CERN Accelerator School Superconductivity for Accelerators … · 2017-07-08 · (4.2K) is...

CERN Accelerator School Superconductivity for Accelerators

Current Leads, Links and Buses

A. Ballarino, CERN

Erice, Sicily, Italy

3 May 2013

CAS, Erice, 3 May 2013 A. Ballarino

SC Devices in SC Accelerators Magnets

RF Cavities

Current Leads

SC Bus-Bar

SC Links

Beam Instrumentation (based on SQUIDS, Superconducting Quantum Interference Devices)

Auxiliaries Quench Protection

Cold Diodes (semiconductors)

Energy Extraction

Post Mortem System

Interlocks……..

2


Current Leads

Conventional leads

HTS Leads (Bi-2223, Y-123)

Bus-Bar

Nb-Ti bus-bar

Superconducting Links (MgB2)

Protection

Outline

FROM

TO

FROM

TO

3


Leads and Bus-Bar in a Magnet Circuit

RT

LHe

4


Single Magnet

Individual magnet operated at LHe temperature

Bus-Bar (Nb-Ti)

Energy Extraction Power

Converter

Cryostat

Magnet

DFB

Current

Leads

HTS

5


Powering the LHC Machine

8000 Superconducting magnets

1700 Electrical circuits

More than 3 MA of current

More than 3000 current leads (from 60 A to 13000 A)

More than 2000 km of Nb-Ti bus-bar

More than 50000 interconnections

6


Sector 1

5

DC Power feed

3 DC Power

2

4 6

8

7

LHC

Sector

arc

cryostat

For superconducting magnets, no DC powering across

LHC Interaction Points

Powering of LHC machine

7


Magnets Individually Powered

Imax = 19500 A L = 14 H

E-stored = 2.7 GJ Energy extraction (50 m)

LHC Dipole Orbit Corrector CMS Solenoid

Imax = 55 A L=7 H

E-stored=9.2 kJ No energy extraction

inner = 6 m Length = 12 m

=79 mm

8


LHC Main Dipole Circuit

Power

Converter

Energy

Extraction:

switch closed

Magnet 1 Magnet 3 Magnet 153

Magnet 2 Magnet 152 Magnet 154

Magnet 5

Magnet 4

Energy

Extraction:

switch closed

DFB DFB

LHC powered in eight sectors, each with 154 dipole magnets (1232 dipoles)

Time for the energy ramp is about 20 min (Energy from the grid)

Time for discharge is about the same (Energy back to the grid)

9


Current Leads

10


Current leads

Current leads are usually the dominant source of heat leaking into the magnet cryostat

Objective of a current lead design: minimisation of the heat leak introduced by the transmission of a given current

Sources of heat: thermal conduction from room temperature to cryogenic environment and ohmic loss

11


Thermal Conductivity in Metals

In metals, the principle thermal conduction mechanisms are electronic and lattice

K = Kl + Ke

Kl = lattice conductivity Ke = electron conductivity

In pure metals, electron contribution is dominant

Ke = (1/3) Cv <v> l

<v> = velocity of electrons

l = mean free path of electrons

Cv = eletronic specific heat per unit volume

12


Thermal Conductivity in Metals

<v> = 2 (EF/m)1/2

l = <v>

Ke = (1/3) Cv <v> l

n = number of electrons per specific volume

m = mass of electron

kB = Boltzmann constant

= mean free time

T = absolute temperature

EF = Fermi Energy

<v> = average speed

13


Free electrons also conduct electricity in metals, high thermal conductivity gives low electrical resistivity

Wiedemann-Franz Law

L= Lorenz number

= 1/

k = L T

14


Wiedemann and Franz (1853): ratio of thermal to electrical conductivity has about the same value for different metals at the same temperature

