PERFORMANCE OF Nb/Cu CLAD SEAMLESS SC CAVITY MADE FROMA PIPE BY HOT&COLD ROLLING AND DEEP-DRAWING
I. Itoh, Nippon Steel Co., 20-1 Shintomi Futtsu-shi, Chiba-ken, Japan
K.Saito, H.Inoue, KEK Accelerator Lab., 1-1 Oho Tsukuba-shi, Ibaraki-ken, Japan
W.Singer, DESY, 22603 Hamburg, Germany

Abstract

2.2

Nippon Steel Co. has successfully developed
Cu/Nb/Cusandwichedseamless
pipesforsuperconducting rf L-band cavities using deep-drawing
and spinning of a Cu/Nb/Cu clad sheet which wasfabricated by a hot rolling method. DESY formed singlecells with TESLA shape from the clad pipes using hydroforming. KEK electron-beam welded Nb pipes on thecells and made surface treatments including barrel
polishing, heat treatment and electropolishing. As a resultof the vertical test, excellent performance :Eacc,max=39.0MV/m, Q0,max= 1.67×1010 at 1.5K and
1.3GHz was then achieved in the first test.

Prior to deep-drawing, the clad sheet was annealed inN2 gas mainly for Cu softening, and deep-drawn by foursteps to a cup. Spinning was then performed on it. Aftercutting its bottom, a clad seamless pipe was made. It was
250mm long and 133mm in inner diameter, and is shownin Fig.2. During the deep-drawing and spinning processes,
intermediate and final annealings in N2 gas were made on
it for Cu softening. Fig.3 shows the wall thickness
distribution of a Cu seamless test pipe after deep-drawing
and after spinning. The Cu pipe was fabricated from a
4.0mm thick Cu sheet to make sure fabricating processes.
It is clearly shown that the thickness at the pipe’s flange

Fabrication of Cu/Nb/Cu clad seamless pipes

1 INTRODUCTION

Two Cu sheets

So far sc cavities have been fabricated from Nb sheets
by deep-drawing and EB welding. However this process
looks to be too expensive for the future applications likeTESLA, FEL and ERL projects. On the other hand,
Nb/Cu clad seamless cavities have a lot of benefits for sccavities: highly reliable performance and cost-effective
fabrication[1]. This excellent idea was realized in ourrecent measurement. Here we present the measurement
result and a new fabrication method of Cu/Nb/Cu
sandwiched seamless pipes for sc cavities.

Cu box2

CU/NB/CU CLAD SEAMLESS PIPES

Nippon Steel Co. has unique original technology tofabricate NbTi/Nb/Cu multilayer composite sheets andseamless pipes for superconducting magnetic shielding[2][3]. This technology was applied to produce Cu/Nb/Cuclad sheets and seamless pipes. Cu/Nb/Cu sandwichedsheets were fabricated by an airtight-cladding andhot&cold rolling method. Then clad seamless cups wereformed by deep-drawing and spinning technology. Aftercutting the cups’ bottoms, clad seamless pipes were made.
The fabricating processes are shown in Fig.1.

Nb sheet

Assembling
EB Welding
Hot Rolling
Cold Rolling
Deep Drawing
Spinning

FigureAnnealing
1: Fabricating processes ofsandwiched clad seamless pipes.

Cu/Nb/Cu

2.1 Fabrication of Cu/Nb/Cu clad sheet
After preparing a box and lid made of Cu sheets of 4nine class, an Nb sheet with RRR = 250 was inserted into
the Cu box, and a Cu/Nb/Cu sandwiched slab wasassembled. The slab was EB welded at the seam between
the box and lid, and vacuum-sealed (canning), then it was
hot rolled and cold rolled to a 4.3mm thick clad sheet. At
the time Nb thickness was about 0.75mm.

Figure 2: Cu/Nb/Cu clad seamless pipe made by cladding,
hot&cold rolling, deep-drawing and spinning.

after deep-drawing is larger than that of the sheet by over50%, and gradually decreases parallel to the axis to its
bottom direction. But after spinning, very uniform
thickness could be achieved. This tendency could also beobserved with the Nb/Cu clad pipes. Their thickness
increase at the flange was almost the same as 50%.

better value of 214 could be achieved by the same
vacuum annealing after etching the sample surface by CP,
when nitrogen concentration decreased to around 25ppm
in both cases. Nitrogen concentration aside,concentrations of H and O did not change so much. H
was less than 6ppm and O was less than 90ppm in all
samples. Therefore, for our fabricating method ofCu/Nb/Cu pipes, it was confirmed that their Nb was very
resistant to gas contamination and degradation in RRR
value during the processes.
3NB/CU CLAD SEAMLESS SC CAVITIESDESY has successfully developed hydro-forming
technology for L-band Nb seamless cavities and Nb/Cuclad seamless cavities[4]. KEK possesses excellent
technology for treating Nb inner surfaces including
electropolishing successfully developed by itself[5].
3.1 Fabrication of Nb/Cu single-cell cavities

