Gianluigi Ciovati Jefferson Lab - CERNGianluigi Ciovati Jefferson Lab Sept. 20 th-25 , 2009, Berlin,...

High field Q-slope and the baking effect

Gianluigi Ciovati

Jefferson Lab

Sept. 20th-25th, 2009, Berlin, Germany

G. Ciovati, High field Q-slope and the baking effect, SRF’09, Berlin, Sept. 20th-25th, 2009

Outline

• Q-drop and baking effect: the essential facts

• Models to describe the cavity excitation curves

• “Oxygen pollution” model: new results against it

• Hot-spots due to magnetic vortices

• Recent results on Nb sample measurements

• New results on cavity heat treatments

• Conclusions


Q-drop and baking effect: essential facts

1E+09

1E+10

1E+11

0 20 40 60 80 100 120 140 160 180

Bp (mT)

Q0

before bake

120C/9h bake

T = 1.7 K

Quench

G. Ciovati, P. Kneisel and A. Gurevich, Phys. Rev. STAB 10 (2007) 062002

Q-drop

typical Q-drop onset field

Single cell

OC CEBAF shape

Large-grain Nb

ILC

equator

G. Eremeev et al., SRF’03, MoP18

Temperature map during Q-drop

iris

iris

• Q-drop non-uniform hot-spots due to high surface magnetic field

• Low-temperature (120 °C) baking reduced intensity of the hot-spots,

higher Q maintained at higher field

– Its effectiveness depends on the material/processing combination

G. Ciovati, High field Q-slope and the baking affect, SRF’09, Berlin, Sept. 20th-25th, 2009

Models to describe the cavity

excitation curves


Gurevich model

• The effect of “defects” with reduced

superconducting parameters is included in

the calculation of the cavity Rs

Hot-spot

Hot-spots

H(t)

TTm

T0

Ts

coolant

xd

• This non-linear Rs is used in the heat

balance equation

1

22

2

0

21 1 4

p

b

u e

Bg u g u u

B

0

0 2

0

0

1 1p

p b

Q eQ B

g B B

g related to the No. and

intensity of hot-spots

Q0(0)

Bb0

low-field Q0

quench field

Fit parameters:

A. Gurevich, Physica C 441 (2006) 38


Weingarten model

• ns0 defects per unit volume at the Nb

surface with lower critical field (B0)

<< Bc1(Nb)

• The size of the nc region increases

with field, above B0a

x

l

Energy loss

Energy gain

“weak” sc spot

Superconductor

2 2 2 2

3 2 200 0

4 1 2 31 1

3 2 3 4s

p p pnl

p s

p c c c

B B BBR B fn

B B B B l

Low-field Q-increase Medium and high-field Q-slopes

Fit parameters: ns0, B0, Rres, (Ginzbug-Landau parameter)

W. Weingarten, SRF’07, TuP16Poster TUPPO052


Models comparison with data

1.E+09

1.E+10

1.E+11

0 20 40 60 80 100 120

Bp (mT)

Q0

Data

Gurevich model

Weingarten model

Gurevich model Weingarten model

Q0(0) 5.51010 Rres (nW) 5

Bb0 (mT) 115 B0 (mT) 1.5

g 0.18 ns0 (1/m2) 21010

r2 0.960 1.767

r2 0.985

Fit parameters values

Both models give good agreement with experimental data

Data from: G. Ciovati, J. Appl. Phys. 96 (2004) 1591.


What are the “defects” invoked by

both models?


“Oxygen pollution” model

Oxide cluster

l

After baking

Nb2O5

Suboxides (NbO2, NbO)

Interstitial oxygen

l

Before baking• Surface analysis of Nb samples shows high concentrations of

interstitial oxygen (up to 10 at.%) at the Nb/oxide interface

• Interstitial oxygen reduces Tc and the Hc1

• The calculated O diffusion length at 120°C/48h is 40 nm

Magnetic vortices enter the surface at the reduced Hc1, their viscous motion dissipating energy

Interstitial oxygen is diluted during the 120°C baking, restoring the Hc1 value for pure Nb

Calculated oxygen concentration at

the metal/oxide interface as a

function of temperature after 48h

baking

G. Ciovati, Appl. Phys. Lett. 89 (2006) 022507


Oxygen pollution model: shortcomings

• The Q-drop did not improve after 400°C/2h “in-situ” baking, while O

diffuses beyond l1

• The Q-drop was not restored in a baked cavity after additional baking in 1

atm of pure oxygen, while higher O concentration was established at the

metal/oxide interface2

• Surface analysis of single-crystal Nb samples by X-ray scattering revealed

very limited O diffusion after baking at 145°C/5h3

The model cannot explain the following experimental results:

