Quality requirements and control of high purityniobium for superconducting RF cavities

W. Singer *, A. Brinkmann, D. Proch, X. Singer

Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany

Abstract

Niobium is a mainly used material for the fabrication of superconducting accelerating RF cavities. The proposed

eþe� linear collider TESLA demands about 500 tons of niobium. High purity Nb should reach the TESLA specifi-

cation. The required high thermal conductivity can be additionally improved by the post-purification. The quality

control includes the residual resistivity ratio measurement, microstructure analysis, analysis of interstitial and metallic

impurities, hardness measurement, tensile test, examination of the surface roughness and search for clusters. An eddy

current scanning system with rotating table is applied for diagnostic of cracks and foreign material inclusions in nio-

bium sheets. 100% of sheets are eddy current tested. Synchrotron fluorescence analysis and neutron activation analysis

used for supplemental non-destructive identification and investigation of detected defects. More than 1000 niobium

sheets for TESLA test facility were examined. A SQUID based scanning system has potentially higher sensitivity as a

conventional pick up coil. First prototype of SQUID apparatus demonstrates sensitivity sufficient to detect inclusions

as small as 0.1 mm in diameter.

� 2002 Elsevier Science B.V. All rights reserved.

PACS: 74.70.Ad

Keywords: High purity niobium; RF cavities; TESLA

1. Introduction

Niobium is the favourite metal for the fabrica-

tion of superconducting accelerating cavities. The

majority of these cavities are manufactured from

niobium sheet material. The resonators are oper-

ated well below the transition temperature of ni-

obium (9.2 K). A high thermal conductivity in thecavity wall is needed to guide the dissipated radio

frequency (RF) power to the liquid helium cool-

ant. In the case of bulk niobium cavities this re-

quires niobium of exceptional purity with residual

resistivity ratio ðRRRÞ > 300. The Nb material

must be free of foreign inclusions or metallurgical

defects down to a scale of 50 lm.Considerable care must be applied during han-

dling or machining the Nb parts in order to avoid

any additional contamination. The conventionalfabrication way of bulk niobium cavities is deep

drawing of half-cells from sheet material and

electron beam welding. Final cleaning of the fin-

ished cavity by chemical or electrochemical meth-

ods and rinsing with ultraclean high-pressure

water are essential steps to achieve a defect-free

*Corresponding author. Tel.: +49-408-9982775; fax: +49-

408-9981970.

E-mail address: waldemar.singer@desy.de (W. Singer).

0921-4534/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-4534(02)02208-6

Physica C 386 (2003) 379–384

www.elsevier.com/locate/physc

inner Nb surface such as needed in RF cavities at

high fields.

The production of niobium for cavities is a

challenge for the fabrication process: stringent

vacuum requirements during electron-beam melt-

ing of the ingots, clean and well controlledconditions during sheet rolling, cutting and re-

crystallization heating. Adequate procedures have

been developed by some Nb producers. Open

communication between these companies and the

user laboratories was very helpful to reach the

required specification.

Up to now a total amount of about 25 tons of

high purity niobium has been purchased for thefabrication of superconducting cavities. During

the last years typically 2 tons/year have been or-

dered. There is worldwide interest in new super-

conducting accelerators for elementary particle

physics and fourth generation synchrotron light

sources, in particular free electron lasers in the

ultraviolet and X-ray regime. Therefore a contin-

uously growing demand for high purity Nb isexpected. The proposed linear collider project

TESLA requires 500 tons of high purity niobium

at a fabrication schedule of three years [1].

2. Niobium specification

The purity of Nb purchased is important bothin terms of dispersed impurity content and inclu-

sions from manufacturing steps, such as rolling.

Inclusions on the RF surface play the role of

normal conducting nucleation sites for thermal

breakdown. Dissolved impurities serve as scatter-

ing sites for the electrons not condensed in Copper

pairs. These impurities lower the thermal conduc-

tivity and thereby limit the maxim tolerable sur-face magnetic field.

