LHC Interconnect Simulations and FRESCA Results L. Bottura, A. Verweij, G. Willering Based on work and contributions of many MAC, 27.04.2010

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

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

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

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

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)

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

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

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)

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)

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

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

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

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)

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

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, 113-119, 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

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

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

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

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

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

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

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

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

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

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)

Additional material

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

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

Run 090813.15 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

Run 090813.20 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

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)

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

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

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

Scaling of results - power law