Steady-State Heat Transfer in the LHC Superconducting Coils:
Status of Experiments and Modeling
Pier Paolo Granieri
Karlsruhe Institute of Technlology (KIT)
Institute for Technical Physics (ITEP)
pier.granieri@kit.edu
KIT – Universität des Landes Baden-Württemberg und
nationales Forschungszentrum in der Helmholtz-Gemeinschaft
Workshop on Beam-Induced Quenches
September 15-16, 2014
CERN
www.kit.edu
Acknowledgements
D. Richter, D. Tommasini, P. Fessia, B. Baudouy, L. Bottura, L. Rossi,
P. Lebrun, R. van Weelderen, D. Santandrea, T. Koettig, J. Bremer, L.
Dufay-Chanat, D. Cochet, S. Prunet, J.-L. Carpano, M. Breschi, C.
Meuris, B. Jeanneret, D. Leroy. N. Kimura, T. Nakamoto, A. Four, F.
Lentijo, A. Mapelli, P. Petagna, J-B. Bureau, S. Luzieux, F. Beauvais,
A. Bonasia, P. Jacquot, S. Clement, A. Benfkih, R. Bruce, B.
Salvachua, S. Redaelli, L. Skordis, F. Cerutti, A. Lechner, M. Sapinski,
B. Auchmann, A. Verweij, L. Esposito. B Colijn
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Heat transfer in SC coils
Steady-state (mW/cm3)
slow losses
Dominant mechanism:
Heat transfer from cable to He bath (through
cable electrical insulation)
Experiments and modeling ongoing
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Pier Paolo Granieri
Transient state (mJ/cm3)
fast losses
Dominant mechanism:
Local heat transfer from strand to He inside
the cable
No conclusive experiments (yet) 
numerical codes
Institute for Technical Physics (ITEP)
Outline
Steady-State heat transfer in LHC cables and quench limits
1) Historical overview
2) Investigations performed in the last years: 2008 – 2013
3) Most recent advances: 2014
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Outline
Steady-State heat transfer in LHC cables and quench limits
1) Historical overview
2) Investigations performed in the last years: 2008 – 2013
3) Most recent advances: 2014
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Historical overview (1/3)
CEA-Saclay (1991 – 1999): first development of the stack method
measure the heat transfer through the cable’s electrical insulation
Quench limit estimation at 7 TeV : 5 mW/cm3
insulation “assumed non porous to helium”,
Tmargin of 1.2 K (8.65 T)
“But a real insulation has helium porosities, …“
SSC
LHC
Increasing permeability
C. Meuris, “Heat transport in insulation of cables cooled by superfluid helium”, Cryogenics, vol. 31, pp. 624–628, 1991.
C. Meuris, B. Baudouy, D. Leroy, B. Szeless, “Heat transfer in electrical insulation of LHC cables cooled with superfluid helium”, Cryogenics, vol. 39, pp. 921–931, 1999.
B. Baudouy, Etude des transferts de chaleur dans les isolations électriques de câbles supraconducteurs d’aimant d’accélérateur refroidi par hélium superfluide, Ph.D. dissertation, 1996.
L. Burnod, D. Leroy, B. Szeless, B. Baudouy, C. Meuris, “Thermal modeling of the LHC dipoles functioning in superfluid helium”, Proc. EPAC ‘94, London, England, pp. 2295–2297, 1994.
B.J. Jeanneret, D. Leroy, L.R. Oberli, T. Trenkler, Quench levels an transient beam losses in LHC magnets, LHC Project report 44, CERN, Switzerland, 1996.
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Historical overview (2/3)
CEA-Saclay (1995-2000): drum experiment:
(1)
(1)
Espacement
Measurement of 1D heat transfer
Better understanding of heat transfer
Seconde couche
Première couche
Vu
Vue en perspective
0,25
Q isol
Q HeII
Q calcul
T i - T b (K)
0,2
0,15
0,1
Mesure
0,05
 A 

Ti
He II : Q HeII  L1 3  (T) dT
 T
Conduction : Qisol = ²T/R
b
Calcul : Qisol + QHeII
0
0
0,05
Qm
0,1
0,15
0,2
Q*
0,25
Q (W)
B. Baudouy, M.X. François, F.P. Juster, C. Meuris, “He II heat transfer through superconducting cables electrical insulation”, Cryogenics, vol. 40, pp. 127–136, 2000.
B. Baudouy, Etude des transferts de chaleur dans les isolations électriques de câbles supraconducteurs d’aimant d’accélérateur refroidi par hélium superfluide, Ph.D. dissertation, 1996.
