QUALITY FACTOR MEASUREMENTS OF THE ULTRAMET 3 GHzCAVITY CONSTRUCTED USING CHEMICAL VAPOUR DEPOSITION∗

D. L. Hall , D. A. Gonnella, M. Liepe†

Cornell Laboratory for Accelerator-based Sciences and Education, Ithaca, NY 14850, U.S.A.V. A. Arrieta , S. R. McNeal , Ultramet Corporation, Pacoima, CA 91331, U.S.A.‡ §

AbstractA seamless 3 GHz bulk niobium cavity constructed by

Ultramet using rapid chemical vapor deposition (CVD)techniques has been tested on the vertical SRF test standat Cornell. The cavity received a 25 µm buffered chemicalpolish (BCP) and 700C heat treatment for 4 days. Firsttest results gave an intrinsic quality factor of Q0 = (1.55±0.12)×107 and (2.00±0.15)×107 at 4.2 K and 1.5 K, re-spectively. A second BCP removed 100 µm of material, af-ter which test results improved to Q0 = (7.59±1.52)×107

and (4.16±0.31)×108 at 4.2 K and 1.5 K. During the firsttest poor coupling to the input amplifier impeded tests ataccelerating fields >0.2 MV/m, while during the secondtest the cavity quenched at 1.3 MV/m when operating at1.5 K. An optical inspection of the cavity after the secondtest revealed the presence of at least 4 pits on the upperhemisphere suggesting an area of higher than average sur-face resistance that may have contributed to the low fieldquench via thermal runaway. The potential of CVD as aconstruction method for SRF cavities is discussed.

INTRODUCTIONA seamless 3 GHz ILC-style SRF bulk niobium cavity

was delivered to Cornell in September of 2012 for testing.The cavity was produced using a fabrication process in-corporating rapid chemical vapor deposition (CVD) tech-niques developed by Ultramet, Inc. (Pacoima, CA USA),under research funded by the U.S. Department of Energy.The cavity was subjected to two vertical performance tests,the results of which are presented in this paper. Also pre-sented are RRR measurements of three samples of UltrametCVD niobium conducted by Cornell in July 2012.

Ultramet’s CVD-based cavity fabrication methodologyuses low cost commercially available niobium source ma-terials, and allows for the construction of seamless niobiumcavities and components that require a minimal amountof assembly welds, includinge achieved. Furthermore,XPS measurements conducted by Ultramet on CVD nio-bium samples confirm that contamination from the mandrelof the as-fabricated niobium is confined to a region <25µm from the surface [1]. This suggests that CVD niobiumcavities will require only minimum chemical treatment to

∗Work supported by U.S. Department of Energy Phase 1 SBIR GrantDE-SC0002721

† dlh269@cornell.edu‡ victor.arrieta@ultramet.com§ shawn.mcneal@ultramet.com

achieve peak performance.

CAVITY CONSTRUCTION METHODThe cavity is formed by depositing niobium, using pro-

prietary CVD techniques developed by Ultramet, onto asacrificial interlayer metal-coated graphite mandrel, suchas those seen in Fig. 1 (A). The mandrel, immediately afterapplication of the interlayer metal, is shown in Fig. 1 (B).

For the scaled-down 3 GHz ILC cavity tested, a two-layer CVD niobium process was adopted for cost-savingpurposes; first, a thin initial layer of higher quality niobiumwas deposited on the mandrel, after which the surface wassanded using a CNC grinder. Subsequently, a second layerof standard grade niobium was deposited to extend the av-erage thickness of the cavity walls to 3 mm for the sake ofstructural integrity. The cavity immediately after CVD isshown in Fig. 1 (C).

To prevent contamination of the niobium surface layerwith hydrogen and other impurities, the mandrel must bethoroughly out-gassed prior to the CVD process. Uponcompleting the niobium deposition, the outside of the cav-ity is machined on a CNC lathe to achieve uniform wall

Figure 1: (A) Graphite mandrel prototypes machined usingCNC methods. (B) The mandrel immediately following ap-plication of the sacrificial interlayer metal. (C) The cavityimmediately after CVD. (D) The completed cavity, afterremoval of the mandrel and exterior finish. Photographscourtesy of Ultramet, Inc.

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thickness, after which the mandrel is removed by chemicalmethods. The cavity in its final as-fabricated form prior toleaving Ultramet for Cornell is shown in Fig. 1 (D).

