Workshop on Beam losses, heat deposition and quench levels for LHC magnets, Geneva, 3-4 March 2005

Liquid helium heat transferin superconducting cable

insulation of accelerator magnets

B. BaudouyCEA saclay

2BB / 4 March 2005

The context

•Heat transfer from the conductorto the cold source define thetemperature margin

•Electrical insulation is the largestthermal barrier against cooling

•For LHC, there are two limits to extract the load of 10 mW/cm3

– Full helium cooling– Full “conductive” cooling

• Tconductor~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/cm3

3BB / 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 Nb3Sn magnet

– Fiberglass tape + Ceramic precursor

– Smaller Porosity (d~0.1 µm, ε?, th=400 µm)

– k≈4 10-2 W/Km (kkapton≈10-2 W/Km) @ 2 K

Courtesy of F. Rondeaux (CEA)

4BB / 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

5BB / 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

6BB / 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)

11 mm

1.47 mm

CuNi strand wires

thermometer

Stycast

electrical insulation tapes

7BB / 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

8BB / 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 drumKimura et al. [6] with stack

•Superfluid regime– Landau– Turbulent

•Mixed regime– HeII + Conduction

•Multiple phases– He I + He II

9BB / 4 March 2005

Heat transfer analysis with the stack

•Saclay numerical model

•KEK physical model : Only Helium

Kimura et al. [6] with stack

uniformly heatedconductor

internal helium

insulator 1

insulator 2

longitudinal conduit

gap

insulator

Tb

Tb

neighbour

small

faceinsulatio

n

small face insulation

large face insulation

Meuris et al. [3]

10BB / 4 March 2005

Heat transfer analysis with the drum

•Study on conventional insulations

– d~10 m, channel length ~ mm

– He II in // conduction + Kapitza

0

0,05

0,1

0,15

0,2

0,25

0 0,05 0,1 0,15 0,2 0,25

Mesure

He II :

Conduction :

Calcul :

T i -

T b

(K

)

Q (W)

Qm

Q*

Qisol

QHeII

Qcalcul

QHeII

AL1 3

(T) dTTb

Ti

Q

isol + Q

HeII

Qisol

= ²T/R

2

3 4 3n

s

AT q q

s T

0

5 1012

1 1013

1,5 1013

2 1013

1,5 1,6 1,7 1,8 1,9 2 2,1 2,2

0,1 MPa

Pvs

ƒ(T

) (

W3 m

-5K

-1)

Température (K)

For Large T, He II HT < Conduction HT

Baudouy et al. [5]

11BB / 4 March 2005

•Heat transfer experiments (2005 – 2006)

– How does Nb3Sn 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)

12BB / 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/cm3 range)

3Vacuum tube

First layer Second layer

Conductors

Spacers Insulation

Heat paths considered in parallel

– Small face path

– Large face path

13BB / 4 March 2005

He II Heat transfer method (2/2)

Première couche recouvrement à 50%

Deuxième couche

Espacement de l'enrubannage de la deuxième couche

Insulation1D 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)

14BB / 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

HeatersMaekawa [7]

15BB / 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

16BB / 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.