Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 1

Superconducting RF cavity R&D for future accelerators

C. M. GinsburgFermilab, Batavia, IL 60510 USA

High-beta superconducting radiofrequency (SRF) elliptical cavities are being developed for severalaccelerator projects including Project X, the European XFEL, and the International Linear Collider(ILC). Fermilab has recently established an extensive infrastructure for SRF cavity R&D for futureaccelerators, including cavity surface processing and testing and cavity assembly into cryomodules.Some highlights of the global effort in SRF R&D toward improving cavity performance, and FermilabSRF cavity R&D in the context of global projects are reviewed.

I. INTRODUCTION

Substantial effort is being expended in the questfor high gradients in superconducting radiofrequency(SRF) cavities for ongoing and proposed projects: (1)the test/user facilities STF (KEK), NML (Fermilab),and FLASH (DESY), (2) the European XFEL cur-rently under construction, (3) Project X at Fermi-lab, and (4) the International Linear Collider (ILC).The common requirements or choices for the high-gradient cavities in these projects are gradients of atleast 23 MV/m, β=1, elliptical shape and an acceler-ating mode frequency of 1.3 GHz. Recent SRF R&Dhighlights in the pursuit of high cavity gradients andthe status of Fermilab SRF infrastructure develop-ment are reviewed.

II. ACHIEVING HIGH GRADIENT

Achieving high gradient in SRF cavities for currentprojects requires pure niobium, an excellent surfacequality, and a good geometry.

For the ILC [1], the niobium sheets from which cav-ities are formed must have a residual resistivity ratio(RRR) of at least 300, consistent with very high purityniobium, to ensure good superconducting properties.

Because RF fields occupy the first ∼40 nm of theinner cavity surface for these cavities, as shown inFig. 1, the quality of the innermost surface is criti-cal and must be carefully controlled both during cav-ity fabrication and test preparation. To avoid the in-clusion of contaminants or defects apparent after thesheet manufacturing, eddy current scanning (ECS) ofall sheets is performed before cavity fabrication. ECShas proven to be a very useful feedback mechanismfor material vendors, constributing substantially to anoverall material quality improvement in recent years.After fabrication, the cavity inner surface must bevery smooth, with no inclusion of foreign particles,or topological defects such as bumps or pits or sharpniobium grain boundaries, which could increase sur-face resistance locally. In addition to the strict qualityassurance for the material, the introduction of dust orother microscopic contaminants must be avoided afterthe final surface preparation, to prevent field emission

which can become dark current during machine opera-tion and cause undesirable heating in superconductingelements and damage to accelerator components.

FIG. 1: Simplified drawing of an SRF cavity surface.[Courtesy of Hui Tian.]

In selecting a cavity geometry for optimum perfor-mance, cavity cell shapes are typically optimized forlow peak surface magnetic field (Hpeak) and low peaksurface electric field (Epeak) relative to the gradient(Eacc), as well as ease of surface processing and fabri-cation.

Many cavities have reached 35 MV/m or more inthe last decade [2, 3], particularly single-cell cavitiesof varying elliptical shapes and 9-cell Tesla-shape cav-ities, as shown in Fig. 2.

III. SURFACE PROCESSING

The surface treatment intended to maximize cavityperformance, developed for the ILC [4], includes ini-tial preparation steps to remove ∼150 µm from theinner surface using electropolishing (EP). This ini-tial removal step may be performed with centrifugalbarrel polishing (CBP) or buffered chemical polishing(BCP) at some labs; however, the maximum gradi-ents reached with BCP as the initial preparation stepare lower than those achieved using the other meth-ods. Cavities then undergo an 800C annealing step,to drive hydrogen from the surface. The final prepa-ration steps include degreasing with detergent, an-

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2 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009

FIG. 2: Maximum cavity gradient as a function of timefor 1-cell cavities of non-standard shapes (top, courtesy ofKenji Saito) and of 9-cell cavities of Tesla shape (bottom,courtesy of Lutz Lilje). All tests were performed at 2K.(Data are from KEK, Cornell, JLab, and DESY.)

other light electropolishing (∼20 µm), a high pressurerinse (HPR) with ultrapure water, drying in a class-10cleanroom, and then evacuation and low-temperaturebaking (120C) for about 48 hours after the final as-sembly with couplers. Additional surface treatmentsto address field emission, which occur after the finalEP, will be described later. The primary methods formaterial removal during surface preparation are CBP,EP and BCP.