Among the metallic impurities, tantalum is

found in the highest concentration (typically about

500 lg/g). Tantalum is difficult to separate from

Nb because both elements have very similar

chemical properties. The impurity level of �500lg/g is normally not harmful since tantalum is a

substitutionaly dissolved in the lattice and doesnot significantly affect the electronic properties as

do interstitial impurities. Tantalum could prove

dangerous, if it clusters become a normal con-

ducting spot. Next in abundance among substitu-

tional impurities are the refractor elements, such as

tungsten, zirconium, hafnium, and titanium, usu-

ally found at the level of 10–50 lg/g.Among the light, interstitially dissolved impu-

rities, oxygen is dominant due to the high affinity

that Nb has for oxygen above 200 �C. The othercommon interstitials are carbon, nitrogen and

hydrogen. The content of hydrogen should be kept

small (less then 3–5 lg/g) to prevent the hydride

precipitation and degradation of the Q-value of

the high RRR cavities under certain cool down

conditions (hydrogen Q0 decease). Interstitialimpurities generally are more dangerous than

substitutional impurities such as tantalum. The

electron scattering on the interstitial impurities

influences mostly the RRR, which can be calcu-

lated for example by empirical formula

RRR ¼Xi

fi=ri

!�1

;

where the fi denote the fractional contents of im-purity i (measured in lg/g) and the ri the corre-

sponding resistivity coefficients which are listed in

Table 1.

To obtain the RRR one must add the resistance

contributions for each impurity element in parallel

to the resistance contribution from phonons. The

contributions of the phonons are temperature de-pendent, so that the highest theoretical RRR for

Nb is 35.000 [2].

The content of the light elements can be re-

duced during the electron beam melting stages of

the ingot. Multiple melts and progressive im-

provements in the furnace chamber vacuum have

led to a steady increase in the RRR of commercial

Nb over the last decade from 30, typical of ‘‘re-actor grade’’ Nb, to 300; even RRR ¼ 600 nio-

bium can be produced on an industrial scale [3].

Four to six melting steps generally are necessary to

Table 1

Weight factor ri of some impurities for RRR calculation

Impurity atom i N O C H Ta

ri in 104 lg/g 0.44 0.58 0.47 0.36 111

380 W. Singer et al. / Physica C 386 (2003) 379–384

reach the RRR ¼ 300 level with few lg/g of oxy-

gen and nitrogen. Intermediate and final recrys-

tallization annealling for 1–2 h at 700–800 �C in a

vacuum furnace at a pressure of �10�6 mbar is

required in order to reach full recrystallization,

uniform small grain and the mechanical propertiesdemanded for the cavity production. The gas

content achieved by melting can be kept during

manufacture of the semifinished products by

careful handling [4]. The quality control includes

the RRR-measurement, microstructure analysis,

analysis of interstitial and metallic impurities,

hardness measurement, tensile test, examination of

the surface roughness and search for clusters [5].The main aspects of niobium specification can

be seen in the Table 2.

The material should have a high thermal con-

ductivity in order to stabilize against breakdown at

normal conducting spots.

The thermal conductivity of the Nb in the

completed cavity can be additionally improved by

the post-purification (called often solid state get-tering). The getter metal mainly titanium or yt-

trium is vapor deposited on the surface of niobium

at high temperature. The interstitial impurities as

oxygen, nitrogen or carbon build compounds with

getter material, because the bonding enthalpy of

this metal to O, N and C is higher as of Nb. The

building of the compounds between getter material

and interstitial impurities reduce the concentrationof interstitials at the surface of Nb and create a

concentration gradient between surface and bulk

of Nb. On the other hand the high temperature

intensifies the diffusion of the interstitial impurities

from inside to surface and as result purify the bulk

of the niobium. A definite slope at the impurities

distribution take place inside of Nb after post-

purification, that can be calculated with help of

second Fick�s diffusion law (Fig. 1) [6]. The im-

purity distribution produced during post-purifica-

tion causes a definite RRR behaviour. The latest

can be observed experimentally (Fig. 2). The pu-

rification heat treatment also homogenizes the Nb.This is indicated by the reduction of magnetic flux

pinning centers as shown by magnetization mea-

surements [7]. The temperature and duration of

the purification annealing depends on the evapo-

ration rate of the getter material and diffusion rate

of the impurities. This technique is in principle

capable to improve the RRR by factor of 10 (see

[8]).Pure titanium is applied for post-purification

of TESLA cavities with annealing parameters of

�1400 �C for 4 h. The RRR of nine cell resonators

reaches normally values of 500–600.