1 G. Eremeev, Ph.D. Dissertation, Cornell University, 20082 G. Ciovati, P. Kneisel and A. Gurevich, Phys. Rev. STAB 10 (2007) 0620023 M. Delheusy, Ph.D. Dissertation, University of Paris-Sud IX, 2008


Magnetic vortices as sources of “hot-spots”

• Oscillation of magnetic vortices, pinned near the Nb

surface during cool-down across Tc, cause localized

heating

• Periodic motion of vortices pushed in & out of the Nb

surface by strong RF field also cause localized

heating

Theoretical calculations show that:

A. Gurevich and G. Ciovati, Phys. Rev. B 77 (2008) 104501


High-field losses due to vortices

The local dissipated power is different in the case of isolated vortices vs. nearly-spaced

vortices

Local dissipation due to vortex motion produces a long-range hot-spot

Overheating due to vortex motion

0 0( 0, ) , coth4

s BCS

yR x y R T y r

d

d: wall thickness

1

6

11

16

21

26

-2 -1 0 1 2

y/dR

s/R

BC

S

r0

local dissipated power

A. Gurevich and G. Ciovati, Phys. Rev. B 77 (2008) 104501


Eliminating vortex hotspots by thermal gradients

• Thermal force acting on the vortex:

• The condition fT > JcF0 gives the critical gradient which can depin vortices:

Taking Bc1 = 0.17T, Jc = 1kA/cm2 and T = 2 K for clean Nb yields |T|c 1.5 K/mm

2

0

1 12

c c c

cc c

J J TT

H T B T

10* , * c

T

Hf s T T s

T

Vortices in Nb may be moved by moderate thermal gradients

Any change of thermal maps after applying local heaters indicate that some of the hot-spots are due to pinned vortices

A. Gurevich, talk TU104 at SRF’07 Workshop, http://web5.pku.edu.cn/srf2007/home.html


Baseline

Experimental results

T=2.0K

1E+09

1E+10

0 20 40 60 80 100 120

Q0

Bp (mT)

baseline After heating

Q-drop, no field

emission Quench

Thermometry board Heaters providing |T|0.5 K/mm

After heating

• heaters location

0

2

4

6

8

10

12

14

16

18

0 0.05 0.1 0.15

Sen

sor

No

.DT (K)

Baseline After heating

G. Ciovati and A. Gurevich, Phys. Rev. STAB 11 (2008) 122001


What we learned from the experiments

• Small No. of heaters, small area of the cavity surface is affected

• Heat supplied was marginally sufficient to displace vortices

(estimated 1.5 K/mm at 2 K, only applied 0.5 K/mm)

• No drastic change in Q0(Bp) performance

• Changes in T-maps indicate re-distribution of

vortices over larger area

A stronger thermal gradient directed outward could push vortices deeper into the bulk


Microwave Studio simulation done by J. Mondal

Optical

fiber

½ cell TESLA cavity,

TE011 mode, f=3.3GHz

• Hot-spots will be identified by an array of thermometers on flat plate

• Laser will be scanned over hot-spot locations to push pinned vortices into the bulk

• Laser Scanning Microscopy to identify “lossy” regions with m-space resolution can be done “in-situ”

with the same apparatus

1 W, =0.5mm, 532 nm laser beam

ANSYS Simulation done by G. Cheng

Nb, 3 mm thick

Vacuum

2K, LHe

Laser shining on the

inner cavity surfaceHigher T

Push pinned vortices deeper into the bulk

Local laser heating to move pinned vortices

Talk TUOBAU02


What impurities or defects in Nb

pin vortices or favors their entry

into the surface?

• By now, I may have convinced you that a plausible cause of the hot-spots are magnetic vortices…

Recent results on Nb sample measurements…


Surface analysis of “hot-spot” samples

Samples from regions of high and low RF losses were cut from single cell

cavities and examined with a variety of surface analytical methods.

