An approach to characterize Lorentz force transfer function for superconducting cavities

Superconducting radio-frequency (SRF) cavities are preferred in many modern accelerator facilities to reduce the operation cost of the RF power systems. SC cavities are usually sensitive to small mechanical perturbations because of their narrow bandwidth. These perturbations such as microphonics and Lorentz force detuning (LFD) can significantly detune the SRF cavity and therefore not only deteriorate the cavity field stabilities but also increase the RF power requirements. Piezo-electric actuators are widely adopted to suppress the cavity detuning caused by LFD and microphonics. Many advanced detuning compensation strategies were proposed to suppress the microphonics and LFD in recent years. The dynamic models of the LFD and piezo actuator are essential in these activities. Generally, the models of LFD can be identified by modulating the cavity field with sinusoidal signals and then measuring the corresponding steady-state amplitude and phase response of the cavity. However, for some SRF cavities, the cavity field may becomes unstable because of the presence of the ponderomotive effects, leading to a unacceptable measurement result. We present a new approach to identify the dominant modes of the LFD model based on a grey-box models estimation method. The validity of the approach was demonstrated by comparing the model output and measurement results on SRF cavities of real accelerator facilities.