Ch 4 Superconducting MagnetsCh. 4 Superconducting Magnets · Ch 4 Superconducting MagnetsCh. 4 Superconducting Magnets Thomas J. Dolan ASIPP H f iASIPP Hefei 2011 Superconductivity

Ch 4 Superconducting MagnetsCh. 4 Superconducting MagnetsThomas J. DolanASIPP H f iASIPP Hefei2011

SuperconductivitySuperconductorsStabilizationStabilization Coil protectionCoil designLarge coilsLarge coilsMagnetic energy storage

1

2From Dr. Matthias Noe, Karlsruhe Summer School, 2008

From Dr Matthias Noe Karlsruhe Summer School 2008

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From Dr. Matthias Noe, Karlsruhe Summer School, 2008

Domain of Superconductivity

T K B TT, K B, T

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Discovery of Superconductors

Kamerlingh Onnes 1911

Dolan SWIP 2009 5From Dr. Matthias Noe, Karlsruhe Summer School, 2008





TheoryTheoryUsually electron-lattice collisions resistivity

Bardeen-Cooper-Schrieffer:Electron pairs can be coupled by phonons(lattice vibrations) Interact only with each other,

not with lattice no resistivity.

In Cu e-phonon-e interaction is weak, no coupling.

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At T=0, all electrons are pairedBond energy 2 = 3.5 kTc Tc = “critical temperature”

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At T>0, some pair bonds are brokenMore unbound electrons Fewer scattering states available

(Pauli Exclusion Principle) weaker electron-phonon interactions weaker electron pair binding weaker electron pair binding

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At T=Tc , all pair bonds are brokenc p superconductivity lost

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Energy Gapgy pAt T=0 all electrons are paired

energy gap 2(0) = 3.5 kTc “critical temperature”

Raising T, J, or B heating some pairs split

Each pair split more free electrons fewer possible scattering states for paired e

(Pauli exclusion principle)(Pauli exclusion principle). weaker e-phonon-e coupling, reduced energy gap .

As T increases 0.

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Coherence Length

1

0

pairedfraction

ns/n

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0s


Meissner Effect -- Diamagnetism

d2B/dx2 = B/L2

“London penetration depth”L = (omec2/nse2)1/2 = 5.3x106 ns

-1/2

If L = constant, then B = Bo exp(-x/ L)

If ns = 1028 m-3, then L = 5x10-8 m

Bo

B(x)o

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L

Flux Quantization

Magnetic fluxon o = h/2e = 2.07x10-15 Wb (Weber)

Fluxon is a tube of normalconductivity embedded inthe superconductorthe superconductor.

Tiny ferromagnetic particlesattracted to magnetic fluxonson the surface of superconducting Pb-In alloy.superconducting Pb In alloy.

1 m

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Type I and Type II Superconductors

Type > 21/2 Lyp LMost pure metals Critical magnetic field Bc = Bco [1 - (T/Tc)2] T = critical temperatureTc critical temperatureType I have Bco < 0.1 T poor for magnets

T II < 21/2 Type II < 21/2 LAllow fluxon penetration into superconductorFluxon penetration begins at “lower critical field” Bc1c1Can carry higher J at higher B than Type I.

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Flux Penetration into Type II

Flow of vortex currents Around fluxons Around fluxons o.

Each fluxon is in a l i (l )normal region (low ns).

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Fluxon overlap reduces ns

Spatial variation of superconducting electron density ns

At “upper critical ppfield” Bc2 fluxon overlap large n smallns small superconductivitylost.

Bc2 ~ o/(2)2

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Fluxons “pinned” in lattice defectsLorentz JxB force pushesfluxons sideways,

t i d b l tti

lattice defects

restrained by latticedefects.

flIf J is high, fluxons canmove from one site to another(“flux creep”) which generates heat

fluxons

( flux creep ), which generates heat.

If many fluxons move, “flux jump” loss of superconductivity

Many lattice defects needed to prevent flux jump.

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Need for Coil StabilizationFlux jumps local “normal” region J2 heat generation

Inductance prevents current decay (L d/dt voltage)

Heat nearby region normal more heat spreadHeat nearby region normal more heat spread

Need to prevent “quench” (spread of normal region).

Large stored energy could melt coil or rupture cryostat.

