WS-Summary based on common

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WS-Summarybased on common Close-out by session
chairs
Thermomag-07 A CARE-HHH workshop on Heat
Generation and Transfer in Superconducting
Magnet 19-21 November 2007 Paris
B. Baudouy, A. Siemko, D. Tommasini, R. van
Weelderen
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Goals of Thermomag-07

• Minimizing and evacuating heat is one of the main
  challenges for the next generation of
  superconducting magnets for high intensity
  particle accelerators such as the IR magnets for
  the LHC luminosity upgrade and the fast cycled
  magnets for FAIR, PS2, SPS
• The WS aims at reviewing the present knowledge on
  heat transfer in superconducting magnets and
  identifying a common thermal design basis
• Identify the state of the art on
• Cooling techniques (fluids and regimes)
• Heat transfer mechanisms
• Modeling of heat transfer from coils to cooling
  system
• Heat transfer experiments
• Identify a common set of thermal design criteria

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Participation

• TOTAL 33
• CERN 13
• INFN 4
• GSI 4
• CEA 3
• EPFL 3
• Wroclaw Univ. 1
• ENEA 1
• KEK 1
• Twente Univ. 1
• JINR 1
• EFDA 1

All researchers directly or indirectly working on
the subject were present
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Monday, 19 November 2007

• 0930-gt1400    Morning Introduction and heat
  generation (B. Baudouy)
• 0930 Welcome by Antoine DAËL (CEA)
• 0940 Introduction to the workshop by
  Bertrand BAUDOUY (CEA)
• 0950 Cryogenics for superconducting magnets by
  Luigi SERIO (CERN)
• 1030  Thermal design criteria for various
  cooling schemes by Rob VAN WEELDEREN (CERN)
• 1110  Break
• 1130 Beam induced losses by Elena WILDNER (CERN)
• 1200 Cable and magnet losses by
  Luca BOTTURA (CERN)
• 1230 Lunch
• 1400-gt1800    Afternoon Heat transfer (D.
  Tommasini)
• 1400 Mechanisms of heat extraction through cable
  insulation by Bertrand BAUDOUY (CEA)
• 1430  Cable in conduit and thermal budget at
  Nuclotron by Alexandre KOVALENKO (JINR, Dubna)
• 1500 Nb3Sn versus NbTi in He II by
  Davide TOMMASINI (CERN)
• 1530 Heat and mass transfer in superfluid helium
  through porous media by Hervé ALLAIN (CEA)
• 1600 Break
• 1630 Modeling of quench levels induced by steady
  state heat disposition by Dariusz BOCIAN (CERN)
• 1700 Modeling of cable stability margin for
  transient perturbations by Pier Paolo GRANIERI
• 1730 Discussion

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Tuesday, 20 November 2007

• 0900-gt1400    Morning experimental results
  (R. van Weelderen)
• 0900 Transient Thermohydraulics measurement in
  cooling channels for the Iseult magnet by
  Philippe BREDY (CEA Saclay)
• 0930 Design criteria for cable in conduit
  conductions in relation with expected
  disturbances by Jean-Luc DUCHATEAU (CEA)
• 1000 First results of experiments at WUT by
  Maciej CHOROWSKI (WUT)
• 1030 Experience at CEA (30') Jaroslaw POLINSKI (C
  EA Saclay)
• 1100 Break
• 1115  Experience at CERN by David RICHTER (CERN)
• 1145 Experience at KEK (30') Nobuhiro KIMURA (KEK
  )
• 1215 Diversity of heat transfer requirements for
  FAIR magnet applications by Marion KAUSCHE (GSI)
• 1245 Lunch
• 1400-gt1830    Afternoon round table
  closeout
• 1400 Round table (1h30') Andrzej SIEMKO (CERN)
• 1530 Break
• 1600 Close out (30') Davide TOMMASINI (CERN)

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General considerations (1)
The subject is not new, but is taking a NEW
importance for projects like FAIR, and the LHC
upgrades (injectors IR) Heat deposition
modeling (E. Wildner/CERN, INFN, LARP) need
improved feedback from magnet designers, and
thermal simulation criteria) (IR-)Magnet
structure cooling (R. van Weelderen/CERN,
LARP) values for 50 W/m up to 100 W/m are still
in the constructible range, major limit at coil
to bath thermal pathway Fast ramp magnets for
PS2, SPS, FAIR (L. Bottura/CERN, M. Kausche/GSI,
A. Kovalenko/JINR) A reasonable internal heat
load target per unit length for future
superconducting ring accelerator magnets is in
the range of 5 W/m to 10 W/m with hollow
conductors 100 W/m is achievable
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The debris-cone
IP
Triplet
Absorber
19 m
1.7 m
23 m
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Simulation codes

• Fluka
• "FLUKA a multi-particle transport code",A.
  Fasso, A. Ferrari, J. Ranft, and P.R.
  Sala,CERN-2005-10 (2005), INFN/TC_05/11,
  SLAC-R-773
• Geant
• Nuclear Instruments and Methods in Physics
  Research A 506 (2003) 250-303, and IEEE
  Transactions on Nuclear Science 53 No. 1 (2006)
  270-278.
• Mars
• Mokhov, N. V. The MARS code system user's guide.
  Fermilab-FN-628, Fermi National Accelerator
  Laboratory (1995).

