Modelling Parallel-Connected, No-Insulation High-Tc Superconducting Magnets

The charging/discharging delays in superconducting coils wound without insulation (NI coils) are a major drawback of the technique. While removing the insulation improves safety margins, the increase in the characteristic time constant tau(c) can make a coil unfit for a particular purpose. It is widely accepted for instance that NI coils will not be used in ac applications where tau(c) similar to 1/f. To decrease tau(c) of the NI coils, the same length of superconductor can be wound/connected in parallel rather than in series-decreasing the inductance L, and hence the time constant tc, while maintaining the number of amp-turns IopN. Here we investigate the effect of parallel connecting coils in a magnet using a 2D axially symmetric model which captures all the necessary electromagnetic properties of the HTS NI coils. These properties include: critical current anisotropy J(c)(B, theta), turn-to-turn conductivity, as well as winding parallelism. Our modeling results show that the parallel connected magnet experiences magnet-wide shielding current effects. Whilst these shielding currents affect field homogeneity - the model enables this effect to be quantified. Furthermore, shielding currents are not an issue when running NI coils in saturated mode. The modeling work presented here provides a simple initial example of how magnet designers may approach designing, optimizing, and operating high current, HTS NI coils.