Superconducting LHC Magnets characteristics and qualification in SM18 test station

Superconducting LHC Magnets – Characteristics and Qualification in SM18 Test StationLHC Seminar for CERN Guides, 6th May 2004, [email protected]

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Superconducting LHC Magnets Superconducting LHC Magnets characteristics and qualification in SM18 test characteristics and qualification in SM18 test

stationstationMarco Buzio (AT/MTM)

ContentsContents

1.1. Introduction: overview of LHC magnet systemIntroduction: overview of LHC magnet system2.2. Superconducting cables and magnetsSuperconducting cables and magnets3.3. The LHC cryodipolesThe LHC cryodipoles4.4. Cryogenic testing in SM18Cryogenic testing in SM18

4.1 Power tests4.2 Field quality


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AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements

This talk gives some highlights on the work done by many people in the AT and TS department over many years.

Special thanks to:

L Bottura, V Chohan, A Masi, JG Perez, P Pugnat, S Sanfilippo, A Siemko, N Smirnov, W Venturini Delsolaro (AT/MTM)

M Pojer, L Rossi, D Tommasini (AT/MAS)

J Axensalva, JP Lamboy, B Vuillerme, L Herblin (AT/ACR)


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Overview of the LHC Magnet SystemOverview of the LHC Magnet SystemOverview of the LHC Magnet SystemOverview of the LHC Magnet System

Magnet TypeMagnet Type FunctionFunctionField/Field/

Current/Current/Magn. lengthMagn. length

N.N. ManufacturerManufacturer

Main DipoleMain DipoleMBMB

steer beams steer beams around the ringsaround the rings

8.33 T8.33 T11850 A11850 A

14.3 m14.3 m12321232

Alstom (F)Alstom (F)

Ansaldo (I)Ansaldo (I)

Noell (D)Noell (D)

Main QuadrupoleMain Quadrupole

MQMQfocalize beamsfocalize beams

233 T/m233 T/m

11850 A11850 A3.1 m3.1 m

~ 400~ 400 Accel (D)Accel (D)

Multipole CorrectorsMultipole Correctors

““spool pieces”spool pieces”(e.g. MCS, MCDO)(e.g. MCS, MCDO)

Compensate field Compensate field errors in main errors in main

magnetsmagnets

0.1 – 3 T0.1 – 3 T

50-550 A50-550 A

0.15 – 1.5 m0.15 – 1.5 m

~ 3600~ 3600Antec (E)Antec (E)

Tesla (UK)Tesla (UK)KECL, CAT (In)KECL, CAT (In)

Compton Compton Greaves (In)Greaves (In)Sigmaphi (F)Sigmaphi (F)

etc…etc…

Orbit correctorsOrbit correctors(e.g. MCBH, MCBV)(e.g. MCBH, MCBV)

Adjust beam orbitAdjust beam orbit

(hor/vert)(hor/vert)

~ 2800~ 2800

Lattice correctorsLattice correctors(e.g. MQS, MQT, (e.g. MQS, MQT,

MS, MO)MS, MO)

Adjust beam Adjust beam parameters (e.g. parameters (e.g.

tune, chromaticity)tune, chromaticity)

N

S

N

N

S

S

N

S

S

N N

S


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Overview of the LHC Magnet SystemOverview of the LHC Magnet SystemOverview of the LHC Magnet SystemOverview of the LHC Magnet System

•Magnet lattice = ½ cost of LHC, 10 yr R&D

•Challenge: large-scale, advanced technology transfer to industriesextensive tests needed at CERN (~10% of magnet cost)

Cryodipoles in SMA18

Short Straight Section in SM18

Different types of correctors


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StatusStatusStatusStatus

• Baseline includes cold tests for all main magnets, details to be finalized

• 1 octant of dipole cold masses delivered, ~120 cold tested (only two rejected)

• 6 Short Straight Sections assembled and tested

• cold test rate expected to ramp up from 8 to 14 magnets/week as soon as all 12 benches completed (Q3 2004)

• first dipole installation tests foreseen in June

• End of cold test phase expected Q3 2006.


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Why superconducting magnets ?Why superconducting magnets ?Why superconducting magnets ?Why superconducting magnets ?

