sarah aull secondary electron yield of srf materials

Surface Resistance of a bulk-like Nb Film

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

The Quadrupole Resonator

sarah.aull@cern.ch 2

• 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

• RF-DC-compensation

• 𝑅S~𝑃dc1−𝑃dc2

𝑃t

3

Calorimetric Method

Helium bath

• 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

sarah.aull@cern.ch 4

Sample Preparation

EBW 1

EBW 2

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 5

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

valente@jlab.org 6

sarah.aull@cern.ch 7

Penetration Depth Measurement

λ(0K) [nm]

400 MHz 40 ± 2

800 MHz 38 ± 1

1200 MHz 38 ± 1

Bulk-like film in the clean limit

ℓ* [nm] RRR

144 ± 20 53 ± 7

* with λL = 32 nmand ξ0 = 39 nm

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

sarah.aull@cern.ch 8

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

• 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.

sarah.aull@cern.ch 9

Q-Slope: film vs. bulk

2.5 K

4 K

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

sarah.aull@cern.ch 10

Thermal Cycling

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

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

sarah.aull@cern.ch 11

Influence of the Cooling Conditions

400 MHz, 2K, 5 mT

Thermal cycling acts on the Q-slope:

The faster the cooling the flatter the slope.

sarah.aull@cern.ch 12

Influence of the Cooling Conditions

400 MHz, 2 K

• 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.

sarah.aull@cern.ch 13

Conclusions for the ECR film

• 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.

sarah.aull@cern.ch 14

Comparison with HIPIMS coating

• Quarterwave, 100 MHz

• For more RF results, see The influence of cooldownconditions 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 15

Comparison with HIE Isolde

sarah.aull@cern.ch 16

Comparison between QPR, 1.3 GHz and HIE Isolde

RRR Geometry Cooling Grain size

QuadrupoleResonator: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 stress

Oxidation

Roughness

…

• 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.

sarah.aull@cern.ch 17

Conclusions for Nb films

Backup

sarah.aull@cern.ch 18

Electron Cyclotron Resonance

No working gas

Ions produced in vacuum

Singly charged ions 64eV

Controllable deposition energy with Bias voltage

Excellent bonding

No macro particles

Good conformality

Generation of plasma3 essential components:

Neutral Nb vaporRF power (@ 2.45GHz)

Static B ERF with ECR condition

m

eB