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

Abstract

A concept of a high-power magnetron transmitter utilizing the vector addition of signals of 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, including linacs for Accelerator Driven System (ADS). The transmitter performance was verified in experiments with CW, S-Band, 1 kW magnetrons. A wideband dynamic control of magnetrons, required for the superconducting linacs, was realized using the magnetrons, injectionlocked 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 transfer function magnitude characteristics of single and 2-cascade magnetrons in the phase modulation domain, by measurements of the magnetrons phase performance and by measurements of spectra of the carrier frequency of the magnetrons. At the ratio of power of locking signal to output power of ≥ -13 dB (in 2-cascade scheme per magnetron, respectively) we demonstrated a phase modulation bandwidth of over 1.0 MHz for injection-locked CW single magnetrons and a 2-cascade setup, respectively. The carrier frequency spectrum (width of ~ 1 Hz at the level of <-60 dBc) of 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 control of output power of the transmitter model has been first experimentally demonstrated using two CW magnetrons, combined in power and injection-locked by the phasemodulated signals. The experiments with the injection-locked magnetrons adequately emulated the wideband dynamic control with a feedback control system, which will allow to suppress parasitic modulation of the accelerating field in the SRF cavities, resulted from mechanical noises, phase perturbations, caused by cavity beam loading and cavity dynamic tuning errors, low-frequency ripples of the magnetron power supplies, etc. The magnetron transmitter concept, tests of the transmitter models and injection-locking of magnetrons by phase-modulated signals are discussed in this work.

An efficient magnetron transmitter for superconducting accelerators

“…Phase modulation in injection-locked magnetron transmitters was studied in Ref. [4] using 2.45 GHz, 1 kW magnetrons operating with 5ms pulses. Both single-cascade and 2-cascade magnetron were examined using the setup shown schematically in Fig 1. The 2-cascade configuration provides the same bandwidth of the phase control at lower power of the injection-locking signal.…”

“…The first method of the fast power control, [4], was realized in a two-channel transmitter with a combining of the output signals by a 3-dB hybrid combiner. The method allows any value of Q L .…”

Resonant interaction of the electron beam with a synchronous wave in controlled magnetrons for high-current superconducting accelerators

“…The wideband phase control required for an SRF cavity powered by a magnetron rf source was first studied in detail using the phase modulation technique with various configurations of transmitters in the pulsed regime with CW, 2.45 GHz magnetrons [13]. A pulsed modulator could power the two magnetrons; it used a partial discharge of a storage capacitor providing pulse duration of 5 ms, with voltage droop of about 0.4% and a negligible ripple.…”

“…Both magnetrons were fed by the same modulator. Experiments demonstrated operation of the 2-cascade magnetron, injection locked at the average of the offset frequencies, at attenuator values in the range of 9-20 dB [13].…”

“…9, were measured in the phase modulation domain by the Agilent MXAN9020A signal analyzer at various power levels of the injection-locking signal. The measurements have been performed at a magnitude of the phase modulation of 0.07 rad., and at a magnetron output power of P Out ≈ 450 W [13]. The measurements with the 2-cascade magnetron were carried out at an attenuator value of ≈13 dB, i.e., the second cascade (tube) operated at P Lock ≈ 25 W. In Figs.…”