What contributions have the authors mentioned in the paper "Metallographic analysis of 11 t dipole coils for high luminosity-large hadron collider (hl-lhc)" ?

The high luminosity upgrade for the Large Hadron Collider ( HL-LHC ) marks an important milestone as it will be the first project where Nb3Sn magnets will be installed in an accelerator. In this paper, the authors present direct evidence of cracking of the brittle Nb3Sn filaments in a prototype dipole that resulted in degraded magnet performance. The authors show that the stresses experienced in the damaged coil are above the critical damage stress of Nb3Sn conductor, as evidenced by a measured Cu stabilizer hardness of 85 HV0. The authors also show that once the collaring procedure issue was rectified in a subsequent dipole, the Nb3Sn filaments were found to be undamaged, and the Cu stabilizer hardness values were reduced to the expected levels. This paper provides a post-mortem verification pathway to analyze the damage, provides strand level mechanical properties, which could be beneficial for improving model prediction capabilities. This method could be applied beyond Nb3Sn magnets to composite designs involving high work hardening materials.

(Open Access) Metallographic analysis of 11 T dipole coils for High Luminosity-Large Hadron Collider (HL-LHC) (2021) | Shreyas Balachandran

Superconductor Science and Technology

PAPER • OPEN ACCESS

Metallographic analysis of 11 T dipole coils for High Luminosity-Large

Hadron Collider (HL-LHC)

To cite this article: Shreyas Balachandran et al 2021 Supercond. Sci. Technol. 34 025001

View the article online for updates and enhancements.

This content was downloaded from IP address 194.12.169.3 on 22/02/2021 at 10:28

Superconductor Science and Technology

Supercond. Sci. Technol. 34 (2021) 025001 (9pp) https://doi.org/10.1088/1361-6668/abc56a

Metallographic analysis of 11 T dipole

coils for High Luminosity-Large Hadron

Collider (HL-LHC)

Shreyas Balachandran

, Jonathan Cooper

1,2

, Orion B Van Oss

, Peter J Lee

,

Luca Bottura

, Arnaud Devred

, Frederic Savary

, Christian Scheuerlein

 and Felix Wolf

Applied Superconductivity Centre, NHMFL, FSU, Tallahassee, FL 32310, United States of America

Department of Mechanical Engineering, College of Engineering, FAMU-FSU, Tallahassee, FL 32310,

United States of America

Department of Physics, Columbia University, New York, NY 100027, United States of America

European Organization for Nuclear Research (CERN), CH-1211 Geneva, Switzerland

E-mail: shreyasb@asc.magnet.fsu.edu and christian.scheuerlein@cern.ch

Received 4 September 2020, revised 9 October 2020

Accepted for publication 28 October 2020

Published 8 January 2021

Abstract

For next-generation accelerator magnets for elds beyond those achievable using Nb–Ti, Nb

is the most viable superconductor. The high luminosity upgrade for the Large Hadron Collider

(HL-LHC) marks an important milestone as it will be the rst project where Nb

Sn magnets

will be installed in an accelerator. Nb

Sn is a brittle intermetallic, so magnet coils are typically

wound from composite strands containing ductile precursors before heat treating the wire

components to form Nb

Sn. However, some mechanical assembly is still required after the coils

have been heat-treated. In this paper, we present direct evidence of cracking of the brittle Nb

laments in a prototype dipole that resulted in degraded magnet performance. The cracking of

the Nb

Sn, in this case, can be attributed to an issue with the collaring process that is required in

the assembly of dipole accelerator magnets. Metallographic procedures were developed to

visualize cracks present in the cables, along with quantitative image analysis for location-based

crack analysis. We show that the stresses experienced in the damaged coil are above the critical

damage stress of Nb

Sn conductor, as evidenced by a measured Cu stabilizer hardness of 85

0.1

, which is higher than the Cu stabilizer hardness in a reference Nb

Sn cable ten-stack that

was subjected to a 210 MPa transverse compression. We also show that once the collaring

procedure issue was rectied in a subsequent dipole, the Nb

Sn laments were found to be

undamaged, and the Cu stabilizer hardness values were reduced to the expected levels. This

paper provides a post-mortem verication pathway to analyze the damage, provides strand level

mechanical properties, which could be benecial for improving model prediction capabilities.

