1 Development of Remountable High Temperature Superconducting Magnet Prof. Hidetoshi Hashizume Tohoku University.

1

Development of Remountable Development of Remountable High Temperature High Temperature

Superconducting MagnetSuperconducting Magnet

Prof. Hidetoshi HashizumeProf. Hidetoshi Hashizume

Tohoku UniversityTohoku University

2

1. Concept of remountable HTS magnet

Magnet can be mounted and demounted iteratively.

Magnet can be fabricated in short parts.

It is easy to access inner structural components.

Making the fabrication easy

Making maintenance easy

Remountable Superconducting MagnetRemountable Superconducting Magnet

High Temperature SuperconductorHigh Temperature Superconductor

HTS can be used in relatively high temperature. Specific heat: Large (c T3 in cryogenic environment.)

HTS is robust against heat generation at jointing parts.

Long cable is NOT necessary.High Performance / Low Cost Cable

Remountable HTS Magnet

Spherical Tokamak

FFHR

3

2. Mechanical joint of HTS tape

Mechanical joint of HTS cable must be developed.

Development of remountable HTS magnet

Just Jointed,Mechanically

HTS Tape

Butt Joint

JJoint // ab-plane

Current Flowing Direction

Lap Joint

JJoint // c-axis

Current Flowing Direction

HTS Tape

Just Jointed,Mechanically

The but joint method is suitable for remountable HTS magnet.

4

3. Fundamental study on butt joint of HTS tape3-1. Experiment of butt joint of HTS tape

Current-Resistance Characteristic

There exists dependence of joint resistance on current.

Joint Resistance: 3.6 (at 60A)

ResultExperimental Apparatus

Voltage Tap

BSCCO 2223 Tape

Vinyl Chloride Board

(IC: 67A, w: 4.0mm, t: 0.26mm)

JointRegion

Vinyl Chloride Board

Contact force : Thermal contraction

30mm

0

1

2

3

4

0 20 40 60 80

Single-Layered TapeThree-Layered Tape

Re

sist

an

ce

(

)

Current (A)

Jointing Resistance is almost inversely proportional to the contact area.

5

3. Fundamental study on butt joint of HTS tape3-2. Performance analysis in different filament location

3-2-1. Composition of HTS tape

SuperconductingFilament

Ag (Stabilizer)

Ag AlloySeveral tens of

superconducting filament

Ag (Stabilizer)+

Cross Section

Ag (Stabilizer)

SuperconductingFilament Ideal Case

Real Case

Same

Different

Ag SuperconductorContact Surface

Filament Location in Butt Jointing

Electromagnetic field analysis is performed to evaluate the influence of filament location

BSCCO 2223 Tape

6

3. Fundamental study on butt joint of HTS tape3-2. Performance analysis in different filament location

3-2-2. Analytical model and numerical scheme

0.22

2102

9.0

10

-32.475

10-

2

3.375

10-

2

0.06 0.060.1

AgGapSuperconductor

0.22

2102 9.0

10

-32.475

10-

2

3.375

10-

2

0.06

0.06

0.1

AgGapSuperconductor

Model 1 Model 2

2-D FEM

Current Vector Potential Method Critical State Model : Bean Model

Governing Equation : Faraday’s Low

TJ Current DistributionJoule heating loss

J = 44.8% of JC

to avoid quenching

Gap Region : Assumed Material to Treat Contact ResistanceGap Resistance　 2.39 10-6 m (230in butt jointing of one-layer HTS tape)

7

3. Fundamental study on butt joint of HTS tape3-2. Performance analysis in different filament location

3-2-3. Analytical result

JC

Ag

SC

Gap

JC

Ag

SCGap

Model 2

Model 1

Current distribution in SC regionagrees with critical state model

Current expands near the gap region

SC filament location does not affect the butt jointing performance

Model Joule Heat (W)

Model 1Model 2 6.56

6.56

Joule heating loss

Contact condition is dominantIncrease of contact point is effective to reduce joint resistance for example with metal-plating.

