transport performance and ac loss of superconducting

Brookhaven Technology Group

Transport performance and AC loss of superconducting cables comprised of

exfoliated YBCO filaments

U.S. DOE Office of Science SBIR Phase I award DE-SC0018737, Phase II award DE-SC0013856

Slowa Solovyov, Saad Rabbani, Zachary Mendelson, and Paul Farrell

Brookhaven Technology Group Inc., Stony Brook, NY 11794

Tim Haugan

Write-Patterson Air Force Base

www.brookhaventech.com

CEC-ICMC 2019, Hartford CT, July 21 - 25 2019 1


Outline

▪ ExoCable technology

▪ Mini coil test results: 77 K and 25 K

▪ AC loss: effect of transposition

▪ Flux dynamics at 25 K

▪ Conclusion and future work



Motivation: we need defect tolerant cable

CEC-ICMC 2019, Hartford CT, July 21 - 25 20193

Across-tape defects

Along-tape defects

Deposition malfunction

Epitaxy failure

Some defects emergeduring coil operation

Courtesy of Anatolii PolyanskiiNHMFL

✓ Avoiding defects in YBCO layers is difficult

✓ Some defects are hidden, get revealed only after coil tests


Solution: electrically coupled cable

2G wire stack BTG exfoliated filament stack

Buffer

Substrate

Sharing current path

Sharing current path


We are solving the following problems:▪ Single-filament magnets proven difficult to protect against burnout

▪ Substrate prevents efficient current sharing

▪ Multifilamentary cable is far more expensive than a single tape

▪ Not compatible with epoxy impregnation


BTG filament, two directions for current sharing


YBCO, Ag layer, 1.2 µm

Solder 10 µm

Copper 75 µm

Solder 5 µm

Current sharing directions

✓ The filament stack is YBCO-metal composite


Multi-filamentary cable architecture


Copper stabilizerNichrome cladding Solder YBCO

✓ Electrically connected filaments are the key element


Current transfer after the re-flow step


✓ Exfoliated stack enables current sharing within < 5 mm

10-7

10-6

10-5

10-4

10-3

60 65 70 75 80 85 90 95 10010

-7

10-6

10-5

10-4

10-3

Vo

lta

ge, (

V)

Current, I (A)

160 165 170 175 180 185 190 195 200-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

Solder coated

filament

He

at f

low

, F (

W/g

)

Temperature, T (oC)

Un-coated filament, signal x100

Solder re-flow

temperature

0 20 40 60 80 10010

2

103

104

105

106

107

180oC

185oC

190oC

Inte

r-fi

lam

en

t re

sist

ivit

y,

(n

·c

m2)

Re-flow duration, t (min)

Re-flow temperature:

Before reflow

After reflow

1 cm

8 cm

Calorimetry Optimization of the resistance

1 cm

8 cm


Test coil manufacturing process


Dry wound Vacuum impregnation, Stycast 1266

Upper current lead

Voltage taps Voltage taps

Cooling collar attached

12 coils, over 100 meters of cable tested


77 K performance after re-flow and impregnation


✓No Ic and n-value degradation after multiple rapid cool-downs to 77 K

✓Solder reflow significantly reduces the winding noise, but reduces Ic by 8%

1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

C5, onel layer, W14

C3, six layers

C3, one layer

C1, one layer

Cri

tica

l cu

rre

nt,

Ic (

A)

Number of cycles, room temperature - 77 K

C2, three layers

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

10-7

V/m

10-6

V/cm

10-7

V/m

10-6

V/cm

Solder re-flow

Three layer coil I20

Epoxy-impregnation

As-wound

Solder re-flow

Six layer coil I17

Epoxy-impregnation

Vo

ltag

e, V

(m

V)

Applied current, I (A)

As-wound

Cycling RT to 77 K


AC loss: Simplified flux penetration model into a coupled cable, highly anisotropic filament


Cross-filament loop (coupling currents)Cross-filament dipole

Persistent current loop (in-plane currents)In-plane dipole (same as in pancakes)

Radial field, Hr

Axial field, Ha

Solenoidal field Hs

In-plane dipoleShields Hr

Subtracts from Hs

Cross-filament dipoleShields Ha shields in the windingAdds to Hs in the bore


High field AC loss and field error (winding magnetization)un-transposed (flat cables) cables


0 1 2 3 4 5 6 7 8 9 10 11 12 130.0

0.2

0.4

0.6

0.8

2.4 mm BTG cable

4 mm SP tape stack

Loss

at

10

0 H

z, E

l(W/m

A c

ycle

)

Cable width (mm)

12 mm SP tape

0 20 40 60 80 100 120 1400

20

40

60

80

100

Fused stack

Loss

, (W

/m)

Frequency, Hz

0.59 T

Wraped stack

0.6 Tesla AC loss measurement

✓ AC los and field error is reduced proportionally to the filament width

0 50 100 150 2000.0

0.1

0.2

0.3

0.4

0.5

Coil current (A)

3d cycle

2nd cycle

Ma

gne

tic

fie

ld h

yste

resi

s,

0H

(m

T)

1st cycle

0 50 100 150 2000.000

0.005

0.010

0.015

0.020

0.025

0.030

C3, six layers

C1, one layer

C2, three layers

No

rmal

ize

d h

yste

resi

s,

H/H

ma

x

Coil current, I (A)

Double pancake

12 mm tape


Effect of transposition on AC loss


0 20 40 60 80 100 120 1400.0

0.1

0.2

0.3

0.4

0.5

0.6

4 filaments, flat

AC

loss

(J/

cycl

e/m

)

Frequency (Hz)

