Thermal runaways in LHC main circuit interconnections: Experiments Gerard Willering

1. Technology Department Thermal runaways in LHC main circuit interconnections: ExperimentsGerard Willering With contributions of: TE/MPE: Arjan Verweij TE/MSC-CI: N. Bourcey TE/MSC-TF: M. Bajko, G. Deferne, G. Dib, M. Charrondiere TE/MSC-SCD: L. Bottura, D. Richter, G. Peiro, C. Scheuerlein, S. Heck TE/MSC-LMF: P. Fessia, K. Chaouki, R. Principe, S. Triquet EN/MME: T. Regnalia, P. Perret TE/EPC: G. Hudson, M. Cerqueira EN/ICE: A. Rijllart, D. Kudryavtsev TE/CRG: V. Benda and many more... Gerard Willering – Splice review – 18 October 2010 - CERN

2. Technology Department Contents Experiments to the effects of defected and consolidated LHC main bus splices are conducted in three phases 1. Thermal runaways in interconnections with defects August 2009 – Februari 2010 5 quadrupole busbar samples in test station FRESCA Goal: Validation of model → safe operating current before consolidation 2. Proof of principle of the consolidation with shunts March 2010 – June 2010 4 quadrupole busbar samples in test station FRESCA Goal: Validation of model → Proof of principle of the consolidation proposal 3. Final validation of the consolidation with shunts in a realistic test setup March 2010 – October 2010 2 dipole busbar samples inbetween two special SSS magnets Goal: Validation of model and final validation of the shunts Gerard Willering – Splice review – 18 October 2010 - CERN

3. Technology Department Defect preparation Definition of a defect: Stabilizer discontinuity Non-stabilized cable with a specified length Gamma-ray image of sample 1, indicating the single-sided defect. 2 1 1 Preparation of the defect. 30 mm non-stabilized cable Guaranteed by Kapton tape Important parameters Rcable = 1.3 µΩ/mm Rquad-busbar = 0.1 µΩ/mm Radd = Rmeasured - R8cm RRRbus RRRcable Gerard Willering – Splice review – 18 October 2010 - CERN

4. Technology Department Sample preparation Sample 2A Single-sided defect Discontinuity Discontinuity Sample 2B Double-sided defect Heater Test layout with normal LHC pieces and geometry and with lots of instrumentation (RQ circuit) Gerard Willering – Splice review – 18 October 2010 - CERN

5. Technology Department Test station FRESCA • - In the FRESCA teststation the sample length is limited to 1.7 m, which gives 0.8 meter of busbar on each side of the interconnection. • 24 Voltage taps • 10 Thermocouples • 5 heaters • The ends of the busbars are thermalized (a lot of copper in direct contact with helium). • Measurements are performed with constant current. • Due to limitations of the test station (Helium volume, length of sample, vincinity of the current leads) the quadrupole interconnections are chosen to test. Gerard Willering – Splice review – 18 October 2010 - CERN

6. Technology Department Typical measurement data Thermal Runaway 7 kA, 43+32 µΩ defect Voltage in non-stabilized cable Temperature in non-stabilized cable Temperature in busbar • Fingerprint of a local thermal runaway: • - Relatively low busbar temperature. • Accelerated voltage increase in the non-stabilized cable. • Main characteristic: Thermal runaway time trun Gerard Willering – Splice review – 18 October 2010 - CERN

7. Technology Department Thermal runaway time • - Except for sample 3B, all samples would melt within 1 second with a current of 12 kA. • The MIITs (kA^2/s) for an exponentially decaying current with timeconstant τ is reached by a constant current in t = 0.5*τ. • For the quadrupole circuit with τ = 20 s, we can correlate the safe currents for the sample conditions with a cross-section at trun = 10 s. • - Although there is a correlation, safe currents can not be drawn from the measurements. Gerard Willering – Splice review – 18 October 2010 - CERN

8. Technology Department Measurement characterization 27 W 16 W The current at trun versus the additional resistance R add shows a good correlation. The allowed power at 10 K is between 16 and 27 W. Since we varied the applied field on the sample, the effective Radd varied giving us a wider range in measurements. Therefore more than 5 points (number of samples) are shown. Gerard Willering – Splice review – 18 October 2010 - CERN

9. Technology Department Melt-down of a non-stabilized cable To perform multiple thermal runaway measurements, the current is cut-off when the maximum temperature reaches in between 100 and 300 K. Out of 175 run-aways we did, we choose the smallest defect of 20 mm at 9 kA to demonstrate that the incident can be reproduced. In fact, each of the 175 measurements would lead to a melt-down. Gerard Willering – Splice review – 18 October 2010 - CERN

