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Physica C 441 (2006) 89–94

Seamless/bonded niobium cavities q

W. Singer *

DESY, Notkestrasse 85, 22607 Hamburg, Germany

Abstract

Technological aspects and performance of seamless cavities produced by hydroforming are presented. Problems related to the fabri-cation of seamless cavities from bulk niobium are mainly solved thanks to the progress of the last years. The highest achieved acceleratinggradients are comparable for both seamless and welded versions (ca. 40 MV/m) Nevertheless further development of seamless cavities isdesirable in order to avoid the careful preparation of parts for welding and get reliable statistic.

Fabrication of NbCu clad cavities from bimetallic tubes is an interesting option that gives new opportunity to the seamless technique.On the one hand it allows reducing the niobium costs contribution; on the other hand it increases the thermal stability of the cavity. Thehighest accelerating gradient achieved on seamless NbCu clad single cell cavities (ca. 40 MV/m) is comparable to the one reached on bulkNb cavities. Fabrication of multi-cell NbCu cavities by hydroforming was recently proven.� 2006 Elsevier B.V. All rights reserved.

Keywords: Cavity; Seamless; Hydroforming; Fabrication; Accelerating gradient

1. Introduction

The traditional fabrication procedure of elliptical SRFcavities consists of deep drawing and electron beam weld-ing (EB) of the half cells. This procedure is well estab-lished and has about 30 year of industrial fabricationexperience. It is well known that RRR degradation inwelding areas of conventional cavities can be critical forthe performance [1]. In the last few years improvementof the material quality control, preparation for EB weldingand the welding parameters allowed to reach acceleratinggradients close to the theoretical limit by applyingadvanced cavity treatment techniques such as electropol-ishing (EP) [2].

Nevertheless a fabrication method which will avoid thewelding is very complementary from two aspects. Firstthe seamless cavity does not have the risk of equator weld

0921-4534/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2006.03.133

q We acknowledge the support of the European Community-ResearchInfrastructure Activity under the FP6 ‘‘Structuring the EuropeanResearch Area’’ program (CARE, contract number RII3-CT-2003-506395).

* Tel.: +49 4089 982775; fax: +49 4089 981970.E-mail address: waldemar.singer@desy.de

contamination and therefore could improve the reliabilitytoward achieving high gradients. It is to expect that thestatistic in performance of seamless cavity for the series willbe better. Second a lower cost of fabrication can beexpected.

Especially interesting is the combination of the seamlesstechnique with NbCu bonding, i.e. the fabrication of cavityfrom bimetallic bonded NbCu tube by seamless technique.

The advantages of this option can be easily pointed out.

• cost effective: allows saving a lot of Nb (ca. 4 mm cavitywall has only ca. 1 mm of Nb and 3 mm Cu). This canbe essential for large projects like ILC;

• the bonded Nb layer has still microstructure and proper-ties of bulk Nb (the competing sputtered niobium layersdoes not have such advantages);

• the well developed treatment of the bulk Nb such as buf-fered chemical polishing (BCP), EP, annealing at800 �C, bake out at 150 �C, high pressure water rinsing(HPR), high power processing (HPP) can be applied(excluding only post purification at 1400 �C);

• high thermal conductivity of Cu improves thermalstabilization;

Fig. 1. The tube necking machine.

90 W. Singer / Physica C 441 (2006) 89–94

• stiffening against Lorentz-force detuning and micro-phonics can be easily done by increasing the thicknessof the Cu layer.

The development of the seamless technique mostly forthe TESLA shape cavity was pursued in the last years atINFN by Palmieri (spinning) [3,4] and at DESY (hydro-forming) [5]. Fabrication of a seamless cavity of theTESLA shape, with a ratio of equator diameter to irisdiameter of about three, is a challenging task and severalyears of development were necessary to build the multi-cellcavities. The spinning fabrication technique and perfor-mance of the spun cavities is described in details in Ref.[3]. A nine-cell cavity was produced. A new spinningmachine was introduced recently [4].

This paper presents the current status of the hydroform-ing development and contains the fabrication technique,first RF test results, remaining problems and perspectives.

