student: jose ferradas university of geneva & cern mdt

Student: Jose Ferradas University of Geneva & CERN MDT

Supervisors: Paolo Ferracin Lawrence Berkeley National Lab.

Carmine Senatore University of Geneva

Ezio Todesco CERN TE-MSC-MDT

27/03/2020

MDT Engineering Meeting – Scientific topics in the COVID-19 era

Main authors (In alphabetical order):H. Bajas, M. Bajko, L. Bianchi, L. Brouwer, B. Castaldo, M. Duda, S. Emami, F. J. Mangiarotti, M. Guinchard,

S. Izquierdo, J.V. Lorenzo, J.C. Perez, E. Ravaioli, E. Tapani Takala and G. Vallone

Jose Ferradas TroitinoMDT Seminar

The main objective of the doctoral research is to perform an exhaustive analysis of themechanical behavior of a Nb3Sn superconducting magnet during a quench.

The topic is tackled from three different sides:

1. Numerical simulation of the magnet mechanics during a quench using Finite ElementAnalysis.

2. Analysis of the experimental data collected during magnet cold tests’ for CERN magnets

Mechanical instrumentation

Electromagnetic instrumentation

3. Characterization of the conductor electro-mechanical limits at University of Geneva.

Study of the cable behavior from a single wire experiment. It includes:

Strands’ electro-mechanical characterization under transverse loads

Strands’ electro-mechanical characterization under axial loads

Stress-Strain measurements

PREFACE

Introduction to the Thesis’ Topic













PREFACE


SEMINAR 1













PREFACE


SEMINAR 2


SEMINAR 1 - OUTLINE

1.- Introduction – Quench processes and superconducting

magnets

6.- Model validation: Mechanical behavior during quench

3.- MQXF – The low-β quadrupole for HL-LHC

4.- 2D & 3D ANSYS APDL Tools

5.- 2D FE Analysis of a quench heater protected magnet

2.- Finite element modelling for the analysis of the

magnet mechanics during a quench

7.- Conclusions


New high-field Nb3Sn accelerator magnets arepushing the boundaries of magnet design andquench protection towards new limits.

Their large stored energy and current densities posenew challenges for the community…

INTRODUCTION

Quench processes and superconducting magnets

Courtesy of L. Bottura


Scaling of the energy per unit length with the bore

field

Maximum magnetic field for state of the art

superconducting accelerator magnets


… but also the electro-mechanical limits of the conductor become a parameter of extremeimportance.

INTRODUCTION


60 80 100 120 140 160 180

0.0

0.5

1.0

1.5

2.0

2.5

F = 0.5 kN

F = 15 kN

unload to 0.5 kN

@ 4.2 K, 19 T

Elec

tric

fie

ld [

V/c

m]

Current [A]

Example of Nb3Sn PIT strain sensitivity for a round strand tested under transverse loads:

Group of Applied Superconductivity

University of Geneva


The coupling of the above-mentioned characteristicsduring a quench is a case of special interest that addsnew complexity for the design of superconductingmagnets beyond 10 T.

INTRODUCTION


Thermo-mechanical stressesQuenchRisk of magnet damage / conductor degradation

The study of quench processes and their intrinsic

mechanics becomes essential!

We need tools capable of predicting the mechanical response of the magnet during a quench

Digression on thermal stresses: Thermal buckling in railway tracks

Source: [Business Insider]



At the same time one cannot forget that the accurate simulation of quench transients is areal multiphysics effort:

- Electromagnetic study ~ Safe voltage levels

- Thermal analysis ~ Safe temperature level

- Mechanical analysis ~ Safe stress levels

Important lessons from the past…

INTRODUCTION


This is the result of a chain of events triggered

by a quench in an LHC bus-bar

Courtesy of Marta Bajko

Damage in HL-LHC coil as a result of an

electrical fault

Courtesy of Paolo Ferracin

This is the result of a quench in

the pre series magnet during

its qualification test

Courtesy of Marta Bajko


SEMINAR 1 - OUTLINE


magnets







7.- Conclusions


Early 2000s – Development of the first APDL models in the other side of the ocean:

Lawrence Berkeley National Lab:

2003 – S. Caspi et al.: “Calculating Quench Propagation With ANSYS®”

2004 - P. Ferracin et al.: “Thermal, Electrical and Mechanical Response in Nb3Sn Superconducting Coils”

3D Coupled thermal-electric transient analysis. Conductor block with smeared material properties. Transition is defined asa jump in resistivity at Tcs. Not varying magnetic field was included.

