srf limitations - universidad de guanajuato
TRANSCRIPT
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Jean Delayen
Center for Accelerator Science
Old Dominion University
and
Thomas Jefferson National Accelerator Facility
SRF LIMITATIONS
First Mexican Particle Accelerator School
Guanajuato 26 Sept – 3 Oct 2011
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Outline
• Residual resistance
• Multipacting
• Field emission
• Quench
• High-field Q-slope
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The Real World
11
10
9
8 0 25 50 MV/m
Accelerating Field
Residual losses
Multipacting
Field emission
Thermal breakdown
Quench
Ideal
RF Processing
10
10
10
10
Q
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Losses in SRF Cavities
• Different loss mechanism are associated with
different regions of the cavity surface
Electric field high at iris
Magnetic field high
at equator
Ep/Eacc ~ 2
Bp/Eacc ~ 4.2 mT/(MV/m)
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Characteristics of Residual Surface Resistance
• No strong temperature dependence
• No clear frequency dependence
• Not uniformly distributed (can be localized)
• Not reproducible
• Can be as low as 1 nΩ
• Usually between 5 and 30 nΩ
• Often reduced by UHV heat treatment above 800C
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Origin of Residual Surface Resistance
• Dielectric surface contaminants (gases, chemical
residues, dust, adsorbates)
• Normal conducting defects, inclusions
• Surface imperfections (cracks, scratches,
delaminations)
• Trapped magnetic flux
• Hydride precipitation
• Localized electron states in the oxide (photon
absorption)
Rres is typically 5-10 n at 1-1.5 GHz
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Trapped Magnetic Field
A parallel magnetic filed is expelled from a
superconductor.
What about a perpendicular magnetic field?
The magnetic field will be concentrated in normal cores where it is
equal to the critical field.
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Trapped Magnetic Field
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Trapped Magnetic Field
• Vortices are normal to the surface
• 100% flux trapping
• RF dissipation is due to the normal
conducting core, of resistance Rn
2
ires n
c
HR R
HHi
= residual DC
magnetic field
• For Nb:
• While a cavity goes through the superconducting transition, the ambient
magnetic filed cannot be more than a few mG.
• The earth’s magnetic shield must be effectively shielded.
• Thermoelectric currents can cause trapped magnetic field, especially in
cavities made of composite materials.
0.3resR to 1 n /mG around 1 GHzDepends on material treatment
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Trapped Magnetic Field
A fraction of the material will be in the normal state.
This will lead to an effective surface resistance
For Nb:
While a cavity goes through the superconducting transition, the ambient
magnetic filed cannot be more than a few mG.
The earth’s magnetic shield must be effectively shielded.
In cavities made of composite materials, thermoelectric currents can cause
trapped magnetic field.
/ cH H
/n cH H
0.5 to 1 n /mG around 1 GHzeff
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Trapped Magnetic Field
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Rres Due to Hydrides (Q-Disease)
• Cavities that remain at 70-150 K for several hours (or slow cool-down, < 1
K/min) experience a sharp increase of residual resistance
• More severe in cavities which have been heavily chemically etched
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Hydrogen: “Q-disease”
• H is readily absorbed into Nb
where the oxide layer is
removed (during chemical
etching or mechanical
grinding)
• H has high diffusion rate in
Nb, even at low
temperatures.
