Copper cavities with a thin niobium film as used in the large electron positron collider LEP would be also attractive for future linear colliders, provided the decrease of the Q-value with the accelerating gradient can be reduced. We aim at extracting the important parameters that govern this decrease. The dependence on the RF frequency is studied by exciting 500 MHz and 1500 MHz cavities in different modes. In addition we combined RF measurements for two 1500 MHz cavities of different RF performance with microscopic tests (AFM, TEM) on samples cut out of the same cavities. Their micro-structural characterisation in plan-view allows to extract the grain size and the defect densities

Bloess, D

Bosland, P

Durand, C

Mahner, E

Mayer, J

Nakai, H

Van Loyen, L

Weingarten, Wolfgang

CERN Document Server

  Metallurgical Analysis and RF Losses in SuperconductingNiobium Thin Film CavitiesD. BLOESS, C. DURAND, E. MAHNER, H. NAKAI† and W. WEINGARTEN CERN - European Organization for Nuclear ResearchCH - 1211 Geneva 23 (Switzerland)P. BOSLANDC.E. Saclay, F - 91191 Gif-sur-Yvette Cedex (France)J. MAYERMPI für Metallforschung, Seestr. 92, D - 70174 Stuttgart (Germany)L.VANLOYENIFW, Helmholtzstr. 20, D - 01069 Dresden (Germany)                                                † Visitor from KEK, 1-1, Oho, Tsukuba, Ibaraki 305 (Japan)Manuscript received August 27, 1996.Abstract — Copper cavities with a thin niobium film - as usedin the large electron positron collider LEP - would be alsoattractive for future linear colliders, provided the decrease ofthe Q-value with the accelerating gradient could be reduced.We aim at extracting the important parameters that governthis decrease. The dependence on the RF frequency is studiedby exciting 500 MHz and 1500 MHz cavities in differentmodes. In addition we combined RF measurements for two1500 MHz cavities of different RF performance withmicroscopic tests (AFM, TEM) on samples cut out of thesame cavities. Their micro-structural characterization inplan-view allows to extract the grain size and the defectdensities.I. INTRODUCTIONSuperconducting (sc) accelerating cavities are at theheart of the particle accelerators recently commissioned orupgraded: the recirculating electron linear acceleratorCEBAF (presently 4 GeV) at Thomas Jefferson NationalLaboratory and the electron positron collider LEP2(presently 80.5 GeV) at CERN. The CEBAF accelerator isoperated at 1.5 GHz and based on niobium sheet metalcavities, whereas CERN’s LEP2 RF system at 352 MHz is(nearly exclusively) built around copper cavities with athin niobium film inside. This choice is not accidental:niobium sheet metal cavities provide larger acceleratinggradients and Q-values above frequencies of, say, 1 GHz,whereas thin film cavities give larger Q-values at lowerfrequencies. However, the Q-value in niobium thin filmcavities degrades with the RF field amplitude faster than inniobium sheet cavities.The aim of this paper is to contribute to theunderstanding of this degradation - or in other words - whydo the RF losses increase more than quadratically with theRF field amplitude (non quadratic losses NQL).II. EXPERIMENTALA. Scope of experiment:Experimental results on NQL are summarized in [1] andwill not be repeated here. Analyzing data from differentcavities [2], the frequency dependence of NQL wascompatible with both a linear and a quadratic law.Therefore, we measured it again in the same cavity.Secondly, the upper critical magnetic field Bc2 obtainedfrom the trapped magnetic field dependence of the surfaceresistance was much larger than the one obtained fromsamples. We therefore cut samples out of two cavities withdifferent RF performance, determined Bc2 again, andseveral micro-structural parameters.B. Frequency dependence of NQL:We have partly re-analyzed and partly taken new dataon two 500 MHz mono-cell cavities tested in the TM010and TM011 modes, and on two 1500 MHz mono-cellcavities