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
Outline • 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 P.P. Granieri - Quench limits 16/9/2013
Quench limits steady-state , mW/cm 3 transient state , mJ/cm 3 (slow losses, > 1-10 s) (fast losses) Local heat transfer from strand Dominant Heat transfer from cable to He bath to He inside the cable (through cable electrical insulation) mechanism No conclusive experiments (yet) Experiments and modeling ongoing: we rely on numerical codes : heat transfer through cable’s • electrical insulation (stack method) 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 • The deduced quench limits refer to a similar to QP3 (Arjan, • uniform heat deposit over the cable Bernhard) P.P. Granieri - Quench limits 16/9/2013
Deduction of cable steady-state quench limits • For steady-state beam losses, a quench occurs if T cable exceeds T cs (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 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 Raw data: - LHC MB and EI4: D. Richter, P.P. Granieri et al. Magnets”, IEEE Trans. Appl. Supercond. 23 submitted for publication - SSC: C. Meuris, B. Baudouy et al. - Nb 3 Sn: P.P. Granieri et al. P.P. Granieri - Quench limits 16/9/2013
Results along the azimuthal direction 6.5 TeV, 4.5 x 10^11 protons/s Collimator 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 16/9/2013
Results as a function of I op , 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 2013 collimation quench test: 4 TeV, 1.63 x 10^12 protons/s Collimator settings: TCP7 @ 6.1 σ , TCS7 @ 10.1 σ LHC collimation Review 2013: Experiment: S. Redaelli, B. Salvachua, R. Bruce, W. Hofle, D. Valuch, E. Nebot http://indico.cern.ch/conferenceOtherViews.py?vi FLUKA simulations: F. Cerutti, E. Skordis ew=standard&confId=251588 P.P. Granieri - Quench limits 16/9/2013
Current vs. previous estimations of steady-state quench limits • Summary of the determined steady-state cable quench limits Operating Beam Quench limit Magnet (mW/cm 3 ) current (kA) energy (TeV) 6.8 4 58 MB 11 6.5 49 11.8 7 47 • Previous estimations, at 7 TeV beam energy: • Jeanneret, Leroy et al. (Note 44, 1996) : 5 mW/cm 3 conservative hypotheses of an insulation “assumed non porous to helium”, and a T margin 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/cm 3 some mechanisms of heat transfer were neglected: the He II heat transfer through the insulation micro-channels, and the plateau at the boiling temperature P.P. Granieri 16/9/2013
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 P.P. Granieri - Quench limits 16/9/2013
" Near steady-state " cable quench limit • 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 16/9/2013
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 9.5 8.5 Temperature [K] 7.5 6.5 5.5 4.5 3.5 2.5 1.5 15:15.8 15:33.1 15:50.4 16:07.7 16:25.0 16:42.2 16:59.5 Time [min:sec:0] P.P. Granieri - Quench limits 16/9/2013
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 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 o f “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 16/9/2013
Backup slides P.P. Granieri - Quench limits 16/9/2013
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 T cs (I op , B) • cable location within the coil cross-section • strand location within the cable cross-section 4) Compute the heat extracted at T cs (I op , 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 P.P. Granieri - Steady-state quench limits 13 7/19/2013
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 T T He II h h strands K T T T He I h h Sat HeI Nucleate T T h h h Sat s h , nucl boil . . Boiling E E h Film film lim film Boiling E E h Gas gas gas lat Steady-state heat transfer between cable and external He bath • • From experimental results (see first part of the talk) P.P. Granieri - Quench limits 16/9/2013
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 limits 16/9/2013
Comparison to 2013 Q6 QT MQM, 4.5 K Heat deposit ~ ns I = 2000 A, no quench I = 2500 A, quench Quench limit mid-plane: 20 mJ/cm 3 Quench limit mid-plane: 23 mJ/cm 3 Quench limit pole: 18.5 mJ/cm 3 Quench limit pole: 21.8 mJ/cm 3 2013 Q6 quench test Very good agreement Experiment: C. Bracco, M. Solfaroli, M. Bednarek, W. Bartmann Simulations: A. Lechner, N. Shetty P.P. Granieri - Quench limits 16/9/2013
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. Wenninger Simulations: A. Lechner, F. Cerutti P.P. Granieri - Quench limits 16/9/2013
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