Low level radio frequency controller for superconducting third harmonic cavity at SSRF

Abstract

BackgroundThe superconducting third harmonic cavity has been developed independently in Shanghai Synchrotron Radiation Facility (SSRF) and passed beam tests. The cavity electric field needs to be precisely controlled during operation to achieve the goal of stretching beam cluster and improving beam life.PurposeThis study aims to design a digital low level radio frequency (DLLRF) controller for superconducting third harmonic cavity at SSRF.MethodsThe hardware of controller was based on a field-programmable gate array (FPGA) board and a front-end board whilst in-phase/quadrature (I/Q) demodulation techniques were implemented in the software of controller. A synergistic strategy was adopted for driving the stepper motor with slow tuning speed and piezoelectric ceramic with fast tuning speed, and an algorithm for the quench detection for passive cavity. Finally, experimental tests were performed to verify the effectiveness of this designed DLLRF.ResultsWhen the state is in top-up mode over 120 mA, the amplitude stability has improved form ±5% with open loop to less than ±1% with close loop, the voltage of piezo has varies smoothly and stably within 120 V, and the beam life has improved more than doubled.ConclusionA digital low level radio frequency controller for the superconducting third harmonic cavity has been designed and satisfies the requirements for SSRF.

Keywords

• low level radio frequency
• third harmonic cavity
• amplitude control
• stability