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TrappedFluxMeasurementsandThermalBoundaryResistanceAnalysisforanECRfilm
Thin Film Workshop 2016Sarah Aull
Acknowledgements toA. Miyazaki, W. Venturini Delsolaro and the HIE-Isolde team (CERN)A.-M. Valente-Feliciano (JLab)
WhereweleftatSRF2015
FirstNb/Cucoatingbyenergeticcondensationshowed• astronglymitigatedQ-slope• (still)lowtrappedflux
sensitivity• strongdependenceonthe
cooldowndynamics
Summary
15/09/2015 [email protected] 13
withaQ-Slopecomparabletobulk
niobiumat4K
severelyaffectedbythermalcurrents
stilllesssensitivetotrappedflux
Abulk-likeNb/Cufilm…
ThermalBoundaryResistanceModel
• SmalldefectsattheNb-CuinterfaceleadtomicroscopicquencheswhichcausetheNb\CuQ-slope.
• Q(Eacc)curvecanbeconvertedintoadistributionfunctionofthermalcontactresistances.
VPalmieriandRVaglio,Supercond.Sci.Technol.29(2016)015004
Tc
RNb/Cu
Nb
Cu
Integrationyieldsdetachedsurfacearea:I =
Zf�RNb/Cu
�dRNb/Cu
3
ThermalBoundaryResistanceModel
• SmalldefectsattheNb-CuinterfaceleadtomicroscopicquencheswhichcausetheNb\CuQ-slope.
• Q(Eacc)curvecanbeconvertedintoadistributionfunctionofthermalcontactresistances.
VPalmieriandRVaglio,Supercond.Sci.Technol.29(2016)015004
Tc
RNb/Cu
Nb
Cu
Integrationyieldsdetachedsurfacearea:I =
Zf�RNb/Cu
�dRNb/Cu
Conversion is not a fit
Difficult to test experimentally
3
ExamplesofDetachedSurfaces
CourtesyB.B
arbo
raCo
urtesyR.Valizad
eh
CourtesyR.Valizad
ehCo
urtesyP.Jac
ob
ECR
Sputtering Sputtering
Sputtering
ThermalContactResistanceoftheECR1
• CompareTBRanalysisforRFmeasurementsontheECRcoatingatdifferenttemperaturesandfrequencycombinations.
400 MHz
800 MHz
2.5 K
4 K
Detachedsurfacearea:
I =
Zf�RNb/Cu
�dRNb/Cu
ThermalContactResistanceoftheECR1
• CompareTBRanalysisforRFmeasurementsontheECRcoatingatdifferenttemperaturesandfrequencycombinations.
400 MHz
800 MHz
2.5 K
4 K
Detachedsurfacearea:
I=(3.4±0.7)10-5«(10-3-10-4)
I =
Zf�RNb/Cu
�dRNb/Cu
ThermalContactResistanceoftheECR1
• CompareTBRanalysisforRFmeasurementsontheECRcoatingatdifferenttemperaturesandfrequencycombinations.
400 MHz
800 MHz
2.5 K
4 K
Detachedsurfacearea:
I=(3.4±0.7)10-5«(10-3-10-4)
I =
Zf�RNb/Cu
�dRNb/Cu
Same result for all curves supports
validity of TBR model
TestingtheThermalContactResistanceModel
• CompareTBRanalysisforRFmeasurementsonbulkNbreferenceatdifferenttemperaturesandfrequencycombinations.
400 MHz
800 MHz
2.5 K
4 K
Detachedsurfacearea:
I =
Zf�RNb/Cu
�dRNb/Cu
TestingtheThermalContactResistanceModel
• CompareTBRanalysisforRFmeasurementsonbulkNbreferenceatdifferenttemperaturesandfrequencycombinations.
400 MHz
800 MHz
2.5 K
4 K
Detachedsurfacearea:
I =
Zf�RNb/Cu
�dRNb/Cu
No meaningful
result for bulk Nb
TBRAnalysisforHIE-Isolde
CompareQ(E)at2.5Kand4.5K:
HIE-Isoldecavitiesshownalsosamedistributionfunctionforfordifferenttemperatures
7CourtesyAkiraMiyazaki(CERN)ThinFilmWorkshop2016
TBRAnalysisforHIE-Isolde
CompareQ(E)at2.5Kand4.5K:
HIE-Isoldecavitiesshownalsosamedistributionfunctionforfordifferenttemperatures
7CourtesyAkiraMiyazaki(CERN)ThinFilmWorkshop2016
Consistent with • thermal boundary resistance being
a main Q-slope contributor • mitigated Q-slope for improved
microstructure due to energetic condensation.
