Filled Thermoplastic Based Coating for Tailored Contact Resistance in HTS Coils

We present a systematic method to tune the turn-to-turn contact resistance$R_{\text{ct}}$in high-temperature superconducting (HTS) solenoids by applying a polyvinyl butyral (PVB) thermoplastic coating filled with silver-coated copper flakes (SCCF). The filler type and its volume fraction determines the accessible resistance range in the final coil.$R_{\text{ct}}$strongly affects the charging dynamics and quench resilience of non-insulated (NI) HTS magnets, hence it is important to be able to control this parameter in a reliable and reproducible way. The coating is applied prior to winding in a continuous dip coating process and results in a dry polymeric film on the tape. After winding, the coating layers are fused above$60\,^\circ \text{C}$and the final$R_{\text{ct}}$is adjusted through the temperature and the duration of the heat-treatment. This method gives access to a wide range of$R_{\text{ct}}$. Values from about 50 to$10^{6}\,μ \Omega \cdot \text{cm}^{2}$can be obtained with the right choice of filler type, concentration and heat-treatment conditions. An 80-turn demonstrator solenoid fabricated using this approach and tested at$\text{77}\,\text{K}$achieved the targeted$R_{\text{ct}}$of around$70\,μ \Omega \cdot \text{cm}^{2}$. A finite-element model, calibrated with the measured field-decay time, was used to extract the average turn-to-turn resistance and validate the tuning method.