8

3. Fundamental study on butt joint of HTS tape3-3. Dependence of joint resistance on current

3-3-1. Assumption of flat contact surface

The dependence can not be obtained

No Temperature Increase

No Flux Flow State( No decrease of JC )

Result

Dependence of resistance on current

Flux flow

Heat generation

Flat contact surface

Jointing region

Electromagnetic Field AnalysisThermal Analysis

Joule Heating LossJointing Resistance

Model 3

Superconductor

Ag Gap(Ag)

Gap(SC)

Gap Resistance: 2.8 10-8 m (Jointing Resistance at 5A)

Thermal Conductivity Specific Heat of Assumed Material

substituted by those of materialsbesides the assumed material

9

Decrease of JC

High J

Expression of decrease of JC

Depth: 0.2mm Depth: 0.4mm

Current density becomes larger than critical current density

Decrease of current flowing area Decrease of critical current density =

3. Fundamental study on butt joint of HTS tape3-3. Dependence of joint resistance on current

3-3-2. Assumption of notch and/or degradation

Degradation of superconducting filamentNotched contact surface

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3. Fundamental study on butt joint of HTS tape3-3. Dependence of joint resistance on current

3-3-3. Analytical result

Dependence of jointing resistance on the transport current

notched contact surface

Depth = 0.4mm

Reason of the dependence

Degradation of SC filament

Actual depth

2.6

2.8

3.0

3.2

3.4

3.6

3.8

0 10 20 30 40 50 60 70

Experiment

Calculation_without notch

Calculation_0.2mm

Calculation_0.4mm

Re

sist

an

ce

(

)

Current (A)

Protection of joint surface is important for example with metal-plating.

11

4. Study on butt joint of laminated HTS cable4-1. Test cable and experimental set-up

Rod10-Layered

BSCCO 2223 Cable

Voltage TapJoint RegionCoolant: Liquid Nitrogen

Silver RatioCritical Current

2.290 A (77 K)

HTS Cable

Specification of BSCCO 2223 TapeCritical Current: 400A

Joint

Load

2.1

4.2

60V

Loading Area: 4.54.5Unit: mm

10-LayeredBSCCO 2223 Cable

Low TemperatureSolderContact Surface

30, 40, 45, 50, 60

4.2mm

2mm

12

4. Study on butt joint of laminated HTS cable 4-2. Stress-resistance characteristic

Enlarge

Joint resistance becomes almost constant (160 ~ 200MPa)

begins to increase slightly (200MPa~ )

Improvement of contact condition

degradation of HTS cable >

As compressive stress increases

There exists optimum stress

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0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 100 200 300 400 500

10.4 MPa41.5 MPa83 MPa124.5 MPa

R

Jo

int [

]

I [A]

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500

41.5 MPa83 MPa124.5 MPa

Join

t R

esis

tan

ce

[

]

Current [A]

0

20

40

60

80

100

120

140

0 100 200 300 400 500

10.4 MPa41.5 MPa83 MPa124.5 MPa

Join

t R

esis

tan

ce

[

]

Current [A]

4. Study on butt joint of laminated HTS cable 4-3. Current-resistance characteristic

Current-resistance characteristic in Dry Joint

Thermal Quench

IC

Current-R characteristic

Joint Resistance 4

Temperature increase disappears

- No decrease of critical current- No change of resistance gradient

Joint resistance can be reduced small enough to prevent quench occurrence due to temperature increase caused by heat generation below 500A

14

0

5

10

15

20

25

30

0 50 100 150

62 MPa 101 MPa

No

rmal

ized

Res

ista

nce

[x1

0-11

m2]

Shear Stress [MPa]

0

5

10

15

20

25

30

0 50 100 150 200

30 deg40 deg45 deg50 deg60 deg

No

rmal

ized

Res

ista

nc

e [x

10-1

1

m2]

Normal Stress [MPa]

-0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10

SC/SCSC(Ag)/SC(Ag)

R

Jo

int [

]

Iteration Number

2

4

6

8

10

12

14

16

0 20 40 60 80 100 120

Dry JointSC(Ag)/SC(Ag)_15mSC(Ag)/SC(Ag)_50mSC(Ag)/SC(Ag)_100m

Join

t R

esis

tan

ce

[x1

0-6

]

Normal Stress [MPa]

Protective effectwith silver-plating

Reducing resistance Preventing degradation

Angle Dependence

Normalized resistance increases when joint surface angle is larger than 45.