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Cryogenic Coil StabilizationCurrent sharing by copper, rapid heat removal By helium coolant. Stekly number

= resistivityI = currentI currentL = conductor lengthA = conductor area

i bl h t fl 4 kW/ 2q = maximum removable heat flux ~ 4 kW/m2

S = helium coolant contact area

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Adiabatic Coil StabilizationTiny filaments flux jump heat too small to make T>TTiny filaments flux jump heat too small to make T>Tc

If d ~ 10 m and To ~ 10 K, then Js < 3x1010 A/m2

Filaments must be “transposed”(braided) to prevent current loops between adjacent filamentsloops between adjacent filaments.

Higher J and dJ/dt than cryogenic

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stabilization.

Tape Wound CoilsThin layer of brittle Nb3Sn on ductile Cu ribbon.

Nb3SnCu

“Dynamic stabilization”

D i f i fl jDamping of magnetic flux jumps + heat removal

Difficult to wind in complex shapes

Canada magnet failureCanada magnet failure.

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Need for Coil Protection

Quench – 1 GJ coil energy dissipated in small volume gy pmelting, coolant pressure, possible vessel rupture.

Broken circuit arcs puncturing insulation coil caseBroken circuit arcs, puncturing insulation, coil case.

Short circuit to ground – current limiting resistor

Coolant channel blockage quench more probable

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Protection CircuitProtection Circuit

Quench raises voltage current flows throughQuench raises voltage, current flows through external resistor (bars of iron cooled by water).

R 0 1 Ohm I = 10 kA V = 1000 VRext ~ 0.1 Ohm, I = 10 kA V = 1000 V.

If L = 30 Henry, current decays with time constant

L/R = 300 s.

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Magnet Safety Analysis

Many potential accidents

Arcs

F lt d t tiFault detection

Local hot spot detectionp

Magnetic energy dump

Organic insulators H2

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Fault DetectionQuench R , T , He pressure,

Sh t i it R t d b l dShort circuit R to ground , unbalanced currents

Open circuit Voltage B changeOpen circuit Voltage , B change

Coil movement Position sensors

Coolant pump or tube

Flow rate , pressure , quench

Dewar Pressure

Refrigeration T Dolan SWIP 2009 28

Refrigeration T

Low-Temperature Supereconductors

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Critical Currents of NbTi/Cu Wires

mm


Strain Degradation of Nb3Sn Conductor

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Comparison of Nb3Sn with NbTi


Hybrid Magnets

Superconducting coil

Normal copper coil

Much higher B thanMuch higher B than Superconducting alone

Much less power thanMuch less power than copper coil alone

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Summary - Superconductivity

Superconducting domain inside Tc, Bc, Jc surface

Quantized fluxons -- tubular normal regions

JxB force pushes fluxons heat generation

Lattice defects pin fluxons, inhibit motion

Stabilization by current sharing magnetic damping tinyStabilization by current sharing, magnetic damping, tiny

filaments, heat removal

H b id il lt hi h BHybrid coils ultrahigh B

Coil protection to prevent melting

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Coil DesignCoil Design

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Coil Design ConsiderationsB field Required N IC d t i I t bili ti i di j i tConductor size, I, stabilization, winding, jointsProtection Fault detection, damage reductionHeat removal Coolant channel, flow rate, pumpingStructural JxB stress, thermal stress, support, pp

windingCryogenics Heat load, refrigerationRadiation damage

Neutron and gamma doses, effects on resistivity and insulators

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g y

Choice of Conductor


Cable in Conduit




Nb3Sn multifilamentary conductorNb3Sn multifilamentary conductor




ITER Coils18 Nb3Sn TF coilsBmax = 11.8 TC bl i d it d tCable-in-conduit conductor; wind-react process6540 tons. 150,000 km.,

Nb3Sn Central Solenoid 9 m

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ITER Coil Systemy


ITER TF Coils

Dolan SWIP 2009 47From Neil Mitchell, SOFT 2008

ITER TF Coils


TF Coil Winding Pack


TF Coil Connections





Insulating the TF CoilInsulating the TF Coil


Installation into TF Coil Case


ITER Toroidal Field RippleITER Toroidal Field RippleITER may need TF ripple < 0.5% to attain Q = 10.ITER may need TF ripple 0.5% to attain Q 10. With 18 TF coils ripple > 0.5%Use Fe inserts between coils.

BB

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TF Coil Case Manufacture

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Poloidal Field Coils

Control plasma shape & position

6 NbTi pancake coils

Cable-in-Conduit conductors.

5 will be wound large5 will be wound large coil-winding building

E tra coils madeExtra coils made.

