High-power magnetron transmitter as an RF source for superconducting linear accelerators

A concept of a high-power transmitter utilizing the Continuous Wave (CW) magnetrons, injection-locked by phase-modulated signals, and intended to operate within a wideband control feedback loop in phase and amplitude, is presented. This transmitter is proposed to drive Superconducting RF (SRF) cavities for intensity-frontier GeV-scale proton/ion linacs, such as the projected Fermilab proton linacs or linacs for Accelerator Driven System (ADS). The transmitter consists of two 2-cascade injection-locked magnetrons with outputs combined by a 3-dB hybrid. The transmitter performance was modelled using CW, S-Band, 1kW magnetrons. A wideband dynamic control of magnetrons, required for the superconducting linacs, was realized using the magnetrons, injection-locked by the phase-modulated signals. The capabilities of the magnetrons injection-locked by the phase-modulated signals and adequateness for feeding of SRF cavities have been verified by measurements of the magnetrons phase performance, by measurements of the transfer function magnitude characteristics of single and 2-cascade magnetrons in the phase modulation domain, and by measurements of spectra of carrier frequency of the magnetron. At the ratio of power of locking signal to output power of ≥−13dB (in 2-cascade scheme per magnetron) a phase modulation bandwidth is over 1.0MHz for injection-locked CW single magnetrons and a 2-cascade setup. The carrier frequency spectra (width of ~1Hz at the level of −60dBc) measured with the magnetron, injection-locked by a phase-modulated signal, did not demonstrate broadening at wide range of magnitude and frequency of the phase modulation. The wideband dynamic management of output power of the transmitter model has been first experimentally demonstrated using combined in power magnetrons, injection-locked by the phase-modulated signals. Experiments with the injection-locked magnetrons adequately emulated the wideband dynamic control with a feedback control system, which will allow to suppress all known parasitic modulation of the accelerating field in the SRF cavities. The magnetron transmitter concept, tests of the transmitter models and injection-locking of magnetrons by phase-modulated signals are discussed in this work.