Transcript ELectron Cloud 2007 - Stanford University

Quick Review of Secondary
Electron Yield Studies to
Mitigate Electron Cloud Effect in
Accelerator
F. Le Pimpec (PSI)
R. Kirby, F. King, M. Pivi (SLAC)
MULCOPIM 08
Valencia, Spain 2008
Outline
• EC mechanism
→ impact on circulating beam
• Luminosity & non Luminosity affecting solution
• Need for theoretical understanding
• Machine & Lab solution :
→ Technical remedies
→ Surface approach (Lab studies)
→ In-situ machine studies
• Summary
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Electron cloud mechanism
Beam Damage
U. Irizo Ariz
RHIC (up) & LHC (down) multipacting schema
G. Rumolo
F. Ruggiero
e-cloud can also arise without multipacting :
25.09.2008 Primary e produced by Ions and Photons
And it could be
even worse…
F. Le Pimpec 3
Possible Remedies
• Reduce the number of charges by bunch
→ lower the luminosity – Sorry not acceptable
• Increase the gap between bunches
→ lower the luminosity – Sorry not acceptable
• Remove some bunches
→ lower the luminosity – Sorry not acceptable
• Get a surface which does not emit much electrons
upon photon, ion or electron bombardment
→ Killing emitted electrons at the emission point : ex Winding
solenoid (NA everywhere), clearing electron electrodes…
→ Play with the surface material : Nature, Geometry, Chemistry
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Understanding the problem
• Need for theoretical understanding of the ECE
● many codes have been developed for accelerators : POSINT,
ECLOUD, EPI, PEI … Code repository URL which includes ESA ESTEC®
(http://oraweb.cern.ch/pls/hhh/code_website.startup)
● General EC webpage at CERN :
(http://ab-abp-rlc.web.cern.ch/ab%2Dabp%2Drlc%2Decloud/
)
• Understanding of :
● Build-up of the cloud (model : SE- Yield- Energy spectrum, UHV chber  properties …)
● Instability of the running beam due to EC
→ Need experimental input on SEY – also near 0 eV incident electron
energy
• EPAC 2004 : Comparison of some codes (THPLT017 )
“ Build-up and instability simulation codes can produce results that vary by factors 3–
100. The differences reflect a strong sensitivity to modeling details. ”
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SEY at 0 eV !!!
Buildup of electron cloud depending
of the parameterization taken at 0 eV
incident electron energy; simulation
for LHC (SLAC-TN-04-046)
R. Cimino et al,
See Ecloud’04
proceedings
Can Low-Energy Electrons Affect
High-Energy Physics Accelerators?
R. Cimino et al
Phys. Rev. Lett. 93, 014801 (2004)
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Technical Solutions
• Clearing Electrodes
→ Potent device, as long as their curing effects on EC does not create new
problems (wakefield, gas desorption, ion instabilities …)
→ “At ECL2 , the optimum configuration of clearing electrodes was investigated,
their impedance for different layouts estimated by two independent programs,
GdfidL and HFSS, respectively, and the clearing efficiency explored in simulations.
“ (CERN-2007)
• Winding Solenoids :
→ Very effective techniques in field free region, but not easy to wind everywhere
(usually Bz <50 gauss).
→ Since year 2000 more than 95 % of the drift region of KEKB are covered.
“Blow up is now almost suppressed up to 1.7 A (3.06 RF bucket spacing)”
Y. Suetsugu (ILCDR 2008 – Cornell University)
solenoid windings
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PEP 2
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Technical Solutions
• Photon Killer :
CERN – LHC
→ In LHC the sawtooth is used to avoid reflection of
photons far away from the emission point. EC is confined
where emitted photons strike the wall
→ The other approach is to use an Antechamber with
photon absorber where photons are trapped. Technique
commonly used in most e+/e- machine.
Photons
Be window
Y. Tanimoto, KEK EPAC08
25.09.2008
Pumping with NEG or with
distributed TiSp pumps
CERN
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Surface studies
So far “we” haven’t found
a material with an as
received max below 1.5
TiZrV/Al
NIM A (551),187, 2005
Nb
We must play tricks :
heating, particles
bombardment, roughness
change …
As received, TiZrV getters and
TiN samples have a max > 1.6
Thermal treatment is effective
in reducing the SEY, but you
need to be able to heat up the
device.
NH et al Appl. Phys. A 76, (2003)
As Received = air exposed
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Electron conditioning
Electrons from the e-cloud can condition surfaces.
Hopefully in a reasonable time scale !
NIM A (551),187, 2005
CERN (NH) - 2000
e-
However, as the e-cloud disappears the conditioning dies off !
