Prapaiwan Sunwong

• General background – characteristics of superconductor

• Material selection and cable structure

• Multipole magnets

• Generation of multipole fields

• Magnet function and coil structure

• Insertion devices

• General design requirements

• Superconducting magnet at SPS

• Concluding remarks

Talk OutlineTalk Outline

IntroductionIntroduction

High bending field is required for

High energy

Compact machine

http://home.web.cern.ch

LHC

[ ]ρBE 3.0=

Superconducting CharacteristicsSuperconducting Characteristics

1. Zero resistanceDiscovered by Onnes in 1911– solid mercury exhibitsvanishing resistance below 4.2 K.

2. Meissner effectExclusion of magnetic flux fromits interior – discovered in 1933by Meissner and Ochsenfeld.

Critical TemperatureCritical Temperature

YBCO

www.ccas-web.org/superconductivity/

Critical Magnetic FieldsCritical Magnetic Fields

Type I

Type II

Nb3Al

Keys, 2002

Critical Current DensityCritical Current Density

Keys, 2002

Nb3Al

Critical Surface Phase DiagramCritical Surface Phase Diagram

Applications of SuperconductivityApplications of Superconductivity

• Superconducting electromagnets (low Tc)

• Medical uses – MRI scanners

• Scientific research – NMR

• Transportations – MAGLEV trains

• Fusion tokamak – ITER

• Particle accelerators

• Josephson junction devices – SQUID

• Low-loss power cables (high Tc)

• Magnet current leads (high Tc)

• Electric motors, generators, fault current limiters

Why Superconducting Magnets?Why Superconducting Magnets?

Type Advantages DisadvantagesPermanent • Compact

• Low cost ( in small low field magnets)• No utilities required• No maintenance• Simple to operate• Can result in very precise fields

• Constant field (mostly)• Limited in field

Resistive • Variable field• No need for complicated cryogenic or vacuum systems• Can be built in house or through existing industrial base• Relatively low capital cost

• Limited in field (up to ~ 2 T)• May require large amounts of electrical power and cooling water• Possible large operating costs for power & water

Superconducting • High and variable field• Lower operating costs• Reliability• Cold beam tubes yield very high vacuums• Can be made compact

• High capital costs• Limited industrial base• Requires complicated ancillary systems – cryogenics, vacuum, quench protection

www.magnet.fsu.edu

Material SelectionMaterial Selection

• Alloy of niobium and titanium extremely ductility

• Tc ≈ 9 K, Bc2 ≈ 15 T (vary with composition, 46.5% Ti optimum)

• Ic is influenced by microstructure (flux pinning)

• Copper stabiliser (RRR ≥ 100)

- mechanical stability

- electrical bypass

- heat sink

• Multifilamentary wire

• Typical filament diameter 5 – 50 μm

• Typical wire diameter 0.3 – 1.0 mm

• Twisted filament/wire reduce coupling between filaments

for ac field or during field sweep

NbTi WiresNbTi Wires

ATLAS strand

LHC MQY duadrupole strand

LHC dipole strand

RutherfordRutherford--type Cablestype Cables

Filaments(6 μm each)

Wire/strand(6,300 filaments)

Rutherford cable (36 strands)

http://lhc-machine-outreach.web.cern.ch

• Transposed cable: every wire changes places with every other wire along the length of the cable, to decouple the wires with respect to their own self field and promote a uniform current distribution.

• Rutherford-type cables can be compacted to a high density (88 – 94 %) and rolled to a good dimensional accuracy.

Rutherford Cables ManufactureRutherford Cables Manufacture

Martin Wilson’s lecture

Multipole MagnetsMultipole Magnets

Dipole

Quadrupole

Resistive magnets Superconducting magnets

Generation of Multipole FieldsGeneration of Multipole Fields

)cos()( 0 φφ mII = , m = order of multipole

)sin(2

),(

)cos(2

),(

100

100

θμθ

θμθθ

mar

aIrB

mar

aIrB

m

r

m

−

−

⎟⎠⎞

⎜⎝⎛−=

⎟⎠⎞

⎜⎝⎛−=

PR

θr

x

y

beam axis

φa

current in z direction

In superconducting magnet, field shape is defined by position of each conductor (that carries current) in the coil.