2. Fabrication technique

For hydroforming one starts with a tube of a diameterintermediate between iris and equator. The forming proce-dure consists of two stages: reduction of the tube diameterin the iris area and expansion of the tube in the equatorarea.

Two aspects should be taken into consideration for thechoice of the initial tube diameter. On one hand the workhardening at the equator should be moderate. Thereforea larger initial tube diameter is preferred, easing the secondstage of hydroforming. On the other hand enlargement ofthe tube diameter will increase the roughness at the iris areaafter diameter reduction. Comparison of the hardness dis-tribution HV10 with the inside surface roughness of thehydroformed cavities gives a hint for the choice of the tubediameter for hydroforming. Experience shows that a tubediameter between 130 and 150 mm is close to optimum.The single cell and multi-cell seamless cavities have beensuccessfully fabricated starting both from ID = 130 mmand ID = 150 mm.

The arithmetic mean roughness, Ra, inside the cavity isnormally several lm larger compared to that of deepdrawn cells. In many cases the surface roughness wasreduced to acceptable values after standard BCP or EPtreatment. Generally the combination of hydroformingwith successive centrifugal barrel polishing (CBP) beforeEP or BCP would be complimentary. CBP reduces the sur-face roughness and is a good pretreatment for EP. It iswell known that the EP is more successful when startingfrom a smother surface. CBP is more cost effective andenvironmentally friendlier than EP or BCP. If CBP as pre-treatment is applied, thinner Nb layer can be removed byEP or BCP (ca. 50 lm instead of 150–200 lm). CBP wassuccessfully applied by KEK as cavity pretreatment proce-dure [6]. The new machine for 1–3 cell cavities withimproved technological process was developed at DESY[7].

The development of the tube diameter reduction at theiris area (necking) demanded even more efforts than itsexpansion (hydroforming itself). Several necking methods(hydraulic necking, electromagnetic strike necking, roundknead, spinning) were tested before the current neckingprocedure was established. The best necking results wereachieved by using a specially profiled ring moving in radialand axial directions. A tube necking device for experimentson a laboratory level based on this principle was developedand constructed at DESY (Fig. 1).

The device is built for necking of Nb and NbCu tubesand is foreseen for single cell, double cell or three cell cav-ities. The machine consists of several transversally orientedplates. Four hydraulic cylinders are fixed on the plates. Allcylinders are equipped with position and pressure sensors.The computer control of the device allows reproducibilityof the necking parameters. The first experiences haveshown a good operation of the machine. The necking ofthe NbCu and bulk niobium tubes at the end as well asat the tube (iris) of the tube can be routinely done.

During hydraulic expansion of the equator area aninternal pressure is applied to the tube and simultaneouslyan axial displacement, forming the tube into an externalmold. The hydraulic expansion relies on the use of the cor-rect relationship between applied internal pressure andaxial displacement, assuming that the plastic limit of thematerial is not exceeded.

The hydraulic two-dimensional bulging of a disk intothe spherical form is very helpful [8], especially because itgives the opportunity to check the anisotropy of plasticproperties. However, taking into account that duringhydroforming the main elongation occurs in the circumfer-ential direction, a conventional tensile test on samplesfollowing this orientation can be sufficient.

A numerical simulation of the hydraulic expansion ofthe tube is very useful. At the beginning of the seamlessdevelopment, a lot of simulations were done with the finiteelement code ANSYS, assuming the elasto-plastic behaviorin accordance with stress–strain curve and isotropic hard-

Fig. 2. The hydroforming machine.

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ening rules [5]. The calculations were carried out on thebasis of the experimentally determined strain-stress charac-teristic of tubes to be hydroformed and resulted in the rela-tion between the applied internal pressure as a function ofthe axial displacement and radial growth (path of theexpansion) for the hydroforming process. The FEM simu-lation gives the starting point for the hydroforming param-eters, which can be additionally tuned on the base ofhydroforming tests, from comparison between the theoret-ical and experimental growth of the tube diameter.

