Preliminary Experimental Investigation of Graded Insulation on the Performance of HTS Pancake Coils

This work investigates a preliminary implementation of the ‘graded approach’ to the transverse resistance in HTS pancake coils. This method aims to find an optimal winding approach by combining key advantages of the no-insulation (NI) and metal-insulation (MI) coils in terms of operation stability and dynamic response, through a sectional variation of the insulation level. Unlike previously reported Partial Insulation (PI) approaches, this method does not rely on a regular insulation pattern throughout the entire winding but rather allows for targeted insulation placement based on qualitative criteria. Three pancake coils were realized and tested: an NI coil, an MI coil, and a graded-insulation (GI) coil with sectionally varied stainless steel tape layers to modulate transverse resistance. All coils were tested under the same conditions in liquid nitrogen, undergoing variable current ramps and fast discharges. Multiple voltage taps and a Hall sensor were implemented in each coil to monitor their respective responses. Additionally, electrical simulations were used to model the current distribution across the coil turns. While this study does not include quench or stability analyses, it provides a first experimental insight into the graded concept, offering a potential foundation for future developments in coil optimization. The distinctive features of each coil configuration are analyzed through a comparison of the model and test results, highlighting qualitatively the possible benefits of the graded approach in specific charging and discharging scenarios.