Table 2

Technical specification for niobium applied for the fabrication

of 1.3 GHz superconducting cavities

RRR >300

Grain size �50 lmYield strength >50 N/mm2

Tensile strength >100 N/mm2

Elongation at fracture 30%

Vickers hardness 6 50

Content of the main

impurities lg/gTa6 500; O6 10; N6 10

C6 10; H6 2

Fig. 1. Oxygen distribution from the center to the surface of

Nb sheet after post-purification.

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500 2000 2500 3000

RR

R

Layer position on the cross-section of sample z(µm)

Fig. 2. RRR distribution inside of Nb sheet after post-purifi-

cation.

W. Singer et al. / Physica C 386 (2003) 379–384 381

An example of Nb thermal conductivity as de-

livered and after refining can be seen in Fig. 3.

The simplified relationship between RRR andthermal conductivity (the thermal conductivity at

4.2 K in W/mK: kð4:2 KÞ � 0:25RRR) allows

avoid costly measurements of thermal conductivity

and use RRR for it rough estimation.

In order to control the Nb quality ‘‘in situ’’

during every stage of cavity fabrication and treat-

ment a new non-destructive method of ac RRR

measurement was developed at DESY [9].The technique involves two concentric coils

positioned close to the object. A current with

definite frequency is created in the primary coil;

the magnetic field of this coil induces eddy current

in the metal. The signal induced in the pick up coil

is a function of the material impedance. The su-

perconductive jump of the signal is utilized for

RRR identification. For elimination of the induc-tive voltage, which the primary coil creates in the

pick up coil without test material, two identical

contrary directed pick up coils are used. Accuracy

of �10% can be achieved by using of standard

samples for calibration.

3. Diagnostic and quality control

A frequent limitation of the field gradient in the

cavities is due to the thermal instabilities caused by

defects (cluster of foreign materials, microcracks,

rests of the oxides and so on). Temperature map-

ping reveals isolated hot spots away from the EB

welding seam and demonstrates the importance of

the search for defects in Nb sheets.

Some methods of non-destructive diagnostic asX-ray radiography, neutron radiography, neutron

activation analysis (NAA), X-ray fluorescence

analysis, ultrasonic- and eddy current inspection

has been taken into consideration. The eddy cur-

rent method was chosen as most suitable for 100%

sheets scanning (rather fast, sensitive to different

sorts of defects with a high resolution). An eddy

current scanning system was developed in collab-oration with BAM (Berlin) and installed at DESY.

The upgraded system can be seen in Fig. 4.

The system rotates the Nb sheet continuously;

the scanning probe is placed like the tangential

arm of a record player. The applied two frequency

principle gives the possibility to separate the sur-

face and bulk signal contribution. Scanning with

high frequency (about 1 MHz) allow to detect thesurface irregularities and the low frequency test

(about 150 kHz) can find the bulk inclusions. The

apparatus picks up both (high frequency and low

frequency) signals simultaneously. All Nb sheets

foreseen for TESLA test facility cavities were eddy

current tested (about 1000). Sheets with irregu-

larities were sorted out. Open communication with

Nb manufacturer led to continues reduction of the

Fig. 3. Measured heat conductivity of samples from the nio-

bium sheets used in the TESLA cavities: before and after the

1400 �C heat treatment (RRR ¼ 270 and 500 respectively).

Fig. 4. Eddy current scanning system for Nb sheets.

382 W. Singer et al. / Physica C 386 (2003) 379–384

number of detected defects from series to series of

Nb production.

A supplemental non-destructive identification

of defects was done by NAA and X-ray fluores-

cence analysis SURFA. The first method is more

efficient for analysis of layers close to the surface(with a penetration depth between few lm and few

hundred lm), NAA delivers the information about

bulk Nb and demonstrates very high sensitivity to

Ta inclusions in Nb.