A special machine for hydroforming experiments on alaboratory level was designed and built [5]. It has a waterhydraulic system to apply the internal pressure in the tubeand an oil hydraulic system for the cylinder movements.The computer control system for the hydroforming wasdeveloped to allow a stepwise as well as a continuoushydraulic expansion.

Picture of the hydroforming machine can be seen in Fig. 2.Two additional aspects contributed to success in the

hydraulic forming of the Nb cavities. The first one wasthe application of periodic stress fluctuation (pulse regime).Previous tensile tests have shown that elongation beforenecking can be increased almost by 30% applying a pulseregime, in comparison with a monotonic increase of stress.

The second one was the use of the correct strain rate forniobium during the hydraulic expansion. The experimentshave shown that the deformation procedure should berather slow. By keeping the strain rate below 10�3 s�1,a 10% higher strain before necking can be obtained.

3. Seamless tubes for hydroforming

Seamless Nb or NbCu tubes suitable for hydroformingare not available on the market and a special developmentis necessary. Required elongation at break greater than30%, small and uniform grains in the final shape arechallenging.

Development of seamless Nb tubes for hydro-forming was done and is being continued in collaboration

with scientific institutes and industrial companies. Severalfabrication methods (spinning, back extrusion, for-ward extrusion, flow forming and deep drawing) wereanalyzed.

The experiences have shown that properties of tubesback extruded from the ingot pills are not adequate forhydroforming of bulk Nb cavities. Areas with rather biggrains (500–1000 lm) dramatically reduce the elongationbefore necking and yield a rough inside surface after hydro-forming. It seems that the amount of deformation is notsufficient to reduce the few cm-size grains of the ingot tosmall and uniform grains of ca. 50 lm size in the final tubeas required for hydroforming.

Much better results can be achieved starting from the ca.10 mm thick sheet having already rather small and uniformgrain structure. The tubes can be produced by deep draw-ing or spinning and show much higher strain before onsetof necking. Combination of spinning or deep drawing withflow forming allows improving the surface and significantlyreducing the wall thickness variations. Shiny surfaces andsmall wall thickness variations (less then ±0.1 mm) canbe achieved.

Fabrication of seamless NbCu clad tubes is a subject ofspecial efforts. Requirements to mechanical properties ofbulk Nb tubes are not so challenging in this case becausethe plastic properties of Cu plays a dominant role duringhydroforming of such bimetallic tube. Back extruded seam-less Nb tubes can be tolerated from a mechanical proper-ties point of view. The main question is how to make thecladding of Nb with Cu without reduction of the Nb purityand cladding quality.

Different cladding procedures were checked. First of allthe explosive bonding was applied. The bonding takesplace by an explosively driven, high-velocity angularimpact of two metal surfaces at very high speeds creatinghuge contact pressure.

The tubes have been produced in two steps:

• Explosive bonding of back extruded seamless Nb tube(ca. 4 mm wall thickness) with oxygen free Cu tube (wallthickness ca. 12 mm).

• Flow forming into NbCu tube of 4 mm wall thickness(ca. 1 mm Nb, 3 mm Cu).

Another cladding way of bimetallic NbCu tubes wasdeveloped at KEK together with the industry (hot bond-ing) [9] and at DESY together with Fa. NuTech. The mainidea is to protect the Nb from contamination at high tem-peratures by a Cu shield; the vacuum of 10�5�10�6 mbar iscreated between Nb and Cu layers and is air tight welded(canning). It can be three concentric tubes Cu/Nb/Cu orthree plates Cu/Nb/Cu. After that, a tube with smaller wallthickness can be produced from such ‘‘sandwich’’ using hotrolling or hot excursion procedures providing at the sametime the cladding. Combination with standard procedureslike spinning or flow forming allows getting the requiredtolerances in the wall thickness.

Fig. 3. Q vs Eacc for cavity 1K2 after BCP and EP. (Nb100, post purifiedat 1400 �C). Filled circles—Jlab (Kneisel) [5]; open circles—KEK (Saito)[5].

