1. LHC Interconnect Simulations and FRESCA Results
L. Bottura, A. Verweij, G. Willering
Based on work and contributions of many
MAC,

2. Outline Summary of experimental results on controlled defects
Samples tested
Results and scaling
Can we burn it ?
Update on simulations for machine conditions
“The Fix”
Experimental proof-of-principle
A validation experiment for “The Fix”
Conclusions

3. A real defect in interconnect
Courtesy of C. Scheuerlein, TE-MSC
A real defect in interconnect
QBBI.A25L4-M3-cryoline-lyra side (+30 μΩ)
31 mm
16 mm
Unmelted Sn foil

4. Ideal defect sample, produced to simulate a thermal runaway
Non-stabilized cable length
Wedge
Bus-bar
Joint
Poor electrical contact
U-profile
Gap in the stabilizer
FRESCA Sample 1
Sample of interconnect with a ≈ 45 mm soldering defect introduced for testing purposes

5. Outline Summary of experimental results on controlled defects
Samples tested
Results and scaling
Can we burn it ?
Update on simulations for machine conditions
“The Fix”
Experimental proof-of-principle
A validation experiment for “The Fix”
Conclusions 5

6. Samples tested -1/2
5 interconnections on 3 hair-pin samples, incorporating 7 different type and size of defects
Ldefect between 20 and 47 mm
RRRcable between 90 and 180
RRRbusbar between 160 and 290
All samples were built from RQ busbars (minimum Cu cross section)

7. Samples tested - 2/2 X-rays by courtesy of J.-M. Dalin
Sample 1 (63 mW)
Sample 2A left (32 mW)
Sample 2A right (43 mW)
Sample 2B (42 mW)
Sample 3A right (56 mW)
Sample 3A left (44 mW)
Sample 3B (32 mW)
X-rays by courtesy of J.-M. Dalin

8. Samples characterization
Defect
R8 additional resistance
Ldefect
(mm)
Rdefect,293K
(µΩ)
Rdefect,10K
RRRdefect
RRRbusbar
R8Megger (µΩ)
Rvoltagetaps (µΩ) 1 60 63 47
0.35
180
≈ 290
2A-1 30 32 27 36
0.29
120
≈ 270
2A-2 45 43 35
0.51 90
2B-2 44 42
0.30
160
3A-1 57
56.0 39 52
0.31
170
≈ 190
3A-2
43.8 40
0.28
140
3B-1
32.4
20.5
0.22
≈ 160
Wide range of defects represented in the samples tested 8

9. Thermal runaway time vs. current
Data fitted by
trunaway = a*(I-Isteadystate)-b
dump= 5 s
Isteady-state
Isteady-state ranges from 6.2 kA (3.7 TeV) to 8.3 kA (4.9 TeV)

10. Scaling of results Varying B
Results can be scaled by plotting the runaway current at a given, relevant time (e.g. 5 s) vs. the additional resistance of the largest defect at 10 K (scaling can be derived from a power balance)

11. Comments
The results collected provide a consistent, and quite complete database for the validation of analytic models
Although not directly applicable to LHC due to the test conditions (limited length of busbar, constant current, and other details), they give good indication of the range of safe operation

12. Sample 3B (32 mW)
Can we burn it ?
The shortest defect (3B) was quenched at 9 kA, increasing the protection voltage Vth
Vth ≈ 500 mV
Estimated
Thot ≈ 800 K
Vth ≈ 400 mV

13. Imaging…
Before test
After test
50 K
50 K
> 1360 K

14. Yes we can ! The complete burn-out of the cable can happen:
In presence of relatively small defects (L ≈ 20 mm)
At modest operating currents (Iop ≈ 9 kA)
Very quickly (t < 1 s)
Very locally (< 5 mm)
With very large temperature gradients (dT/dx ≈ 105 K/m)

15. Outline Summary of experimental results on controlled defects
Samples tested
Results and scaling
Can we burn it ?
Update on simulations for machine conditions
“The Fix”
Experimental proof-of-principle
A validation experiment for “The Fix”
Conclusions 15

16. Simulation model recall
Model developed by A. Verweij, first analyses presented at Chamonix-2009:
A.P. Verweij, Busbar and Joints Stability and Protection, Proceedings of Chamonix 2009 workshop on LHC Performance, , 2009
1-D heat conduction with:
Variable material cross section to model the local lack of stabilizer
Temperature dependent material properties
Heat transfer to a He bath through temperature dependent heat transfer coefficient
Various adjustments and cross-checks performed against other models (1-D and 3-D) and experimental results
Most recent results presented at Chamonix-2010:
A.P. Verweij, Minimum Requirements for the 13 kA Splices for 7 TeV Operation

