Modelling Parallel-Connected, No-Insulation High-${\rm T}_c$ Superconducting Magnets

Abstract

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 $\tau _c\sim 1/f$. 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 $\tau c$, while maintaining the number of amp-turns $I{op}N$. 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.