Mitigation Plans for Insulation Failures and Arcing in the ITER Magnets

The ITER magnet system contains 50 GJ of energy in 3 main coil sets and feeders. Lesson learned from arc incidents in JT-60SA and CS module 3 cold test show the consequences of insulation failure. There are three major insulation challenges in ITER magnets: bulk insulation in the coil, ground insulation and high voltage wiring. The most issues are found at the vacuum interface with Paschen breakdown in poor vacuum leading to arcs from ground insulation and the risk of high voltage wire insulation cracking. High voltage arcing can potentially be a safety concern in ITER due to arcing to the vacuum vessel. The mitigation focuses in 3 main areas: Prediction: Simulation of arc development within the cryostat to assess potential damage and if necessary implementation of extra protection; Prevention: Minimisation of weak areas, notably high voltage (HV) wire exits from ground insulation. As a result of failure root cause analysis, chemical corrosion between coil resin and wire insulation is a concern being investigated, with if necessary quick “universal” solutions to repair or reinforce predicted weak areas. Vacuum monitoring in the cryostat is being improved to identify potential Paschen conditions in advance; Early Detection: Implementation of regular intermediate global electrical tests under controlled Paschen conditions at low temperature to detect breakdown with high sensitivity of leakage currents (∼10 µA) which may in turn require room temperature water cooled busbars to be easily isolated. At least initially, in-cryostat cameras or other spark detection methods are needed to localise failures and repair methods need to be pre-qualified.