Current distribution

Magnetic field

θBrB

B

θr

x

y

Current distribution can be created by multiple intersecting circles/ellipses carrying constant current densities (J) in differentdirections.

The field inside the current free region is computed by superimposing the field produced by the conductors.

Circular conductor:

Elliptical conductor:

⇒ Difficult to fabricate⇒ Use of current shells for practical constant-CSA conductors

Generation of Multipole FieldsGeneration of Multipole Fields

)cos()( 0 φφ mII =

21

20

21

10 ,

aaxaJB

aayaJB yx +

=+

−= μμ

2 ,

2 00xJByJB yx μμ =−=

+J-J

y

x

1a2a

Magnet Function and Coil StructureMagnet Function and Coil Structure

Dipole • m = 1

• Uniform field for bending

• Intersecting (overlapping) circles

• Intersecting (overlapping) ellipses 2

0JdBB yμ

−==

21

20 aa

daJBB y +−== μ

B

Quadrupole• m = 2

• Intersecting ellipses

• Gradient field for focusing

Magnet Function and Coil StructureMagnet Function and Coil Structure

xaa

aaJB

yaa

aaJB

y

x

21

210

21

210

)(

)(

+−

=

+−

=

μ

μ

y

a1

a2

Sextupole• m = 3

• Intersecting ellipses

• For chromaticity correction

( ) ...

...222 +−=

+=

yxSB

SxyB

y

x

Liu, 2011

Some novel designs (for pure multipole fields)

Sextupole Octupole

Magnet Function and Coil StructureMagnet Function and Coil Structure

Major ProjectsMajor Projects

USPAS Course on Superconducting Accelerator Magnets, 2003

TevatronTevatron

Bottura, 2011

Major ProjectsMajor Projects

USPAS Course on Superconducting Accelerator Magnets, 2003

LHCLHC

Bottura, 2011

LHC TwinLHC Twin--aperture Dipoleaperture Dipole

cds.cern.ch

Insertion DevicesInsertion Devices

Undulator

K ≤ 1, θ ≤ 1/γ

- many alternating low-field magnetic poles- strong interference effects to increase photon flux

Wiggler

K > 1, θ > 1/ γ

Multipole wiggler- several periods to increase photon flux- less important interference effects

Wavelength shifter- one period with high field center pole (usually 5-6 T)- very short-wavelength radiation

http://pd.chem.ucl.ac.uk/pdnn/inst2/insert.htm

2c E

BK

u

u

λλ

λ

∝

∝Parameter

Critical wavelength

Helical undulator Planar undulator

Superconducting helical undulator for ILC (bifilar helix design)

UndulatorUndulator

YuryIvanyushenkov, ASD Seminar, 2013

Argonne National Laboratory’s planar superconductor undulator

Period length switching for hybrid superconducting undulator/wiggler

UndulatorUndulator

Grau, 2010

BK uλ∝

The iron yoke and poles of a CESR superconducting wiggler magnet for ILC

Multipole wigglerMultipole wiggler

Superconducting wiggler at NSLS

Superconducting wiggler at DLS

Wavelength ShifterWavelength Shifter

Prototype SWLS at NIRS

Total power distribution of SWLSat SPS (1.2 GeV, 200 mA)

YuryIvanyushenkov, ASD Seminar, 2013

Country Organization ActivityTaiwan TLS, TPS SC wigglers, R&D on SCUs

Russia Budker Institute SC helical undulator for HEP;SC wavelength shifters;SC wiggler

France ACO, Orsay SCU

Germany ANKA SCU for Mainz Microtron, R&D on SCUs

ACCEL Two SCUs (for ANKA and for SSLS/NUS, Singapore)

