Prototype of Wire Scanner System for Beam Profile Measurement for Free Electron Laser

Beam profile measurement via wire scanner is a semi-invasive method, which has advantage of less beam influence compared with a profile target, and is widely used in particle accelerator all over the world. Wire scanner is adopted to be applied to Shanghai High Repetition Rate X-ray Free Electron Laser aNd Extreme Light Facility (SHINE). For this purpose, a prototype of wire scanner composed of a wire target, a set of mechanical mechanism, a beam loss monitor and a set of electronics was designed and developed. A tungsten wire with an outer diameter of 20 μm was used as the wire target, and the safety of this tungsten wire was proved by thermal simulation. The beam loss monitor in the wire scanner prototype consists of a quartz fiber that generates Cherenkov light, and a photomultiplier tube. The data acquisition module was developed based on the ADQ14AC-4C data acquisition platform from Teledyne SP Devices, which is equipped with ADC chip with nominal bit of 14 bit, effective bit of about 9 bit, and analog 3 dB bandwidth of 1.2 GHz. The principle and specific design of the prototype were introduced in this paper. The optimization scheme of installing a cavity beam position monitor (CBPM) upstream of the wire target to accurately compensate the beam center position and the beam charge was also mentioned. In order to accurately evaluate performance of the prototype, we measured the beam profile simultaneously by arranging wire scanner system and a YAG target adjacent to each other in Shanghai Soft X-ray Free Electron Laser (SXFEL). The comparison of the experimental results show that the wire scanner can be used for the online semi-invasive beam profile measurement of the 0.84 GeV electron beam with a bunch charge of 500 pC. The resolustion of the prototype wire scanner is better than 30 μm, and especially less than 10 μm in the direction of low beam jitter. When the beam position changes greatly, the measurement accuracy of the prototype wire scanner is sharply improved by the CBPM system. Furthermore, the relative charge quantity calculated by the CBPM reference cavity signal is used to compensate the beam loss signal, and the measurement result of the prototype wire scanner is slightly improved. The results of the profile target agree with those of the prototype wire scanner well which verify the function of the device applied to SXFEL. The prototype of wire scanner system has laid a good technical foundation for SHINE construction, and at the same time, wire scanner system with CBPM can be used as a supplement for beam profile measurement in SXFEL.