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. Sapinskifor discussing QT results & analysis

Collimation Working Group, September 16, 2013

Deduction of steady-state cable quench limits for the LHC main dipoles

P.P. Granieri - Quench limits

• Steady-state vs. transient quench limits

• Deduction of steady-state quench limit for the LHC MB cable• Method• Results and comparison to collimation quench test• Previous quench limit estimations • What can we do to improve the quench limit computation?

• "Near steady-state" cable quench limit

16/9/2013

Outline

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Quench limitssteady-state, mW/cm3 (slow losses, > 1-10 s)

transient state, mJ/cm3

(fast losses)

Heat transfer from cable to He bath(through cable electrical insulation)

Experiments and modeling ongoing:• heat transfer through cable’s electrical

insulation (stack method)

• The deduced quench limits refer to a uniform heat deposit over the cable

Local heat transfer from strand to He inside the cable

No conclusive experiments (yet) we rely on numerical codes:

P.P. Granieri - Quench limits

Dominant mechanism

0-D (ZeroDee):• uniform heat deposit and

field over cable cross-section• no longitudinal direction

1-D (THEA):• single strand experiencing a

heat deposit and field variation along its length

• similar to QP3 (Arjan, Bernhard)

P.P. Granieri - Quench limits

Deduction of cable steady-state quench limits

• For steady-state beam losses, a quench occurs if Tcable exceeds Tcs (4 - 5.5 K for the LHC MB)

• The cable quench limits depend on• Heat extraction:

• cable cooling within the magnet• mechanical pressure, if Nb-Ti coil• stack heating configuration

• Operating conditions:• transport current• magnetic field, thus cable and

strand considered

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Method reported in: P.P. Granieri and R. van Weelderen, “Deduction of Steady-State CableQuench Limits for Various Electrical Insulation Schemes with Application to LHC and HL-LHCMagnets”, 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

Results along the azimuthal direction

16/9/2013

6.5 TeV, 4.5 x 10^11 protons/sCollimator settings (relaxed): TCP7 @ 6.7 σ, TCS7 @ 9.9 σ

Heat deposit comes from simulations by R. Bruce, B. Salvachua, S. Redaelli, L. Skordis, F. Cerutti, A. Lechner, A. Mereghetti

P.P. Granieri - Quench limits

Results as a function of Iop, and comparison to 2013 collimation QT

• most critical regions considered, i.e. mid-plane for MB• in agreement with the LHC collimation quench test performed in 2013

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Experiment: S. Redaelli, B. Salvachua, R. Bruce, W. Hofle, D. Valuch, E. NebotFLUKA simulations: F. Cerutti, E. Skordis

LHC collimation Review 2013: http://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=251588

2013 collimation quench test:4 TeV, 1.63 x 10^12 protons/sCollimator settings: TCP7 @ 6.1 σ,

TCS7 @ 10.1 σ

Current vs. previous estimations of steady-state quench limits

• 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”

• Bocian et 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

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Magnet Operating current (kA)

Beam energy (TeV)

Quench limit (mW/cm3)

MB6.8 4 58

11 6.5 49

11.8 7 47

P.P. Granieri

P.P. Granieri - Quench limits

What can we do to improve the computation of steady-state quench limits?

• Perform heat transfer measurements at different bath temperatures• e.g. for a bath at 2.1 K the steady-state quench limit is nearly half the value at 1.9 K

• Obtain a deeper insight of the He II heat transport mechanisms occuring in the inter-layer region

• Extend the study to the whole coil/magnet, since there might be other regions saturating before the coil inner layer considered so far

• Numerical modeling of the coil, in order to simulate the actual heat deposit profile that cannot be

experimentally reproduced in a lab

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P.P. Granieri - Quench limits

"Near steady-state" cable quench limit

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• Steady-state heat transfer conditions are reached after a few seconds, depending on cable, heat transfer, He temperature, etc

• For non steady-state mechanisms we need to rely on numerical codes:

P.P. Granieri - Quench limits

What else can we do to improve the computation of quench limits?

• Besides what stated few slides ago, perform transient heat transfer measurements• Prelimirary results: 1.5 s to reach 90% of the steady-state temperature• More analyses will be performed

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15:15.8 15:33.1 15:50.4 16:07.7 16:24.9 16:42.2 16:59.51.5

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P.P. Granieri - Quench limits

Conclusion

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• 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 coil mechanical 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 near steady-state conditions

P.P. Granieri - Quench limits

Backup slides

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P.P. Granieri - Steady-state quench limits 13

Deduction of cable steady-state quench limits: the method

1) Experimentally correlate heat extraction and strands temperature• heating configuration of the cables: typically heating 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 coil geometry• only the innermost cables’ small face is in direct contact with the He II bath• the outermost small face can be, depending on the magnet design, in contact with He

3) Compute Tcs (Iop , B)• cable location within the coil cross-section• strand location within the cable cross-section

4) Compute the heat extracted at Tcs (Iop , B)• at the pressure corresponding to the cable location within the coil cross-section

• LHC dipole (MB): pressure varying btw 50 MPa (mid-plane) to 5 MPa (pole)• HL-LHC IR quad (MQXC): pressure varying btw 120 MPa (mid-plane) to 25 MPa (pole)• HL-LHC IR quad (MQXF): no pressure

7/19/2013

P.P. Granieri - Quench limits

Heat transfer models• Transient heat transfer between strands and He inside the cable

• From experimental results of each He phase. But the model of the whole process should be validated

• Steady-state heat transfer between cable and external He bath• From experimental results (see first part of the talk)

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P.P. Granieri - Quench limits16/9/2013

Comparison to 2013 ADT-fast loss QT

2013 ADT-fast loss quench 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 limits16/9/2013

Comparison to 2013 Q6 QT MQM, 4.5 K

Heat deposit ~ ns

Very good agreement

I = 2000 A, no quenchQuench limit mid-plane: 23 mJ/cm3

Quench limit pole: 21.8 mJ/cm3

I = 2500 A, quenchQuench limit mid-plane: 20 mJ/cm3

Quench limit pole: 18.5 mJ/cm3

2013 Q6 quench test Experiment: C. Bracco, M. Solfaroli, M. Bednarek, W. Bartmann Simulations: A. Lechner, N. Shetty

P.P. Granieri - Quench limits16/9/2013

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. WenningerSimulations: A. Lechner, F. Cerutti