Many laboratories around the world, notably CEBAF, CERN, DESY and KEK, after a period of research and development, are presently or have recently been involved in the industrial production of a large number of RF superconducting cavities. CERN, instead of using the standard bulk niobium technique, has developed a new Nb/Cu technology (niobium film deposited by magnetron sputtering on copper). The aim of this paper is to present the transfer of this technology to three European firms [Ansaldo, CERCA and Siemens (now ACCEL)]. Emphasis will be placed on the major challenges to industry of mastering the very complex procedure (which requires high quality control at every stage of the production) needed to achieve a very demanding final RF performance [Q(6 MV/m) = 3.4¥109 at 4.5 K]

Chiaveri, Enrico

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LARGE-SCALE INDUSTRIAL PRODUCTION OF SUPERCONDUCTINGCAVITIESEnrico Chiaveri, CERN, Geneva, SwitzerlandAbstractMany laboratories around the world, notably CEBAF,CERN, DESY and KEK, after a period of research anddevelopment, are presently or have recently beeninvolved in the industrial production of a large numberof RF superconducting cavities.  CERN, instead of usingthe standard bulk niobium technique, has developed anew Nb/Cu technology (niobium film deposited bymagnetron sputtering on copper).  The aim of this paperis to present the transfer of this technology to threeEuropean firms [Ansaldo, CERCA and Siemens (nowACCEL)].  Emphasis will be placed on the majorchallenges to industry of mastering the very complexprocedure (which requires high quality control at everystage of the production) needed to achieve a verydemanding final RF performance [Q(6 MV/m) = 3.4×109at 4.5 K].1  INTRODUCTIONIn the last two decades many laboratories around theworld decided to develop the technology ofsuperconducting accelerating cavities.  The aims were toincrease the accelerator energy and to save electricalconsumption.  At first such cavities were made ofniobium sheet metal but, unfortunately, the acceleratingfields which could be obtained were limited byquenches.  Since the level of power loss of a cavity islimited by the thermal conductivity of its wall, theefforts of the designers were concentrated on improvingthe Nb purity, in order to increase this thermalconductivity at liquid helium temperature.An alternative solution consists of replacing Nb withcopper as the material for cavity construction anddepositing a thin Nb film (~1.2 µm) on the copper(Nb/Cu cavities).  This approach offers inherentadvantages:  considerably  higher stability againstquenching, insensitivity to small magnetic fields and ahigher quality factor than that of solid Nb at a givenfrequency and working temperature (4.2-4.5 K).  It isevident that by replacing bulk Nb with an Nb layer animportant saving is achieved,  even allowing for theadditional cost of the more elaborate fabricationprocedure.  An interesting feature is the possibility ofreplacing Nb with a wider choice of superconductingmaterials [1].After a period of research and development,including the manufacture of a prototype series, CERNdecided in 1990 to transfer this technology to industry.It awarded the contract for manufacturing the cavitiesand modules (four cavities assembled together) to threeEuropean companies, stipulating RF acceptance tests at4.5 K.  The decision to split the production of the Nb-coated cavities between three manufacturers was takenbearing in mind that the procurement of thesesuperconducting cavities was the most critical aspect ofthe LEP upgrade program (entailing the mastery of manydifferent complex technologies) and also in view of thevery tight delivery schedule.For the LEP upgrading project the total number ofNb/Cu cavities is 256 (64 modules) providing 2.6 GV asthe accelerating voltage Eacc.2  RESEARCH AND DEVELOPMENT ATCERN2.1  CavitiesIn 1980 a development program aiming at theproduction of coated SC RF cavities was started atCERN.  The coating was achieved by sputtering usinginitially a bias diode configuration [2].  At the end of1984  a series of 15 single-cell 500 MHz cavities wasproduced, demonstrating that the accelerating fields andQ values are higher than those of bulk Nb. A newelectrode design was developed based on a cylindricalmagnetron sputtering configuration.  This offered severaladvantages:  increased sputtering rate, better adhesionand uniform thickness of the film.  Using this technique,a small series of LEP cavities (350 MHz) (12 cavities,three modules) was produced and installed in the LEPtunnel in order to obtain information on long-termperformance and to check reproducibility ofperformance.  One of the major problems met in theresearch and development program was the substratepreparation before Nb coating.  