Secondary Electron Yield for SRF Materials Sarah Aull
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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)
• Primary electron travels through the material, creating secondaries
• Most secondaries are produced at the end of the primary path
• Penetration depth primary energy• Probability of emission decreases
exponentially with depth XS
• : Few secondaries, but easy emission• : 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
• Sample can be transferred to the XPS setup under vacuum so that the surface 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, but
helium 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.
C Nb O Rest0
10
20
30
40
50
60
47
5
38
10
23
7
57
13
32
18
45
55
4743
4
BCP BCP degreased EP degreased EP sputtered
Atom
ic C
ompo
sition
[%]
NbTiN
• Kindly prepared by A-M Valente-Feliciano, JLab• NbTiN on Nb via HIPIMS
C 1s N 1s Na 1s Nb 3d5 O 1s Ti 2p30
10
20
30
40
50
33
9
2
11
41
35
35
0
29
23
9
before sputteringafter sputtering
Atom
ic C
ompo
sition
[%]
Nb3Sn
• Kindly prepared by Sam Posen, Cornell• Nb3Sn on Nb via reactive evaporation
• Nb3Sn cavity did not reach multipacting band yet
C 1s O 1s Nb 3d5 Cu 2p Sn 3d50
10
20
30
40
50
60
26
48
8
2
15
5
55
26
1
13
before sputteringafter sputtering
Atom
ic C
ompo
sition
[%]
MgB2
• Kindly provided by X.X. Xi, Temple University• No sputtering to avoid further oxidation• Formation of MgO will increase the SEY.
B 1s C 1s Cl 2p3 F 1s Mg 2s O 1s0
10
20
30
40
50
1821
1 1
16
43
17
25
20
15
411018c1018d
Atom
ic C
ompo
sition
[%]
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).
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