Core Technologies in HTS Coils for Particle-Therapy Accelerators: PRPD-Based Defect Diagnostics and Fast-Ramping Field Response

This paper proposes a non-destructive quality control method for High-Temperature Superconducting (HTS) coils based on Phase-Resolved Partial Discharge (PRPD) analysis. To ensure the safety of expensive REBCO tapes during high-voltage testing, stainless-steel (SS304) tapes with identical geometric dimensions ($0.32\rm{ mm}$thickness,$4\rm{ mm}$width) were utilized as electromagnetic surrogates. We investigated the PRPD signatures of various winding defects, such as protrusions and layer shifts, in pancake coil configurations. The results demonstrate that each defect type produces distinct “Phase-locked clusters,” providing a reliable diagnostic fingerprint for early-stage failure detection. Furthermore, this study evaluates the insulation performance of polytetrafluoroethylene (PTFE) spray coating as a method for achieving tailored insulation levels. Experimental results, including charging delay and field decay analysis, confirm that PTFE-coated coils exhibit dielectric stability comparable to conventional Kapton-insulated coils while offering superior flexibility in controlling the insulation grade. Given that medical particle accelerators operate under high-speed ramping ($L di/dt$), the localized field enhancement identified by PRPD serves as a critical early warning system to prevent quenches caused by mechanical stress or transient voltages. This study establishes a baseline for extending PRPD-based diagnostics to complex 3D HTS magnet systems, ensuring initial quality and long-term operational reliability.