Superconducting Magnets for the LHC - cas.web.cern.ch · Superconducting Magnets for the LHC Lucio...

Superconducting Magnetsfor the LHC

Lucio Rossi

CERN – TE department

CERN Accelerator School

26 February 2009 - Divonne

Lucio Rossi - Superc. Mag. 2

A few references

Basic Superconductivity:M. Tinkham, Superconductivity, Gordon & Breach Publisher

A.C. Rose-Innes, E.H. Rhoderick, Introduction to Superconductivity, Pergamon Press

W. Buckel, Superconductivity, Fundamental and Applications, VCH Publisher

J. Evetts (editor), Coincise Encyclopedia of Magnetic and Superconducting Materials, Pergamon Press

H.W. Weber High Tc Superconductivity, Plenum Press

Applied Superconductivity M.N. Wilson, Superconducting Magnets, Clarendon Press Oxford

H.A. Brechna, Superconducting Magnet Systems, Springer Verlag

K.-H. Mess, P. Schmüser, S. Wolff, Superconducting Accelerator Magnets, World Scientific

B. Seeber (editor), Handbook of Applied Superconductivity, IoP Publishing

L. Dresner, Stability of Superconductors, Plenum Publ. Corp.

Y. Iwasa, Case Studies in Superconducting Magnets, Plenum Publ. Corp.

Previous CERN Academic Training• 1999 : Ph. Lebrun - Superfluid Helium

• 2000 : L. Rossi - Superconducting Magnets

• 2002 : D. Larbalestier - Superconducting Materials

CAS School on SC and cryogenics for accelerator and detectors, Erice 2002,

Report CERN 2003-


Content

• Accelerators and magnet shape• Accelerator Magnets : basic design, magnet types• The heart: superconductors (and superconductivity)• Mechanical structure• Making it suitable for LHC: Field Quality• Alignment issues• Snapshot at construction in Industry• Quench and stability


Circular accelerator: magnet festival

E 0.3 B R

Tev, T, km

LHC:

18 km MB

2.5 km MQ

2 km of

other

Quads

> 8000 Sc

Magnets!!!


Rationale for using SC

• The LHC has a circumference of 26.7 km, out of whichsome 20 km of main superconducting magnets operating at 8.3 T. Cryogenics will consume about 40 MWelectrical power from the grid.

If the LHC were not superconducting:

• If it used resistive magnets operating at 1.8 T(limited by iron saturation), the circumferencewould have to be about 100 km, and the electrical consumption 900 MW (a good-size nuclear power plant), leading to prohibitive capital and operation costs.


Accelerators and Magnet Shape

• The cost and the difficulties scales as the amount of the stored energy: Vol B2/2 0

• Rush for high fieldsmall volume of field

• Thin field tubes that follow the particle trajectories

• Dipole to bend• Quadrupoles to focus• Sextupoles and Octupoles• Correctors (from Dip to Decap)• Correctors are mostly local

Detector Magnets


• Magnetic fields needed for - electric charge identification- momentum spectrometry- p = mv = q B; = q/p B L

BL is often the comparison parameter

• If momentum analysis is done by tracking inside the field volume:

- p/p 1/BL2 large volume better than high field - Field homogeneity appreciated but NOT critical (field knowledge of 0.1% usually suffices)


Accelerator Magnets Basic Design - I

In practice the above current distributions are approximate, so the field contains also higher order harmonics (see later).It can be shown that if the cos(n ) is approximate by step function, there is a “magic” angle that makes nil the first higher order harmonics.

Intersecting ellipses generate

uniform field.

Two intersecting ellipses, rotated

of 90 , generate a perfect

quadrupole fields:.

ba

JbdB y

0

xba

baJB

yba

baJB

y

x

)(

)(

0

0

All these configurations follow:

Js = J cos( ) , Js = J d cos(2 ), …

d = coil thickness

a, b, ellipses parameter


Uniform shell current density with

cut to nul the first higher order

harmonic.

