Modelling parallel-connected, no-insulation high-$T_c$ 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 $\tau _c\sim 1/f$ . 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.