1. The HiLumi LHC Design Study (a sub-system of HL-LHC) is co-funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404. 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-pair cable concept optimized for tape conductors (MgB 2, 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 22K Twisted-pairs Inter-pair joints

4. Thermal Runaway Current  Variation in the critical current among the twisted-pairs are expected due to 1.The fluctuation in Ic uniformity in the tapes over long length and the small random degradation by mechanical twisting process 2.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  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. band-heater

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

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.  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 Intra-pair Quench Propagation Velocity

9.  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 At high current Intra-pair Quench Propagation Velocity

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 1.Single 18-wire sub-cable characterisation: runaway current at different T and current transfer to copper; 2.Quench propagation and MQE in single wire 3.Quench propagation in sub-cable: inter-wire propagation and current sharing with copper