Measurements of the stability margin of a Nb3Sn cable-in-conduit conductor

The stability margin of a single quadruplex of Nb₃Sn/Cu composite strand enclosed in a round stainless-steel tube was measured by using an inductive heater whose duration time is around 0.1 ms. The strand had a diameter of 1.04 mm and a copper fraction of 69%. The fraction of helium in the cable space was 34.5%. Measurements of the stability margin were carried out by varying the transport current, external magnetic field, and initial helium pressure. The helium flow rate was always zero. The limiting transport current, which is the boundary value between the upper and lower stable regions similar to NbTi cable-in-conduit conductors, was observed. The limiting-current density over the cable space inside the conduit at 12 T was 100 A/mm², corresponding to 58% of the critical current. This value does not depend on the initial helium pressure in the range of 3-12 atm. Observed upper and the lower stability margins were more than 200 mJ/cm³ strand and less than 60 mJ/cm³ strand, respectively. The heat transfer coefficient in the cable-in-conduit conductor is estimated to be around 0.1 W/cm²K