Vacuum Pressure Impregnation Process in Superconducting Coils: Best Practice

The technology associated with the design and construction of superconducting magnets has evolved dramatically over the last three decades leading to an overall increase in their complexity. Their use has moved from particle physics through medical applications such as MRI to the very large magnets demanded by fusion technology, where the optimal quality of the HV insulation is a critical step in achieving the design performance. The Vacuum Pressure Impregnation (VPI) technique-a particularly refined case of the more widely known Vacuum Assisted Resin Transfer Mould (VARTM)-has become the most common process for the consolidating electrical insulation of large superconducting magnets. To achieve success with the VPI process demands that detailed attention is paid to many steps, namely: coil drying, resin degassing and coil impregnation. Commercial sensitivity has meant that there has not been a well-published exchange of information and therefore many details of the VPI process may vary in different organizations. The present paper aims at filling this gap and will discuss in detail: (1) all required steps with the main risks in each of them, (2) commonly used methods in each of the steps for optimized control of the process and (3) tooling to minimize risk of failure during impregnation.