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Historical overview (3/3)
CERN (2007):
Real piece of coil
Heat generated by DC currents through
resistive inter-strand contacts
CERN (2009):
Network model
Tests, where heat was generated using a
inner heating apparatus
Quench limit estimation at 7 TeV :
17 mW/cm3
12-
some mechanisms of heat transfer were
neglected: the He II heat transfer through the
insulation micro-channels, and the plateau at
the boiling temperature (see slide 12)
D. Richter et al., “Evaluation of the transfer of heat from the coil of the LHC dipole
magnet to helium II”, IEEE Trans. Appl. Sup., vol. 17, no. 2, pp. 1263–1268, 2007.
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Pier Paolo Granieri
D. Bocian, B. Dehning, A. Siemko, “Quench limit model and measurement for steady state
heat deposit in LHC magnets”, IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 2446–2449,
2009.
Institute for Technical Physics (ITEP)
Conclusion (1st part)
The need for experimental tests was clear from the beginning, due to
the unknown He distribution in the cable´s insulation and in the coil
Beneficial effect of He II was assessed
Quench limits based on conservative assumptions: no He II
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Outline
Steady-State heat transfer in LHC cables and quench limits
1) Historical overview
2) Investigations performed in the last years: 2008 – 2013
3) Most recent advances: 2014
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Pier Paolo Granieri
Experiments and Modeling
Institute for Technical Physics (ITEP)
Experimental technique
New specificities wrt previous stack-type
experimental setups (CEA, KEK, CERN)
thermometry
real cable structure
controlled pressure
heating configurations
P
Cable
center
Cable edge
Stack sketch by S. Pietrowicz
N. Kimura et al., “Heat transfer characteristics of Rutherford-type superconducting cables in pressurized He II”, IEEE Trans. Appl. Supercond., vol. 9, no. 2, pp. 1097–1100, 1999.
P.P. Granieri, P. Fessia, D. Richter, D. Tommasini, “Heat transfer in an enhanced cable insulation scheme for the sc magnets of the LHC luminosity upgrade”, IEEE Trans. App. Sup., 20, 2010.
P.P. Granieri, Heat transfer between the superconducting cables of the LHC accelerator magnets and the superfluid helium bath, Ph.D. dissertation, 2012.
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Experimental results
Different regimes, depending on the He phases, in
parallel with polyimide conduction
Uniform cable temperature below Tλ
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Effect of mechanical pressure
Does the heat extraction from the cable (the He II porosity) depend on the applied pressure?
P
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Numerical modeling
Quantitative analysis of Nb-Ti coils,
accounting for anisotropic coupled 3-D
thermal mechanisms in the superfluid
helium region
Unknown dimensions:
◦ thsmall
◦ Alarge
◦ Achannel (%)
Sketch by B. Baudouy

Small face:

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Pier Paolo Granieri
Large face:
Institute for Technical Physics (ITEP)
Numerical modeling
Quantitative analysis of Nb-Ti coils, accounting for anisotropic 3-D thermal
mechanisms in the superfluid helium region
determine heat fluxes in coil cross-section
estimate characteristic dimensions
Steady-state heat balance equations solved at the model nodes
2-D cables cross-section
3-D shape of channels
Coupled heat transfer mechanisms:
polyimide conduction
He II dynamic regimes
(channel size, heat flux)
classical He II laws assumed
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Numerical results
The model allows to identify the different contributions to the heat transfer
through the LHC insulation
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Heat transfer in He II through µ-channels
In modeling, He II classical heat transport laws were assumed to
be valid in narrow channels
A study is ongoing to validate such hypothesis (collaboration with
CEA-Saclay), making use of MEMS techniques
ΔT
Q
Q
So far, Landau and Gorter-Mellink regime
measured down to 10 μm thickness
P.P. Granieri, B. Baudouy, A. Four, F. Lentijo, A. Mapelli, P. Petagna and D. Tommasini, “Steady-state heat transfer through micro-channels in pressurized He II”, Adv. in Cryogenic
Engineering.
P.P. Granieri, Heat transfer between the superconducting cables of the LHC accelerator magnets and the superfluid helium bath, Ph.D. dissertation, 2012.
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Deduction of cable steady-state
quench limits
For steady-state beam losses, a quench
occurs if Tcable exceeds Tcs
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
P.P. Granieri, R. van Weelderen, “Deduction of Steady-State Cable Quench Limits for Various Electrical Insulation Schemes with Application to LHC and HL-LHC Magnets”, IEEE Tr. App. Sup.