Slippage occurs because shear stress becomes larger than normal stress.

4. Study on butt joint of laminated HTS cable 4-4. Angle dependence and plating effect

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4. Study on butt joint of laminated HTS cable4-5. Analytical evaluation with structural analysis

4-5-1. Analytical model (1)

6 loading system are compared

• Normal stress distribution on joint surface

• Intensity of stress concentration

• Buckling with outward force

Structural analysisStructural analysis 3D elastic-plastic deformation analysis

ModelWidth w (mm)

Radius r (mm)

Bottom loading

A 4.4 0 ×

B 4.4 0 ○

C 4.4 2.0 ×

D 4.4 2.0 ○

E 4.4 3.0 ×

F 2.0 2.0 ×

HTS cable

w

r

Top rod

Bottom rod

Evaluating

Bottom loadingO

Bottom loading

16Unit [mm]

Schematic view of analytical modelSchematic view of analytical model

○ ○ 計算領域上面図計算領域上面図

○ ○ 計算領域側面図計算領域側面図

x

y z

Loaded area

Calculating area

20.0

HTS cable

2.0

4.2

Joint surface

Analytical objectAnalytical object ：：10-layered laminated HTS cable

Length： 20mm, Width： 4.2mm, Thickness： 2.0mm

• Symmetric restraint condition • yz-plane ： ux=0

• zx-plane ： uy=0

1/4 of real object is calculation area

Calculation areaCalculation area

ElementElement

Bottom plane conditionBottom plane condition

Joint surfaceJoint surface

Loading by rodLoading by rod• Displacement is given to nodal point of cable.

Regarding Assumed cross-section replicated at yz-plane as joint surface.

• Curvature at rod edge：curve-like displacement is given.

Assuming that the surface is jointed ideally.

• Slippage between layers is considered on yz-plane.

• 20-node rectangular element

• Model B, D ：Load is given as in case of Upper plane.

• Except Model B, D ：z-directional displacement is constrained.t

4. Study on butt joint of laminated HTS cable4-5. Analytical evaluation with structural analysis

4-5-2. Analytical model (2)

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Stress concentration at rod edge affects normal stress distribution on joint surface.

Loading from bottom side is effective to obtain uniform stress distribution.• Stress distribution in Model B is more uniform than that in Model A

• The same tendency is obtained in Model C and Model D

• Stress distribution is concentrated at upper side in Model F

t

y

n

Joint surface HTS cable

Calculating area

• Comparing Model A, B, F (rod=250MPa)

Normal stress on joint surfaceNormal stress on joint surface

Influencing the joint resistance

Comparison in term of loading direction (A, B)

Comparison in term of rod width (A, F)

Nonuniform distributionNonuniform distribution

Joint surface is degraded partiallyJoint surface is degraded partiallyeven if under the optimum stress even if under the optimum stress

4. Study on butt joint of laminated HTS cable4-5. Analytical evaluation with structural analysis

4-5-3. Evaluation of normal stress distribution on joint surface

Model A Model B Model F

150MPa

50MPa

18

○ Equivalent stress distribution on zx-plane

Model Ar =0

Model Cr =2

Model Er =3

Evaluating Influence of rod edge curvature on intensity of stress concentration

Large curvatureLow stress concentration

Stress is dispersed at part of Stress is dispersed at part of rod edge curvature.rod edge curvature.

