Phase Control of RF Cavities
Particle accelerators use superconducting radio frequency (RF) cavities that create extremely large electromagnetic fields to accelerate charged particles. The latest accelerators require an unprecedented level of precision in terms of particle energy, which translates into accelerating field amplitude and phase within error bounds of 0.01 % and 0.01°, respectively. To save money, it is possible to split the output of one high power controlled RF source to multiple cavities. However, in practice, all cavities are slightly different and experience different disturbances in operation. Because of an inability to quickly modulate the phase and amplitude of the individual split high power RF signals, the fields of an entire multi-cavity system are averaged and treated as one entity on which feedback control is performed at the low power input to the high power RF amplifier. The issue is compounded by the severe electrical loading that the RF cavity experiences during operation. Radiation pressure causes Lorentz force detuning, which shifts each cavity’s resonance peak, amplitude, and phase of its accelerating field in a unique way. Piezo tuners have been used to counteract Lorentz force detuning of individual cavities. This paper studies RF cavity phase control via piezo tuners. The controller designed is capable of quickly modifying the natural frequency of a cavity as a tool for modulating the phase of an RF signal. The approach is validated in hardware with a Niobium coated single-cell copper TESLA-type RF cavity.
KeywordsParticle accelerator RF resonator Resonance control Lorenz force detuning Phase shifting
The authors would like to thank Tsuyoshi Tajima for lending the RF cavity and Mark Prokop for lending the RF measurement equipment used for this study. We would also like to thank Lawrence Castellano and Phil Torrez for their help in setting up the RF experiment.
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