Lorentz (1872): the proportionality constant is L = 2.4510-8 WK-2

NB We did not consider lattice thermal conduction

Wiedemann-Franz Law

k = L T

15


Optimization of a Current Lead

k = L T L = 2.4510-8 WK-2

There is a minimum heat leak associated with the transmission of a given current

This minimum heat leak is independent on

conductor’s properties

A good electrical conductor has high thermal conductivity

16


)2C

T2H

(T2I0

L2H

QC

Q

k(T)

T0

Lρ(T)

dx

dTk(T)AQ(T)

Conduction-Cooled Current Lead

minC,Q 0

HQ

)2C

T2H

(T2I0

LminC,

Q

)2C

T2(T2I0

L-2C

QQ(T)

H

C

T

T )2C

T2(T2I0

L-2minC,

Q

K(T)

opt)

A

L(

kA

W47

I

minC,Q

dxdx

dTk(T)A

dxd

dx

dTk(T)A

dx

dTk(T)AQ(T)

2IA

dxρ(T)

02IA

1ρ(T)

dx

dTk(T)A

dxd

opt)

A

IL( Shape Factor

17

1W @ 4.2 K Wmin = 70 W


Multiple-stage cooling

Conduction-Cooled Current Lead

Cryo-cooler A stand-alone cooler providing intermediate temperatures or

Heat exchangers using cryogen at the temperatures available in the cryogenic system

)2C

T22

(T2I0

LminC,

Q

TC

T2

T1

TH

Qc

18


LHC Dipole Corrector Current Leads

50 K

20 K

1.9 K

RT

19


LHC Dipole Corrector Current Leads

47 W/kA 2.8 W @ 60 A

20


Self-cooled Current Lead

LHe

RT

In steady state conditions and neglecting kHe:

Self-cooling conditions

LCmQc

Qc

m

21


Material properties

Temperature Dependence of Material Properties

22


h, T=

QC,min(LHe) = 1.04 W/kA

Self-Cooled Current Lead

QC,min(4.2K) is independent on material properties

BUT

The optimum geometry , i.e. the shape factor, depends on material properties

LHe

RT

Optimum Shape Factor

LCmQc

See “Superconducting Magnets”, M. Wilson, Chapter 11

23


Conventional self-cooled leads

SS=Stainless Steel Cu ETP=Electrolytic-Tough-Pitch Copper

24


The temperature profile depends on material properties

The thermal performance in stand-by conditions (0 A) depends on material

properties (kC(T), with A and L defined)

Conventional self-cooled lead Q=0

Measured

Calculated

I=Iopt=18 kA Cu, RRR=80

x=L

T(K)

x (m)

18.4 W

RRR

25


Conventional vs HTS leads

Room temperature

LHe temperature

QA = 47 W/kA

A B C

HTS

QB = 1.04 W/kA QC = ?

THTS

26


HTS Current Lead-Resistive Section

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

20 30 40 50 60 70 80 90THTS (K)

m (

g/s

kA

)

THe=80 K

THe=70 K

THe=50 K

THe=20 K

THe=5 K

LHe

RT

THe THTS

m

Forced He gas flow cooling

27


0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

20 30 40 50 60 70 80 90

THTS (K)

SF

. 10

7 (

A/m

)

THe=5 K

THe=20 K

THe=50 K

THe=70 K

THe=80 K

LHe

RT

THe THTS


SF =

28


LHe

RT

THe THTS


I nominal A optimum

Q (x/L=1)=0

29

THTS

(K

)



30


LHC and ITER HTS Current Leads

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

20 30 40 50 60 70 80 90THTS (K)

m (

g/s

kA

)

THe=80 K

THe=70 K

THe=50 K

THe=20 K

THe=5 K

LHC

HTS leads

ITER HTS leads

LHC: from 60 A to 13000 A ITER: from 10000 A to 68000 A 31


K. Onnes, 1911

K(T)(T) = LT

0

WFL abolished

Low K(T)

