In a systematic study of the RF response of superconducting niobium cavities operated in their fundamental TM010 mode at 1.5 GHz, magnetic flux trapping has been used as a tool to diagnose the presenc e of pinning centres. In addition to bulk niobium cavities the study covers copper cavities, the inner walls of which are coated with 1.5 µm thick niobium films grown by magnetron sputtering in a nobl e gas atmosphere. The use of different gases (Xe, Kr, Ar and Ne) or gas mixtures has made it possible to vary the concentration of noble gas atoms in the films. Film contamination is characterised by an electron mean free path l calculated from the results of systematic measurements of the penetration depth at T = 0 K, l0, and from RRR measurements made on samples prepared under similar conditions as the cavity films

Benvenuti, Cristoforo

Calatroni, Sergio

Campisi, I E

Darriulat, Pierre

Durand, C

Peck, M A

Russo, R

Valente, A M

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsCERN   EST/97-08 (SM)MAGNETIC FLUX TRAPPING IN SUPERCONDUCTING NIOBIUMC. Benvenuti, S. Calatroni, I. E. Campisi1 , P. Darriulat,C. Durand2 , M.A Peck, R. Russo, A.M. ValenteAbstractIn a systematic study of the RF response of superconducting niobium cavities operatedin their fundamental TM010 mode at 1.5 GHz, magnetic flux trapping has been used as atool to diagnose the presence of pinning centres. In addition to bulk niobium cavitiesthe study covers copper cavities, the inner walls of which are coated with 1.5 µm thickniobium films grown by magnetron sputtering in a noble gas atmosphere. The use ofdifferent gases (Xe, Kr, Ar and Ne) or gas mixtures has made it possible to vary theconcentration of noble gas atoms in the films. Film contamination is characterised by anelectron mean free path " calculated from the results of systematic measurements ofthe penetration depth at T = 0 K, O0, and from RRR measurements made on samplesprepared under similar conditions as the cavity films.Paper presented at the Eighth Workshop on RF SuperconductivityAbano, Italy, 6-10 October 1997Geneva, SwitzerlandDecember 1997                                                       1 On leave from TJNAF, Newport News, Virginia, 23606, USA2 Present address: Laboratori Nazionali di Legnaro, I-35020 Legnaro, PD, Italy-2-1. IntroductionIn a systematic study of the RF response of superconducting niobium cavitiesoperated in their fundamental TM010 mode at 1.5 GHz, magnetic flux trapping has beenused as a tool to diagnose the presence of pinning centres. In addition to bulk niobiumcavities the study covers copper cavities, the inner walls of which are coated with 1.5 µmthick niobium films grown by magnetron sputtering in a noble gas atmosphere. The use ofdifferent gases (Xe, Kr, Ar and Ne) or gas mixtures has made it possible to vary theconcentration of noble gas atoms in the films. Film contamination is characterised by anelectron mean free path " calculated from the results of systematic measurements of thepenetration depth at T = 0 K, O0, and from RRR measurements made on samplesprepared under similar conditions as the cavity films.2. Experimental Apparatus and MethodA sketch of the apparatus is shown in Fig. 1. The average RF magnetic field (HRF) andsurface resistance (Rs) are obtained from spectrum analyser measurements of theincident, reflected and transmitted RF powers. Flux trapping is achieved with the help of asolenoidal coil wound around the cryostat which produces a magnetic field parallel to thecavity axis and uniform over its volume. The coil is surrounded by a Mu-metal cylinderwhich shields the cryostat from the earth field to better than 0.01 G. Care has been takento prevent the presence of ferromagnetic materials inside the cryostat.Fig. 1In practice, each measurement implies draining the cryostat and letting the cavitytemperature increase above Tc, to typically 20 K, switching the solenoid on at the desiredfield level Hext, adjustable between 0 and 9 G, and refilling the cryostat with liquid helium(Fig. 2, path 1).