Fermi National Lab:

2001 - Yamada et al.: “Quenches and resulting thermal and mechanical effects on epoxy impregnated

Nb3Sn high field magnets”

2002- Yamada et al.: “ 2D/3D Quench Simulation using ANSYS for Epoxy Impregnated Nb3Sn High Field

Magnets”

2003 -Yamada et al.: “ 3D ANSYS Quench Simulation of Cosine Theta Nb3Sn High Field Dipole Magnets”

Only the thermal part is solved in ANSYS. At the end of each time step, solution is stopped and the electric part is solvedwith implemented analytical formulas. Then the solving is restarted.

FE Modelling

The magnet community needs tools capable of predicting the mechanical response of the magnets during a quench…

Often, new is forgotten old!


Later in Europe…

CERN:

2018 - Jose Vicente Lorenzo, Hugo Bajas et al.: “Quench Propagation Velocity and Hot SpotTemperature Models in Nb3Sn Racetrack Coils”

Sort of combines the two previous. 3D Coupled-physics transient analysis is solved. At the end of each time step,solution is stopped and coded analytical formulas add some of the quench physics that are not possible tomodel with commercial ANSYS. Varying magnetic fields and other phenomena can be included.

University of Tampere (T. Salmi, A. Stenvall and J. Zhao):

2017 - J. Zhao et al., “Mechanical behavior of a 16 T FCC dipole magnet during a quench.,”2018 - J. Zhao et al., “Mechanical stress analysis during a quench in CLIQ protected 16 T dipolemagnets designed for the future circular collider.,”

In the framework of EuroCircol, the method combines 2D COMSOL simulations from CERN MPE with 2Dmechanical simulations in ANSYS.

Again recently in the US…

Lawrence Berkeley National Lab:

L. Brouwer – Developed special ANSYS User Defined Elements. They allow to incorporate intoANSYS APDL solver the most important physics of superconductivity! A fantastic tool, 2D for themoment.

FE Modelling


Mechanical model

Thermal-electric model

Electro-magnetic

model

Reviving the idea of performing complete 2D and 3D analysis ofthe magnet mechanics during a quench, using commercialANSYS APDL at CERN.

The HL-LHC MQXF Low-β quadrupole magnet, our playgroundfield.

First, magnet protected with quench heaters (placed in the outer layer of the coils). Then, add other protection systems (CLIQ). Characterize the magnet response.

The method should combine the necessary ANSYS multi-physicsmodels. They will provide the input for the mechanical model.

The campaign on electro-mechanical characterization of Nb3Sn strands should determine the limits at which the magnet can operate safely.

Our approach

3D MQXF magnet mechanical model(G. Vallone, 2017)

FE Modelling


SEMINAR 1 - OUTLINE


magnets







7.- Conclusions


Nominal gradient: 132.6 T/m

Nominal current: 16470 A

Peak field in the conductor at Inom : 11.4 T

Aperture: 150 mm

Magnet outer radius: 630 mm

Stored energy at Inom: 1.17 MJ/m

Fx / Fy (per octant) at Inom = +2.47 / -3.48 MN/m

Fz (Whole magnet) at Inom = 1.17 MN

MQXF

MQXF Low-β quadrupole magnet

Courtesy of Giorgio Vallone

Mechanical concept: Aluminium shell pre-loaded with bladders.

First time for an accelerator magnet.


