First demonstration of microphonic control of a superconducting cavity with a fast piezoelectric tuner

Superconducting cavities exhibit a high susceptibility to mechanical vibrations due to their narrow bandwidth of operation. The resulting modulation of the resonance frequency (typical amplitudes are, in the absence of mechanical dampers, a few tens of Hz at a modulation frequency of up to a few hundred Hz) can exceed the cavity bandwidth leading to a perturbation of the amplitude and phase of the accelerating field, which can be controlled only at the expense of rf power. It is therefore highly desirable to control the resonance frequency of the cavity with a fast controller. A fast mechanical tuner based on piezoelectric or magnetostrictive actuator appears very attractive, since its tuning is done simply by a micrometric deformation of the resonator geometry. In the past these tuners have been limited by mechanical resonances in the transfer function to a modulation bandwidth of about 1 Hz. With modem control theory and high speed DSPs and FPGAs it is now possible to design complex controllers which allow high gain up to several hundred Hz. In this paper we present first results of fast microphonics piezoelectric control for a superconducting quarter wave resonator. Microphonics at 42 Hz (inner conductor) are controlled despite a large mechanical resonance a 662 Hz in the actuator transfer function.