• H precipitates to form a
hydride phase with poor
superconducting properties:
Tc=2.8 K, Hc=60 G • At room temperature the required
concentration to form a hydride is
103-104 wppm
• At 150K it is < 10 wppm
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Cures for Q-disease
• Fast cool-down
• Maintain acid temperature below 20 C during BCP
• “Purge” H2 with N2 “blanket” and cover cathode with
Teflon cloth during EP
• “Degas” Nb in vacuum furnace at T > 600 C
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Q0 Record
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Multipacting
• No increase of Pt for increased Pi during MP
• Can induce quenches and trigger field emission
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Multipacting
Multipacting is characterized by an exponential growth in the number of
electrons in a cavity
Common problems of RF structures (Power couplers, NC cavities…)
Multipacting requires 2 conditions:
• Electron motion is periodic (resonance condition)
• Impact energy is such that secondary emission coefficient is >1
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One-Point Multipacting
One-point MP
Cyclotron frequency:
Resonance condition:
Cavity frequency ( g) = n x cyclotron
frequency
Possible MP barriers given by
n: MP
order
The impact energy scales as 2 2
2
g
e EK
m
+ SEY, (K), > 1 = MP
Empirical formula: 0
0.3Oe MHznH f
n
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Two-Point Multipacting
Empirical formula:
0
0.6Oe MHz
2 1nH f
n
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Two-Side Multipacting
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Multipacting Simulatiom
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Secondary Emission in Niobium
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Secondary Emission in Niobium
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MP in SRF Cavities
Early SRF cavity
geometries (1960s-’70s)
frequently limited by
multipacting, usually
at < 10 MV/m
“Near pill-box” shape
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MP in SRF Cavities
Electrons drift to equator
Electric field at equator is 0
MP electrons don’t gain energy
MP stops 350-MHz LEP-II cavity (CERN)
“Elliptical” cavity shape (1980s)
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Cures for Multipacting
• Cavity design
• Lower SEY: clean vacuum systems (low partial pressure
of hydrocarbons, hydrogen and water), Ar discharge
• RF Processing: lower SEY by e- bombardment (minutes
to several hours)
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Recent Examples of Multipacting
1.E+09
1.E+10
1.E+11
0 2 4 6 8 10 12 14 16 18 20
Qo
E (MV/m)
SNS HB54 Qo versus Eacc Multipacting limited at 16MV/m 5/16/08 cg 2008
MP
Field
emission
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Field Emission
• Characterized by an exponential drop of the Q0
• Associated with production of x-rays and emission of dark current
1.E+09
1.E+10
1.E+11
0 5 10 15 20
Qo
E (MV/m)
SNS HB54 Qo versus Eacc
0
0
0
0
1
10
100
1000
10000
0 5 10 15 20 25
mR
em
/hr)
E (MV/m)
SNS HTB 54 Radiation at top plate versus Eacc 5/16/08 cg
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DC Field Emission from Ideal Surface
Fowler-Nordheim model
6 2 9 3/21.54 10 6.83 10exp
: )
:
:
2 Current density (A/m
Electric field (V/m)
Work function (eV)
EJ
E
J
E
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Field Emission in RF Cavities
Acceleration of
electrons drains cavity
energy
Impacting electrons produce:
• line heating detected by
thermometry
• bremsstrahlung X rays
Foreign particulate
found at emission
site
FE in cavities occurs at fields that
are up to 1000 times lower than
predicted…
5/26 9 3/21.54 10 ( ) 6.83 10exp
:
:
Enhancement factor (10s to 100s)
Effective emitting surface
kE
JE
k
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How to Investigate Field Emission
FE in SC Cavity
Electron trajectory T-map
Dissection and analysis
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Dissection and SEM
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C, O, Na, In Al, Si
Stainless steel
Melted
Melted
Melted
Example of Field Emitters
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DC Field Emission Microscope
Carbon Carbon
Emission
Current
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Type of Emitters
Smooth nickel particles emit
less or emit at higher fields.
Ni
V
•Tip-on-tip model
explains why only 10%
of particles are emitters
for Epk < 200 MV/m.
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Tip-on-tip Model
• Smooth particles show little field emission
• Simple protrusions are not sufficient to explain the measured
enhancement factors
• Possible explanation: tip-on-tip (compounded enhancement)
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FE onset vs. Particulate Size
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Enhancement by Absorbates
Adsorbed atoms on the surface can enhance the tunneling of electrons
from the metal and increase field emission
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Intrinsic FE of Nb
Single-crystal Nb samples showed FE
onset higher than 1 GV/m.
The work function was obtained from the
I-V curves:
= 4.05 17% eV for Nb (111)
= 3.76 27% eV for Nb (100)
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Cures for Field Emission
• Prevention:
– Semiconductor grade acids and solvents
– High-Pressure Rinsing with ultra-pure water
– Clean-room assembly
– Simplified procedures and components for
assembly
– Clean vacuum systems (evacuation and venting
without re-contamination)
• Post-processing:
– Helium processing
– High Peak Power (HPP) processing
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Helium Processing
• Helium gas is introduced in the cavity at a pressure just
below breakdown (~10-5 torr)
• Cavity is operating at the highest field possible (in heavy