CopperCopper NbTiNbTiResistivityResistivity 1.5·101.5·10-8-8 mm 0 (DC)0 (DC)

Current densityCurrent density 5-10 A/mm²Pure NbTi @ 1.9K: Pure NbTi @ 1.9K: 2500 2500 A/mmA/mm²²

LHC Cable:LHC Cable: 400 A/mm400 A/mm²²

Cable cost Cable cost 25 €/kA·m25 €/kA·m 25-50 €/kA·m25-50 €/kA·m

Max. Max. temperaturetemperature

< 90°C< 90°C(air or water cooled)(air or water cooled)

< 4.2 K< 4.2 K(LHe cooled)(LHe cooled)

Max. magnetic Max. magnetic fieldfield (stress-limited)(stress-limited) ~10 T @ 1.9K~10 T @ 1.9K

Non-linearitiesNon-linearities Non-linear ferromagnetic Non-linear ferromagnetic hysteresishysteresis Diamagnetic effect, rate dependenceDiamagnetic effect, rate dependence


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Superconducting vs. resistive: field qualitySuperconducting vs. resistive: field qualitySuperconducting vs. resistive: field qualitySuperconducting vs. resistive: field quality

Field quality determined by coil geometry

geometry not accessible for direct measurements at working temperature (cryostat/beam pipe) results must be extrapolated

coils must be shimmed during production, errors extremely difficult to correct

conductor positioning errors of ~ 25 m provoke relative field errors ~ 10-4

Field quality determined by iron pole shaping

geometry accessible for direct measurements

Measurement/shimming can be iterated

large conductor positioning errors tolerable

Field depends on the homogeneity of material magnetic properties


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Stability of superconductor: quenchesStability of superconductor: quenchesStability of superconductor: quenchesStability of superconductor: quenches

• A quench is a sudden thermodynamic transition to the normal resistive state, as the material crosses the critical surface Bcr(Jcr,T)

• Superconducting wire is intrinsically unstable: a quench can be triggered locally by the deposition of a few J (very low heat capacities at 1.9K !), which may be released by: - cable movements (few m) magnetic friction, Lorentz forces, mechanical friction - cracking of resin - radiation from the beam

• Quench stability is achieved by making the SC into thin filaments embedded in a conductive matrix.

• The performance of a magnet is degraded w.r.t. material properties due to manufacturing process, non-uniform field and current distributions

•Quench detection and active magnet protection are necessary to avoid excessive localized heat deposition (global margin for LHC dipoles ~ 85%)

T

B

J

Critical surface of NbTi


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LHC superconducting cableLHC superconducting cableLHC superconducting cableLHC superconducting cable

• Total: 400 tons NbTi, 7000 km

• Rutherford type structure

• 28 or 36 strands per cable, twisted to minimize linked flux during field ramps

• up to 8800 7 m Ø filaments per strand, embedded in Cu to achieve thermal stability(minimize Joule heating + maximize heat transfer after a quench)

• SC cross-section as high as 60% of the total to increase current density relatively low stability

• insulated with barber-pole wrapped polymide to allow for high LHe penetration (90% filling factor)

• keystoned + a different design for each coil layer (to allow current grading)

15 mm

Cu

NbTi


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LHC superconducting cableLHC superconducting cableLHC superconducting cableLHC superconducting cable

One LHC superconducting cable carries up to 13000 A …..

… which is equivalent to about 10 conventional power lines …

… or this thick bunch of resistive copper cables.


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Cryodipole: overviewCryodipole: overviewCryodipole: overviewCryodipole: overview

Instrumentation connector


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Dipole coilsDipole coilsDipole coilsDipole coils

x

y

+J -J

ideal cos() current distributionover a circular profile giving anuniform field inside the circle …

… and the optimized approximationmaking use of a discrete conductor

Cu spacer blocks

Quench Heaters (inner)

Quench Heaters (outer)

Outer layer (lower B, higher J)

Inner layer (higher B, lower J)

Beam pipe

NbTi cable


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Lorentz forcesLorentz forcesLorentz forcesLorentz forces

1.70 MN/m

0.75 MN/m

125 kN/coil

Total current 1 MA

• Magnetic coils tend to expand under the effect of self-forces

• Coils in the two apertures are attracted

• Stress levels depend on field strength, thermal contraction, level of pre-compression and width of any gaps


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Energy storedEnergy stored Energy storedEnergy stored

The total energy stored in the magnetic field of each dipole is = 7 MJ …dVBLI

volumemagnet

2

_

2

22

… that is equivalent to the mechanical energy necessary to lift a massof 32 tons to the height of 22 m …

… or the chemical energy contained in about 500 gr of delicious Swiss muesli !

