LHC Machine Advisory Committee Meeting no. 23 , June 13 th 2008

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LHC Machine Advisory Committee Meeting no. 23, June 13th 2008

Surface treatments and coatings

for the mitigation of electron clouds in the SPS

Sergio Calatroni, Paolo Chiggiato, Pedro Costa Pinto, Mauro Taborelli

Coatings, Chemistry and SurfacesTS-MME

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Electron Clouds in the SPS

Effects of electron clouds (EC):

Pressure rises generated by electron induced desorption. Increased heat load. Emittance growth. Beam instability: head-tail instabilities and bunch-to-bunch coupling. Interference on the electrodes of beam pick-up monitors

EC instabilities are a limiting factor for the SPS as LHC injector. To avoid the built-up of EC, simulations indicate that the maximum secondary electron yield of the beam pipe walls shall be less than 1.3.

In 1999, for the first time, EC were identified in the SPS with an LHC-beam configuration.

G. Arduini et al.LHC Project Report 423

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Mandate of the SPS-U Working Group

This Study Team should: identify limitations in the existing SPS and propose and study possible solutions …

The ultimate goal is to reliably provide the LHC with the beam required for

reaching ten times the nominal luminosity.

A Design Report shall be published in 2010 describing the proposed actions and their estimated cost and planning.

Our main role: to find out a thin film (SPS compatible) with max < 1.3.

Subsequently, to prepare a viable strategy for the coating of about 1000 SPS stainless steel vacuum chambers during the shut-down periods.

AB dept: G. Arduini , F. Caspers, K. Cornelis, E. Metral, G. Rumolo; E. Shaposhnikova; F. ZimmermannAT dept: E. Mahner; B. Henrist.TS dept: S. Calatroni; P. Chiggiato; M. Taborelli; Ch. Vallgren

Elena Shaposhnikova, http://paf-spsu.web.cern.ch/paf-spsu/

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Ti-Zr-V Coatings: the Case of the LHC’s LSS

Most of the Long Straight Sections of the LHC are coated with Ti-Zr-V :

more than 1200 vacuum chambers were produced;

about 15 Kg of Ti-V-Zr is spread over 6 Km of LHC beam pipe;

vacuum commissioning almost concluded.

Low SEY are obtained for Ti-Zr-V coatings after heating in vacuum.

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Ti-Zr-V Coatings: the Case of the LHC’s LSS

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Heating Temperature Limitations

Ti-Zr-V activation must be carried out at temperatures higher than 180°C; the nominal heating temperature for the LHC’s LSS is 230°C.

These temperatures are too high for some future CERN projects:

SPS-Upgrade: Embedded in the magnets, the vacuum chambers cannot be heated.

CLIC positron damping ring: heating temperatures shall be presumably limited to 150°C because of

the complex installed devices, mainly SC wigglers and beam monitors.

PS-2: Maximum heating temperature to be defined, but most likely as low as

possible.

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Thin Films and Treatments: Possible Candidates - 1

Possible Solutions

To abandon the seek of low SEY surfaces and opt forclearing electrodes installed along the vacuum chambers.

Presented at the last MAC by T. Kroyer

To find out thin films with an intrinsically low SEY.

To render the surface rough enough to block secondary electrons.

… or both combined

Lower activation temperature NEG

No need of heating once in vacuum

By machining By chemical or electrochemical methodsBy coating

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SEY high for insulators (for example MgO, CsI, Al2O3 and diamond)

SEY low for light metals (Ti) and graphite

Strongly dependent on surface cleanliness, oxidation and roughness

Strongly dependent on impinging electron dose.

Many clean elements and their compounds fulfill the max<1.3 imposed by the SPS.

But when exposed to the air their SEY increases steeply, resulting in max higher than about 1.5:

oxides formation and water adsorption.