This method could be applied beyond Nb

Sn magnets to composite designs involving high work

hardening materials.

Keywords: Nb

Sn, HL-LHC, dipole magnets, damage analysis

(Some gures may appear in colour only in the online journal)

1. Introduction

Sn is the most viable conductor for high eld acceler-

ator magnets in the range of 10–16 T for High Energy Phys-

ics research [1]. Beyond the Large Hadron Collider (LHC)

the next generation Future Circular Collider (FCC), a 100

TeV hadron-hadron collider, is expected to use both Nb

dipoles and quadrupoles operating at 16 T, 4.2 K [2]. Chal-

lenges involved in the construction of 16 T Nb

Sn magnets

include the development of production quality conductors that

1361-6668/21/025001+9$33.00 1

Publishing Ltd Printed in the UK

This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License.

Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Supercond. Sci. Technol. 34 (2021) 025001 S Balachandran et al

can carry a demanding current density, J

, of 1500 A mm

−2

(16 T, 4.2 K) and magnet designs incorporating stress manage-

ment to control the high stresses experienced by Nb

Sn con-

ductors during magnet construction and under the inuence of

Lorentz forces during operation. Recently achieved increases

in the upper limits of J

in Nb

Sn strand indicate that suf-

ciently high J

to meet FCC requirements is now possible

[3, 4]. Furthermore, an experimental prototype Nb

Sn dipole

magnet has recently generated the signicant milestone eld

of 14 T. Further design testing is now underway for a 15 T tar-

get eld, where conductor degradation in high-stress regions

is a major concern [5].

A crucial step in the development of high eld Nb

magnets is the major upgrade of the LHC, called High

Luminosity LHC (HL-LHC). Tens of Nb–Ti magnets will be

replaced with Nb

Sn dipole and quadrupole magnets using

state-of-the-art Nb

Sn conductor. The 11 T dipole magnets

[6] are the rst Nb

Sn magnets that will be installed in a

particle accelerator. They will provide the same integrated

ﬁeld as the standard 8.3 T LHC dipoles over a shorter length,

which allows the installation of additional collimators in the

LHC arcs.

Stress management is a critical issue in Nb

Sn magnets

due to the brittle Nb

Sn phase. Although the conductors are

wound and then reacted to minimize mechanical stresses on

the Nb

Sn laments, there remain several avenues leading to

conductor damage. Improper assembly may result in over-

stressing and coil damage, thermal expansion mismatch of

the coil, and magnet constituents can cause damage during

thermal cycling, Lorentz forces generated during powering

may overstrain the Nb

Sn. FE simulations can provide use-

ful but approximate predictions of the stress distribution in the

magnet during the different assembly and operational condi-

tions; however, the prediction of precise stress levels remains

a challenge.

During cold testing of an 11 T dipole short model mag-

net at CERN, a critical current limitation of the coils was

detected. It was suspected that the reason for this degrada-

tion was fracturing of some of the Nb

Sn laments as a res-

ult of excessive mechanical stress during the collaring of the

coils. A post-mortem (destructive post magnet testing) metal-

lographic analysis technique has been developed in order to

provide more in-depth insight, at the conductor level, into the

degradation and complexity in stress states. This approach was

previously demonstrated in Nb

Sn cable-in-conduit magnet

sections tested for the ITER project [7–9].

This paper provides a roadmap for different aspects related

to damage and microstructure in Nb

Sn model dipole coils. A

comparison between multi-axial loaded dipole sections versus

uniaxially loaded cable ten-stack sample sections indicate

variations in the damage behavior. A technique to resolve the

effects of stresses on the lament level using Cu microhard-

ness measurements s presented in this paper. This technique

could be applied to future model magnets and also provide

granular data to provide realistic stress values for the devel-

opment of stress prediction models for complicated magnet

assemblies.