HTS Current Lead-HTS Section

32


BSCCO 2223, Bi-2223, Bi(2223), 1-G wire

YBCO, Y-123, 2-G wire, REBCO (RE=Rare Earth Ba-Cu-O), Coated Conductor (thin layer of superconductor on a substrate), REBCO coated conductor

Not to get lost when you will found different acronyms in the literature

High Temperature Superconductors

33


0

0.5

1

1.5

1 10 100 1000 10000

(IL/A)/104 (A/m)

Q4

.2 K

(W

/kA

)

Vapour-cooled

Stainless steel

Vapour-cooled

Copper ETP

Over

cooling

Over

cooling

Sub

cooling

Sub

cooling

0.1

1.04

HTS Current Lead

Conventional vs HTS Leads

34


Thermal Conductivity Bi-2223

BSCCO: Bi-Sr-Ca-Cu-O

Bi-2223 sintered polycrystals

Anisotropy of K(T) because of anisotropy of crystal structure

35



Bi-2223

Fill factor 30 %

Y-123

4 mm

0.2 mm 0.2 mm

4 mm

0.2 mm

4 mm

4 mm

0.1 mm

Fill factor 1 % (1 to 3 m of Y-123)

Ag Matrix Bi-2223 filaments

Metallic substrate Buffer layer Y-123 layer

36


Thermal Conductivity Ag-Au Alloy

1

10

100

1000

10000

1 10 100 1000

Temperatura (K)

K (

W/m

K)

Ag

1.0 % Au

2.9 % Au

11 % Au

30 % Au

T (K)

N.B. The thermal conductivity of an alloy is not a weighted average of its elemental constituents

37



Year

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Tra

nsi

tion T

emper

ature

(K

)

0

20

40

60

80

100

120

140

160

180

Liquid Nitrogen 77.4K

Liquid He 4.2K

HgBa2Ca2Cu3Ox

at 30GPa

HgBa2Ca2Cu3Ox

Tl2Ba2Ca2Cu3O10

(BiPb)2Sr2Ca2Cu3O10

YBa2Cu3O7-x

LaBaCuONb3Ge

Nb3(AlGe)Nb3SnNbPb

Hg

LNeon (27 K) LHydrogen (20.3 K)

MgB2

2001

7.9 K 9.2 K

92 K

110 K

125 K

38 K

38

THTS ?


Ic(T) Dependence – Bi-2223

Self-field conditions

39


Ic(B) Dependence – Bi-2223

HTS: Highly Anisotropic Materials

40


Operating temperature THTS

1.00

1.50

2.00

2.50

3.00

3.50

4.00

20 30 40 50 60 70 80 90

THTS (K)

mH

e (

g/s

)

0

1

2

3

4

5

6

7

8

9

QH

TS

(W

)

mHe 20 K

QHTS

I = 70 kA

Higher THTS More HTS conductor Higher heat load at 4.2 K

41


LHC Current Leads: Saving

Conventional leads HTS leads

Heat load into LHe 1.1 W/kA 0.1 W/kA

Exergy consumption 430 W/kA 150 W/kA

Exergy consumption

(% conv. lead)

100 35

Total exergetic power 1290 kW 450 kW

Current = 3 MA

42


LHC HTS Current Leads

HTS Part

13000 A LHC Lead

43


LHC HTS Current Leads

THe=20 K

THe=4.2 K

THTS=50 K

44


LHC HTS Current Leads in LHC Tunnel

45


Persistent-Current Mode Operation

Long time operation of magnet at steady fields

Demountable leads

Superconducting switch

Need of VERY low resistance joints in the superconducting circuit to guarantee uniformity of current in time – required current stability in LHC main dipole circuit 1 ppm

Lead

SC Switch

SC Magnet

Heater

46


Buses, Links and their Protection

47


A conventional self-cooled current lead needs to be protected against thermal run-away

The resistive and the HTS part of a HTS lead need to be protected against respectively thermal run-away and resistive transition

In most cases, the superconducting bus needs to be protected against resistive transition

