Feasibility Study for High Power RF – Energy Recovery in Particle Accelerators

When dealing with particle accelerators, especially in systems with travelling wave structures and low beam loading, a substantial amount of RF power is dissipated in 50Ω termination loads. For the Super Proton Synchrotron (SPS) at Cern this is 69 % of the incident RF power or about 1 MW. Different ideas, making use of that otherwise dissipated power, are presented and their feasibility is reviewed. The most feasible one, utilizing an array of semiconductor based RF/DC modules, is used to create a design concept for energy recovery in the SPS. The modules are required to operate at high power, high efficiency and with low harmonic radiation. Besides the actual RF rectifier, they contain additional components to ensure a graceful degradation of the overall system. Different rectifier architectures and semiconductor devices are compared and the most suitable ones are chosen. Two prototype devices were built and operated with up to 400 W of pulsed RF power. Broadband measurements – capturing all harmonics up to 1 GHz – were done in the time domain with a fast sampling oscilloscope. The data was processed by a Matlab script, which allowed to extract the efficiency, the reflected power spectrum and the complex reflection coefficient, all dependent on input power level. Systematic errors were compensated by an open-, short-, load- calibration and the reflection coefficient was plotted in a smith chart, making the measurement set up equivalent to a single port, large signal vector network analyzer, operating with high power RF pulses. The RF/DC converter prototype with two Schottky diodes reached 88.7% efficiency at an input power level of 284 W.