yulia trenikhina tem studies of niobium hydrides participants in superconducting niobium cavitiey...

ILLINOIS INSTITUTE OF TECHNOLOGY

TEM studies of cavity cutouts from EP niobium SRF cavities prepared by different treatments.

Yulia Trenikhina

SRF Workshop10/07/2014


Outline

Cutouts from Nb EP 120°C baked/not baked cavities (HFQS):•TEM diffraction: room and cryogenic T•Direct observation of Nb nanohydrides for the 1st time•High Resolution TEM: no oxidation along grain boundaries

Nitrogen doping for high Q0 (MFQS):

•Treatment characterization: Nb nitrides on the surface, nitrogen doping deeper.•TEM diffraction at room and cryogenic T: Nb hydrides precipitation is the cause?

Are Nb nanohydrides responsible for HFQS and MFQS?


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)Se

nsor

num

ber

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

Effect of 120°C on Q0


Cutout (d=11mm, t=3mm)

Origin of Hot and Cold cavity cutout

Hot spot: from EP cavity Cold spot: from EP+120°C

baked cavity

~10 µm~

3 µm

“useful near-surface area”

Cu grid

SEM of FIB sample


Room T Comparison of Hot and Cold spot

[113] [011]

[-111]

[001]

NED: Hot (not baked) and Cold (baked) spot at room T

Electron diffraction: only Nb at room TH in solid solution (α-phase)

~10 µm

~3

µm

“useful near-surface area”

Cu grid

SEM of FIB sample[100]


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_ _ _ _

Hot (not baked) spot at 94K 120°C baked stop at 94K

Nb+ε(Nb4H3)

Nb+β(NbH)

Nb +ε,β

Nb hydrides precipitation

Cryogenic T investigations of cavity cutouts

Nb

NO Nb hydrides precipitation

Nb

Diffraction mapping with low intensity beam


NED: Nb hydrides precipitation in all cutouts, amount and/or size of NbHx is different

44%-68% probed spots

26%-29%probed spots

Hot (not baked) at 94K

Cryogenic T investigations of cavity cutouts

Diffraction with brighter beam, better S/N

120°C baked stop at 94K


grain 1

grain 2

SEM image of GB

Grain boundary investigation

HRTEM: No visible oxide layer along GB

HRTEM

No evidence!

J. Halbritter, SRF 2001

MATERIAL SCIENCE OF Nb RF ACCELERATOR CAVITIES:WHERE DOWE STAND 2001?

J. HalbritterForschungszentrum Karlsruhe, Institut für Materialforschung I

Postfach 3640, 76021 Karlsruhe , Germany

AbstractThe rf losses, especially actual level and increase with

rf fields, limit most stringently the application ofsuperconducting rf cavities. This is due to the neededcooling power to be supplied locally to the high field re-gion causing rf breakdown. The rf losses are due to twosources based on different physics: dielectric rf lossesproportional to REE!2 and shielding current losses pro-portional to RHH||2. Material science wise intrinsic lossesRBCS are separate from extrinsic, rf residual losses Rres.The separation of Rres(T,f,H) from the BCS lossesRBCS(T,f,H) yields the quasi-exponential increases of theelectric surface resistance with the electric field E! per-pendicular to the surface "RE(E!) # exp (-c/E!) and thepower law increases of the magnetic surface impedanceswith the magnetic field H|| parallel to the surface "RH(H||)# (H||)2n (n = 1, 2. .). By Nb/Nb2O5-y interfaces of externaland internal surfaces RHres(T,f) and REres(f,E!) can beexplained quantitatively by localized states nL of Nb2O5-yin close exchange with extended states nm of Nb. Espe-cially, the Q-drop # 1/RE(E!) and its reduction by EP-and BCP-smoothening and by UHV anneal at T$100°Care well accounted for by interface tunnel exchange. TheUHV anneal not only reduces surface scattering and REbut also enforces the RBCS(T, 1.3 GHz, H < 10 mT)-dropand reduces RBCS(T, $ GHz, $ 10 mT) by more than afactor 2. The interrelations RBCS-drop, RE and RBCS withthe material science of Nb2O5-y/Nb interfaces, e.g., by EPor UHV anneal, will be elucidated.