CBP is a standard technique developed for cavitiesat KEK in which abrasive small stones are placed intoa cavity with water to form a slurry and the cavityis rotated. The KEK CBP machine and a diagramshowing the CBP process are shown in Fig. 3. As acentrifugal process, material is preferentially removedfrom the equator region. Since standard cavities havean equator weld, CBP is very effective in smoothingthe weld.

EP is an electrolytic current-supported material re-moval, and has been developed for use on cavities by

FIG. 3: The KEK CBP machine (top) and a diagramshowing the CBP process (bottom). [Courtesy of KenjiSaito.]

KEK in collaboration with industrial partners, andadopted at most labs. The EP process is complemen-tary to CBP because the material removal is preferen-tially on the iris. The ANL EP machine and a diagramshowing the EP process are shown in Fig. 4. In thiscase, the niobium cavity functions as an anode, andan aluminum cathode is inserted on the cavity axis.The electrolyte is HF(40%):H2SO4 in a ratio of 1:9 byvolume. Some sulfur remains on the surface after EPand will cause field emission unless it is removed.

BCP involves filling a cavity with a combinationof HF(40%):HNO3(65%):H3PO4(85%) in a 1:1:2 ra-tio by volume. BCP has been shown to cause hydro-gen contamination at the surface; this problem canbe mitigated by using the appropriate proportion ofacid and buffer and by keeping the temperature belowabout 15C. BCP is rather less expensive than EP andis often sufficient to produce cavities attaining gradi-ents as high as about 25 MV/m. However, it tendsto enhance grain boundaries which may degrade theperformance of standard fine-grain cavities.

An improvement to the average maximum gradi-ent was seen after the previous BCP standard was re-placed with EP, as shown in Fig. 2. The comparison

Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 3

FIG. 4: The ANL EP machine (top, courtesy of MichaelKelly), and a diagram showing the EP process (bottom,courtesy of Lutz Lilje).

of best test results of DESY cavities treated with BCPand EP is shown in Fig. 5, revealing that the maxi-mum gradient achieved with BCP is ∼25-30 MV/m,whereas the maximum gradient achieved with EP israther higher at ∼35-40 MV/m. A comparison of theresulting surface smoothness is shown in Fig. 6. Al-though it is possible now to achieve gradients in 9-cellcavities higher than 35 MV/m, there are several fac-tors which limit cavity performance.

IV. REDUCING FIELD EMISSION

One of the factors which limits cavity performanceis the presence of field emission. Four recent fieldemission studies have been performed: flash EP (alsoknown as fresh EP), dry ice cleaning, degreasing, andethanol rinsing. In tests at KEK using six single-cellIchiro-shape cavities, a 3 µm EP using fresh acid afterthe final EP was studied [5]. A gradient improvementto both average and rms was observed. Furthermore,the treatment was found to increase the gradient atwhich field emission turns on. Dry ice cleaning is a less

FIG. 5: Comparison of cavities processed with BCP (top)and EP (bottom). The EP’d cavities reach noticeablyhigher gradients. [Courtesy of DESY.]

FIG. 6: Comparison of a sample surface after BCP (left)or EP (right). The EP’d surface is noticeably smootherthan the BCP’d surface. [Courtesy of DESY.]

developed but promising alternative being developedat DESY [6]. It reduces or eliminates contaminatingparticles in several ways: they become brittle uponrapid cooling, they encounter pressure and shearingforces as CO2 crystals hit the surface, and they arerinsed due to the 500 times increased volume after sub-limation. Also, LCO2 is a good solvent and detergentfor hydrocarbons and silicones, etc. Dry ice cleaningis a dry process which leaves no residues, because theloosened contaminants are blown out the ends of the

4 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009

cavities by the positive pressure, and can be performedin a horizontal orientation. The DESY dry ice nozzleis shown in Fig. 7. Dry ice cavity cleaning might bepossible after coupler installation, a procedure whichrisks introducing field emission. Improved field emis-sion characteristics have been seen in single-cell cavitytests, and an extension of the system to 9-cell cavitiesis planned.

FIG. 7: The DESY dry ice nozzle. [Courtesy of DESY.]

