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RREVIEW ONEVIEW ON
QQ--DDROP ROP MMECHANISMECHANISM
Bernard VISENTIN
International Workshop on Thin FilmsInternational Workshop on Thin Films
9th - 12th October 2006
2Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Definition of Used ParametersDefinition of Used Parameters
Q0 Quality Factor (figure of merit) G Geometry Factor RS Surface Resistance
Eacc Accelerating Field,average electric field seen by particlecrossing cavity gap L
L
zacc dzEL
E0
1
Scycledissipated
stored
R
GQ
/0 2
C1-10 (O1) 1,3 GHz - BCP
1E+09
1E+10
1E+11
0 10 20 30Eacc (MV/m)
Q0
pas d' électronspas rayons X
limitation :- puissance RF- quench thermique
Baking : ~ 110 -120 °C / 2 days ( under UHV )
Annealing : ~ 800 °C - remove Hydrogen from Nb bulk
~ 1350 °C ( +Ti ) - remove Oxygen and improve thermal conductivity (bulk)
3Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Thin Film Cavities & Q-DropThin Film Cavities & Q-Drop
Wuppertal – Nb3Sn / Nb – 1.5 GHz
Vapor Deposition Technique
G. Müller et al.- EPAC (1996)
Saclay – Nb / Cu – 1.5 GHz
Magnetron Sputtering in Ar
P. Bosland et al.- ASC (1998)
( no field emission, no quench only RF power limitation )
CERN – Nb / Cu – 1.5 GHz
Magnetron Sputtering in Kr
V. Arbet-Engels et al.- NIMA (2001)
Advantages to use Thin Film Technology for SRF Cavities :Reduced Cost – New Superconducting Material (higher Tc & Hsh)
severe Q-drop limits High Gradient Performances Eacc < 25 MV/m
Eacc=15 MV/m
4Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
1E+09
1E+10
1E+11
0 10 20 30 40
Eacc (V/m)
Qo
S110 - Thin Film Nb/Cu @ 1,5 GHz ............... ( scaled to 1,3 GHz )
C110 - Bulk Nb Cavity - BCP or EP - @ 1,3 GHz
PowerLimitationPower
Limitation
Thin Film & Bulk CavitiesThin Film & Bulk Cavitiesvery steep Q-drop exists on Bulk Cavities (BCP or EP)
5Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
very steep Q-drop exists on Bulk Cavities (BCP or EP)
Thin Film & Bulk CavitiesThin Film & Bulk Cavities
It can be cured by baking : limitations in Q0 and Eacc can be exceeded
reason why R&D has been more extended for Nb bulk cavities
1E+09
1E+10
1E+11
0 10 20 30 40
Eacc (V/m)
Qo
S110 - Thin Film Nb/Cu @ 1,5 GHz ............... ( scaled to 1,3 GHz )C110 - Bulk Nb Cavity - BCP or EP - @ 1,3 GHz
C110 - Buffered Chemical Polishing + Baking
C103 - ElectroPolishing + Baking
PowerLimitation
QuenchThermalQuench
PowerLimitation
B. Visentin et al .- EPAC (1998) & SRF (1999)
6Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Thin Film R Thin Film R & & D on CavitiesD on Cavities
R&D gave up now at CERN and at Saclay since 2001
but still continues in Europe (CARE program) and USA (JLab, Cornell)
IPJ Poland / INFN – Nb / Cu – 1.3 GHz
Cylindrical UHV Arc Discharge
magnetic filter –droplet)
J. Langner - CARE Report (2005)
JLab – Nb / Cu – 500 MHz
E-beam evaporation
+ ECR plasma (Nb ionization)
G. Wu - Argonne Workshop (2004) & SRF (2005)
7Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Application of Thin Film CavitiesApplication of Thin Film CavitiesCERN Technology
Large Size (< 700 MHz) with Thick Wall ( 6 mm )Specifications for Low Gradients ( < 15 MV/m)
LHC : 400 MHz
S 3rd H C : 1500 MHz
SOLEIL : 352 MHz
: 200 MHz - fact. - coll.
