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EPFL Applied Superconductivity Magnetic Field Mapping

 

Dynamic Field Mapping

Knowledge of the dynamic current distribution in the superconductor is a key element for understanding the conductor properties and for quality control purposes. It is also of great importance for the determination of the ac losses in high-temperature superconducting (HTS) tapes. By measuring the magnetic field just above the conductor surface and after performing suitable data processing, a 2D map of the current distribution within the conductor can be calculated.

  

Experimental set-up

For the data series acquisition, we have employed a high resolution 16-channel digital lock-in amplifier.

The individual Hall probes are positioned 600 μm apart along the sample width. In order to enhance the spatial resolution, a mobile Hall-probe array has been designed that can be mechanically moved laterally and longitudinally along the sample. The displacement is activated by step motors. The dynamic measurement is then repeated each time the sensors move by one step.

  

Reconstruction of the current distribution

In order to obtain the map of the current density corresponding to the measured field profile in the y-direction, a method based on Ampere’s theorem for calculating the current distribution in HTS tapes, using certain assumptions for the current density in the superconductor, has been especially developed for this purpose.

  

A result for applied sine current with amplitude of 70 A is given in the next figure for ωt = 2π . The plot gives a comparative example of the measured and calculated field profiles: the cross-symbols are the results obtained from the Hall-probe sensors and the solid line is the field corresponding to the reconstructed current distribution.

 

Next figures show the current distributions, calculated at wt = 3p/4, 3p/2, 7p/4, 5p/2, and 3p with the Finite Element Modeling software and the one obtained with the reconstruction method, based on the Hall probe measurements.

 

Bertrand Dutoit,   Last updated: september 2011,   © 2003-2011 EPFL-I&C-SCIICBD 1015 Lausanne