A 15 thin films development for SRF applications

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A 15 thin films development for SRF applications K. Ilyina 1*, G. Rosaz 1,

A 15 thin films development for SRF applications K. Ilyina 1*, G. Rosaz 1, S. Calatroni 1, W. Vollenberg 1, J. B. Descarrega 1, A. Lunt 1, A. Gerardin 1, F. Leaux 1, A. Sublet 1, M. Taborelli, 1 W. Venturini-Delsolaro 1, M. Bonura 2 & C. Senatore 2 1. CERN, Route de Meyrin, CH-1211 Geneva 23, Switzerland 2. Univeristy of Geneva, 24 rue du Général-Dufour, CH-1211, Geneva 4, Switzerland K. Ilyina 1

Motivation … to develop innovative coating techniques and investigate new materials for superconducting accelerating

Motivation … to develop innovative coating techniques and investigate new materials for superconducting accelerating cavities capable of providing the desired characteristics, in addition to operational cost reduction. Proposed solutions: q Replace expensive Nb bulk cavities with coated copper ones. ü Copper cavities offer high thermal conductivity at low temperature, which should greatly help to increase the stability of the cavity against breakdown. ü Nb coated copper cavities are successfully used in CERN ( LEP, LHC and HIEISOLDE machines). q Replace Nb with superconductor with superior parameters (A 15 intermetallic compound) K. Ilyina 2

Nb 3 Sn T c R High critical temperature Tc = 18. 3 K

Nb 3 Sn T c R High critical temperature Tc = 18. 3 K (Nb ~ 9. 3 K) Small surface resistance Rs ~ 0. 4 nΩ @ 4. 2 K and 500 MHz (Nb ~ 45 nΩ) Lines of equal BCS surface resistance Rs. BCS for films in the plane ( n, TC) at T = 4. 2 K and 500 MHz (Nb ~ 45 n ) s Stoichiometry control. A 15 phase is reachable only in very narrow region of Sn % SC properties appear only after annealing Challenges . Temperature should be high enough to trigger the SC but not too high to not melt Cu and not destroy brazing of the flanges. Binary phase diagram of the Nb-Sn system [1] J. Charlesworth, I. Mac. Phail, and P. Madsen, J. Mater. Sci. 5, 580 (1970). K. Ilyina 3

Low temperature coatings • Characterisation as-deposited • Annealing • Characterisation after annealing High temperature

Low temperature coatings • Characterisation as-deposited • Annealing • Characterisation after annealing High temperature coatings • Characterisation as- deposited • Possible additional annealing & characterisation K. Ilyina 4

Low temperature coatings Deposition and annealing systems Annealing Atomic Sn percentage for CERN synthesised

Low temperature coatings Deposition and annealing systems Annealing Atomic Sn percentage for CERN synthesised samples varies in the region from 20% to 35% We are able to produce films with right composition K. Ilyina 6

Presence of A 15 phase. XRD analysis & morphology Low temperature coatings as-deposited P

Presence of A 15 phase. XRD analysis & morphology Low temperature coatings as-deposited P 1=1*10 -3 mbar 100 nm after annealing Confirming presence of superconducting A 15 phase Absence of non-superconductive phases - Nb. Sn 2, Nb 6 Sn 5 100 nm 7

High temperature coatings Coating pressure From 1*10 -3 mbar to 5*10 -2 mbar Coating

High temperature coatings Coating pressure From 1*10 -3 mbar to 5*10 -2 mbar Coating temperatures 650 o. C – 735 o. C K. Ilyina 8

Analysis of high temperature coatings XRD Confirming presence of superconducting A 15 phase P

Analysis of high temperature coatings XRD Confirming presence of superconducting A 15 phase P 1=1*10 -3 mbar Coating temperature 710 o. C SEM FIB cross section Pt Dense, crack free grainy pattern Nb 3 Sn Cu 100 nm 200 nm K. Ilyina 8

Determination of Tc Tconset Best achieved Tc value is 16. 5 K Magnetic moment

Determination of Tc 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. K. Ilyina 9

Tc vs Composition CERN samples Expected dependence [2] A. Godeke. Superconductor Science and Technology,

Tc vs Composition CERN samples Expected dependence [2] A. Godeke. Superconductor Science and Technology, Vol. 19, N. 8 No strong dependence from Sn% on CERN samples Another parameter screens the expected dependency K. Ilyina 10

Critical temperature dependence on sputtering and annealing parameters Low and high temperature coatings Low

Critical temperature dependence on sputtering and annealing parameters Low and high temperature coatings Low temperature coatings process lead to higher critical temperature values no matter coating parameters HT coating + annealing post coating Could be a good combination by modulating temperature and duration Best Tc values achieved for the samples coated at intermediate pressure K. Ilyina 11

Copper substrate influence K. Ilyina 12

Copper substrate influence K. Ilyina 12

Low temperature coatings Cu Sn Nb Pt Nb 3 Sn PAr=1*10 -2 mbar 1

Low temperature coatings Cu Sn Nb Pt Nb 3 Sn PAr=1*10 -2 mbar 1 µm Cu Tann=800 o. C Pt 1 µm Cu Nb Sn Nb 3 Sn PAr=5*10 -3 mbar 1 µm Cu Tann=750 o. C 1 µm STRONG Cu interdiffusion within the coated layer NOT RF COMPATIBLE K. Ilyina 13

High temperature coatings Cu Sn Nb 2. 5 µm 100 nm PAr=1*10 -3 mbar

High temperature coatings Cu Sn Nb 2. 5 µm 100 nm PAr=1*10 -3 mbar 2. 5 µm Cu 1 µm Sn Nb 1 µm 100 nm PAr=5*10 -2 mbar XPS ANALYSIS NEEDED TO CONFIRM IF Cu SIGNAL IS REAL OR JUST BACKGROUND K. Ilyina 14

Possible solutions q Thin buffer layer (Ta, Nb) Could possibly cause cracking of the

Possible solutions q Thin buffer layer (Ta, Nb) Could possibly cause cracking of the surface in RTC. HTC – crack-free surface. q Development of appropriate annealing protocols K. Ilyina 15

Conclusions Achievements: • • Presented technology allow to synthesise films with desired composition after

Conclusions Achievements: • • Presented technology allow to synthesise films with desired composition after coatings Presence of superconducting A 15 phase is confirmed Best Tc 16. 5 K There are recipes for both LTC and HTC, allowing to produce crack-free grainy pattern Future action: • • • Get rid of copper in Nb 3 Sn layer (buffer layer, high temperature coatings) New annealing protocols development Improvement of the superconducting properties of the films Test of the RF properties of the film Continue study for second A 15 material – V 3 Si K. Ilyina 16

Thank you for your attention! K. Ilyina 17

Thank you for your attention! K. Ilyina 17