1 INTRODUCTIONThe technology to produce Nb rf accelerator cavities

with gradients above Eacc $ 10 MeV/m and Qo (2K; 1GHz) $ 1010 is now in hand and delivered by industry, asthe result of more than 30 years intensive research and de-velopments started first at Stanford [1] and Karlsruhe [2].But still progress is needed to fulfill the ever growingdesire of, e.g., the high energy physics community [3].Highlights in the recent progress are discussed below.Progress usually starts with a name, like Q-decease or Q-drop, followed by quantification and then the decease canbe overcome. The progress to be reported in Nb cavitiyperformance is based on basic research carried through bygroups at CEBAF, DESY, INFM, KEK and SACLAY,discussed in Sects. 4 and 5.Where stands the materials science of superconducting

Nb rf cavities and surfaces at the moment? Firstly, Nbfree of inclusions, like big Ta- or NbOx-lumps, with a dcresistance ratio RRR > 200 is now available [3].

Secondly, high pressure (80 bar) water rinsing (HPR) [4]is able to reduce the dust on Nb surfaces sufficiently.Thirdly, we are left with intrinsic Nb corrosion yieldingafter electropolishing (EP) or buffered chemical polishing(BCP), followed in both cases by HPR, some inhomoge-neities, as sketched in Fig. 1.

1nm

NbC H -OHx y

H O-OH2 1nm

NbO (x 0.02)x $

Nb O2 5-y

NbO (x 1)x %

Fig. 1: Nb surface with crack corrosion by oxidation by Nb2O5volume expansion (factor 3). Nb2O5-y-NbOx weak links/segregates(y, x < 1) extend up to depths between 0.01 – 1/ 1-10 µm forgood – bad Nb quality and weak - strong oxidation [8].Embedded in the adsorbate layer of H2O/CxHyOH (& 2 nm)being chemisorbed by hydrogen bonds to NbOx(OH)y,adsorbate covered dust is found. This dust yields enhancedfield emission (EFE [7]) summarized in Sect. 3.1.

Crucial are the scales: Nb is coated by less than 0.5 nmNbOx(x %1) and by 1 – 3 nm Nb2O5-y covered with hydro-gen bonded H2O/CxHy (OH)z of similar thickness.Optimal superconducting Nb properties have to hold, atleast, in a penetration depth 'H(T<Tc/2) $ 40 nm. Anotherscale is the dimension of Cooper pairs (F $ 60 nm, wheremetallic defects in Nb have to be much smaller in size tosustain overall good Nb properties. Similarly on Nb2O5,dust and protrusions have to be reduced in amount andsize to get good electric field properties. This neededhomogeneity in and on Nb is achieved now yielding Eacc &30 MeV/m The scales of 1 - 10 nm for Nb cleanliness andhomogeneity have to be compared to accelerator lengthsof 0.1 -1 km, showing a reproducibility over 10 order inmagnitude being now achieved. This, by itself, is a bigachievement. But as we see from semiconductor industry,there is still more to come. As material scientist, I will notdwell on this but I want to outline old and new Nb resultsand understandings, where, in my view, progress, hasbeen made in the last years and will be made in the nextyears.

The 10th Workshop on RF Superconductivity, 2001, Tsukuba, Japan

292


Possible effect of 120°C bake

NbOx

Nb2O5

Nb

H

NbOx

Nb2O5

Nb

H-vacancycomplex

EP cavity EP cavity after 120˚C bake

Mild vacuum 120˚C bakeIntroduction of H-Vac complexes

Less/no NbHx precipitation

A. Romanenko, C.J. Edwardson, P.G. Coleman, and P.J. Simpson Appl. Phys. Lett. 10, 232601 (2013)B. Visentin, M.F. Bathe, V. Moineau, and P. Desgardin, Phys. Rev. ST Accel. Beams 13, 052002 (2010)

~40 nm


Standard state-of-the art preparation

Nitrogen doping => up to 4 times higher Q!