-3-Fig. 2Hall probes distributed along an outer meridian of the resonator are used to measure thenormal magnetic field component in the vicinity of the superconductor’s surface. As the cavity iscooled down below Tc, it crosses the transition from normal state to mixed phase when thetemperature T is such that the upper critical field, Hc2(T) = Hext. At this time the magnetic fluxcondenses in a fluxon lattice having a density of 4.8 × 104 mm-2 per gauss of Hext , too large to resultin a detectable signal on the 5 mm2 area of each of the Hall probes. When the temperaturedecreases further and enters the Meissner phase, the Hall probe signal will remain unchanged ifthe flux is fully trapped, as is usually the case. If, on the contrary, the flux is partly or completelyexpelled the Hall probe signal will be affected. For each cavity the field discontinuity 'H detected byeach of the Hall probes (with the solenoid on) when crossing the transition upwards is measured intwo different configurations, one ('Hon) when the cavity has been cooled down in Hext z 0, the other('Hoff) when the cavity has been cooled down in Hext = 0.Fig. 3-4-The parameter H = 'Hon/'Hoff measures the fraction of magnetic flux expelled from thefilm as it becomes superconducting. When the flux is fully trapped, H = 0. However, as theHall probes see only a very small fraction of the cavity area, the extrapolation of this resultto the whole surface is only valid if the film is sufficiently homogeneous. In practice, otherfeatures give confidence that this is indeed the case. When H deviates from 0, a situationobserved in the case of heat-treated bulk niobium cavities, it is only possible to concludethat a change in fluxon density occurred over the cavity area covered by the Hall probes.However, in this case it is not possible to reliably deduce from the measurement of H thevalue of the flux trapping efficiency averaged over the whole cavity area. Examples ofsuch measurements are illustrated in Fig. 3 which displays the magnetic field H measuredin one of the Hall probes as a function of temperature. A typical film with full flux trappingis illustrated in Fig. 3a, and a heat-treated bulk niobium cavity with incomplete fluxtrapping in Fig. 3b. The accuracy of the H measurement is about 1%.As a by-product, the jump of 'Hoff across Tc provides a measurement of the criticaltemperature, the width of the transition being only slightly affected by the temperaturegradient between the upper and lower parts of the resonator, which is kept below4 mK/cm over the cavity length.A small superconducting coil, attached to the external resonator wall at the equator, isused to obtain a relative measurement of the lower critical field at T = 1.7 K, Hc1. It canproduce a field of up to 20 kG on the film surface. When applied on a cavity cooled downin Hext = 0 (the perfect Meissner state) its penetration inside the film signals the transitioninto the mixed phase, occurring when the field generated by the superconducting coilreaches Hc1, and results in a sudden increase of the surface resistance (Fig. 2, path 2).Fig. 4 illustrates a typical measurement. The knee of the curve, obtained from a linear fitto ln Rs versus I (the current in the superconducting coil), defines Hc1. While the methoddoes not allow an accurate measurement of the absolute value of Hc1 because of the badgeometry, it provides a good relative measurement when comparing one cavity withanother, the geometry being the same in both cases. The dependence of Hc1 on the meanfree path is illustrated in Fig. 5 where Hc1-2 is displayed as a function of O02, both quantitiesbeing normalised to the values taken in the clean limit, with O02 = Oclean2 (1+S/2([0/")) and [0being the BCS coherence length. The line, drawn as a reference, corresponds to adependence of the form Hc1-2 = O04/3.Fig. 4 Fig. 5-5-3. Flux trapping efficiencyWith the exception of heat-treated bulk niobium, complete trapping was alwaysobserved, H < 0.2% to 99% confidence level. Moreover, the trapped state was found to berelatively robust against external perturbations. It was unaffected by switching on and offthe solenoid, by the presence of RF power in the cavity, by quenches (corresponding to abrief local increase of the temperature above Tc), and did not significantly evolve overperiods of several hours. However, for bulk niobium the measurement method preventsan accurate localisation of the pinning centres. In particular, pinning may occur on theouter surface of the cavity wall and the fluxons may be free to move on the inner surfacewhere the RF field penetrates.Heat-treated (1000°C or more) bulk niobium cavities display a lower