Approximation of cos with coil blocks (left) and multiple shells (centre) and of intersecting ellipses (from Wilson book).

DEVIATION FROM PERFECT FIELD: 10-4 (0.01%) that is taken as the unit.

Accelerator MagnetsBasic Design - II


Accelerator MagnetsBasic Design - IV

Reality

e.m. forces

NOT SELF-

SUPPORTING

How to contain

them

More difficult in

twin magnets!

Joverall 500 A/mm2 ! e.m. forces are not kept by conductors but tend to torn apart the winding.

Principle

coil-collars-cylinder structure


Outer Yoke

Force

Inclined

Force

Collar/Yoke

Forces

Shrinking

Cylinder

Iron Yoke

Packing rods

Austenitic Steel

Collars

Collaring rods

Coils

Ft=340 tonnes

• The approach chosen by LHC is to have a design where most of the force can be taken by collars

• Then to have a vertical gap almost closed at RT and the outer skin welded such as to have a strong prestress

• Part of this is lost during cool-down but there is enough to assure azimuthal prestress of the coil up to 9 T and to have at least radial contact radial between collar and yoke

• The choice of stainless steel has released the tolerances on the skin stress: ideal 150 15 MPaWe can live even if all tolerance goes in the too low or too high direction.


Dip X-sectTwin Concept

1.9 K, 8.3 T

15 m long

56 mm bore


• Cu-NbTi cable @ 1.9 K. Gop = 223 T/m• e.m. forces kept by collars only (iron is a mere flux return yoke)

• Two-in-one concept (two apertures are de-coupled both magnetically and mechanically).

• 3.5 m long, coil ap.56 mm, straight, alignment given by inertia tube• Correctors are fixed on the inertia tube

MQ

Main Quad

CERN- CEA

collaboration

MQ X-sect


MQY wide aperture quadrupole

• Four-layer, graded shell coil.

• Free standing collars, fully

supporting the forces.

• Two-in-one iron yoke.

70 mm ID coil

G = 160 T/m at 4.5 K

I = 3620 A

E = 141 kJ/m/aperture

Lmag = 3.4


CERN-Oxford Inst. (MQY)

Fermilab (MQXB)

KEK (MQXA)

Three 70 mm quadrupoles developed for the LHC IRs


The zoo of the 6000 SC corrector magnets

MCS Sextupole Magnets

MOOctupoleMagnets MQSXA

QuadrupoleOctupoleSextupoleMagnets

MCDODecapoleOctupoleMagnets

Superconductivity

• Zero resistance !!

• But at low temperature

• And within certain limit of field and current

• Concept of critical surface


f3(J,B,T0) f2(J,T,B0)

f1(B,T,J=0

)

J

B

T


Who provide the current: type II superconductors

Flux penetration in the material is in quanta:

= h/2e 2 10-15 Wb

B

J

F

Lorentz force : Fp = -Jc B : to avoid

movements and heating it is needed a pinning

given by defects.

NbTi: Fp max 15 GN/m3 (or 15 N/mm3 !!) Jc

3 GA/m2 (3000 A/mm2 ) at 5 T


Practical Materials

Long journey from material discovery to

magnet application

Criterion Number

Superconducting 10,000

Tc 10 K .and. Bc2 10 T 100

Jc 1 GA/m2 @ B > 5 T 10

Magnet-grade superconductor 1

From science to technology

Tc vs year


Critical field vs temperature(zero current)



Niobium-Titanium

Critical surface of NbTi (from Wilson textbook)

Critical current of best Cu/NbTi with

typical 3 T field shift at superfluid

helium (INFN-LASA lab, february 2000)

Critical current density vs field measured on

NbTi multiflamentray wire at 4.22 and 2.17 K

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

0 1 2 3 4 5 6 7 8 9 10 11 12

Btot (T)

Jc(A

/mm

2)

LHe HeII

10

100

1000

10000

0 5 10 15 20 25 30 35 40 45

Applied Field (T)

JE

(A/m

m²)

