Progress and R/D challenges for FCC-ee SRF

EPJ Techniques and Instrumentation Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 https://doi.org/10.1140/epjti/s40485-023-00094-5 REVIEW Open Access Progress and R/D challenges for FCC-ee SRF W. Venturini Delsolaro1*,M.Garlasche 1, F. Peauger1,G.Rosaz 1,I.Karpov 1, L. Zhang2,A.M.Valente Feliciano3,S.A.Udongwo 5,A.Bianchi 1,G.Bellini 1, L.M.A. Ferrera1, C. Pereira Carlos1,L.VegaCid 1,S.Leith 1, T. Proslier4, S. Gorgi Zadeh1,M.Timmins 1, M. Therasse1,T.Koettig 1,S.Atieh 1, O. Brunner1and F. Gerigk1 *Correspondence: Walter.V[email protected] 1European Organization for Nuclear Research (CERN), CH-1211, Geneva, 23, Switzerland Full list of author information is available at the end of the article Abstract The FCC-ee machines present a huge challenge for the RF systems, which need to be adapted to very diverse beam conditions going from moderate energy and high current for the Z machine to high energy and low beam current for the ttbar. This inverse scaling results naturally from a fixed budget for the synchrotron radiation, which the SRF cavities need to compensate. A global solution was elaborated for the FCC Conceptual Design Report (Abada in Eur Phys J Spec Top 228):261–623, 2019), and is referred here as the baseline. Recently, further studies have led to a new optimized baseline, still based on traditional elliptical cavities. In parallel, a novel concept, named the Slotted Waveguide ELLiptical (SWELL), was proposed with the potential of greatly simplified logistics and reduced costs. Under several aspects, all these changes call for enhanced performance of the RF systems. A vigorous R&D program has therefore continued since the publication of the CDR, with the aim of pushing the performance and demonstrating the feasibility of a more advanced baseline and, more recently, of the SWELL option. The progress and challenges of this ambitious program were presented in the dedicated SRF sessions at FCC week 2022 (FCC week 2022 website, 2022,https://indico.cern.ch/event/1064327/timetable/)and are summarized in this paper. Keywords: Future Circular Collider; RF superconductivity; Accelerating cavity 1 Machine parameters relevant to RF and revised SRF cavity baseline As detailed in the FCC Conceptual Design Report (CDR) [1], the RF system of the FCC lepton machines is designed to provide a constant RF power of 50 MW per beam at four different energies going from 45.6 GeV to 182.5 GeV (see Table 1). Thedesign choicesoftheacceleratingcavitiesallowtocopewithbeamcurrentsandRF voltages varying by two orders of magnitude. Accelerating cells operating on the TM010 mode with elliptical shapes have been adopted for their ability to operate at high accelerating gradient (for the H and ttbar energies), hence limiting the overall length of the RF sections. Fortheoperationathigh beamcurrents(Z andWmodes), thepowerhandlingcapacity of the fundamental power couplers, which equip each cavity, will be the limiting factor. High beam current operation will also induce higher order modes in the cavity. It is thus © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicatedotherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 2 of 13 Table 1 RF-relevant machine parameters mandatory to damp and extract them to avoid beam instabilities and overheating of the cavity surroundings. In the CDR, the LHC frequency of 400 MHz and its first harmonic (800 MHz) were chosen for the design of the accelerating cavities. Single cell 400 MHz cavities hosted in four-cavity cryomodules and each equipped with four coaxial higher mode couplers have beenconsidered forthe Zmachine.Eachcavity operatesat1MWRFpowerin CWmode bymeansofasingleLHCtypecoaxialfundamentalpowercoupler.FortheWandHenergies,thestrategydescribedintheCDRconsistsinreplacingthesinglecellcavitiesby4-cell 400 MHz cavities and install progressively additional cryomodules during shutdowns to reach the desired energy. This approach is inspired by the construction and operation of the LEP RF system. The CDR targets for the FCC-ee are: 10 MV/m accelerating gradient and1MWRFpowermaximumforeach4-cell400MHzcavity.Toreachtheenergyofthe ttbar, 5-cell 800 MHz cavities are added to the 400 MHz RF system, all operated on-crest at 20 MV/m. It is important to mentionthatthe high energy boosterallowingthetop-upinjection of the beam into the collider is a third ring in the tunnel which needs a dedicated RF system working in pulsed mode and reduced beam current. In the CDR, 4-cell 400 MHz cavities operating at the same RF voltage as for the collider were considered for this additional ring. Since the publication of the CDR, investigations have continued