tested in the TM010 and TM210 modes. Thesemodes were chosen as our fixed coupling probe gavenearly identical coupling to the microwave line after cooldown.A problem to be solved is closely related to the differentfield distributions of the two modes. One measures thedependence of the Q-value, Q = w U/P  ( w =2p f  =frequency, U = stored energy, P = dissipated power), onthe RF field amplitude, for example the acceleratinggradient Ea, or the peak magnetic surface field Bp. The Q-value is an average quantity, as it is composed of RFlosses at different locations of the cavity surface withdifferent local RF field amplitudes. What one likes toextract is the surface resistance Rs somewhere on thecavity surface as a function of the RF surface magneticfield amplitude B at the same location - tacitly assumingthat  Rs depends only on B and not on the location. Aboveall one is interested in the slope Rs’ of Rs(B).In a cavity excited in the fundamental mode the RFmagnetic field B  is approximately the same nearlyeverywhere and identical to its peak value Bp. In a goodapproximation the average surface resistance ‹Rs› (Bp) isidentical to Rs (B). In a cavity excited in a higher mode,however, this is no longer true. The slope Rs’ is notidentical with the one extracted from the Q(Bp)measurement, a fact which gives rise to a systematiccorrection (cf. appendix).02004006008001000120014000 10 20‹Rs› [n W ]Bp [mT](a)498 MHz4.2 K916 MHz55 – 5 n W /mT33.5 – 0.3 n W /mT02004000 20 40‹Rs› [n W ]Bp [mT](b)910 MHz495 MHz4.2 K2.35 – 0.04 n W /mT1.17 – 0.04 n W /mT02004006008000 20 40 60‹Rs› [n W ]Bp [mT](c)2791 MHz1503 MHz1.8 K4.7 – 0.2 n W /mT1.84 – 0.04 n W /mT0400800120016000 20 40 60‹Rs› [n W ]Bp [mT](d)2780 MHz1499 MHz1.5-2.0 K8.1 – 0.5 n W /mT4.2 – 0.1 n W /mTFig. 1: ‹Rs› vs. Bp measured for two modes in four differentniobium thin film mono-cell cavities (a-d).Fig. 1 displays the average surface resistance ‹Rs› vs.the peak RF magnetic surface field amplitude Bp for fourdifferent cavities and two different modes. The numbersgive the slope a  of a linear approximation to the datapoints. The lower and upper bound of the linear range ischosen such that the error D a   is minimized. The ratio g  ofthe slopes Rs’ = a /b  (cf. appendix) for the two differentmodes is compared with their frequency ratio and itssquare (Table I). The error in g   comprises the errors Daand Db . The latter is based on the computer simulation ofthe field distribution in a cavity. Results from differentcodes (URMEL and SUPERFISH [3]) deviated by 7%. Wetherefore estimated the error Db   = – 14 %.TABLE IRATIO g  OF Rs’ FOR DIFFERENT  MODESCavity f1[MHz]f2[MHz]f2 / f1 (f2 / f1)2 Ratio ofslopes ga 498 916 1.8 3.4 1.9 –  0.4b 495 910 1.8 3.4 2.3 –  0.4c 1503 2791 1.9 3.4 4.2 –  0.9d 1499 2780 1.9 3.4 3.2 –  0.7C. Analysis of samples cut from cavities:Samples were cut from two 1500 MHz niobium filmcavities (we call them “as received”), the RF results ofwhich were quite different (Fig. 2 and Table II). They aretaken from ref. 4, except for the slope a . It is determinedas described before to facilitate the comparison.<Rs> [n W ]05000 5 0Bp [mT]1.8 K <Rs> [n W ]050010000 5 0Bp [mT]1.8 KFig. 2: <Rs> vs. Bp for  two 1500 MHz niobium film cavities,TABLE IICOMPARISON OF RF PERFORMANCE OF CAVITIES #1 AND #2(UPPER PART) AND  METALLURGY OF THE CORRESPONDINGSAMPLES (LOWER PART)Physical quantity Cavity #1 Cavity #2BCS surface resistance [nW ], 4.2 K 405 –  11 400 –  15Residual surface resistance Rres [nW ] 32 –  5 75 –  5Critical temperature Tc [K] 9.506– 0.016 -Slope Rs’ [nW /mT], 1.8 K 1.2 –  0.1 6.3– 0.3Penetration depth l 0 [nm] a) 82– 3 -Critical temperature Tc [K] b) 9.61 - 9.79 9.35 - 21.74    (after immersion in HNO3) 9.16 - 9.21 9.15 - 9.19Upper critical field Bc2 [T], 4.2 K 1.50– 0.05 2.6– 0.1   (after immersion in HNO3) 0.84 –  0.02 0.80 – 0.02“Structure” length c)  [nm] (AFM) 200 100Roughness Ra[nm] (AFM) 3 3Grain