上海硬X射线自由电子激光装置（SHINE）的一台磁体长度为4 m、周期长度16 mm、磁气隙为5 mm的超导波荡器样机目前已完成集成和测试。本文针对这种超小间隙的超导波荡器，提出了一种基于双霍尔探头的磁场点测量方法，描述了其测量原理，并对样机做了磁场测量和优化实验。通过分段调节其2 m段超导磁体的激磁电流对磁场进行优化，使该段在100 A激磁电流下的相位误差小于5°满足设计要求。实验结果表明了本文提出的磁场测量和优化方法是有效的。

A superconducting undulator (SCU) prototype with a magnet length of 4m is being developed for the SHINE (Shanghai High Repetition Rate XFEL and Extreme Light Facility) project. The period length of the undulator is 16mm, the period number is 250, and the pole gap is 5mm. A point measurement system with three Hall probes will be used to measure the magnetic fields in the gap. The Hall probes fixed on a sledge will experience temperatures of about 4.2K to 50K when they move through the undulator gap. A temperature-dependent calibration system for these hall probes is necessary. The paper introduces the configuration of the cryogenic calibration system, and the result for the sensitivity and nonlinearity of one Hall probe from 2.8K to 300K over a magnetic field range of ±1.9T.

BackgroundIn the Shanghai High Repetition rate XFEL (X-ray free electron laser) and Extreme Light (SHINE) facility, the vertical linear polarization laser is generated by using 40 planar superconducting undulators (SCUs) with a period length of 16 mm, length of 4 m, and a gap of 4 mm. At present, the Hall probes are the most reliable method for measuring the undulator magnetic field whilst the positioning accuracy of the sensitive center of the Hall probe is one of the main factors affecting the accuracy of magnetic field measurement.PurposeThis study aims to calibrate the position of the Hall probes' sensitive region for magnetic field measurements of SCU with high-precision.MethodsThe experimental platform for magnetic field point measurement of SCUs was introduced in details, a sledge with three mounted Hall probes and a retro-reflector were applied for magnetic field measurement. By flipping the sledge, the lateral distance between the sensitive centers of the Hall probe and each other were obtained, so did the lateral distance between the sensitive centers of the Hall probe and the apex of the pyramid prism. Therefore, the position of the Hall probes' sensitive region and center of the retro-reflector were calibrated.Results & ConclusionsThe positional calibration of the Hall probes has an accuracy higher than ±10 μm, which meets the requirements for magnetic field measurement.

MAX phase materials have shown a series of interesting, even sometimes unusual, properties and exhibited combined attributes of both metals and ceramics, which are due to their “layered ternary transition metal carbides” structures. In the process of studying the multistage phase transformation of 312-MAX phase materials, we found that there were some confusions and mistakes when describing the crystal structures of the β-phase in the existing literature. In order to clarify their structural and physical properties, the β-phase Ti3SiC2 materials have been prepared by using 500 keV He2+-ion irradiation-induced phase transformation and examined by first-principles calculations and Rietveld analysis of grazing incidence x-ray diffraction patterns. Two accurate descriptions of the β-Ti3SiC2 structure are given here. In order to avoid confusion again, it is recommended to use one of the two descriptions uniformly. In addition, some physical properties parameters of β-Ti3SiC2 have been calculated and compared, which also confirmed the correctness of the structure description. Finally, the vacancy formation energies of β-Ti3SiC2 have been predicted and discussed in detail from the points of phase stability and transformation for the first time.