Surface Resistance of a bulk-like Nb Film

Sarah Aull, Anne-Marie Valente-Feliciano, Tobias Junginger and Jens Knobloch

sarah.aull@cern.ch 2

The Quadrupole Resonator

• Resonant frequencies: 400, 800, 1200 MHz

• Same magnetic field configuration for all frequencies

• Bmax ≈ 60 mT• Temperatures 1.8 -20 K• Sample:

• 75 mm diameter• Equipped with a dc heater and 4

temperature sensors

361 mm

Sample

sarah.aull@cern.ch 3

• RF-DC-compensation

Calorimetric Method

Helium bath

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• OFHC copper substrate:• mechanically polished• Electron beam welded to Nb ring (EBW 1)• 12 μm electro polishing• Rinsing with ultra pure water at 6 bar

• Shipped to Jefferson Lab for coating• Shipped back to CERN, EBW to support

structure (EBW 2)• Rinsing with ultra pure water at 6 bar• Mounted in the quadrupole resonator

Sample Preparation

EBW 1

EBW 2

valente@jlab.org 5

Deposition Conditions

Cu substrate • OFHC Cu • Mechanical polishing + electropolishing• Final sulfamic acid rinse for cu passivationDeposition Conditions• ECR• Bake & coating temperature: 360 °C• Total coating time: 60’Dual ion energy:• 184 eV for nucleation/early growth• 64 eV for subsequent growth• Hetero-epitaxial film Nb on OFHC Cu

Typical Cu substrate

valente@jlab.org 6

Film characterization

Witness sample Nb/(11-20) Al2O3 Tc= 9.36 ± 0.12 KRRR = 179

Diffraction on Nb/Cu witness sample:EBSD IPF map and XRD pole figure show very good crystallinity and grain sizes in the range of the typical Cu substrate

sarah.aull@cern.ch 7

Penetration Depth Measurement

λ(0K) [nm]400 MHz 40 ± 2

800 MHz 38 ± 11200 MHz 38 ± 1

Bulk-like film in the clean limit

ℓ* [nm] RRR

144 ± 20 53 ± 7* with λL = 32 nm

and ξ0 = 39 nm

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R(T) curve consistent with a film with RRR 50 and a reduced energy gap (might be due to strong oxidation)

R(T): comparison with bulk Nb

Rres [nΩ] Δ [K]

400 MHz 46.6 ± 0.8 14.2 ± 0.3

800 MHz 79 ± 2 14.8 ± 0.2

1200 MHz 156 ± 11 15.1 ± 1

mean 14.6 ± 0.2

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• Q-Slope of Nb film is linear for B > 5 mT for temperatures up to 4 K.

• Q-Slope of the Nb film is significantly stronger than for bulk Nb (1 order of magnitude)

RRR is unlikely the cause for the strong Q-slope of Nb films.

Q-Slope: film vs. bulk2.5 K

4 K

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• Thermal cycling: warm up the sample to the normal conducting state and cool down under different conditions.

Thermal Cycling

Thermal cycling does not affect the (low field) BCS contribution.

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• Influence on the surface resistance: Slow uniform cooling increases RS by more than a factor 2.

Influence of the Cooling Conditions

400 MHz, 2K, 5 mT

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Thermal cycling acts on the Q-slope: The faster the cooling the flatter the slope.

Influence of the Cooling Conditions

400 MHz, 2 K

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• This bulk-like Nb film shows significantly different behaviour than bulk Nb with the same RRR:• In contrary to bulk Nb: cooling fast and with a high temperature gradient

leads to lower surface resistance.• Lowest surface resistance was achieved by quenching.

• The Q-Slope of the film is much more severe than the one of bulk Nb. Therefore low RRR is unlikely the cause for strong Q-slopes in Nb film cavities.

• The cooling conditions act on the Q-Slope, leading to better performance after fast cooling.

Conclusions for the ECR film

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• Single cell 1.3 GHz Cu cavity + EP• Coating by Giovanni Terenziani• RF Cold test by Tobias Junginger• For more RF results of this cavity, see:

HIPIMS Development for Superconducting Cavities, Giovanni Terenziani & Tobias Junginger

• Cooling rate derived from temperature slope at Tc

• Lower RS for fast cooling and smaller temperature gradient.

• Thermal cycling influences the Q-Slope as well.

Comparison with HIPIMS coating

15

• Quarterwave, 100 MHz• For more RF results, see The

influence of cooldown conditions at transition temperature on the quality factor of niobium sputtered quarter-wave resonators, Pei Zhang

• Surface resistance increases for larger temperature gradients.

• Cooling rate has no significant influence on RS.

Courtesy of Pei Zhang

Comparison with HIE Isolde

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Comparison between QPR, 1.3 GHz and HIE Isolde

RRR Geometry Cooling Grain size

Quadrupole Resonator:ECR

Lower RS for fast cooling with T gradient

53 disc conduction tens of microns

1.3 GHz: HIPIMS

Lower RS for fast cooling with small T gradient

21 elliptical Bath cooled

30 nm

HIE Isolde:Diode sputtering

Lower RS for small T gradients

15 QWR conduction 200 nm – 1 μm

depending on thickness

Unknown

Influence of grain size

Influence of geometry

Thermal currents

Influence of stressOxidation

Roughness

…

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• As for bulk Nb: The cooling conditions, speed and/or spatial gradient, influence the RF performance.

• Different film projects are difficult to compare due to different coating techniques and geometries.

• Optimum cooling procedure to minimize the low field RS is accompanied by a flattened Q-Slope.

• Further conclusions require dedicated experiments, where spatial and temporal gradients and thermal currents can be controlled independently.

Conclusions for Nb films

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Backup

Electron Cyclotron Resonance

No working gas Ions produced in vacuum Singly charged ions 64eVControllable deposition energy with Bias voltageExcellent bonding No macro particlesGood conformality

Generation of plasma3 essential components:

Neutral Nb vaporRF power (@ 2.45GHz)

Static B ERF with ECR condition

m

eB