YBCO Insert Tape (B|| Tape Plane)

YBCO Insert Tape (B Tape Plane)

MgB2 19Fil 24% Fill (HyperTech)

2212 OI-ST 28% Ceramic Filaments

NbTi LHC Production 38%SC (4.2 K)

Nb3Sn RRP Internal Sn (OI-ST)

Nb3Sn High Sn Bronze Cu:Non-Cu 0.3

YBCO B|| Tape Plane

YBCO B Tape Plane

2212

RRP Nb3Sn

Bronze

Nb3Sn

MgB2

Nb-TiSuperPower tape used

in record breaking

NHMFL insert coil 2007

18+1 MgB2/Nb/Cu/Monel

Courtesy M. Tomsic,

2007

427 filament strand

with Ag alloy outer

sheath tested at

NHMFL

Maximal JE for

entire LHC NbTi

strand production

(CERN-T. Boutboul

'07)

Complied from

ASC'02 and

ICMC'03 papers

(J. Parrell OI-

ST)

4543 filament High Sn

Bronze-16wt.%Sn-

0.3wt%Ti (Miyazaki-MT18-

IEEE’04)

Domain of iron

dominated

magnets

Je vs field for all practical superconductor


E-J curve

Transition at fixed temperature: V = k In , so we have to adopt a criterion to define Ic.

Electric field. Ic is the current generating an electric field Ec= 10-5 V/m E = Ec (J/Jc)n

Resistivity. Ic is the current showing an apparent resistivity of c = 10-14 m.

The exponent n, called also n-value or n-index, is related to the homogeneity of the

material or of the superconducting properties. For good superconductors n 30 – 60 or

more. Near critical surface, B > 0.9 Bc2 the n-values drops down to 20 or below.

8T

0

5

10

15

20

0 200 400 600 800 1000 1200

Jsc (A/mm2)

E(µ

V/m

)

c =10-14

m

Ec=10 µV/m

5T

1

10

100

100 1000 10000

log Jsc

log

Ec =10

-14

Ec=10 µV/m


Superconductors are not stable!

I

T TLHe

Iop

Superconductors are NOT stable against perturbation albeit very small.

E of J are enough to drive superconductor normal!

Heat capacity drops at low temperature (T<< TDebey) :

C T3 T = E/ C. So even small E generates sensible T

operating point of the magnet beyond critical surface QUENCH

TEMPERATURE

CURRENT

Ic curve

Imagnet

All current in sc

Current

in Cu

Current

in sc

TC

S

TC

All current in

Cu

TEMPERATURETCTCS

JOULE

POWER

Electrodynamic stability: intimate

contact between the superconductor and a

good conductivity material.

Adiabatic (or intrinsic)stability: to cure

the flux rearrangement that generates heat

Direct cooling : LHe and more HEII are

very good coolant, capable to remove

heating in milliseconds! Latent heat 10-

1000 times that of solid specific heat!


Superconductor of LHC


-250

-200

-150

-100

-50

0

50

100

150

200

250

-1 -0.5 0 0.5 1

B (T)

M (

mT

)

shielding current layer in a superconducting

filament (persistent currents)

M 1/Bn

n =

0.5...1.5

M JcDeff

Courtesy of L. Bottura

Magnetization and fine filaments


+ I - I

B

m

Multipoles generated areproportional to 1/B

n

If M is symmetric only allowedmultipoles are present

Courtesy of L. Bottura

Effect of magnetization


Magnetization for LHC NbTi

Strands:Width of the magnetization loop of all manufacturers (all 01, 02 billets)

15

20

25

30

35

40

45

50

1 51 101 151 201 251 301 351 401 451 501 551 601 651 701 751 801 851 901 951100110511101115112011251130113511401145115011551Strand identifier (ordered sequentially)

Wid

th o

f th

e m

agnetization loop (

mT

)