Having curvature at rod edge is effective Having curvature at rod edge is effective to disperse stress concentrationto disperse stress concentration

Model A, C, E (Loading from upper side)

4. Study on butt joint of laminated HTS cable4-5. Analytical evaluation with structural analysis

4-5-4. Evaluation of intensity of stress concentration

Model Br =0

Model Dr =2

Dual loading Stress is dispersed

• Stress is dispersed with the curvature.

• Stress is dispersed with dual loading.

Model D is the best to avoid Model D is the best to avoid stress concentrationstress concentration

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○z-component of stress on zx-plane

HTS cable has layered structure buckling with layer peeling

Model A Model B

Outward force occurs locally.

• Outward force can peel layer especially in Model B.

• Cable supporting area in Model B is smaller than that in Model A

There exists possibility of buckling in dual loading

Cable is unstable

Model A 　 r=0, Top loading Model B 　 r=0, Dual loading

4. Study on butt joint of laminated HTS cable4-5. Analytical evaluation with structural analysis

4-5-5. Evaluation of buckling with outward force

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5-Layered BSCCO 2223 Cable

Voltage Tap

Remountable Coil Bobbin

Jointing Region

5-Layered BSCCO 2223 Cable

Voltage Tap

Remountable Coil Bobbin

Jointing Region

4 Jointing Regions

5. Prototype of remountable HTS magnet

101

102

0 50 100 150 200 250 300

Joint 1 (1st Iteration)Joint 2 (1st Iteration)Joint 3 (1st Iteration)Joint 4 (1st Iteration)Joint 1 (2nd Iteration)Joint 2 (2nd Iteration)Joint 3 (2nd Iteration)Joint 4 (2nd Iteration)

Join

t R

esis

tan

ce

[x1

0-6

]

Current [A]

Layer number Winding number Outer radius Operating temperature Joint number

5285 mm

77 K4

Specification

Solenoid type

Photograph of prototypeCurrent-resistance characteristic

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6. Summary1. Fundamental study on butt joint of HTS tape

- Joint resistance of 3.6was achieved at 60A.

2. Study on butt joint of laminated HTS cable

- Joint resistance was reduced small value enough to prevent thermal quench and decrease of critical current at 77K below 500A.

- Silver-plating is effective to improve and protect joint surface condition.

3. Fabrication of prototype of remountable HTS magnet- Performance of prototype of remountable HTS magnet was evaluated in liquid nitrogen environment.

- Dependence of joint resistance on current was confirmed. According to numerical analysis, that is caused by degradation of superconducting material near joint surface.

Protecting joint surface with metal-plating is important.

Feasibility of remountable HTS magnet was demonstrated.

- Joint resistance is decided by reduction of contact resistance and degradation of superconducting material with increase of joint stress.

- Joint resistance became stable when angle of joint surface was 30 to 45.

- How to obtain uniform stress distribution and to avoid stress concentration and buckling are indicated by structural analysis.

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7. Future works8-1. Development of independent heat removal system

(a) Porosity: 70% Fiber diameter: 90μm (b) Enlarged view

Characteristic of metal porous media・ High thermal conductivity　・ Enlargement of heat transfer area・ Transport of latent heat with capillary force

Heat removal system with metal porous media

0 0.2 0.4 0.6 0.8 10

0.01

0.02

0.03

0.04

0.05

0.06

流速[m/s]

熱伝

達率

[MW

/m2 /K

]

0.78MW/m2

1.67MW/m2

2.44MW/m2

3.34MW/m2

4.51MW/m2

5.37MW/m2

6.25MW/m2

Experimental set-up in case of water Result in case of water

Coolant：Water LN2

Velocity [m/s]

He

at

tra

nsf

er

coe

ffic

ien

t [M

W/m

2/K

]

23

8. Future works8-2. Design of cooling channel with metal porous media

Preventing thermal quench with heat transfer enhancement

Introducing cooling technique with metal porous media to remountable HTS magnet

Evaluation of rheological characterization and heat transfer characteristic when liquid nitrogen flows thorough metal porous media.

Issues

Design and fabrication of jointing region with metal porous media

Metal Porous Media

Insulator

HTS CableConduit

Flow of LN2

Design Proposal