carried out with circuit models are shown, with control schemes customized for the NBI operation. The performance of the different solutions in steady-state, dynamic and anomalous conditions are discussed, with particular focus on BD events. Finally, thermal analyses on the converter are carried out, to verify whether natural convection of air can he a suitable cooling method for the power components.