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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.
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Surface Resistance of a bulk-like Nb Film
Sarah Aull, Anne-Marie Valente-Feliciano, Tobias Junginger and Jens Knobloch
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
• 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
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
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
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)
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.
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.
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.
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.
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.
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.
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
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.
Conclusions for Nb films
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
