Yulia Trenikhina SRF-15

Yulia TrenikhinaSRF 2015Whistler, Canada09/16/2015

Nanostructure of the penetration depth in Nb cavities: debunking the myths and new findings

Yulia Trenikhina SRF’15 Whistler, Canada2

50 100 150 200 250 300 350

2

4

6

8

10

12

14

16EP + 120C baking, Bpeak = 119 mT

Angle (deg)

Sens

or n

umbe

r

1016254063100159252400

∆T (mK)

3X0-10

50 100 150 200 250 300 350

2

4

6

8

10

12

14

16Electropolished, Bpeak = 119 mT

Angle (deg)

Sens

or n

umbe

r

1016254063100159252400

∆T (mK)

310-10

50 100 150

1

10

100

Nb 310-10 Nb 3X0-10

∆T (m

K)

B (mT)

50 100 150109

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:EPEP+800C bake+BCPEP+800C bake

EPEP+120C

Yulia Trenikhina SRF’15 Whistler, Canada

Cavity diagnostics: EP vs. EP+120C baked

3

We used three different elliptical cavities made of highpurity niobium for our studies: two fine grain (!50 lm)cavities of TESLA shape24 with the fundamental TM010

mode at the resonant frequency f0 ¼ 1:3GHz, and one largegrain (!10 cm) cavity of Mark-III shape with f0 ¼ 1:5GHz.Fine grain cavities were electropolished for #120 lm mate-rial removal, and one of the cavities was additionally bakedunder vacuum at 120 $C for 48 h. The large grain cavitywas chemically polished (BCP) instead of electropolishing.Before rf measurements all cavities were high pressurewater rinsed, which is a standard treatment to avoidfield emission caused by microparticles. Using standardphase-lock techniques,25 the intrinsic quality factor

Q0 / 1= !Rs!Rs ¼

xl0ÐVH2dVÐ

ARsðHÞH2dA

" #was measured for each of

the cavities at 2 K as a function of the peak surface mag-netic field Bpeak. Fig. 1(a) shows corresponding excitationcurves Q0ðBpeakÞ for both fine grain cavities. The Q0ðBpeakÞ

curve for the large grain cavity was similar to the unbakedfine grain cavity.

A custom-built temperature mapping system similar to

that described in Ref. 26 was attached to the outside of the

cavity walls during the measurements. Spatial maps of the

temperature difference DT ¼ Ton ' Toff with and without rf

power in the cavity were obtained at different rf field levels.

Such DT is proportional to the local power dissipation

Pc / RsðHÞH2, where RsðHÞ is the local surface resistance

and H is the local magnetic field. In Fig. 1(f), the magnitude

of the local magnetic field at each sensor location and char-

acteristic temperature maps are shown for baked and

unbaked fine grain cavities at the same Bpeak ¼ 119mT.

Individual DTðBÞ for samples 1-EP and 3-EP-MB circled on

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 for4-BCP would be expected to be the same. All mild baked

TABLE I. List of cutout samples investigated with VEPAS (EP, electropolishing; BCP, buffered chemical polishing; MB ¼ mild baking ' vacuum bake at120 $C for 48 h).

Grain size Cavity/sample treatment Sample name Microwave dissipation

50lm EP 120lm 1-EP Strong increase from !100mT (Fig. 1(b))

50lm Nb-310-10 þMB 1-EP-MB Not measured

50lm EP 120lm 2-EP Mild increase from !100mT

50lm Nb-240-10 þMB 2-EP-MB Not measured

50lm EP 120lm þ MB 3-EP-MB Low up to 160mT (Fig. 1(b))

50lm EP 120lm þ BCP 50lm 4-BCP Not measured

50lm EP 120lm þ BCP 50lm þMB 5-BCP-MB Not measured

!10 cm BCP 120lm 6-BCP Strong increase from !100mT

!10 cm BCP 120lm þ MB 7-BCP-MB Not measured

FIG. 1. (a) Results of Q0ðBpeakÞ measurements on fine grain electropolished cavities before and after mild baking; (b) individual DTðBÞ for samples 1-EP and3-EP-MB; (c) temperature mapping boards attached to the outside cavity walls; three boards are removed for demonstration purposes; (d) single individualboard with sensor numbering; (e) schematic showing how angle is measured around the rotational cavity axis; 36 boards are uniformly spaced with 10$ separa-tion; (f) local magnetic field B at sensor locations and unfolded temperature maps of DT / Rs for unbaked and baked fine grain electropolished cavities atBpeak ¼ 119mT.

232601-2 Romanenko et al. Appl. Phys. Lett. 102, 232601 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.126.32.13 On: Fri, 07 Mar 2014 02:54:31

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 2-5nmNbOx 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

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Nb hydrideprecipitation

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-nHcomplex

EP cavity 120˚C-baked EP cavity

Less/no NbHx precipitation!

~50 nm

Yulia Trenikhina SRF’15 Whistler, Canada7

30

NUANCE Center*exploring the inner space

TEM Sample Preparation with FIB @ Ben Myers - 2009

Lift-Out (4 of 4)L

0°

TiltL

Raise Omniprobe

~10�mL

Retract Pt GIS needleL

Send Omniprobe

to Eucentric

HighL

Retract Omniprobe

FIB

FIB

37

NUANCE Center*exploring the inner space

TEM Sample Preparation with FIB @ Ben Myers - 2009

Cleaning (1 of 2)J

Tilt into sample +/-

3-5°J

Application: SiJ

Shape: RectangleJ

Size: ~10�m (X) x ~2�m (Y)J

5kV, ~46pAJ

~2-5min per side

+/-

FIB -

~52+5°

tilt FIB -

~52-5°

tilt

SEM

22

NUANCE Center)exploring the inner space

TEM Sample Preparation with FIB ? Ben Myers - 2009

E-beam PtE

0°

Tilt

E

Application: Pt e-DepE

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

~3 μ

m

SEM images during the FIB sample preparation

TEM sample preparation: focused ion beamC

u grid

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 TH 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 precipitationAnnealing 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:EPEP+800C bake+BCPEP+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 GBPt layer

Nb2O5

HRTEM image of GB


EELS study of Nb oxides in EP and EP+120C baked cavities

14

120x103

100

80

60

40

20

0

cou

nts,

arb

. uni

ts

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

EP1

23

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!

Documents

Yulia Trenikhina SRF-15