size [nm] (TEM) 100 - 500 50 - 200Defect density [nm-3] (TEM) 10-4 10-3RRR (10 K) 13.0 – 0.5 9.0– 0.5Film thickness [m m] 3 3a) two fluid model fit  b) width of the transition  c)  for explanation cf. Fig. 51) The upper critical field Bc2 and the criticaltemperature Tc: Bc2 was measured at 4.2 K by two coilswith coplanar windings and the sample in between. Thewhole set-up is exposed to a static magnetic field ( 8 T scmagnet) directed parallel to the sample’s surface. Whenthe primary coil is fed with a small AC current, the pickupsignal in the secondary coil depends on that part of thesample surface which cannot be encircled by a closed scpath. The cleanest signal (Fig. 3) was obtained for aniobium film on a copper substrate (thinned down to1 mm), an AC magnetic field of 50 mG at 10 Hz. Thecritical temperature Tc was measured with the sameapparatus at vanishing static magnetic field by slowlywarming up the sample.0.0 0.5 1.0 1.5 2.00.00.51.01.52.02.5U [mV]B [T]9.5 9.6 9.7 9.8 9.9 10.00.00.51.01.52.02.5U [mV]T [K]0.0 0.5 1.0 1.5 2.0 2.5 3.00.00.51.01.52.02.5B [T]5 10 15 20 25 300.00.51.01.52.02.5T [K]Fig. 3: Pickup signal for samples #1 (left) and sample #2 (right)vs. the static magnetic field (top row) and vs. the temperature(bottom row) Bc2 is defined as the magnetic field above whichFor a similar set of samples the copper substrate wasremoved by immersion in diluted nitric acid. The resultschanged markedly (Fig. 4). Bc2 and Tc were the same forthe two samples and much closer to the values of bulkniobium.0.0 0.2 0.4 0.6 0.8 1.00.00.51.01.52.02.5U [mV]B [T]0.0 0.2 0.4 0.6 0.8 1.00.00.51.01.52.02.5B [T]9.0 9.1 9.2 9.3 9.4 9.50.00.51.01.52.02.5U [mV]T [K]9.0 9.1 9.2 9.3 9.4 9.50.00.51.01.52.02.5T [K]Fig. 4: Same as Fig. 3 for similar samples after immersion innitric acid to remove the copper substrate.2) Measurements with an Atomic Force Microscope(AFM) and a Transmission Electron Microscope (TEM):Fig. 5: AFM pictures of samples #1 (top) and #2 (bottom). Wecall the average distance of the grooves “structure length” todistinguish it from the grain size. It turns out that it is equal to theThe AFM measurements were performed with theExplorer AFM of TopoMetrix at IFW Dresden, the TEManalysis was done with the Zeiss EM 912 Omega at MPIStuttgart.The image resolution of the AFM pictures is 300 X 300pixels. The AFM pictures show a “structure” with acharacteristic length of about 200 nm in sample #1, abouttwice the one of sample #2 (Fig. 5). This length can beinterpreted as a typical grain size. The samples have aboutsimilar average roughness (Ra = 3 nm).The TEM measurements were performed on sampleswhich were cut parallel (plan view) and perpendicular(cross-section) to the niobium film plane. Standardpreparation techniques involving mechanical polishing andion beam thinning the specimens revealed a columnargrain growth perpendicular to the interface. The rod-shaped like Nb grains reach dimensions of up to severalm m in length, which is basically the deposited filmthickness. The diameter of the columnar grains can best bestudied in the plan view specimens. Typical micrographsfor cavities #1 and #2 are shown in Fig. 6 together with areference sample made from bulk niobium. The typicalgrain sizes in the plane parallel to the film are 100 - 500nm for sample #1 and 50 - 200 nm for sample #2. Fromthe micrographs a high density of defects is also evident.To ensure that the defects are not introduced duringspecimen preparation we have also studied the bulk Nbsample which was prepared in exactly the same way. Thedefects presumably consist of dislocations, dislocationloops and point defect agglomerates. Mean defect densitiesper nm3 were estimated to be 10-4 for sample #1 and 10-3for sample #2. These values were obtained by measuringmean distances between defects in several micrographsand are based on an estimated specimen thickness of 20 -30 nm. The defect density in the bulk Nb reference