Shigeki KATO

1
In-situ Measurement of Secondary Electron Yields
at Positron Ring of the KEKB

• Shigeki KATO
• ? Summary of Laboratory Results
• ? Experimental Setup at LER
• ? Sample Materials and Experimental Procedures
• ? Experimental Result of SEYs
• ? Dependences on Materials and Primary Electron
  Energy
• ? Comparison before and after Exposure to
  e-Cloud
• ? Comparison w/ Solenoid Field and w/o Solenoid
  Field
• ? Additional Experimental Result of SEYs at Lab
• ? SEYs of CO Saturated NEG Surface
• ? Incident Angle Dependence of SEYs
• ? Summary

2
Summary of SEYs and Material Surfaces at KEK
Laboratory

• Electron or ion dose effects of dmax at different
  copper surfaces (mainly CERN data) were
  reasonably explained with Electron Beam Induced
  Surface-graphitization.
• More e- Dose More Graphite Lower SEY
• Electron Beam Induced Surface-graphitization was
  found at most metallic surfaces.
• Carbon source was found to be carbonatious
  contamination on as-received metal surfaces.
• This graphitization still occurs even for
  sputtered clean copper owing to electron beam
  irradiation or heating in UHV because of carbon
  diffusion from sub-surface and bulk.

3
In-situ Measurement System of SEYs at LER
? SEMS (Secondary Electron Measurement System)
has been built at Fuji straight of LER.
ltFeaturesgt
? UHV In-situ Measurements of SEYs at Surfaces
Exposed to e-Cloud at LER ? Primary Electron
Beam 50eV5KeV, Beam Scan Capability ? Quick
Sample Exchanging Capability with Loadlock
Chamber ? Electron Activity Monitoring Close to
Sample _at_ Beam Chamber
It does not seem cost effective system! What is
the reason a sample exposed to e-Cloud cannot be
measured existing SEMS of lab after its transfer?
4
Example (NEG) of Non In-situ SEY Measurement
? SEY
SEY of NEG --gt e-Cloud Exposure at LER
--gtTransfer via Air --gt SEY of NEG dmax1.96
dmax1.87
In-situ dmax0.98
? Meaningless measured value is given due to air
exposure.
? Change of Surface State Observed Using XPS
? Surface is heavily contaminated due to air
exposure. ? Surface history is almost reset.
? In-situ Measurement in UHV is Always Mandatory!
5
In-situ SEMS at Fuji Straight Section at LER
Isolation Valve
Electron Source
Y
Positron Beam Chamber
X
Z
Three Sample Manipulators
Base Pressure 10-8 Pa
6
SEMS at LER (cntn'd)
Y
Samples
Hole (1.5mmf) of Electron Monitor
Sample
Faraday Cups
Z
Sample Surface
7
Samples Experimental Procedure
? Exposed Samples
? Experimental Procedure

• Sample Loading to SEMS and Baking at 180?
• Measurement of SEYs in UHV (Ambient Temperature,
  ? Incident Angle)
• Sample Loading from SEMS to LER Beam Chamber
• Exposure to Electron Cloud for Two Weeks during
  Physics Run
• Sample Transfer Back to SEMS and Measurement of
  SEYs

? Conditions of SEY Measurement ? Different
Materials ? Comparison before and after
Exposure ? Comparison w/ or w/o Solenoid Field
8
SEYs after Exposure to e-Cloud w/o Solenoid Field
Primary Electron Energy Dependence of SEYs for
Copper, TiN and NEG
At Lab After e- Irradiation
At LER After Exposure
At LER Before Exposure
? All as-received Samples High dmax (NEG
Highest dmax)
? After Exposure to e-Cloud Drastic Decease
ofdmax for All (Copper Lowest dmax)
? Results obtained at LER are almost consistent
with those results obtained at Lab.
? Surface Analyses e- beam induced
graphitization was found for copper surface
exposed to e-Cloud as found in lab experiment. (
In lab, the same graphite formation at TiN
surface graphite and carbide formation at NEG
surface were found.)
9
SEYs after Exposure to e-Cloud w/ Solenoid Field
Primary Electron Energy Dependence of SEYs for
Copper and NEG
? With Solenoid Field Slightly Largerdmax for
All --gt Slow Aging of Surfaces with
Less e-Cloud --gt Less e- Beam
Induced Graphite and/or Carbide Formation
10
SEYs of CO Saturated NEG Surface ( lab )
? SEY of gas saturated NEG surface should
increase because of pumped contamination. It
is important to know it for designing. ? CO
saturation was made until NEG pump speed
deteriorates down to1/100.
CO Saturated NEG
Non Gas-saturated NEG
? After Heating Large Increase of dmax --gt More
e-Cloud at Beginning of Beam Operation.
? After e- Beam Irradiation Slight Increase of
dmax due to Less Graphite and/or Carbide
Formation.
? In This Case, CO Saturation is Small ( only 0.4
at! ) More Serious Issue when Repeat Use
11
Dependence of SEYs on Incident e- Beam Angle (
lab )
? Larger SEYs Sallower Incident Angle of e-
Beam. ? This dependence is important for both
mag. field and field free spaces.
SEY Results for e- Beam Irradiated Surfaces
Close to Real Surface State
60
dmax 30 Up
dmax 10 Up
0
? Lower Z Materials Less Enhancement of dmax
with Sallower Incident Angle.
12
Dependence of SEYs on Incident Angle (cntn'd)
? Sallower Incident Angle of e- Beam Larger SEYs
SEYs Results for As-received Surface
? Metal Surfaces High SEYs and Large
Enhancement (x3) of SEYs with Sallower
Incident Angle at Initial Stage
? Graphite Surfaces Low SEYs and Small
Enhancement (x2) of SEYs in a Wide Range of
Incident Angle
? Proposal Test Installation of Graphite Wall
to Beam Chamber
13
Summary

• SEMS (Secondary Electron Measurement System) has
  been built at Fuji straight of LER.
• Results obtained at LER were almost consistent
  with those results obtained at Lab.
• e- beam induced surface-graphitization was found
  for copper surface exposed to e-Cloud as found in
  lab experiment.
• Slightly largerdmax of NEG and copper with
  solenoid field would be explainable based on slow
  aging of surfaces without e-cloud due to less e-
  beam induced graphite and/or carbide formation.
• After heating of NEG surface, large increase of
  dmax would cause more e-cloud at beginning of
  beam operation.
• 1st CO Saturation of only 0.4 at of NEG showed
  remarkable increase of dmax . Repeat use of NEG
  will cause further increase of dmax.
• Larger SEYs with sallower incident angle of e-
  beam should be recognized well.
• At metal surfaces, high SEYs and its large
  enhancement with sallower incident angle at
  initial stage were shown.
• At graphite surface, low SEYs and its small
  enhancement in a wide range of incident angle
  were found.
• Test installation of graphite wall to beam
  chamber was proposed.

Plan Measurement of SEY Change after Long Term
Exposure to e-Cloud and Exposure to H2
or CO Gas during Beam Operation.
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14
Fin
Thanks for Technical Assistance Michiru
NISHIWAKI Michiru SHIRAI and Tsuneyuki
NOGUCHI
e-Cloud
e Bunch
15
(No Transcript)
16
Typical Exposure to e-Cloud for Two Weeks
Without Solenoid Field
LER Beam Current ?Idt 500Ah

• Monitored
• Electron
• Dose (?)
• 3x10-4Ah
• --gt 4x1020e-/cm2