Eddy Current Quality Control of Soldering in Superconducting Current-Carrying Joints with Allowance for the Effect of Cross Section Variations on Testing Results

Abstract

We propose a dual-frequency eddy current procedure for quality control of soldering in superconducting current-carrying joints of electromagnets. The advantage of the procedure is that the quality control result is independent of inevitable manufacture- and soldering-induced variations in the cross section of the joint being tested. The measurements are taken in two stages with an eddy current device using a transformer-type probe with a U-shaped core. At the first stage, measurements are taken at a high frequency, with the eddy current device readings depending only on the cross section of the joint but not on the degree of soldering. At the second stage, measurements are taken at a low (main) frequency, with the eddy current device readings depending on both the soldering of superconducting cables with each other and the outer section of the copper stabilizer. The degree of soldering in the tested joint is evaluated taking its cross-sectional area into account, based on the results of measurements at the two frequencies. Prior to testing, the eddy current device is pre-configured using three sample joints: two 100%-soldered samples of the same thickness but different heights and one sample with 0% degree of soldering.