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PF Coil ClampsPF Coil Clamps


PF Coil WindingsPF Coil Windings


PF Coil WindingPF Coil Winding


Winding ITER PF Coils


Stacking Double Pancakes


Central Solenoid

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From Neil Mitchell, SOFT 2008

CS Coil Module


Central Solenoid LeadsTransformer to induce plasma current I ~ 15 MA6Nb3Sn cable-in-conduit coils

Vertical pre-compression structure

Pulsed coils fatigue lifePulsed coils fatigue life4 K Current leads 300 K




Correction CoilsCorrection CoilsEdge Localized Modes (ELMs)“Resonant magnetic perturbations” pedge < ELM levelResonant magnetic perturbations pedge ELM levelFeedback control

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TF Coil Procurement



2.2 MW 0.63 MW


HTS Current Lead



Large Helical Device (LHD)ℓ=2 helical coils

6 circular PF coils

10 field periodsp

National Institute for Fusion Science,Toki Japan

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Toki, Japan

Coils inside LHD cryostaty

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LHD CoilsPoloidal Coils inner middle outer Helical Coils

Inner diameter, m 3.2 5.4 10.4 Major radius 3.9 m

Outer diameter, m 4.2 6.2 11.6 Minor radius 0.975 m

W i ht t 16t 25 45 W i ht t 65Weight, ton 16t 25 45 Weight, ton 65

B max T 6.5 5.4 5.0 Bmax, T 6.9

Current, kA 20.8 21.6 31.3 Current, kA 13, ,

# turns 240 208 144 # turns 450

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LHD Coil Winding Machineg

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L H li lLarge HelicalDevice,Toki, Japan

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I t i f LInterior of LargeHelical Device,Toki, Japan

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Wendelstein 7-X (W7X) Stellarator Coils and PlasmaCoils and Plasma

50 modular coils

Plus 20 circular coils

Each 3.5 m high

5 field periods

Ring support structureg pp

Greifswald, Germany

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Wendelstein 7-X Cryostat

245 ports for plasma heating & diagnostics. Thermally insulated tubes: vacuum vessel cryostat ports

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Thermally insulated tubes: vacuum vessel cryostat ports

W 7-X Coil Production

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W7X Coil after Heating

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W7X Modular Coil

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First Assembly Rig6-ton coil vacuum vessel

segment2nd vessel segment brazed on2nd vessel segment brazed on

Thermal insulation installed

More coils & vessel segments Half-module (5 modular Half module (5 modular

+ 2 auxiliary coils)

Bolt on support ringBolt on support ring

Adjust alignments

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50-ton Half-Module50 ton Half Module

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Second Assembly HarnessHalf-module is hoisted and joined to other half-module

The support ring segments bolted togetherpp g g g

Plasma vessel halves are brazed. 100-ton module

24 coil leads brazed onto coils, insulated, leak-tested

He coolant tubes are connected, leak-checked

Magnet coil instruments & connecting cables installedg g

Completed module leaves assembly jig after 28 weeks

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W 7-X Outer Shell

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Third Assembly – Experiment HallModule hoisted into bottom shell of the outer vessel;

connections and supports are attached. pp

Lifted onto machine foundation, attached to supports.

Top shell of the outer vessel put on and brazedTop shell of the outer vessel put on and brazed.

~60 ports inner vessel outer vessel connected, insulated.

Divertor plates, heat shields, cryopumps installed.

Five modules joined: brazing plasma vessel & outer vessel.

Magnets connected to power supplies, He supplies,

cooling pipes.

Dolan SWIP 2009 91Repeated control measurements and leak tests.

W 7-X InstallationsMicrowave heatingElectric power supplyElectric power supplyCryogenicMachine controlPl di ti tPlasma diagnostic systems

~ 2014 -- uncooled divertor, short pulses, full power.

~ 2017 -- water-cooled divertor 30 minute duration(limited by the external heat-rejection system)(limited by the external heat rejection system).

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Superconducting Magnetic Energy StorageGoals: to store electrical energy for:Variations between day and nighttime demandSolar and wind power plants.High energy particle acceleratorsPulsed fusion power plantsp p

Potentially better than pumped hydrostorage, compressed air and batteriesair, and batteries.

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5000 MWh SMES System5000 MWh SMES System

R 0 6 kR = 0.76 kmBmax = 6.8 TH = 15 m108 turnsI 768 kAPressure on rock =Pressure on rock =

3.9x105 N/m2

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Summary – Superconducting MagnetsSummary Superconducting Magnets


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Documents

Ch 4 Superconducting MagnetsCh. 4 Superconducting Magnets · Ch 4 Superconducting MagnetsCh. 4 Superconducting Magnets Thomas J. Dolan ASIPP H f iASIPP Hefei 2011 Superconductivity