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Results, what we consider

• Cable
• We make the binning for the scoring so that it
  corresponds to a maximum volume of equilibrium
  for the heat transport (cable transverse size,
  with a length of around 10 cm, value to be
  confirmed)
• Total power deposited in the magnets
• Important to know the volume of the magnet (the
  model has to be realistic)
• The power deposited per meter of magnet

N.B. For the total energy deposited we need a
realistic design of the magnet
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CLASSIFICATION OF HEAT EXTRACTION PATHS?
Heat transfer
?Tcoil typically 80-90 mK available down from
2.17 K max ?Tcoil-freeA (radial)? typically
60-70 mK available around 2.050 K ?TfreeA-bHX
(longitudinal)? typically 80-90 mK available
around 1.98 K about 160 mK remains for heat
transfer to cold source and up to cold compressors
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Specific Conductive Cross section (cm2/W/m
m3/4)?
Aspec is in the range of 0.3 to 1.1 cm2/W/m m3/4
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CLASSIFICATION OF HEAT EXTRACTION PATHS?

• Example
• NED dipole/Q1 LHC inner triplet upgrade, 100
  W/m, up to 5 m longitudinal heat extraction
  length, Tbath1.935 K, ?T85 mK, Aspec 0.55
• --gt A470 cm2 to be made in the yoke
• Assuming 15 of the cold mass volume
  taken up by the coil, which is what needs to be
  condcuted our radially over 0.05 m at
  Tbath2.020 K, ?T65 mK, Aspec 0.75
• --gt A0.21 cm2 to be provided in the collar
  yoke laminations every 10 cm
• Conclusion values are still in the
  constructable range

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Summary - 1/2

• It is always beneficial to minimize AC loss,
  compatibly with protection, stability (transient
  heat balance) and current distribution

Current Distribution
The tri-lemma of the optimum pulsed
superconducting cable design (PERITUS DELINEANDI
OPTIMORUM DUCTORUM) (courtesy of P. Bruzzone,
ECOMAG-05)
Heat Balance Protection and Stability
AC Loss
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Summary - 2/2

• The best compromise of AC loss, current
  distribution, heat transfer, and cost can only be
  found in conjunction with the specific needs of
  the accelerator system and magnet design
• A reasonable internal heat load target per unit
  length for future superconducting ring
  accelerator magnets is in the range of 5 W/m to
  10 W/m
• Higher values are not economically interesting
• Lower values may bear too much complication in
  the cable design
• The above target may be largely exceeded, for
  specific applications and locations, and over
  short lengths

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General considerations (2)
For Rutherford cables for a long time the only
systematic experience on thermal transfer from
cable to helium bath was in CEA, limited to 1.9
K, and some activity in KEK Now there is very
recent activity at KEK, CERN and Wroclaw
University of Technology Work on Ceramic
insulating schemes (CEA) Classical
high-conductive schemes (RAL), Highly porous
kapton wrapping schemes (CERN, WUT)
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Electrical insulation

• Historical insulation 2 wrappings
• First wrapping in polyimide with 50 overlap
• Second wrapping in epoxy resin-impregnated
  fiberglass with gap
• The LHC insulation work 2 wrappings
• First wrapping in polyimide with 50 overlap
• Second wrapping in polyimide with polyimide glue
  with gap
• Current LHC Insulation 3 wrappings
• First 2 wrappings with no overlap
• Last wrapping with a gap
• Innovative insulation for Nb3Sn magnet
• First wrapping 50

Courtesy of F. Rondeaux (CEA)
Baudouy 1, Meuris 2 and Puigsegur 3
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NED Innovative insulation

• One wrapping with 50 overlap
• Heat treatment of 100 h at 660 C
• 10 MPa compression only !
• 5 conductors heated

LHC
SSC
Baudouy 8
Increasing permeability
Courtesy of F. Rondeaux (CEA)
?T5 mK _at_ 150 mW through 5 conductors
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NED Conventional insulation

• Glass-fibre epoxy insulation developed by RAL
• Determination of ? and Kapitza resistance
• ? 4 times lower than kapton
• Rkapitza identical

Canfer 7
NED conventional insulation
LHC
SSC
Increasing permeability
NED Ceramic insulation
Baudouy 8
?T5 mK _at_ 150 mW through 5 conductors
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Enhanced porosity
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Tests Results/1
Vertical compression 10 MPa
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Heat transfer tests
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General considerations (3)

