1.3-GHz Superconducting Radio
Frequency Cavity Materials
and their Evaluation
Ashish KUMAR
CASA, Accelerator Laboratory,
High Energy Accelerator Research Organization (KEK)2021/09/09
2021/09/09

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Contents
1. International Linear Collider
2. 1.3 GHz SRF Cavities

3. Materials for SRF Cavity Assembly
4. Pressure Vessel Compliance for SRF cavities
5. Methodology to Evaluate Mechanical Properties
6. Mechanical Properties
7. Recent Developments in Niobium Material Technology
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International Linear Collider

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International Linear Collider©Rey.hori / KEK International Linear Collider is an electron-positron collider accelerator.
 1.3 GHz SRF Cavities accelerates the electrons and positrons to near speed of light.

 7800 1.3 GHz SRF cavities in 850 cryomodules to attain centre of mass energy of 250 GeV.
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1.3 GHz SRF Cavity

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1.3 GHz SRF Cavities

Tesla Cavity

Tesla-like Cavity

 9-cell standing wave
structure 9-cell standing wave
structure Almost 1 m in length Optimized for reduction in
Hpk / Eacc.

 Lowest TM mode
resonates at 1300 MHz.

 Thicker end plates.

 Employed in Eu-XFEL(DESY), LCLS-II (SLAC).

 Employed in STF2 (KEK)2021/09/09

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SRF Cavity Assembly©Rey.hori / KEK

Cited from: Phys. Rev.ST Accel. Beams 3,092001 (2000)
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Materials for SRF Cavity
Assembly

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Materials for SRF Cavity Assembly

Materials

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Niobium

SRF Cavity

Titanium

Helium Jacket

NbTi Alloy

End plates / End
Flanges9

Niobium Nb has a body-centred cubic (bcc) crystal
structure and a melting point of 2,468 °C.
 Extracted from mines mostly in Brazil.
 It is choice of material for SRF cavity.
 It becomes superconducting at 9.2 K.
 33rd most abundant material on earth.
 Largest mines are in Brazil.

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CBMM Brazil, Araxa Mine

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Melting and Purification of Nb
 Melting and purification of Nb by electron beam
furnace.
 Melted in vacuum of < 3E-1 Pa for the first melt and< 1E-3 Pa for last melt to obtain high purity Nb.
 Molten globules drop into water cooled copper
crucible to form an ingot.

 Gases and impurities with lower melting
temperatures than Nb evaporates.
 Most of the impurities are present on the skin of
the ingot and is machined away for a purer ingot.
Cited from: arXiv:1501.07142 [physics.acc-ph]
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Melting and Purification of Nb
EBM Furnace

 Several companies can produce high purity refractory
metals in larger quantities.
 ATI Wah Chang (USA), Cabot (USA), W.C. Heraeus(Germany), Tokyo Denkai (Japan), Ningxia (China),
CBMM (Brazil), H.C. Starck (Germany, USA).
 Leybold furnace with 500 kW nominal power melts
the Niobium at 40 – 50 kg/hr (CBMM).

 Another 1.8 MW nominal power melts Nb at 90 –120 kg/hr and pressures < 1E-3 mbar (CBMM).

Cited from: AIP Conference Proceedings 927, 191 (2007)
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Nb Ingot12

Nb Disk Manufacturing

FG Nb

LG Nb
Cited from: Nuclear Instruments and Methods in Physics Research A
774 (2015) 133–150

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Direct Slicing of LG Nb

Cited from: AIP Conference Proceedings 1352, 79 (2011)

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Cited from: AIP Conference Proceedings 927, 191 (2007)14

Niobium for 9-Cell 1.3 GHz SRF Cavity
9-Cell 1.3 GHz
Nb SRF Cavity

•
•
•
•

Conventional Material

R & D Material

FG Nb

LG Nb

Grain size < 50 μm
Isotropic mechanical properties.
Uniform and adequate properties.
High Cost.
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•
•
•
•

Grain size > 1 cm.
Anisotropic mechanical properties.
Issue with pressure vessel clearance.
Low Cost.
15

Hydrogen Degassing
 Hydrogen degassing is necessary to avoid Q-disease.

 Cavities are heat treated at temperatures > 500 °C
under vacuum conditions < 10-3 Pa).
 Molybdenum or tungsten resistive heating elements
provides radiation heating.
 Ultra-high vacuum inside the furnace is created withcryo-pumps.
 Current standard for SRF cavity is 800 °C for 2-3 hours.
Ultra-high vacuum furnace at KEK
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Induction Furnace at Jlab, USA Hot zone is made of Niobium and the cavity is treated by black-body radiation.
 Cu induction coil instills eddy current in the Nb to produce heat.