92 W. Singer / Physica C 441 (2006) 89–94

Excellent results in the cavity performance were reachedfor both kinds of tubes: explosively bonded and hotbonded. We do not have enough statistics for comparisonof explosive bonding with hot bonding. It seems that thequality of hot bonding is more reliable; better statistic inthe cavity performance should be expected.

Softening by annealing deserves special attention forNbCu clad option. Because of the large difference in recrys-tallization temperature, it is not possible to reach duringappropriate annealing, an optimum for the hydroformingplastic properties in both materials. For example, Cu canbe fully recrystallized by annealing at 560 �C for 2 h withthe grain size about 30 lm and acceptable for hydroform-ing elongation before rupture (about 35–40%). Niobiumhas after such annealing a deformed structure without pro-nounced grains. The recrystallization temperature of Nb israther high (ca. 800 �C), and annealing of NbCu bimetalliccomposition at this temperature will lead to significantgrain growth in Cu.

The high plastic properties of Cu play a leading role inthe forming process of NbCu clad cavities. The hard andless plastic Nb layer is much thinner. Nb follows the Cuduring forming because of the tight bonding. Nevertheless,undesired effects cannot be completely avoided. The ten-dency of cracks forming at iris area during necking is espe-cially dangerous.

The situation can be improved by suitable alloying ofthe Cu. It is well known that small additions of some met-als (Hf, Ti, Cr, Zr, Mg, Sn, Mn, Al) increase the recrystal-lization temperature of Cu. It seems that the alloyCu0.15%Zr available on the marked could be a good can-didate for replacing the pure Cu in NbCu clad tubes. Thisalloy has the desired mechanical properties after annealingat temperatures required for Nb recrystallization.

Unfortunately the alloying of Cu will increase the num-ber of scattering centers for electrons and reduce the ther-mal conductivity. However, experiments have shown thatthe thermal conductivity of Cu0.15%Zr remains highenough ca. 150 W/m K at 4.2 K (comparable with that ofhigh purity Nb [1]). In addition the thermal conductivitycan be increased by special heat treatment. The NbCu0.15%Zr clad tube has been produced and is ready forhydroforming.

4. Bulk Nb cavities

Several single cell cavities have been manufactured sofar without intermediate annealing from spun or deepdrawn tubes with ID 130 or 150 mm (one of them was fab-ricated at the company BUTTING with a similartechnique).

It turned out that the forming is possible without inter-mediate constraint (mould of the intermediate shapebetween the tube and final cell). Nevertheless an intermedi-ate constraint is desirable in order to reach stable andreproducible cavity fabrication taking into account varia-tion of plastic properties within the tube. Finally the cavi-

ties were calibrated by increasing the internal pressure upto 500 bar after the hydroforming was completed.

One of the best results of hydroformed bulk Nb singlecell cavities can be seen in Fig. 3 [5]. Cavity 1K2 demon-strates excellent performance. Accelerating field (Eacc) isca. 43 MV/m with a high Q-value of >1.5 · 1010. The accel-erating field of the cavity 1BT1 reached about 39 MV/mwithout post purification annealing.

The development of hydroformed cavity fabrication hasdemonstrated that high performance levels in both Q-valueand accelerating gradient can be achieved.

One of the important steps of the seamless developmentis to demonstrate that the technology allows fabricatingnot only single cell but also multi-cell cavities.

The main problem in the multi-cell fabrication was thereduction of the tube diameter in the iris area (necking).In the first stage the necking was done by spinning usingan industrially available spinning machine and a ‘teachand playback’ procedure. For single cell cavity this proce-dure works. Necking of the multi-cells was less successful.For the long tube it was difficult to achieve the uniformwall thickness at the necking area. The irregularity in thewall thickness on iris area up to 1–2 mm even caused a holeduring material removing by EP or centrifugal barrel pol-ishing in some multi-cell cavities. Therefore we had todevelop a special necking procedure and constructed a spe-cial necking device described in Section 2. The wall thick-ness variation reachable with this device does not exceed0.2 mm.