It turned out that in orderto avoid “peel-off” of the Nb film any contaminationmust be avoided;  in other words a cavity which has atotal surface of about 6 m2 must be as clean as the siliconwafers of 20 cm2 surface used in the manufacture ofVLSI (Very Large Scale Integration) integrated circuits.A special chemical treatment was developed at CERNfor the whole cavity, and the most critical phases ofproduction, such as magnetron installation on the cavity,rinsing and drying, were carried out in a class 100 cleanroom.2.2  Cryogenic ComponentsIt became apparent quite early that the final cryostatdesign needed some novel construction concepts toFig. 1.  Vacuum and helium tanksminimize the costs, to meet the requirements of highaccessibility to all critical parts such as couplers and tomaximize the average accelerating field.For these purpose a special vacuum vessel wasdesigned and developed at CERN [3].  This consists of acylindrical structure made of aluminum alloy; the mainsupporting frame of the vessel is made of two end ringsand three longitudinal plates which are welded to thetwo rings. Two stainless-steel-sheet sealing envelopesare wrapped around the cylindrical structure. This kindof design allows easy access to all cavity componentsand it turned out that this was very important during theproject because, due to technical difficulties and delayexperienced with the couplers, RF and HOM (HigherOrder Mode) couplers had to be installed at CERN onthe modules already accepted.Another cryogenic component developed at CERN isthe helium tank which encloses the cavity.  The designwas aimed at minimizing the volume of liquid helium(safety in the LEP tunnel).  The tank is made of 2 mmthick stainless-steel sheet welded throughout.  Its mainbody consists of two half-shells which match the shapeof the cavity. The seal between the cavity and the heliumtank is achieved at the stainless steel ™Conflat flangeswhich equip all the ports (beam tubes and couplers).Thus the copper seals of all ™Conflat flanges merelyseparate the ultra-high machine vacuum (inner cavityvolume) from the thermal insulation vacuum.3  PRODUCTION SEQUENCE3.1  Cavity ProductionThe major challenge for CERN was to help industryin mastering very quickly (six months) many differenttechnologies, i.e. electron-beam (EB) welding, ultra-highvacuum, chemical cleaning, Nbsputtering, clean-room operation andprovision of high-purity waterfacilities.  In most cases even largeindustrial companies can master onlysome of these technologies.The cavity half cells are producedby lathe spinning.  The beam tubesare rolled, EB welded longitudinallyand ball-extruded to create the powerand HOM coupler ports.  The beamtubes, as well as the half cells, aredegreased, electropolished (120 µmn–butanol, phosphoric acid) andrinsed with demineralized water. Theelectropolishing is applied in twosteps (~ 60 µm each); between thesethe surface is inspected visually andif necessary surface flaws areremoved.  ™Conflat type flanges arebrazed to the coupler and beam tube ports.  All parts arejoined by EB welding using the internal gun technique.The entire cavity is degreased, filled with sulphamicacid, chemically polished (~ 20 µm, sulphamic acid, n-butanol, hydrogen peroxide and ammonium citrate),rinsed with sulphamic acid, high-purity water(18 MΩ/cm) and alcohol and dried under clean laminarair flow (Fig. 2).Fig. 2.  Cavity in clean roomFor Nb sputtering the magnetron cathode is mountedand dismounted on the cavity in a class 100 clean room.After coating the cavity is closed, rinsed with high puritywater (18 MΩ/cm), dried by pumping and vented withfiltered (0.2 µm pore size) dry nitrogen gas.  The cavityis then ready to be shipped to CERN for the first RF testin a vertical cryostat.  If the cavity does not attain therequired performance, an attempt is made to localize thedefects by temperature mapping and to recover thecavity by helium processing (24 hours), or if necessaryby water rinsing (at low or high pressure).  If this isunsuccessful the defective Nb layer is chemicallyremoved and the cavity returned to the manufacturer fora second or even a third Nb coating. In view of the verydemanding contractual RF specifications [Q at 6MV/mof at least 3.4 × 109 at 4.5 K] the intermediate RF test onthe vertical cryostat was found to be absolutelyfundamental for the production sequence.Temperature mapping and visual inspection werealso necessary tools for detecting and classifyingdifferent kinds of defects.  By far the greatest number of“hot spots” detected on cavities during the RF tests werecaused by non-uniform interface structure which reducedthermal contact between the Nb layer and the copperbulk [4].  