WP6 Characterisation of HiLumi SC Link

1. WP6 Characterisation of HiLumi SC Link Edward A Young, Jess Spurrell, Yifeng Yang University of Southampton

2. Twisted-pair cables and packed assembly • Novel twisted-paircable concept optimized for tape conductors (MgB2, Y-123 and Bi-2223). A. Ballarino “ Alternative design concepts for multi-circuit HTS link systems”. IEEE Trans. on Applied Supercond.21 pp. 980-984, 2011

3. Thermal Runaway Current Twisted-pairs Inter-pair joints 22K

4. Thermal Runaway Current • Variation in the critical current among the twisted-pairs are expected due to • The fluctuation in Ic uniformity in the tapes over long length and the small random degradation by mechanical twisting process • Temperature gradient along the helium flow • The relevant stability indicator under such a operation circumstance is the thermal runaway current, which ensured by successive current trains as shown in the example below: • At 22K, the cable assembly first exhibited thermal runaway at 1400A in strands (red half circles below) #44 (pair #22) and #20 (pair #10). Prior to the thermal runaway, both strands exhibit the highest voltages just below the runaway current • The thermal runaway propagates along the strands to the respective inter-pair joints (red circle below) in 0.4-0.6s, giving an estimated longitudinal thermal runaway propagation velocity of ~ 5m/s.

5. Thermal Runaway Current: Local Distribution band-heater • A band-heater was installed in the middle of the assembly for exploring the spread/distribution of the thermal runaway current among the twisted-pair strands. • More strands could be brought into thermal runaway by a higher steady-state heating. • The example above shows thermal runaway t in 7 strands triggered by 4.2W applied to the band heater. Some directly triggered (red half circle, some indirectly by intra-pair propagation (blue half circle) • Longitudinal propagation reached to the 5 respective inter-pair joints. • The spread of the runaway current is rather small.

6. Thermal Runaway Current: Local Distribution Inter-pair joints Twisted-pairs

7. Intra-pair Quench Propagation Velocity • A spot-heater was installed on strand #35 of pair #18 to induce localised quenches in a single strand. • The objective was to trigger quench in the co-twisted strand through radial heat diffusion of the normal zone. spot-heater

8. Intra-pair Quench Propagation Velocity • As expected, intra-pair quench propagation velocity is slow and  I. • The longitudinal length of the normal zone is nonetheless moderate (~25-50cm) • The hot spot is detectable (~5mV) to avoid radial quench. Large current steps Small current steps

9. Intra-pair Quench Propagation Velocity At high current • A hot spot on a single strand can lead to inter-pair quench by radial propagation!! • Difficult for detection before intra-pair quench • But possible to prevent inter-pair quench

10. Minimum Quench Energy MQE Search MPZ is rather small: V strand is only 1.5-5 times of V hot spot.

11. Next: Round Wire Cables Single 18-wire sub-cable characterisation: runaway current at different T and current transfer to copper; Quench propagation and MQE in single wire Quench propagation in sub-cable: inter-wire propagation and current sharing with copper