trappingefficiency, with H taking different values for different Hall probes, spanning the whole rangebetween 0 and 1 and going hand in hand with a lack of reproducibility of the Rsmeasurements in successive trapping cycles. Unfortunately, an analysis of the nature ofthe fluxon movements when crossing the mixed phase would require much more powerfulinstrumentation than that available in the present experiment. The small area covered byeach of the Hall probes, comparable to that of a grain in the heat-treated case, and thebad geometry (the probes being on the outer side of the 3 mm thick niobium wall while theRF field penetrates from the inner side) prevent a reliable interpretation of the data in suchcases.4. Fluxon-induced lossesThe fluxon-induced resistance, Rfl, is obtained from the difference between data measuredwith and without trapped flux. It is best measured at T d 1.7 K where the BCS resistance hasbecome negligible. Its dependence on HRF and Hext is observed to take the formRfl (1.7 K) = (R0fl + R1fl HRF) Hext.This is illustrated in Figs. 6 and 7 with data corresponding to films grown in an argonatmosphere (RRR # 15, O2 # 2.5).051015202530354045500 5 10 15 20 25 30Hrf [mT]Rfl / Hext [nΩ/G]0246810120 2 4 6 8 10 12Hext [G]R fl / ( R0 fl + R1 fl Hrf ) [G]Fig. 6 Fig. 7Above a cavity-dependent threshold, possibly correlated with field emission, Rfl is oftenobserved to increase faster with HRF. The proportionality to Hext , indicating that the lossesinduced by different fluxons are uncorrelated, remains valid as long as Rfl does not exceed afew hundred nanoOhms, above which a steeper Hext dependence is usually observed.-6-Films may occasionally display a threshold in Hext, Hthr, below which Rfl (1.7 K)vanishes. Such films are usually of bad quality, as shown by the large value of theirresidual resistance. A possible explanation of this particular feature is the presence of alarge density of small normal conducting defects (such as pin holes or cracks reachingdown to the copper substrate) having dimensions in excess of the coherence length. Fluxtrapped in such defects does not induce any additional RF loss as the defect was alreadynormal conducting before trapping. The threshold in Hext would then correspond to thesaturation of such defects, above which Rfl (1.7 K) would increase in proportion to thenumber of additional trapped fluxons, i.e. to Hext-Hthr.Within the linear range the parameters R0fl and R1fl can be used to characterise fluxon-induced losses at low temperature. Fig. 8 illustrates their evolution when the mean freepath (each data point is labelled with the associated value of O02 ) is varied. Minimal lossesare observed when the pinning is strongest, corresponding to barely overlapping pinningcentres in the thin superficial layer where the RF penetrates. For higher concentrations ittakes less energy for a fluxon to jump from one centre to a nearby one under the influenceof the RF field, whilst for lower concentrations some fluxons which are not individuallypinned are free to move away from their equilibrium position in the globally pinned lattice.In both cases important flux-flow losses are generated.Fig. 8The temperature dependence of Rfl is illustrated in Fig. 9 for films grown in an argonatmosphere (O2 # 2.5). The ratio kfl = Rfl (4.2 K) / Rfl (1.7 K) is a convenient measure of theamount of temperature dependence. As a function of the mean free path (Fig. 10) itdisplays a maximum at "#[0 , where Rfl is minimal.11.522.531.5 2 2.5 3 3.5 4T [K]Rfl (T) / Rfl (1.7K)Fig. 9 Fig. 10-7-5. ConclusionThe data presented in this short note illustrate how magnetic flux trapping can beused as a tool in the study of the superconducting properties of niobium films. Twoparameters, R0fl and R1fl , are usually sufficient to describe fluxon-induced RF losses. Theyspan nearly two orders of magnitude when the mean free path is varied from the cleanlimit to " # 0.1 [0. They provide information complementary to that obtained from theknowledge of the mean free path, as the pinning and electron scattering mechanisms areof different natures. They can be measured accurately and contribute importantinformation to the understanding of low Tc superconductivity.

Magnetic flux trapping in superconducting niobium

Magnetic flux trapping in superconducting niobium

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