2. Experimental methods

For the current study, we have analyzed two 11 T dipole coil

cross sections, which were rough-cut using a band saw after

disassembling the magnets after cold testing and then preci-

sion cut with a diamond wire saw. The coil cross-section and

the coil collaring procedure are discussed in detail in [10].

11 T dipole cable ten-stack samples [11] were also evaluated.

Figures 1(a)–(e), provides a visualization of the dipole section

at various stages of disassembly. An 11 T dipole collared aper-

ture containing two coils is shown in gure 1(a). Figure 1(b)

shows a photograph of the short model coil #109 after magnet

cold test and disassembly. The dipole design used in the 11 T

dipole magnet is indicated in the top view of the solid model

in gure 1(c). The sectional view (Section AA) in gure 1(d)

shows the various components that make up the dipole seg-

ment, including (a) Nb

Sn cables, (b) austenitic stainless steel

316L loading plate which presses against the loading pole,

) wedges. The insulat-

ing layers and epoxy impregnation that is present in the dipole

cross-section are not indicated in the gure here. The 11 T

dipole Nb

Sn coils are segmented into six different conductor

blocks, as numbered in gure 1(d). To evaluate metallographic

details of cross-sections, different dipole coil sections were cut

as shown in gure 1(e). The orientation of the samples pol-

ished from the dipole section corresponded to the plane con-

taining the axial and transverse directions. The nomenclature

of the sections consists of the conductor block number follow-

ing the word section, and the corresponding row cut from the

section. As indicated in gure 1(e). Section 1-R1, shows the

section from coil block #1, the rst row cut from coil block #1

indicated by R1. The two-layer Nb

Sn segments in the cross-

sections are indicated as Row 1–9, with each row consisting

of two Nb

Sn conductor segments.

2.1. Details of 11 T dipole coils for this study

For this metallographic study, two coils were selected for ana-

lysis, short model coil #109, which showed severe critical cur-

rent degradation, and 11 T dipole series coil GE-C02 that did

not show current degradation.

After collaring coil #109 was assembled in 11 T twin aper-

ture dipole MBHDP102, as shown in gure 1(a). Cold tests

of MBHDP102 revealed a substantial critical current limita-

tion at the mid-plane turns of coil #109, and the n-value was

also strongly suppressed [10, 12], both indications of pos-

sible Nb

Sn lament breakage. The critical current limitation

probably caused the relatively low quench currents of magnet

MBHDP102.

The critical current deterioration, which was detected over

an extended length in the straight part of the coil, was pre-

sumed due to excessive mechanical stresses applied during

the collaring of the coil [10]. Section AA in gure 1(c) is

a coil cross-section in which lament damage is expected.

Coil #109 was de-collared and central section cuts were made

for metallographic examination. A central section is shown in

gure 1(d) was diamond wire saw cut and analyzed further.

Supercond. Sci. Technol. 34 (2021) 025001 S Balachandran et al

Figure 1. Segments of the 11 T dipole coil at various stages of disassembly, (a) cross section of collared 11 T dipole coil assembly made of

two coils, (b) 11 T dipole short model coil #109, (c) top view of a 11 T dipole coil, (d) cross-sectional view of section AA, including the cut

dipole segment of Coil #109 used in this study, (e) isometric view of the dipole section to visualize the coil segments analyzed by

metallography.

The second coil investigated here (GE-C02) was the second

series 11 T dipole coil collared in the 11 T dipole aperture

HCMBH_C001-01000001. There was a difference between

manufacturing procedures used in GE-C02 versus coil #109.

The series coil GE-C02 has an optimized insulation scheme,

and the collaring procedure was improved to reduce the mech-

anical stress on the Nb

Sn conductor during collaring. Finite

element simulations of the stress distribution in the coil dur-

ing the Collaring procedure indicate that the highest stresses

are experienced at the coil midplane on the Nb

Sn cables in

section 1-R1 [13].

