# Presentation: 524726

## 
Pierre Bauer 

Fermilab

SMTF Collaboration Meeting Oct. 5,6,7, 2005 
Materials R&D / WG1

- Pierre Bauer
- Fermilab
- SMTF Collaboration Meeting Oct. 5,6,7, 2005
- Materials R&D / WG1

- SRF Materials Research Proposal

## This proposal is based on the original idea by A. Gurevich from the University of Wisconsin. 

**This proposal is based on the original idea by **_**A. Gurevich**_** from the University of Wisconsin. **

## Q-DROP & BAKING EFFECT

**Examples of Q-drop at CEA, JLab and DESY;**

**B. Visentin - SRF03**

**L. Lilje, EPAC 04**

## EXPERIMENTAL FACTS ON Q-DROP

**Baking, often removes Q-drop, and it does this more efficiently in EP treated cavities! The baking effect is stable in air (exposure doesn’t affect it) and in-sensitive to HF treatment (which results in a complete re-building of oxide layer).**
**Baking reduces BCS resistance by up to a factor of 2, increases residual resistance slightly! The increase in BCS resistance is related mostly to a decrease of mean free path, as given for instance by an increase of oxygen contamination. That implies that the first 100 nm in the bulk are in the “clean limit” before baking.**
**The decrease of mean free path was also measured by Giovati with the frequency shift method at 7-9K. There is an alternate theory for the change of BCS resistance according to the weak layer model.**
**Thermal maps indicate that there are hot spots during the Q-drop region. The hot spots disappear after baking.**

## The baking effect is gradual until a saturation is reached. Heating at a temperature beyond ~160°C does not produce a baking benefit. Oxypolishing, anodizing experiments further strengthen the case for a diffusion driven process because they show that a removal of ~100 nm restores the before baking condition (increase of BCS resistance, Q-drop),..etc. Heat treatment profiles which result in comparable (calculated) “oxygen profiles” produce more or less the same result (I.e. removal of Q-drop, decrease in BCS resistance) in the bulk indicating that there is a strong connection with oxygen content! Gigi’s oxygen diffusion model!

**The baking effect is gradual until a saturation is reached. Heating at a temperature beyond ~160°C does not produce a baking benefit. Oxypolishing, anodizing experiments further strengthen the case for a diffusion driven process because they show that a removal of ~100 nm restores the before baking condition (increase of BCS resistance, Q-drop),..etc. Heat treatment profiles which result in comparable (calculated) “oxygen profiles” produce more or less the same result (I.e. removal of Q-drop, decrease in BCS resistance) in the bulk indicating that there is a strong connection with oxygen content! Gigi’s oxygen diffusion model!**
**Also single crystal cavities have Q-drop and baking effect, albeit with higher Q-drop onset field (a trend which also appears from small grain to larger grain (post-purified) material). TE mode cavities also show Q-drop and baking effect, so it’s a magnetic field effect! Some cavities show no Q-drop, even without baking!**
**Bc3 measurements show an increase in Bc2 with baking – indicating an increase of disorder and contamination.**
**Frequency effect: Q-drop onset field appears to increase with frequency.**

## Q-DROP NOT EXPLAINED BY KNOWN SURFACE RESISTANCE CONTRIBUTIONS

## Q-DROP MODEL IWEAK SUPERCONDUCTING LAYER MODEL

**2-superconductor model, with the high purity bulk Nb being covered by a thin (~2 nm) layer of “low-grade” (contaminated) Nb.**
**The Q-drop arises when the low critical magnetic field  (WHICH?) is reached in the “weak” layer. **
**After baking the “weak layer” dissolves into the bulk. While this removes the Q-drop, it does so at the expense of a slight increase of surface resistance (?)**

**G. Ciovati – SRF05**

**Oxygen contamination model**

**Vortex penetration model**

## Q-DROP MODEL IIMAGNETIC FIELD ENHANCEMENT MODEL

**Grain edges quench at medium field because they reach the critical field earlier due to field enhancement effects; ****	**
**Field enhancement factor distribution consistent with Q-drop at ~20 MV/m needs to peak around 1.4, with narrow sigma; **
**Knobloch assumes that the quenched region does not grow (?)**

**J. Knobloch – SRF99**

**Vortex penetration model**

## Q-DROP MODEL IIIVORTEX PENETRATION MODEL

**Vortex penetration as a cause for Q-drop and quench; **
**Which type of vortex – A, J or “mixed”?**
**	**
**Estimate of power loss: ????? consistent with experimental Q-drop?????; **
**Field of first vortex entry?**
**Material non-uniformity?**
**Disentangling topology from “chemical” effects?**

**M. Rabinowitz – J. Appl. Phys 42-88 1971 **

**Currently the preferred model!**

## HOT SPOT MODEL

**Hot spot model: small (********m) “defects” can explain Q-drop (growth by thermal diffusion); The basis of this model is surface non-uniformity!**

**A. Gurevich – SRF05**

**G. Ciovati – SRF05**

## SUMMARY Q-DROP MODELS

_**Weak-layer model**_
**    ****uniform layer effect vs hot spot?**
**    ****nm-layer – proximity coupling?**
**    ****Safa model ultimately needs VPM**
_**Vortex Penetration Model**_
**    ****flux penetration below bulk Hc (or Hsh)**
**    ****due to: **
** ****weak/contam. layer or weak spots (GBs?)**
** ****topology (field enhancement)**
_**MFE model**_
**    ****good for explaining BCP-EP difference!**
**    ****Baking effect?**
**    ****Maybe a “second-order” effect?**

**Model hierarchy??**

_**Hot spot model**_
**    ****small (********m) “defects” can explain Q-drop**
**    ****???? **

_**Reduction of BCS resistance**_
_**Oxygen Diffusion**_

## FUNDAMENTAL SRF R&D PROPOSAL I

**Chasing hot spots: cut out hot spot and submit to analysis;**

## FUNDAMENTAL SRF R&D PROPOSAL II

******      ****What is the difference between hot-spot and normal spot samples?**
******      ****What is the interstitial oxygen concentration profile in the first 100 nm from       **
**           ****the surface in the hot-spot and normal samples?**
******      ****What is the thickness of the oxide layer in the hot-spot and normal samples? **
******      ****What is the exact chemical composition of the hot-spots and normal samples **
**           – ****are there metallic inclusions in either of them?**
******      ****What is the magnetic field of first flux penetration in both types of samples?**
******      ****What is the bulk Bc1, Bc2 in both sets of samples? What is the surface Bc2 **
**           ****and Bc3 in both types of samples?**
******      ****What is the critical RF field of both types of samples?**
******      ****What is the RF surface resistance in both types of samples?**
******      ****What is the electron mean free path in the surface layer of both types of **
**           ****samples?**

## FUNDAMENTAL SRF R&D PROPOSAL III

**Chemical analysis: **
**	****3D AP, XPS, other chemical analysis**
**Superconducting Properties:**
**	****STM (gap-profiles), magneto-optics (magnetic properties), low power ****	****micro-wave microscopy, high power RF surface resistance, transport**
**Topology**
**	****SQUID microscopy, profilometry**
**Thermal & Mechanical Properties**

## ANY OTHER IDEAS 

**FOR ANALYTICAL TESTS **
**OF HOT SPOTS??**