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Improved understanding of Kibble balance magnets

The BIPM Kibble balance team reports, in two recently published papers in Metrologia[1, 2], a detailed study of an effect that is relevant for Kibble balances: the effect of the coil-current on the magnetic field produced by the permanent magnet. The exact size of the effect depends on the specific geometry and dimensions of each magnetic circuit, but in general has to be taken into account at the present level of uncertainty of Kibble balances.

At present, all Kibble balances use permanent magnets to produce the magnetic field. The BIPM-type construction has been widely adopted in other Kibble balance experiments (Figure 1). A basic assumption is that the magnetic field is the same for the weighing and the velocity measurements. In the first paper[1], it had been shown that the magnetic profile can be significantly affected by the coil-current in the weighing phase. The magnetic flux generated by the coil passes through the magnet and produces an additional contribution to the magnetic field at the coil position. This mechanism leads to a significant slope of the vertical magnetic field profile, which had been demonstrated by an analytical model and an experimental study. A correction due to the current asymmetry and the vertical position change between the two steps of the weighing phase needs, therefore, to be carefully considered.

The linear coil-current effect has been further investigated in the new publication[2], particularly in an original one-mode Kibble balance where the current is present in both the weighing and the velocity phases. It has been found that the magnetic profile change determined by the velocity measurement is twice that deduced from the weighing measurement (Figure 2). Several experimental methods to determine the coil-current effect, in order to apply a correction, were validated. A compensation technique is also presented to minimize this effect in the BIPM-type magnet. Calculations based on finite element analysis showed that the technique can efficiently neutralize the effect and can be adopted in future Kibble balances.

These papers are the most significant contributions that the author Dr S. Li, a research fellow at the BIPM since September 2016, has made to the field. Dr S. Li recently won the ACES (Applied Computational Electromagnetics Society) Early Career Award for his earlier work on the modelling of the second order nonlinear coil-current effect.

  1. Li S., Bielsa F., Stock M., Kiss A., Fang H., A permanent magnet system for Kibble balances, Metrologia, 2017, 54, 775-783.
  2. Li S., Bielsa F., Stock M., Kiss A., Fang H., Coil-current effect in Kibble balances: analysis, measurement, and optimization, Metrologia, 2018, 55, 75-83.
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