An additional and progressive rise is recorded during months of stay in the air, eventually leading to max higher than 2: airborne hydrocarbon

adsorption. N. Hilleret, EPAC 2000 proceedings

Either in situ heating or particle bombardment (conditioning) is necessary to recuperate lower SEY values: surface degassing + possibly formation of a graphite layer

Ding, Tang, and Shimizu, J. Appl. Phys. 89 (2001) 718

9N. Rey Whetten, J. Appl. Phy. 34(1963)771

The ideal film material :

has intrinsically low SEY; is not prone to adsorb water vapor, oxygen and hydrocarbons; can be easily deposited on stainless steel beam pipes; is compact, smooth and not inclined to produce dust; is UHV compatible; has possibly low resistivity.

Graphite could be a good compromise.


SEY of graphite

!

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Production and Characterization of a-C Thin Films - 1

Goal: to produce low SEY carbon films, ideally graphite thin films.

However, carbon films are neither pure graphite nor pure diamond. In general they are amorphous (lack of long-range order). Locally, the carbon orbital hybridization can be diamond-like (sp3) or graphite-like (sp2).

Our aim consists in producing amorphous carbon (a-C) films with the highest fraction of sp2 hybridization

+without heating the vacuum chamber during coating (SPS constraint)

Source: Wikipedia ‘hybridization’

sp3 sp2

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Production and Characterization of a-C Coatings - 2

Typically graphite amorphous carbon films have: [S.R.P. Silva, EMIS data review]

a very low density (1.2 – 1.5 g cm-3); very low bandgap (0 – 0.6 eV); very low sp3 content (less than 30%); they are compact, soft and blackish.

Magnetron sputtering is an effective coating technique for the production of graphite amorphous carbon films.

beam pipewall

graphite rod

+

+

-U

Despite that, the reflected neutralized plasma ions can hit the growing film thereby influencing the sp3 C content.

Sputtered C atoms have an energy of few eV.

Magnetron sputtering does not provide enough energy to the deposited C atoms to produce a highly packed diamond-like film:the displacement energy of C atoms in graphite is about 20 eV.

dischargegas sputtered

C atoms

B

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Production and Characterization of a-C Coatings - 3

electron gun

Ip = Is + Ic

= Ic / (Is + Ic)

Sample

Ip

Ic

AIs

A

Ic

XPSSEY

measurement

Additional characterizations:- Morphology by SEM;- Thermal and electron stimulated degassing;- FTIR and Raman spectroscopy;- RBS and ERDA.- Resistivity

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SEY of a-C - 1

0.0

0.5

1.0

1.5

2.0

2.5

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Primary Electron Energy [eV]

SE

Y

a-C (discharge gas Ne)

St. Steel

max=1.34

max=2.13

a-C film produced by magnetron sputtering on a 50 cm long, 10 cm diameterst. steel vacuum chamber (2 h exposure in the air)

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SEY of a-C - 2

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

0 100 200 300 400 500 600 700 800 900

Primary Energy (eV)

SE

Y

Measured at ICMM-CSIC in Madrid by I. Montero Herrero

Kr discharge gas

Ne discharge gas

Influence of the discharge gas: a-C deposited on Cu coupon samples

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SEY of a-C - 3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 100 200 300 400 500 600 700 800 900


SE

Y

C-Kr/Cu (April 2008)

C-Kr/Cu (June 2008)

Influence of a long exposure in the air:Kr as discharge gas

by I. Montero Herrero

3 days

2 months

max=1.97

max=1.42

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SEY

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 100 200 300 400 500 600 700 800 900


SE

Y

C-Ne /Cu (April 2008)C-Ne/Cu (June 2008)

SEY of a-C - 4

Influence of a long exposure in the air:Ne as discharge gas

by I. Montero Herrero

3 days

2 monthsmax=1.47

max=1.14

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SEY of a-C - 5

Influence of the discharge gas

Kr is more implanted in the graphite cathode than Ne.

Implanted Kr is sputtered by Kr ions and implanted in the carbon film (sometimes it can be detected by XPS in the C film: detection limit about 0.5%).

C has a higher sputtering yield when bombarded with Ne than Kr; this result in a higher deposition rate.

