Two phase helium cooling characteristics in Cable-in-Conduit Conductors

Cable-in-Conduit Conductors (CICCs) are validated base conductors for fusion grade superconducting magnets. In practice, the CICC acts as a narrow cryostat that provides adequate cryostability with the direct contact of coolant fluid to conductor from inside. The superconducting magnets are cooled using forced flow (FF) supercritical helium or two phase (TP) helium, which flows through the void spaces of the CICC. Thermo-hydraulics studies using supercritical helium in single phase flows are well-known and established. On the other hand, the TP cooling with helium is still an area of active research. There are perceived risks of the TP cooled CICC running into flow chocking and possible thermoacoustic oscillations leading to flow instabilities; and hence traditionally are not favoured. This research work addresses the fact that TP cooling is an exciting cooling and operation regime with inherent parametric benefits. Our present study is an analytical study of forced flow two phase helium cooling in CICC wound superconducting magnets. The TP flow provides greater cryostability by the latent heat of helium compared to enthalpy, as in case with supercritical helium cooling of CICC. Analysis carried out predicts significant gains with TP cooling on a prototype CICC, which is circular in cross-section and appropriate for fusion devices for high magnetic field applications. These formalisms are general and extendable to any CICC wound magnets. This paper describes the analysis and advantages of the TP cooling over single phase cooling of a CICC.