SLS
8Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
•Not a fundamental limitation : improve cleanness during process (substrate, sputtering,…)
•Granular Superconductor Theory : Josephson fluxon penetrationin weak links (grain boundaries oxidized sputter island)
•Thermal resistance at superconductor-substrate interface•Energy Gap dependence (H)
1E+09
1E+10
1E+11
0 10 20 30 40
Eacc (V/m)
Qo
PowerLimitationPower
Limitation
Lot of Theories and Experiments have been
performed on Nb Bulk cavities
Situation Review in bulk case( past + latest results )
Where do we standto understand Q-Drop origin ?
Hope to clear upthe Thin Film issue ???
J. Halbritter - Workshop of the Eloisatron Project (1999) B. Bonin - Supercond. Sci. Technol. 4,257 (1991)
V. Arbet - Engels et al.- NIMA (2001)
not enough data on Thin Film Cavities
Q-Drop Origin ( Thin Film )Q-Drop Origin ( Thin Film )
V. Palmieri - SRF (2005)
9Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Three different slopes in bulk Nb Cavity at
Low Medium High Field
Q-Drops for Bulk CavityQ-Drops for Bulk Cavity
Associated Theories
NbOX Clusters Surface Heating - RS(T) I.T.E , M.F.E. … etc. …
C1-16 ( EP )F1 - no baking
1E+09
1E+10
1E+11
0 10 20 30Eacc ( MV/m )
Q0
10Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
D1-22 ( EP )
1E+09
1E+10
1E+11
0 10 20 30Eacc ( MV/m )
Q0
B1 - EP
B2 - Baking
C1 - HF 50'
C2 - Baking
quenchfield
emission
RFpower
Theory : NbOx Clusters in Nb
localized states inside energy gap (Rs)
Baking : Q-Slope enhancement Additional Clusters ( O Diffusion )
HF Rinse (10%) : initial Q-slope restored Phenomenon localized
at Ox./Nb Interface
Nb2O5 + 10 HF → 2 H2NbOF5 + 3 H2O
C1-03 ( EP @ KEK - Tests @ Saclay)I1 = E5 + air exposure 46 months
+ 20' HF + HPR1E+09
1E+10
1E+11
0 10 20 30 40Eacc ( MV/m )
Q0
E5 : ( 120°C / 60 h )
I1 : HF chemistry ( 20 mn )
quench
Low Field Q-DropLow Field Q-DropJ. Halbritter – SRF Workshop (2001)
B. Visentin – Argonne Workshop (2004)
11Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Theory
quadratic dependence :
linear dependence : hysteresis losses due to Josephson fluxons in weak links (oxidation of grain boundaries)
Experimental Checking: quadratic and linear dependence at JLab & DESY (x-cells)
only quadratic dependence at Saclay (1-cell)
Medium Field Q-DropMedium Field Q-Drop
G. Ciovati - Argonne Workshop (2004)
J. Halbritter – 38th INFN Eloisatron Project Workshop (1999) & SRF (2001)
)()1(2
22
2
0 KBCSCC
PSS R
d
kTRB
B
BRR
PS bBaR
12Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
( Medium + High ) Field Q-Drop( Medium + High ) Field Q-Drop
)()()1(2
222
2
0 KBCSCC
PSS R
d
kTRB
TC
B
BRR
Thermal Model Refinement non linear correction due to RF pair breaking
Experimental Checking
Thermal Feedback Model
with linear or non linear RBCS
before and after baking.
P. Bauer et al. – SRF (2005)
A. Gurevich – Argonne Workshop (2004)
better fit with non linear model but not enough to explain
the high field Q-drop
13Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
High Field Q-Drop ( 6 theories )High Field Q-Drop ( 6 theories )
H. Safa - SRF (2001)
J. Halbritter – Eloisatron Workshop (1999)
B. Bonin - SRF (1995)
J. Knobloch - SRF (1999)
E. Haebel – TTF Meeting (1998)
A. Didenko – EPAC (1996)
Diffusion (O, Imp.) : “ Interface Tunnel Exchange ”
“ Bad Superconducting Layer
”
“ Granular Superconductivity ”
Surface Roughness : “ Magnetic Field Enhancement
”
High Field (T, Hpeak) : “ Thermal Feedback ”
“ Energy Gap Dependence
H”
14Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
High Field Q-Drop ( 6 theories )High Field Q-Drop ( 6 theories )
H. Safa - SRF (2001)
J. Halbritter – Eloisatron Workshop (1999)
B. Bonin - SRF (1995)
J. Knobloch - SRF (1999)
E. Haebel – TTF Meeting (1998)
A. Didenko – EPAC (1996)
Diffusion (O, Imp.) : “ Interface Tunnel Exchange ”
“ Bad Superconducting Layer
”
“ Granular Superconductivity ”
Surface Roughness : “ Magnetic Field Enhancement
”
High Field (T, HP) : “ Thermal Feedback ”
“ Energy Gap Dependence
H”
15Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
J. Halbritter - SRF (2001)
& IEEE Trans. on Appl. Supercond. 11, (2001)
RF field on
metallic surface
metalcleanfornegligibleZemissionelectroncausesE
seriesTaylorHHRRZHE
CHH ...1 22*0
Dielectric oxide layer on metal enhancement of ZE by I.T.E.