The aperture, including coil GE-C02, was cold tested in

a ‘hybrid’ magnet assembly where the second aperture was

not powered. Powering tests after the rst magnet cool down

showed excellent quench performance, and in particular, no

critical current degradation at the coil mid-plane was observed.

However, after an additional thermal cycle, the coil perform-

ance was degraded, due to a critical current (I

) limitation at the

coil ends (making the magnet available for destructive post-

mortem analysis). The degradation at the coil ends was pre-

sumably caused by warming the magnet too fast. For metal-

lographic examination, the coil samples were extracted in the

central- straight part of GE-C02 (similar to the coil section in

#109). In the central section of GE-C02, no lament damage

is expected [10].

2.2. Description of 11 T dipole cable ten-stack samples

The ten-stack samples are made of ten 11 T dipole cables

(nominal width and mid-thickness are 14.7 mm and 1.25 mm,

Supercond. Sci. Technol. 34 (2021) 025001 S Balachandran et al

Figure 2. Ten-stack Nb

Sn coils in three different states: (a)

#28-compression loaded to 210 MPa (change to 210 MPa in the

gure) along the direction transverse to the Nb

Sn wire

cross-section, (b) #33-compression loaded to 210 MPa along the

axis of the superconductor wire, and c) is an unloaded ten-stack.

respectively [14]), which are stacked alternatingly in order to

compensate for the keystone angle of 0.79

◦

. The Rutherford

cables are made of 40 Restacked-Rod Process type Nb

strands with 0.7 mm nominal diameter, which were pro-

duced by Oxford Instruments Superconducting Technology

(now Bruker-OST). The Rutherford cables have a 25 µm thick

stainless steel core, and they are surrounded by a 0.15 mm

thick cable insulation made of mica tape and S2/E-glass ﬁber.

The Nb

Sn formation heat treatment was performed with a

ramp rate of 50

◦

C h

−1

and was comprised of three iso-

thermal plateaus of 210

◦

C-48 h, 400

◦

C-48 h and 665

◦

C-

75 h. The ten-stack samples are designed to be comparable

to dipole cable sections; the wire volume in the ten-stack

samples that were analyzed is 72%. The Cu in this sample

is assumed to be fully annealed after the prior heat treatment

to a peak temperature of 665

◦

C. More information about the

ten-stack sample production and mechanical loading has been

published in [15].

Ten-stack samples No 28 and No 33 (gure 2) were uni-

axially loaded to compressive peak stress of 210 MPa. Ten-

stack #28 was loaded in the transverse direction (corresponds

to gure 1(e)), and No. 33 was loaded in the axial direc-

tion. Unloaded sample #57 was used as a reference in this

study.

Composite theory predicts iso-strain in the composite con-

stituents under axial loading [16]. This has been conrmed

experimentally for the Nb

Sn laments and Cu matrix in dif-

ferent Nb

Sn wires by high energy synchrotron x-ray dif-

fraction [16], and in ten-stack samples by neutron diffraction

[8]. At 210 MPa (sample 28), the macroscopic axial ten-stack

stresses in the Nb

Sn and Cu are about 450 MPa and 100 MPa,

respectively [17].

Transverse loading theory predicts iso-stress [18] in the

composite constituent materials [19]. Neutron diffraction

measurements conrm that under transverse macroscopic ten-

stack stress of 210 MPa (sample 33), the stress in the Nb

laments and in the Cu matrix is very similar and correspond

to the externally applied stress of 210 MPa [11].

