Roadmap on the Development of Klystron Modulators for ESS

The European Spallation Source (ESS) will require, by its completion, a pulsed Linac capable of delivering a proton beam with a peak power of 125MW, a pulse width of 2.86ms and a pulse repetition rate of 14Hz. The required klystrons will be fed by 33 high power long pulse modulators, each rated for 115kV/100A; 3.5ms/14Hz. Due to their high power levels, not only the quality of the pulses delivered is of great concern, but also the power quality on the AC grid represents a considerable challenge. Additional constraints like cost, footprint, efficiency, reliability/maintainability made it impossible to procure a solution already available in the market by the time of decision. This contribution is focused on the modulators development roadmap for ESS. It describes the obtained results and lessons learned from the first phase of the project, where two commercial solutions were procured and tested leading to unsatisfactory results. Concurrent to the commercial approach, ESS decided to launch an internal R&D project aiming at developing a new class of modulators, particularly suited for long pulse and high power applications. The related topology (SML: Stacked Multi-Level) is modular and based on several HV modules connected in series, using High Frequency Transformers fed from primary low voltage inverters. In order to cope with power quality requirements, Active Front Ends are used at the first stage of the capacitor chargers. A reduced scale prototype, capable of powering one 704MHz/1.6MW_pk klystron near to nominal, was built and successfully validated on a dummy load and on a klystron. The series production of the first batch of 12 full scale modulators, capable of feeding 4 klystrons in parallel, was outsourced on a built-to-print basis. The first full scale unit was recently tested at full power on a HV dummy load. Experimental results of both the prototype and full scale unit are presented and discussed.