( localized states of Nb2O5-y and density of state of Nb )
with electron diffusion at NbOx - Nb2O5-y interface
I.T.E. quantitative description of Q-slope
E
C
E eR*
80:
:* ERRbyfittednallyconvention
factortenhancemenfieldelectricE
ITE reduction by :
•smoothening surface ( EP )
( * and E° )
•baking : Nb2O5-y vanished - better interface
( reduction of localised states )
valueonsetEatstarting
1E+09
1E+10
1E+11
0 10 20 30Eacc (MV/m)
Q0
C1-16 ( 1.3 GHz )
no electronsno X-rays
RFpower
RH
RE
E°
Interface Tunnel ExchangeInterface Tunnel Exchange
16Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Magnetic Field EnhancementMagnetic Field Enhancement
J. Knobloch - SRF (1999) microstructure on RF surface
( surface roughness - step height 10 m )
magnetic field enhancement
normal conducting region if
factor ( BCP )
Hm
Cm HH
5.26.1 m
K. Saïto - PAC (2003)
electromagnetic code + thermal simulation Q0(Eacc)
Q-slope origin
the most dissipative G.B. quench (equator)
EP : ( HC/m= 223 mT ) m=1
BCP : ( HC/m = 95 mT ) m=2.34
17Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
HF Rinse
- used to suppress field emission -
does not affect baking benefit
Experimental High LightsExperimental High Lights
B. Visentin - Argonne (2004) & SRF(2005)
1E+09
1E+10
1E+11
1E+12
0 10 20 30 40Eacc (MV/m)
Q0
D1-22 ( EP - Saclay A1 )C1-03 ( EP - KEK 9 )C1-15 ( BCP - Saclay I1 )C1-16 ( BCP - Saclay P1 )C1-10 ( BCP - Saclay N1 )
Q-slopes before baking ( BCP and EP cavities )
RF PowerLimit
C1-03 ( EP @ KEK - Tests @ Saclay)I1 = E5 + air exposure 46 months
+ 20' HF + HPR1E+09
1E+10
1E+11
0 10 20 30 40Eacc ( MV/m )
Q0
E5 : ( 120°C / 60 h )
I1 : HF chemistry ( 20 mn )
quench
Baking: Definitive Treatment
air exposure for 4 years - HPR
High Field Q-drop
Similarity between BCP and EP cavities (before baking) In contradiction with M.F.E.
theory
Baking: Universal Treatment
fine, large, single crystal, clad, shape
w / wo annealing @ 800 or 1350 °C
EP (>40 MV/m) or BCP chemistry In contradiction with I.T.E. theory
18Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Eacc > 40 MV/m
( TESLA like shape - fine grains)
BCP chemistry instead of EP chemistry
Some Exceptional OccurrencesSome Exceptional Occurrences
P. Kneisel - SRF (1995)
Baking Resistance
B. Visentin - EPAC (2006)
48 h baking
B. Visentin - EPAC (2002)
C1 19
1E+09
1E+10
1E+11
0 10 20 30Eacc ( MV/m )
Q0
B1 : BCP - no HTC1 : Sdt UHV Baking (110°C / 60h in cryostat)E1 : BCP after HT 800°C / 2hF1 : Fast Argont Baking + HF (145°C / 3h in stove)F2 : Sdt UHV Baking (110°C / 60h in cryostat)
T. Saeki – TTC Meeting @ KEK (2006)
W. Singer - SRF (2001)
Nb/Cu clad cavity
(after baking) Niobium Cavity C1-15
1E+09
1E+10
1E+11
1E+12
0 10 20 30 40
Eacc ( MV/m )
Q0
before baking
after baking @ 110 °C / 60 h
QuenchRF Pow erLimitation
19Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Theories / Experiments ConfrontationTheories / Experiments ConfrontationB. Visentin - SRF (2003) – updated at Argonne Workshop (2004)
Y / N = theory in agreement / contradiction with experimental observation N+ = undisputable disagreement with experiment
20Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
G. Eremeev, H. Padamsee - EPAC (2006)
Fine and large grain
cavities @ 1.5 GHz / BCP
Large grain (G.B.= white lines)
Fine grain
Global heating - Large spread out ( fine grain )
Hot spots for large grain cavity
Grain boundaries not involved in Q-drop
L. Lilje - SRF (1999)
Where do we stand now ?Where do we stand now ?