to address some criticalities of the baseline solution. From the longitudinal beam dynamics point of view, operation at the Z pole is the most challenging, due to high beam current and large number of bunches. With the present single cell cavity design, coupled bunch instabilities are supressedbyseveralwell-knownRFfeedbacktechniquesorbysynchrotronradiation.Transient beam loading effects, for instance due to abort gaps in the beam structure, are well mitigated and HOM power losses are kept manageable. In LEP, the 4-cell 350 MHz cavities were found limited to about 7 MV/m accelerating gradient in average over large statistics. The huge superconducting surface (∼5m 2) implies a higher probability to have defects during fabrication, which would induce field emission,Q0degradationsorquenches.Improvementofsuchlargecavitiesisnotobvious and requires significant SRF infrastructure upgrades at CERN. It is not evident that the surface treatments recipes required to reach high gradients can be adapted to this cavity type. In the new proposed baseline, 4-cell cavities are replaced by new 2-cell cavities at the same400 MHzfrequency.Thisnewoptionfacilitatesthemanufacturingandsurfaceprocessing. Theamountof RF powerper cavitycan be divided bytwo, relaxing thespecification of the FPC. Each cell is now well coupled with HOM couplers placed on the adjacent Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 3 of 13 Table 2 Revised parameter table beampipe,dampingthehighfrequencymodesinamoreeffectiveway.Thisconfiguration isbetterfortheWoperationwherethebeamcurrentof135mAcanalsobeconsideredas high. To limit to total length of the RF system, the accelerating gradient specified for the 2-cell cavities has been increased to 12 MV/m. Achieving this level of performance constitutes the main objective of the R&D program for the next years. The use of the 5-cell 800 MHz elliptical cavities and their performance specifications are also being revisited. These cavities will equip the RF system of the booster except for the Z machine. An increase of the nominal accelerating gradient from 20 to 25 MV/m was introduced to allow a drastic reduction of the total number of cryomodules. Thanks to the 2-cell 400 MHz cavity and its ability to operate at high current, the alignment of the cryomodule to use a common RF system for both beams can be performed at the level of the H energy. Table 2summarizes the revised RF parameters, taking into account the up-to-date beam characteristics. 2 Cavity optimization for the revised baseline For the Z and W machines, the specificity in the cavities RF design is the cancellation of longitudinal higher order modes (HOMs). Thanks to an advanced multi-parameter optimization process, the shape of the ellipses is chosen to lower the peak electromagnetic surface fields while eliminating any high frequency resonant longitudinal modes below thecut-offfrequencyofthebeampipe.ThispreventsresonantexcitationofHOMsbythe beam which would generate kilowatts of parasitic RF power in the cryomodules. In the revised baseline, most of the efforts have been focused on the 2-cell 400 MHz cavity design. At the nominal accelerating gradient of 12 MV/m, the peak electric and magnetic surface fields are respectively Ep= 24.4 MV/m and Bp = 76.1 mT. With a geometric factor G of 196.3 , achieving a minimum Q0of 3e9at nominal field requires a maximumsurfaceresistanceRsof65n.Withfourhook-typecouplersintegratedon the beam pipes (two on each side), the transverse HOMs are sufficiently damped at about 60 k/m, which is far below the beam instability threshold of 700 k/m at the W working point. Cavity studies will be pursued in the next years to continue optimizing the shape of the three cavity types. The damping schemes with coaxial couplers will be deeply analysed to ensure that mechanical errors do not jeopardize their functionality. Complementary Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 4 of 13 evaluationslikemultipacting analyses and frequency sensitivitytoLorentzforcesandmicrophonics will be also addressed. 