1. 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 CHATS-AS 2011 12 th – 14 th October, CERN, Geneva, Switzerland

2. CHATS-AS 2011 - Pier Paolo Granieri2 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

3. CHATS-AS 2011 - Pier Paolo Granieri3 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

4. CHATS-AS 2011 - Pier Paolo Granieri4 Main bus bar interconnections  The analysis is based on this design of the main bus bar interconnections (without shunt)

5. CHATS-AS 2011 - Pier Paolo Granieri5 Main bus bar interconnections  Good joints: R el (RT) ~ 12 µΩ  Incident: ◦ 1) bad contact between SC cables ◦ 2) transverse lack of solder ◦ 3) interruption between joint and bus stabilizer.

6. CHATS-AS 2011 - Pier Paolo Granieri6 The additional resistance  The R 16 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 R add up to ~ 60 µΩ  The additional resistance can be correlated to the length of the defect by the following equation evaluated at room temperature: R add = R 16 – R 16,good R 16,good ~ 12 µΩ for MB ~ 19 µΩ for MQ

7. CHATS-AS 2011 - Pier Paolo Granieri7 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

8. CHATS-AS 2011 - Pier Paolo Granieri8 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

9. CHATS-AS 2011 - Pier Paolo Granieri9 THEA parametric analysis  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 τ Dump ◦ Copper Residual Resistivity Ratio RRR ◦ Spatial distribution of the lack of SnAg ◦ Helium cooling capability

10. CHATS-AS 2011 - Pier Paolo Granieri10 Adiabatic model results I = 11850 A B = 0.474 T τ det = 0.2 s τ Dump = 100 s RRR (cable/bus) = 80 - 100 Gap = 7 mm Gap = 8 mm L = 2 m current Main Bending x = 0 mx = 2 m T MAX < 500K T MAX > 500K

11. CHATS-AS 2011 - Pier Paolo Granieri11 Adiabatic model results  Aim: finding the critical defect length  Minimum gap leading to Tmax > 500 K  Stability as a function of the τ Dump :

12. CHATS-AS 2011 - Pier Paolo Granieri12 Adiabatic model results  Stability as a function of the RRR:

13. CHATS-AS 2011 - Pier Paolo Granieri13 Adiabatic model results  Spatial distribution of the lack of solder  Main bending: I = 11850 A, τ det = 0.2 s B = 0.474 T, τ Dump = 100 s RRR (cable/bus) = 80 - 100 meltingGap = 8 mm stable Gap 1 = Gap 2 = 4 mm  The split defect exhibits better stability with the same total length Gap 1 Gap 2

14. CHATS-AS 2011 - Pier Paolo Granieri14 Adiabatic model results  Stability as a function of the spatial distribution of the defect:

15. CHATS-AS 2011 - Pier Paolo Granieri15 Heat transfer in the bus bar region Extrapolation at high ΔT He-II contribution  The same parametric studies have been repeated modeling cooling with He II  Bus bar htc derived from tests

16. CHATS-AS 2011 - Pier Paolo Granieri16 Non adiabatic model results  Stability dependence on the cooling conditions: R additional [µΩ] 3.5 TeV (τ dump : 100 s) 7 TeV (τ dump : 100 s) Adiabatic everywhere 4210 He-I 97.517 He-II 11631  Increase of the acceptable Radd by a factor of 2-3

17. CHATS-AS 2011 - Pier Paolo Granieri17 Non adiabatic model results  Critical gap: 24 mm Burn out time ranges from 0.5 s to 8 s  In the stable cases the Bus Bar recovers to 1.9 K  The longer the defect the longer the recovery time

18. CHATS-AS 2011 - Pier Paolo Granieri18 Non adiabatic model results  Stability as a function of the τ Dump : ◦ the effect is negligible: short burn-out time

19. CHATS-AS 2011 - Pier Paolo Granieri19 Non adiabatic model results  Stability as a function of RRR:  With cooling the RRR is relevant: improved longitudinal conduction favors heat extraction towards helium

20. CHATS-AS 2011 - Pier Paolo Granieri20 Non adiabatic model results  Stability as a function of the spatial distribution of the defect:

21. CHATS-AS 2011 - Pier Paolo Granieri21 Summary 1: main results of the parametric study Adiabatic vs. τ Dump Relevant 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 The heat transfer significantly improves stability

22. CHATS-AS 2011 - Pier Paolo Granieri22 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 h bus bar h splice

23. CHATS-AS 2011 - Pier Paolo Granieri23 Fresca experimental analysis 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

24. CHATS-AS 2011 - Pier Paolo Granieri24 Fresca experimental analysis of defective interconnections  Scheme of the experimental setup: no connection btw: - SC cable & bus bar - bus bar & U/flat profile

25. CHATS-AS 2011 - Pier Paolo Granieri25 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

26. CHATS-AS 2011 - Pier Paolo Granieri26 Results without current  Heater M turned on: End of boiling of the He closer to heater M Start of boiling of the He far away from heater M End of boiling of the He far away from heater M

27. CHATS-AS 2011 - Pier Paolo Granieri27 Results with current  Defect  interruption of Cu stabilizer circuit  I forced to flow in non-stabilized SC cable  large power generated in case of a quench  High sensitivity wrt transversal resistances  tuning of contact thermal and electrical resistances 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

28. CHATS-AS 2011 - Pier Paolo Granieri28 Results with current  Balance btw heat generation ahd heat extraction:

29. CHATS-AS 2011 - Pier Paolo Granieri29 Summary 2: analysis of tested defective interconection  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