Expansion of the necked tubes caused less problems.Several double and three cell bulk Nb cavities of theTESLA shape were successfully hydroformed at DESY.The expansion at the equator area can be done in two dif-ferent ways: all cells simultaneously (see Ref. [5]) or succes-sively one cell after the other. Both options were realizedfor three cells. The second option is probably more realisticfor hydroforming of a nine cell cavity from one piece. It

W. Singer / Physica C 441 (2006) 89–94 93

allows making such fabrication more reliable even if somevariation of tube mechanical properties will take place.

5. NbCu clad cavities

Few single cell NbCu clad cavities were fabricated. Boththe necking and the expansion were done at DESY withoutintermediate constraint. The calibration at 500 bar wasdone afterwards. Some of the cavities were additionallyannealed at 560 �C for 2 h before calibration in order tomake them softer (Fig. 4).

The end tubes with the flanges were connected with theNb layer of the cavity by EB welding. The welding of 0.7–1 mm thick Nb layer of the cavity with the 2 mm thickwall of Nb of the end tube in principle does not causeany problem and provide sufficient stiffness for the singlecell cavity. The Cu should be very carefully removed fromthe cylindrical part of the cavity before welding otherwise aleak tight welding is not guaranteed. The stiffness of theconnection can be additionally increased by using ColdGas Dynamic Spray System developed at University ofthe Federal Armed Forces in Hamburg (H. Kreye). Thesmall purity degradation of Nb during spraying (fromRRR = 300 to RRR = 250), the microstructure and prop-erties of sprayed Cu layer being close to bulk Cu (porosity

Fig. 4. Double cell NbCu clad cavities recently produced at DESY fromKEK tubes (no cracks on the inside surface).

Fig. 5. The best result achieved in a single-cell NbCu clad cavity produced at DLab: 180 lm BCP, annealing at 800 �C, baking at 140 �C for 30 h, HPR (Kne

ca. 1%, small oxidation electr. conduct. ca. 80% of bulkCu), make this technique very attractive for applying toNbCu cavities.

The hydroforming technique allows also to fabricatemulti-cell cavities from Nb/Cu clad tubes. Several dou-ble-cell cavities were produced recently at DESY fromsandwiched Cu/Nb/Cu tubes delivered from KEK. Rathersmall frequency deviation from cell to cell (within 1 MHz)was achieved.

Some NbCu clad cavities were already prepared and RFtested at 2 K.

An excellent result was achieved at Jefferson Lab withcavity 1NC2 from explosively bonded tube even withoutelectropolishing (Fig. 5). At the achieved accelerating gra-dient, Eacc = 40 MV/m, the Q-value is ca. 1010 [5].

One of the NbCu cavities (NSC–3) produced at DESYfrom sandwiched hot roll bonded tube (Nippon SteelCo.) was prepared and RF tested at KEK (Fig. 6). Excel-lent result of ca. 39 MV/m shows that very good cavities

ESY from explosively bonded tube. Preparation and RF tests done at Jeff.isel) [5].

Fig. 6. Q(Eacc) curve of the NSC-3 cavity: barrel polishing, CP(10 lm),annealing 750 �C ·3 h, EP(70 lm) (Saito) [9].

94 W. Singer / Physica C 441 (2006) 89–94

can be fabricated not only from explosively bonded butalso from hot roll bonded tubes.

Unfortunately one peculiarity of bimetallic NbCu cavi-ties behavior remains open, namely Q degradation duringfast cool down, RF processing or quench (trapping of mag-netic flax caused by thermo-coupling effect). The phenome-non is not completely understood. Similar effect wasobserved on Nb3Sn cavities and in sputtered NbCu cavities.The Q degradation is relatively moderate (less than oneorder of magnitude in Figs. 5 and 6) especially for ratherthick Nb layers of ca. 1 mm thickness. The degradationcan be cured by warm up above Tc and slow cool down. Itseems that annealing and hot bonding should contribute tothe reduction of the thermal coupling effect due to diffusion.

6. Conclusions

Several single-cell and multi-cell bulk Nb and NbCuclad cavities have been fabricated at DESY by hydro-forming.