10. Technology Department Melt-down of a non-stabilized cable With an increased protection cut-off voltage the thermal runaway was conducted until the cable melted over the full width over a length of 1.5 to 3 mm. - The temperature was at least 1360 K to melt the copper in the cable. - Remarkably, at the moment of melt-down, the thermocouple in the busbar 15 mm from the hotspot only measured 50 K. Sample 3B LNSBC = 21 mm Radd = 27 μΩ I = 9 kA trun = 13 s Gerard Willering – Splice review – 18 October 2010 - CERN

11. Technology Department Content Experiments to the effects of defected and consolidated LHC main bus splices are conducted in three phases 1. Thermal runaways in interconnections with defects August 2009 – Februari 2010 5 quadrupole busbar samples in test station FRESCA Goal: Validation of model → safe operating current before consolidation 2. Proof of principle of consolidation with shunts March 2010 – June 2010 4 quadrupole busbar samples in test station FRESCA Goal: Validation of model → Proof of principle of the consolidation proposal 3. Final validation of the consolidation with shunts in a realistic test setup March 2010 – October 2010 2 dipole busbar samples inbetween two special SSS magnets Goal: Validation of model → safe operating current before consolidation Gerard Willering – Splice review – 18 October 2010 - CERN

12. Technology Department Shunt preparation First try: Discontinuity of the copper was not guaranteed due to solder creep in the voids. Second try: Discontinuity guaranteed by cutting away part of the stabilizer • Important parameters for shunts: • Thickness of the shunt • Non-soldered shunt length (see with white arrows). Gerard Willering – Splice review – 18 October 2010 - CERN

13. Technology Department Shunt preparation 0 mm 11 mm Sample 4 without shunt 7 mm 5 mm Sample 4 with shunt Sample 4 3 mm thick shunts Shunts reduced to 1.5 mm thickness Gerard Willering – Splice review – 18 October 2010 - CERN

14. Technology Department Result on measurements with shunts • Runaway time for the shunted samples much higher than for non-shunted samples. • All the shunted samples can carry 13 kA for more than 24 seconds. • The same data, but the MIITs are calculated (kA2*s) • The shunted samples with 1.5 and 3 mm thick shunts can handle the MIITs of 15.5 kA with τ = 20 s. • These samples do not have the worst case parameters and not the worst case conditions. Therefore no direct conclusions for LHC conditions. Gerard Willering – Splice review – 18 October 2010 - CERN

15. Technology Department Content Experiments to the effects of defected and consolidated LHC main bus splices are conducted in three phases 1. Thermal runaways in interconnections with defects August 2009 – Februari 2010 5 quadrupole busbar samples in test station FRESCA Goal: Validation of model → safe operating current before consolidation 2. Proof of principle of consolidation with shunts March 2010 – June 2010 4 quadrupole busbar samples in test station FRESCA Goal: Validation of model → Proof of principle of the consolidation proposal 3. Final validation of the consolidation with shunts in a realistic test setup March 2010 – October 2010 2 dipole busbar samples inbetween two special SSS magnets Goal: Validation of model → safe operating current before consolidation Gerard Willering – Splice review – 18 October 2010 - CERN

16. Technology Department Preparation of final validation test • Goal: Test in realistic conditions of a worst case scenario, with a non-soldered shunt length of 8 mm and low RRR values. • 2 Special SSS spare magnets are connected to the testbench in SM18. • In total 35meter of RQ busbar and 35 meter of RB busbar. • Two instrumented RB (M3) interconnections. • No magnets in the test-circuit 2 interconnections Quadrupole lines Quadrupole lines Quench stopper Dipole lines Dipole lines 2 interconnections Gerard Willering – Splice review – 18 October 2010 - CERN

17. Technology Department Preparation of final validation test • Test conditions are rather special: • First time 2 magnets in serie on the test-bench -> Test bench elongation • The quench needs to be stopped between M3 and M1 line. • Additional copper strips (Lyra) for cooling • Large, 30 liter reservoir for helium • Instrumentation wire feed-through-box • Heating power of more than 300 W for long time with • significant loads on cryogenic system Gerard Willering – Splice review – 18 October 2010 - CERN

18. Technology Department RRR measurements • High precision measurements on resistance are important for the validation of shunt and model. • In the test the U-profile/wedge have a low RRR • - In the tests the shunts have a much lower RRR than foreseen for the LHC conditions since they are not annealed shunt shunt U-profile/wedge Gerard Willering – Splice review – 18 October 2010 - CERN

19. Technology Department Expected results Thermal runaway measurements on the interconnection with the largest non-soldered length (8 mm). What to expect in the test conditions? I > 16 kA @ τ = 100 s Figure from A.Verweij (chamonix 2010 workshop and first splice review) Gerard Willering – Splice review – 18 October 2010 - CERN