01B

01E

02B02C

02G 02K

02D

Cable Magnetization (CERN)Control limits are TDB strand limits

20

21

22

23

24

25

26

27

28

29

30

B10

001

B10

009

B10

017

B10

025

B10

033

B10

041

B10

049

B10

057

B10

065

B10

073

B10

081

B10

089

B10

097

B10

105

B10

113

B10

121

B10

129

B10

137

B10

145

B10

153

B10

161

B10

169

B10

177

B10

185

B10

193

B10

201

B10

209

B10

217

B10

225

B10

233

B10

241

B10

249

B10

257

B10

265

Map Id

Wid

th o

f th

e m

ag

neti

zati

on

lo

op

(m

T)

at

T=

1.9

K, B

=0.5

T

01B cable

Due to field

imperfection

generated by M:

Rejection of

conductor

Limit in the dynamic

range of the magnets

In LHC Dfil= 6-7 m

Courtesy of S. LeNaour


dB/dt

resistive

contact Rc at

cross-over

point

induced eddy currents

in the loop I -dB/dt

and I 1/Rc

superconducti

ng path in the

strands

Needs for 10-20 kA cable for

protection

Needs very high packing factor: 90% !!

Needs a system simple that keep

strands The strand are fully

transposed

BUT field changes

over a period !

Ends problems

Junctions

BICC

Rutherford cable


Controlling the contact resistance

CERN has developed

the controlled

oxidation method

Coating wire with 0.3-

0.5 of SnAg then H.T.

cable in air

What are the acceptable

limits ?

Too low (< 15-20) gives

field errors (ad He

consumption

Too high (>100-200) may

raise instability or current

distribution

Rc measured by CERN on the cables for the inner dipole layer

0

20

40

60

80

100

120

140

160

180

200

B10001

B10034

B10067

B10100

B10133

B10166

B10199

B10232

B10265

B10298

B10331

B10364

B10397

B10430

B10463

B10496

B10529

B10562

E00005

E00039

E00072

E00105

E00139

Rc [

µO

hm

]

average Rc = 42

µohm for the

production of

ALSTOM cables

average Rc =

33 µohm for


Snap-back phenomena-1

.2-1

-0.8

-0.6

-0.4

-0.2

0

0 500 1000 1500 2000time (s)

b3 (

units)

0.05 A/s

0.5 A/s0.25 A/s

0.1 A/s

•during the injection platform of an LHC dipole, the

sextupole error term integrated over the magnet

length decays with a time constant of ~ 1000sec.

•when the ramp is started, the error term 'snaps back'

to its earlier value

•it is difficult to find a time constant of 1000 seconds

in any of the usual coupling modes

-20

-10

010

20

30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Field (T)

Magnetization (m

T)

Injection + 20 mT

- 20 mT

•BICCs produce a field component which alternates

20 mT along the magnet

•imagine the hysteresis curve of NbTi filaments

subjected to this oscillation

•a 20 mT increase produces very little change in

filament magnetization

•a 20 mT decrease produces a large change in filament

magnetization

•thus the hysteresis curve acts as a 'rectifier' enabling

the oscillating BICCs to produce a dc level

After Rob Wolf


LHC MB X-sect: conductor (Rutherford cable)

Conductor

position

optimization:

Control of harmonics

Balance of margin

among blocks

Stable against

inevitable errors

Minimum shear among

conductors

Balance between T

margin of inner/outer


Cu Spacer

Precise at 20 m

Used to steer

production

Change of Cu wedge

0.2-0.5 mm of inner

wedges in July 2001

(3 CM, 15 CC).

Effect in 2002.

~35 CM old Xsect.

