# Presentation: 224389

## Session 1 : fundamental limits

- Nearly “perfect” cavities can be achieved
- Flux line penetration (AG) vs superheating (HP)
- Time scales of GL domain vs flux line vs GHz
- What is the vortex penetration mechanism ?
- Tc and Bc remain relevant parameters to maximize performance
- Materials with higher Bc
- Taking advantage of thin superconductors to raise Bc1(ML) above Bc(Nb)
- Heat transfer: Kapitza issues obstruct new ideas to add more interfaces?
- Are there ways to reduce the present requirements of near perfection?

- SRF materials workshop

## Session 1 : Fundamental of SRF

- We know GB are weak links (but dc vs rf...)
- All optimized cavities have similar Qslope and quench, with or w/o GB !
- Even these have hot spots.

- SRF materials workshop

- Beyond Niobium
- Theory and experiments starting to join
- How do we verify that success happens?

- SRF materials workshop

## Session 2 : Material properties and characterization

- Surface Studies
- SIMS, XPS are complementary – need combined and coordinated studies
- TEM, scanned probe microscopy – need resources, very targeted studies
- Surface morphology – mind the scale!
- New probes (scanning RF, strip resonator, Bc3)
- Area probes average; local probes may miss important details

- SRF materials workshop

## Session 2 : Material properties and characterization

- Surface Studies, cont.
- Problem areas are local; but over what scale?
- Some things thought to be problems simply aren’t (?!)
- Oxygen -- cavities with various mixtures of Nb2O5, NbO2, NbO behave the same
- Mapping and post-mortem techniques :
- Hot spots, TMap: only radar for problems?
- Volume of interest: -5nm to -10 nm under the surface ?
*Interesting ideas: dissecting cavities, study mono/bicrystals Laser-produced hot spots, Laser as in-situ probe *

- SRF materials workshop

## Session 2 : Material properties and characterization

*RF / Superconducting properties*
- Coupling with surface analysis!
- Comparison between single samples and whole cavities should be relevant
- Mushroom test platform: access to RF properties, comparison against DC
- Are there other parameters “easy” to measure that could give us better prediction of the cavity behavior?
- Bc3
- Thermal transfer: influence of annealing, grain boundaries....

- SRF materials workshop

## Session 3: New materials for the future

- T/Tc matters for Rs, so higher Tc helps even if the superconductor is not perfect
- Reprise: Are there ways to reduce the requirements of perfection?
- New techniques are promising not only for making superconductors or multilayers...
- Conformality due to flowing vapors and surface catalyzation of desired chemical reaction
- Want line compounds – MgB2
- ... and provide new versatility
- We are on the eve of a new era (functionalized coatings?)
- We are involving new people (chemists)!  Continue!

- SRF materials workshop

## Session 4: Surface processing

- How heavily do “dry” treatments modify the surface?
- “molecular peening” should cause damage and reduce RRR
- Temperature and surface mobility should heal damage
- What are consequences of resolidification of sputtered / molten ejecta?
- What are consequences of energetic implants due to plasma?
- New surface treatments shouldn’t create a new problem!
- Interesting ideas: chemical-mechanical polishing of sheets or 1⁄2 cells => reduced etching afterwards

- SRF materials workshop

## Session 5: Niobium

- Single crystals and very small grains are desirable (for forming and flatness);  Forming and roughness (BCP) difficulties relate to “large grains” at a scale between those above.
- Is there no way out?
- Do we want a certain texture of Nb?
- Can we relax some of the specs (e.g. Ta); do we need to strengthen some?
- Damage layers !

- SRF materials workshop

- SRF materials workshop

## Session 4: Innovative processing methods

- How can we overcome the variability of the present surface preparation processes
- Systematic studies of the damage layer is needed
- Modification of the fabrication process ?
- What do we need to address? Surface removal (what thickness, why), particle contamination, surface modification regarding secondary emission, field emission... Are all these issues compatible between themselves?
- Do we need complementary or alternative techniques?

*Interesting ideas: chemical-mechanical polishing of sheets or 1⁄2 cells => reduced etching afterwards*

- SRF materials workshop

## Session 5: Niobium production

- What knowledge do we need to get reproducible production of the adequate material
- How can we practically get reproducible production of the adequate material
- Are there specific textures/ orientations with more favorable behavior (forming, magnetic transition field, work function....), how can we optimize it.
- What do we still need to know? (e.g. recrystallization behavior @ welding, cold mechanical resistance of welded parts...)

- SRF materials workshop

## Session 2 : Material properties and characterization

*RF / Superconducting properties*
- New techniques in sight
- RF microscopy
**It’s RF  ! It’s local (chasing hot spots) !**
- We can combine it with plenty of other techniques
- High field data ?
- Point contact Spectroscopy
- SC properties near the surface
- Magnetometry? (Bc3)
- Samples vs cavities issue is still a hot topic
- But now that we know how to prepare routinely  clean cavities...

*Interesting ideas: role of interfaces, non linearities*

- SRF materials workshop