In the quest of new materials for SRF applications, the secondary electron yield (SEY) needs also to be taken into consideration. A high SEY holds the risk that multipacting becomes again a main performance limitation of an SRF cavity. In the worst case, a too high SEY makes a material completely unsuitablefor an RF exposed surface. This talk will discuss general aspects of the role of the surface condition and present SEY measurements on different SRF relevant materials, i.e. MgB2, Nb3Sn and NbTiN.

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

3. sarah.aull@cern.ch
• RF-DC-compensation
• 푅S~
푃dc1−푃dc2
푃t
3
Calorimetric Method
Helium bath

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

18. Backup
sarah.aull@cern.ch 18

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
 