Development of Fast High-Power Rf Vector Modulator Employing Tem Ferrite Phase Shifters

Recent charged-particle accelerator projects employ high power RF distribution systems to power superconducting radio frequency (SRF) accelerating cavities. Each cavity usually requires several hundred killowatts of pulsed power at relatively high duty cycle (roughly up to 10% in high-power proton linear accelerators) in low UHF bands for production of the particle beam. So far, one amplifier to one cavity configuration is common, since precise vector control of RF is done at the low power inputs of the amplifiers. If a fan-out configuration that feeds many cavities with a single very high-power amplifier klystron is realized, significant cost savings can be achieved in construction and installation. The fan-out configuration, however, requires independent control of RF amplitudes and phases to the cavities at high power level. A prototype high-power RF vector modulator for such applications has been built and tested. The vector modulator employs a quadrature hybrid and two fast ferrite phase shifters in square coaxial transverse electromagnetic (TEM) transmission lines. The square coaxial format can provide the power-handling capability and thermal stability. RF properties of the design and results of high power system testing of the design are presented.

I. Introduction

A RF vector modulator delivers independent control of amplitude and phase of RF at the output. Fanning out one high-power amplifier output to many load cavities with independent amplitude and phase control may save construction costs of RF systems in large scale charged particle accelerators by 20–30 %. The feasibility of fast high-power RF vector modulators have been investigated and TEM mode designs recently been demonstrated at various power levels [1]. Other high-power vector modulator designs have also been proposed and tested [2], one of them employing two reflective-type phase shifters mounted to a WR-2300 waveguide [3]. Figure 1 shows the possible architectures of the vector modulators: consisting of two hybrids (either 0 and 180 degree hybrids or two quadrature hybrids) and a pair of phase shifters to form a transmissive type; and a hybrid (either with a quadrature or a 180-deg hybrid) and two short-circuited phase shifters to form a reflective type. Placing an image plane at the center of the phase shifters delivers the reflective type. Fig. 1 Construction of vector modulators. (a) Transmissive type with two hybrids; imposing an image plane at the center delivers (b) Reflective type either with a quadrature hybrid or a 180-deg hybrid.