Stability analysis of the interconnection of the LHC main superconducting bus bars

1. CHATS-AS 2011 12th – 14th October, CERN, Geneva, Switzerland Stabilityanalysis of the interconnection of the LHC main superconducting bus bars P. P. Granieri 1,2, M. Breschi 3, M. Casali 3, L. Bottura 1 1 CERN, Geneva, CH 2 EPFL-LPAP, Swiss Federal Institute of Technology, Lausanne, CH 3 University of Bologna, Italy Acknowledgments: M. Bianchi, G. Willering, A. Siemko

2. Outline Stability analysis of the LHC main SC bus bar interconnections Model description Effect of parameters in adiabatic conditions Effect of parameters with heat transfer to helium Analysis of dedicated measurements Thermal model Electric model CHATS-AS 2011 - Pier Paolo Granieri

3. Outline Stability analysis of the LHC main SC bus bar interconnections Model description Effect of parameters in adiabatic conditions Effect of parameters with heat transfer to helium Analysis of dedicated measurements Thermal model Electric model CHATS-AS 2011 - Pier Paolo Granieri

4. Main bus bar interconnections The analysis is based on this design of the main bus bar interconnections (without shunt) CHATS-AS 2011 - Pier Paolo Granieri

5. Main bus bar interconnections Good joints: Rel (RT) ~ 12 µΩ Incident: 1) bad contact between SC cables 2) transverse lack of solder 3) interruption between joint and bus stabilizer. CHATS-AS 2011 - Pier Paolo Granieri

6. The additionalresistance The R16 electrical resistance measurements is measured over a 16 cm length across the splice to detect splices with a high excess resistance in the NC state Measured Radd up to ~ 60 µΩ The additional resistance can be correlated to the length of the defect by the following equation evaluated at room temperature: ~ 12 µΩ for MB R16,good Radd = R16 – R16,good ~ 19 µΩ for MQ CHATS-AS 2011 - Pier Paolo Granieri

7. THEA model THEA is a multi-physics model: Heat conduction in solid components Compressible flow in cooling channels Current distribution in electrical components Bus Bar and Interconnection model single homogeneous thermal element two components, Nb-Ti and Cu initial T = 10 K  quench already developed Adiabatic boundaries The current distribution is neglected CHATS-AS 2011 - Pier Paolo Granieri

8. THEA model Defect Model: the contemporary presence of transverse and longitudinal lack of solder is considered in calculations Worst case for stability: the whole current is forced to flow in the SC cable copper matrix - Defect modeled as a reduction in the Cu cross section - Neglecting the Cu not in contact with the SC cable makes the domain symmetric - Thermal approximation: loss of heat capacity CHATS-AS 2011 - Pier Paolo Granieri

9. THEA parametricanalysis Results of convergence study Mesh with Δx < 0.5 mm for the fine mesh region and Δx < 5 mm for the coarse mesh region Time steps Δt < 10 ms are necessary to catch the solution features Stability analysis as a function of manufacturing quality, operating conditions and protection system parameters: Current dump time τDump Copper Residual Resistivity Ratio RRR Spatial distribution of the lack of SnAg Helium cooling capability CHATS-AS 2011 - Pier Paolo Granieri

10. Adiabatic model results I = 11850 A B = 0.474 T τdet = 0.2 s τDump = 100 s current RRR (cable/bus) = 80 - 100 Main Bending L = 2 m x = 0 m x = 2 m Gap = 8 mm Gap = 7 mm TMAX < 500K TMAX > 500K CHATS-AS 2011 - Pier Paolo Granieri

11. Adiabatic model results Aim: finding the criticaldefectlength Minimum gap leading to Tmax > 500 K Stability as a function of the τDump : CHATS-AS 2011 - Pier Paolo Granieri

12. Adiabatic model results Stability as a function of the RRR: CHATS-AS 2011 - Pier Paolo Granieri

13. Adiabatic model results Spatial distribution of the lack of solder Main bending: I = 11850 A, τdet = 0.2 s B = 0.474 T, τDump = 100 s RRR (cable/bus) = 80 - 100 melting Gap = 8 mm stable Gap1 = Gap2 = 4 mm The split defect exhibits better stability with the same total length Gap1 Gap2 CHATS-AS 2011 - Pier Paolo Granieri

