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Conditioning:. The carbon story Differences for low T vs RT?. M.Taborelli , CERN, 22/11/2013. Copper in the lab: effect of electron irradiation , RT. C. Carbon first decreases (ESD) and then increases (?). Cu. Auger peak (arb.u.). O. Scheuerlein ,Taborelli 2002. 2.5KeV.

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slide1

Conditioning:

The carbon story

Differences for low T vs RT?

M.Taborelli, CERN, 22/11/2013

slide2

Copper in the lab: effect of electron irradiation , RT

C

Carbon first decreases (ESD)

and then increases (?)

Cu

Auger peak (arb.u.)

O

Scheuerlein ,Taborelli 2002

2.5KeV

1.6E-6 1.6E-5 1.6E-4 1.6E-3 1.6E-2 1.6E-1

dose [C/mm2]

The increase is NOT influenced by the pressure of light species (CO or CO2or CH4) in the residual gas

(consistent with their short sojourn time) at least up to 1e-7 mbar range.

Confirmed also by conditioning in presence of CH4

At low T this could be different: the sojourn time is larger, so the depostion from the residual gas might be an issue….if there is residual gas

Pbase =10E-9 mbar

10E-7 mbar CO

10E-7 mbar CO2

C-KLL peak area

Irradiation time (min)

slide3

StSt in the lab, RT: conditioning, not reproducible….

StSt316LN

unbaked

vac. system

(no C increase)

unbaked vac. system,

(C increase)

unbaked vac. system,

e-gun taken apart and cleaned

(C increase)

Is this non-reproducibility typical of StSt? It put at least some questions on the origin of the carbon!

M.Taborelli, CERN

slide4

Carbongrowthisnecessary to decrease the SEY

StSt316LN

Another argument : the δmax of clean copperis 1.3 (same for clean copperoxide), but conditionedcopperreaches 1.1

slide5

Energy dependence of conditioning, RT

Necessary dose depends on impingingelectronsenergy

«Final» SEY depends on energy

The carbonislessgraphitic for irradiatingatlowenergy

R.Cimino PRL 2012

slide6

Energy dependence of ESD (RT)

ESD energy dependence

(Hilleret, Vorlaufer)

ESD energy dependence

(Redhead, Hobson)

May be this explains part of the energy dependence of conditioning: low energy does not “clean” the surface.

Or the energy dependence is just similar ; therefore the conditioning process involves excitations in the eV range, so no marked temperature dependence is expected….except for the influence of re-adsorption!

slide7

Conditioning of copper in SPS and EPA (RT): SEY in situ I

Sample in line of sight of the LHC type proton beam in SPS,

SEY measured in situ without venting

(V.Baglin et al 2000)

Conditioning with photoelectrons (sample +100V)

V.Baglin

slide8

2

δmax

1

10-2

10-6

10-4

10-8

Dose Clb/mm2

Conditioning of copper in the laboratory (500eV electrons from an electron gun) (RT)

LHC Proj. rept. 472

V.Baglin et al.

N.Hilleret

The effect is quite similar, but the Emax has a different behaviour!

It goes lower than the SEY of bare copper (1.3), down to 1.1

M.Taborelli, CERN

slide9

Multipacting in e-cloud monitor : no e-gun!

B-field 1.2KGauss

vacuum chamber

liner

W wire

RF powered

multipactingcurrent

48 stripsspaced 1.25 mm

80 cm length

Simultaneousmeasurement of electron-current on the collector, reflected RF power and pressure riseduring an RF power-ramp

slide10

Conditioning on multipactingsystem (RT) :

Wire stretched in an e-cloud monitor, powered with RF

Measurement of multipacting current in the monitor

Conditioning: current as a function of produced dose on StSt liner

Decrease of the dose per cycle

conditioning

Current [mA]

conditioning

Nr of cycles

Next question: measurement of the kinetic energy of impinging electrons

slide11

Copper in the lab: Conditioning at low T

10K

400eV,

10-2 C/mm2

R.Cimino

R.Cimino

Scrubbing 300eV

Scrubbing 300eV

25ML H2O

4.5K

25ML CO

4.5K

Preliminary, only one run….. to be confirmed

A.Kuzucan

A.Kuzucan

Dose [C/mm2]

slide12

Facts and questions (limited by my knowledge!):

  • graphitic carbon on the surface is necessary to have sufficiently low SEY
  • (we know that this is material dependent and electron energy dependent)
  • Difference in lab conditioning and machine conditioning for Emax evolution: is it significant?
  • No direct measurements showing that we have carbon by electron irradiation at low T, but it is likely from SEY of cold scrubbing data and from the energy dependence of the process….
  • Conditioning with SEY below 1.3 in machines (measured in situ):
  • PEPII (TiN, NEG), presence of ions
  • EPA: attracted photoelectrons
  • CESR TA: photons
  • Obvious: in an accelerator the beam does not allow to cumulate the dose linearly as a function of beam-time, since the SEY decrease induces a current decrease: with the e-gun in the lab the situation is not the same
slide13

