1. Workshop on Beam losses, heat deposition and quench levels for LHC magnets, Geneva, 3-4 March 2005 Liquid helium heat transfer in superconducting cable insulation of accelerator magnets B. Baudouy CEA saclay

2. 2 BB / 4 March 2005 The context Heat transfer from the conductor to the cold source define the temperature margin Electrical insulation is the largest thermal barrier against cooling For LHC, there are two limits to extract the load of 10 mW/cm 3 –Full helium cooling –Full “conductive” cooling T conductor ~4 K [1] Previous works focused on the thermal paths –Creating paths between the conductors by wrapping with gaps or with dry fiber glass tapes, taking out the epoxy resin… –No complete work on the solid material (holes, conductive insert) NED load is 50 to 80 mW/cm 3

3. 3 BB / 4 March 2005 The 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 Nb 3 Sn magnet –Fiberglass tape + Ceramic precursor –Smaller Porosity (d~0.1 µm, ε?, th=400 µm) –k≈4 10 -2 W/Km (k kapton ≈10 -2 W/Km) @ 2 K Courtesy of F. Rondeaux (CEA)

4. 4 BB / 4 March 2005 Principle of the experimental model To Model the construction, geometrical, mechanical, electrical and thermal configurations of a superconducting coil Stack (Saclay and KEK) [2], [3], [4] –Insulated with real electrical insulation –Polymerization heat treatment –Under mechanical constraints –With thermal load reproducing the load in the magnet Drum (Saclay) [5] –Study the transverse heat transfer (small faces) –Real insulation –No mechanical constraints –1D heat transfer

5. 5 BB / 4 March 2005 Saclay’s Stack Stack of five insulated conductors under mechanical constraint Conductor in stainless steel heated by Joule effect Polymerization under 50 to 150 MPa at 130°C to 170°C He II and He I Real cable Stainless steel cable Thermometers

6. 6 BB / 4 March 2005 KEK’s Stack Stack of six insulated conductors under mechanical constraint Conductor in Cu-Ni(10%) strands Ø 0.8 mm (w=11 mm x h=1.5 mm) Courtesy of N. Kimura (KEK)

7. 7 BB / 4 March 2005 Experimental results with the stack (1/2) Saclay tested two types of insulation in the stack experiment and the drum experiment –All polyimide with gap –2 or 3-layer insulations –Dry fiber glass + polyimide (second layer) –Polyimide=Kapton® KEK tested all polyimide insulations –2 or 3-layer insulations –Polyimide=Peek® and Kapton® Current CERN insulation material is Apical® –Kapitza resistance and thermal conductivity @ 2 K –Just tested at Saclay

8. 8 BB / 4 March 2005 Experimental results with the stack (2/2) Epoxy Resin fills up the helium path Dry fiber thermally decouples the conductors Very small paths of helium for all polyimide insulations with gaps due to overlapping Meuris et al. [3] Baudouy et al. [5] with drum Kimura et al. [6] with stack Superfluid regime –Landau –Turbulent Mixed regime –HeII + Conduction Multiple phases –He I + He II

9. 9 BB / 4 March 2005 Heat transfer analysis with the stack Saclay numerical model KEK physical model : Only Helium Kimura et al. [6] with stack Meuris et al. [3]

10. 10 BB / 4 March 2005 Heat transfer analysis with the drum Study on conventional insulations –d~10  m, channel length ~ mm –He II in // conduction + Kapitza For Large  T, He II HT < Conduction HT Baudouy et al. [5]

11. 11 BB / 4 March 2005 Heat transfer experiments (2005 – 2006) –How does Nb 3 Sn conductor insulation behave in helium environment? –How much heat can be transferred through it? Two types of insulation are considered –glass fiber tape, vacuum-impregnated with epoxy resin –“innovative” insulation (glass fiber tape + ceramic) At least four cooling schemes are envisioned –Pool boiling He I at 4.2 K –Superfluid helium at 1 atm Construction of He II double bath Cryostat –Double bath cryostat by Wroclaw University of Technology in Poland –4.2/1.8 K Heat exchanger by CEA Saclay NED Heat transfer work package (2005-2006)

12. 12 BB / 4 March 2005 He II Heat transfer method (1/2) Characterize thermal performances in He I and II of samples representative of insulated conductors in magnet coils subjected to static heat deposition (in the 50-to- 80 mW/cm 3 range) 3 Vacuum tube First layerSecond layer Conductors Spacers Insulation Heat paths considered in parallel –Small face path –Large face path

13. 13 BB / 4 March 2005 He II Heat transfer method (2/2) Insulation 1D transverse HT through the small face Conductors 1D longitudinal HT (and transverse!) through the large face Stack = Drum + Conduit  Stack experiment 1D transverse HT (Drum set-up) 1D longitudinal HT (Conduit experiment)

14. 14 BB / 4 March 2005 The drum and conduit experiments Pressure sensor Insulation Indium joint Heater Temperature sensor Feedthrough Vacuum DP 190 glue Cea/ Saclay SIS Vacuum or insulators P T Heaters Maekawa [7]

15. 15 BB / 4 March 2005 Stack : Representative of real magnets? Is the stack experiment accurate to predict heat transfer in magnet? –A good tool to compare insulation systems –A good tool to understand heat transfer and improve the insulation systems Heat loads generated at KEK on 1-m model A new experiment or an experimental set up is needed to validate the simulation of heat transfer by the stack experiment Kimura et al. [6] with stack

16. 16 BB / 4 March 2005 References [1] Burnod L, Leroy D, Szeless B, Baudouy B, and Meuris C.Thermal modelling of the L.H.C. dipoles functioning in superfluid helium. Proceedings of 4th EPAC 1994.p. 2295-2297. [2] Meuris C. Heat transport in insulation of cables cooled by superfluid helium. Cryogenics 1991; 31: 624. [3] Meuris C, Baudouy B, Leroy D, and Szeless B. Heat transfer in electrical insulation of LHC cables cooled with superfluid helium. Cryogenics 1999; 39: 921-931. [4] Kimura N, Kovachev Y, Yamamoto A, Shintomi T, Nakamoto T, Terashima A, Tanaka K, and Haruyama T. Improved heat transfer for Rutherford-type insulated cables in pressurized He II. Proceedings of Maget technology 1998.p. 1238- 1241. [5] Baudouy B, François MX, Juster F-P, and Meuris C. He II heat transfer through superconducting cables electrical insulation. Cryogenics 2000; 40: 127-136. [6] Kimura N, Yamamoto A, Shintomi T, Terashima A, Kovachev V, and Murakami M. Heat transfer characteristics of Rutherford-type superconducting cables in pressurized He II. Ieee Transactions on Applied Superconductivity 1999; 9: 1097- 1100. [7] Maekawa R. and Baudouy B. Heat transfer through porous media in the counterflow regime of He II. Proceedings of Cryogenic Engineering Conference 2003.p. 983-990.