LHC MB X-sect: copper wedges

Profile II

-80

-60

-40

-20

0

20

40

Batc

h P

RO

T

Batc

h A

Batc

h 1

Batc

h 2

Batc

h 3

Batc

h 4

Batc

h 5

Batc

h 6

Batc

h 7

Batc

h 8

Batc

h 9

Batc

h 1

0

Batc

h 1

1

Batc

h 1

2

Batc

h 1

3

Batc

h 1

4

Batc

h 1

5

Batc

h 1

6

Batc

h 1

7

Batc

h 1

8

Batc

h 1

9

Batc

h 2

0

Batc

h 2

1

Batc

h 2

2

Batc

h 2

3

Batc

h 2

4

Batc

h 2

5

Batc

h 2

6

Batc

h 2

7

Batc

h 2

8

Batc

h 2

9

Batc

h 3

0

Batc

h 3

1

Batc

h 3

2

Batc

h 3

3

Batc

h 3

4

Batc

h 3

5

Batc

h 3

6

Batc

h 3

7

m P1out

P2out


Polyimmideinsulation

Around cable and

around coils

Important

elements are

dimensions, 3%

of thickness, and

creep (Apical

creeps less than

kapton)

Rutherford Cables InsulationGround isolation

Four layers 125AH

LHC MB X-sect: conductor and ground Insulation, Interlayer

Inter

layer

To allow

HEII to

flow

-2 layers of Apical 200 AV insulation

-1 layer Pixeo to glue cables together at 185°C (-0,+5 critical)


LHC MB X-sect: insulated CBT

CBT

Cold bore tube

StSt tubes

Insulation done at

CERN

Special Insulation

technique > 20 kV

Clearance between

coils and insulated

CBT is about 0.5 mm

over the 15 m length


LHC MB X-sect: Quench Heater

Strips of stainless

steels partially

coated with copper

to adjust resistance

Encapsulated like a

sandwich in two

foils of 75 m of

polyimide

Fired by current

pulse, heat must

diffuse from strip to

coils in 20-50 ms !!


LHC MB X-sect: Collars

Collars and collaring are the main controllers of the final coil shape


LHC MB X-sect: magnetic insert

Introduced to

ease the

mechanical

assembly

It serves for FQ

By tapering we

cured unwanted

quadrupole and

octupole

components

The iron InsertPunched

together with

the yoke

lamination


The iron yoke:

Stray field

15% field

increase (but

big gain in

protection)

If saturates

affect FQ

(sextupole)

Trim of

magnetic

length

LHC MB X-sect: yoke laminations

One supplier for

the steel 45,000

tons

Precise vertical gap

Temperature probe

Regular

Nested


LHC MB X-sect: Bus Bars & fillers

Bus Bars

Quad BB

Focusing

Quad BB

Defocusing

Corrector

circuit bus

bars on top

of Quad BB

Dipole

circuit BB

160 km of

main

BusBars!!

We provide:

-technology

-SC 02 cables

-Polyimmide

foils and tapes


LHC MB X-sect: Shrinking cylinder and support

Two half

shells,

welded on

the magnet

Many

difficulties

Curvature

released from

1 to 2.5

mm: still not

OK.

To c ure this

we went to

sorting

Precise support


LHC MB X-sect: beam screen and HXT

Copper Heat Exchange Tubes

HEII satur.

Beam

Screen

Inserted at

CERN just

before

insertion in

the tunnel


3D

Inner layer lyre side

End Spacers: critical for Quench

Two slightly different design (in cable

profile)

Only pre-series (4-5 suppliers can do)


3D

Connection side 3


LHC MB - end partCBTs and Yoke


LHC MB -end partend plate


LHC MB-end partBus Bars postioning


LHC MB -end partShrinking cyilinder


LHC Main Dipole -end partCu HXT


LHC Main Dipole -end partCorrector Magnets (spool pieces)

Assembly

in CMAs

is purely

mechanic

al

(tolerance

s of B axis

wrt mech.

frame

given by

supplier


LHC Main Dipole -end partEnd covers

One supplier

Powder

technology


LHC Main Dipole -end part« Cold foot »

Very accurate positioning


LHC Main Dipole -end partBellows and N-line


Interconnection between two superconducting magnets

6 superconducting bus

bars 13 kA for B, QD, QF

quadrupole

20 superconducting bus

bars 600 A for corrector

magnets (minimise

dipole field harmonics)

42 sc bus bars 600 A for corrector

magnets (chromaticity, tune, etc….)