Fine grain
21Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Hot spots ( large grain cavity )
Reduced after baking
Q-slope restored by 40 V anodization
1E+09
1E+10
1E+11
0 5 10 15 20 25 30Eacc (MV/m)
Q0
Baseline 40 V anodization1st 120C 12h bake 2nd 120C 12h bake
T = 1.7 K
1
4
7
10
13
16
19
22
25
28
31
34
S1 S
3 S5 S
7 S9 S11 S
13 S15
0
0.02
0.04
0.06
0.08
0.1
0.12
T (K)
AzimuthBottom Iris
Top Iris
Equator
Q0 = 6.1 109
Eacc = 30.3 MV/mLarge-grain single-cell
1
4
7
10
13
16
19
22
25
28
31
34
S1 S
3 S5 S
7 S9 S1
1 S1
3 S1
5
0
0.03
0.06
0.09
0.12
0.15
0.18
T (K)
AzimuthBottom Iris
Top Iris
Equator
Q0 = 2.9 109
Eacc = 24 MV/m
Large Grain : Hot SpotsLarge Grain : Hot Spots
G. Ciovati - LINAC (2006)
22Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Hot Spot TheoryHot Spot Theory
A. Gurevich - SRF (2005)Localized sources of dissipation
caused by defects:
• grain boundaries (vortex penetration)
• precipitates
• non uniform surface oxide layer
Cavity surface with hotspots (dark grey)caused by smaller defects of radius r0 (black)
Hot spots consequences:
• non linear effect
• reduce breakdown magnetic field HC
• increase high field Q-drop
23Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Open Issue : Oxygen RoleOpen Issue : Oxygen RoleCorrelated problem to the Q-drop existence , why baking suppress it ?
Cavity Baking Interstitial Oxygen diffusion
),,(2/
0
tTC
eDD
Dt
xerfcCC
SRTE
S
a
Oxygen Diffusionin Niobium
0,0
0,2
0,4
0,6
0,8
1,0
0 20 40X ( nm )
C/CS
100 °C / 3h
100°C / 60h
analytic solutions2nd Fick's law
semi infinite solid : C(0,t) = CS
Improved modelwith decomposition of oxide layer
G. Ciovati - SRF (2005)S. Calatroni - SRF (2001)
0
0.2
0.4
0.6
0.8
1
1.2
100 120 140 160 180 200
c(0, 48 h)u(0, 48 h)v(0, 48 h)
Oxy
gen
conc
entr
atio
n (a
t. %
)T (°C)
From oxide decomposition
Initial interstitial oxygen
minimum of O for 140°C at x=0near of optimum baking parameter 120°C
24Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Oxygen Oxygen involvedinvolved in Q-slope in Q-slope
SIMS measurements on samples.