This is clearly related to failures in thechemical polishing, water rinsing and drying of thecopper surface before coating.3.2  Module ProductionFig. 3.  Assembly in clean roomAfter the intermediate test at CERN described above,whereby the quality of the Nb coating was checked, thecavities were returned to industry for assembly intomodules (four cavities, 12.5 m total length).  The criticaloperation was the connecting of the large diameter(Ø = 24 cm) beam tube bellows which had to be done ina class 100 clean room (Fig. 3). Best results are obtainedif this operation is done in as short a time as possible tominimize the risk of contamination.  After mounting ofthe ancillary equipment (tuners, helium piping etc.)within the cryostat, the modules are sent to CERN,without the HOMs and RF couplers, for final acceptance.4  ANALYSIS OF THE INDUSTRIALPRODUCTION4.1  Cavity ProductionThe most critical step in cavity production is thesubstrate preparation before Nb sputtering which, if notproperly done, usually results in “peeling off” of the Nbfilm.  Detached areas are typically of the order of somesquare millimeters which might seem small compared tothe 6 m2 of inner surface but is unfortunately sufficientfor completely spoiling a cavity. Possible failures duringsurface preparation could be related to stains of chemicalproducts on the surface, insufficient degreasing, liquidretention in pits, areas with varying copper structureintroduced during the lathe-spinning or the electro-polishing, or mechanical damage to the surface.Another possible mechanism of defect creation isforeign particles sticking on the cavity surface beforeand/or after coating.  These can be either dust particlesintroduced during the manipulation or operation of thecavity or metal splinters from gaskets or tooling. In orderto assure the success of this delicate phase of cavityproduction, a very careful quality protocol of procedureswas agreed upon with the three firms.  It must bestressed that the visual inspection of the cavity afterchemical treatment (and after grinding off of anylocalized defects) is a critical step. In fact the mostimportant component of the  technology transfer was toformulate a precise quality control procedure.Figure 4 shows the best performances among the 200cavities already tested vertically so far. The Q-factorsare consistently higher than the specifications (by 25%on average). Figure 5 shows the performance of theaccepted cavities (Q at 6 MV/m) as a function of theyear of production.  There is a clear trend towards highervalues showing that the firms gradually learned how toproduce better and better cavities.1100 2 4 6 8 10Ea[MV/m]Q[1E9]Vertical test at 4.5K3.4 at 6 MV/mAcceptance valueFig. 4. Performance of the best LEP cavities3.003.203.403.603.804.004.204.404.604.8093949501234567Accepted CavitiesQ(6MV/m)*109939495QAV = 4.03QAV = 3.92QAV = 3.72Fig. 5.  LEP cavity performance  vs. year of productionAt present the success rate after a single coating isabout 75%, but at certain periods of the seriesproduction, one company managed to produce 20consecutive flawless cavities.4.2  Module AssemblyMost of the problems encountered up until the finalassembly were related to clean-room technology.Working in a clean room requires not only a sequence ofprecise procedures but also the development of a specialbehavioral “culture”.  Companies who succeeded increating this “culture” with competent people andadequate equipment showed a zero rejection rate!Companies whose personnel were less well trained andwhere the equipment was inadequate experienced arejection rate as high as 50%.  At present, all threecompanies can reach the required performance levels, insome cases even without helium processing.  Nosignificant degradation of the Q vs. Eacc curve wasobserved [1].When there was doubt concerning possible dustcontamination during the assembly operations in theclean room, it was found important not to continue theoperation, but rather to recheck the cavities, possiblyrecovering them by rinsing before completing theassembly of a full module.  It should be kept in mindthat the module assembly is the last critical step in afour- to five-month fabrication sequence, and that onlyabsolutely minimal risks should be accepted.5  CONCLUSION AND FUTUREThe new technology developed at CERN, Nb sputter-coating of copper cavities, has now become an industrialreality.  Three European firms are able to produce SCcavities (from sheet metal to the final product) havingvery high level RF performance at liquid heliumtemperature. It has to be emphasized that thanks to thetransfer of this technology from CERN to the threefirms, they can now handle different techniques such aselectron-beam welding, ultra-high vacuum, clean-roomwork, the implementation of large chemical and high-purity water plants, sputter-coating etc.We have achieved strict adherence to our productionplan by three separate companies.   