… or the thermal energy sufficient to melt 5 kg of steel …

… or the electrical energy needed to light a 100W bulb for 20 hours …

However … WARNING: electrical insulation can be irreversibly damaged by sparks containing less than a J !!

However … WARNING: electrical insulation can be irreversibly damaged by sparks containing less than a J !!


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Dipole cold massDipole cold massDipole cold massDipole cold mass

Austenitic (non-magnetic) steel collars

Alignment pins

“Lyre”-shaped current leads

Beam pipe + kapton insulation

Corrector spool pieces

Shrinking cylinder (welded under compressionto confer curved shape)

Beam screen

20K GHe cooling conduit

Pseudo-random cryopumping holes


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Two-in-one conceptTwo-in-one conceptTwo-in-one conceptTwo-in-one concept

Distance and parallelism of the two beam rings are guaranteed Construction errors cannot be corrected tighter tolerances

Overall: higher cost effectiveness

Part of the magnetic flux returning through the yoke goes to enhance the field in the other aperture (+10%) Magnetic symmetry is broken Field errors are coupled

Number of magnets to build is halved Space occupied in the tunnel is considerably reduced Difficulty, cost and risk of construction are increased

collars

iron yoke


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Magnet test sequence Magnet test sequence (essential steps)(essential steps)Magnet test sequence Magnet test sequence (essential steps)(essential steps)

Cold mass arrives at SMA18

SMA18

- Electrical & leak tests- Instrumentation setup- Cryostating- Preparation for cold tests

SMA18

- Electrical & leak tests- Instrumentation setup- Cryostating- Preparation for cold tests

SM18

- Cold HV insulation - Quench protection system- Training- Magnetic measurements- Special tests (short sample limit, geometry …)

SM18

- Cold HV insulation - Quench protection system- Training- Magnetic measurements- Special tests (short sample limit, geometry …)

SM18/SMI2

- Warm HV insulation- Fiducialization (geometry)- Special magnetic measurements(polarity, field direction)- Inserion of beam screen- Preparation for storage

SM18/SMI2

- Warm HV insulation- Fiducialization (geometry)- Special magnetic measurements(polarity, field direction)- Inserion of beam screen- Preparation for storage Long-term storage

in Prevessin

6 + 6 cryogenic test benchesCryogenic Feed Box

Scanning machine for SSS

Long coil shaft system for MBs

13kA power converters

VME acquisition and control racks


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-50

0

50

100

150

200

250

300

350

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

Time [hours]

Tem

pera

ture

[K]

-50

0

50

100

150

200

250

300

350

-2 0 2 4 6 8 10 12 14 16

Time [hours]

Tem

pera

ture

[K]

Thermal cycling on cryogenic test benchesThermal cycling on cryogenic test benchesThermal cycling on cryogenic test benchesThermal cycling on cryogenic test benches

• cool-down and warm-up greatly accelerated (2 weeks in LHC)

• LHe bath at ambient pressure is made inside cold mass (40 kg LHe/dipole)

• Heat exchanger carries innovative bi-phase superfluid He flow to subcool down to 1.9K

warm-up with He @ 320K

Cool-down

LHe @ 4.2K SF LHe @ 1.9KGHe @ 80K

SF LHe

G/LHe


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Power testsPower testsPower testsPower tests

• HV insulation tests: up to 3kV between coils, ground and quench heaters

• Instrumentation tests: DAQ system and magnet instrumentation (voltage taps, T sensor) verified and calibrated

• Quench protection system test: quench heaters are discharged in various combinations at the 1.5 kA level to verify time constants, max. induced voltages and currents, etc …

• Training: field is repeatedly ramped up, and possibly quenched, until the ultimate field level is reached (9T).The location of the quench is systematically measured to assess quality of construction.

