Development of a long pulse high power klystron modulator for the ESS linac based on the Stack Multi-Level topology

A novel Stacked Multi-Level (SML) modulator topology optimized for long pulse and high average power applications has been developed at ESS. It utilizes six identical modules connected in series at the HV output side and fed in parallel from the low voltage side. Each one is formed by one HF inverter, one step-up transformer, one HV rectifier bridge and one HV passive filter. They are supplied, in groups of two, from three capacitor banks which in turn will be charged from the low voltage electrical grid by using three groups of active AC/DC and DC/DC converters. Industrial standard power electronic components are used at the primary stage, which are placed in conventional electrical cabinets. Only few special components (transformers, rectifiers, filters) are required to be placed in an oil tank. A technology demonstrator rated for 115kV/20A and 3.5ms/14Hz is at the final phase of construction. The main power conversion circuit and regulation principles will be described and details on the design and construction of the main subsystems will be given. Simulation and experimental results will be given showing the achieved performance in terms of HV pulse quality and AC grid power quality.