A modular and compact long pulse modulator based on the SML topology for the ESS Linac

The ESS Linac project, phase I, requires 12 long pulse klystron modulators with compact footprint, high pulse power and improved quality both on the output pulse waveform and on the input AC power line. Conventional long pulse modulators are typically based on HV pulse transformers and commonly exhibit poor efficiency, low power density, large footprint and cost, with still limited performance on pulse rise time, pulse flat-top accuracy and AC power line quality (flicker, current harmonic distortion, power factor). This paper presents the Stacked Multi-Level (SML) klystron modulator topology, a novel modular concept based on the association of several HV modules in series at their output, each formed by a high voltage and high-frequency transformer, a HV diode rectifier bridge and a low pass filter. Each HV module is fed from a low voltage power electronic inverter at ground potential since the transformer provides the required galvanic isolation between primary and secondary windings. This topology is believed to better suit the application and better satisfy ESS requirements, directly addressing the mentioned shortcomings of conventional topologies. The development and validation of this new concept has included the design and construction of a reduced scale prototype with the potential of delivering long (3.5 ms) and high quality pulses (0-99% rise time <120 μs and flat top ripple <0.15%) with pulse amplitudes up to 115 kV and pulse power up to 2 MW, while maintaining excellent AC grid power quality (low flicker operation < 0.2%, sinusoidal current absorption with total harmonic distortion < 3%, and unitary power factor). The paper describes the main features of the topology and the main design aspects, presenting results both from simulation models, including parasitic elements, and from an experimental setup