• Activities in thermal modeling, needing
  experimental support and validation
• Heat and mass transfer through superfluid porous
  media (H. Allain/CEA, B. Baudouy/CEA)
• working experiment, modeling reasonable but in
  every regime still issues to be resolved
• General magnet cooling, steady state and
  transient (M. Chorowski/WUT, R. van
  Weelderen/CERN)
• Based on ANSYS ICEM, CFX and dedicated Helium
  modules Superfluid helium conduction module
  under development, code comparison with
  analytical literature data has started
• Supercritical Helium cooling modeling (FAIR)
• Modeling of quench levels by steady state beam
  loss heat load (D. Bocian/CERN, A. Siemko/CERN)
• Equivalent resistance network model Validation
  of model with magnets at 4.5 K within 20,
  validation of model at 1.9 K not completed
• CICC stability (J. L. Duchateau/CEA)
• Stekly criterion not adequate for conductor
  design for fusion applications, the less copper,
  the higher the stability limit

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The Stekly criterion in question
TF ITER conductor prototype manufactured by Nexans
a non copper section Anoncu, a copper section Acu
and an helium section AHe. In a project like
ITER the optimum composition of the conductor
components is calculated through the so- called
design criteria.   The recent review of the ITER
project has led to some interrogation about the
systematic use of the Stekly criterion to
calculate the copper section of the ITER PF NbTi
coils.  
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The Stekly criterion in question
The Stekly criterion imposes that the copper
section of the cable has to be adjusted such as
?lt1 to be in the so-called well-cooled region
with ? being the Stekly parameter The Stekly
criterion expresses that when the strands are
taken at a temperature above Tc by a disturbance,
the CICC can be stable and can recover by
evacuating the power generated in copper as it is
in communication through heat transfer with an
infinite bath whose temperature is at T0 if the
criterion is respected. In practice in case of
NbTi, the application of the Stekly criterion can
lead to very high copper to non copper ratio
increasing the price of conductor
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Conclusion
No, Stekly criterion is not adequate to design
conductors for fusion application. Contrary to
Stekly criterion, it has been demonstrated that,
for a given composite allocation, the stability
limit in energy for a disturbance (100 ms, long
length of CICC) is a decreasing function of the
copper content The less copper, the highest
the stability limit !  The particular role of
?He and ?comp is highlighted thanks to a
simplified approach which demonstrates that the
critical energy is essentially linked to the
current sharing temperature in poor cooled
regime. The crucial role of h is linked to these
two parameters. Copper is necessary for
intrinsic dynamic stability but also for short (1
ms) mechanical disturbances applied to small
length of CICC (?1 cm).
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General considerations

• THERMOMAG is the first workshop dedicated to
  heat transfer in superconducting accelerator
  magnets
• not yet a fully synergic activity between teams
• the subject is not new, but is taking a NEW
  importance for projects like FAIR, and the LHC
  upgrades (injectors IR)
• for Rutherford cables for a long time the only
  systematic experience on thermal transfer from
  cable to helium bath was in CEA, limited to 1.9
  K, and some activity in KEK
• very recent activity at KEK, CERN and Wroclaw
  University of Tec
• activities in modeling, needing experimental
  support and validation
• activities in development of new insulation
  schemes/materials

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Thermal design criteria for accelerator magnets

• we believe the following principles can be used
  as guidelines
• introduce thermal design in early stage
• lattice magnets designed for lt 10 W/m (includes
  5 for beam losses) to be economical
• in HeII (50-100 mm aperture order)
• up to 50 -100W/m, for short (up to 40m) magnet
  strings hard limit basically cable insulation
• insulation porosity dominates heat extraction
  from cable through insulation
• below 9T temperature margin may not be an
  argument for Nb3Sn
• in supercritical helium or two phase (50-100 mm
  aperture order)
• with Rutherford cables 30 W/m, however further
  limited by cable insulation
• with hollow conductors 100 W/m are achievable
• for heat loads gt 2 W/m look for alternatives in
  Nb3Sn, or NbTi CICC, or need of specific studies

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Desirable work

• critically review and organize heat transfer
  experience data
• characterize heat exchange in supercritical He
• fundamental experiments in narrow channels/voids
  in all regimes
• investigate role of porosity in supercritical
  helium
• investigate applicability and benefits of high
  thermal conduction insulation especially in
  supercritical He
• continue the effort in development of insulation
  schemes/materials
• continue and consolidate the effort in modeling
• Strengthen communication beam loss calculation
  teams with magnet and thermal designers
• design experiments to validate models in the
  different regimes
• all this can be done ONLY by an efficient network
  of collaborations

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Proposed initiatives

• we define a community list
• web site on heat transfer with integrated
  database
• each laboratory writes a short report of
  on-going activities by Jan 31st
• this is circulated within the community list
  with feedback for an organized work with
  tentative list of deliverables
• by Feb 28th we agree on a proposal for
  deliverables
• we will then try, where applicable, to get
  formal agreement
• we issue a status report by June 30th 2008
• we meet again in autumn 2008
• A dedicated coordinator could improve the
  effectiveness of the community