Jens knobloch open discussion
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Transcript of Jens knobloch open discussion
Improved RBCS theory
Improved theoretical models now allow some prediction of cavity behavior
But! Can we use this theory for films?
f
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S. Aull, this workshop. Nb film2.5 K
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?
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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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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
Impact on cooldown conditions
QWRs @ CERN (100 MHz)
6Courtesy of Pei Zhang
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j
Nb film
Comparison Theory with Measurements
Is a quantitative (theoretical analysis) possible with thin films?
I believe we are still a long way off …
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Xiao et alTheory
„Bulk-like“ film(Aull et. al)
Vogt et alBulk Nb (TESLA)
Frequency 1500 MHz 1200 MHz 1300 MHzλ(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
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. 9
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
• Modified design at HZB: - similar but with higher fields - 125 mT demonstrated so far- 433 MHz - 1.3 GHz- Double resolution- Demountable sample (hopefully!)
• SEY measurements at CERN and U. Siegen
- If high, QPR will let you know!
• Theory at JLAB, Cornell, ODU• Material analysis @ JLAB and ?
10Calorimetry chamber(large domain Nb)
Hollow quadrupolerods (Nb)
Resonator body (Nb)
Frame(SS, Ti)
Coaxial gap
Sample
Pole shoes
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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