Protection of leads and bus-bar

48



Magnet 1 Magnet 2

Power Converter

Magnet 154

Magnet i

When one magnet quenches, quench heaters are fired for this magnet. Resistance is switched in the circuit

The current in the quenched magnet decays in about 200 ms – it flows flows through the bypass diode

Quench

Heater PS

49



Magnet 1 Magnet 2

Power Converter

Magnet 154

Magnet i

The time constant of the LHC Main Dipole circuit is 107 s. This “rapid” current decay is obtained by switching an external resistance into the circuit

If the leads or bus-bar quench, the time constant for the discharge is given by the circuit (107 s for the LHC Main Dipole chain) 50


Protection-LHC Bus-Bar

LHC Main Dipole Bus-Bar Stabilizer

(=107 s)

LHC Main Quadrupole Bus-Bar Stabilizer

(=40 s)

LHC Main Dipole Lyra

Bus-Bar: Nb-Ti Rutherford Cable/Strand with copper stabilizer

Rutherford Cable

51


BSCCO 2223 Superconductor with stabilizer: Ag-Au matrix of tapes plus stainless steel supporting structure

Protection-LHC HTS Current Leads

Long circuit time constants may make the use of HTS leads not appropriate for that specific application. Ex. ATLAS toroid leads (20.5 kA) are conventional (slow discharge of circuit: 3 hours).

Bi-2223 Stack of Tapes Stainless Steel

Nb-Ti

50 K 4.2 K

L = 0.5 m

52


Energy Stored in LHC Dipole Magnets

E dipole = 0.5 L dipole I 2

dipole

Energy stored in one dipole operating at 7 TeV with

11850 A is 7.4 MJoule

For 154 dipoles in one sector, E 1.2 GJoule

For all 1232 dipoles in the LHC, E 9 GJoule (good reason for dividing the powering into 8 sectors)

400 kg of TNT

Energy must be dissipated in the resistor and not in magnets, bus-bar or current leads

53


Effect of 7.4 MJ in a Dipole Coil

54


LHC Nb-Ti Bus-Bar and Interconnections

55


Protection-LHC Bus Bar LHC Main Dipole Splice

Highly resistive splice Quench in bus-bar Detection of voltage Energy discharge in resistor

BUT

56


Highly resistive splice + non-continuity of stabilizer

Protection-LHC Bus Bar LHC Main Dipole Splice

Localized over-heating until melting of the cable and local discharge of the circuit energy

LHC Incident, 19th Sept 2008, Sector 3-4 57


Superconducting Links

LHe

58


LHe

GHe Power Converterr

Power Converterr

SC cables GHe

59


Superconducting Links for LHC Upgrade

Itot = 120 kA N =22 cables MgB2 Round Wire

Ex. Development for LHC

60


HTS Power Transmission Cables

HTS cables for integration in the grid

AC cables for operation in the network

First and second generation HTS conductors

LN2 operation

Cables operated at up to max 4000 A (IRMS)

One or there cables in the cryogenic envelope

Horizontal transfer

High-voltage

61


Superconducting Links for LHC Upgrade

SC links for the LHC machine Quasi-DC operation

Also MgB2 conductor

GHe operation

Cables operated at up to 20 kA

Multi-cable ( 50 high-current cable) assemblies

Horizontal + Vertical ( 80 m) transfer

1.5 kV – 2 kV electrical insulation

62


Cables for Superconducting Links

High-current and low-field

High temperature (5 K to 25 K)

High temperature margin

Compact cables and compact multi-cable assemblies

Rutherford cables: why not, but they require round conductor with good mechanical properties (bending radius wire diameter)

The cost of the conductor should be a small fraction of the cost of the system

63


M. Noes , CAS, Erice 2013

64


SC Link Test Station at CERN

LHe (4.2 K)

GHe (5 K to 70 K)

20 kA

65


Thanks for your attention !

CERN Accelerator School Superconductivity for Accelerators … · 2017-07-08 · (4.2K) is...

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