After baking,
O concentration is modified:
• increased for multiple grain
• reduced for large grain
J. Kaufman, H. Padamsee - SRF (2005)
Q-slope restored after oxide layer thickening
( anodization 40 V)
1E+09
1E+10
1E+11
0 5 10 15 20 25 30Eacc (MV/m)
Q0
Baseline 40 V anodization1st 120C 12h bake 2nd 120C 12h bake
T = 1.7 K
G. Ciovati - LINAC (2006)
Already observed at Cornell (30 V – 60 V)
H. Padamsee - Argonne (2004)
Some observations are in agreement…
25Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Oxygen Oxygen involvedinvolved in Q-slope ( cont. ) in Q-slope ( cont. )
Fast Baking :
Based on a equivalence in terms of interstitial oxygen diffusion:
110 °C / 60 h ↔ 145 °C / 3 h
B. Visentin - SRF (2005)
O Diffusion( Semi Infinite Solid )
C/CS = erfc ( … )
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
0 10 20 30 40 50 60
X ( nm )
C/CS
110°C/60h
145°C/3h
C1-09 ( BCP cavity )fast UHV baking
1E+09
1E+10
1E+11
1E+12
0 10 20 30Eacc ( MV/m )
Q0
V2 : baking 145 °C / 3 hours
V1 : no baking
quench
26Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
Oxygen Oxygen not involvednot involved in Q-slope in Q-slope
SIMS measurements
any noticeable difference before (A) and after UHV baking (8 & 3)
( multiple grain samples)
(only for baking at high temperature in air)
B. Visentin - EPAC (2006)
NbO/Nb
0,00
0,01
0,10
1,00
10,00
0 20 40 60 80 100
Abrasion Depth (nm)
Inte
nsi
ty (
a.u
.)
A Reference
8 UHV baking 110°C/60h
3 UHV Fast Baking 145°C/3h
5 Air Baking 145°C/3h
B. Visentin - Argonne (2004)
Q-slope not restored after formation of new
oxide layer ( HF rinse )
C1-03 ( EP @ KEK - Tests @ Saclay)I1 = E5 + air exposure 46 months
+ 20' HF + HPR1E+09
1E+10
1E+11
0 10 20 30 40Eacc ( MV/m )
Q0
E5 : ( 120°C / 60 h )
I1 : HF chemistry ( 20 mn )
quench
Or after anodization 5 V – oxide thickness x2 ( 10 nm )
H. Padamsee - Argonne (2004)
But controversy exists in experimental results arguing for the non involvement…
27Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
SIMS Analysis and BakingSIMS Analysis and BakingEfficient Q-slope improvement by Baking ( Q0 vs. Eacc )
if there is any noticeable O diffusion in Nb ( SIMS analyses )- UHV or Argon atmosphere : oxygen free - no diffusion from surface- T / (110 °C / 60 h ↔ 145 °C / 3 h) upper limit before diffusion from NbOx
C1-09 ( BCP cavity )fast baking with IR heaters
1E+09
1E+10
1E+11
1E+12
0 10 20 30Eacc(MV/m)
Q0
V1 - No BakingV2 - UHV Baking 145°C/3hY2 - Air Baking 145°C/3h
quench
C1 09 (BCP cavity)fast baking in stove
1E+09
1E+10
1E+11
1E+12
0 10 20 30Eacc(MV/m)
Q0
AE1-No BakingAE2-Argon Baking 145°C/3hAF1-Air Baking 145°C/3h quench
Nb2O+/Nb2
+
1E-04
1E-03
1E-02
1E-01
1E+00
1E+01
0 10 20 30 40 50
Abrasion Depth (nm)
Inte
nsit
y (a
.u.)
A - Reference3 - UHV Baking 145°C/3h5 - Air Baking 145°C/2h
Nb2O+/Nb2
+
1E-04
1E-03
1E-02
1E-01
1E+00
1E+01
0 10 20 30 40 50
Abrasion Depth (nm)
Inte
nsit
y (a
.u.)
A - Reference
D - Argon Baking 145°C/3h stove
C - Air Baking 145°C/3h stove
B. Visentin – TTC Meeting @ KEK (2006)
Debate is still open :
If oxygen is not involved, which is responsible ?
Strong correlations
between
RF results and
SIMS analyses
28Bernard Visentin International Workshop on Thin Films - Legnaro - October 2006
CONCLUSIONCONCLUSIONBulk Cavity :
no substrate (only Nb) - BCP (reproducibility) – (EP) - Baking (cure)
lot of experimental data (worldwide) and theories since 1998
not sufficient to understand HF Q-Drop origin
noose is tightening (theoretical explanations rejected) - in progress
Thin Film (Nb/Cu) : more difficult
Substrate + Thin Film
Lot of parameters to adjust or different process for coating
(pressure and gas, bias, atomic Nb, ionic assistance, Nb ions… )
Very important for the SRF future (new superconducting material)
Split-up between substrate and thin film issues is necessary
Nb substrate (BCP) - no matter what the absolute RF performances are -
Coating parameters Optimization in terms of relative RF performances.
RF tests on Nb substrate before coating same substrate
Annealing possible ( hydrogen, oxygen contributions to Q-drop )