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Cited from: Review of Scientific Instruments 83, 065105 (2012)17

Titanium Material of choice for the helium jacket of the SRF cavity.
 Lower coefficient of thermal expansion and similar to Nb.
 Retains strength and ductility at cryogenic temperatures.
 Sufficient mechanical strength for the helium jacket.

 Commercially available pure Ti can be used, JIS class 1 and class 2.

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Ti Helium Jacket/Tank

Ti end plate18

Niobium-Titanium Alloy
 It is an alloy of Nb and Ti and mostly used as a type II superconductor for
superconducting magnets.
 Tesla cavity assembly employs NbTi for conical discs (end plates) of the end-group.
 NbTi’s mechanical properties are on par to Ti at high annealing temperatures.

 For Tesla-like cavity assembly, instead of NbTi pure Ti is used for end plates.

End plates for Tesla cavity
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End flange

19Pressure Vessel Compliance

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1.3 GHz 9-Cell SRF Cavity Assembly
Niobium SRF Cavity with its Titanium
jacket is considered as a pressure vessel
Design must be cleared by high-pressure
gas safety authorities
Determine maximum allowable stressand buckling pressure using ANSYS
Nb and NbTi not listed as a material forhigh pressure vessel design
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Cited from: The International Linear Collider: A
Global Project, arXiv:1903.01629 [hep-ex]

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Steps Involved in Preparing
Documentation
 Description of the SRF cavity assembly.
 Mechanical properties of the materials at room and in liquid helium temperatures.
 Mechanical properties of welded joints like Nb-Nb, Nb-Ti, Ti-Ti welds etc.

 Stress and buckling analysis of the cavity assembly using CAE software at maximum
allowable working pressure (0.2 MPa) and tuner displacement.
 Cavity fabrication information.
 Pressure test and examination reports.
 Documentation to summarize above items to be submitted to the high-pressure gas
safety authority.
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Methodology for Material
Characterization

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Material
Characterization

Material characterization
Chemical Analysis

Impurities

RRR Measurement

Purity of the material

Mechanical
Properties

Tensile Testing24

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Residual Resistivity Ratio (RRR)
 Indicates the level of purity of a material.
 Purer the material higher the thermal conductivity.
 For SRF cavities high thermal conductive wall (atleast 10 W/m-K at 2.0 K) necessary to guidedissipated RF power to He II.
ρ 293 KRRR for Nb =
ρ 9.3 K
ρ 293 KRRR normal conductors =ρ 4.2 K

 Nb classified as: low (<100), medium (<300) andhigh RRR (>300).
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Cited from: arXiv:1501.07142 [physics.acc-ph]25

Tensile Testing Material is subjected to uni-axial tension until failure.
 Engineering stress (F/L*W) versus engineering strain (∆L/L) is determined.

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Tensile Testing Young’s Modulus (E) – Stiffness of thematerial in tension
Stress𝐸=(𝑖𝑛 𝐸𝑙𝑎𝑠𝑡𝑖𝑐 𝑅𝑒𝑔𝑖𝑜𝑛)
Strain 0.2% Yield Strength (0.2% Y.S) – Stressindicating the limit of elastic behavior
 Tensile Strength (T.S) – Maximum stressor stress before failure of the specimen
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Tensile Testing Poisson's Ratio (𝜈) – absolute value of theratio of longitudinal strain to the transverse
strainLongitudinal strain
Δ LΤL
𝜈=
=
Transverse strain−Δ WΤW Elongation – Measure of ductility of amaterial
Elongation (%) = 100 × ∆Lf ΤL

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Tensile Testing Machine At KEK, Shimadzu’s® Autograph AG-5000C
tensile test machine is utilized.
 Room and cryogenic temperature tests (withcryostat) can be performed.
 Stroke length of approximately 1.5 m.
 Rated for ± 50 KN of force.

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Strain Gages
 Its resistance varies w.r.t applied force.
 Metal’s electrical resistance changes when it
deforms.
 It transforms the strain applied into
proportional change in the resistance.

 120 Ω Strain gages are most widely used.

Cited from: https://www.kyowaei.com/eng/technical/strainbasic_course/index.html

∆R
∆L
=𝑘
= 𝑘. 𝜀R0L

𝑘 is the gage factor
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Cited from: Karl F. Hoffman, An Introduction to measurements using
strain gages, Hottinger Baldwin Messtechnik GbbH, Darmstadt
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Extensometer Measures average strain in the gage section.
 Clip-on type doesn’t need independent mounting
device.

 Uses a pair of knife edges pressed on thespecimen to define the gage length.
 At gage length the circuit is balanced, and material
under tension produce output voltage.
 Can measure strains upto 100% and is available forwide range of gage length (10 – 200 mm).

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Wheat stone Bridge and Bridgebox
 Measures unknown resistance compared to known
resistance values.
 Allows measurements down to milli-ohms.