samplewas lower by about three orders of magnitude confirmingthat specimen preparation is not a source of the high defectdensities measured for the samples from the cavities.Fig. 6: TEM pictures  (from top left to bottom right) in plan viewof bulk niobium reference sample (RRR = 300) sample #13) Measurements with a Scanning Electron Microscope(SEM) and a Light Microscope (LM): All pictures (Figs. 7and 8) display etching patterns from the copper substrate.There are coarse and fine structures visible, which aresimilar for the samples #1 and #2. The SEM picturessuggest the presence of holes, which is not confirmed for alarger magnification yet.Fig. 7: SEM pictures of sample #1 (left) and sample #2 (right).The image width is 40 m m.Fig. 8: LM pictures of sample #1 (left) and sample #2 (right). Theimage width is 1 mm.III. DISCUSSIONThe data are compatible with a linear dependence of thesurface resistance on frequency below 1.5 GHz(Table I). This fact confirms earlier data obtained on aseries of 350 MHz and 1500 MHz cavities [1]. In addition,the data suggest a quadratic dependence above 1.5 GHz,which may signify that near a crossover frequencybetween 1.5 and 3 GHz another loss mechanism becomesdominant. How can it be understood?The analytic expression for NQL [2] is (besidesnumerical factors near unity) equivalent to that of AClosses in a superconductor (Bean model). They aredescribed in linear approximation in B by the surfaceresistance (with the critical current density jc of Bean’sformula [5] replaced by Bc2/(m 0l)),  R l BB Ts cf T B=p ( )23 0 2w m( , )14 34. (1)l is the depth of the surface layer, which contributes toNQL. Rs is proportional to the fraction f of metalbecoming normal-conducting (nc) with increasing B. Theprecise analytic form of f is not known. f depends on thetemperature and is here approximated by the ratio of theRF magnetic field B and the upper critical magnetic fieldBc2 of the film, f(T, B) = B/Bc2 (T). A linear increase ofthe surface resistance with frequency reflects the fact thatthe losses are generated during one RF cycleloss mechanism must appear. It has the same physicalbasis as the BCS losses and hence increases with thesquare of the frequency. It rises from the fact that a thin ncsurface layer or grain boundaries [6] feel the voltage fromthe inductance of the sc electrons. In the two fluid model,for a surface fraction f(T) of nc electrons, thecorresponding surface resistance for sufficiently lowtemperatures (T<Tc/2) isR f Ts n= ( )12 03 2m 02s l w (2)(s n = conductivity at 10 K, l 0 = penetration depth, f(T) =(T/Tc)4) [7]. Therefore, above some frequency w c, the RFlosses described by (2) exceed those described by (1).By equating (1) and (2), and the replacement f(T)fi f(T,B) in (2), one obtains for the depth l of the lossy surfacelayerl n=p34 03m 0s l w c. (3)With a typical conductivity for niobium at roomtemperature (7.6·106 ( W m)-1) and RRR = 10, one obtains aconductivity of s n = 0.8·108 ( W m)-1 at 10 K. By insertinga crossover frequency close to the observed value, w c =2p ·3·109 s-1, l 0 = 85 nm [8], one finds for the depth l = 3nm. We point out that this number gives only the order ofmagnitude. It implies that the sensitive region is confinedto the uppermost surface layer. This is a reassuringconsequence, because it is compatible with the observationthat the NQL are sensitive to the surface [9].The “as received” sample #1 (good RF performance)has a lower Bc2 with a sharper transition, a smallertransition width in Tc, a factor of two larger structurelength (AFM), a larger grain size (TEM), a larger RRRvalue and one order of magnitude lower defect density,compared to sample #2 (poor RF performance). RBCS andthe niobium average surface roughness are quite similar.Immersion in nitric acid not only removes the coppersubstrate, but also oxidizes the niobium surface. Afterapplying this treatment, the two samples #1 and #2become very