Conditioning:

The carbon storyDifferences for low T vs RT?

M.Taborelli, CERN, 22/11/2013

1.6E-6 1.6E-5 1.6E-4 1.6E-3 1.6E-2 1.6E-1 dose [C/mm2]

Auge

r pea

k (a

rb.u

.) C

O

CuCarbon first decreases (ESD) and then increases (?)

Irradiation time (min)

C-KL

L pe

ak a

rea

Pbase =10E-9 mbar10E-7 mbar CO10E-7 mbar CO2

The increase is NOT influenced by the pressure of light species (CO or CO2 or 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

Copper in the lab: effect of electron irradiation , RT

Scheuerlein ,Taborelli 20022.5KeV

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

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

M.Taborelli, CERN

1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+001

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

518eV baked

unbaked 1

unbaked 2

unbacked 'quickly vented'

Approx. Dose [C/mm2]

max

. Sec

onda

ry e

miss

ion

yiel

d

unbaked vac. system(no C increase)

unbaked vac. system,e-gun taken apart and cleaned(C increase)

unbaked vac. system, (C increase)

StSt316LN

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

Carbon growth is necessary to decrease the SEY

StSt316LN

Another argument : the δmax of clean copper is 1.3 (same for clean copper oxide), but conditioned copper reaches 1.1

Energy dependence of conditioning, RT

Necessary dose depends on impinging electrons energy

«Final» SEY depends on energy

The carbon is less graphitic for irradiating at low energy

R.Cimino PRL 2012

Energy dependence of ESD (RT)

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!

ESD energy dependence(Hilleret, Vorlaufer)

Sample in line of sight of the LHC type proton beam in SPS,SEY measured in situ without venting

Conditioning with photoelectrons (sample +100V)

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

(V.Baglin et al 2000)

V.Baglin

δmax

10-210-6 10-4

Dose Clb/mm2

2

10-81

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

N.HilleretThe effect is quite similar, but the Emax has a different behaviour!

LHC Proj. rept. 472V.Baglin et al.

M.Taborelli, CERNIt goes lower than the SEY of bare copper (1.3), down to 1.1

W wireRF powered

multipacting current48 strips spaced 1.25 mm

vacuum chamber B-field 1.2KGauss

80 cm length

liner

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

Simultaneous measurement of electron-current on the collector, reflected RF power and pressure rise during an RF power-ramp

Conditioning on multipacting system (RT) :Wire stretched in an e-cloud monitor, powered with RFMeasurement of multipacting current in the monitor

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

conditioning Decrease of the dose per cycle

Nr of cycles

conditioning

Curr

ent [

mA]

Next question: measurement of the kinetic energy of impinging electrons

Copper in the lab: Conditioning at low T

25ML H2O25ML CO

4.5K4.5K

Preliminary, only one run….. to be confirmed

Scrubbing 300eV Scrubbing 300eV

10K

400eV, 10-2 C/mm2

R.Cimino

R.Cimino

A.KuzucanA.Kuzucan

Dose [C/mm2]

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 ionsEPA: attracted photoelectronsCESR 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

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)

Conditioning of copper in EPA: SEY in situ II

attracted photoelectrons, not e-cloud electrons

Hilleret, Baglin, Henrist

Synchrotron radiation normalSynchrotron radiation 45° to normal (no direct photons)

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

Conditioning in CESRTA: SEY in situ III

TiN TiN

Conditioning in machines, CESRTA: SEY in situ III

Conditioning by photons from the circulating beam of electrons

Conditioning in PEPII: SEY in situ IV

PEPIIM.Pivi

TiN40mC/mm2

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 !

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

Conditioning (scrubbing) of StSt in SPS: SEY after HV transfer

Transversal position in the beampipe [mm]

SEYm

ax

sample

beam

RF shield extracted from SPS

M.Taborelli, CERN

SPS-quad RF shield

SPS quadrupole RF shield: SEY after transfer in air

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

Thickness of the black layer: sputtering profile in XPS

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

ESD coefficient and SEY decrease: cleaningESD dominated by par H2 and CO: hydrocarbons are cracked (graphitization)

(V.Baglin, N.Hilleret, G.Vorlaufer 2002)

Copper (cleaned for UHV and air exposed)

Norm

alise

d de

sorp

tion

yiel

d

Norm

alise

d SE

Y __

_, __

_Re

lativ

e SE

Y

Rela

tive

deso

rptio

n co

efficie

nt1.6E-9 1.6E-7 1.6E-5 1.6E-3 1.6E-1 [C/mm2]

M.Taborelli, CERN

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

Electron induced bulk diffusion of C?

Good and bad carbon: can we distinguish them?

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

irradiated

air exp.

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

M.Taborelli, CERN

Scheuerlein ,Taborelli 2002

Molecules adsorbed reversibly :

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

M.Taborelli, CERN

dt τ

sojourn time τcurrent density per area jcross-section for cracking σ molecules impinging per unit area and time Zsticking coefficient s

- Physisorption, light molecule : CO, 1e-7 mbar, sojourn time 1e-11 s, cross section 1E-16 cm2 gives 1E-9 ML/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)

Model based on adsorption and cracking by the beam:

dNcracked = Nrev x j x σ dt

Growth rate:

For 1mA/cm2

Cracking of the residual gas by e-beam

Density of molecuels