CoolDownDynamics:ThinFilmWorkshop2014
SimulationofCoolDownDynamics
• HeattransfersimulationwithCOMSOLtostudycooldowndynamicsintheQuadrupoleResonatorgeometry.
• TemperaturedependentmaterialpropertiesforNb,Cuandstainlesssteel.
SimulationofCoolDownDynamics
• HeattransfersimulationwithCOMSOLtostudycooldowndynamicsintheQuadrupoleResonatorgeometry.
• TemperaturedependentmaterialpropertiesforNb,Cuandstainlesssteel.
the slower the cool down, the bigger the
temperature gradient across the surface
InfluenceofCoolDown
• Thelowfieldsurfaceresistanceincreasesforslowcooling,resp.large(r)temperaturegradients
• Thehigherthelowfieldresistance,thestrongertheQ-slope.
• Theinfluenceofanambientfieldisnegligible.
400MHz,2K
InfluenceofCoolDown
• Thelowfieldsurfaceresistanceincreasesforslowcooling,resp.large(r)temperaturegradients
• Thehigherthelowfieldresistance,thestrongertheQ-slope.
• Theinfluenceofanambientfieldisnegligible.
400MHz,2K 400MHz,2K
FluxTrappingorFluxExpulsion?
Asolenoidunderthesampleallowstrappedfluxstudies.
FluxTrappingorFluxExpulsion?
Asolenoidunderthesampleallowstrappedfluxstudies.
Allambientfieldistrappeduptoathresholdfieldabovewhichfieldcanbeexpelled
FluxTrapping• Belowthethresholdfield,allfieldistrappedindependentlyof
cooldowndynamics.
• Thedependenceoncooldowndynamicsissignificantlystrongerthanlossesduetotrappedflux.
FluxTrapping• Belowthethresholdfield,allfieldistrappedindependentlyof
cooldowndynamics.
• Thedependenceoncooldowndynamicsissignificantlystrongerthanlossesduetotrappedflux.
400MHz,2K
FluxTrapping• Thedependenceoncooldowndynamicsissignificantly
strongerthanlossesduetotrappedflux.
• ECRcoatingstillshowslowtrappedfluxsensitivity
SECR (400MHz) = (0.022± 0.001) n⌦/µT
400MHz,2K
SECR (800MHz) = (0.075± 0.009) n⌦/µT
FluxTrapping• Thedependenceoncooldowndynamicsissignificantly
strongerthanlossesduetotrappedflux.
• ECRcoatingstillshowslowtrappedfluxsensitivity
SECR (400MHz) = (0.022± 0.001) n⌦/µT
400MHz,2K
SECR (800MHz) = (0.075± 0.009) n⌦/µT
What causes the strong surface resistance dependence on cool
down dynamics?
ECR2• Measureforthreedifferentthermal
cycles.
• Fitwith
• Findunchangedsuperconductinggap—>re-runfitwithcommon∆
400MHz
RS (T ) =ABCS
Texp
✓�
kBT
◆+Rres
RS (400MHz, 10mT, T )
EffectofThermalCycling
• BCSfactorstronglyincreaseswithresidualresistance
•
• Hinttowardsstrongmagneticfields?Duetothermalcurrents?
ABCS ⇠ !2 · ` · � (T, `)3�4
Conclusion• AnalysisofvariousRFdataincombinationwithFIB-SEMsupports
thatadhesionplaysthekeyroleincuringtheQ-slopeinNb/Cu.
• ConsistentpictureforNb/Cu:TemperaturegradientswhencrossingTcincreaselowfieldsurfaceresistance.
• Magneticfluxexpulsiononlyabovethresholdfield.
• Nocorrelation(sofar)betweencooldownrate/temperaturegradientandamountoffluxexpulsion.
• Effectoftemperaturegradientsignificantlystrongerthantrappedflux.
• Thermalcyclingappearstochangetheeffectivepenetrationdepth.