One of several degreasing R&D studies is that of aKEK 9-cell Ichiro-shape cavity which was processedand tested at KEK and JLab [7]. In an initial test atJLab, the cavity produced substantial field emissionwhich was observed at gradients above 15 MV/m. Af-ter the cavity was ultrasonically cleaned with a 2%Micro-90 solution and standard HPR, the field emis-sion was substantially reduced or even eliminated.

An extensive study of ethanol rinsing after the finalEP was carried out at DESY [8]. Out of 33 cavitiesused in the study, 20 were treated using the standardprocedure and 13 with an additional ethanol rinse af-ter the final EP. The number of tests in which fieldemission was observed was substantially reduced. Inaddition, the maximum gradient was somewhat im-proved, from 27± 4 MV/m without ethanol to 31± 5MV/m with ethanol rinse. Since field emission wasfound to be reduced so substantially, the ethanol rinseis now part of the standard DESY cavity treatment.

V. MANUFACTURING AND SHAPESTUDIES

The vast majority of cavities which have been pro-cessed and tested in the last decade are Tesla-shapefine-grain cavities [9]. Some fundamental changesto the standard Tesla-shape fine-grain cavities whichhave been under investigation include changes to thefabrication techniques, material composition and cav-ity shape.

Because electron-beam welding is a substantial por-tion of the cavity fabrication cost, and may be a source

of surface defects which cause premature quenches(described later), reducing the number of equatorwelds required in cavity fabrication would be advan-tageous. Recently, a 9-cell cavity was fabricated atDESY using a hydroforming technique [10]. The cav-ity was built from three 3-cell hydroformed units, soit had only two iris welds and two beampipe welds.The surface was treated with the standard proce-dure including ethanol rinse. The cavity performancewas comparable to cavities built using standard tech-niques.

Material is wasted, and a lot of time is required, toroll sheets and stamp out the disks from which fine-grain cavities are made. In principle, it should bepossible to save manufacturing costs by slicing an in-got directly into large-grain sheets. It may also bepossible to achieve high gradients using BCP only,since the performance of large-grain cavities should beless sensitive to the grain boundary enhancement seenin fine-grain cavities, as long as the grain boundariesare strategically located in low surface-field regions.Recent experience at DESY with large-grain cavitiesshows their performance is comparable to fine-graincavities. It is still unclear whether BCP will be suf-ficient or whether EP will be necessary. Many 1-celltests have shown high gradients, in a range compa-rable to fine-grain cavities. Recent tests on three 9-cell large-grain cavities [11] showed gradients compa-rable to fine-grain cavities as well. Effective large-grain ingot cutting methods are being pursued in in-dustry. Single-crystal niobium cavities have been diffi-cult to produce, because it is difficult to produce largediameter single-grain ingots. Six single-cell single-grain cavities of varying shape and fabrication tech-nique were fabricated, processed and tested recentlyby a JLab/DESY collaboration [12], with performancefound to be comparable to that of fine-grain cavities.Unless substantial performance improvement is seen,the difficulty of producing single-grain cavities maynot justify the effort required. In both the large-grainand single-grain cases, further study of crystal ori-entation effects is needed. For a summary of recentlarge-grain and single-grain cavity work, see Ref. [13].

It may be possible to increase the RF breakdownmagnetic field of superconducting cavities by creat-ing a multilayer coating of alternating insulating lay-ers and thin superconducting layers [14]. By usingmultilayers thinner than the RF penetration depth,the critical magnetic field can be increased from nio-bium Hc1 to one similar to high-Tc superconductors,thereby significantly improving the maximum gradi-ent achievable. Recently, a cavity was prepared withsuch a composite surface and tested [15]. A 10 nmlayer of Al2O3 was chemically bonded to the niobiumsurface of an existing single-cell cavity using atomiclayer deposition. The surface was then covered by a3 nm layer of Nb2O5. The cavity showed promisingearly results, and further study is underway.

Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 5

Cell accelerating length and equator diameter arefixed by β and frequency respectively. However, thedetails of the shape may be optimized for low fieldemission (low Epeak/Eacc) and reduced sensitivity tothe fundamental maximum surface magnetic field (lowHpeak/Eacc); see, e.g., Ref. [16]. Excellent results onsingle-cell elliptical cavities have recently been ob-tained. A comparison of three shapes of ellipticalsingle-cell cavities is shown in Fig. 8. A re-entrantshape cavity built and tested by Cornell Universityrecently reached 59 MV/m [17], as shown in Fig. 9,setting a world record for the type of cavities de-scribed in this paper. Excellent results have also beenachieved with an Ichiro shape cavity at KEK, witha record of 53.5 MV/m [2]. Furthermore, 46.7 ± 1.9MV/m was reached on six single-cell cavities with op-timized surface treatment parameters [5]. Anotherlow-loss shape single-cell cavity which was processedand tested by a DESY/KEK collaboration reached47.3 MV/m [18]. It is rather more difficult to man-ufacture an excellent multi-cell cavity than an excel-lent single-cell cavity, and the very high gradients seenin single-cell alternative-shape cavities have not beenachieved in 9-cell versions yet. One 9-cell Ichiro-shapecavity, without endgroups, which was processed andtested by a KEK/JLab collaboration reached up to 36MV/m [19].

FIG. 8: Single-cell 1.3 GHz elliptical cavities: Cornell re-entrant (left), Tesla (center) and Ichiro (right).

VI. UNDERSTANDING CAVITY BEHAVIOR

Quenches and field emission appear as hot spotson the outer cavity surface. Temperature mapping(T-map) systems have been used at nearly all of thelabs active in this field for many years to study thesephenomena [20]. The existing T-map systems primar-ily use Allen-Bradley carbon resistors as thermal sen-sors, as developed at Cornell [21]. T-map systemscommonly in use for many years at DESY include afixed type for single-cell cavities with 768 sensors [22],and a rotating system for 9-cell cavities with 128 sen-sors [23]. A fixed thermometry system for 9-cell cavi-ties with four sensors around each equator and a few

FIG. 9: The record SRF 1-cell cavity gradient. [Courtesyof Cornell University.]

sensors on the endgroups has been developed at KEKand was used for tests of STF Tesla-like cavities [24].Second sound detection has been used at ANL forquench location on split-ring resonators [25]. A selec-tion of new hot spot detection systems are described;these systems vary in coverage, flexibility, and in thenumber of cavity tests required to extract useful T-map information.

A system of Cernox temperature sensors has beendeveloped at Fermilab [26] and used for quench loca-tion. Up to 32 sensors may be attached as neededto suspect locations; therefore the system is very flex-ible but also time consuming to install, typically re-quiring several cavity test cycles to conclusively locatequenches. It is highly portable and suitable for anycavity shape. An example installation of this systemon a 9-cell cavity is shown in Fig. 10, in which a hardquench at 16 MV/m was observed, accompanied by atemperature rise of about 100 mK, above the 2K op-eration, over about 2 sec in sensors #3 and #4 beforethe quench. The quench was seen on all sensors.

New multi-cell T-map systems have also been de-veloped. A new 2-cell system at JLab was recentlycommissioned [27], with 160 Allen-Bradley sensors in-stalled around each of the two equators. Understand-ing cavity behavior requires two cooldowns: one tomeasure the TM010 passband modes and determinewithin two the limiting cells, then a second after theT-map installation to measure the temperature dis-tribution of the limiting cell. At LANL, a new 9-cellT-map system has been developed [28] which employs4608 Allen-Bradley sensors and a multiplexing schemeto map an entire 9-cell cavity in a single test yet limitthe number of cables leaving the cryostat. The pre-

6 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009

FIG. 10: Fermilab Cernox thermometry system installedon a 9-cell cavity in the region of a pre-determined hotspot.

liminary results are very promising. A 9-cell T-mapsystem at Fermilab using 8640 diodes as a multiplexedthermometer array is under development [29]. All ofthese multi-cell T-maps are specifically designed forthe Tesla cavity shape.

Cornell has developed a new quench location sys-tem using second sound sensors [30]. Second sound isa thermal wave which can propagate only in superfluidhelium. It is generated when a heat pulse is trans-mitted from a heat source, such as a quench, throughsuperfluid helium. In this system, eight sensors detectthe arrival of the temperature oscillation, and the lo-cation is determined from the relative timing of thearrival of the oscillation from different sensors. Thissimple system is suitable for any cavity shape or num-ber of cells and can locate quenches in a single cavitytest. The detector is shown in Fig. 11, and an exam-ple of the data used to triangulate the location of thesecond sound source is shown in Fig. 12.