Nominal gradient: 132.6 T/m

Nominal current: 16470 A

Peak field in the conductor at Inom : 11.4 T

Aperture: 150 mm

Magnet outer radius: 630 mm

Stored energy at Inom: 1.17 MJ/m

Fx / Fy (per octant) at Inom = +2.47 / -3.48 MN/m

Fz (Whole magnet) at Inom = 1.17 MN

MQXF

MQXF Low-β quadrupole magnet

Courtesy of Paolo Ferracin

Mechanical concept: Aluminium shell pre-loaded with bladders.

First time for an accelerator magnet.


SEMINAR 1 - OUTLINE


magnets







7.- Conclusions


ANSYS APDL Tools

Adds to the mechanical models alreadyavailable: multi-physics FE Models able toreproduce quench events.

Details in : [1] , [2]

Thermal-Electric (Quench Propagation) andMechanical models coupled together.

Inter-Filament Coupling Losses (IFCL)implemented. Further development possible.

Disclaimer: These tools have been fundamentallybuilt for the analysis of the magnet mechanics duringquench. They are not “another quench code”!

Quench

Heater Detail

The developed ANSYS APDL package:

[2] J. Ferradas Troitino et al, “On the magnet mechanics during a quench: 2D Finite element analysis

of a quench heater protected magnet”

[1] J. Ferradas Troitino et al, “3D Thermal-Electric Finite Element Model of a Nb3Sn Coil During a Quench”


Modelling strategy

3D Thermal-Electric

quench simulation

E.M. forces from

2D/3D

Electromagnetic

model

Mechanical

model

2D/3D ROXIE

electromagnetic

model

Magnetic field on each conductor (Load Lines)

E.M. Forces

Temperature distribution

ANSYS APDL Tools


Overview on the multiphysics thermal-electric models

ANSYS APDL Tools


3D (1mm) MQXF Thermal-Electric symmetric model

Adiabatic boundary conditions

Direct coupled-physics elements

Surf./Surf. contact elements

Exploit the quadrupole symmetry.

Material properties as function of

temperature and magnetic field.

Conductor simulated as a block with

smeared properties.

IFCL included

ANSYS APDL Tools



3D (1mm) MQXF Thermal-Electric full model

Same characteristics. Geometry

expanded.

Allows the study of failure cases and hot

spot effect.

ANSYS APDL Tools


The model could be validated using the extensive experimentalcampaign for MQXFS magnets:

Quench Integral (QI) tests

QI ~ Less than 10% difference w.r.t. experimental data in all tests

S5 – Low RRR

Measured QI is lower than simulations (Also

for other codes!)

S6b – Low RRR ~ S5

Lowest QI measured up to date.

High measured resistance for the coils with Bundle Barrier: very fast

discharge.

(Not included in the model)

S4 – Closest to REF

One HF strip not present in the

experimental case. This explains the fastest

discharge in the model.

Validated against experimental measurements Simulation results

obtained under the

model assumptions.

An “error bar” should be

always considered!



ANSYS APDL Tools

[3] H. Bajas “MQSXFS5 Test Report, EDMS: 2165441”


The model could be validated using the extensive experimentalcampaign for MQXFS magnets:

Quench Heaters (QH) tests : MQXFS4

Simulation results

obtained under the

model assumptions.


always considered!

Delay precision ~ Around 1 ms at Inom

Thermal behaviour matched

Validated against experimental measurements



ANSYS APDL Tools


It also gives an insight on how the reference conductor propertiesrepresent the different magnets produced:

Quench Heaters (QH) tests : All magnet tested

Simulation results

obtained under the

model assumptions.


always considered!

Validated against experimental measurements

ANSYS APDL Tools


3D (Full length) MQXF Thermal-Electric full model

Full 3D coil geometry including coil ends

Topology simplified: QH are not included.

The delays to quench for each turn are

extracted from the 2D, and imposed here.

In view of 2D results, where the structure

remains cold, just the coil is simulated.


