Frequency shift observer for an SNS superconducting RF cavity

In contrast to a normal conducting RF cavity, a superconducting RF cavity is very susceptible to shifts in its resonance frequency. The main sources of the shift are Lorentz force detuning and microphonics. In a spallation neutron source, to compensate for the frequency shift, a feedforward control is to be applied. In this paper, as an initiative step, a frequency shift observer is proposed which is simple enough to be implemented with a digital signal processor in real time. Simulation results of the proposed frequency shift observer show reliable performance and acceptable computation time for the real time implementation.

I. Introduction

The spallation neutron source (SNS) linac to be built at Oak Ridge National Laboratory (ORNL), TN, consists of a combination of low energy normal conducting (NC) accelerating structures as well as higher energy superconducting RF (SRF) structures. In order to provide the stable cavity field so that the beam obtains the full power from the input RF power, the RF control systems for both the NC and SRF portions of the linac are designed to maintain the cavity field at a specific amplitude and phase.