field emission regime)
• Duty cycle is adjusted to remain thermally stable
• Field emitted electrons ionized helium gas
• Helium ions stream back to emitting site
– Cleans surface contamination
– Sputters sharp protrusions
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Example of Field Emitters
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Helium Processing
Copper
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Helium Processing in CEBAF
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Helium Processing in CEBAF
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Practical Limitations (CEBAF)
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High Peak Power Processing
Power = 1.5 MW
Pulse Length = 250 us
= Q0
QL
QL = L
= Q0
QL
QL = L
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High Peak Power Processing
local melting leads to formation of a plasma
and finally to the explosion of the emitter
→ “star bursts” caused by the plasma
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High Peak Power Processing
For field emission free
Ep (pulsed) = 2 Ep (cw)
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High Peak Power Processing
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Issues with HPP
• Reduced Q0 after processing
• No experience with HPP above
Eacc = 30 MV/m in 9-cell cavities
• Very high power required
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Thermal Breakdown (Quench)
Localized heating
Hot area increases with field
At a certain field there is a thermal runaway, the field collapses
• sometimes displays a oscillator behavior
• sometimes settles at a lower value
• sometimes displays a hysteretic behavior
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Thermal Breakdown
Thermal breakdown occurs when the heat generated at the hot spot is
larger than that can be transferred to the helium bath causing T > Tc:
“quench” of the superconducting state
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Quench Mechanism
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Thermal Breakdown: Simple Model
Heat flow out through a spherical surface:
24 2 T
Tr Q
r
Breakdown field given by
(very approximately):
4 ( )T c btb
d d
T TH
r R
T: Thermal conductivity of Nb
Rd: Defect surface resistance
Tc: Critical temperature of Nb
Tb: Bath temperature
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Thermal Conductivity of Nb
RRR is the ratio of the resistivity at 300K and
4.2K (300 )
(4.2 )
KRRR
KRRR is related to the thermal conductivity
For Nb: ( 4.2 ) / 4 ( )-1 -1W. m . KT K RRR
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Note: Htb has nearly no dependence on TB < 2.1 K
Numerical Thermal Model Calculations
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Magneto-thermal Breakdown
• Quench location identified by T-mapping
• Morphology of quench site reproduced by replica technique
Local Magnetic Field Enhancement:
Quench when H > Hc
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,max c RF
acc
m p acc
r HE d
H E
r 1, reduction of the local critical field within
the penetration depth, due to impurities or
lattice imperfection
d, thermal stabilization parameter √
m > 1, geometric field enhancement factor
Magneto-thermal Breakdown: Maximum Eacc
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Type of Defects
Cu
0.1 – 1 mm size defects cause TB
Surface defects, holes can also cause TB
No foreign materials found
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Optical Inspection
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Defects Seen by Optical Inspection
DESY
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Defects Seen by Optical Inspection
DESY
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Cures for Quench
• Prevention: avoid the defects
– High-quality Nb sheets – Eddy-current scanning of Nb sheets
– Great care during cavity fabrication steps
• Post-treatment:
– Thermally stabilize defects by increasing the RRR
– Remove defects: local grinding
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Post-purification for Higher RRR
Disadvantages:
• > 50 m material removal necessary
after heat treatment
• Significant reduction of yield strength of
the Nb
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Post-purification
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Post-purification
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How High of RRR Value is Necessary?
100 m
diameter nc
defect
9-cell ILC cavities
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Defect Repair: Local Grinding
Polymond + water for grinding
Polymond: diamond particles in a resin
(particle size = 40 ~ 3 um)
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Defect Repair: Local Grinding
Before Grinding
Bump
After Grinding and EP 50um
Removed the bump
Quench at Eacc=20 MV/m
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Summary on Quench
• Big improvement in Cavity fabrication and treatment
less foreign materials found (at limitations <20MV/m only)
• Visual inspection systems are available
• Many irregularities in the cavity surface are found with this systems
during and after fabrication and treatment
pits and bumps
weld irregularities
• Often one defect limits the whole cavity
• Some correlations are found between defects and quench locations
at higher fields
But often no correlation between suspicious pits and bumps
and quench location
• At gradient limitations in the range >30 MV/m defects are often not
identified
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High-Field Q-Slope (“Q-drop”)
11
10
9
8 0 25 50 MV/m
Accelerating Field
Residual losses
Multipacting
Field emission
Thermal breakdown
Quench
Ideal
RF Processing
10
10
10
10
Q
Q-drop
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Q-drop and Baking
1E+09
1E+10
1E+11
0 4 8 12 16 20 24 28 32 36
Eacc [MV/m]
Q0
T=2K T=2K after 120C 48h bake
