Investigation of mechanical degradation

mechanism onto the quench heater insulation

to Nb3Sn coil

TE-MSC-LMF

Felix Wolf, Arnaud Foussat and Jan Petrik

17.04.2019

Content

Mechanical degradation of insulation system between quench

heater and Nb3Sn cable due to compressive load

Samples

Test machine

Measurement results

Mechanical degradation of insulation between quench heater and

Nb3Sn cable due to mixed load (compression, shear, bending)

Samples

Test machine

Measurement results

2

Findings on MBHP001 - CR06-07 Initial insulation resistance in range of ∼ 2-3 GOhms.

Direct shorts provoked in collared coil state was

found in same cross sections on paired coils,

residual resistance of few Ohms.

All in LF strip region

Type of direct short defect created in

extended Hi-pot test on CR07, ( hole

diam 2 mm, dumped E tot > 1kJ)

Short after successive C

bank discharges in CR06

3Short detail after peeling test

Last discharge before

burn through

Courtesy of Jan Petrik, 11/01/2019

https://indico.cern.ch/event/778100/

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Introduction

3

Objective: Study of failure mechanism of the

insulation system between quench heater and cable under compressive load

Test: Compressive test + online leakage

current measurement

Sample: 11T Nb3Sn Rutherford cable (H150 C02

244A)

Mica insulation + fibre glass braiding (identical to coil 108)

18-stack with cancelling keystone angle

Impregnated quench heater (11T Non-Conform 1st Trackwise piece 08.03.2018)

Test machine: 180 kN hydraulic press, equipped with

calibrated load cells Burster type 8526 (0-200kN)

Steel press tool, insulated with multilayer 0.125mm polyimide film

Electrical measurement device (Megger MIT 1025 and Keithley 6487)

Applied load (0-110 MPa)

Press tool

Quench heater (~30µm) on polyimide film (50µm)

Reacted cable 18-stack

Press support

Impregnation (CTD-101K)

ℎ=

15

.6 m

m

𝑏 = 26.45

mm

Multilayer insulation to ground (8x125µm)

Multilayer insulation to ground (8x125µm)

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Compressive load

Quench heater on polyimide film

Cable 18-stack

Impregnation 75 mm

V

Press tool

4

Ceramic binder curing Cycle 80oC (1h), 150oC (2h)

Reaction heat treatment Cycle 20oC/h to 650oC (20h) (Bldg. 927)

Impregnation New mould to apply radial and transversal compression

Polymer-Lab. (771)

Samples with varying insulation thickness 2 x 250 µm (one cut for microscopy)

2 x 250-200 µm

3 x 200 µm (one cut for microscopy)

3 x <150 µm (one cut for microscopy)

Tests: Break down voltage (Megger MIT 1025)

Leakage current with Keithley 6487

RHT mould with clearance variation for up to 18 cables,

enables ceramic binder application

Prepared sample in the impregnation mould which

enables radial and transversal compression

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Status of sample preparation for compressive load scenario

5

Sample ID insulation thickness

Quench heater – Cable [µm]

QHC01 ~250

QHC02 ~250

QHC03 ~200

QHC04 200-250

QHC05 ~200

QHC06 200-250

QHC07 <150

QHC08 ~200

QHC09 <150

QHC10 <150

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Sample overview and test equipment

Sample with electrical connections installed in the press gap

Test machine with measurement equipment

Overview of the manufactured samples

6

0.1 mm 0.1 mm 0.1 mm

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Microscopic analysis of the insulation thickness

150µm insulation CR06

250µm insulation QHC02

Contact pressure <2.5 MPa (local)

150µm insulation CR06

150µm insulation QHC10

Contact pressure >2.5 <10 MPa (local)

CR06 QHC02 QHC05CR06 CR06 QHC10

150µm insulation CR06

200µm insulation QHC05

Contact pressure >2.5 <10 MPa (local)

Samples prepared with varying insulation thickness adjusted by impregnation mould clearance

Insulation thickness determined by microscopy

Contact pressure determined with LW Prescale film

7

Leakage current measurement at 500V without load of sample QHC07

(<150µm), determined resistance about 1 TΩ.

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Leakage current measurement without applied pressure

Leakage current decreases towards

an plateau value

Leakage current is very sensitive to:

Ambient conditions

Time gap between

measurements

0.0E+00

2.0E-10

4.0E-10

6.0E-10

8.0E-10

1.0E-09

0 20 40 60 80 100 120 140 160 180

Cu

rre

nt

[pA

]

Time [sec]

QHC07_01 QHC07_02 QHC07_03

QHC07_04 QHC07_05 QHC07_08

QHC07_09 QHC07_10 QHC07_11

QHC07_12 QHC07_13

1000

800

600

400

200

0

8

Leakage current measurement at 500V with varying load and

varying load ramp rate of sample QHC07 (<150µm), determined

resistance about 1.5 TΩ.

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Leakage current measurement with applied pressure

Leakage current plateau after 25 minutes

No change in the leakage current if the load

ramp is below 0.1 kN/s

No permanent degradation of the insulation

up to 60 MPa

Reversible changes of the resistance if the

load ramp is above 0.5 kN/s

9

Sample QHC07 (Insulation thickness < 150µm) No permanent degradation of the insulation

up to 60 MPa

Leakage current increases before the

sample looses mechanical strength

Leakage current increases above an applied

load of 90 MPa

Sample failure

(reduction of

mechanical strength)

Leakage current measurement at 500V with load ramp

rate 0.13MPa/s QHC07 (<150µm insulation thickness).

