New Considerations About Stability Margins of NbTi Cable in Conduit Conductors

The design of NbTi cable in conduit conductors (CICCs) according to the criterion of the limiting current often leads to high copper content, with a Cu/nonCu ratio inside the multifilamentary composites in the range of 6 to 8. This high copper content makes NbTi multifilamentary composites difficult and expensive to manufacture, especially if the required filament diameter is in the range of 5-10 mum. Moreover, a detailed analysis of the cable in conduit stability for expected disturbances or heat deposition on a long length of conductor shows that copper plays, in practice, no role in the case where current is uniformly distributed. An analytical approach is given to demonstrate this statement and propose a cheaper and reliable method to identify the drivers of the stability margins. Certainly copper is needed in large amounts for protection, but this copper can be segregated in dedicated strands. Typical copper content of 60% to 70% in the strands is sufficient for intrinsic dynamic stability and to stabilize the CICC against short mechanical disturbances in the range of 1 ms for the time constant affecting short lengths of CICC (10 mm). Stability issues in NbTi CICCs might also be linked to current distribution effects where contact resistance between strands plays a leading role.