ANSYS APDL Tools


3D propagation also extensively validated (ASC 2018)

QPV in a cable experiment

Simulation of a real training quench

[1] J. Ferradas Troitino et al, “3D Thermal-Electric Finite Element Model of a Nb3Sn Coil During a Quench”

ANSYS APDL Tools


Overview on the mechanical models

ANSYS APDL Tools


2D & 3D MQXF Mechanical model

PLANE82 – structural 2D quadratic

Surf./Surf. contact elements (CONTA172/TARGE169).

Plane stress assumption, verified against 3D models and

measurements.

Exploit the quadrupole symmetry. Also full one available.

Linear elastic material properties.

Coil simulated as a block with smeared properties (isotropic).

Adds:

Half-magnet length, longitudinal

symmetry B.C.

Longitudinal loading

Courtesy of Giorgio Vallone

ANSYS APDL Tools

[4] G. Vallone et al., “Summary of the Mechanical Performances of the 1.5 m Long Models of the Nb3Sn

Low-β Quadrupole MQXF”


Very good agreement along the experimental campaign, both 2D and 3D.

Assembly and magnet powering

Loading

Cool-Down

PoleShell

Mechanical models also deeply validated

ANSYS APDL Tools

[5] G. Vallone et al., "Modelling Coil-Pole Debonding in Nb3Sn Superconducting Magnets for Particle

Accelerators”


SEMINAR 1 - OUTLINE


magnets







7.- Conclusions


In last presentation, we showed the 2D analysis of MQXF magnet in nominal operation conditions…

Just small reminder of what we learnt here!

2D MECHANICS OF A QH PROTECTED MAGNET



Case study: Nominal current training quench

15 ms Detection + Validation time example

o Inner layer pole turn quenched(Highest peak field conductor)

Magnet protected with outer layer QH:

o Reference MQXF conductor parameters.o Results: 34 MIITS → Peak T during the disch. = 300 K

Adiabatic estimation of hot spot temp.

RRR =150Cu2SC = 1.2

Ref. Crit. Surf. Param. (5% degrad. cabling)



2D Mechanical results: Stress evolution

Focus on the pole region (The hottest block in this case):

o Additional azimuthal compression to the inner layer pole turn block

(Compared to Cool down). The plot below shows the interface between coil and pole.

Singularities coming from non-linear contact definition.

Better to study the stresses inside the block: (Next slide)Selected

Elements



2D Mechanical results: Stress evolution

Quench (End of the discharge) to Cool Down difference:

o Contours = End of the discharge – Cool down

Azimuthal

40 to 30 MPa additionalcompression to the poleblock !

Remember: To complete the study, we also performed a parametric study for analyzing the influence of quench position, conductor parameters, protection parameters, etc!



Three main conclusions

This was a reference case at nominal current, just protected with QH. Conservative.

For it, under the modelling assumptions, we could learn:

The peak stress during a quench in the coil block is not defined by the hotspot temperature. Rather than that, it is governed by an averagetemperature in the coil blocks:

Dissipated energy

In global terms, a performed parametric study shows that the mechanicalresponse for a quench at nominal current does not change if the designparameters are kept within the established tolerances.

Therefore, this response would be almost the same for all magnets tested up to now!

The quench transient adds an additional azimuthal compression to thecoil, due to the fast thermal expansion, which cannot be neglected.

Most of magnets tested up to now include a dump resistor (part of the energy extracted) or CLIQ (not yet studied): The exact peak stress value

will come later, just keep in mind the physics for now!


SEMINAR 1 - OUTLINE


magnets







7.- Conclusions


Model Validation

Can we validate these results?


Model Validation


MQXFS Mechanical Instrumentation: MME Mechanical Measurements Lab.

MQXFS magnets are instrumented with Strain Gaugesand Optic Fibers in the winding poles and Al Shell.

Strain Gauges were the baseline configuration formech. measurements in the magnet.

Synchronized FBG were added for validation purposesstarting with MQXFS5, now becoming also a baselineprocedure for MME Mechanical meas. lab.

An extensive campaign on mechanicalinstrumentation validation was performed, proving theaccuracy of the system down to 1.9 K.