Proposed experimental works

  • Cold SEY:
  • Do we need more data on conditioning of cold samples by irradiation in presence of residual gases?
  • restart the cold SEY system (in 1-2 months)
  • couple a cold SEY system with the new XPS to enable for in situ surface analysis (implies additional financial and possibly manpower support, at least 1 year)
  • Low Ep data:
  • Measuring with higher bias on the present SEY system enables to go down to about 15eV (tested on carbon, comparison with data from M.Belhadj)
  • Setting up an SEY system working in sample/Faraday cup mode and shorter distance gun/sample might allow to go to lower Ep (6 months)
  • Conditioning with RF:
  • It is a situation more close to the beam case than an electron gun, in terms of “dose to time” relation: should we build a low T setup? (cold magnet!)
  • Insert a copper liner in the RF multipacting system (3-6 months)
slide15

Conditioning of copper in EPA: SEY in situ II

Hilleret, Baglin, Henrist

attracted photoelectrons, not e-cloud electrons

slide16

Conditioning in CESRTA: SEY in situ III

Synchrotron radiation 45° to normal (no direct photons)

Synchrotron radiation normal

TiN

TiN

Conditioning with the synchrotron radiation of a circulating electron beam (no e-cloud)

Emax increases

slide17

Conditioning in machines, CESRTA: SEY in situ III

Conditioning by photons from the circulating beam of electrons

slide18

Conditioning in PEPII: SEY in situ IV

PEPII

M.Pivi

Same for normal impinging synchrotron radiation and 45°off

NO carbon after conditioning: scrubbing due to ions? Just concluded from the fact that there is no carbon?

(S.Kato: TiN with SEYmax=1 and 44% C, for Cu SEYmax=1 with 91% C)

Emax increases !

TiN

40mC/mm2

slide19

RF shield extracted from SPS

Transversal position in the beampipe [mm]

Conditioning (scrubbing) of StStin SPS: SEY after HV transfer

A sample left 6 months in the SPS and measured in the lab shows the trace of the conditioned central region (e-cloud region)

beam

SEYmax

sample

M.Taborelli, CERN

slide21

Thickness of the black layer: sputtering profile in XPS

  • Estimated thickness: 60nm (from calibration on Ta2O5) or 50 times more than airborne carbon contamination (from comparison of sputtering time)
slide22

Correlation with SEY on an received copper surface exposed to an e-beam :

Copper (cleaned for UHV and air exposed)

1.6E-9 1.6E-7 1.6E-5 1.6E-3 1.6E-1 [C/mm2]

(V.Baglin, N.Hilleret, G.Vorlaufer

2002)

Relative SEY

Normalised SEY ___, ___

Normalised desorption yield

Relative desorption coefficient

ESD coefficient and SEY decrease: cleaning

ESD dominated by par H2 and CO: hydrocarbons are cracked (graphitization)

M.Taborelli, CERN

slide23

Electron induced bulk diffusion of C?

Some experiments (S.Kato, M.Nishiwaki, KEK) indicate that this is possible.

There are some 10 ppm of C in the bulk

ESD on 13C implanted copper shows that C migrates to the surface upon electron irradiation

M.Nishiwaki, S.Kato,AVS2002

slide24

Good and bad carbon: can we distinguish them?

Auger C-KLL

Airborne hydrocarbon contamination increases the SEY; it is modified by the e-beam to graphitic carbon

irradiated

air exp.

Scheuerlein ,Taborelli 2002

”graphitization (S.Kato)”: hydrocarbons are changed in graphitic carbon which has a low SEY

M.Taborelli, CERN

slide25

Model based on adsorption and cracking by the beam:

Molecules adsorbed reversibly :

dNrev = Z x s x (1-Θ) - Nrev - Nrevx j x σ

dtτ

sojourn time τ

current density per area j

cross-section for cracking σ

molecules impinging per unit area and time Z

sticking coefficient s

dNcracked = Nrev x j x σ

Growth rate:

dt

For 1mA/cm2

  • Physisorption, light molecule : CO, 1e-7 mbar, sojourn time 1e-11 s, cross section 1E-16 cm2 gives 1E-9ML/h
  • Large molecule as tiny trace: Molecule of M=100 at 1e-10 mbar, huge sojourn time gives a growth of 0.01 ML/h
  • Physisorption, large molecule : Hydrocarbon, 1e-7 mbar, with M=100 and sojourn time = 0.1 s, gives 10ML/h (this is what happens often in SEM)

M.Taborelli, CERN