Sindhunil Barman Roy 1 and Ganapati Myneni 2

1. Superconducting Niobium Materials for Radio-Frequency Cavity Applications Sindhunil Barman Roy1 and Ganapati Myneni2 1Raja Ramanna Centre for Advanced Technology, Indore, India 2Jefferson Lab. Virginia, USA

2. Existing method for Niobium Materials qualification • Current method mainly relies on improving the residual resistivity ratio (RRR) of the Nb. • Apparently it is based on the belief that impurity elements degrade superconducting properties of Nb. High RRR (>300) seemingly signifies high purity level of Nb !! • High RRR Nb + right cavity shape + chemical treatment  Low extrinsic (+ surface) defects, so cavity loss reduces. • Niobium refinement process is very expensive.

3. Residual Resistivity Ratio (RRR) • RRR gives an idea of the defects in a metal • Defects in a metal do not mean impurity elements alone, but also encompass point defects, line defects (dislocations), grain boundaries etc. • High RRR, however, does not necessarily say how good (or bad) are the superconducting properties of a material • Gives indirect information on thermal conductivity of the normal state via Wiedemann-Franz law • Thermal conductivity in the superconducting state is non-trivial RRR = R300K/R10K

4. Role of dislocations/strain on the performance of SCRF cavities • This overview of the mechanical and physical metallurgy associated with the production of SRF cavities clearly shows that dislocations are an omnipresent facilitator for and detractor of the performance of cavities. • There is only a small amount of knowledge about the underlying physics of dislocations on functional performance. Processing paths can be optimized to obtain desirable microstructures for forming, but there is also evidence that dislocations affect thermal conductivity and rf currents on the interior surface. • Perhaps a recrystallized single-crystal or large grain cavity with few dislocations aligned with the direction of heat flow may lead to optimal and reproducible performance of a cavity. What is the effect of strain/disorder on the superconductivity ??

5. Role of dislocations/strain on the performance of SCRF cavities RRR of Nb material in the formed SCRF cavity will be significantly different from the RRR of starting Nb-sheet metal. So will be the thermal conductivity !

6. Thermal conductivity of the Nb superconducting state • Thermal conductivity of the superconductivity state is more relevant than that in normal state • Thermal conductivity in the SC state of Nb wih intermediate level of impurity and their magnetic field dependence is not well studied until recently.

7. SCRF Materials R&D: Approach of a Condensed Matter Physicist Superconducting transition temperature, Superconducting Critical Fields and Surface Resistance are the most important parameters Question we are asking: What is the tolerable level of elementalimpurities in sustaining the superconducting and other relevant materials properties of Niobium, which are required for obtaining best performance in a Superconducting Radio Frequency (SCRF) cavity ?

8. Critical Fields in a Superconductor • External magnetic field is totally expelled below a lower critical field limit HC1. • In a type-I superconductor above Hc1 normal state is reached. • In a type-II superconductor magnetic field penetrates the materials above HC1 in the form of quantized flux lines; the material remains superconductor until a upper critical field HC2 • HC1<H<HC2 => Abrikosov lattice or Vortex state => important for high critical current (Jc) applications e.g. SC magnets. • H < HC1 => Meissner state. • HC1 determines the limit of gradient in a SCRF cavity

9. Surface Resistance in a Superconductor Quality factor of a SCRF cavity is inversely proportional to surface resistance Response of a superconductor in ac field is described by two fluid model: • Cooper pairs form superfluid. • Unpaired electrons form normal fluid → source of power dissipation in ac field. BCS Surface resistance • Surface resistance depends exponentially on temperature. • Surface resistance depends to the square of frequency.

10. Influence of impurity on Surface Resistance In a real material like Nb λ = λL √ (ξ0/ξ) , where, ξ0 and ξ are coherence lengths in the pure and real material respectively, and ξ-1 = ξ0-1 + l-1 Niobium For l >> ξ0 → RBCSClean l For l << ξ0 → RBCSDirty l-1/2 W. Weingartnen; Appl. Supercond.

11. Characterization of Nb materials for SCRF cavity applications should be done in terms of superconducting transition temperature, lower critical magnetic field and superconducting surface resistance

12. Effect of Ta and Fe impurities in Nb materials Nb materials R&D at RRCAT : What is the tolerable Ta-impurity level in Nb for SCRF cavity applications ?

13. Nb materials used in the RRCAT investigations • These Nb materials are prepared at Tokyo Denkai in multiple stages of refinement • Same materials have been used for preparation of SCRF cavities at Jefferson Lab • Four sets of materials from different stages of refinement have been received • Each set contains materials, as received from the vendor, and also subjected to different stages of processing (as given to a typical SCRF cavity in Jlab) like, buffer chemical polishing and subsequent thermal annealing and baking. • Some ideas of the impurity levels are available from chemical analysis. They are definitely below 2 at%. • No information available, on the actual distribution of the impurities. • We want to find the correlation between the superconducting properties of Nb and the impurity level, especially whether this is significant for SCRF cavity performance

14. XRF studies of Nb Materials using Indus-2 Synchrotron Source • The samples are in the shape of • roughly 2mm x 2mm x 2mm cube • Superconducting properties • have been studied on the same • sample. • We study XRF spectrum focusing • SR beam separately on each • cube face. This hopefully will • give an idea, if there is gross • inhomogeneity in the impurity • distribution. Nb sample Mono SR beam SDD

15. XRF Spectrum of pristine Niobium sample • Nb samples with various Ta impurity contents as received from vendor • XRF spectrum of a particular cube face • Measurement time 500 seconds; repeated 3 times • Excitation energy : 17 keV • Beam current 70-75 mA. • XRF spectrum taken for all the faces of the sample cube