The mechanical instrumentation was then used tovalidate the mechanical models and to obtainessential information from the magnet tests.

… nevertheless, the instrumentation has never beenused to study what happens during a quench.

The reason: quench is an extremely fast transient. Themechanical instrumentation was conceived from thebeginning for steady-state! We never intended to usethem for transitory phases! For example: temperaturecompensation issues!

Model Validation

[6] M. Guinchard et al., "Techniques of mechanical

measurements for CERN Applications and Environment"

[7] A. Chiuchiolo et al., "Strain Measurements With Fiber Bragg Grating Sensors in the Short Models of the HiLumi

LHC Low-Beta Quadrupole Magnet”

[8] M. Guinchard, L. Bianchi., "Mechanical Strain Measurements Based on Fiber Bragg Grating Down to Cryogenic Temperature – Precision

and Trueness Determination"


Our new analysis indicate that we must re-consider these statements

The MQXFS “Quench” model can give important information on the system’s temperaturedistribution during a quench… and how temperature can affect the measurements!

For example, looking at an extreme case in terms of hot-spot (Inner pole turn quenched at Inominal,only QH protected):

After 0.5 s the magnet is fully discharged and the adiabatic “quench” model

predicts a temperature below 20 K where the SG/FBG are placed!

This should be very conservative since Helium cooling is neglected

Model Validation


Model and experimental results

What does the model predicts as pole and shell response during a quench?

For the sake of understanding, a generic model result is shown below. We introducethe concept of the “Quench delta”.

Preload

Cool Down

Powering

*Adiabatic, not in scale!

Quench

Delta in Stress due to Quench

The “Quench delta” definition: A GENERIC MODEL RESULT

Preload

Cool Down

Powering

Quench

Model Validation



What does the experimental measurements show?

An example: Strain Gauges and FBG measurements are plot together for MQXFS5,where both systems are available.

The agreement between both systems is almost perfect!

FBG and SG show the same delta!

But different behavior during re-

cooling!

Delta in Stress due to Quench

Difficult to see, but two lines per

color!

Model Validation

One coil selected



For the experimental signals. Essential to distinguish 3 different phases:

Keep in mind: Two systems (FBG 10x sensitive to temperature)

1. Assembly and magnet powering: Measurement system designed for this. (Scale of minutes)

2. Quench: Very fast transitory phase (Scale of milliseconds). Thanks to high. freq. acquisition

and based on the modelling result where the pole stays cold during the discharge, we can

use this data.

3. Re-cooling: Slower transitory phase (Scale of tenths of seconds). Definitely, the measurement

system is not designed for this: presence of high temperature effects. Not really trustful.

Model Validation

(1)

(2)

(3)



Perfect agreement between systems

for the range of interest !(Green)

SG in black

FBG in magenta

TAKEOVER:

The nature of both system is

completely different. The fact that

they show the same delta confirms a

physical effect!

As we said, we neglect the re-cooling transitory phase. It is an

interesting topic for a separated meeting! The Mechanical

Measurements Lab. is currently studying this effect.

Model Validation



Just a last point confirming our analysis:

The response of the compensator is coherent with the physics behind the transient.

It comes back to the cool down value after the quench in our time range of interest, just

the effect of magnetic field is removed! .

Model Validation



The excellent agreement between SG and FBG has been verified for:

1. Different quenches2. Measured strain (Compensated and not-compensated)3. Different acquisition systems

Model Validation



How both experimental and numerical results compare to each other?

Strain analysis during MQXFS5 Quench Integral tests

Magnet is ramped to a selected current level, and all OL-QH arefired with nominal powering parameters

Cleanest case for Experimental-Model comparison (Nouncertainty on quench location, no hot-spot, etc.)

FEM model catches accuratelythe behavior. Difference in theorder of less than 5 MPa.

Important to note that the thermalexpansion coefficient plays a bigrole in the results. Up to date thereare not dedicated measurementsfor our HL-LHC magnets.

