A megawatt class compact power combiner for solid-state amplifiers

Abstract

We propose a compact multiport two-stage combiner capable of handling peak power up to 10 MW at the UHF band and suitable for particle accelerator applications. The detailed electromagnetic and thermal simulations of the combiner operating at the ESS specifications of 400 kW at 352 MHz are presented. At the first stage, the power is combined to a 100 kW level by means of a non-resonant 12-way radial combiner, which is assumed to be fed by 8 kW solid-state amplifiers. At the second stage, a waveguide combiner with T-shape couplers separated by a half-wavelength of the fundamental waveguide mode is used in order to bring the combined power to the required level. The combiner is broadband and has a relative power non-uniformity less than 5% over a 10 MHz frequency band around the central frequency. The size of the proposed combiner is several times smaller than the existing ones. We also present low-power measurement results of a prototype of the radial combiner.

Keywords:

• solid-state amplifier
• broadband power combiner
• electric breakdown

Acknowledgements

The authors wish to thank J. Jacob (ESRF) for helpful discussions. The authors are also grateful to M. Langlois (ESRF) and E. Montesinos (CERN) for their useful comments, to M. Noor for the mechanical drawings and colleagues from the FREIA group for encouraging this work.

Notes

1 We performed simulations of such a combiner and found that the electric field strength for a 100 kW output signal is close to 3 MV/m, which is approximately equal to the electric breakdown threshold in a static electric field.[13] The breakdown threshold is somewhat higher at RF frequencies and depends on the surface quality [14–16] but we will consider the worst case scenario and hold 3 MV/m as a conservative estimate for the maximum allowed electric field strength.

2 The isolation is somewhat smaller for one of the input ports despite the complete symmetry. We attribute this artefact to a numerical error.

3 According to the mechanical drawing, the weight of the overall radial combiner is around 20 kg so that the heat capacity is high due to the mass and large surface of couplers along with good thermal connection of hot parts to the main body of the combiner.

4 Note that input coaxial lines can handle only a few MW without pressurization so that extra care is needed to go beyond a MW level.