RRR RESULTS FROM CVD SAMPLESIn July 2012 Cornell performed RRR measurements of

three samples of CVD niobium provided by Ultramet. Eachsample was deposited using different parameters in theCVD process (time, temperature, pressure, reactor design,etc.). The results of the RRR measurements are given intable 1. These, together with the RRR measurements per-formed separately by ORNL on behalf of Ultramet [2-4],demonstrate that the CVD process can be optimized to pro-duce high quality niobium with a bulk RRR >200, on parwith high-RRR niobium used in traditional cavity fabrica-tion.

CAVITY PREPARATION AND RESULTSOF THE FIRST TEST

The as-fabricated cavity included beam pipes and wasreceived without beam pipe flanges; these were fabricatedand added at Cornell upon arrival. The cavity received a5 µm buffered chemical polish (BCP) in preparation of theflanging operation, after which two reactor-grade niobiumindium-seal flanges were electron-beam welded at eitherend of the cavity. The CVD niobium responded well tobeam welding and demonstrated good structural integrity.An additional 10 µm of material was removed via BCP toremove any possible contaminants introduced during thewelding process.

The cavity then received a 700C bake in a UHV furnacefor 4 days, followed by a further 10 µm BCP. Since workcarried out separately by Ultramet with samples of CVDniobium indicate that surface impurities from the fabrica-tion process are present only to a depth of <25 µm , thestandard 100 µm BCP etch was not performed for the firsttest.

Cavity preparation culminated in a final low temperaturebake to improve the BCS surface resistance of the niobium.Although the bake was intended to be 120C for 48 hours,a failure in the furnace control system instead caused thecavity to be baked at 210C for 3 hours. The cavity re-ceived a final high pressure rinse before being mounted onthe test stand for a vertical test.

Results for the first test measuring intrinsic quality fac-tor Q0 against accelerating field Eacc are shown in Fig. 2.

Table 1: Results of the RRR measurements at Cornell onCVD deposited niobium. Each sample was fabricated us-ing different sets of critical parameters in the CVD process.

Sample No RRR1-1 (129± 24)2-1 (253± 44)3-1 (47± 9)

0 0.05 0.1 0.15 0.2

107

108

1.5 K

4

Figure 2: Quality factor Q0 vs. accelerating field Eacc

measurements made during the first vertical test. Poor cou-pling to the cavity impeded testing at fields >0.2 MV/m.

Values of Q0 were taken at temperatures of 1.5 K and4.2 K. Initial results demonstrate no discernible differencebetween the two temperatures, although poor coupling be-tween the input antenna and the cavity impeded testing atfields greater than 0.2 MV/m. Results from the first testindicate a Q0 of (2.00 ± 0.15) × 107 at 1.5 K, sub-parfor a 3 GHz cavity when compared to standard fabrica-tion methods. The measured surface residual resistance of(1.4±0.1)×10−5 Ω suggests the presence of surface con-taminants and/or areas of poor superconductivity or normalconductivity. The cavity was removed from the vertical teststand and preparations were made for a second test.

POST-TEST CHEMISTRY AND RESULTSOF THE SECOND TEST

To remove the surface contamination suggested by theresults of the first test, the cavity was subjected to a 100µm BCP etch. No other chemistry or further baking wasperformed, and the cavity was returned to the test stand fora second test. Measurements taken at 1.5 K indicate a Q0 of(4.45±0.33)×108 at 1.1 MV/m, and a measurement takenat <0.5 MV/m shows a low-field Q0 of (7.59±1.52)×107

at 4.2 K. Accordingly, the average residual surface resis-tance improved to (6.50± 0.49)× 10−7 Ω. The Q0 at 4.2K indicates a BCS resistance of (2.91 ± 0.22) × 10−6 Ω,which is in good agreement with the expected BCS resis-tance for niobium of 2.49×10−6 Ω obtained using SRIMP[5].

A quench at approximately 1.3 MV/m impeded testingat higher fields despite the improved coupling factor. A Q0

vs. Eacc curve for the second test is shown in Fig. 3.

OPTICAL INSPECTION OF THE CAVITYINTERIOR AND ANALYSIS OF RESULTS

An optical inspection of the cavity was performed af-ter the second test to investigate for possible causes of the

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quench at 1.3 MV/m observed during the second test. Anumber of pits and surface defects were found on the upperhemisphere of the cavity in a band parallel to the equa-tor, as shown in Figs. 4 (A) and (B), ranging in size fromsmall indentations to pits approximately 0.2 mm across and0.05–0.1 mm deep. Epoxy casts of the inside of the cavityconfirmed that the larger pits are visible to the naked eye,although the small size of the cavity impeded the creationof a reliable cast from which to obtain depth measurements.