Leakage current measurement

vs. applied pressure.

Damaged sample after 110 MPa applied load.

10

Sample QHC10 (Insulation thickness < 150µm)

Sample prepared with higher compression

(extra 100µm polyimide shim).

Leakage current increases with at load

above 60 MPa.

Time dependent leakage current increase,

even at decreased load.

Clear change of the leakage current

behaviour without applied load after 80MPa

loading.

Reduction of

mechanical

strength

Increase of current before

mechanical failure

0.0E+00

2.0E-10

4.0E-10

6.0E-10

8.0E-10

1.0E-09

0 200 400 600 800 1000 1200 1400

Cu

rre

nt

[pA

]

Time [sec]

QHC10_01

QHC10_02

QHC10_03 after 80 MPa

QHC10_04 after 80 MPa

1000

800

600

400

200

0

Leakage current measurement at 500V with load ramp

rate 0.13MPa/s QHC07 (<150µm insulation thickness).

Leakage current measurement at 500V without load of sample

QHC10 (<150µm), determined resistance about 5 TΩ.11

Outlook

Investigation of the MQXF insulation system without mica

Target insulation thickness (205µm) 145µm S2 glass + 50µm QH polyimide

First set of samples prepared for impregnation (H160C0238A RRP108/127)

Next batch of samples prepared for reaction heat treatment (H16EC0247C PIT)

Simplified model of the impregnated MQXF sample with quench heater, soldered

electrical connections and residual epoxy block

Impregnation mould for the investigation of the MQXF impregnation system.

12

Content

Mechanical degradation of insulation system between quench

heater and Nb3Sn cable due to compressive load

Samples

Test machine

Measurement results

Mechanical degradation of insulation between quench heater and

Nb3Sn cable due to mixed load (compression, shear, bending)

Samples

Test machine

Measurement results

13

Objective: Study the failure mechanism of the insulation

between quench heater and cable under

mixed load (compression, shear, bending)

Test: 3 Point bending test + online leakage current

measurement

Sample: 11T Nb3Sn Rutherford cable (H150 C02

244A)

Mica insulation + fibre glass braiding (identical

to coil 108)

2-stack with cancelling keystone angle

impregnated quench heater (small scale

production for the test with PCIB lab)

Test machine: Collaboration with TE-EN-MME

Electrical measurement device (Keithley

6487)

𝜀 =6 ∙ ℎ ∙ 𝑤

𝑙2, ε 𝑤 = 0.16mm = 0.1%

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil 3 Point bending test Applied Load (0-1000 N)

Quench heater (~30µm) on polyimide film (50µm)

Reacted cable 2-stack

Steel bar (Stainless steel 3 mm)

ℎ=

3.8

mm

𝑏 = 15.4 mm

Multilayer insulation to ground (8x125µm)

Multiplayer insulation to ground (8x125µm)

Applied Load

Sliding contact

𝑙 =60 mm𝐹𝐴 𝐹𝐵

𝐹

𝑤V

Impregnation (CTD-101K)

14

𝑤 = mid-span deflection

𝑙 = support span

ℎ =beam tichness

Strain calculated at the outer surface:

Estimated thermal strain difference between the quench heater insulation and copper is in the

order of 0.1% to 0.4% between RT and cool down

15

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Estimation of thermal contraction between 300K – 1.9K

Material Thermal expansion

(300-1.9K) [1]

Kapton -0.4%

Copper -0.32%

G10 (wrap) -0.25%

G10 (normal) -0.7%0.

1 m

m

0.1 mm

CopperPolyimide

S2+Epoxy

[1] Jack W. Ekin, Experimental Techniques for Low-Temperature Measurements : Cryostat Design, Material Properties, and Superconductor Critical-Current Testing,

Oxford : Oxford Univ. Press, 2006.

Microcopy of a coil segment from CR06 showing the

insulation system between quench heater and coil.

Literature thermal expansion data of selected material.

∆𝜀th = 0.08%

∆𝜀th = 0.07%

(∆𝜀th = 0.38%extrema)

Quench heater production

Production in the CERN PCB-Lab

(EP-DT-EF)

Ceramic binder curing

Cycle 80oC (1h), 150oC (2h)

Reaction heat treatment

Cycle 20oC/h to 650oC (20h)

Impregnation

First tests performed on 28. Feb. 2019

in collaboration with EN-MME

Open mould enables

ceramic binder application

RHT mould with clearance

variation for 3 two stacks

Impregnation mould with prepared sample and shims for

clearance adjustment

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Status of sample preparation for 3-point bending test

16

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Test structure for 3-point bending test

Test sample supported on a 3mm steel bar installed in the 3-point bending structure.

17

Keithley 2001

(Voltage

measurement)

Keithley 6487

(Picoammeter +

Voltage source)

UTS 2000 test machine

with 200kN load cell

Video extensometer

3 Point bending

test structure

PC with LabView

for DAQ

3-Point bending test machine with measurement equipment for the leakage

current determination and the video extensometer and light source.

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil Test equipment for the 3-point bending test

18

First testes performed, to investigate the strain in the insulation system as developed during cool down.

Strain determined by the sample deflection, measured with the extensometer.

Change in the leakage current occurs with the reduction of the mechanical strength with 0.5%

Further samples prepared for measurements

Investigation of mechanical degradation mechanism onto the quench heater insulation to Nb3Sn coil First test results of the combined 3-point bending and compression test

3-Point bending test sample under applied load. Leakage current measurement in comparison to

strain during 3-point bending test.19

Strain targetLeakage current increase

Thank you for your

attention.

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