Improved RBCS theory

Improved theoretical models now allow some prediction of cavity behavior

But! Can we use this theory for films?

f

2

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?

3

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?

4

Impact of cooldown conditions @ HZB

5

• 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

7

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 …

8

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

11