Additional investigations are necessary to clarify the role of the discharge gas. Tests with Ar in progress…

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Morphology of a-C

Very smooth surface Substrate defects smoothedby the a-C film

100 nm

Electron Microscopy Analysis

a-C coating are used in the industry to produce very smooth surfaces

Typical thickness ≈100 nm

SEM images by TS-MME-MM section

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Dust Production of the Deposited a-C Film

0,0E+00

2,0E+04

4,0E+04

6,0E+04

8,0E+04

1,0E+05

1,2E+05

1,4E+05

1 3 5 7 9 11 13 15 17 19

0.3um before coating

0.3um after coating

Particle diameter in the range 0.3m and 0.5m Particle diameter larger than 0.5m

0.0E+00

1.0E+03

2.0E+03

3.0E+03

4.0E+03

5.0E+03

6.0E+03

1 3 5 7 9 11 13 15 17 19

0.5um before coating

0.5um after coating

After gentle hammering no further particles produced.

In progress: coated vacuum chambers are stored under vacuum to check particle production after several weeks.

run# (min) run# (min)

Fine dust particles are always produced during sputtering. As a matter of fact, about 10 times more particles are detected in a 2-m long vacuum chamber after coating.

The particles are removed by pumping.

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a-C Carbon: Ongoing Activities

The optimization of the sputtering variables for the production of low SEY a-C coatings is in progress.

It requires the variation of the following parameter:

sputtering current and voltage ;

deposition rate and film thickness;

discharge gas pressure;

substrate treatments and temperature during the coating.

inclusion of other elements, namely N and B.

The resulting SEY variations will be measured.

Ripalda et al.J. Appl. Phys., Vol. 92, No. 1, 1 July 2002

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Rough Morphologies -1

Very rough Ti, Zr, Al, Inconel ,Cu … surfaces can be obtained by chemical or electrochemical attack.

Rough surface

Smooth surface

A rough surface can reducesecondary electron emission.

However, discouraging results have been obtained for stainless steel.

(Electro)chemical attacks are hardly applicable to the existing SPS vacuum chambers.

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Rough Morphologies - 2

0

0.5

1

1.5

2

2.5

3

Zr smooth Zr rough

SE

Y(A

ES

)

500eVThe relative improvement is some 15%.....which means that a layer having max=1.35 would result in a max of 1.15.

Once coated by a thin layer of a-C, this structure could provide SEY values well below the imposed threshold.

Primary electron energy = 500eV

(Zr Fast Sputtering Deposition)

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0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

0 200 400 600 800 1000 1200 1400 1600 1800 2000


SEY

a-C

a-C/ rough Zr

a-C onto a rough Zr coating

(aC/Zr Fast Deposition)

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Evaporation of metals in a relatively high pressure of a rare gas is known to produce very rough and porous films. Already mentioned in the literature, gold black has been produced and characterized.

0

0.5

1

1.5

2

2.5

smooth Au black Au

SE

Y(A

ES

)

500eVPrimary electron energy = 500eV

Gold black deposited by evaporation in Kr (0.5 mbar)

(Gold Black)

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Rough Morphologies - 5 (Gold Black)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Primary Electron Energy[eV]

SE

Y

black-Au run 1

black-Au run 2

black-Au run 1 after6 days in air

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a-C on gold black

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0 200 400 600 800 1000 1200 1400 1600 1800 2000


SE

Y

No dependence on the time of exposure to the air

(a-C / Gold Black)

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Rough Morphologies - 6 (high pressure sputtered Cu)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Energy [eV]

SE

Y

Measured after 2 h exposure to the air. Longer exposure ongoing…

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UHV Compatibility

10-9

10-8

10-7

10-6

0 100 200 300 400 500

P

[T

orr

N2 e

q.]

Heating Temperature [°C] (2 h)

24 h heating

sputtered a-C

Stainless Steel (vacuum fired)

Electron Stimulated Desorption of a-C filmse- current: 1 mA

e- energy: 500 eV

Water vapor outgassing:

a-C are very smooth, therefore water desorption should not be a problem.

