Sarah aull secondary electron yield of srf materials
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Transcript of Sarah aull secondary electron yield of srf materials
Secondary Electron Yield for SRF Materials
Sarah Aull
Looking back to the SRF13
• 500 nm MgB2 on a Nb substrate (deposited by Chris Yung at STI)
• Strong multipacting on 1st RF test
• After new rinsing: even stronger multipacting + „burn marks“ in high E field regions
• XPS measurements show only 70% MgB2
Cause for multipacting?
Emax
Secondary Electron Yield (SEY)
• SEY =# 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛
• Primary electron travels through the material, creating secondaries
• Most secondaries are produced at the end of the primary path
• Penetration depth 𝑅𝑚𝑎𝑥~𝐸0 primary energy
• Probability of emission decreases exponentially with depth XS
• 𝑅𝑚𝑎𝑥 ≪ 𝑋𝑆: Few secondaries, but easy emission
• 𝑅𝑚𝑎𝑥 ≫ 𝑋𝑆: Many secondaries, but low emission
• SEY is closely connected to the electrical conductivity
• Metals:
• internal secondaries scatter mainly with free electrons
• Vacuum barrier is in the order of 10 eV
• low SEY: 0.5 (Li) – 1.8 (Pt); SEY(Nb) = 1.3
• Insulators:
• Internal secondaries scatter with phonons and defects
• Vacuum barrier is in the order of 1 eV
• High SEY: 4 – 15 (MgO)
• The SEY of alloys ranges usually between 1.5 and 3
Literature values usually refer to pure material,
not the technical surfaces!
SEY of (non) conductors
• If the primary electron arrives in a grazing angle, secondaries are more likely emitted ( higher SEY)
• If the surface is rough, emitted secondaries can be reabsorbed ( lower SEY)
• Oxides and contamination on the surface might influence the SEY significantly
• Contamination: hydrocarbons, condensed water and gases (especially on a cryogenic surface), foreign material
Influence of the Surface
SEY setup at CERN
• SEY measurement
• under UHV
• at room temperature
• with normal angle
• SEY =𝐼collector
𝐼sample+𝐼collector
• Sample can be transferred to the XPS setup under vacuum so that thesurface condition is not altered.
• The XPS setup includes a sputter ion gun (Argon)
• XPS measurements were performed with every SEY measurement (before and after sputtering) to estimate the cleanliness of the surface
• Sputtering removes contaminants but also changes the chemical composition of the surface!
• Few nm were sputtered off for removal of the carbon peak
XPS & Sputtering
Nb
• Gases will condense on the cold surface
• RF conditioning will not remove nm of material, buthelium processing might
• Angular dependence might play an important role
• It is unknown if the SEY changes below Tc
The SEY data before and after sputtering serves as a bad case and good case scenario!
From SEY data to multipacting in a cavity
SEY of technical bulk Nb
• Both samples cut from same Nb sheet.
• Carbon and oxides have strong impact on the SEY.
47
5
38
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7
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C Nb O Rest
Ato
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BCP BCP degreased EP degreased EP sputtered
NbTiN
• Kindly prepared by A-M Valente-Feliciano, JLab
• NbTiN on Nb via HIPIMS
33
9
2
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C 1s N 1s Na 1s Nb 3d5 O 1s Ti 2p3
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before sputtering
after sputtering
Nb3Sn
• Kindly prepared by Sam Posen, Cornell
• Nb3Sn on Nb via reactive evaporation
• Nb3Sn cavity did not reach multipacting band yet
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15
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C 1s O 1s Nb 3d5 Cu 2p Sn 3d5
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before sputtering
after sputtering
MgB2
• Kindly provided by X.X. Xi, Temple University
• No sputtering to avoid further oxidation
• Formation of MgO will increase the SEY.
1821
1 1
16
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25
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B 1s C 1s Cl 2p3 F 1s Mg 2s O 1s
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1018c
1018d
Conclusion
• The SEY of technical surfaces need to be considered for SRF applications.
• SEY of NbTiN & Nb3Sn comparable to Nb (after sputtering).
• Validation through RF tests is however needed.
• MgB2 needs a non-dissipating passivation with low SEY.
Acknowledgements
• Thank you for providing and preparing samples:
• MgB2 for the QPR: Chris Yung, STI
• MgB2 for SEY: Xiaoxing Xi, Temple University
• NbTiN; Anne-Marie Valente-Feliciano, Jefferson Lab
• Nb3Sn: Sam Posen, Cornell
• Nb: Nuria Valverde Alonso, CERN
• BCP/EP: Serge Forel & Leonel Ferreira, CERN
• Mauro Taborelli for access to the SEY setup
• Mounir Mensi and Holger Neupert for performing the measurements with me (and answering all my questions).