• Special tests: e.g. provoked quenches at 4.2K to assess the current-carrying capability of the cable (short sample limit)

• Provoked quench at 7T: empty He before warm-up

Quenches are normally originated in the heads(complex 3D geometry + looser tolerances)Quenches originating in the straight part may point to fabrication defects


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Training example: good dipoleTraining example: good dipoleTraining example: good dipoleTraining example: good dipole

D1_

Low

_S01

D2_

Up_

S13

D2_

Low

_S12

D2_

Low

_S12

D1_

Low

D2_

Low

_S12

7

7.25

7.5

7.75

8

8.25

8.5

8.75

9

9.25

9.5

9.75

10

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Quench Number

Mag

net

ic F

ield

at

Qu

ench

B [

Tes

la]

HCMBB-A001-01000001.T1 Ultimate Field = 9T Nominal Field = 8.34 Tesla

2nd thermal cycle 3rd thermal cycle

No Quench

Training Quench

Nominal field

Ultimate field

Memory effect

detr

ain

ing


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Training example: bad dipoleTraining example: bad dipoleTraining example: bad dipoleTraining example: bad dipole

Training Quenches at 1.8K

T=2.179KT=2.21KT=2.186K

7

7.25

7.5

7.75

8

8.25

8.5

8.75

9

9.25

9.5

9.75

10

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Quench Number

Mag

net

ic F

ield

at

Qu

ench

B [

Tes

la]

Ultimate Field = 9T Nominal Field = 8.34 Tesla Ansaldo1 2nd thermal cycle

Nominal field

Ultimate field

No memory effect


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Magnetic measurementsMagnetic measurementsMagnetic measurementsMagnetic measurements

• Loadline: local and integrated transfer function and harmonics as a function of steady-state excitation level

• LHC cycle: quantify dynamic effects as they will occur in the real machine cycle (magnet is put in a reproducible clean state by means of a previous quench and current pre-cycle)

• Coupling currents: systematic study of dynamic effects as a function of ramp rate and powering history

• Field direction: average direction of the field w.r.t. cryostat fiducials

• Magnetic axis: position of the axis (locus of B0) w.r.t. cryostat fiducials, used to define the precise position of the magnet in the tunnel and to assess the geometry in cold conditions.

why magnetic measurements ?

To qualify/accept magnets: - minimum level of performance must be guaranteed- harmonics measurement may spot construction errors

For the machine: reliable operation requires detailed knowledge of: - transfer function to within 200 ppm- harmonics up to decapole to within 50 ppm- field direction to within ±1 mrad- dynamic effects modelling database

why magnetic measurements ?

To qualify/accept magnets: - minimum level of performance must be guaranteed- harmonics measurement may spot construction errors

For the machine: reliable operation requires detailed knowledge of: - transfer function to within 200 ppm- harmonics up to decapole to within 50 ppm- field direction to within ±1 mrad- dynamic effects modelling database


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0.705

0.7075

0.71

0.7125

0.715

0 5000 10000Current (A)

Tran

sfer

func

tion

(T/k

A)

MBP2N1

Non-linear effectsNon-linear effectsNon-linear effectsNon-linear effects

Superconducting filament magnetization (persistent eddy currents)

• large hysteresis with relative errors of the order of 10-3 at low field (injection)

• hysteresis depends on temperature, current and current history

• main field and multipoles affected in different ways

Iron saturation

• affects only small area in the collar (B>2T)

• relative errors of the order of 10-2 at high field

• additional multipoles generated

Linear regime (geometric contribution)

• field is proportional to the current (can be computed with Biot-Savart’s law)

• the T.F. depends only on the coil geometryinjection


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Dynamic superconductor effectsDynamic superconductor effectsDynamic superconductor effectsDynamic superconductor effects

Normal sextupole during ramps

-3

-2

-1

0

1

2

0 2 4 6 8 10

field (T)

B3

-B3

geom

etric

(G

auss

@ 1

0 m

m)

35 A/s50 A/s

10 A/s

20 A/s

MTP1N2

0

1

2

3

4

5

0 500 1000 1500

time from beginning of injection (s)

b3 (

units

@ 1

7 m

m)

500

700

900

1100

1300

1500

dipo

le c

urre

nt (

A)

Coupling currents effects

• finite inter-filament and inter-strand resistance (RC) gives rise to loops linked with changing flux

• multipole errors Ḃ, RC-1

• hysteresis depending in a complex way upon field level, temperature and powering history

Decay and snap-back

• superconductor magnetization and coupling currents interact in a complex way to give long-term logarithmic time dependence effects (field decay)

• hysteresis branch switching may occur at the end of a decay phase to cause sudden current redistribution and additional multipole errors (snap-back)


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Magnetic measurement equipment: twin 15m harmonic coil shaftsMagnetic measurement equipment: twin 15m harmonic coil shaftsMagnetic measurement equipment: twin 15m harmonic coil shaftsMagnetic measurement equipment: twin 15m harmonic coil shafts

