Pier Paolo Granieri, TE-CRG

1. Collimation Working Group, September 16, 2013 Pier Paolo Granieri, TE-CRG Ack.: R. van Weelderen, L. Bottura, D. Richter, P. Galassi, D. Santandrea and S. Redaelli, R. Bruce, B. Salvachua, F. Cerutti, E. Skordis, A. Lechter, M. Sapinski for discussing QT results & analysis Deduction of steady-state cablequench limits for theLHC main dipoles

2. Outline P.P. Granieri - Quench limits • Steady-state vs. transientquenchlimits • Deduction of steady-state quenchlimit for the LHC MB cable • Method • Resultsand comparison to collimation quench test • Previousquenchlimit estimations • Whatcanwe do to improve the quenchlimit computation? • "Nearsteady-state"cablequenchlimit

3. Quenchlimits • transient state, mJ/cm3 • (fastlosses) • steady-state, mW/cm3 • (slow losses, > 1-10 s) • Dominant • mechanism • Heattransferfromcable to He bath • (throughcableelectricalinsulation) • Experiments and modelingongoing: • heattransferthroughcable’selectricalinsulation (stackmethod) • The deducedquenchlimitsrefer to a uniformheatdepositover the cable • Local heattransferfromstrand • to He inside the cable • No conclusive experiments (yet)  • werely on numerical codes: • 0-D (ZeroDee): • uniformheatdeposit and field overcable cross-section • no longitudinal direction • 1-D (THEA): • single strandexperiencinga heatdeposit and field variation alongitslength • similar to QP3 (Arjan, Bernhard) P.P. Granieri - Quenchlimits

4. Deduction of cablesteady-state quenchlimits Method reported in: P.P. Granieri and R. van Weelderen, “Deduction of Steady-State Cable Quench Limits for Various Electrical Insulation Schemes with Application to LHC and HL-LHC Magnets”, IEEE Trans. Appl. Supercond. 23 submitted for publication Raw data: - LHC MB and EI4: D. Richter, P.P. Granieri et al. - SSC: C. Meuris, B. Baudouy et al. - Nb3Sn: P.P. Granieri et al. P.P. Granieri - Quench limits • For steady-state beamlosses, a quenchoccurs if TcableexceedsTcs(4 - 5.5 K for the LHC MB) • The cablequenchlimitsdepend on • Heat extraction: • cablecoolingwithin the magnet • mechanical pressure, if Nb-Ti coil • stackheating configuration • Operating conditions: • transport current • magneticfield, thuscable and strandconsidered

5. Resultsalong the azimuthal direction 6.5 TeV, 4.5 x10^11 protons/s Collimator settings (relaxed): TCP7 @ 6.7 σ, TCS7 @ 9.9 σ Heatdepositcomesfrom simulations by R. Bruce, B. Salvachua, S. Redaelli, L. Skordis, F. Cerutti, A. Lechner, A. Mereghetti P.P. Granieri - Quench limits

6. Results as a function of Iop, and comparison to 2013 collimation QT 2013 collimation quench test: 4 TeV, 1.63 x10^12 protons/s Collimator settings: TCP7 @ 6.1 σ, TCS7 @ 10.1 σ LHC collimation Review 2013: http://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=251588 Experiment: S. Redaelli, B. Salvachua, R. Bruce, W. Hofle, D. Valuch, E. Nebot FLUKA simulations: F. Cerutti, E. Skordis P.P. Granieri - Quench limits • mostcriticalregionsconsidered, i.e. mid-plane for MB • in agreement with the LHC collimation quench test performed in 2013

7. Current vs. previous estimations of steady-state quenchlimits P.P. Granieri • Summary of the determined steady-state cable quench limits • Previous estimations, at 7 TeV beam energy: • Jeanneret, Leroy et al. (Note 44, 1996) : 5 mW/cm3 conservative hypotheses of an insulation “assumed non porous to helium”, and a Tmargin of 1.2 K (8.65 T) “But a real insulation has helium porosities, and a better understanding of heat transfer requires an experimental approach” • Bocianet al. (2009 ): 12-17 mW/cm3 some mechanisms of heat transfer were neglected: the He II heat transfer through the insulation micro-channels, and the plateau at the boiling temperature