3 R&D challenges for the revised baseline As mentioned, the new baseline calls for higher nominal gradients and unloaded quality factorsintheSRFcavities.The400MHzsystemshallrelyonthe Nb/Cutechnologywhile for the 800 MHz, we envisage a more traditional solution based on bulk Nb. The strategy adopted is todeveloptheNbcoatings on 1.3 GHz mono-cells, which are cheapertomanufacture and offer higher turnaround for quick feedback on new coating and preparation methods. At the same time, practising at 1.3 GHz will keep open the possibility to adopt Nb/Cu also for the 800 MHz system, if the performance targets at high field can be met reproducibly. Nb thin films on copper havealready shown [3] to sustain high accelerating fieldsatQ0inthe 1010 rangeat 1.3 GHz. However,in thesecoatings,realisedwiththeDC magnetron sputtering method, the surface resistance rapidly increases with the RF field, which so far limited the application of Nb/Cu to low-to-medium gradient accelerators. Thisphenomenon, known astheQ-slope problem[4],isstillanopensubjectofresearch: several theories and models have been proposed, none of which is fully satisfactory over all the parameter space accessible to the experiments. In recent years, progress was made inthemitigationoftheQ-slope,byusingenergeticcondensationtechniquessuchasHigh Power Impulse Magnetron Sputtering (HIPIMS) [5]. Along with the unwanted Q-slope, Nb films display several attractive features, like an optimized BCS surface resistance, typically a factor 2 lower than bulk Nb, which makes 4.5 K operation (where the BCS component is dominant) possible at higher frequencies than for bulk Nb. 4BeyondNb Asindicatedabove,reducingtheBCSsurfaceresistanceisnecessaryifoperationatsuperfluidheliumtemperaturesistobeavoided,withtheconsequentsavingsintermsofcapital andrunningcostsforalargeinstallation.Therefore,newSRFmaterialswithhigherTcare actively investigated. Among these, Nb3Sn is the prime candidate, and the SRF community has been developing it for a long time with very encouraging results, albeit not yet mature for operation in a real accelerator. Thus far, the main development of Nb3Sn for SRF applications uses bulk Nb as a substrate on which the Nb3Sn A15 phase is grown by thermally diffusing Sn from vapours at high temperature. Nb3Sn cavities produced by thermal diffusion have reached fields in excess of 20 MV/m at 1.3 GHz and 4.5 K [6]. One of the issues for improving their performance is to guarantee enough thermal stability to avoidpremature quenches.Itis thereforeinteresting to explorethepossibility to formthe A15 phase on a high thermal conductivity substrate like copper. Work in this direction is ongoing in several laboratories, including CERN. 4.1 The 1.3 GHz program In the past few years, Nb films on copper deposited with HIPIMS were optimized first on smallsamplesmeasuredwiththeQPR,andthenonTESLA shape1.3GHzsinglecell cavities. Initially, progress was slow due to the lack of suitable substrates. Actually, porosities atweldlocationandotherdefectsjeopardizedresults.Totacklethisissue,CERNlaunched the manufacturing of high quality substrates along three different lines: from the experiencewithHIEISOLDE[7]itwasknownthatelectronbeamwelding(EBW)ofcoppercan Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 5 of 13 Figure 1 Some steps of bulk machining of a 1.3 GHz cavity induce defects which spoil the final RF performance, therefore seamless techniques were pursued.Inparallel,electro-polishingofellipticalcoppersubstrateswasimplementedand the sputtering systems and RF test stands were optimized. Once new substrates became available, the program changed pace. In the next paragraphs the main achievements will be briefly described. 4.1.1 Innovative substrate manufacturing Tosetastandard,machiningoutofabulkcopperblockwasusedtoproduceafewseamless mono cells. In Fig. 1, we show the tooling for internal machining, the machined cell, and the finished cavity after welding of the cut off tubes. The process was implemented in CAM and diamond finishing was applied to obtain a final roughness Ra<15μm. Shape accuracy within 20 μmwasachieved. Other seamless manufacturing techniques for elliptical cavities include hydroforming, and Cu electrodeposition on an aluminium mandrel, which is subsequently removed chemically. In parallel, in order to improve the quality of EB welding of copper, specifically for the equatorialweldofthecell,toolingand initial testshavebeendeveloped,sotorealiseafull penetration weld from the inner cavity side. The above-mentioned activities are performed while taking into consideration process up-scalabilitytolargercavitysizes,suchas400MHz.Inparticular,hydroformingisinvestigated as a potential candidate for series production in industry. Manufacturing studies go hand in hand with related R&D. In such framework, finite element analyses are performed in order to benchmark and optimise processessuchashydroforming,spinningandmachining.Tothisaim,specificmaterialandfailuremodelsare Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 6 of 13 being created. Furthermore, experimental studies are performed in order to understand the impact of different processes (both shaping and material removal) on the quality of coating and RF performance. 