Improvements of the DIII-D Cryosystem from Analysis of Failure Modes and Their Effects

The cryogenic system at General Atomics' DIII-D Tokamak fusion facility provides cryopumping for the fusion vessel and neutral beams, cooling for the superconducting magnets for electron cyclotron heating, and cooling for the deuterium pellet injector. The operation of the fusion facility requires a reliable cryosystem, since upsets can result in significant downtime. A failure modes and effects analysis of the cryosystem was performed to identify and evaluate potential failures. This paper describes the methodology used for identifying the potential failures, based on likelihood of occurrence and consequence to operations. This paper further describes the analysis developed to understand these failure modes, and the improvements and testing to reduce the likelihood and the consequences of these failures. This failure analysis focused on maintaining the helium liquefier operation by providing reliable backup power and cooling for the helium compressors, and resulted in several recommendations which improved the liquifier's availability.