Minimizing coupling loss by selection of twist pitch lengths in multi-stage cable-in-conduit conductors

The numerical code JackPot-ACDC (van Lanen et al 2010 Cryogenics 50 139-48, van Lanen et al 2011 IEEE Trans. Appl. Supercond. 21 1926-9, van Lanen et al 2012 Supercond. Sci. Technol. 25 025012) allows fast parametric studies of the electro-magnetic performance of cable-in-conduit conductors (CICCs). In this paper the code is applied to the analysis of the relation between twist pitch length sequence and coupling loss in multi-stage ITER-type CICCs. The code shows that in the analysed conductors the coupling loss is at its minimum when the twist pitches of the successive cabling stages have a length ratio close to one. It is also predicted that by careful selection of the stage-to-stage twist pitch ratio, CICCs cabled according to long twist schemes in the initial stages can achieve lower coupling loss than conductors with shorter pitches. The result is validated by AC loss measurements performed on prototype conductors for the ITER Central Solenoid featuring different twist pitch sequences.