8. Whatcanwe do to improve the computation of steady-state quenchlimits? P.P. Granieri - Quench limits • Performheattransfermeasurementsatdifferent bath temperatures • e.g. for a bath at 2.1 K the steady-state quenchlimitisnearlyhalf the value at 1.9 K • Obtain a deeper insight of the He II heat transport mechanismsoccuring in the inter-layer region • Extend the study to the wholecoil/magnet, sincetheremightbeotherregionssaturatingbefore the coilinner layer consideredso far • Numericalmodeling of the coil, in order to simulatethe actualheatdeposit profile thatcannotbe experimentallyreproduced in a lab

9. "Nearsteady-state"cablequenchlimit P.P. Granieri - Quench limits Steady-state heattransfer conditions are reachedafter a few seconds, depending on cable, heattransfer, He temperature, etc For non steady-state mechanismsweneed to rely on numerical codes:

10. Whatelsecanwe do to improve the computation of quenchlimits? P.P. Granieri - Quench limits • Besideswhatstated few slidesago, performtransientheattransfermeasurements • Prelimiraryresults: 1.5 s to reach 90% of the steady-state temperature • More analyses willbeperformed

11. Conclusion • We presented a general method to determine steady-state quench limits of SC magnets, by measuring heat transfer on cable stacks while taking into account the cable cooling within the magnet, the coilmechanical and operating conditions • The method was successfully applied to the LHC main dipole magnets, providing an improvement w.r.t. previous steady-state quench limits estimation • good agreement with LHC collimation quench test performed in 2013 at 4 TeV • Calculations of “near steady-state” quench limits have been presented • Recommendations on how to improve the quench limit computation • In steady-state conditions • In nearsteady-state conditions P.P. Granieri - Quench limits

12. Backup slides P.P. Granieri - Quench limits

13. Deduction of cablesteady-state quenchlimits: the method P.P. Granieri - Steady-state quench limits 1) Experimentallycorrelateheat extraction and strandstemperature • heating configuration of the cables: typicallyheating all the cables • as a function of the mechanical pressure (for He II porous Nb-Ti coils) • in different positions of the cable (center vs. edge) 2) Scale the heat extraction to the coilgeometry • only the innermostcables’ small face is in direct contact with the He II bath • the outermostsmall face canbe, depending on the magnet design, in contact with He 3) ComputeTcs (Iop , B) • cable location within the coilcross-section • strand location within the cable cross-section 4) Compute the heatextractedatTcs (Iop , B) • at the pressure corresponding to the cable location within the coil cross-section • LHC dipole (MB): pressure varyingbtw 50 MPa (mid-plane) to 5 MPa (pole) • HL-LHC IR quad (MQXC): pressure varyingbtw120 MPa (mid-plane) to 25 MPa (pole) • HL-LHC IR quad (MQXF): no pressure

14. Heattransfermodels He II He I Nucleate Boiling Film Boiling Gas P.P. Granieri - Quench limits • Transientheattransferbetweenstrands and He inside the cable • Fromexperimentalresults of each He phase. But the model of the wholeprocessshouldbevalidated • Steady-state heattransferbetweencable and external He bath • Fromexperimentalresults (see first part of the talk) strands

15. Comparison to 2013 ADT-fastloss QT 2013 ADT-fastlossquench test Experiment: D. Valuch, W. Hofle, T. Baer, B. Dehning, A. Priebe, M. Sapinski Simulations: A. Lechner, N. Shetty, V. Chetvertkova P.P. Granieri - Quench limits

16. Comparison to 2013 Q6 QT I = 2500 A, quench Quenchlimitmid-plane: 20 mJ/cm3 Quenchlimitpole: 18.5 mJ/cm3 I = 2000 A, no quench Quenchlimitmid-plane: 23 mJ/cm3 Quenchlimitpole: 21.8 mJ/cm3 2013 Q6 quench test Experiment: C. Bracco, M. Solfaroli, M. Bednarek, W. Bartmann Simulations: A. Lechner, N. Shetty Very good agreement P.P. Granieri - Quench limits MQM, 4.5 K Heatdeposit ~ ns

17. Comparison to 2010 wire scanner QT 2013 wire scanner quench test Experiment: B. Dehning, A. Verweij, K. Dahlerup-Petersen, M. Sapinski, J. Emery, A. Guerrero, E.B. Holzer, E. Nebot, J. Steckert, J. Wenninger Simulations: A. Lechner, F. Cerutti P.P. Granieri - Quench limits