For rough surfaces, water desorption could be a major hindrance.

Outgassing measurements are planned.

a-C coating

Schematic view of the ESD system

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Grooves

Grooves, calculation of L.Wang at SLAC on the geometryobtained on copper at the CERN workshop

100 200 300 400 500 600 7000.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Energy (eV)

SE

Y

0=1.50,Height=1.0mm, =150, B=0

Courtesy of S.Atieh and J.M.Geisser

0 100 200 300 400 500 600 7000.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Energy (eV)

SE

Y

0=1.50,Height=1.0mm, =15o

B=0.2TeslaB=2 Tesla

B=0

Assuming max=1.5

1mm

15ºReduction also for B=0 so that we can measure SEY in the lab

B=0.2T

B=2T

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NEG Films of Lower Activation Temperature

Some studies have already been pursued in this direction: the Ti-Zr-Nb, Zr-Fe and the Ti-Zr-Cr systems have been partially analyzed.

Interesting results have been highlighted for some Zr-Cr alloys produced by sputtering;

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100

Ratio of the O1s peak area after and before heating at 250°C (1h)

O1s

sig

nal

dep

leti

on

Cr concentration [at %]

Magnetron sputtered Cr-Zr

However, at the best, they reproduce the typical features of TiZrV.

Other alloys are expected for the next months.

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SPS E-cloud Experimental Tests

Ecloud experiments are in progress in the SPS point 5 ECX5.

Stainless steel, a-C film (Kr discharge gas) and activated Ti-Zr-V will be tested. The ecloud signal will be detected by strip detectors [G. Arduini et al., Proceeding EPAC 2002].

We do not expect remarkable results for this a-C film: its SEY should be about 1.5

Better coatings have been produced since the installation in the SPS.

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Implementation in the SPS - 1

Coating pace: about 1000 chambers in 3 years during shutdowns, namely 2 arc sectors per year.

About 90 chambers per month : 4 to 5 per day.

On the coating bench, a batch of 5 chambers remains 2 days:

• first day in the afternoon for installation and pumping overnight; • second day for coating, • third day in the morning for disassembling.

Two coating benches are necessary!

ECX5 cavern in the SPS underground is available.The cavern is a 20-meter diameter cylinder. Room for two coating benches and a storage area have to be fitted in it.

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We can profit of the experience acquired during the refurbishment of the cooling circuits of 255 dipole magnets, running over three years :

• Repairing pace: 4 to 5 magnets per day• Buffer of 10 magnets in ECX5.• Handling.• Synchronized logistic.

Courtesy of S. Sgobba TS-MME-MM

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LHC cold bore cleaning machine: it should be available for the cleaning of the SPS beam pipes.

If the cleanliness of the vacuum chamber walls is unacceptable for the film adherence, cleaning have to be considered.

Courtesy of L. Ferreira TS-MME-CCS

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Magnets configuration in the coating bench.

The alternatives are:

Magnets laid in horizontal position:

simpler handling; the graphite cathodes have to be supported in the vacuum chambers; only one bench can be installed in ECX5.

Magnets placed vertically (experience acquired with the LSS beam pipes):

special frame to hold the magnets (≈15 tons each); two benches can be installed in ECX5; easier insertion and holding of the graphite cathodes.

Preliminary design in progress…

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Conclusions

Bare and a-C coated rough surfaces have being considered (black coating and fast sputtering). Their compatibility with the unbaked vacuum system of the SPS has to be assessed.

Since the beginning of the year, in the frame of the SPS-U working group, we have been seeking thin film coatings and surface treatments to mitigate electron clouds in the SPS. We intend to profit from the experience acquired during the coating of the ≈ 1200 LHC-LSS vacuum chambers.

Sputter coated a-C thin films are good candidates. Their properties depend strongly on the sputtering parameters. At the present, an optimization is in progress to reduce their max .

A preliminary design for the coating bench for the about 1000 vacuum chambers of the SPS arcs is on the starting blocks…

Documents

LHC Machine Advisory Committee Meeting no. 23 , June 13 th 2008