4.1.2 Electro-polishing of copper cavities The copper substrate surface is critical for the quality of the deposited Nb film, and it needs to be processed after the manufacturing steps to remove the mechanically damaged layer. Chemical polishing was applied in the past (LEP, LHC, HIE ISOLDE), as it is simplertoimplementforcomplexshapes.Itisknown,however,thatelectro-polishing can produce smoother surfaces as it naturally tends to preferentially etch the peaks of the surface roughness. Modelling and optimization of electro-polishing for copper cavities was the subject of recent work presented at FCC week 2022. Strong focus is put on the combined effects of electrochemistry and fluid dynamics. This study has shown that the electro-polishingplateaucanbesignificantlyshiftedduetothechemicalsflow.Thisbeing taken into account, a model is proposed to obtain a universal description of the process in order to ease its scale-up and enable the surface treatment of 400 MHz elliptical cavities meant to be used in the FCC-ee machine. The electro-polishing of 1.3 GHz cavities has already proven to lead to very high quality Nb thin films and the next step consists of transferring this know-how to actual 400 MHz cavities in view of reaching the FCC-ee performance target. 4.1.3 HiPIMS coatings,Nb film optimization Itisonlyafterthecoppersubstrateisundercontrolthatworkcanstarttotweakthecoating parameters in order to find optima in the superconducting characteristics. In fact, since PVD coatings are deposited in out-of-equilibrium conditions, and since the SRF performance is determined by the properties of a thin layer of the order of a few tens of nm, there isa virtually infinite panoply ofoutcomes depending on coating parameterslike the substratetemperature,thesputtering gas, the workingpressure, thebiasvoltage, etc. The mainproblemistoidentifyestimatorsofthefinalRFperformance,whichcouldbereadily measuredon small samples. Recentresultshaveshown thatthe critical currentdensity of Nb thin films can be used as a proper indicator of film quality. This is backed by in-depth material analyses highlighting the impact of dislocations onto the film’s capacity to trap magnetic vortices. The latter being partly responsible of the quality factor degradation, it isofgreatimportancetocontroldefectdensityviatuningofthecoatingparameters.Ithas been shown that the energy of impinging Nb ions during the coating process is of prime importance and an optimum value has been identified and leads, in combination with a proper surface treatment, to high performance Nb films in 1.3 GHz resonators achieving bulk-like behaviour up to acceleratingfields of the order of 10 MV/m. In view of relaxing therequirementson thesurfacetreatmentprocessit has also beendemonstratedthatthe coating can be performed in two steps. The first step uses high ion bombardment energy, which results in a planarization of the layer whose surface roughness becomes independent of the substrate. Monte-Carlo simulations proved to be an efficient tool to predict such planarization effect. Soon the impact of such two-step process will be evaluated on actual RF resonators. Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 7 of 13 4.1.4 Cavity testing results 1.3 GHz single cell cavities are tested at CERN in a vertical cryostat, using standard RF equipment. The cavity is driven on resonance by means of a Phase Lock Loop (PLL). A variable coupler is used to minimize measurement uncertainty by maintaining critical coupling conditions throughout the measurement. The standard measurement program includes scans of Q vs E, at 4.2 K and at 1.8 K, as well as Q vs temperature at fixed field, to extrapolate the residual surface resistance, and of resonance frequency vs temperature to extract the penetration depth. In Fig. 2, we show the Q vs E curves at 4.2 K of 15 cavities tested between January 2021 and April 2022 and the 400 MHz target values scaled to 1.3GHz.Wstands for welded,BM for machinedfrom bulkandLforelectrodeposited.In general, the surface resistance at low field matches the theoretical minimum as function ofmeanfreepath,whichisforeseenby theMatthis–Bardeenexpressions,andtheQslope is small. The field limitation below 10 MV/m comes from administrative limits with near to zero threshold for the radiation interlock in the CERN cryolab. Forthe FCC program aspresentlydefined,CERN envisagestouse Nb coated cavities at 4.2K,thereforetheachievedresultsarealreadysatisfactory,iftransposedatthelowerfrequencies of interest. However, our efforts aim to understand and study the contributions to the residual resistance, which dominates at lower temperatures. Bringing these under control would enable application of the Nb/Cu technology