2.3. Metallography

For metallographic evaluation, the samples were mounted in

a clear epoxy-hardener mixture, which was cured at room

temperature (RT). Subsequent sectioning was performed using

a wafering saw with the sample glued to a base plate to

avoid the danger of damage that might be caused by clamping

the sample directly. The samples were mechanically polished

using SiC pads with a decreasing grit size of 320, 400, 600,

800, and 1200. The load on the sample during the grinding

process was 12 N, and the cross-sectional area of the mount

was ~500 mm

. After the grinding process, the samples were

polished using diamond slurries of successively decreasing

particle size (5 µm, 3 µm, 1 µm). The nal polishing step

used a 50 nm colloidal silica solution with a pH ~ 11 in a

Vibromet

vibratory polisher. This polishing procedure is a

standard metallographic sample preparation technique; how-

ever, the low loads are essential here to maintain the integ-

rity of the sample. The samples were lightly etched in a 1:1:

HF:H

O solution for 10 min to make the cracks more visible

by light microscopy. In order to remove staining from the etch,

the samples were returned to the vibratory polisher for 30–

45 min, after which the samples were cleaned and dried for

further imaging.

Imaging of the cracks was performed using an Olympus

BX41M reected light microscope combined with an Olym-

pus UC50 digital camera and Olympus Stream capture soft-

ware. Backscattered Electron (BSE) images were obtained

using a Zeiss ESB- FESEM. Analysis of shape and crack dens-

ity was performed using FIJI-ImageJ [20, 21], combined with

custom routines for superconductor analysis [8].

Vickers microhardness tests were performed on the pol-

ished cross-sections of the dipole and ten-stack samples using

a LECO 300 AT hardness tester. The load dependence on the

hardness was veried over the complete range of 10 g–1 kg on

the unloaded ten-stack samples. For hardness comparisons, a

100 g load with a dwell time of 15 s was used in this study.

The hardness measurements are designated HV

0.1

XX, where

HV signies Hardness Vickers (scale), and the subscript 0.1,

means the applied load in kg, and XX is the hardness number

derived from the diagonal indent measurements.

Metallography and hardness measurements in this paper

focus on section 1-R1 in dipole coils #109, and GE-C02, where

the stresses on the Nb

Sn cables are predicted to be maximum

from available simulation results [13]. A complete descrip-

tion of the metallographic process development and compar-

isons between different coil sections are underway and will be

presented elsewhere.

3. Results

3.1. Nb

Sn crack distribution in coil #109

Section 1-R1 of Coil #109 was mounted, polished, etched, and

imaged. Cracks were observed in this section with varying

densities based on location. Crack morphologies vary in the

lament, as shown in gure 3. The commonly found cracks are

Metallographic analysis of 11 T dipole coils for High Luminosity-Large Hadron Collider (HL-LHC)

Figures

Citations

Quantum Machine Learning in High Energy Physics

AC loss and contact resistance in highly flexible rebco cable for fusion applications

Onset of Mechanical Degradation due to Transverse Compressive Stress in Nb<sub>3</sub>Sn Rutherford-Type Cables

Onset of Mechanical Degradation due to Transverse Compressive Stress in Nb3Sn Rutherford-Type Cables

Origin of the 50 Hz harmonics in the transverse beam spectrum of the Large Hadron Collider

References

NIH Image to ImageJ: 25 years of image analysis

Fiji: an open-source platform for biological-image analysis

Berechnung der Fließgrenze von Mischkristallen auf Grund der Plastizitätsbedingung für Einkristalle .

The plastic deformation of metals

The hardness of solids

Related Papers (5)

Nb 3 Sn 11 T Dipole for the High Luminosity LHC (CERN)

Development Status of a 2-m Model Magnet of Beam Separation Dipole for the HL-LHC Upgrade

Assembly Tests of the First Nb 3 Sn Low-Beta Quadrupole Short Model for the Hi-Lumi LHC

Intra-beam Scattering and Luminosity Evolution for HL-LHC Proton Beams

DESIGN CONCEPT AND PARAMETERS OF A 15 T Nb3Sn DIPOLE DEMONSTRATOR FOR A 100 TEV HADRON COLLIDER

Frequently Asked Questions (1)

Q1. What contributions have the authors mentioned in the paper "Metallographic analysis of 11 t dipole coils for high luminosity-large hadron collider (hl-lhc)" ?