 4 resistors in series-parallel connection.
 2 diagonally opposite terminal to supply excitation
voltage and other two to measure output voltage.
 Output in mV / V.

Kyowa® DB-120A Bridgebox
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Wheat stone Bridge and Bridgebox
Ratio of output tosupply voltage

Vo
R1
R3
=
−
Vs R1 + R 2 R 3 + R 4

Balanced Wheatstonebridge condition

∆R
∆L
=𝑘
= 𝑘. 𝜀R0L

R1 R 3=R2 R4Vo k= ε1 − ε2 + ε4 − ε3Vs 4Strain Gage
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Configurations of Strain Measurement

Quarter bridge

No bending strain cancellationor temperature compensationk
Vo = Vs (ε0 )42021/09/09

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Half Bridge Configuration

Half bridge

Temperature
Compensation onlyk
Vo = Vs (ε0 )42021/09/09

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Half Bridge Configuration

Half bridge
Bending strain cancellation
only
Room temperature testingk
Vo = Vs (ε0 )22021/09/09*Output voltage is doubled 36

Full Bridge Configuration

Full bridge

Bending strain cancellation &temperature compensation
Cryogenic temperature testingk
Vo = Vs (ε0 )22021/09/09*Output voltage is doubled

37Strain Amplifier Output voltage from a Wheatstone bridge circuit is small.

 Electrical amplification for measurement and recording.
 Kyowa® strain amplifiers are used to supply bridge voltage
and amplification of the bridge output voltage.

 Bridge excitation voltage of 2 V and 0.5 V.
 Maximum output voltage of 10 V with maximum strain of9999 micro-strains, i.e 1% strain.
 Internal gain can be set from 200x to 2000x.

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Mechanical Properties

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Why Mechanical Characterization?
 Cavity is fabricated at room temperature conditions and operated in LHe.
 Materials should have requisite strength to handle the procedures involved in cavity
fabrication, such as press forming, trimming, welding etc.
 Materials should be able to handle stress generated due to large temperature variation.
 SRF cavity assembly must be able to withstand conditions set by high-pressure gas safety
authorities for pressure vessels.
 Materials for cavity assembly such Nb, Ti and NbTi are known to be weakest at room
temperature conditions.

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Room Temperature Tensile Tests

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Sample in the jig of tensile test machine
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Necking of Aluminum sample
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Low Temperature Tensile Testing SRF cavity assembly
temperature is 2.0 K.

operational

 Mechanical properties of metals
changes drastically from 300 K to 2.0 K.
 A custom-built cryostat at KEK to dip
tensile test specimens in liquid helium.

 Three specimens can be tested in one
cooldown cycle.

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Specimen for Tensile Testing

Cryogenic temperature tensile testing specimen

Room temperature tensile testing specimen
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Preparation of Tensile Test Specimen

Wire EDM Cut

Chemical PolishingTensile test specimen

Oven for bonding
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Annealing

Strain Gage Bonding45

Niobium Mechanical Properties
 Mostly properties of FG Nb will be presented.
 FG Nb is a standard material that is considered while designing the SRF cavity.

 Data presented here is for the annealed specimens.
 Employed for Eu-XFEL (DESY), LCLS-II (SLAC) and STF-2 (KEK).

 LG Nb is mainly a research material and currently not used for large scale accelerators.
 LG Nb properties have been studied in details at KEK but the data is still under publication.

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Stress-strain curve for FG Nb
 Isotropic mechanical properties (independent of location of tensile test specimen).
 However, properties do deteriorates with annealing.

At Room Temperature
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In Liquid Helium (4.21 K)
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Effect of Annealing on Nb
 Material properties deteriorates withannealing at higher temperatures.
 Least deterioration at 600 °C without any
loss in hydrogen degassing performance.
 However, 800 °C is considered as an ideal
temperature due to better flux expulsion.

 Annealing relieves residual stresses andhelps in recrystallization too.

Cited from: AIP Conference Proceedings 671, 227 (2003)
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Mechanical Properties of FG Nb

Tensile Strength w.r.t temperature
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Yield strength w.r.t temperature

Cited from: H. Nakai et al., “Tensile test of materials at low temperatures for ILC Superconducting
RF cavities and cryomodules”, Teion Kogaku (J. Cryo. Super. Soc. Japan) Vol. 48 No. 8 (2013)49

Mechanical Properties of FG Nb

Young’s modulus w.r.t temperature
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Elongation w.r.t temperature

Cited from: H. Nakai et al., “Tensile test of materials at low temperatures for ILC Superconducting
RF cavities and cryomodules”, Teion Kogaku (J. Cryo. Super. Soc. Japan) Vol. 48 No. 8 (2013)50

LG Nb Mechanical Properties
 Anisotropic mechanical properties for LG Nb due to grain orientation.
 T.S can be as low as 66 MPa to as high as 124 MPa.

