Yulia Trenikhina SRF-15

Yulia Trenikhina
SRF 2015
Whistler, Canada
09/16/2015
Nanostructure of the penetration depth
in Nb cavities: debunking the myths
and new findings

Yulia Trenikhina SRF’15 Whistler, Canada2
16
EP + 120C ba
50 100 150
2
4
6
8
10
12
14
16
Electropolish
Angl
Sensornumber
50 100 150
109
1010
1011
FG
FGB
Q0
Bpeak
(mT)
(a)
(b)
(c)
(d)
HFQS elimination in FG
EP cavities after 120°C
bake
HFQS-producing treatments
High Field
Q-slope
HFQS recipes:
EP
EP+800C bake+BCP
EP+800C bake
EP
EP+120C

Yulia Trenikhina SRF’15 Whistler, Canada
Cavity diagnostics: EP vs. EP+120C baked
3
the peak surface mag-
orresponding excitation
cavities. The Q0ðBpeakÞ
the maps are shown in Fig. 1(b). Samples 2-EP and 6-BCP
had DTðBÞ curves of the shape similar to 1-EP, and that for
4-BCP would be expected to be the same. All mild baked
s on ﬁne grain electropolished cavities before and after mild baking; (b) individual DTðBÞ for samples 1-EP and
tached to the outside cavity walls; three boards are removed for demonstration purposes; (d) single individual
EP and 120C bakedEP (not baked)
T-maps @ 28 MV/m

EP vs. EP+120C baked: what is causing HFQS?
4
few monolayers of hydrocarbons
Nb2O5 2-5nm
NbOx x<2.5
Nb bulk
magnetic ﬁeld
penetration
depth @2K
<100 nm
Near-surface layer of SRF cavity
after typical processing
What in the near-surface is causing different dissipation character?

Hypothesis for the HFQS problem: what is the difference?
5
,&-$ *'<$ (.8'.,.%*')$ 1#'8)"$ &,$ ).3#$ .'$ )"#(#$ #';.-&'T
3#')(D$@&I#;#-B$-#%&32.'*).&'$*'=$20221#$'0%1#*).&'
).&'$ =.*8-*3$ ("&I'$ .'$ A.8D P4*6D$ !"
("&I($P$H"*(#(O
>FP
@*."2D" K.&2.03T@<=-&8#'$H"*(#$=.*8-*3$-#H&-)#=$2<$`*'%"#()#-$*'=$G.)-#$^FV_$4*6$*'=$5S hN$#/0.1.2-.03$H-#((0-#$*'=$H&)#')
"<=-.=#$H"*(#($2*(#=$&'$)"#$=*)*$&,$A-&33$*'=$#"'$^P9_$,&-$)"#$α *'=$α i β -#8.&'($*'=$`*'%"#()#-$*'=$G.)-#$^FV_$,&-$)"#$β i
Nb hydride
precipitation
Tool:
T-dependent TEM electron diffraction
phase characterization
Why to look at Nb-H system?
1. H concentration in near-surface
of cutouts > 10 at.%
2. ε, β are NOT superconducting
Difference in precipitation state of
Nb-H in the near-surface?

Hypothesis: Nb hydrides and HFQS
6
• Normal conducting Nb hydrides are SC by proximity effect up to Hcritical = HFQS
onset (A. Romanenko et.al., Supercond. Sci. Technol. 26, 035003 (2013));
• Major role of vacancies and vacancy-hydrogen complexes in the 120C baking
effect (A. Romanenko et.al., Appl. Phys. Lett. 10, 232601 (2013)).
NbOx
Nb2O5
Nb
H
NbOx
Nb2O5
Nb
V-nH
complex
EP cavity 120˚C-baked EP cavity
Less/no NbHx precipitation!
~50 nm