Figure 3: Wall thickness distribution along axis
direction of Cu seamless test pipe.
2.3 RRR

and concentrations of H, N and O in Nb

The clad sheets underwent high temperature heating inan air atmosphere before hot rolling, and the clad
seamless pipes underwent a few intermediate annealingsin N2 gas. Therefore RRR degradation was investigatedfrom the viewpoint of the effect of gas contaminations inNb material on RRR. The results are shown in Fig.4.
Measured RRR values of an as-received Nb sheet were
244 and 261. But RRR of a sample taken from aCu/Nb/Cu clad seamless pipe decreased to 157 after Cu
resolving by dipping in HNO3. After resolving, itsconcentration of nitrogen increased from about 25ppm ofas-received Nb sheet to 210ppm. But its RRR recovered
to 202 by vacuum annealing at 750℃ for 3 hrs, and a
Figure 4: Effect of treating processes of Nb sample on

DESY formed the Cu/Nb/Cu clad seamless pipes tosingle-cells without beam pipes for L-band cavity shapeby hydro-forming. Then KEK accomplished fabricationof single-cell cavities by removing their inner copper
layers, EB welded Nb pipes on them and made a few
inner surface treatments including barrel polishing,chemical polishing cleaning, vacuum annealing at 750℃for 3 hrs, and electropolishing of about 70μm. Fig.5
shows the inner surface of Nb layer after resolving theinner Cu layer by nitric acid. After hydro-forming, a few
small cracks, which were about 15mm long, were foundon the surface of the inner Cu layer at the cell’s equator.
Their directions were perpendicular to its periphery. But
after resolving the inner Cu layer, the cracks wereconfirmed by microscope inspection of the Nb innersurface to have been completely eliminated. Fig.6 shows
the finished single-cell seamless sc cavity with Nb pipes.
3.2 Performance of Nb/Cu clad seamless sc cavity
The cold test was performed at 1.5K and 1.3GHz with
one of the Nb/Cu clad seamless sc cavities. The measured

0.5mm

RRR and H, N and O concentrations in Nb
material of Nb/Cu pipe.

Figure 5:Inner surface of Nb layer of Nb/Cu clad
seamless cell after resolving inner Cu layer
by nitric acid.

5CONCLUSION

It was confirmed that our new approach to reliable clad
seamless sc cavities has many advantages. (1)The deepdrawn and spun pipe had a good workability for hydroforming. (2)Sandwiched structure has a resistance against
cracking during the hydro-forming. (3)A good bonding
can be achieved by hot rolling with large area reduction
at high temperature. The first sc cavity made from thepipe showed an excellent performance. Therefore thisnew approach is very promising to yield a cost-effective
and reliable fabrication technology for Nb/Cu clad
seamless sc cavities.
Figure 6: 1300MHz Nb/Cu clad seamless sc cavity.
result (●) is shown in Fig.7. Excellent performance ofEacc,max=39.0MV/m with Q0 of 7.13×109 was achieved.
And Q0,max = 1.67×1010 was achieved with Eacc =4.01MV/m. However the Q value decreased somewhat
after the first quench. In the second measurement,
Eacc,max was not changed, and Q0-value at that time
dropped to 4.80×109. In addition, Q0,max =1.33×1010
was obtained with the same Eacc of 4.02MV/m.

6 ACKNOWLEDGMENTS
The authors would like to thank K.Umemori andK.Harada of KEK for their supports of fabrication andevaluation of the cavities. The cold test was supported by
the Cryogenics Science Center of KEK, which suppliedliquid He. We would like to make a special thank toJ.Kusano of JAERI for his support in our early R&D. We
would like to thank Tokyo Denkai Co. Ltd. and Nomura
Plating Co. Ltd. for their helps and supports. We would
also like to thank K.Fujisawa of Mitsui Bussan MS Co.,
M.Fukuda of Nittetsu Shoji Co., T.Oda, Y.Yamashita
M.Ishii, T.Senuma, M.Kikuchi, H.Fujii, H.Otsuka andT.Narutaki of Nippon Steel Co. for their support inadvancing this R&D.
REFERENCES

QuenchFigure 7: First result of high gradient performanceof Nb/Cu clad seamless sc cavity.
4DISCUSSIONSThe new fabrication method for Cu/Nb/Cu sandwichedseamless pipes for single-cell sc cavities was proved tohave a high potential for excellent performance. The
second cavity was also finished, and the measurement isuinderway. By these two cold test, we would like toconfirm the excellent performance in statistics. There are a
few issues in this method that still need to be solved. Since itis relatively hard to make a long pipe by our method, tube
drawing will be needed afterward for a seamless cladcavity with larger cell numbers than single-cell including
nine-cell. As the next step, we are planning to fabricate
long pipes for multi-cell sc cavities. In addition, weconsider jointing between clad pipes to increase
productivity for future mass production of drawn pipes.

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