similar with respect to Bc2 and Tc. Therefore,the discrepancy in Bc2 and Tc observed in the “asreceived” samples #1 and #2 could again be a surfaceeffect. But one cannot exclude an effect of the NbCuinterface.These observations may be explained by a commonfeature, confirming previous results [9]: the uppermostfilm surface is a mixture of phases containing extreme typeII niobium with a    very short coherence length   (“disorderedniobium” according to R. Vaglio [10]). A deeperdiscussion has to be presented elsewhere.IV. CONCLUSIONThe frequency dependence of Rs’ allows an estimate ofthe depth l of the surface layer giving rise to NQL (severalnm). The comparative measurements on samples cut fromcavities with different RF performance are in accordancewith the following explanation.To the authors’ best knowledge the features observed inniobium thin film cavities - as summarized in ref. 1 - canbe explained as follows: the film surface has a coherencelength, which spans the interval from that of bulk niobiumdown to several nm. The NQL appear when the RFmagnetic field amplitude B exceeds the lower critical fieldthe niobium surface). They are induced in a - compared tothe penetration depth - thin surface layer by the gradualdestruction and re-establishment of superconductivity (orequivalently entry and exit of magnetic flux) within oneRF half cycle and the dissipation into the lattice of the heatinvolved. They are described by (1) for frequencies belowabout 1 GHz and for B > Bc1. As an example, with f = 352MHz, l = 3 nm, Bc2 = 1.5 T, T = 4.2 K, one obtains Rs’ =1.2 n W /mT for the LEP cavities, which is the observedvalue [1].APPENDIXThe geometry factor G is defined asG=dvBdaBwm 022òò, (4)and the Q-value (as a function of the peak surfacemagnetic field amplitude Bp) is defined asQ Bd B Bd R B Bppp( ) = ( )( )òòwva2s2, (5)where the two integrals are taken over the volume V andthe surface A of the cavity. The average surface resistance‹Rs› is defined asR B GQ Bs p p( ) = ( ). (6)With a parametrisation of the local surface resistance Rsas a function of the local surface magnetic field amplitudeB,R B R R Bs s s( ) = + × +0 ' ... , (7)one finds  R B R Rd B BB d B BBs p s spp pp( ) = + ( )( ) × +òò0'...aa32b1 244 344, (8)which is abbreviated asR B R Bs p s p( ) = + × +0 a .... (9)The slope a  of the average surface resistance ‹Rs› islinked with the slope Rs’ of the local surface resistance RsasRs'=ab. (10)The b  factors were computed with URMEL and aresummarized in Table III.TABLE IIICORRECTION  FACTOR FOR SLOPE Rs’Mode (Frequency [MHz]) bTM010 (500) 0.96TM011 (910) 0.85TM010 (1500) 0.93TM210 (2790) 0.56ACKNOWLEDGMENTSWe thank D. Boussard for his comments when readingwell as the preparation of several samples were performedby R. Russo, whom we thank gratefully.REFERENCES[1] W.Weingarten,Proc.7thWorkshopRFSuperconductivity, Saclay,France, 17 - 20 Oct. 1995, ed. B. Bonin, Gif-sur-Yvette 1996, p. 129,to be published in Part. Acc..[2] C. Durand et al., IEEE Trans. Appl. Supercond. 5 (1995) 1107.[3] URMEL : T. Weiland, DESY 82-015 and DESY M-82-24.SUPERFISH: K. Halbach and R. F. Holsinger, Part. Acc. 7 (1976)213.[4] P. Darriulat et al., ibid. ref. 1, 467.[5] Cf. e.g. A. M. Portis, Electrodynamics of High TemperatureSuperconductors, Singapore 1993, p. 85.[6] J. P. Carini et al., Phys. Rev. B37 (1988) 9726.[7] Cf. e.g. J. H. Hinken, Superconductor Electronics, Berlin, Heidelberg1989, p. 22.[8] R. J. Donnelly, “Cryogenics”, in Physics Vade Mecum, ed. H. L.Anderson, AIP 1981.[9] Ph. Bernard et al., Proc. 3rd EPAC Berlin, March 24 - 28, 1992, eds.H. Henke, H. Homeyer, Ch. Petit-Jean-Genaz, Gif-sur-Yvette 1992,p. 1269.[10] C. Camerlingo et al., Phys. Rev. B31 (1985) 3121.

Metallurgical analysis and RF losses in superconducting niobium thin film cavities

Metallurgical analysis and RF losses in superconducting niobium thin film cavities

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