The technique developed so far allows the production ofmulti-cell cavities. A good reproducibility of the cavityshape can be easily achieved. The main task left in the fab-rication technique is its industrialization.

It is shown that very good RF performance can beachieved with hydroformed seamless cavities. The mea-sured accelerating field of ca. 43 MV/m in cavity 1K2 isone of the highest ever achieved in a single-cell cavity.The aim of the development is the fabrication of a nine-cellcavity of the TESLA shape. The DESY hydroformingmachine is a laboratory equipment built to check the per-spectives of hydroforming for cavity applications. The sizeof this machine is not sufficient to hydroform a nine cellcavity from whole tube. We proposed to fabricate a nine-cell cavity assembled from three pieces of three cells. Exten-sion of the hydroforming procedure to the nine-cellfabrication from one piece does not have any particularproblems; especially because it was proven that the hydro-forming can be done cell by cell. We still don’t have enoughexperience in the fabrication of seamless Nb tube of thelength about 1.8 m required for hydroforming of thenine-cell cavity from whole piece.

The fabrication of seamless bimetallic NbCu cavitiesdeserves special attention. The combination of hydroform-ing with tube cladding is a very promising technology. Itreduces significantly the cavity material costs. A rough esti-mate shows that the NbCu clad cavities of TESLA shape

will be 30% cheaper than standard bulk Nb cavities, assum-ing equal cost of the end groups. The RF performance ofthe best hydroformed NbCu clad cavities is comparableto the best bulk niobium cavities. Degradation of the qual-ity factor Qo due to thermocurrents is an open issue andneeds further investigation.

Acknowledgements

The development continues in tight collaboration withKEK, JLab, INR and INFN. A significant contributionto the success of hydroforming development was done byP. Kneisel and K. Saito due to state of the art preparationand RF tests of the hydroformed cavities. I would like toexpress gratitude to my colleagues at DESY who workedwith me in the last years and significantly contributed tothe development of hydroforming technology, especiallyto I. Jelezov, X. Singer and G. Weichert.

References

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ing of metals: applications to copper and niobium, in: Proc of the 11thWorkshop on RF Superconductivity SRF2003, Lubeck, Germany, 8–12 September 2003, WeTO2.

[3] V. Palmieri, Advancements on spinning of seamless multicell reentrantcavities, in: Proc of the 11th Workshop on RF SuperconductivitySRF2003, Lubeck, Germany, 8–12 September 2003.

[4] V. Palmieri, Progress on spun seamless cavities, in: Proc of the 12thWorkshop on RF Superconductivity SRF2005, Ithaca, USA, 10–15July 2005.

[5] W. Singer, H. Kaiser, X. Singer, G. Weichert, I. Jelezov, T.Khabibuline, A. Skasyrskaia, P. Kneisel, T. Fujino, K. Saito,Hydroforming of superconducting TESLA cavities, in: Proc. of the10th Workshop on RF Superconductivity, Tsukuba, Japan, FA009, 6–11 September, 2001, pp. 170–176.

[6] T. Higuchi, K. Saito, Hydrogen absorption in EP of niobium,hydrogen in materials and vacuum systems, in: 1st InternationalWorkshop, AIP Conf. Proc., vol. 671, 11–13 November 2002, pp. 203–219.

[7] G. Issarovitch, D. Proch, X. Singer, D. Reschke, W. Singer, Devel-opment of centrifugal barrel polishing for treatment of superconduc-ting cavities, Proc. of the 11th Workshop on RF SuperconductivitySRF2003, Lubeck, Germany, 8–12 September 2003.

[8] I. Gonin, J. Jelezov, H. Kaiser, W. Singer, Hydroforming test of backextruded Nb Tube, in: Proc. of the 8th Workshop on RF Supercon-ductivity, 6–10 October 1997.

[9] I. Itoh, K. Saito, H. Inoue, W. Singer, Hot roll bonding method forNb/Cu clad seamless SC cavity, in: Proc. of the 11th Workshop onRF Superconductivity SRF2003, Lubeck, Germany, 8–12 September2003.