Li Shisong, Schlamminger Stephan, Wang Qing
Department of Engineering, Durham University, Durham DH13JT, United Kingdom.
National Institute of Standards and Technology (NIST), Gaithersburg, 20899 MD, United States.
IEEE Trans Instrum Meas. 2020 Oct;69(10). doi: 10.1109/tim.2020.2981983.
Permanent magnets together with yokes to concentrate the magnetic flux into a cylindrical air-gap are widely employed in Kibble balances. These experiments require a uniform magnetic flux density along a vertical path, typically a substantial fraction of the length of the air-gap. Fringe fields that are present at both ends of the air-gap limit the region where the flux density does not change more than a certain relative fraction (here: 5 × 10) of the flux density in the center of the magnet system. By simply adding an iron ring with a rectangular cross-section to the inner yoke at each end of the air gap, the effects of the fringe fields can be counteracted, and, hence, the length of the region, where the flux density remains within a given tolerance band is increased. Compared to the alternative, employing a taller magnet, the proposed method yields a magnet system with an extended region of a uniform field without significantly increasing the mass of the magnet system. Potential applications include compact and table-top Kibble balances. We investigate possible adverse effects on the performance of the magnet system caused by the additional rings: magnetic field strength, coil-current effect, and a dependence of the radial field on the radial position in the field. No substantial disadvantage was found. Instead, the method presented here outperformed previously suggested methods to improve the radial dependence of the radial field, e.g., shorter outer yoke. In summary, adding rings to the inner yoke improves the uniformity of the field without a detrimental effect to function, cost, and form factor of the magnet system.
永久磁铁与轭铁配合使用，将磁通量集中到圆柱形气隙中，在基布尔天平中得到广泛应用。这些实验要求沿垂直路径有均匀的磁通密度，通常是气隙长度的很大一部分。气隙两端存在的边缘场限制了磁通密度变化不超过磁体系统中心磁通密度的某个相对分数（此处为5×10）的区域。通过在气隙两端的内轭铁上简单地添加一个具有矩形横截面的铁环，可以抵消边缘场的影响，因此，磁通密度保持在给定容差带内的区域长度增加。与采用更高磁体的替代方案相比，所提出的方法产生了一个具有扩展均匀场区域的磁体系统，而不会显著增加磁体系统的质量。潜在应用包括紧凑型和台式基布尔天平。我们研究了附加环对磁体系统性能可能产生的不利影响：磁场强度、线圈电流效应以及径向场对场中径向位置的依赖性。未发现实质性缺点。相反，此处提出的方法在改善径向场的径向依赖性方面优于先前提出的方法，例如更短的外轭铁。总之，在内轭铁上添加环可提高场的均匀性，而不会对磁体系统的功能、成本和外形尺寸产生不利影响。
