CLIC Vacuum Issues (2) Paolo Chiggiato Vacuum, Surfaces and Coatings TE department

Paolo Chiggiato,TE-VSC 1

CLIC Vacuum Issues (2)

Paolo ChiggiatoVacuum, Surfaces and Coatings

TE department

Content:

• Vacuum problems in CLIC.

• Known solutions:• Bakeout.• Lumped pumping.• Linear pumping.• Distributed pumping: NEG coatings.• Amorphous carbon coatings.• Scrubbing (conditioning).

• Possible developments.

CLIC seminars


- Synchrotron radiation induced desorption (LEP).

- Pressure instabilities (ISR).

- Electron clouds and electron induced desorption (SPS).

- Ion trapping (e- synchrotron light sources).

- Limited vacuum conductance (undulator vacuum chambers, ESRF).

Each of these obstacles has been already surmounted in the past. The challenge for the CLIC vacuum is the simultaneous combination of some of these problems in the same beam pipe. DR vacuum requirements are the most demanding.

In this respect, the CLIC vacuum requirement is far to be conventional. As a consequence, non-conventional solutions have to be proposed. Some of them demand a vigorous development in the next years.

Vacuum problems in CLIC


P ≤ 10-9 Torr

P=10-8 Torr P=10-8 Torr

P=10-9 Torr P=10-9 Torr


P ≤ 10-9 Torr

P ≤ 10-9 Torr

Pressure requirements (?) with beam


Synchrotron radiation bombardment


Pressure instabilities



Ion Trapping



Electron cloud phenomena


dCR=1.1


Limited Vacuum Conductance



Any system requiring a pressure lower than 10-8 Torr in a few days of pumping should be baked in-situ.

Effects of bakeout:- Lower outgassing: from roughly 10-10 Torr l s-1 cm-2 of water vapor to 10-12-10-13 Torr

l s-1 cm-2 of H2

- Lower desorption yields:

- Lower SEY: but always higher than the required threshold for DR.

Present Solutions: Bakeout

10-4

10-3

10-2

10-1

100

150 300

Deso

rptio

n yi

elds

[mol

ecul

es/e

lect

ron]

24 hours bakeout temperature [°C]

H2

CH4

316LN

electropolished

cleanedelectropolished

cleaned

10-3

10-2

10-1

100

150 300

COCO

2

Deso

rptio

n yi

elds

[mol

ecul

es/e

lect

ron]

24 hours bakeout temperature [°C]

316LN

electropolished

cleaned

Electron energy: 500 eV


Present Solutions: Lumped Pumping

10

Pres

sure

Circulating beam

LP max

P min

The average pressure is quickly limited by the chamber conductance when increasing the spacing between the pumps: the benefit of increasing S is minimal.

It is more efficient to increase the number of pumps i.e. decrease L. But this can be very expensive or not possible due to space limitation.

Pumps located at regular distances from each other give rise to a parabolic pressure profile with minima at the pumps location. The average pressure is given by:

Pav

QLS 1

S6CL

QL2

6C

Q is the degassing rate per metre of pipe.

C is the conductance of one metre of pipe,

S is the pumping speed of each pump

for small aperture


Present Solutions: Linear Pumping

When a pumping speed exceeding a few hundreds of litres per second is required, linear pumping is the obvious choice.

PEP-II vacuum chamber

LEP vacuum chamber cross section

Two kinds of linear pump have been used:

1. Integrated ion pumps (they make use of the magnetic field of the machine bending magnets)

2. NEG strips (St101 or St707)


Present Solutions: Distributed Pumping-1

Electron Beam

Cooling and Heating channels NEG coating

NEG films do not need space, electric power, insulation and feedthroughs (simplified design).

After activation a NEG film surface is very clean resulting in a large pumping speed and reduced degassing (both thermal and ion/radiation/electron induced).

NEG films trap the gas coming from the substrate material.

Low SEY.

The film coating is considered a part of the vacuum chamber.



About 15 Kg of Ti-V-Zr are spread over 6 Km of LHC beam pipe



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

Lower SEY should be obtained by increasing the roughness of the coating. This is obtained by:

1. Increasing the substrate roughness, for example through chemical attack.

2. Optimizing the sputtering process.



Substrate temperature during coating

100 °C 300 °C

SEY measurements expected in the next months

In any case, for NEG films, the low SEY can be obtained only after in-situ activation at temperatures higher than 175°C.


