Anomalous behaviour in ICCS

ICCS conductors behave anomalously. Several anomalies have been documented: quenches driven by pulsing fields (referred to as ramp-rate limitations), and energy margins that show a transition between a “well-cooled” and an “ill-cooled” regime (marking a transition between high energy margin at low transport current and low energy margin at higher currents). In this paper, a novel model will be described that attempts to interpret experimental data. In the experiments, 3×3×3 ICCS Nb₃Sn sample coils have been tested for stability to ramping fields and transport currents. Three such coils have been tested, one made of triplets made of all SC strands, a second with one copper and two SC strands and a third one with one steel and two SC strands. The analysis deals mainly with unpressurized cases, but the model also applies to supercritical He. The hypothesis is that non-uniform helium and superconductor distribution in the conductor is the cause of the “anomalous” behaviour. The energy margin as a function of the transport current has also been calculated, and it shows a transition. The limiting current is determined by the highest local fraction of bunching (averaged over a cross section). The model, however, has difficulty explaining multiple ramps