Context and Summary of Baking Knowledge

• cavity is assembled, ready for RF test

• full equipped with antennas, thermal sensors

• inner part of the cavity is maintained under vacuum

• baking at 120°C for 2 days

“In-Situ” Baking

Ultra High Vacuum

120 °C / 48 hours

I n- S i tu Ba k i n gPu m p in gVe n t R e s i s t iv eH e a t e rT h e r m a lR e gu l a t io nP t 1 0 0T h e r m a lS e n so r sCe r n o x R Fc a b l e

i n Cr y o s ta t w i t h He1,E+09

1,E+10

1,E+11

0 5 10 15 20 25 30

Q0

Eacc ( MV/m )

C1-05 Chemical Etching (BCP)

Sans étuvage

Etuvage 110°C / 48h

1E+09

1E+10

1E+11

0 10 20 30 40 50

Q0

Eacc ( MV/m )

C1-03 - Electropolishing (EP)

J1 = EP / no baking

I1 = EP + UHV Baking 120°C/48h

quench

“Fast” Baking

Correspondence with 110°C / 60h

O Diffusion

( Semi Infinite Solid )

C/CS = erfc ( … )

0,4

0,5

0,6

0,7

0,8

0,9

1,0C/CS

110 °C / 60 h

145 °C / 3 h

160 °C / 1 h

UHV or Argon (1 atm.)

145 °C / 3 hours

Positron Annihilation Spectroscopy

PAS - DBS (Doppler broadening of positron annihilation radiation)

e+ → thermalisation (1ps)

→ diffusion

→ trapping in open-volume defects (voids, vacancies, VNb clusters , dislocation lines, grain boundaries…)

→ annihilation with e-→ 2γ (511 keV) → γ detection

γDoppler broadening of annihilation line, only due to e- momentum in propagation direction

(e+ momentum is negligible due to thermalisation)

sharpness parameter S = AS/A0 is determined by annihilations with valence electron (low momentum)

wing parameter W = AW/A0 is determined by annihilations of core electrons (high momentum) and gives

chemical information of the annihilation site

e+ is easily trapped in vacancy (repulsion by lattice ions) increases its lifetime and annihilation is preferentially

done with valence electron (lack of core electron), smaller momenta, smaller Doppler shift, narrow

curve, S↑ W↓

positron implantation profiles

Experimental Results

bernard.visentin@cea.fr

POSITRON ANNIHILATION SPECTROSCOPY

ON NIOBIUM SAMPLES

Bernard VISENTIN - CEA Saclay DSM / IRFU / SACM - 91191 Gif / Yvette - FRANCE

Marie France BARTHE, Virginie MOINEAU, Pierre DESGARDIN - CNRS / CEMHTI - 45071 Orléans cedex 2 - FRANCETUPPO047

Slow Positron Beam Facility in Orléans

Positron source (22Na)

Moderator (polycrystalline W foil - 4 µm): e+ (1.10-4@ 3eV)

Extraction (48 eV) – Acceleration (0 to 25 keV)

Beam diameter: 2 mm

γ Detection: high purity Ge detector

Reference sample: UO2Vacuum pressure: 5.10-8mbar

after baking : S ↑ & W↓ : vacancy number ↑

whole sample but particularly at the RF layer5 0µµ

0, 3 7 70, 3 7 90, 3 8 10, 3 8 30, 3 8 50, 3 8 70, 3 8 90, 3 9 1 0 5 1 0 1 5 2 0 2 5 3 0LowMomentumFractionS En e r g y ( k e V )S in g le Cr y s ta l S (E ) 1234

0, 0 6 30, 0 6 50, 0 6 70, 0 6 90, 0 7 10, 0 7 30, 0 7 50, 0 7 7 0 5 1 0 1 5 2 0 2 5 3 0Hi ghMomentumFractionW En e r g y ( k e V )S in g le Cr y s ta l W (E ) 1234

0, 4 1 00, 4 1 5ionS S Ca n dF G Sa m p le s 12optical µ−scope picture

of fine grain sample C

(after 120 µm BCP)

Makhovian profiles in Niobium(after thermalization & before diffusion)

0, 3 7 50, 3 7 70, 3 7 90, 3 8 10, 3 8 30, 3 8 50, 3 8 70, 3 8 9 0, 0 6 1 0, 0 6 6 0 , 0 7 1 0, 0 7 6LowMomentumFractionS H i g h M o m e n tu m F r a c t i o n WS in g le Cr y s ta l S (W ) 1234

with z0 = zm/Γ(3/2)

and A = 2.95 µg/cm2.keV-α , α = 1.7 , ρ N b = 8.57 g/cm3(A, α) material dependent – Monte Carlo calculation

P (z, E) = 2z/z02 exp-(z/z0)

2

0, 0 00, 0 10, 0 20, 0 30, 0 40, 050, 0 60, 070, 0 8 0 5 0 1 0 0 1 5 0P (z ,E ) D e p t h z ( n m )2 ke V3 ke V5 ke V8 ke V 11 01 0 01 0 0 0 0 5 1 0 1 5 2 0 2 5MeanIm plantationde pth (nm ) S lo w Po s it ro n E n e r g y ( k e V )z m = A Eα

/ρ

Abstract :Since the ‘baking effect” discovery, a part of the Saclay R&D dedicates one’s efforts to

understand this effect and its correlated question about the High Field Q-slope origin. Experiments on “fast

baking” in Oxygen free atmosphere and SIMS analyses have shown that interstitial oxygen diffusion cannot be

involved in baking phenomenon.

