Design and performance of the LLRF control system for CSNS linac

Abstract

The China Spallation Neutron Source (CSNS) linac is designed with beam energy of 80 MeV and a peak current of 15 mA in the first phase. It consists of RFQ, two bunchers of medium-energy beam transmission line, four DTL tanks and one debuncher of linac-to-ring beam transmission line. Correspondingly, eight online RF power sources are used to power these accelerators. In order to stabilize the amplitude, phase and resonant frequency of the RF accelerating field, and minimize beam loss, we have established digital low-level RF (LLRF) control system. The LLRF system includes RF reference line, analog module (AM), clock distribution module, digital control module (DCM), high-power protection module, timing and RF interlock module and so on. The DCM is mainly responsible for the stability of the RF field amplitude and phase, and RF interlock module can quickly cut off the RF drive in case of arc in the RF transmission system, VSWR over threshold or cavity vacuum fault and so on. During beam commissioning, all of eight online units of LLRF control system were operating stably and reliably. The amplitude and phase variations of the linac fields have been achieved about ± 0.4% and ± 0.15° with 10-mA beam loading, much better than the design requirements of ± 1% in amplitude and ± 1° in phase. With the help of this system, we achieved stable operation under different beam loads. Also, many important progresses have been achieved in the LLRF control system for a more convenient operation and a higher stability performance. This article describes the design and implementation of the LLRF for CSNS linac.