FIG. 11: The Cornell second sound sensor (left, courtesyof Cornell University) and a diagram showing the functionof the sensor (right, courtesy of Genfa Wu).

A resurgence of interest in optical inspection oc-curred recently when several cavities with hard quenchlimitation in the 15-20 MV/m range, were observed to

FIG. 12: Cornell data showing the arrival of second soundwaves at three different sensors. [Courtesy of Zachary Con-way.]

have surface defects correlated with hot spots, usinga new optical inspection system developed by KyotoUniversity/KEK with a clever lighting technique andexcellent resolution. This optical system [31] consistsof an integrated camera, mirror, and lighting systemon a fixed rod; the cavity is moved longitudinally andaxially with respect to the camera system. The light-ing is provided by a series of electroluminescent strips,which provide lighting with low glare on the highlypolished surface. These strips can be turned off andon, and the resulting shadows studied for an effective3D image of the depth or height of the defect. Thecamera resolution is about 7 µm. A photograph ofthe Kyoto/KEK camera system is shown in Fig. 13.An optical inspection system employed regularly atJLab [19] includes a Questar long-distance microscopeand mirror to inspect any cavity inner surface. Thissystem uses electroluminescent lighting. Additionaloptical inspection systems are in use or under devel-opment at other labs.

Because many of the cavity-limiting surface defectsfound by thermometry and optical inspection are lo-cated in the heat-affected zone of the electron-beamwelds, weld properties are under intense scrutiny. Un-derstanding which weld parameters might contributeto creating such defects, and ultimately eliminatingthe defect occurrence, is critical to improving the yieldof high-gradient cavities. Note that cavities with suchdefects are largely, but not exclusively, from new cav-ity manufacturers. One group has attempted to repro-duce these features on samples [32], to permit theirsystematic study. In time, study of cavity surfacequality may improve cavity quality in a manner simi-

Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 7

FIG. 13: The KEK/Kyoto University cavity inspectioncamera system. [Courtesy of Yoshihisa Iwashita.]

lar to the improvement seen in niobium sheet qualityresulting from regular eddy current scanning.

VII. FERMILAB INFRASTRUCTURE FORSRF DEVELOPMENT

At Fermilab, the SRF infrastructure has been sub-stantially built up in recent years. The key pro-gram goals are the achievement of high gradientswith high yields for future accelerators, the testingof cavities, and the ramp up of the production ratesneeded to construct cryomodules for Project X andILC R&D. The infrastructure which has been builtto achieve these goals includes facilities for single-cellcavity R&D, an ANL/FNAL joint cavity processingfacility, a vertical cavity test facility, a horizontal teststand, a cryomodule assembly facility, and a cryomod-ule test facility.

The ANL/FNAL cavity processing facility providessurface processing and assembly of cavities for verticaltest, for both 1-cell and 9-cell elliptical cavities. Re-cent photographs of the primary facility components:EP, ultrasonic degreasing, high-pressure ultrapure wa-ter rinsing, and assembly support and vacuum leaktesting, are shown in Fig. 14. All of these componentsare now in routine operation.

At the FNAL vertical cavity test facility (VCTF),more than 40 cavity tests have been performed inFY08/FY09, where a test is defined as a cryogeniccycle. The test cavities have been primarily 9-celland single-cell 1.3 GHz elliptical cavities, with two325 MHz single-spoke resonators included. The testshave primarily been for instrumentation development,e.g., variable input coupler, thermometry, cavity vac-uum pumping system, and for cavity diagnostic testsfor vendor development. Upgrades are planned to in-crease the cavity test throughput to more than 200

FIG. 14: ANL/FNAL join facility infrastructure: (clock-wise from top left) EP, ultrasonic degreasing, HPR andassembly stands. [Courtesy of Michael Kelly and DamonBice.]

cavity tests per year by October 2011, to supportProject X and ILC R&D. Some of the cavities recentlytested at the VCTF are shown in Fig. 15. A summaryof the vertical test results of 9-cell cavities is shown inFig. 16.

