1. Electrical Tests of HTS Twisted-pair Cables With Helium Gas Cooling Task 5. High Tc superconducting link Partners CERN R&D of HTS twisted pair cables Bruker Energy & Supercon Technologies (BEST) R&D and Production of HTS Tapes Columbus Superconductors R&D and Production of HTS Tapes Institute of Cryogenics University of Southampton (SOTON) R&D of Cable operation and tests

2. Conductor Specs and Cables in Liquid Cryogens ConductorsManufacturer Conductor I C (A), 77K Cable I C (A), 77K Cable IC (A), 4.2K Type-1Bi-2223Bruker HTS852601220 Type-2Bi-2223Sumitomo1804902880 Type-3YBCOSuperPower902904410 Type-4YBCOAMSC903003220 Type-5MgB 2 Columbus330 @30K4260

3. Test Setup at SOTON 2m long cryostat with two inner vessels for He gas flow; Independent flow control for each vessel Copper/HTS current leads up to 3000 A Multiple channel instrumentation for voltages and temperatures Interlocks for quench protection

4. Cooling Configurations and Critical Current Measurements U NI - FLOW : Positive temperature gradient along the cable in the flow direction; Analogue to typical cable/bus-bar applications; Ic onset always at the warm end; B I - FLOW : Improved temperature uniformity along the cable; Suitable for assessing cable homogeneity and intrinsic properties such as V-I;

5. Critical Current Measurements I C D ETERMINATION : Whole cable I C undefined in the presence of a temperature gradient for uni-flow. DC measurement of slow current ramp not suitable for gas cooled long length cables as heat transfer may be insufficient to maintain a stable isothermal condition in the vicinity of I C ; A semi-transient protocol using square current pulses (1-10 s) was adopted; S TABILITY NEAR I C : An important requirement for the cable application;

6. Current Tests in Uni-flow: An Example of Type-3 Cable at 50 K Uni-flow results in a temperature gradient along the cables, so that the warm terminal end is first to develop voltage; (Note the different V scales) Voltage initially only appeared in a ¼ of the cable length (400mm); Thermal runaway at currents 1060 A, faster at the resistive terminals; Cables #1 and #2 exhibit similar behaviour indicating excellent reproducibility for cable production. Overall Warm terminal Cable Cold terminal Cable #1Cable #2 superimposed V1V1 V2V2 V3V3 V4V4 T1T1 T 1.5 T2T2 T3T3

7. Current Tests in Bi-flow: An Example of Type-1 Cable at 30 K With bi-flow, voltages are developed along the whole cable. Although the two terminals had different contact resistances, their nonlinear resistance due to superconductors at higher currents are comparable. Overall Current lead terminal Cable Inter-cable terminal V1V1 V2V2 V3V3 V4V4 T1T1 T 1.5 T2T2 T3T3

8. V-I Characteristics can be obtained using the semi-transient method Typical V (T) traces at different I pulses Type#3 Type#1 Type#2 Contact resistances were also obtained for different temperatures. They are broadly consistent across different types, consistency can be improved.

9. Critical Current vs Temperature

10. Thermal stability near I C exists V1V1 V2V2 V3V3 V4V4 T1T1 T 1.5 T2T2 T3T3 Type#3 Type#1 Type#2

11. Conclusions 1.Tests on the twisted-pair cables carried out successfully on different types with He gas cooling. 2.Different cooling configurations were studied, uni-flow as in typical operation condition and bi-flow for near isothermal condition. 3.Homogeneity and reproducibility of cables confirmed with bi-flow. Consistency with results from measurements in liquid cryogens. 4.Thermal stability near I C were confirmed for all the types, including MgB 2. 5.More work needed to understand the current sharing behaviour of MgB 2. 6.Cables sufficiently robust for HTS links