Model Validation



Measured and simulated delta in the pole confirms an extra compression forthe coil due to the thermal expansion during quench.

The shown delta concerns the pole, the only estimation of what the coil seescomes from the model!

Pole Az. Delta: 41 MPa / Pole Az. Stress @Quench = 154 MPa / Coil Az. Peak = 145 MPa

For the QI test at Nominal Current:

Model Validation



Measurements on different magnets confirms the model outcome:

The mechanical response during a quench (for a coil block!) is not defined by the hotspot temperature. Rather than that, it is governed by an average temperature in thecoil blocks:

Dissipated energy / Coil enthalpy

Model Validation

Different magnets, different conductors (different QI) : Same energy stored, same mechanical response!



Results of the 3D model (not shown in thepresentation) are consistent with the presented2D cases !Indeed, these QI tests can be seen as a pure 2Dcase of uniform coil temperature!

The model cannot be validated using real trainingquench data: Uncertainty on quench positionversus the mechanical instrumentation one.

However, since we have shown that the model isvalidated for the presented cases, the numericalpredictions must apply as well for the trainingquenches!

The yellow star is the result from the simulation ofa training quench in MQXFS5!

MQXFS5 Training

Model Validation



Extremely interesting information from training quenches! And in this case, the 3D simulation shows the importance of the Hot Spot in quench

mechanics!

Analysis will be published soon…!

STAY TUNED!

Model Validation

MQXFS5 Training MQXFS5 QI tests


SEMINAR 1 - OUTLINE


magnets







7.- Conclusions


SEMINAR CONCLUSIONS

Main outcome

First of all, this is the result of a joint effort, which profits from the expertise of each team. Thanks to all!

The essential collaboration between MDT, MME and TF made possible this analysis!

Numerical simulations show that, during a quench, the thermal and

electro-magnetic transients generate new peak stresses in the coils.

These new stresses build-up on top of the magnet pre-load.

In the case of MQXF, they remain below 175 MPa for the cases

studied up to now. Further cases may need to be studied.

The extensive set of data from mechanical measurements acquired

during quench tests, can be used to validate the model outcome.

This last experimental data combines different measurement systems

(electrical SG and optical FBG), which show an almost perfect

agreement.

Last verifications are being performed, but with the data available

up to the date, experimental results confirm the model predictions.

Jose Ferradas TroitinoMagnet Design and Technology

Long experience with the mechanical models: Validated 2D

and 3D models

In 2D, different strategies to model the connection between the

pole and the coil

1. “Separation/no-sliding”

Separation allowed with very high friction always

2. “Glued”

Bonded contact

3. Only for quench simulations, a third option: “Low friction” quench model:

Separation allowed with very high friction during current ramp

Separation allowed with 0.2 friction during quench

Results are identical to case 1 (Only changes during quench)

Different strategies for coil material properties can also be

adopted: Linear elastic / Bilinear

Quench Modelling Tools

3D & 2D Mechanical Models in ANSYS APDL




UltimateNot Powered

The unloading mechanism in ANSYS:


“Glued”

Bonded contact



Ultimate

Ultimate




“Separation/no-sliding”

Separation allowed with very high friction always

Ultimate

Ultimate


Our new analysis indicate that we must re-consider these statements

Additionally, below 50 K thermal strain for titanium is rather constant

At the end of the quench

discharge and at the position of

the SG/FBG:

The difference in strain using

the mechanical strain or the

total one (including thermal) is

less than 10 µstr in the ANSYS

model.

-2.00E-03

-1.80E-03

-1.60E-03

-1.40E-03

-1.20E-03

-1.00E-03

-8.00E-04

-6.00E-04

-4.00E-04

-2.00E-04

0.00E+00

0 50 100 150 200 250 300 350

Linear Expansion [m/m]

NIST

ANSYS

Under these assumptions and material properties, we claim that the

study of the measured compensated strain (or even the non-compensated

one) is meaningful and must be considered!

Model Validation



Model Validation

5 µStr

student: jose ferradas university of geneva & cern mdt

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