14. Adiabatic model results Stability as a function of the spatial distribution of the defect: CHATS-AS 2011 - Pier Paolo Granieri

15. Heattransfer in the bus bar region • The same parametric studies have been repeated modeling cooling with He II • Bus bar htc derived from tests Extrapolation at high ΔT He-II contribution CHATS-AS 2011 - Pier Paolo Granieri

16. Non adiabatic model results Stabilitydependence on the cooling conditions: • Increase of the acceptable • Radd by a factor of 2-3 CHATS-AS 2011 - Pier Paolo Granieri

17. Non adiabatic model results • In the stable cases the Bus Bar recovers to 1.9 K • The longer the defect the longer the recovery time • Critical gap: 24 mm Burn out time ranges from 0.5 s to 8 s CHATS-AS 2011 - Pier Paolo Granieri

18. Non adiabatic model results Stability as a function of the τDump : the effect is negligible: short burn-out time CHATS-AS 2011 - Pier Paolo Granieri

19. Non adiabatic model results Stability as a function of RRR: With cooling the RRR is relevant: improved longitudinal conduction favors heat extraction towards helium CHATS-AS 2011 - Pier Paolo Granieri

20. Non adiabatic model results Stability as a function of the spatial distribution of the defect: CHATS-AS 2011 - Pier Paolo Granieri

21. Summary 1: main results of the parametricstudy The heat transfer significantly improves stability Adiabatic vs. τDumpRelevant effect RRRLow impact for high currents Relevant impact for low currents Heat Transfer Limited effect due to short burn out times Relevant impact at all current levels due to an improved heat removal from the hot spot The splitting of the defect improves stability CHATS-AS 2011 - Pier Paolo Granieri

22. Outline Stability analysis of the LHC main SC bus bar interconnections Model description Effect of parameters in adiabatic conditions Effect of parameters with heat transfer to helium Analysis of dedicated measurements Thermal model Electric model hsplice hbus bar hbus bar hbus bar hbus bar hsplice CHATS-AS 2011 - Pier Paolo Granieri

23. Frescaexperimentalanalysis of defective interconnections Defective ICs were experimentally investigated in FRESCA Sample 2B: MQ IC with one-side defect, 35 mm long He-I bath Pictures courtesy of G. Willering, TE-MSC CHATS-AS 2011 - Pier Paolo Granieri

24. Frescaexperimentalanalysis of defective interconnections Scheme of the experimental setup: no connectionbtw: - SC cable & bus bar - bus bar & U/flat profile CHATS-AS 2011 - Pier Paolo Granieri

25. THEA Model description Thermal model: 3 elements linked through Temperature dependent thermal resistances (SnAg, Polyimide, Fiberglass, He) Heat transfer coefficients Contact thermal resistance Electrical model: 2 elements Contact electrical resistance Htc Thermal / electrical elements: • SC cable SC cable • Bus Bar Bus bar • Heaters CHATS-AS 2011 - Pier Paolo Granieri

26. Resultswithoutcurrent Heater M turned on: Start of boiling of the He far away from heater M End of boiling of the He closer to heater M End of boiling of the He far away from heater M CHATS-AS 2011 - Pier Paolo Granieri

27. Resultswithcurrent Defect interruption of Cu stabilizer circuit  I forced to flow in non-stabilized SC cable large power generated in case of a quench High sensitivitywrt transversal resistances tuning of contact thermal and electricalresistances Y. Lei et al., Measurements of Interstrand Thermal and Electrical Conductance in Multistrand Superconducting Cables”, IEEE Trans. Appl. Supercond., vol. 12, March 2002, pp. 1052-1055 CHATS-AS 2011 - Pier Paolo Granieri

28. Resultswithcurrent Balance btwheatgenerationahdheat extraction: CHATS-AS 2011 - Pier Paolo Granieri

29. Summary 2: analysis of testeddefectiveinterconection Thermo-electrical model of the interconnection of the 13 kA bus bar Based on the definition of local heat trasfer coefficients Successfully analyze measurements in He I bath Thermal model Interconnection non adiabatic Presence of He inside it Electric model Contact thermal and electric resistance btw SC cable and Cu stabilizer CHATS-AS 2011 - Pier Paolo Granieri