Present Solutions: Amorphous Carbon Films-1

Advantages of magnetron-sputtered C films:

• They do not need any in situ bakeout to attain the lowdmax .• Their dmax is lower than that of smooth TiZrV and scrubbed surfaces.• Multiple exposures to air do not increase the dmax if the samples are correctly stored.

• Good adhesion, no loose dust C particles.• Resistive behavior: major impact on the impedance can be excluded.

10 μm 0.2 μm

If those heating temperatures are unsafe, low SEY can be obtained thanks to another coating recently developed at CERN for that purpose, namely amorphous carbon thin film.


Present Solutions: Amorphous Carbon-2

0 400 800 1200 16000.4

0.6

0.8

1.0

1.2

Primary Electron Energy [eV]

SEY

Courtesy of Mauro Taborelli and Christina Yin Vallgren


10-11

10-10

10-9

10-8

10-7

100 101 102

Out

gass

ing

Rate

[Tor

r l s

-1 cm

-2]

Pumping Time [h]

Carbon coated

Bare stainless steel

The water vapor outgassing rate is higher than that of uncoated stainless steel by a factor of 20 after 100 h of pumping.


GasOutgassing Rate

[Torr l s-1 cm-2]H2 3 x 10-13

CH4 2 x 10-16

CO 2 x 10-15

CO2 7 x 10-15

Ne 1 x 10-16

Compared to uncoated stainless steel:H2: about 5 times lowerCO2: at least a factor of 5 higher

Unbaked Baked at 150°C for 24h


Critical Energy20.5 KeV

Angular acceptance4.234 mrad

Photon Flux (E>10eV)2.94x1015 photons (s mA)-1

Beam Energy6 GeV

Typical Beam Current185 mA

31The system is bakes at 300°C (24h). The sample is not baked. The sample is separated from the rest of the system by a gate valve (at the diaphragm position, not pictured in the drawing); it is pumped by an auxiliary TMP during the bakeout of the system.

At the end of the bakeout, the gate valve is opened.

Angle of incidence = 25 mrad

SIP SIP SP

Gate Valve

Turbomolecular pumping group

PG

PG

BAG

PG BAG

QMAFront-End Slits and Absorber

Conductance Diaphragm

PG : Penning Gauge BAG : Bayard-Alpert Gauge QMA : Quadrupole Masss Analyser SIP : Sputter Ion Pump SP : Sublimation Pump

Water or liquid nitrogen cooling circuit

NEG-coated chamber

C coated chamber

Angle of incidence = 25 mrad



The photon desorption yield of the unbaked C coated sample is lower than that of uncoated stainless steel.


Courtesy of Roberto Kersevan



Courtesy of Pedro Costa Pinto


Present Solutions: Scrubbing (Conditioning)-1

Desorption yields, except for H2O, have a power law dependence on the dose D of bombarding particles:

where a is found to vary between 0.6 and 1.

a Do

E=300 eVJ. Gomez-Goñi and A. G. Mathewson

J. Vac. Sci. Technol. A 15, 3093 (1997)

a0.77

The samples were baked at 150 C for 24 h and at 300 C for 2 h

Beam conditioning reduces both desorption yields and SEY


Present Solutions: Scrubbing (Conditioning)-2

Noël Hilleret EPAC 2000


Implementation in CLIC

--NEG coating (possibly rough) + auxiliary IP -- or --a-C and lumped getter pumps + auxiliary IP if

only low T baking is allowed

NEG coating + auxiliary IP NEG coating + auxiliary IP

Lumped pumps Lumped pumps

NEG coating + auxiliary IP or scrubbing


- Rough NEG coatings for lower SEY.

- Lower activation temperature NEG coatings.

- Coating in restricted geometries (two halves solution)

- Optimization of a-C film outgassing.

- Miniaturization of lump pumps.

Possible Developments

Date post:	22-Feb-2016
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CLIC Vacuum Issues (2) Paolo Chiggiato Vacuum, Surfaces and Coatings TE department

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