No differences were found in terms of:

• roughness

• oxide structure

• crystalline orientation

It was found that “hot-spot” samples have a higher density of crystaldefects (i.e. vacancies, dislocations) than “cold” samples

A. Romanenko, Ph.D. Dissertation, Cornell University, 2009

Hot Cold

Local misorientation angle

0° 1° 2°

More in the

next talk…


Possible role of surface HydrogenNRA hydrogen depth profiling on Nb samples

0

0.4

0.8

1.2

1.6

2

2.4

2.8

3.2

3.6

4

-0.01 0.01 0.03 0.05 0.07 0.09 0.11

Depth [m]

H c

on

c [

10

22/c

m3]

#2 #3 - 90C 48hr bake

#4 #5 - 105C 48hr bake

#6 #9 - 120C 48hr bake

• High concentration of interstitial H

has been measured at the metal/oxide

interface of Nb samples after BCP

treatment and is reduced by the low

temperature baking

Before bake

After bake

• Thermal desorption studies showed two hydrogen desorption peaks at 130 °C

and 198 °C, interpreted as hydrogen desorption from surface and subsurface sites1

• Measurements by Positron Annihilation Spectroscopy (PAS) show that the

defect density (vacancies) increases with hydrogen concentration in Nb samples2

• Hydrogen affects the magnetic behavior of Nb by lowering the magnetic

susceptibility for increasing H concentration3

G. Ciovati, J. Appl. Phys. 96 (2004) 1591.

1A.L. Cabrera and J. Espinosa-Gangas, J. Material Research 17 (2002) 2698.2J. Cizek et al., Phys. Rev. B 69 (2004) 224106.

3U. Kobler and J.-M. Welter, J. Less Common Metals 84 (1982) 225.


Passivation of the Nb surface

• Heat-treat the cavity under vacuum at high temperature ( 800°C, 3h) to

reduce lattice defects, hydrogen content and oxide layer thickness

• Passivate the Nb surface by growing a thin nitride layer (10nm thick) at

an intermediate temperature (400°C) to prevent re-absorption of hydrogen

and oxygen during cool-down and water rinses

• The low-temperature baking can be easily added to the process

• Do not apply any chemical etching afterwards!

How can we improve the Nb surface to reduce pinning and maintain a high surface barrier to flux penetration?

This concept had been discussed already in 1971 (P. B. Wilson, “Theory and suggested experimental procedure for the

production of a nitride layer on niobium”, SLAC-TN-71-7, (1971)


Heat treatment process

Cart with N2

bottle and

needle valve

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

3/24/2009

9:07

3/24/2009

11:31

3/24/2009

13:55

3/24/2009

16:19

3/24/2009

18:43

3/24/2009

21:07

Date/Time

Pa

rtia

l P

res

su

re (

mb

ar)

0

100

200

300

400

500

600

700

800

900

Te

mp

era

ture

(°C

)

Hydrogen

Oxygen

Water

Nitrogen

Carbon dioxide

Temperature

• 800°C/3h, admit N2 (10-5 mbar) at

400°C/20min, cooldown

• Nb samples were treated with the

cavities and depth profiling of the

impurities was done at NCSU

2 orders of magnitude lower hydrogen content

No nitride layer was formed

Poster TUPPO061


1E+09

1E+10

0 20 40 60 80 100 120 140 160 180

Q0

Bp (mT)

AES001 Fine-Grain Nb ILC single cell

Baseline (EP)

Heat treated (without Nitrogen)

120C/24h bakeQuench

T=2.0Kf = 1.3 GHz

MP

1E+09

1E+10

0 20 40 60 80 100 120 140

Q0

Bp (mT)

Ningxia Large Grain Nb CEBAF single cell

Baseline (BCP)

Heat treated (with Nitrogen)

120C/12h bakeQuench

T=1.7Kf = 1.5 GHz

Some FE,Quench

Initial results

• Although we were unable to produce the thin nitride layer in the present

“production” furnace an improvement of the Q0 (40%) and Bp,onset (15%)

was achieved.

Eacc(ILC) 32 MV/m Eacc(ILC) 42 MV/m

• A new “cleaner”, more flexible furnace is currently being developed to fully

explore the possible advantages of Nb surface passivation


Conclusions

• Cavity Q0(Bp) curves are well described by modelsconsidering anomalous losses due to defects of “general”nature

• Theoretical calculations and initial experiments suggestthat hot-spots are related to magnetic vortices

– New experiments with local laser heating are plannedto confirm this

• Not clear which impurity/defect favors vortexpinning/entry

– Hydrogen induced vacancy clusters?

– Cavity surface passivation to reduce surface defectsand impurities will be explored with a new furnace


Acknowledgements

Thanks to my collaborators:G. Myneni, P. Kneisel, K. Jordan, Jefferson Lab

A. Gurevich, Florida State University

F. Stevie, North Carolina State University

S. Anlage, University of Maryland

THANK YOU FOR YOUR ATTENTION

Gianluigi Ciovati Jefferson Lab - CERNGianluigi Ciovati Jefferson Lab Sept. 20 th-25 , 2009, Berlin,...

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