thin films, superconducting, superconductivity, SRF, cavities, enzo palmieri, niobium, workshop, rf, sputtering,

1. Improved RBCS theory
Improved theoretical models now allow some prediction of cavity
behavior

2. But! Can we use this theory for films?
f
2
2.5 K
S. Aull, this workshop. Nb film

3. Questions
• Can we even control the material
parameters/treatments/operating conditions
 Production techniques: e.g., spinning, deep drawing, heat treatment,
welding …
 Preparation: EP, BCP, MBP, Plasma arc
 Nitrogen doping
 Operating conditions: e.g., cooldown conditions
• I.e., if I tell you how the material was handled … can you tell me
what it‘s surface resistance will be?
3

4. e.g., Hydrides
Treatment history of the material strongly impacts material
properties … even in „simple“ bulk Nb systems
• Mechanical deformation
• EDM slicing of large-grain material
• Barrell polishing
• First cooldown v. subsequent cooldowns
• BCP v. EP (what about same recipe at different labs?)
• Single grain/large grain v. multigrain
• What does the surface morphology of the hydrides do?
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5. Impact of cooldown conditions @ HZB
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• Res. resistance as fn of temperature gradient during cooldown
• Factor of 8 difference depending on cooldown conditions!
Julia Vogt, SRF 2013 TESLA Cavity results
ΔRres ≈ 8 nΩ

6. Impact on cooldown conditions
QWRs @ CERN (100 MHz)
Courtesy of Pei Zhang 6

7. 7
Influence of the Cooling Condi ons
j
• Influence on the surface resistance: Slow uniform cooling
increases RS by more than a factor 2.
400 MHz, 2K, 5 mT
Nb film
sarah.aull@cern.ch 11

8. Comparison Theory with Measurements
Is a quantitative (theoretical analysis) possible with thin films?
I believe we are still a long way off …
8
Xiao et al
Theory
„Bulk-like“ film
(Aull et. al)
Vogt et al
Bulk Nb (TESLA)
Frequency 1500 MHz 1200 MHz 1300 MHz
λ(0K) 32 nm 38 nm ?
Δ 15.2 K 15.2 K 15.7 K
ℓ 50 nm* 144 nm ?
λL 32 nm 32 nm 32 nm?
ξ0 40 nm 39 nm 39 nm?
T 2 K 2K 2 K
Rs ≈16 nΩ ≈150 nΩ ≈10 nΩ min
Rs
(scaled to 1.5 GHz)
≈16 nΩ 234 nΩ ≈13 nΩ
* Little variation of RBCS with mean free path in the range of interest

9. What should we be doing?
To characterize films/bulk Nb we need
• Ability to characterize RF properties in the full phase space, i.e.,
 frequency,
 wide field range
 Wide temperature ranges
 At high resolution (nOhm and better!)
 rapid turn around!
• Ability to do this with samples
• Ability to do this in a frequency range of interest for cavities (i.e.
not 10 GHz!)
• Need to check the SEY characteristics
• Understand material properties, morphology … & correlate this
with the RF measurements
• Then compare best results with the theoretical predictions!
• Learn to walk before we run! E.g. Nb3Sn … look into samples
before cavities.
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10. RF characterization
QPR is the ideal tool!
• Allows analysis of the materials over the full phase space
• No “Enzo” effect to LHe, but can measure Kap. resistance film-substrate
• Orginial design at CERN
 T = 1.5 K - > 9.2 K
 0 – 60 mK
 400 MHz – 1.2 GHz, nm Resolution
Resonator body (Nb)
• Modified design at HZB:
 similar but with higher fields
Hollow quadrupole
 125 mT demonstrated so far
rods (Nb)
 433 MHz - 1.3 GHz
 Double resolution
Pole shoes
 Demountable sample (hopefully!)
• SEY measurements at CERN
Sample
and U. Siegen
 If high, QPR will let you know!
• Theory at JLAB, Cornell, ODU
• Material analysis @ JLAB and ? Calorimetry chamber 10
(large domain Nb)
Frame
(SS, Ti)
Coaxial gap

11. Finally …
• Once BCS behaves close to the theory, do we (as accelerator
physicists) even care?
... Or will one (for CW operation) then always choose a bath
temperature where BCS no longer dominates?  residual
resistance will be what matters
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