• The origin of the Q-drop is still unclear. Occurs for all Nb material/treatment
combinations
• The Q-drop recovers after UHV bake at 120 C/48h for certain
material/treatment combinations
1.5 GHz single-cell,
treated by EP
No X-rays
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Experimental Results on Q-drop 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
0.1
0.2
0.3
0.4
0.5
0.6
0.7
T (K)
AzimuthBottom
Iris
Top Iris
Equator
Q0 = 2.8 109
Bp = 110 mT
3D Parametric Surface
JLAB
DESY
CORNELL
• “Hot-spots” in the equator area (high-magnetic field)
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Experimental Results on Q-drop
1E+09
1E+10
1E+11
0 20 40 60 80 100 120 140 160
Bpeak [mT]
Q0
TE011
TM010
TE011 after 120C 30h bake
TM010 after 120C 30h bake
T = 2 K
• Q-drop and baking effect observed in both TM010 and TE011 modes. TE mode has
no surface electric field
Q-drop: high magnetic field phenomenon
Onset of Q-drop is higher for
• smooth surfaces
• reduced number of grain boundaries
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Baking works on cavities made of:
• Large-grain Nb (buffered chemical polished or electropolished)
• Fine-grain Nb, electropolished
Smooth surface, few grain boundaries
Smooth surface, many
grain boundaries
100
m
Baking does not work on cavities made of:
• Fine-grain Nb, buffered chemical polished
Rough surface, many grain boundaries
100
m
• Fine-grain Nb, post-purified, BCP
Smooth surface, fewer
grain boundaries
50 m
Baking: Material and Preparation Dependence
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Recipes necessary to overcome the Q-drop, depending on the starting material,
based on current data:
Large grain/Single
crystal niobium Fine grain niobium
BCP EP
Titanization
120 C/12 h
UHV bake 120 C/48 h
UHV bake
Recipe against Q-drop
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r32=Bc3/Bc2: depends on
bake temperature and duration
• Decrease of RBCS due to of l and of
energy gap
• The physics of the niobium surface
changes from CLEAN (l > 200 nm) to
DIRTY LIMIT (l 25 nm 0)
Baking Effects on Low-field Rs and Hc3
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Models of Q-drop & Baking
• Magnetic field enhancement
• Oxide losses
• Oxygen pollution
• Magnetic vortices
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Magnetic Field Enhancement Model
Local quenches at sharp steps (grain
boundaries) when mH > Hc
m: Field enhancement factor
AFM image of a grain
boundary edge
Q0(Bp) calculated assuming
Distribution function for m values
The additional power dissipated by a
quenched grain boundary is estimated to be
17 W/m J. Knobloch et al., Proc. of the 9th SRF Workshop, (1999), p.
77
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MFE Model: Shortcomings
• Single-crystal cavities have Q-drop
• Seamless cavities have Q-drop
• Low-temperature baking does not change the surface roughness
• Electropolished cavities have Q-drop, in spite of smoother surface
The model cannot explain the following
experimental results:
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Interface Tunnel Exchange Model
• Interface Tunnel Exchange (ITE) model
– Resonant energy absorption by quasiparticles
in localized states in the oxide layer
– Driven by electric field
Schematic representation of
the Nb surface Band structure at Nb-NbOx-Nb2O5-y interfaces
-ezE0
ezE0
0 * *
rEe z
1E+09
1E+10
1E+11
0 50 100
Bp (mT)Q
0
T=1.37K
ITE Fit10
9
1010
1011
0 0 0
2 2
2 2
0 0
1
2
p p pc E c E c Ec E c E c EE
s
p p
c c c cR b e e e e e e
E E E E
J. Halbritter et al., IEEE Trans. Appl. Supercond. 11 (2001) p.
1864
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ITE Model: Shortcomings
• The baking effect is stable after re-oxidation
• The Q-drop was observed in the TE011 mode (only magnetic field on the surface)
• The Q-drop is re-established in a baked cavity only after growing an oxide 80 nm thick by anodization
The model cannot explain the following
experimental results:
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Oxygen Pollution Model
Oxide cluster
After baking
Nb2O5
Suboxides (NbO2, NbO)
Interstitial oxygen
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
G. Ciovati, Appl. Phys. Lett. 89 (2006) 022507
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
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• The Q-drop did not improve after 400 C/2h “in-situ” baking, while O diffuses beyond
• 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 interface
• Surface analysis of single-crystal Nb samples by X-ray scattering revealed very limited O diffusion after baking at 145 C/5h
The model cannot explain the following experimental results:
Oxygen Pollution Model: Shortcomings
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Fluxons as Source of Hot-Spots
• Motion of magnetic vortices, pinned in Nb during cool-
down across Tc, cause localized heating
The small, local heating due to vortex motion is amplified
by RBCS, causing cm-size hot-spots
• Periodic motion of vortices pushed in & out of the Nb
surface by strong RF field also cause localized heating
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• The effect of “defects” with reduced
superconducting parameters is included
in the calculation of the cavity Rs
Hot-
spot
Hot-spots
H(t)
T Tm
T0 Ts
coolant
x d • 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
Thermal Feedback with Hot-Spots Model
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Q-drop: Recent Samples Results
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 crystal
defects (i.e. vacancies, dislocations) than “cold” samples
Hot Cold
Local misorientation
angle
0 1 2
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Q-drop: Recent Samples Results
• Zero-bias conductance peak: presence of dissipative pair-breaking layers on the
cavity surface
• Possible source: magnetic impurities (Nb2O5- ?)