Sarah aull secondary electron yield of srf materials
14
Secondary Electron Yield for SRF Materials Sarah Aull
-
Upload
thinfilmsworkshop -
Category
Science
-
view
109 -
download
1
description
In the quest of new materials for SRF applications, the secondary electron yield (SEY) needs also to be taken into consideration. A high SEY holds the risk that multipacting becomes again a main performance limitation of an SRF cavity. In the worst case, a too high SEY makes a material completely unsuitablefor an RF exposed surface. This talk will discuss general aspects of the role of the surface condition and present SEY measurements on different SRF relevant materials, i.e. MgB2, Nb3Sn and NbTiN.
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
10
23
7
57
13
32
18
45
55
4743
4
0
10
20
30
40
50
60
C Nb O Rest
Ato
mic
Co
mp
osi
tio
n [
%]
BCP BCP degreased EP degreased EP sputtered
NbTiN
• Kindly prepared by A-M Valente-Feliciano, JLab
• NbTiN on Nb via HIPIMS
33
9
2
11
41
35
35
0
29
23
9
0
10
20
30
40
50
C 1s N 1s Na 1s Nb 3d5 O 1s Ti 2p3
Ato
mic
Co
mp
osi
tio
n [
%]
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
26
48
8
2
15
5
55
26
1
13
0
10
20
30
40
50
60
C 1s O 1s Nb 3d5 Cu 2p Sn 3d5
Ato
mic
Co
mp
osi
tio
n [
%]
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
43
17
25
20
15
41
0
10
20
30
40
50
B 1s C 1s Cl 2p3 F 1s Mg 2s O 1s
Ato
mic
Co
mp
osi
tio
n [
%]
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).
