status report of the infn surface science laboratory now ...critten/cesrta/ecloud/doc/... · 9-dec-...

9-Dec- 2002 R. Cimino LHC- Vac

Status report of the INFN SurfaceScience Laboratory now at CERN.

• The installed system and its characterization• What has been measured and its implication• What else (side activities)• Future work

R.Cimino and I.R. Collins


The installedsystem

• µ-metal chamber;• En. & angle res. analyser;• Low T manipulator;• LEED - Auger RFA;• Faraday cup.

• For details: see minutesof 18/2/2002 meeting

• or call in 30-1-044


• 3 slots: 1,2,3 mm wide• Gold plated• To measure actual Beam

current and size• Partly coated with

Phosphorous powder to “see”the beam

Thanks to E. Kos

Faraday cup


Characterization of the e- beam

• Find e- gun optimum lens voltages for:• Small spot: less than 0.5 mm in diameter• Position and intensity stability Vs Time, Energy and

Filament current.

• Stable between 50 and 400 eV.• Gun currents between few nAmps and µAmps

==> 20 µC/h/mm2 - 20 mC/h/mm2

• Intense spot with low background


Characterization of the RFA asan electron energy analyser

• On Au sample Vs Electronenergy and Sample Bias:

• Need of a Bias voltage tomeasure Secondaries fromsample and not a superpositionof them and those producedinternally by the grids of theanalyser.

0 10 20 30 40 50 60 70Energy (eV)

-20 Volt Bias

0 Volt Bias

Analyser secondaries

True secondaries


Characterization of the electronsource and sample Bias

• The actual Primary energyimpinging on the sample is:

Ep = Egun + E bias

• It is possible to vary Egunand E bias to measure atlow Ep keeping the Egun ina region where it is stable(>50 eV) and recording EDC.

0 20 40 60 80 100In

tens

ity (

a.u.

)Energy (eV)

- 20 Volt Bias

0 Volt Bias

Ep=80 eV

Ep=100 eV

Ep=50 eV

- 50 Volt Bias

Egun=100 eV


Characterization of EDC line-shapeas a function of analyser resolution• The actual EDC

line shape is animportant

parameter ofthe BIEM

simulation codes.• All measured data

have an An.Res.=1.2 eV

• The measured line-shape depends on the an. Res.

0 10 20 30 40 50 60 70

An. Res=1.2 eVAn. Res=3.8 eVAn. Res=8.0eV

Primary Energy (eV)

Ep=60.3 eV


10 18 26

D 2

As receivedafter WL exposureafter Ar+ sputtering

Kinetic

Inte

nsity

(a.u

.) hn =30 eV

34 42Energy (eV)

x 50

Characterization of EDC line-shapeas a function of analyser resolutionPhotoemission results

(R. Cimino, I.R. Collins, V.Baglin Phys. Rev. Acc. &Beam 2, 63201 99) doshow, on some as-received samples,

very sharpsecondaries.

• Data will be collected with better res. (<0.1 eV)


Measure of Secondary e- YIELD

• At each Primary energy wecan measure Igun (with theFaraday cup) and Isample.

Igun - Isample

Igund =

N.B. System not optimized to measure SEY

0.9

1.0

1.1

1.2

0 100 200 300 400

d

Primary energy (eV)

Au


• Energy Distribution Curves as function of Ep

0 80 160 240 320

Ep=312 eV

Kin. En. (eV)

Secondaries

Reflected electrons

What has been measured on f.s. Cu

0 80 160 240 320

Ep=212 eV

Kin. En. (eV)

Secondaries

Reflected electrons


• Energy Distribution Curves as function of Ep

0 40 80 120 160

Ep=162 eV

Kin. En. (eV)

Secondaries

Reflected electrons


0 60 120

Ep=41 eV

Kin. En. (eV)

Secondaries

Reflected electrons


What has been measured on f.s. Cu• Energy Distribution Curves as function of Ep

0 4 8 12 16Kin. En. (eV)

Ep=11 eVSecondaries

Reflected electrons

0 4 8 12 16

Ep=3.7eV

Kin. En. (eV)

Secondaries

Reflected electrons


What has been measured on f.s. Cu• Integrating the curves

gives the Percentageof Secondaries andReflected electrons

0 20 40 60 80 100 120Kinetic Energy (eV)

Ep=112 eV

Secondaries

Reflected electrons

• To separate “truesecondaries” from“rediffused electrons” isarbitrary and has notbeen considered in thisanalysis.


Secondariesand

ReflectedElectronVERSUS

PrimaryEnergy


0.0

20.0

40.0

60.0

80.0

100.0

0 50 100 150 200 250 300

Primary Energy (eV)

Secondaries

Reflected electrons

Perc

enta

ge



0.0

0.2

0.4

0.6

0.8

1.0

0 50 100 150 200 250 300 350

d

Primary Energy (eV)

d Total

Contribution of reflected electrons to d

Contribution of secondaries to d

• we can single outthe contribution tod of the secondariesand the reflectedelectrons versusprimary energy.

Igun - Isample

Igund =


Implication• Low energy electrons have a long survival time (in agreement

with observation on PSR at LANL…).• In FELs a low repetition rate is supposed to ensure no e- cloud

problems. BIEM has to be considered.• BIEM simulations need to be updated for the LHC.

• Reflected el. are NOT absorbed and do notdirectly contribute to heat load !!!

• However they will be accelerated by thefollowing bunches, gaining energy to bedeposited on the BS.


• There is an on-going collaboration between INFN and INFMto use XMOS, a bending magnet beamline at ELETTRA (TS)to measure the photon reflectivity as recommended by theLHC MAC (forward scattered and diffuse) from the LHCsawtooth structure as well as flat Cu samples to quantifysawtooth efficiency in reducing the presence ofasynchronous source of electrons in the LHC.

• (first beamtime allocated 6-12/Feb. 2003)

What else?


• e- induced EDC of Cu and study of the secondaryversus reflected component at very low primarye- energy (1-300 eV Ep) as a function of:

-Analyser resolution; (Possibly angle of inc. and em.)

- Space charge- Samples and sample spot (as received samples are NOT

identical nor homogeneous)

- At RT and below 20 K (in presence of adsorbed gas on thesurface).

- Possibly Vs Scrubbing (as a function of scrubbing e- energy)

Objectives for next 6 months…..


• Possibly correlate those results with surfacecomposition changes monitored by in situ Augerspectroscopy.


0 4 8 12Kinetic Energy (eV)

E.D.C. Ep=10 eV

Cu at 10 K

100 200 300 400 500

dN(E

)dE

Kinetic Energy (eV)

AUGER at 10 K taken "in situ" after EDCs.

CO

CuOx

Ep=3keV

Cl




•To Give a deeper understanding of the chemical processesoccurring at surfaces, for example: from LHC PR 472:“…Although the phenomenon of conditioning has beenobtained reproducibly on many samples, the exactmechanism leading to this effect is not properlyunderstood. This is of course not a comfortable situationas the LHC operation at nominal intensities relies on thiseffect…”



• Understand the implication of low energyelectrons to the LHC, by setting up anexperiment in order to measure the actual heatload per electron as a function of its energy(10-200 eV).

• Measure the photon reflectivity


status report of the infn surface science laboratory now ...critten/cesrta/ecloud/doc/... · 9-dec-...

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