But we get steady help from the SR (CERN and KEK results)
and some extra help from residual gas ions kicked to the wall
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Ion conditioning
250 eV ion exposure (P~10-8 Torr)
SEY after PGD + Air exposure
e-
CERN (NH) - 2000
NIM A (564),44-50, 2006
PGDs (~500 V - 0.1 Torr) are extremely
efficient in lowering the SEY to pure
metal values in short time (~30 minutes)
Downside : Not so trivial to make a PGD
in-situ
What about CO ion exposure ?
(chemistry)
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Using the beam itself to condition
the wall is of course an option
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Artificial roughness
Simulation
Cu
Cu
 = 60°
M. Pivi et al to be published in JAP
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Artificial roughness & coatings
TiZrV/Al
h x w x l = 5 x 1 x 1 [mm]
SR, ions and electrons do produce electrons & increase the
pressure (PSD, ESD, ISD). In the other hand they cleanup the
1” surface
SEY
and lower the SEY over time. Thence, surfaces in an accelerator
Raster - TiZrV/Al (#8) grooved sample, T=300K, Normal Incidence
experiment
environment might not
behave or condition
as in the lab
1
As received 1st set
As received 2nd set
Activated 210C-2h
Secondary Electron Yield
0.9
0.8
TiZrV/Al
0.7
0.6
0.5
0
250
500
750
1000
1250
Energy : eV
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1500
In-situ machine studies
Most of the major laboratory (ANL, BNL, CERN, KEK, LLNL, SLAC)
are measuring the EC and the distribution in energy of the electron –
usually use of a special RFA device (See ILCDR08 – Cornell University)
PEP-II LER straight
section
e+ 
Connecting Flange
e- detectors :
measure e- Hrztl
distribution & energy
Flat chamber
Groove chamber
Electron detectors
M. Pivi et al
Arc bend B1 magnet upstream
M. Pivi et al
Now samples and even full length UHV chamber are exposed to
accelerator operation. NEG coating was found to not only suppress
EC but SEY results were in agreement with what was measured in lab
(see A. Rossi ecloud’04)
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Accelerator SEY studies
2.5
2.5
2.5
Baked for 75 h at 430K
max=1.90
after e- Beam irradiation
Exposed to e Beam for 2 Weeks
+
(before Exposure to e Beam)
NEG
2
+
(5keV, 1x 1020e- cm-2)
Solenoid Field Off
Emax=250eV
2
2
Emax=250eV
1.5
TiN
max=1.76
Emax=375eV
1
LER
0.5
LER
After Exposure
1.5
TiN
1
max=1.08 Emax=300eV
NEG max=1.05
Emax=275eV
Cu
max=0.96
0.5
Before Exposure
TiN+Cu+NEG@D08BK_EpDep.plot
1000
2000
3000
Primary Energy [eV]
4000
After e- Irradiation
1.5
TiN
1
max =1.04 Emax=300eV
NEG max =0.98
Emax=375eV
Cu
max =0.98
0.5
Emax=275eV
SEY@Lab.
SEY@KEKB D08 Str. Sec.
Normal Incidence
0
Laboratory
Emax=300eV
SEY@KEKB D08 Str. Sec.
0
Secondary Electron Yield
Secondary Electron Yield
Secondary Electron Yield
Cu
max=1.80
5000
0
Normal Incidence
0
1000
2000
TiN+Cu+NEG@D082w_EpDep.plot
3000
Primary Energy [eV]
4000
5000
Normal Incidence
0
TiN+Cu+NEG@XHV2-EB_EpDep.plot
0
1000
2000
3000
4000
5000
Primary Energy [eV]
• After Exposure to beam: Drastic decease of max for all samples.
• Results are almost consistent with those results obtained at Lab.
S. Kato, KEK
Review 2007
Y. Suetsugu in
ILCDR08
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Summary
• ECE still is very stimulating theoretically and experimentally :
Number of workshop and special meetings: Ecloud’07, ECL2,
ILCDR08 .. .
• Material research mainly done in laboratory, in depth SEY
studies at CERN - KEK - SLAC (use of AES - XPS first with air
exposure yet without) especially on coatings.
• Test in accelerator confirms laboratory research - Grooves
and coatings are effective in mitigating EC. Beam-surface
interaction helps conditioning the UHV chambers, as expected.
• This workshop is a good bridge between Space and
Accelerator community
- common problems
- Accelerator physicist were/are unaware of your work
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Acknowledgement
Colleagues from the accelerator
community
B. Henrist, G. Rumolo, M. Furman, H. Hseuh,
F. Zimmermann …
F. Le Pimpec