23.
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
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
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Different quench limit estimations
as a function of the operational current
most critical region considered, i.e. mid-plane
« in agreement » (?) with the LHC collimation quench test performed in 2013
Magnet
MB
2013 collimation quench test:
4 TeV, 1.63 x 10^12 protons/s
Collimator settings: TCP7 @ 6.1 σ,
TCS7 @ 10.1 σ
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Experiment: S. Redaelli, B. Salvachua, R. Bruce, W. Hofle, D. Valuch, E. Nebot
FLUKA simulations: F. Cerutti, E. Skordis
Pier Paolo Granieri
Beam
energy
(TeV)
Quench limit
(mW/cm3)
4
58 – 80
6.5
49 – 57
7
47 - 49
LHC collimation Review 2013:
http://indico.cern.ch/conferenceOtherViews.py?vi
ew=standard&confId=251588
Institute for Technical Physics (ITEP)
"Near steady-state" cable quench limit
(numerical calculation)
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:
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Conclusion (2nd part)
A significant effort has been done in the last 5 years, both from the experimental
and theoretical point of view
We improved a lot our knowledge of
Heat transfer mechanisms in the magnets
Quench limits
Open questions
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
Actual He II distribution in the coil (different from the simpler stack configuration)
Address other LHC magnets than the MB
Non uniform heat deposit ?
Should the study be extended to the whole coil/magnet ? There might be other regions
saturating before the coil inner layer considered so far
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Outline
Steady-State heat transfer in LHC cables and quench limits
1) Historical overview
2) Investigations performed in the last years: 2008 – 2013
3) Most recent advances: 2014
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
New instrumentation technique
Central cable instrumented with:
Bare Chip Cernox TM (CX)
In situ calibrated Au-Fe0.07% - Chromel thermocouples (TC)
Current
Leads
Reference
Cernox
TC´s cold
junction
Sampl
e
holder
Sampl
e
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Pier Paolo Granieri
Helium
transfer
line
Institute for Technical Physics (ITEP)
Experimental results: MB
In the He II region:
He heat transfer mechanisms are well identified
as well as the hydrostatic pressure effect
Homogeneous temperature in the cable
P.P. Granieri, D. Santandrea, T. Koettig, R. van Weelderen, “Heat Extraction from the LHC Main Dipole, Main Quadrupole and MQXA Superconducting Cable”, IEEE Tr. App. Sup., submitted.
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Experimental results: MQ and MQXA
In the He II region:
Assessed effect of Tbath (not constantly 1.9 K in the LHC)
The hydrostatic pressure effect on MQ and MQXA does not perfectly agree with theory, as it does for the
other samples
For higher temperatures, towards Tcs:
Similar behavior of the 3 samples, if power expressed in mW/cm3
Non-homogeneous temperature in the cable
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Coil-like sample – MB
Fishbone
Inner
layer
Ground
insulation
Instrum.
cable
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Pier Paolo Granieri
Heaters
Outer
layer
Institute for Technical Physics (ITEP)
Experimental results: coil-like sample
In the He II region:
Assessed effect of Tbath
Assessed effect of the actual beam loss scenario
A different behavior of the various MB samples is observed
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Conclusion (3rd part)
The instrumentation technique was improved, allowing to use Cernox bare
chips and to in-situ calibrate the thermocouples
Heat extraction was measured for the 1st time on a sample accurately
reproducing the coil
as well as on stack-samples of the most critical LHC magnets:
Main Bending dipoles (MB)
Main Quadrupoles (MQ)
Low-β quadrupole MQXA
To be done in the near future
The coil-like sample will be tested in pressuruzed He II. In this way the magnet
quench limit can be directly calculated without needing assumptions on the He II
distribution
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)
Conclusion
A good comprehension of the SC magnets thermal behavior will become critical for the
next LHC operation at 13-14 TeV
In the last years we have constantly improved our understanding of the steady-state
heat transferthrough experimental and theoretical (modeling) work
However, not all the mechanisms are well understood
Open questions
1) A fully understanding of the experimental results would require:
more statistics
to assess the impact of the instrumentation technique
a deeper comprehension of the He phase transition mechanisms
2) The large amount of results obtained is a nice playground for modeling, which is
needed to fully exploit the experimental results (including determining the quench
limits), to simulate conditions that cannot be experimentally reproduced in a lab, etc
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Pier Paolo Granieri
Institute for Technical Physics (ITEP)