In Fig. 6 the totalnumber of accepted cavities vs. time shows a constantand uninterrupted production rate. Using three suppliersensured a steady production rate even if one or the otherof the companies experienced a temporary loss ofcapacity.05 01 0 01 5 02 0 02 5 02 -Jan -9 21 -Jan -9 31 -Jan -9 41 -Jan -9 51 -Jan -9 63 1 -D e c -9 6Fig. 6.  Number of accepted cavities vs. timeOne of the keys to the success of the project has beenthe availability at CERN of the necessary know-how andequipment for backup if the companies experienced anytemporary disruption of the production.  CERN was ableto check procedures at any time and at any stage ofproduction (chemical treatment, Nb-coating, high-puritywater rinsing etc.) to determine where the problemoriginated. It is of the utmost importance that this systemof CERN-based checks be maintained until the end ofthe LEP upgrading project.It has been demonstrated (Fig. 7) that the Q-values ofNb-coated cavities are superior to those of bulk Nb forthe same frequency and at the same temperature (4.5 K).It should be noted that this comparison is quite fair,since the cavities of both types were produced by thesame company.  Bulk Nb shows thermal quench, whichis not the case for Nb/Cu, and it has to be emphasizedthat none of the 45 modules tested was limited by athermal quench.A further advantage of this technology is itseconomy: comparing the industrial costs of bulk Nb andNb/Cu cavities, it can be concluded that the total cost ofcavity production for the LEP upgrade was reduced byabout 20%. The Nb sputter-coating technique could beapplied for future machines for frequencies up to800 MHz, with accelerating fields between 6 and10 MV/m at 4.2-4.5 K. More than 80% of theindustrially produced cavities for LEP2 had Q ≥ 2.5⋅109at 8 MV/m (4.5 K).∆ Nb/Cu (2012)     O bulk NbCavity a : Cavity b :Cavity c : Cavity d : 1100 2 4 6 8 101100 2 4 6 8 101100 2 4 6 8 10Thermal Quench1100 2 4 6 8 10Thermal QuenchFig. 7.  Comparative performance of Nb bulk and Nb/CuVery encouraging results have also been obtained atCERN for a higher frequency (1.5 GHz)   Anaccelerating field of 13 MV/m and low field Q-value ofQ > 1010 were achieved with a five-cell cavity at1.8 K [5].The success rate of production could probably befurther increased by making some improvements insurface preparation (electropolishing the whole cavity ordeveloping a new chemical treatment).  In additionvisual inspections could be replaced by more objectiveprocedures. In order to avoid expensive and lengthyrecovery operations it is very important, particularly in areal industrial environment, to establish correlationsbetween fabrication steps and cavity performance.I would like to stress that the new technology hasattained sufficient maturity to make it attractive forfuture applications.6  ACKNOWLEDGEMENTSIt is a pleasure to acknowledge the vision,competence and support of the pioneers of the Nb/Cutechnology:  E. Picasso, Ph. Bernard and C. Benvenuti.Many thanks go to D. Boussard for his critical remarksand friendly help in writing this article.   The SC cavitieswould not be an industrial reality without the technicalcompetence and strong motivation of R. Hänni,E. Magnani and A. Scharding.  Thanks are also due toK. Schirm for discussions and help in preparing thispaper.   I am grateful to H.P. Vogel (ACCELInstruments GmbH), J.C. Boutes (CERCA) andP. Gagliardi (Ansaldo Energia SpA) for helping me tounderstand the industrial approach to large seriesproduction.  I am indebted to Prof. V.L. Telegdi for hishelpful advice and critical reading of this paper.REFERENCES[1] E. Chiaveri, ‘Production by Industry of a LargeNumber of Superconducting Cavities:  Status andFuture’, Particle Accelerators 1996, to be published.[2] C. Benvenuti, ‘Superconducting Coatings forAccelerating RF Cavities:  Past, Present, Future’,Proc. 5th Workshop on RF Superconductivity,Vol. I, p. 189, DESY, Hamburg, 1991.[3] R. Stierlin, ‘Development of a Cryostat for the 4-cell 352 MHz SC Accelerating Cavities for LEP’,Proc. 3rd Workshop on RF Superconductivity,Vol. II, p. 639, Argonne National Lab., USA, 1988.[4] K. Schirm, ‘Production of Cavities and Modules’,Proc. 6th LEP Performance Workshop, Chamonix,15-19 Jan. 1996; CERN, Geneva March 1994.[5] D. Bloess et al., ‘Superconducting, Hydroformed,Niobium Sputter-Coated Copper Cavities at1.5 GHz’, 6th Workshop on RF Superconductivity,Vol. II, p. 739, CEBAF, Newport News, Virginia,USA, 1993.

Large-Scale Industrial Production of Superconducting Cavities

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