20. Technology Department Current cycles for test Current cycles for thermal runaway measurements at 1.9 K. LHC cycle: 13 kA, tau = 100 s Very quick recovery of the normal zone Test cycle: 14 kA, τ = 100 s Test cycle: 14 kA, τ = 140 s Test cycle: 14 kA for 22 s, then τ = 140 s. Still no signs of thermal runaway in the most critical shunt!! Therefore we went to constant currents of 13 and 14 kA (power supply limit). Gerard Willering – Splice review – 18 October 2010 - CERN

21. Technology Department Quench behavior at a constant current of 13 kA • No significant heating of the interconnection in 180 s. • No significant heating in the busbar Q9-1 in 180 s. • Normal zone does not enter the Q8 busbars. • Very stable conditions at 13 kA in busbar and interconnection!!! Q9 Q8 Q9-busbar have an RRR ≈ 250 → R9.4meter ≈ 2.3 µΩ at 10 K Q8-busbars have an RRR ≈ 300 → R16.5meter ≈ 3.5 µΩ at 10 K Gerard Willering – Splice review – 18 October 2010 - CERN

22. Technology Department Quench behavior at a constant current of 14 kA • Small temperature increase in the interconnection in 85 s. • The full 35 meter of busbar between the quench-stoppers become normal • Accelerated heating effect in busbars Q8 and Q9-2. • Limitation factor is not the shunted interconnection, but the busbar. • In the straight section the busbars are encapsulated in a G10 casing and close to each other. • - In the region closer to the interconnection superfluid helium is available for cooling. Q9 Q8 Q9-busbar have an RRR ≈ 250 → R9.4meter ≈ 2.3 µΩ at 10 K Q8-busbars have an RRR ≈ 300 → R16.5meter ≈ 3.5 µΩ at 10 K Gerard Willering – Splice review – 18 October 2010 - CERN

23. Technology Department Temperature profiles in the shunts at 13 and 14 kA • Additional proof of thermally stable conditions with measurements by two thermocouples in the shunts of Interconnection 1. T3 T4 Interconnection 1 Interconnection 2 I = 13 kA I = 14 kA Gerard Willering – Splice review – 18 October 2010 - CERN

24. Technology Department Quench propagation velocity Quench propagation velocity – dipole busbar • No propagation below 12 kA (with the busbar cooled by superfluid helium). • Arjan’s calculation (RRR = 200) is a bit more optimistic than the measurements (RRR 250 - 300). Gerard Willering – Splice review – 18 October 2010 - CERN

25. Technology Department Quench propagation velocity Quench propagation velocity – quadrupole busbar • No propagation below about 9 kA (with the busbar cooled by superfluid helium) • At higher currents the measured velocity might be overestimated since the temperature and therefore the resistance can be increased. Gerard Willering – Splice review – 18 October 2010 - CERN

26. Technology Department MIITs >> 30000 kA2s at 13 kA (no sign of thermal runaway) MIITs > 18000 kA2s at 14 kA (Start of thermal runaway in busbars) (LHC 13 kA, 100 s – MIITs = 8500 kA2s LHC 11.8 kA, 100 s – MIITs = 6800 kA2s) Simple and short conclusion: The proposed shunts work! Gerard Willering – Splice review – 18 October 2010 - CERN

27. Technology Department Summary and Conclusion • Thermal runaways in interconnections with defects • - Clear proof of the damage a defect can have with the melted sample. • - Measurements provided largely sufficient experimental data for model validation (by A. Verweij). • - Conclusions on safe current/energy cannot be drawn directly from this measurements, since test conditions are different from machine conditions. • Proof of principle of consolidation with shunts • - Clear improvement of the thermo-electric stability by applying shunts on the samples with defects. • - Boundary conditions of the test-station prohibit direct conclusions on the stability of the consolidated interconnection, but indicates that the principle good. • - Sufficient experimental data for model validation (by A. Verweij). • Final validation of the consolidation with shunts in a realistic test setup • - A consolidated interconnection with a copper shunt having a cross-section of 45 mm^2, a double defect in the interconnection, a non-soldered lenght of 8 mm and an RRR of 160 is more stable than the busbar itself in the straight section. • - In the condition a quench starts in the interconnection itself a continuous current of 13 kA does not show any sign of a thermal runaway in the first 180 seconds. • - At a continuous current of 14 kA provokes an excellerated temperature increase in the encapsulated part of the busbars, with a temperature of about 40 K after 85 s. • - In terms of thermo-electrical stability the shunt is overdesigned. Gerard Willering – Splice review – 18 October 2010 - CERN