16. XRF Spectrum of chemically polished Niobium sample • Nb sample with various Ta impurity content s; sample chemically polished and annealed • XRF spectrum of a particular cube face • Excitation energy : 17 keV • Beam current 25-30 mA. • Measurement time 1000 seconds; repeated 2 times • XRF spectrum taken for all the faces of the sample cube

17. Measured concentration profiles on different faces of Nb sample Pristine Nb sample with lowest Ta impurity

18. Measured concentration profiles on different faces of Nb sample; chemically polished sample Chemicaljy polished Nb sample with lowest Ta impurity

19. Determined average concentrations of impurities in Nb samples

20. Effect of Ta Impurities on the SC Properties of Nb Study of Magnetization versus Temperature of Nb samples. Allows accurate determination superconducting transition temperature (TC) Lo Lowest Ta Intermediate Ta Lowest Ta Intermediate Ta highest Ta highest Ta Nb samples as received from vendor Chemically polished Nb samples • No significant Variation of TC as a function of Ta impurity contents • Perceptible change in TC in the chemically polished sample Supercon. Sci. Tech. (2013)

21. Effect of Ta Impurities on the SC Properties of Nb Red : Lower Ta Blue : Higher Ta Red: Lower Ta Blue : Higher Ta HC2 HC1 HC1 HC2 • Higher Ta impurity only marginally affects HC1 and HC2 • Note there is a significant effect of chemical polishing on both the samples. Supercon. Sci. Tech. (2013)

22. Some more comments on Ta impurities in Nb What happens if Ta impurities forms cluster of micron size and reside as inclusions on the surface of SCRF cavity? Thus create hot spots? • Ta being chemically very similar to Nb, leads to the difficulty of separating it, and Ta and Nb readily forms solid solution. So statistical probability of forming Ta cluster is low, and the probability of such Ta clusters residing on the surface is even lower. • If such Ta clusters actually form, then they are expected to be chemically quite pure and free from topological defects. Thus at the operating temperature of SCRF cavities the electrical resistivity is expected to be quite small. • Ta is actually a superconductor of Tc 4.3K. Thus for a Nb SCRF cavity operating at 2K, normally the Ta clusters are not supposed to become hot spots. • Even if at 2K , at high RF fields such Ta clusters tend to become normal, being surrounded by superconducting Nb they are still likely to remain as superconductors through proximity effect !

23. Summary and conclusion at this stage • Superconducting properties (TC, HC1 and HC2) of Nb samples do not change significantly with variation of Ta impurity contents between 1300 and 130 ppm • Exact elemental impurity ( atomic no. > 11) contents of these Nb samples have been determined . To the best of our knowledge this kind of study, correlating superconducting properties of Nb materials with impurity contents does not exist • BCS surface resistance does not get affected significantly in Nb due to such variation of impurity contents (from literature). Surface roughness does influence. • Thermal conductivity of Nb materials do not vary significantly due to Ta impurity contents (from literature) • We seriously question the necessity of existing degree of refinement of Nb material (from Ta impurities) ? Huge cost implications ! • What about low Z impurities like O, H and N ?

24. Nb materials R&D at RRCAT : Possible role of low Z gaseous impurities in SCRF cavity?

25. Some Results on the possible effects of low Z impurities on the superconducting properties of Nb materials Effect of Buffer Chemical Polishing (BCP) treatment on the TC of Nb samples Samples from Jlab, USA. Conclusions : BCP degrades Tc considerably. Supercon. Sci. Tech. Vol. 21 065002 (2008); Vol. 22 105014 (2009)

26. Some Results on the possible effects of low Z impurities on the superconducting properties of Nb materials Buffer Chemical Polish (BCP) treatment lowers the field at which magnetic flux lines enter the material as compared to that in pristine Nb. SCRF cavity prepared with such BCP Nb would reach maximum 30-35 MV/m → Supercon. Sci. Tech. Vol. 21 065002 (2008); Vol. 22 105014 (2009)

27. Anomalous flux-pinning properties of chemically polished Nb materials • Magnetization hysteresis, hence flux- pinning is less in chemically polished Nb samples. • This is observed in fine grain, large grain and single crystal samples of Nb. • Chemically polished samples is supposed to have Bean-Livingston surface barrier. • The surface of the pristine samples is strained and have more impurity atoms. So it can have enhanced surface pinning. • Absence of flux-jumps in BCP Nb indicates that bulk pinning is affected. Supercon. Sci. Tech. Vol. 21 065002 (2008)

28. Temperature and magnetic field dependence of thermal conductivity of superconducting large grain Niobium • Normal state κ (T) of BCP treated Nb is lower (by 10%) than that of pristine Nb. • Both Tc and HC1 of BCP treated Nb is lower than the pristine Nb. • A small but distinct dip in κ (T) is observed at HC1 Supercon. Sci. Tech. Vol. 25 035010 (2012)

29. Temperature and magnetic field dependence of heat capacity of superconducting large grain Niobium S B Roy et al (unpublished)

32. Elemental Niobium : An Exotic Superconductor • λ/ξ in very pure Nb can be as low 0.84 which is in the crossover regime from type-I to type-II superconductor. • In recent times it is recognized that such superconductors are rather rare, and they belong to a new class type-1.5. Their properties including the flux-line lattice or vortex matter phase are now being explored, • Nb provides an unique opportunity to study such superconductors and its contrast with well studied type-II superconductors, as with varying impurity contents λ/ξ will go well inside the type-II regime. • Nb with varying impurity contents will allow a systematic study of the superconductivity from clean limit to dirty limit. The crossover region, the so called intermediate impurity range is a rather unexplored region. • Flux-line lattice or vortex matter in Nb is already known to be quite interesting. Nb with various degrees of defects/impurities provides an interesting platform for a systematic study on the effect of disorder in vortex-matter.

33. Thank you