Sarah aull surface resistance of a bulk-like nb film
1. Surface Resistance of a
bulk-like Nb Film
Sarah Aull, Anne-Marie Valente-Feliciano,
Tobias Junginger and Jens Knobloch
2. 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
4. • 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
5. Deposition Conditions
Cu substrate
• OFHC Cu
• Mechanical polishing + electropolishing
• Final sulfamic acid rinse for cu passivation
Deposition 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
6. Film characterization
Witness sample Nb/(11-20)
Al2O3
Tc= 9.36 ± 0.12 K
RRR = 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
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 nm
and ξ0 = 39 nm
sarah.aull@cern.ch 7
8. R(T): comparison with bulk Nb
R(T) curve consistent with a film
with RRR 50 and a reduced energy
gap (might be due to strong
oxidation)
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
sarah.aull@cern.ch 8
9. • 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
10. • 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.
11. Influence of the Cooling Conditions
• Influence on the surface resistance: Slow uniform cooling
increases RS by more than a factor 2.
400 MHz, 2K, 5 mT
sarah.aull@cern.ch 11
12. Influence of the Cooling Conditions
Thermal cycling acts on the Q-slope:
The faster the cooling the flatter the slope.
sarah.aull@cern.ch 12
400 MHz, 2 K
13. Conclusions for the ECR film
• 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
14. Comparison with HIPIMS coating
• 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
15. Comparison with HIE Isolde
• 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 15
16. Comparison between QPR, 1.3 GHz and HIE Isolde
RRR Geometry Cooling Grain size
sarah.aull@cern.ch 16
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
stress
Oxidation
Roughness
…
17. Conclusions for Nb films
• 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
19. 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 plasma
3 essential components:
Neutral Nb vapor
RF power (@ 2.45GHz)
Static B ERF with ECR condition
eB
m