Twin Rotating Unit

• 213 Al2O3 sectors carrying 3 rectangular coils each

• cover the full length of the magnet to obtain integral in a single rotation (~ few seconds)

• stiff torsionally, bending at joints to follow the curvature of the magnet

• doubles up as an antenna for quench localization

• resolution = 0.1 T, 50 rad; accuracy = 100 ppm

• 213 Al2O3 sectors carrying 3 rectangular coils each

• cover the full length of the magnet to obtain integral in a single rotation (~ few seconds)

• stiff torsionally, bending at joints to follow the curvature of the magnet

• doubles up as an antenna for quench localization

• resolution = 0.1 T, 50 rad; accuracy = 100 ppm

shafts

coil

SiN flange+ bronze roller

Ti bellows

Micro-cableconnector


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Magnetic measurement equipment: harmonic analysisMagnetic measurement equipment: harmonic analysisMagnetic measurement equipment: harmonic analysisMagnetic measurement equipment: harmonic analysis

G FFTA

A-C

Programmable amplifier

V(t) GV(t) ()

{An,Bn}

“A” coil

“C” coil

Absolute (A) Compensated (A-C)


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Magnetic axis measurement in dipolesMagnetic axis measurement in dipolesMagnetic axis measurement in dipolesMagnetic axis measurement in dipoles

X Y

travelling probe (mole)

LED light spot (optically projected in the centre of the coil)

telescope CCD camera + DSP XY measurement

Telescope coordinate frame

Magnet coordinate frame

Dipole

Reference support

Telescope

Alignment target (fiducial)

Magnetic axisLED light-spot

(coil rotation axis)

Laser tracker

T

S

R

S

T

Quadrupole Configured Dipole(“ugly quad”)

Dipole axis measurement with a warm rotating coil mole + telescope tracker


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Laser Tracker SurveyLaser Tracker SurveyLaser Tracker SurveyLaser Tracker Survey

• Laser tracker to detect 3D position of spherical optical targets (retro-reflector corner cubes) mounted on standard conical receptacles

• Interferometer and phase modulation detector for distance measurement;2 16000 pt. incremental angular encoders for direction measurement

• Nominal accuracy = 5 ppm = 50 m @ 10 mReal-world repeatability = 150 m @ 10 m

• Measurement modes: static, cyclic, dynamic (< 1kHz)

• Used for:- measurement of magnetic axis and field direction of dipoles and quadrupoles, warm and cold (moles, Chaconsa, 15m shafts benches)- geometric survey of auxiliary equipment (reference magnets)

• Laser tracker to detect 3D position of spherical optical targets (retro-reflector corner cubes) mounted on standard conical receptacles

• Interferometer and phase modulation detector for distance measurement;2 16000 pt. incremental angular encoders for direction measurement

• Nominal accuracy = 5 ppm = 50 m @ 10 mReal-world repeatability = 150 m @ 10 m

• Measurement modes: static, cyclic, dynamic (< 1kHz)

• Used for:- measurement of magnetic axis and field direction of dipoles and quadrupoles, warm and cold (moles, Chaconsa, 15m shafts benches)- geometric survey of auxiliary equipment (reference magnets)

LEICA Tracker unit

Taylor-hobson 3.5’’ tooling ball

Retro-reflectorRotating

head


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L Rossi, Superconducting magnets for the LHC main lattice, Presented at: 18th International Conference on Magnet Technology MT 18 , Iwate, Japan , 20 - 24 Oct 2003

M Allitt, R Wolf et al; Status of the Production of the LHC Superconducting Corrector Magnets Presented at: 18th International Conference on Magnet Technology MT 18 , Iwate, Japan , 20 - 24 Oct 2003

L Rossi, Superconducting Cable and Magnets for the Large Hadron Collider Presented at: 6th European Conference on Applied Superconductivity EUCAS 2003 , Sorrento, Napoli, Italy , 14 - 18 Sep 2003

W Scandale, E Todesco et al, Influence of Superconducting Cable Dimensions on Field Harmonics in the LHC Main Dipole, LHC Project Report 693

CERN Accelerator School on Superconductivity in Particle Accelerators, Hamburg 1995, CERN Yellow Report 96-03

The Large Hadron Collider – Conceptual Design, Report CERN/AC/95-05

ReferencesReferencesReferencesReferences

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Superconducting LHC Magnets characteristics and qualification in SM18 test station