Quick Review of Secondary Quick Review of Secondary Electron Yield Studies to Electron Yield Studies to Mitigate Electron Cloud Mitigate Electron Cloud

Effect in AcceleratorEffect in Accelerator

Quick Review of Secondary Quick Review of Secondary Electron Yield Studies to Electron Yield Studies to Mitigate Electron Cloud Mitigate Electron Cloud

Effect in AcceleratorEffect in Accelerator

F. Le Pimpec (PSI)F. Le Pimpec (PSI) R. Kirby, F. King, M. Pivi (SLAC) R. Kirby, F. King, M. Pivi (SLAC)

MULCOPIM 08MULCOPIM 08 Valencia, SpainValencia, Spain 20082008

25.09.2008 F. Le Pimpec 2

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

25.09.2008 F. Le Pimpec 3

U. Irizo Ariz

RHIC (up) & LHC (down) multipacting schema

F. Ruggiero

Electron cloud mechanism

Beam Damage

And it could be even worse…

G. Rumolo

e-cloud can also arise without multipacting : Primary e- produced by Ions and Photons

25.09.2008 F. Le Pimpec 4

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

25.09.2008 F. Le Pimpec 5

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. ”

25.09.2008 F. Le Pimpec 6

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)

Can Low-Energy Electrons Affect High-Energy Physics Accelerators? R. Cimino et al Phys. Rev. Lett. 93, 014801 (2004)

R. Cimino et al, See Ecloud’04 proceedings

25.09.2008 F. Le Pimpec 7

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

PEP 2

25.09.2008 F. Le Pimpec 8

Technical Solutions• Photon Killer :→ 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

Pumping with NEG or with distributed TiSp pumpsY. Tanimoto, KEK EPAC08

CERN

Be window

CERN – LHC

25.09.2008 F. Le Pimpec 9

Surface studies

TiZrV/Al

Nb

NH et al Appl. Phys. A 76, (2003)

As Received = air exposed

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.

So far “we” haven’t found a material with an as received max below 1.5

We must play tricks : heating, particles bombardment, roughness change …

NIM A (551),187, 2005

25.09.2008 F. Le Pimpec 10

Electron conditioning

CERN (NH) - 2000

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

Electrons from the e-cloud can condition surfaces.Hopefully in a reasonable time scale !

e-

NIM A (551),187, 2005

25.09.2008 F. Le Pimpec 11

NIM A (564),44-50, 2006

Ion conditioning

What about CO ion exposure ? (chemistry)

250 eV ion exposure (P~10-8 Torr)

CERN (NH) - 2000

e-

SEY after PGD + Air exposure

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

Using the beam itself to condition the wall is of course an option

25.09.2008 F. Le Pimpec 12

Artificial roughness

SimulationSimulation

CuCu

M. Pivi et al to be published in JAP

= 60°

25.09.2008 F. Le Pimpec 13

Artificial roughness & coatings

1”

h x w x = 5 x 1 x 1 [mm]TiZrV/Al

0 250 500 750 1000 1250 15000.5

0.6

0.7

0.8

0.9

1

Energy : eV

Se

co

nd

ary

Ele

ctr

on

Yie

ld

Raster - TiZrV/Al (#8) grooved sample, T=300K, Normal Incidence

As received 1st set

As received 2nd setActivated 210C-2h

TiZrV/Al

SEY SEY experimentexperiment

SR, ions and electrons do produce electrons & increase the pressure (PSD, ESD, ISD). In the other hand they cleanup the surface

and lower the SEY over time. Thence, surfaces in an accelerator environment might not behave or condition as in the lab

25.09.2008 F. Le Pimpec 14

In-situ machine studiesMost 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)

e- detectors : measure e- Hrztl distribution & energy

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)

Groove chamber

Flat chamber

Electron detectors

Connecting Flange

PEP-II LER straight

section e+

Arc bend B1 magnet upstream M. Pivi et al

25.09.2008 F. Le Pimpec 15

0

0.5

1

1.5

2

2.5

0 1000 2000 3000 4000 5000

Seco

ndar

y E

lect

ron

Yie

ld

Primary Energy [eV]

Normal Incidence

Exposed to e+ Beam for 2 Weeks

max=0.96

Emax=300eV

NEG

TiN+Cu+NEG@D082w_EpDep.plot

max=1.05Emax=275eV

max=1.08

Cu

TiN

Emax=300eV

SEY@KEKB D08 Str. Sec.

Solenoid Field Off

0

0.5

1

1.5

2

2.5

0 1000 2000 3000 4000 5000

Seco

ndar

y E

lect

ron

Yie

ld

Primary Energy [eV]

Normal Incidence

Baked for 75 h at 430K

max=1.80

Emax=375eV

NEG

TiN+Cu+NEG@D08BK_EpDep.plot

max=1.90 Emax=250eV

max=1.76

Cu

TiNEmax=250eV

SEY@KEKB D08 Str. Sec.

(before Exposure to e+ Beam)

0

0.5

1

1.5

2

2.5

0 1000 2000 3000 4000 5000

Seco

ndar

y E

lect

ron

Yie

ld

Primary Energy [eV]

max=0.98

Emax=300eV

NEG

TiN+Cu+NEG@XHV2-EB_EpDep.plot

max=0.98Emax=375eV

max=1.04

Cu

TiN

Emax=275eV

after e- Beam irradiation

Normal IncidenceSEY@Lab.

(5keV, 1x1020e- cm-2)

LaboratoryAfter e- Irradiation

LERAfter Exposure

LERBefore Exposure

S. Kato, KEKReview 2007

• After Exposure to beam: Drastic decease of max for all samples.• Results are almost consistent with those results obtained at Lab.

Accelerator SEY studies

Y. Suetsugu in ILCDR08

25.09.2008 F. Le Pimpec 16

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

F. Le Pimpec

Acknowledgement

Colleagues from the accelerator community

B. Henrist, G. Rumolo, M. Furman, H. Hseuh, F. Zimmermann …