Yulia Trenikhina SRF’15 Whistler, Canada7
30TEM Sample Preparation with FIB @ Ben Myers - 2009
L 0° Tilt
L Raise Omniprobe ~10 m
L Retract Pt GIS needle
L Send Omniprobe to Eucentric High
L Retract Omniprobe
FIB
FIB
J Shape: Rectangle
J Size: ~10 m (X) x ~2 m (Y)
J 5kV, ~46pA
J ~2-5min per side
FIB -
S
22TEM Sample Preparation with FIB ? Ben Myers - 2009
E Application: Pt e-Dep
E Shape: Rectangle
E 15 m (X) x 1.5 m (Y) x 200nm (Z)
E 2-5kV, >1.4nA
E-beam dep at
0° Tilt (SEM)
E-beam dep at
52° Tilt (SEM)
SEM
~10 μm
~3μm
SEM images during the FIB sample preparation
TEM sample preparation: focused ion beam
Cugrid
Pt protective layer
Courtesy of B. Meyer

Nb near-surface at room T
8
[113] [011]
[-111]
EP and EP+120C baked at room T
SAD, NED, SEND: only Nb at room T
H in solid solution (α-phase)
[100]
Pt protective layer
Nb bulk
Nb oxide
200 nm
TEM: low mag overview

Nb near-surface at cryogenic T
9
βε
ε+β
βε
ε β β
ε ε ε+β ε+β
ε ε ε
ε
ε
ε
ε
ε ε
ε
ε
ε+β
ε+β
εε+β β
_ _ _ _
EP at 94K EP+120C baked at 94K
Nb+ε(Nb4H3)
Nb+β(NbH)
Nb+ε,β
Nb hydrides precipitation
Nb
NO Nb hydrides precipitation
Nb

Nb near-surface at cryogenic T continue
10
NED: Nb hydrides precipitation in all cutouts,
amount and/or size of Nb hydrides is different
EP at 94K EP+120C baked at 94K
Nb hydrides
Nb hydrides

Other treatments show Nb hydrides precipitation
Annealing at 800C˚ for 3h+BCP
β β β β
_ ε_ ε
β β β β
β β β β
β β β β
_ ε_ ε
_ ε_ ε
_ ε_ ε
_ ε_ ε
_ _ _ _
ε_ εε
_ ε ε ε
ε ε εε
ε ε εε
ε ε εε
ε ε εε
ε ε εε
ε ε εε
Annealing at 800C˚ for 3h

HFQS recipes:
EP
EP+800C bake+BCP
EP+800C bake
large size or large amount of
Nb hydrides at 94K
=
HFQS-free recipe:
EP+120C bake
Cause of HFQS
12
small/widely spread Nb
hydrides at 94K=

Grain boundaries in EP and EP+120C baked
13
grain 1 grain 2
SEM image of GB
Pt layer
Nb2O
5
HRTEM image of GB

EELS study of Nb oxides in EP and EP+120C baked cavities
14
120x10
3
100
80
60
40
20
0
counts,arb.units
395390385380375370365360355
energy loss, eV
region 1
region 2
region 3
region 4
Nb
Nb oxides
Pt layer
STEM Z-contrast image
Oxygen inward diffusion during the bake
EELS Nb M2,3 edge
EP+120C
EP
1
2
3
4

Nb hydrides in N-doped cavity cutouts and samples
15
• Nb hydrides do precipitate in N
doped cavities and samples!
Most of the probed area is affected by
hydride precipitation in quench spot.
• Do Nb hydrides cause quench in N
doped cavities?
Cryogenic temperature structural
characterization of non-dissipating spot
is needed.
More about N doped
cavities in MOPB055
Quench Spot at 94K
baked with N @800C + 5 μm EP
?

Conclusions
16
• Combination of T-dependent macro- and microscopic
characterization revealed precipitation of Nb hydrides in the state-
of-the-art SRF resonators for the 1st time;
• We showed that micro- and nanoscopic characterization gives a
valuable insight into mechanisms that governs the performance of
SC cavities;
• Routine use of micro- and nanoscopic characterization for the
development of new SC materials and treatments for the SRF
cavities.

Microscopic characterization for new materials/treatments
17
We are working on: Nb3Sn coating of Nb cavities with Cornell
Nb3Sn coated Nb cavity: SEM/EDX on cavity cutouts
More about Nb3Sn in Sam Posen’s
posters TUPB048, TUPB049 and
Yulia’s poster TUPB056.
Thanks for your attention!

Yulia Trenikhina SRF-15

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