Babcock Noell New SCU for ANKA

UK ASTeC, RAL and DL Helical SCU for ILC

Sweden MAX-Lab SC wiggler

USA Stanford Helical SCU for FEL demonstration

BNL R&D on SCUs

LBNL R&D on SCUs

Cornell SC wiggler

NHFML R&D on SCUs

APS R&D on SCUs

Work on superconducting insertion devices around the world

General Design RequirementsGeneral Design Requirements

• Keep it superconductive with a comfortable margin

• Magnet training

- well protected (when quench)

• Reduce heat load

- minimise contact resistance

- vapour-cooled/hybrid current leads

• Good cryogenic system to handle all heating

• Good support structure to handle large Lorentz force

• Cheap and easy to manufacture

• Field quality (uniformity) – relative field error better than 10-4 is required.

• Not degraded by exposure to the high radiation levels

• Well cooling of the chamber and active interlock system

Iron YokeIron Yoke

heat exchanger

bus-bar

saturation control

Wilson’s and Bottura’s lectures

CryostatCryostat

Wilson’s and Bottura’s lectures

Radiative heat transfer ∝ T4

Thermal PropertiesThermal Properties

Ekin, 2007

Quench and ProtectionQuench and Protection

Quench = conversion of magnet energy (LI2/2) into heat inside the volume of

magnet winding which has transited into the resistive state

E = 7.8 × 106 J for LHC dipole magnet

equivalent to the kinetic energy of 26-tonnes magnet

travelling at 88 km/hr

Cause of quenching

• Low specific heat

• Conductor motion (10μm motion of

NbTi

raise local temperature to 7.5 K)

• Resin cracks

• Jc decreases with increasing temperature

Wilson’s lecture

Quench and ProtectionQuench and Protection

3D simulation of quench propagation for a cos theta type magnet

http://research.kek.jp/people/wake/magqt/

Quench and ProtectionQuench and Protection

LHC dipole GSI001

Wilson’s lecture

Safe hot spot temperature = 100 – 150 (300) K

Quench and ProtectionQuench and Protection

ten Kate 2013

1. Normal zone detected 2. Switch opened3. Heater activated

Bypass diodes for magnets connected in series

Training of Superconducting MagnetsTraining of Superconducting Magnets

Several thermal and electrical cycles need to be applied to a new coil before

the optimal performances are obtained.

Wilson’s lecture

LHC short prototype dipoles

Superconducting Magnet at SPSSuperconducting Magnet at SPS

6.5 T Superconducting Wavelength Shifter (from NSRRC, Taiwan)

• Current operating field = 4.0 T at 170 A (maximum field = 6.5 T at 308 A )

• Critical current of NbTi is 428 A at ∽8 T inside the coils.

• Helium consumption = 1.4 L/hr (published value = 1.3 L/hr)

• Hard x-rays radiation used in

macromolecular crystallography

- energy range = 12.7 keV

- flux = 109 photons/s at 100 mA

www.slri.or.th

www.slri.or.th

6.56.5 T Superconducting Wavelength ShifterT Superconducting Wavelength Shifter

Liquid nitrogen

Liquid helium

Cryogenic SystemCryogenic System

www.slri.or.th

Production capacity : 20 L/hr

• From MAX-Lab, Sweden

• Maximum field = 6.4 T at 250 A

• No liquid nitrogen screening

• 10 out of 1482 windings in side pole

were burnt off and replaced by Cu sheet.

• Helium consumption < 5 L/hr (???)

6.46.4 T Superconducting Wavelength ShifterT Superconducting Wavelength Shifter

Wallen, 2002

Concluding RemarksConcluding Remarks

• Magnet is the most important application of superconductivity.

• Superconducting magnets provide high magnetic fields, which are required for

high-energy and/or compact accelerators. NbTi has been used the most.

• Magnetic field profiles from superconducting magnets are determined by position

of superconducting coils, which can be obtained at high accuracy.

• Advantages of superconducting insertion devices:

- High field increases photon energies

- High flexibility

- Smaller period for the same peak field

- New research possibilities