Archive | 2019
SRF Trip Caused by the Tuner in BEPCII
Abstract
The stability and reliability of the Superconducting RF system (SRF) is generally a key issue in a large scale accelerator such as Beijing Electron Positron Collider II (BEPCII). In the past several years, SRF is one of the main factors limiting the availability of BEPCII, and many efforts have been made to fix the SRF troubles. This paper focuses on the details of the SRF trip caused by the tuner, which is one of the most persistent troubles and figured out till the summer of 2018. INTRODUCTION With the advantages of higher accelerating gradient, larger beam pipe and lower RF power consumption, a Superconducting RF system (SRF) has been used in Beijing Electron Positron Collider II (BEPCII) and worked successfully for more than ten years [1]. Yet, the stability and reliability of SRF is not so high and the downtime caused by SRF is one of the main factors limiting the availability of BEPCII [2]. As an example, Figure 1 shows the downtime distribution of BEPCII in the run of collider mode from November 2017 to June 2018. It may be seen that about one-quarter of the downtime is due to the SRF trips [3]. Figure 1: Downtime distribution of BEPCII from November 2017 to June 2018. The SRF system of BEPCII is a very complicated system, and its trips may come from the beam loss, the failure of the high power RF parts, the outgassing and/or the arc of the SC cavities, the oscillation of the LLRF loops, the interlock of the cooling water and/or air, and so on. To analyse the SRF trips and then reduce the trip rate, the appropriate detection, record and even specialized experiments are needed. Three types of equipment are of equipment are routinely used to detect and record the SRF trips in BEPCII. One is the control system of the 250kW power transmitter. This system is well developed by THALES and may record most of the trip causes, especially those come from the high power parts, the outgassing and/or the arc of the SC cavities and the interlock of the cooling water and/or air. The second one is a 16-channel oscilloscope manufactured by YOKOKAWA. This oscilloscope may record at most 16 signals such as the beam current, the accelerating voltage, the loading angle, the forward and reflected RF powers, et al. And, with the trigger function, the oscilloscope can record and show graphically the micro-second scale variations of these signals. By observing the time sequence of the variations of the beam current and the accelerating voltage, it could be distinguished preliminarily which one is the cause of the beam trip, the SRF trouble or the beam loss. The third one is a bunch-by-bunch BPM system built by BI group. With this system, the cause of the beam trip can be clarified further. Many beam trips have been analysed using the second and third equipment, and the results agree well with each other. Among the SRF trips, one kind of trip caused by the tuner is the most persistent trouble, and the details will be discussed in the following sections. TUNER OF BEPCII SRF SYSTEM The tuning system is an essential part of SRF system to adjust the cavity resonant frequency to the RF frequency during beam operation so as to have a perfect transmission of RF power to the beam. It usually consists of a tuner and tuning loop. BEPCII adopts a KEKB-type tuner, stretching the cavity to adjust its resonant frequency. Figure 2 is the schematic diagram of the BEPCII tuner [4, 5]. Figure 2: The schematic diagram of the BEPCII tuner. In practice, the cavity is tuned on the basis of the measured loading angle L , between the klystron current and the cavity voltage. BES, 10.5% Infrastruc, 12.0% Linac, 1..3% SRF, 24.4% PS, 15.0% Control, 3.8% BI, 2.1% Magnet, 7.8% Crygenic, 20.0% Vacuum, 0.6% Operator, 0.9% Unknown, 1.5% ___________________________________________ * Work supported by Natural Science Foundation of China (11575216) † [email protected]. 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-WEPRB031