Discussion with Ultramet suggests that these pits maybe evidence of the BCP etching through the first niobiumlayer and exposing the interface between the first and sec-ond layers of niobium. It is likely that damage to the man-drel locating datum, and/or small disparities in the CNCgrinding profile during coating and machining operationsand the physical geometry of the cavity, led to regions ofthe first niobium layer along the length of the cavity be-ing thinner than expected or even being penetrated, creat-ing microscopic pin holes exposed during cavity prepara-tion etching operations. This is strongly supported by thealigned nature of the pits and their being confined to a nar-row band on the cavity inner surface. Subsequently, thisthin region of the niobium layer interface was the first to beexposed by surface chemistry. Further BCP may exacer-bate existing pits or expose additional pits, and/or artifactsof the CNC grinding process.

The results of the first test suggest the presence of sur-face contaminants. Discussions with Ultramet concludethat the metal interlayer coating on the mandrel may nothave been sufficiently outgassed, hence leading to contam-inants being introduced into the surface niobium layer. Al-though more aggressive surface chemistry removed enoughcontamination to lower the average residual surface resis-tance significantly, it also exposed the pitted interface layerband shown in Fig. 4 (A). It is likely that the surface inthis region possessed a higher residual surface resistance

0 0.5 1 1.5

108

109

1.5 K

Figure 3: Quality factor Q0 vs. accelerating field Eacc

measurements taken during the second vertical test, follow-ing the removal of 100 µm of material via BCP. Only onepoint at 4.2 K was taken during this test, not shown here.

Figure 4: (A) Extent of the band (superposed in red onthe cavity diagram) in which all pits were found. (B) Anexample of one of the pits. The pit shown is approximately0.2 mm in diameter.

which, coupled with possible contaminants left from theCNC grinding process, contributed to a thermal runawaythat caused the quench observed at 1.3 MV/m.

CONCLUSIONThe CVD cavity has been shown to be capable of achiev-

ing a quality factor of (7.59 ± 1.52) × 107 at 4.2 K, closeto the expected BCS value for niobium at 3 GHz; a promis-ing result for a first prototype. Furthermore, the CVD pro-cess has demonstrated that it can create niobium samples ofRRR >200. The results of the tests and subsequent inspec-tion indicate that the current limitations of the cavity lieprimarily in easily adjusted construction methods. In fu-ture cavities, to prevent contamination of the niobium sur-face layer, the mandrel must be thoroughly degassed priorto deposition. Furthermore, Ultramet is currently conduct-ing DOE-funded research to develop improved mandrellingtechniques due to undergo validation at Cornell in Septem-ber of 2013 [6]. The mandrelling techniques under devel-opment will allow for the net deposition and/or substan-tially thicker initial deposits of the RRR>200 CVD nio-bium material, hence avoiding or minimizing the risk ofinterlayer penetration or contamination by machining de-posits.

In summary, the CVD fabrication process shows con-siderable promise with respect to the creation of seamlesssuperconducting structures, with potentials for cost savingsand the construction of complex geometries, and warrantsthe need for further investigation.

REFERENCES[1] Victor A. Arrieta and Shawn R. McNeal, “High-purity nio-

bium superconducting radio frequency cavities”. Final report,Ultramet for the U.S. Department of Energy, Washington,DC. June 2006. Grant DE-FG02-05ER84175.

[2] Victor A. Arrieta and Shawn R. McNeal. “Economical man-ufacture of seamless high-purity niobium”. In Proc. of SRF2011, Chicago. Ultramet for U.S. Department of Energy,November 2010.

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[3] Victor A. Arrieta and Shawn R. McNeal. “Economical man-ufacture of seamless high-purity niobium”. Final report, Ul-tramet for U.S. Department of Energy, Washington, DC,November 2010. Grant DE-SC0002721.

[4] Victor A. Arrieta and Shawn R. McNeal. “Advanced manu-facturing and testing of seamless high purity niobium super-conducting radio frequency cavities”. Final report, Ultrametfor U.S. Department of Energy, Washington, DC, November2012. Grant DE-SC0007489.

[5] J. Halbritter. “Comparison between measured and calcu-lated RF losses in the superconducting state”. Zeitschrift furPhysik, 238:466476, October 1970.

[6] Victor A. Arrieta and Shawn R. McNeal. “BC coating devel-opment for manufacture of superconducting radio frequencyaccelerator cavities”. SBIR proposal (unpublished), Ultrametfor U.S. Department of Energy, Washington, DC, November2010. Grant DE-SC0009544.

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