+ 12 sc bus bars for 6 kA (special

quadrupoles)

13 kA Protection

diodeTo be connected:

• Beam tubes

• Pipes for helium

• Cryostat

• Thermal shields

• Vacuum vessel

• Superconducting

cables


Critical ProcessWinding-Curing-Coil formation

• Coils are cured under press

• Then measured all along 15 m

• Then collared with shims

• Shims influence also prestress and then coil movements (quench)

• Shift of radius of tens of micron as well deformation can easily drive harmonics out of tolerance


Steering production (and check assembly) Field Measurements - CERN supply

It has also helped to detect a number of defects.

It has also been used to detect subtle electrical shorts

Introduced first to steer the FQ toward beam dynamics targets.Note the uniformity among different manufacturers

10.07

10.09

10.11

10.13

10.15

0 200 400 600 800 1000Magnet progressive number

Int

transf

func (

Tm

/kA

)

-40

-20

0

20

40

Units

Firm 1

Firm 2

Firm 3

AT-MAS

Cold mass

upper limit for single magnet (3 sigma)

lower limit for single magnet (3 sigma)

0

5

10

15

20

0 200 400 600 800 1000

Magnet progressive number

Tota

l num

ber

of fo

und d

efe

cts


Stering production with actions

-10

-5

0

5

10

0 100 200 300 400 500 600 700 800 900 1000 1100

Magnet progressive number

b3 inte

gra

l (u

nits)

all

Firm 1

Firm 2

Firm 3

Cold mass

upper limit for systematic

lower limit for systematic

AT-MAS & MTM

Cro

ss

-se

cti

on

2

Cross-section 3


W W

V2

V1

V2

V1

97.26

R 281236

LINE V1

R 281236

97.26

R 281236

LINE V2V2

2

V1

1

14343 LINE V1

14343 LINE V2

9.14

2

V1/V2

V1/V2

Ø 0.6

V1/V2

Ø 2

3 D: curvature measurementsEnd tolerances


LTD 500Mechanical mole

Target

Accuracy:10ppm at 2on static target

BUT 0.2 mm when changing position

Measuring Instruments : Laser tracker (Leica) and moles


Snapshot at industry


The longitudinal welding

• Pre-developed at CERN

• Installed directly CMAs

• Two weldings synchronized

• Root welding STT: high quality very sophisticated control, a world PRIMA for this conditions and austenitic steel

• Problem on the press, now almost over : still quality of welding

• Each CM leak tested 26 bar !!!


Magnet performance and Training Curve

Second Generation Dipoles MBL1N1, MBL1AJ2, MBL1AJ1, MBL1N2, and MBP1A1(all training quenches at superfluid helium)

6.00

6.50

7.00

7.50

8.00

8.50

9.00

9.50

10.00

Ma

gn

etic

Fie

ld a

t Q

uen

ch B

[T]

MBL1N1 MBL1AJ2 MBL1N2

Th

. C

ycle

Fa

st

Th

. C

ycle

Th

. C

ycle

Th

. C

ycle

MBP1A1MBL1AJ1

Th

. C

ycle


The spectrum of disturbances

Continuous Distributed Perturbances:

AC losses (hysteretic and coupling losses, eddy currents)

Intrinsic dissipation due to smooth transition (Iop too near or above Ic!)

Thermal load (vacuum degradation,…). This could be a serious effect in

cryocooled system.

These perturbations are usually predictable and estimate must be done at design level

coupling losses can depends on interstrand resistance, i.e. on manufacture

technique and on prestress and e.m. forces more difficult to evaluate

Continuous Point Perturbances:

Joints inside coils

Release of mechanical energy (hysteresis of the stress-strain relation)

Localised heat input (suspension rods with bad thermal anchoring)

These effects are well understood and predictable (it does not mean easy to cure !)