to all FCC machines, besides offering a valid alternative for high-energy colliders at large. Figure 3shows the Q vs E curves at 1.85 K. At these low temperatures, the BCS component is suppressed, and the non-linear behaviour of the residual surface resistance is highlighted.AlargerscatterintheresultsisapparentfromthecomparisonofFigs.2and3: at low temperature, in some cases, the Q slope is low and even comparable with bulk Nb, whileinothercasesitremainslarger.Table3summarizesthesuperconductingparameters oftheHiPIMS Nb/Cucavitiesmeasuredsofar.Again, wenote the high reproducibility of fundamental quantities such as the critical temperature and the penetration depth, indicatinggoodcontroloftheNbfilmgrowth,whiletheresidualresistanceshowsmorespread results. Since sensitivity to temperature gradients upon crossing Tcis systematically observed, all reported scans were taken after thermal cycles above Tc,whichoptimisethe homogeneity of the superconducting transition. On some of the tested cavities, the measurement was carried out severel times interleaved with thermal cycles up to room temperature. The results were fully reproducible, showing that the coating withstands well the stresses induced by the different coefficient of thermal expansions between copper and niobium. 4.1.5 Thermal mapping Even when the superconducting properties of the coating have been optimised, local imperfections may still limit the cavity performance. Defects such as bad adhesion or local contamination of the Nb film show up as hot spots when the cavity is powered. Thermal mappingisapowerful tooltolocatesuchdefects and gain insightonhowtopreventtheir occurrence.WhilethisisastandardtoolforstudiesonbulkNbcavities,specialcaremust betakeninthecaseofcoppercavities,duetothemuchlargerthermaldiffusivity,resulting in smaller signals for the same amount of dissipated power. A new thermal mapping system for 1.3 GHz cavities was developed at CERN. It comprises 192 Allen Bradley thermometers in contact with the cavity surface and mounted Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 8 of 13 Figure 2 1.3 GHZ HiPIMS Nb/Cu cavities performance at 4.2 K Figure 3 1.3 GHZ HiPIMS Nb/Cu cavities performance at 1.85 K on 12 boards, signal processing is through 12 ADC channels and 6 multiplexers. Figure4 showsaprototypeboard,togetherwiththesimulatedandfirstmeasuredtemperatureprofiles.Firstresultswiththethermalmappingsystemindicatethatdissipationislocalisedat pseudo random locations along the cell, which is an important clue for further optimisation of the coatings. More details on the thermal mapping systems are in [8]. 5 Progress with Nb3Sn on Cu Nb3Sn on Cu was studied at CERN since the start of the FCC study. The approach followed begins by optimizing the A15 phase formation and some key DC superconducting parameters on small samples. Good quality Nb3Sn layers, with Tcup to 16 K, have been Venturini Delsolaro et al. EPJ Techniques and Instrumentation (2023) 10:6 Page 9 of 13 Table 3 Superconducting parameters of 1.3 GHZ HiPIMS Nb/Cu cavities Figure 4 Prototype thermal mapping with a first validation test deposited on copper substrates by DC magnetron sputtering from a stoichiometric target.Copperdiffusion intothegrowingfilmwastackledbysuitablediffusionbarriers,film cracking was mitigated by using Kr as sputtering gas. In this phase, in situ annealing was recognizedtobecrucialinachievingtheexpectedTc[9].Afterthecoatingrecipewasoptimizedonsmallsamples,theRF surfaceimpedancewasassessedbymeansofQuadrupole Resonator measurements. The results indicated a high residual resistance, which significantly increased with the applied RF field strength. Following these first indications, the deposition method was changed to bipolar HIPIMS, with a Ta interlayer. This choice was based on the recent results obtained on 1.3 GHz Nb/Cu resonators. This technique has shown to provide thin films with a Nb:Sn stoichiometry that lies perfectly in the desired rangeoftheA15crystallinephase.Theeffectofcoatingparameterssuchaspressure,temperatureandbipolarHiPIMSbiasvoltageontothecriticaltemperaturehavebeenstudied in depth and thin films with Tcof about 15.5 K have been obtained. The film surface contamination with copper has been highlighted as a critical point and solutions such as thermaltreatmenthavebeenproposed.Inthefollowingyear,theNb3Sn/Cusurfaceresistance will be evaluated and a special effort will be put on pushing thefilms’ Tc to thebulk value of 18.3 K. 6 Atomic layer deposition for SRF cavities Atomic Layer Deposition (ALD) is a highly conformal thin film synthesis technique that enablesanunprecedentedleveloffilmthicknessandcompositionuniformityonarbitrary