Alternative materials and coating techniques for cavities E. A. Ilyina1, G. Rosaz1, W. Vollenberg1, J. B. Descarrega1, A. Lunt1, A. Gerardin1, F. Leaux1, S. Calatroni1, A. Sublet1, M. Taborelli, 1 W. Venturini-Delsolaro1, M. Bonura2 & C. Senatore2   1. CERN, Route de Meyrin, CH-1211 Geneva 23, Switzerland 2. Univeristy of Geneva, 24 rue du Général-Dufour, CH-1211, Geneva 4, Switzerland

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

Motivation … to investigate new materials for superconducting accelerating cavities capable of providing the desired characteristics, in addition to operational cost reduction. Proposed solutions: Replace expensive Nb bulk cavities with coated copper ones Copper cavities offer high thermal conductivity at low temperature, which should greatly help to increase the stability of the cavity against breakdown. Nb coated copper cavities are successfully used in CERN ( LEP, LHC and HIE-ISOLDE machines). Replace Nb thin films with superconductor with superior superconducting properties (A15 intermetallic compound)

Choice of Nb3Sn Advantages and challenges Why is it attractive? High critical temperature Tc = 18.3 K (Nb ~ 9.3 K) Small BCS surface resistance RBCS ~ 0.25 nΩ @ 4.2K and 400MHz (Nb ~ 29 nΩ) Challenges Stoichiometry control. The only superconducting A15 phase is reachable only in very narrow region of Sn % (marked with blue colour) Formation of superconducting A15 phase possible only after high temperature treatment Temperature should be high enough to promote SC but not too high to not destroy the cavity (brazing of the flanges and do not melt Cu substrate) Copper substrate Nb3Sn Binary phase diagram of the Nb-Sn system [1] [1] J. Charlesworth, I. MacPhail, and P. Madsen, J. Mater. Sci. 5, 580 (1970)

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

With/without additional annealing in the same chamber “Room-temperature” coatings High-temperature coatings Characterisation as deposited + Nb:Sn 3:1 Annealing Annealing temperatures 600oC - 800oC Annealing time 24H…72H With/without additional annealing in the same chamber Characterisation after annealing + + Characterisation

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

Presence of A15 phase. XRD analysis & morphology “Room-temperature” coatings as-deposited 100 nm Confirming presence of superconducting A15 phase (crystallite size  70 – 200 nm) Absence of non-superconductive phases - NbSn2, Nb6Sn5, as well as Nb after annealing 100 nm

after annealing Presence of A15 phase. XRD analysis & morphology “Room-temperature” coatings after annealing Cracks after annealing 20 µm All “room temperature” samples are evincing cracks after annealing. Now coating recipe to overcome this problem is found !

Analysis of high temperature coatings XRD Confirming presence of superconducting A15 phase With crystallite size  150 - 400 nm Coating pressure 1*10-3 mbar Coating temperature 710oC SEM top view FIB cross section 100 nm 200 nm Cu Nb3Sn Pt Dense, crack free grainy pattern K. Ilyina

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

Magnetic moment vs. temperature at B = 12 Oe. Tc measurements Measurements are done using SQUID-VSM in collaboration with University of Genève. Tconset Best achieved Tc value is 16.5 K Magnetic moment vs. temperature at B = 12 Oe. The inset shows the temperature derivative of the magnetic moment as a function of the temperature

Tc vs Composition Room temperature coatings process lead to higher critical temperature values no matter coating parameters High temperature coating + annealing post coating Could be a good combination by modulating temperature and duration [2] A. Godeke. Supercond. Sci. Technol.,  19 (2006) R68-R80  Tc values constantly lower for the films coated on copper substrates with respect to the bulk Nb3Sn values Copper substrate influence? Stresses in the film??

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

Micro-strain sensitivity of the critical temperature “Room-temperature” coatings+annealing While uniform residual stress is released, micro strain remains significant. Increase in micro strain in the films leads to the critical temperature depression, while dependence from tin content is not obvious Calculated values from broadening of the diffraction line using Rietveld analysis.

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

Copper substrate could be a reason of Tc depression!!! [3] R. Kampwirth ; J. Hafstrom ; C. Wu IEEE Transactions on Magnetics ( Volume: 13, Issue: 1, Jan 1977 ) Difference in thermal expansion coefficient between substrate and Nb3Sn contribute to disordering effect. Element α (x 10-6) K-1 Cu 16.8 Ta 7.64 Nb 7.02 Nb3Sn 6.3 Nb buffer layer could improve Tc value

“Room temperature” coatings + annealing Copper interdiffusion “Room temperature” coatings + annealing Cu Nb Sn Pt Nb3Sn STRONG Cu interdiffusion within the coated layer Not RF compatible Cu 1 µm 1 µm 1 µm 1 µm Accelerating voltage of 30 kV and imaging current of 2.6 nA 1 µm Nb3Sn Cu Pt Nb Sn High temperature coatings 0.1 At% Possibly RF compatible XPS confirmed the presence of Cu in the layer

Possible solutions against copper and stress influence Thin buffer/diffusion barrier layer (Ta, Nb) Reducing disordering effect, preventing copper diffusion Development of optimized annealing protocols Stress reduction, Tc improvement

Outline of the presentation 1. Motivation of the work 2. Elaboration method 3.1 Crystallinity and morphology of the films 3.2 Critical temperature measurements 3.4 Substrate influence on A15 properties 4. Achievements and future actions 3. Results 3.3 Microstructural properties of the films

Target surface resistance 50 nOhm Conclusions Achievements: Presented technology allow to synthesise films with desired composition Presence of superconducting A15 phase is confirmed Best Tc  16.5 K There are recipes for both “room temperature” and high temperature coatings, allowing to produce crack-free surfaces Established dependence of critical temperature on microstructural properties Future action: Get rid of copper in Nb3Sn layer (buffer layer) Improvement of the superconducting properties of the films Continue study for another A15 material – V3Si Test of the RF properties of the film Target surface resistance 50 nOhm By courtesy of Sarah Aull

Thank you for your attention!