The spectrum of disturbances - II

Transient Distributed/Point Perturbances

Flux jumps. This effect is cured almost definitely for NbTi. Effects

could be seen on NbSn with very high current density and very large effective

filament diameter. This effect can be detected at low field, during current ramp.

Mechanical origin: movements, friction, sudden release of elastic

energy…

crack in the resin

Basically these last two mechanism are now understood, in principle, and

acoustic emission experiments did prove it almost visually.

Still they are less predictable and more difficult to avoid. They depend on

magnet geometry, material properties, local conditions and on many details.

They can depend on magnet history (previous quench, overheating, thermal

induced stress, etc.)


Temperature and enthalpy margins

The main action to take is to have a reasonable energy margin, larger than the

expected energy release, to make unlikely to pass the critical surface: but the

specific heat of solids are pretty low near LHe and we can rely only on T 1-2 K

I

T TLHe

Iop

Volumetric Specific Heat

0.E+00

2.E+04

4.E+04

6.E+04

8.E+04

1.E+05

1.E+05

1.E+05

0 5 10 15 20T [K]

H [

J/(

m3K

)]

Al

NbTi

Cu

G10


Point Disturbances : MPZEnergy density is not the only criterion, since most of the perturbations are localized.

l

T

T0

Tcs

Conductor length

MPZ : the Minimum Propagating Zone

with a simple balance between power

dissipated in the normal zone and heat

conducted along the cable we found:

2

)(2

c

opcs

J

TTkl

If there is no stabiliser, only NbTi, we see that l 1 m H 1 nJ only !!


Stability of LHC

1

10

100

1000

10000

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

4.3K 6T H16

4.3K 6T H8

4.3K 6T H1

1.9K 9T H16

1.9K 9T H8

1.9K 9T H1

If there is no stabiliser, only NbTi, we see that l 1 m H 1 nJ only !!

If each strand hehaved as single wire, H 1-10 J


PROTECTION

A superconducting magnet, whatever the stability margin, it will quench. And magnet

integrity has to be preserved.

When working at current density like in the LHC dipoles, where dissipation per unit

volume following a quench is JCu2 6 10-10 m 1018 A/m2 = 600 MW/m3

Excessive voltage rise insulation breakdown.

Temperature growth melting or serious trouble to insulators and conductor

Temperature gradients excessive stress with subsequent de-training.

Impressive damage caused by a short circuit developed during a quench in a LHC dipole protype


Hot Spot Temperature

Let’s suppose that heat is coming only by Joule effect and

conduction is not significant

)( )(

)()( )()()( 2

0

0

22

md

T

T

TUTJdTT

TCdttJdTTCdtTtJ

m

op

The function U(T) is a computable a

priori, based only on material properties.

If the magnet is discharged on an external

–dumping- resistors, RD, Td=0.5 Lmag/RD.

U vs T

0

1

2

3

4

5

6

7

8

9

10

0 50 100 150 200 250 300

T [K]

U [

10

16A

s /m

4]

LHC dipole

ATLAS

MIITs

The goal is to speed up the quench

propagation by any means, to

avoid too high hot spots:

1) Heater : activated in 20 ms !!

2) Benefit of quench-back

This goes against having LHe

inside the coils (i.e. is against

stability)!


Protection scheme for a dipole string

LHC protection scheme (courtesy of F. Rodriguez Mateos , CERN)


Quench statistics (single magnet test)

Only 2.5% rejected

Half for quench half for electrical

NCs (QH)

Except 1, all repaired successfully


Future for SC magnets ?No new LHC at view but…

LHC luminosity upgrade

Larger and higher gradiend

quadrupoles…

Higher field dipoles

FAIR project at GSI

SIS100 – superferric fast cycled 2.1 T

SIS300 – 4.5 T 0.1 Hz cycled magnets

Superconducting Magnets for the LHC - cas.web.cern.ch · Superconducting Magnets for the LHC Lucio...

Documents

Transcript of Superconducting Magnets for the LHC - cas.web.cern.ch · Superconducting Magnets for the LHC Lucio...