4 filaments, spiral

0.6 Tesla, 77 K

0 20 40 60 80 100 1200.0

0.1

0.2

0.3

0.4

1.5 mm flat

1.5 mm twisted, 40 mmAC

loss

(J/

cycl

e/m

)Frequency (Hz)

0.6 Tesla, 77 K calorimetry

✓ Transposition does reduce AC loss significantly✓ Filament width is the most important factor


Magnetization hysteresis in flat and twisted cable test coils


0 20 40 60 800.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

3d cycle, flat

1st cycle, twisted

3d cycle

twistedHysteresis

Hys

tere

sis,

H

(m

T)

Current, I (A)

Trapped field

1st cycle, flat

✓ At 35 mm pitch twisting resulted in an insignificant reduction of hysteresis


25 K testing-conduction cooled


12” chamber Cryomech compressor Cryomech coldhead

Mounted coilWith scanning Hall probe


Central field hysteresis at 77 and 25 K


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 200 400 600 800 1000 1200

-40

-20

0

20

Ce

nte

r fi

eld

,

0H

(T) 22 K, C3, 6 layer coil

4 A/s

Relaxation

Fie

ld h

yste

resi

s,

0H

(m

T)

Coil current, Icoil

(A)

77 K 22 K

✓At 22 K field dynamics is defined by relaxation at high currents


Flux dynamics at 77 and 25 K


0 200 400 600 800 1000 1200 14001.000

1.001

1.002

1.003

1.004

1.005

1.006

50 A (0.02 T)

100 A (0.05 T)

No

rmal

ize

d f

ield

, H

(t)/

H(0

) (a

.u.)

Time(s)

6 layer coil, 77 K 200 A (0.10 T)

0 1000 2000 3000 4000 5000 60000.0

0.2

0.4

0.6

0.8

1.0

Coil center, 1,000 A

Decay rate, 300 A

Coil outer edge, 300 A

Coil center, 300 A

No

rma

lize

d f

ield

, H(t

)/H

(0) (

a.u

.)

Time (s)

6 layer coil, 25 K

0

2

4

6

8

10 Field

time

de

rivative

H

/t/H

(10

-5/s)

77 K 25 K

✓Completely different field settling profile at 77 and 25 K.


Time profile of magnetic field inside and outside the bore


0 100 200 300 400 500 600

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Radial component, innner

Axial component, innner

Radial component, outer

Axial component, outer

No

rma

lize

d m

agn

eti

c fi

eld

, (a

.u.)

Time (s)

Relaxation at 300 A

Magnet axis

✓The central field reduction is probably explained by the flux diffusion through the

winding, due to heating from the leads at high current


Heat migration from the current leads and effect on the field quality


Current leads

Nichrome heater

0 1000 2000 3000 40000.015

0.016

0.017

0.018

0.019

0.020C1 single layer coil

Fie

ld (

T)

Time (s)

0

10

20

30

40

50

60

70

80

90

Te

mp

era

ture

(K)

✓Winding temperature gradient are responsible for the field decay

Thermocouple


Challenge of conduction-cooled Wiedemann-Franz current leads


0 5 10 15 20 25 300

50

100

150

200

1,000 A

600 A

300 A

100 A

1 m long lead, 20 K

0 A

He

at f

lux

at c

old

pla

te, P

(W)

Lead diameter, d(mm)

✓Limited thermal budget of a conduction-cooled magnet complicates high-current lead design


Proposed cryocooled rectifier


Transformer Rectifier Magnet coil

Cryogenicenvironment

Low voltage, high current leadsHigh voltage, low current leads

Vacuum vessel

AC power

Cryogenicenvironment

Vacuum vesselTraditionalapproach

Proposed Cryo-cooledrectifier


Mechanical design of the current management system


17 KM1

M3

Ls1

Cryo-head

Insulation

RT


Parallel connection of the rectifying elements allows reducing the conduction loss


0 50 100 150 200 250 3000

20

40

60

80

4, 130

3, 170

2, 240

Vo

ltag

e d

rop

on

th

e r

ect

ifye

r (V

)

Load current (A)

1, 520

Room temperature

1 2 3 4

100

200

300

400

500

600

Re

sist

ance

pe

r M

OSF

ET (

)

Number of MOSFETs


0 50 100 150 200 2500

10

20

30

40

50

60 18 K, 2 KHz operation

Half-period at 2 KHz

100 A

150 A

300 A

Inp

ut

po

we

r, P

i (W

)

Discharge time, td (s)

28 W

Recovery of energy stored in the dissipation inductance

IBM Seminar May 9 2019, Yorktown NY 23

-50 0 50 100 150 200

0.0

2.0

4.0

6.0

8.0

Incomplete discharge

Discharge gate signal /10

M1

M2Vo

ltag

e (

V)

Time (s)

Coil

-50 0 50 100 150 200

0.0

2.0

4.0

6.0

8.0Discharge gate signal /10

M1

M2Vo

ltag

e (

V)

Time (s)

Coil

Incomplete discharge

Complete discharge

✓ Energy recovery improves efficiency of the drive


Future development: continuous cabling system development


Proposed Phase II development

Sequential slicing

Cabling from multiple spoolsFeed spool

Cable spool

Filament spoolFilament spool

Tape spool

✓ Direct cabling from a wide tape spool


Conclusion and future challenges

▪ Conclusion: ▪ Demonstrated operation of epoxy-impregnated multi-filamentary

cable in conduction cooled mode

▪ AC loss is not affected by transposition

▪ Winding magnetization at 25 K is strongly affected by thermal flux, for example from current leads

▪Future challenges▪ Filament fusion: we need < 100 n cm2 areal resistivity

▪ Scale-up of the filament handling

▪ Continuous splicing


transport performance and ac loss of superconducting

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