Presence of Niobium vacancy near the surface could contribute to explain such phenomenon. To explore

this way, we have performed experiments on niobium samples by means of positron annihilation radiation

Doppler broadening spectroscopy. For the first time, an increase of vacancy sites is disclosed at the Nb sample

surface after baking. We suggest that the dissociation of vacancy-hydrogen complex is at the origin of the

“baking effect”. This modification is observed in 100 nm depth under the sample surface, an area where the

superconducting RF layer is located.

We have experimentally shown that number of vacancies

increases after baking treatment on SC samples

( VNB-H complex dissociation )

Suggested scenario:

• Hydrogen ( aqueous species) is absorbed during chemistry; VNb-H complex creation

• RF losses: Q-drop (like Nb Hydrides in Q-disease)

• Above 100°C complex dissociation and H diffusion out of RF layer→ Q-drop removed

• Above 120°C Oxygen diffusion in material → VNb-O complex → Q-drop restored

• Additional BCP on fine grain samples B & G (to confirm C result)

• Baking of FG samples (C,B & G) to confirm baking results on SC

• Complementary experiments (ERDA) to follow hydrogen atom

Conclusion

Correspondence with 110°C / 60h

in terms of oxygen diffusion

0,0

0,1

0,2

0,3

0 10 20 30 40 50 60

X ( nm )semi infinite solid

C(0,t) = CS

→ analytic solutions

2

2

0x

CeD

t

C RTEA

∂

∂=

∂

∂2nd Fick's law

tTD

xerfcCtxC S

)(2/),( =

• cavity is treated after HPR in Clean Room

• Infra Red Heaters

• cavity under vacuum or filled with Argon (1 atm. )

• baking at 145°C for 3 hours

1E+09

1E+10

1E+11

0 10 20 30 40

Q0

Eacc ( MV/m )

1DE14 ( Electropolishing @ Henkel )

no Baking

145°C/3h Argon Baking @ Saclay

quench

Field

Emission

RF Power

limitation

1E+09

1E+10

1E+11

0 10 20 30 40 50

Q0

Eacc ( MV/m )

IS#8 ( Ichiro shape )

Electropolishing @ KEK

A1 - EP20um + EP3um + Degreasing

A3 - Argon Baking 145°C / 1+1 hRF Power Limitations

Oxygen Free Atmosphere needed( Ultra High Vacuum or Argon )

if not : RF degradation after Baking

1E+09

1E+10

1E+11

1E+12

0 10 20 30

Q0

Eacc ( MV/m )

C1-09 ( BCP cavity )

fast baking - 145 °C / 3 h

V1 - no baking

V2 - UHV baking

Y2 - Air bakingquench

1,E-04

1,E-03

1,E-02

1,E-01

1,E+00

1,E+01

0 20 40 60 80 100

Intensity (a.u.)

Abrasion Depth (nm)

NbO+/Nb+A Reference

8 UHV baking 110°C/60h

3 UHV Fast Baking 145°C/3h

5 Air Baking 145°C/3h

Similar results for plots of

NbO+, Nb2O+, Nb3O

+, Nb4O+…

• No O diffusion @ 10 nm scale – to be avoided

• O diffusion @ local scale ? (Nb Vacancy filling)

• Vacancy complex dissociation [V.H] ?

• Vacancy diffusion (stage III ) ?

SIMS analyses

on Nb samples

e+ → moderator (W) → 3 eV

→ different behaviour at the sample surface between single crystal & fine grain:

could be explained by a surface pollution for B & G

no hard chemistry after cutting → experiment on fine grain sample C

→ no vacancy migration

→ no vacancy filling (no oxygen diffusion at nm scale)

→ vacancy number increases

Suggestion: Hydrogen role - V.H complexes existence - dissociation after baking

with hydrogen diffusion ( D0=5.10-4 cm2/s → D=2.10-5 cm2/s @ 100°C )

empty vacancies → more traps for positrons

Comments on Results

Results for Fine Grain samples:

surface pollution (B and G samples)

more vacancy in C compared to SC samples

concentration in excess at interface

structures and grain boundaries

µµµµ

m 0, 3 7 50, 3 8 00, 3 8 50, 3 9 00, 3 9 50, 4 0 00, 4 0 5 0, 0 5 6 0, 0 6 1 0, 0 6 6 0, 0 7 1 0, 0 7 6LowMomentumFracti H ig h Mo m e n tu m Fr a c t i o n W ) 3 4CBG(after 120 µm BCP)

Niobium Samples

• single crystal (SC): beam hole off cut from large grain Nb sheet provided by X. Singer – Heraeus RRR 500

• fine grain (FG): Tokyo Denkai - RRR 200

diameter ~ 8 mm

thickness ~ 1 mm

2 0 0

µµµµ

m2 0 0

µµµµ

m2 0 0

µµµµ

m optical

µ−scope (x 50)

Sa m p le N io b iu m C he m is tr y C u t t in g An n e a l in g C he m i s t r y Ba k in g1- 2 S in g le C r ys ta lH e r a e u sR R R 5 0 0 n o Wa te r J e ta b r a s iv ega r n e ts a n d( 1 5 0µµµµ

m ) 8 0 0 ° C– 4 ho u r s1. 1 0- 7 m ba r B C P- 1 0 0

µµµµ

m n o3- 4 1 4 5 ° C / 2 ho u r sA r go n – 1 a t mB F in e G r a inTo k yoDe n ka iR R R 2 0 0 E P5 0 0µµµµ

m Wa te r J e ta b r a s iv ega r n e ts a n d( 1 5 0

µµµµ

m ) H F r in s e ( 1 0 %- 3 0 mn )be fo r e & a f te r8 0 0 ° C– 2 h 3 0 mn1. 1 0- 7 m ba r n o n oG n o 1 4 5 ° C / 2 h o u r sA r go n – 1 a t mC B C P- 1 2 0

µµµµ

m n o