A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication)

1.3 GHz

Nitrogen doping: a breakthrough in Q0

This was the highest Q possible up to last year


LCLS-II spec

• Technology immediately adopted for SLAC

• 100+ single cell tests with high Qs

• 10s of 9-cell tests with the “production” protocol for LCLS-II

–We have 8 nine-cell cavities lined up for the first cryomodule at FNAL

T=2K

N-doping-production-ready


N-doping treatment

I. Reacting bulk niobium cavities with N2 gas (N2 p.p ~ 2x10-2 Torr) at 800°C in UHV furnace for ~20 min followed by 30 min with no N;

II. Material removal via electropolishing (EP) followed by high-pressure water rinsing (HPR).


XRD: hexagonal NbN0.5

120x103

115

110

105

100

95

cou

nts,

arb

.uni

ts

406 404 402 400 398 396 394 392 390binding energy, eV

treatments at 800Cº fit peak A peak B peak C

XPS N2 1s: ~20 at.% of N 150x103

100

50

0

cou

nts,

arb

.uni

ts

214 212 210 208 206 204 202 200 198binding energy, eV

treatment at 800Cº fit doublet A doublet B doublet C XPS Nb 3d:

mixture of NbNx, NbNxOy and Nb2O5

XRD, XPS, SEM: we have NbNx (β-NbN) after the 1st step

Nb samples processed parallel with cavitiesSEM of Nb surface after step I

Investigation of N treatment: step I


~2 µ

m

TEM low mag Nb [113] TEM low mag

NbN0.5

NbN0.5+ NbNx

TEM, NED: NbN0.5+NbNx within at least first 2 µm.Poor SRF performance after step I: Q~ 107

Pt protective layer Pt protective layer

Investigation of N treatment: step Isurface after step I TEM sample


Investigation of N treatment: step I

grainboundary

SEM image

TEM image TEM image

NbNx extend ~2µm along GB


TEM image

XRD, XPS, TEM: NO Nb nitrides after step II

Nb

Investigation of N treatment: step II

TEM image

Pt protective layer

Pt protective layer

Cutout from N treated cavity


Non-doped

Doped

Depth (um)

Interstitial N in NbNitrides

N depth profiles by SIMS

Set of N-doped samples using different temperatures and duration – comparison with the non-doped


SIMS on cutouts

40 ppm of N


N traps H at interstitials close to tetrahedral. No/less NbHx precipitation

• Pfieffer et. al. (J. Phys. F: Metal Phys., V.6(2), 1976);

• Rush et. al. (Europhys. Lett., 48(2), 187-193, 1999);

• Magerl (Phys. Rev. B, V.27(2), 1983)

• Baker et. al. (Acta Metallurgica, V.21, 1973);

• ...

Possible effect of N doping

SEND at 94K: NO Nb hydrides formation in cutouts from N-treated cavities, similar to baked cutout.


First direct cryogenic T observation of Nb nanohydrides in cutouts from EP baked/not baked cavities

• Size/distribution of NbHx define Q0

GB don’t appear to have an oxide layer

Understanding of N doping of Nb on microscopic material level• Possible scenario: Nitrogen traps hydrogen

Our data are inline with proposed model

Conclusions


Acknowledgments

UIUC MRL: Dr. J.Kwon, Prof. J.-M. Zuo, Dr. J. MabonFermilab: Dr. Anna Grassellino

Thank you for your attention!

yulia trenikhina tem studies of niobium hydrides participants in superconducting niobium cavitiey...

Science

c baked cavity

nb nitrides

nb 3x0

cold baked spot

ep cavity cold spot

nb nanohydrides responsible

c bakednot baked cavities

fg ep cavities