Heat management issues A perspective view centered on the design of superconducting devices Opinions of L. Bottura At the mWorkshop on Thermal Modeling

Heat management issuesA perspective view centered on the

design of superconducting devices

Opinions of L. BotturaAt the mWorkshop on Thermal Modeling and Thermal

Experiments for Accelerator Magnets

Sept. 30th, Oct. 1st, 2009

Outline Matters of steady state heat removal

from coils, magnets and other superconducting systems

Stability of superconductors Magnet quenches, and not only




Cooling in steady operation - 1 Coil heat transfer

What is the maximum power that can be removed from a coil ?

Focus on strand-to-helium heat transfer, through the insulation

Mainly motivated by LHC operation, NIT, HFM on the longer term

Magnet heat transfer Same motivation as above, extend the analysis to

the proximity cryogenics

We will hear extensively on both

Cooling in steady operation - 2 Force-flow cooling

Pressure drop, flow and heat transfer in supercritical helium flow

Relevant for the SC link foreseen for the Phase I upgrade (and FCM ?)

For SC link, two options being designed and prototyped

NbTi (Tmax < 5.5 K !) through APUL at FNAL MgB2 (Tmax < 10…15 K) at CERN

A long SC link, possibly vertical geometry, is being designed within the scope of WP 7.5 of EuCARD program

NIT - SC link8 x 0.6 kA

3 kA

14 kA

7 x 14 kA + 7 x 3 kA + 8 x 0.6 kA

Cable R&D by courtesy of A. Ballarino, CERN-TE-MSC

Aha ! FCMInternally cooled cable prototype

CERN/BNG Dstrand: 0.6 mm Jc(4.2 K, 5 T) > 2500 A/mm2

Cu:CuMn:NbTi = 2.4 Deff < 3 m < 1 ms Number of strands = 32 Cable twist pitch < 80 mm Bnom = 2 T Tnom = 4.5 K Inom = 5800 A Ic = 11500 A Tcs > 6.5 K Iop/Ic = 50 % Tmargin > 2.0 K IDpipe = 4.5 mm ODconductor = 7.6 mm Ra > 100 Massflow = 5 g/s Pressure drop (60 m) < 0.1 bar

Present state-of-the-art in pressure drop

The customary approach to friction data modeling is to plot in dimensionless

form f(Re), fit a model, and compare results

Data for small-size CICC’sDcable = 12 mmDstrand = 0.81 mmCable: 3 x 3 x 4 x 4

Kat

hede

r

Alternative approach based on porous media

Straight correlation plot to check accuracy:

Average relative error f ≈ 20 %

A tortuous matter - 1 Tortuosity is the ratio of

the length l of a flow streamline between two points x1 and x2, and the distance of the two points d = | x2- x1 |

Larger tortuosity implies larger pressure drop

Length effect Flow effect

d

l

A tortuous matter - 2 Tortuosity in porous media depends on void fraction

(porosity) - we already take into account this effect In addition tortuosity in CICC’s depends on the

cabling pattern - we do not take into account this effect !

Cable tomography by courtesy of ENEA and PSI

Scales and issues Small temperature increase, up to the temperature margin (< 1

K) The heat transfer process affects coil, magnet and proximity

cryogenics (1 mm …100 m) Slow time scales, comparable to operation (1 s … 1 h) He-I and He-II are both of relevance Today, much of the experimental focus is on cable/coil in

He II, but very little data in all other areas (magnet, powering cables)

Modeling work is lagging behind experimental results, which is normal, but there may be a lack of basic understanding of the dominating physical mechanisms (micro/macro porosity in cables and insulations)




Superconductors stability

Measurements by M. de Rapper, CERN-TE-MSC

Figure 3

Figure 2

Scales and issues Small temperature increase up to the decision

recovery/quench (≈ 1 K) The heat transfer process is local to the strand in the

cable (1 mm …1 cm) Fast time scales for the decision between

recovery/quench (10 s … 10 ms) He-I and He-II are both of relevance We do not have a complete, consistent and

validated model of heat transfer for these processes




Quench propagation Single components (e.g. magnet, bus-

bars (!?!)) require knowledge of heat transfer at the level of the cable

Systems (e.g. strings of magnets) require knowledge of heat and mass transfer at the level of the proximity cryogenics

Clean gap

≈ 45 mm

Quench in an LHC bus-bar Mock-up of defects in the LHC interconnects

are tested to find the boundary between stable quenches (can be protected by a current dump) and thermal runaways (can lead to over-temperatures)

Sample manufactured by C. Urpin, H. Prin, CERN-TE-MSC

Run 090813.15Stable quench: a normal zone is established and reaches approximate steady-state conditions (T ≈ 30…40 K)

stable

Measurements by G. Willering, G. Peiro, A. Verweij, CERN-TE

Run 090813.20Runaway quench: the temperature in the normal zone increases over a time scale of the order of few s to R.T.

runaway

trunaway


Effect of heat transfer @ 1.8 K

Adiabatic interconnect

Wet interconnect


Effect of heat transfer @ 4.3 K

Wet interconnect

Adiabatic interconnect


Heat transfer coefficient

Measurements by D. Richter, CERN-TE-MSCAnalysis by P. Granieri and M. Casali, CERN-TE-MSC

?

A string of LHC magnets model of the regular

LHC cell: D quadrupole and

lattice correctors 3 dipoles F quadrupole and

lattice correctors 3 dipoles

QV9202SIQV920

Pressure evolution computed pressure

Quench propagation reasonable

match of quench

propagation MB3-MB2-MB1

quench propagation MB3-

MB4 too fast

Scales and issues Large temperature increase, potentially up to room-

temperature (≈ 100 K) The heat transfer process is local to the cable in the

magnet (1 cm …1 m) Moderate time scales for the evolution of the

temperature profile during quench (0.1 s … 100 s) He-I (including pressure waves and mass transport)

is of most relevance The process spans several orders of magnitude,

and involves transport phenomena, with uncertainties at each level of the multiple scales

Summary

space

time

tem

pera

ture

10-3

10-2

10-1

110

1001000

10-3

10-2

10-1

110

100

0.1

1

10

100

Quench of magnet and busses

Magnet cooling,Flow issues

Stability

Cable/Coil cooling

Documents

Heat management issues A perspective view centered on the design of superconducting devices Opinions of L. Bottura At the mWorkshop on Thermal Modeling