17. CAVEAT - heat transfer individually adjusted to match results
Model validation
Adiabatic simulation
He-cooled simulation
ff=1.6
ff=1.8
ff=0.9 17

18. Hypotheses used for the simulation of LHC operation
RRRcable = 80
RRRbusbar = 100/160 (no significant impact)
RRRwedge = RRRbusbar
ff = 0.9
tpropagation = 10/20 s
Has an impact on the operating current at the instant of the interconnect quench in case of quench induced by warm gas flow

19. Allowables for safe operation
Best estimate of the maximum allowable additional defect resistance (in ) for safe operation
Energy RB RQ
3.5 TeV 76 80
5 TeV 34 41
7 TeV 11 15 19

20. Outline Summary of experimental results on controlled defects
Samples tested
Results and scaling
Can we burn it ?
Update on simulations for machine conditions
“The Fix”
Experimental proof-of-principle
A validation experiment for “The Fix”
Conclusions 20

21. Proof of principle of “The Fix”
Shunt thickness: 3 mm
Shunt cross section: 45 mm2
Non-soldered shunt length: 6 to 10 mm
Electrically insulated shunt

22. Thermal runaway after repair
CAVEAT - The results obtained may have been partially influenced by (unavoidable) solder drops into the initial busbar discontinuity
Thermal runaway after repair 2A 2B
Double defect (sample 2A):
Isteady state increased from 6.4 to 12.8 kA
Single defect (sample 2B):
Isteady state increased from 8.3 to 11.2 kA

23. Validating “The Fix”
The safest approach remains a global fix of all LHC main bus interconnect
This must be a guaranteed repair that withstands all and any quench condition through the entire life of the machine
It makes sense to invest in a validation experiment to stress (thermally, electrically, mechanically) a repaired interconnect in conditions relevant to the operation of the accelerator
Work is in progress to prepare rapidly such a test set-up

24. Experiment configuration
Use the M1 and M3 line in two DS-SS mounted on a horizontal test bench as test lines with LHC-representative and safe test conditions
Drawing by Philippe Perret

25. Experiment goals
Verify the thermo-electric stability of a defective interconnect and validate the repair solution with initial current up to 14 kA and current dump with exponential time constant  up to 150 s
Cycling tests to investigate (mechanical/thermal) fatigue
Quench propagation in RQ and RB busbars
Characterization of defect and repair resistance to benchmark quality control methods
Good practical exercise for the repair activity to take place in the tunnel

26. Conclusions
The present simulation and experimental database supports the decision to operate at 3.5 TeV (safe)
A simple shunt resolves the issue of thermal runaway, as expected
We will focus next on implementation issues for the fix, and its validation both in controlled testing conditions (vertical cryostat of FRESCA) and LHC-relevant conditions (SM-18)

27. Additional material

28. Vertical sample design
Detailed design by Th. Renaglia, EN-MME
Vertical sample design
Spider
Interconnect (2x)
Spacer
MQ bus-bar

29. Typical quench test Ramp and hold the current Fire the heater(s)
Monitor the normal zone voltage
QD at 100 mV
20 ms delay
Dump in ≈ 100 ms

30. Run
Stable quench: a normal zone is established and reaches steady-state conditions at a temperature such that the Joule heat generation is removed by conduction/convection cooling
stable

31. Run
Runaway quench: the normal zone reaches a temperature at which the Joule heat generation in the normal zone exceeds the maximum cooling capability leading to a thermal runaway
runaway

32. Runs at “0” background field
For identical test conditions, the time necessary to reach the thermal runaway (trunaway) depends on the operating current
trunaway(7.5 kA)
trunaway(8 kA)

33. trunaway vs. Iop
For any given test condition of temperature and background field it is possible to summarise the above results in a plot of runaway time trunaway vs. operating current Iop

34. Effect of Bop (RRR)
An applied magnetic field induces magnetoresistance and reduces thermal conduction  the effect is an increased tendency to thermal runaway

35. Effect of Top
Changing bath conditions (1.9 K vs. 4.3 K) changes the heat transfer, but has no apparent effect on trunaway. The behavior of the sample is nearly adiabatic for this run
data taken during the 2nd cool-down with sealed insulation

36. Scaling of results - power law