WS-Summary based on common “Close-out” by session chairs

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

2. Goals of Thermomag-07 Minimizing and evacuating heat is one of the main challengesfor 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

3. 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

4. Monday, 19 November 2007 • 09:30->14:00    Morning: Introduction and heat generation (B. Baudouy) • 09:30 Welcome by Antoine DAËL (CEA) • 09:40 Introduction to the workshop by Bertrand BAUDOUY (CEA) • 09:50 Cryogenics for superconducting magnets by Luigi SERIO (CERN) • 10:30  Thermal design criteria for various cooling schemes by Rob VAN WEELDEREN (CERN) • 11:10  Break • 11:30 Beam induced losses by Elena WILDNER (CERN) • 12:00 Cable and magnet losses by Luca BOTTURA (CERN) • 12:30 Lunch • 14:00->18:00    Afternoon : Heat transfer (D. Tommasini) • 14:00 Mechanisms of heat extraction through cable insulation by Bertrand BAUDOUY (CEA) • 14:30  Cable in conduit and thermal budget at Nuclotron by Alexandre KOVALENKO (JINR, Dubna) • 15:00 Nb3Sn versus NbTi in He II by Davide TOMMASINI (CERN) • 15:30 Heat and mass transfer in superfluid helium through porous media by Hervé ALLAIN (CEA) • 16:00 Break • 16:30 Modeling of quench levels induced by steady state heat disposition by Dariusz BOCIAN (CERN) • 17:00 Modeling of cable stability margin for transient perturbations by Pier Paolo GRANIERI • 17:30 Discussion

5. Tuesday, 20 November 2007 • 09:00->14:00    Morning : experimental results (R. van Weelderen) • 09:00 Transient Thermohydraulics measurement in cooling channels for the Iseult magnet by Philippe BREDY (CEA Saclay) • 09:30 Design criteria for cable in conduit conductions in relation with expected disturbances by Jean-Luc DUCHATEAU (CEA) • 10:00 First results of experiments at WUT by Maciej CHOROWSKI (WUT) • 10:30 Experience at CEA (30') Jaroslaw POLINSKI (CEA Saclay) • 11:00 Break • 11:15  Experience at CERN by David RICHTER (CERN) • 11:45 Experience at KEK (30') Nobuhiro KIMURA (KEK) • 12:15 Diversity of heat transfer requirements for FAIR magnet applications by Marion KAUSCHE (GSI) • 12:45 Lunch • 14:00->18:30    Afternoon : round table & closeout • 14:00 Round table (1h30') Andrzej SIEMKO (CERN) • 15:30 Break • 16:00 Close out (30') Davide TOMMASINI (CERN)

6. 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”

7. The “debris-cone” IP Triplet Absorber 19 m 1.7 m 23 m

8. 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. TheMARS code system user's guide. Fermilab-FN-628, Fermi National Accelerator Laboratory (1995).

9. 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

10. 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

11. 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]

12. CLASSIFICATION OF HEAT EXTRACTION PATHS‏ Example: NED dipole/Q1 LHC inner triplet upgrade, 100 W/m, up to 5 m longitudinal heat extraction length, Tbath~1.935 K, ∆T~85 mK, Aspec ~0.55: --> A~470 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 Tbath~2.020 K, ∆T~65 mK, Aspec ~0.75: --> A~0.21 cm2to be provided in the collar & yoke laminations every 10 cm Conclusion: values are still in the “constructable” range

13. Pulsed Field Conductor … and cost ! 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

14. 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

15. 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)

16. 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]

17. 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) ΔT=5 mK @ 150 mW through 5 conductors

18. Glass-fibre epoxy insulation developed by RAL Determination of λ and Kapitza resistance Λ 4 times lower than kapton Rkapitza identical NED : Conventional insulation Canfer [7] NED conventional insulation LHC SSC Increasing permeability NED Ceramic insulation Baudouy [8] ΔT=5 mK @ 150 mW through 5 conductors

19. Enhanced porosity

20. Tests Results/1 Vertical compression 10 MPa

21. Heat transfer tests

22. 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”

26. 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.

27. The Stekly criterion in question The Stekly criterion imposes that the copper section of the cable has to be adjusted such as <1 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

28. 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).

29. 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

30. 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 < 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 > 2 W/m look for alternatives in Nb3Sn, or NbTi CICC, or need of specific studies

31. 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

32. 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