Further improvement of the scanning system

for detection of defects in niobium can be done by

the SQUID based methods. SQUID sensors are

more sensitive in comparison with conven-tional eddy current pick up coils. The Institute of

Applied Physics, Universit€aat Gießen and WSK

Meßtechnik GmbH developed in collaboration

with DESY an eddy-current non-destructive sys-

tem based on a niobium dc SQUID, which could

detect inclusions of a volume of as small as 10�12

m3, as well as small defects at the surface of the

niobium sheets [10].A circular coil with a diameter of a few mm

generates eddy currents in the niobium sheet. In-

homogeneities having conductivity different from

that of niobium lead to change in the eddy current

field, which is detected by SQUID. In order to

minimize the excitation field at the location of the

SQUID, usually a gradiometric excitation coil is

used, having the shape of a double D. However,since the inclusions are very small, a relatively

small double-D coil must be used to maximize the

eddy current density at the location of the inclu-

sion. Making small double-D coils with many

turns and high symmetry is not easy, however.

Instead, was used an electrical compensation

scheme in which the field of the circular excitation

coil is compensated electronically at the location ofthe SQUID by feeding part of the excitation cur-

rent through the modulation coil used for flux

locking the SQUID. By carefully adjusting the

amplitude and phase of the compensation current

the excitation field at the SQUID can be com-

pensated by a factor of 1000.

Fig. 5 shows the results of a typical measure-

ment of a niobium sheet with nine artificial surfaceflaws (indentations less then 100 lm in diameter

and depth). The eddy current frequency was 110

kHz and the diameter of the excitation coil was 3mm.

Nine surface flaws could be detected. A mea-

surement of the same Nb using a conventional

eddy current scanning system can be seen for

comparison in Fig. 6. Not all of the surface flaws

could clearly be detected with this system.

References

[1] TESLA - Technical Design Report, 2001, DESY 2001-011/

ECFA 2001-209/TESLA Report 2001-23/TESLA FEL

2001-05.

[2] K. Schulze, O. Bach, D. Lupton, F. Schreiber. Purification

of niobium. Niobium, in: H. Stuart (Ed.), Proceedings of

the International Symposium, San Francisco, USA, No-

vember 8–11, 1981, pp. 163-223.

[3] F. Sch€oolz. How to produce Nb RRR600 on an industrial

scale? Proceedings of the 9th Workshop on RF Supercon-

ductivity, Santa Fe, USA, November 1–5, 1999, pp. 100–

102.

[4] M. H€oormann, The production of high thermal conductivityniobium on a technical scale for high frequency supercon-

ductors, Heraeus, Hanau, 1988.

[5] W. Singer, D. Proch. Technical specification for niobium

applied for the fabrication of 1.3 GHz superconducting

Fig. 5. Two-dimensional distribution of the pick up signal on a

part of a test Nb sheet containing a number of surface flaws,

measured with the SQUID system.

Fig. 6. Same Nb sheet scanned with the conventional eddy

current system.

W. Singer et al. / Physica C 386 (2003) 379–384 383

cavities, RRR 300, Lab-Note 1/96, (Version E), December

2001.

[6] W. Singer, Some aspects of diffusion in niobium by high

temperature gettering, August 1995, TESLA Report 95-

13.

[7] M. Bathe, F. Herrmann, P. Schmueser, Magnetisation

and susceptibility measurements of Nb samples for

cavity production, Proceedings of 8th Workshop on

RF Superconductivity, October 6–10, 1997, pp. 881–

889.

[8] H. Safa, D. Moffat, B. Bonin, F. Koechlin, Advances in the

purification of Nb by solid state gettering with Ti, J. Alloys

and Compounds 232 (1996) 281–288.

[9] W. Singer, D. Proch, Proceedings of 7th Workshop on RF

Superconductivity, October 17–20, 1995, Paris, France,

p. 547.

[10] M. M€uuck, C. Welzel, A. Farr, F. Sch€oolz, W. Singer,

Nondestructive testing of niobium sheets for supercon-

ducting resonators, Proceedings of Applied Superconduc-

tivity Conference, August 4–9, Houston, USA.

384 W. Singer et al. / Physica C 386 (2003) 379–384