The horizontal test stand (HTS), which is used totest 9-cell cavities which have been welded into a he-lium tank, i.e., dressed, using pulsed high power, isnow operational. It has been commissioned for both1.3 and 3.9 GHz cavities. Four 3.9 GHz cavities weretested in 2008; they have been installed in a cryomod-ule which is currently at DESY. Upgrades are plannedto expand the capacity with the addition of a secondcryostat by 2012. A photograph of the HTS is shownin Fig. 17.

Two cryomodule assembly facilities (CAF’s) havebeen built at Fermilab, CAF-MP9 and CAF-ICB. TheCAF-MP9 facility receives dressed cavities and theirperipheral parts, and assembles these into a cavitystring in a class-10 cleanroom. Then, the string as-sembly is installed onto the cold mass and transportedto CAF-ICB. At CAF-ICB, the cavity string is alignedand assembled into the vacuum vessel. After the cry-omodule is complete, it is shipped to the cryomod-ule test facility (NML) for testing. One cryomodule,

8 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009

FIG. 15: Some of the cavities recently tested at the Fer-milab VCTF: (clockwise from top left) SSR1, 9-cell Tesla-shape, and 1-cell Tesla-shape (with diode thermometry)cavities. [Courtesy of Bill Mumper.]

FIG. 16: A summary of maximum gradients from 9-cellvertical tests in the ILC Americas region. The cavitiestested at the VCTF are shown by the black histogram.

CM1, built with cavities from DESY, has been assem-bled in the CAF facilities. Recent photographs of theCAF-MP9 and CAF-ICB, showing steps in the assem-bly of CM1, are shown in Figs. 18 and 19, respectively.

The cryomodule test facility, NML, will test one RFunit, i.e., 3 cryomodules, using a 10 MW RF systemand an electron beam with ILC parameters. VariousProject-X parameters will also be tested with beam in

FIG. 17: A photograph of the HTS, showing a 9-cell 1.3GHz cavity being installed. [Courtesy of Fermilab VisualMedia Services.]

FIG. 18: Recent photographs of the CAF-MP9 facility,showing stages in the assembly of the first cryomodule,CM1. [Courtesy of Tug Arkan.]

this facility. NML will provide the capability to con-duct advanced accelerator R&D for future acceleratorcomponents. Through the end of FY09, the facilityis being prepared to test the first Fermilab-assembledcryomodule, CM1, without beam, requiring comple-tion of the RF system and the cryogenic system. Arecent photograph of the NML facility is shown inFig. 20.

Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 9

FIG. 19: Recent photographs of the CAF-ICB facility,showing stages in the assembly of the first cryomodule,CM1. [Courtesy of Tug Arkan.]

FIG. 20: A recent photograph of the NML facility, indicat-ing where the first cryomodule, CM1, has been installed.[Courtesy of Jerry Leibfritz.]

VIII. CONCLUSIONS AND OUTLOOK

Highlights of the rich R&D activity in the quest forhigh gradient and reduced cost have been described.

Very high gradients have been measured in niobiumSRF cavities: more than 50 MV/m in single-cell cav-ities of various shapes, and more than 35 MV/m inseveral 9-cell Tesla-shape cavities. Several promisingnew results from large-grain 9-cell cavities and hydro-formed cavities address the cost and reliability issuesof these cavities. Several cavities which have been lim-ited to 15-20 MV/m by hard quench have been studiedusing various techniques, and the outlook is good thatuseful information can be fed back to cavity manufac-turers to improve the yield of high-gradient cavitiesover time. Surface treatment is crucial for optimumperformance, and several promising studies on finalpreparation methods have been found to sharply re-duce field emission. Fundamental SRF studies are alsounderway, many showing promising results.

An overview of Fermilab infrastructure in pursuitof SRF R&D has been given. Among the recent ac-complishments, one 1.3 GHz cryomodule using DESYdressed cavities has been built and transported to thecryomodule test facility for testing. Most key infras-tructure components are now in place, with final com-missioning underway.

Acknowledgments

I am grateful to my colleagues for their contribu-tions to the content for this talk, as referenced. Iespecially thank those who graciously permitted meto show their unpublished data.

This manuscript has been authored by Fermi Re-search Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy. TheUnited States Government retains and the publisher,by accepting the article for publication, acknowl-edges that the United States Government retains anonexclusive, paid-up, irrevocable, worldwide licenseto publish or reproduce the published form of thismanuscript, or allow others to do so, for United StatesGovernment purposes.

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