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Cited from: IOP Conf. Ser.: Mater. Sci. Eng. 756 012002

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LG Nb Mechanical Properties Large variation in low temperature mechanicalproperties for LG Nb due to grain orientation.
 T.S can be as low as 400 MPa to as high as 800 MPa.
 0.2% Y.S is difficult to determine due to serrations inthat operational region.

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Examples of some tested specimens
Cited from: IOP Conf. Ser.: Mater. Sci. Eng. 756 012002

FG Nb tensile tested specimens at KEK by Nakai et al.
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Room temperature LG Nb tensile tested specimens by Zhao et al.
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Examples of some tested specimens
FG Nb

LG Nb
LG Nb

Low temperature tensile tested specimens (by me)
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Nb Mechanical Properties w.r.t SRF
Cavity Strength Requirement
Material

Y.S
[MPa]

T.S
[MPa]

Elongation[%]

Material

Y.S
[MPa]

T.S
[MPa]

Elongation
[%]

FG Nb
LG Nb

~50
66-124~150
84-136

~40
> 30

FG Nb
LG Nb

~500
-

800 - 900
400 - 800

~7~6

Tesla-likeEu-XFEL

> 39
> 50

> 120
> 140

> 30
> 30

Tesla-like

For high-pressure gas safety
regulation
2021/09/09> 300

Only necessary for formabilityof SRF cavity half cells
55Titanium Mechanical Properties Sufficient mechanical properties for pressure vessel design.

Tensile Strength w.r.t temperature
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Yield strength w.r.t temperature

Cited from: H. Nakai et al., “Tensile test of materials at low temperatures for ILC Superconducting
RF cavities and cryomodules”, Teion Kogaku (J. Cryo. Super. Soc. Japan) Vol. 48 No. 8 (2013)

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Titanium Mechanical Properties
 Young’s modulus and elongation is maintained in operational range.

Young’s modulus w.r.t temperature
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Elongation w.r.t temperature

Cited from: H. Nakai et al., “Tensile test of materials at low temperatures for ILC Superconducting
RF cavities and cryomodules”, Teion Kogaku (J. Cryo. Super. Soc. Japan) Vol. 48 No. 8 (2013)

57

Ti Welded Joints Mechanical Strength Ti-Ti weld has similar properties to pure Ti.
 Ti-Nb EBW weld has sufficient strength for SRF cavity assembly.

2021/09/09 Cited from: K. Mukugi et al., “Low temperature mechanical properties of Titanium and weld joints (Ti/Ti,
Ti/Nb) for helium vessels”, in Proc. The 10th Workshop on RF Superconductivity, Tsukuba, Japan, 2001.

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NbTi Alloy Mechanical Strength Sufficient mechanical properties for pressure vessel design.

Tensile Strength w.r.t temperature
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Yield strength w.r.t temperature

Cited from: H. Nakai et al., “Tensile test of materials at low temperatures for ILC Superconducting
RF cavities and cryomodules”, Teion Kogaku (J. Cryo. Super. Soc. Japan) Vol. 48 No. 8 (2013)

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NbTi Alloy Mechanical Strength Young’s modulus is constant and elongation is reduced in operational range.

Young’s modulus w.r.t temperature

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Elongation w.r.t temperature

Cited from: H. Nakai et al., “Tensile test of materials at low temperatures for ILC Superconducting
RF cavities and cryomodules”, Teion Kogaku (J. Cryo. Super. Soc. Japan) Vol. 48 No. 8 (2013)60

Recent Developments in
Niobium Material Technology

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Medium Grain Niobium New material manufactured by ATI (USA).
 It has an average grain size of 200-300 microns,with occasional 1-2 mm grains.
 It is expected to have similar properties as FG Nb.

 Cost effective material as it can be directly sliced
after forging.
 Mechanical property characterization is being
conducted at KEK.

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Cited from: A. Kumar et al., MOPCAV004, SRF 21

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Niobium for 9-Cell 1.3 GHz SRF Cavity
Cited from: A. Kumar et al.,MOPCAV004, SRF 21

9-Cell 1.3 GHz
Nb SRF Cavity

Conventional Material

R & D Material

FG Nb

LG Nb

• Grain size < 50 μm• Isotropic mechanical properties.
• Uniform
andadequate
properties.
• High Cost.

• Grain size > 1 cm.
• Anisotropic mechanicalproperties.
• Issue with KHK clearance• Low Cost.

New Material
•

•
•
•
•

ATI MG Nb
Grain size - 200-300
occasional 1-2 mm grains
New material with no data.
Isotropic properties?
Viable for SRF cavity?
Cost reduction w.r.t FG Nb

μm,

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Thank you for your attention!

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