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CLIC seminars. CLIC Vacuum Issues (2) Paolo Chiggiato Vacuum, Surfaces and Coatings TE department. Content: Vacuum problems in CLIC. Known solutions: Bakeout. Lumped pumping. Linear pumping. Distributed pumping: NEG coatings. Amorphous carbon coatings. - PowerPoint PPT Presentation
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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
Paolo Chiggiato,TE-VSC 2
- 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
Paolo Chiggiato,TE-VSC 3
P ≤ 10-9 Torr
P=10-8 Torr P=10-8 Torr
P=10-9 Torr P=10-9 Torr
Vacuum problems in CLIC
P ≤ 10-9 Torr
P ≤ 10-9 Torr
Pressure requirements (?) with beam
Paolo Chiggiato,TE-VSC 4
Synchrotron radiation bombardment
Paolo Chiggiato,TE-VSC 5
Pressure instabilities
Vacuum problems in CLIC
Paolo Chiggiato,TE-VSC 6
Ion Trapping
Vacuum problems in CLIC
Paolo Chiggiato,TE-VSC 7
Electron cloud phenomena
Vacuum problems in CLIC
dCR=1.1
Paolo Chiggiato,TE-VSC 8
Limited Vacuum Conductance
Vacuum problems in CLIC
Paolo Chiggiato,TE-VSC 9
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
Paolo Chiggiato,TE-VSC 10
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
Paolo Chiggiato,TE-VSC 11
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)
Paolo Chiggiato,TE-VSC 12
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.
Paolo Chiggiato,TE-VSC 13
Present Solutions: Distributed Pumping-2
About 15 Kg of Ti-V-Zr are spread over 6 Km of LHC beam pipe
Paolo Chiggiato,TE-VSC 14
Present Solutions: Distributed Pumping-3
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.
Paolo Chiggiato,TE-VSC 15
Present Solutions: Distributed Pumping-4
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.
Paolo Chiggiato,TE-VSC 16
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.
Paolo Chiggiato,TE-VSC 17
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
Paolo Chiggiato,TE-VSC 18
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.
Present Solutions: Amorphous Carbon-3
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
Paolo Chiggiato,TE-VSC 19
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
Present Solutions: Amorphous Carbon-4
Paolo Chiggiato,TE-VSC 20
The photon desorption yield of the unbaked C coated sample is lower than that of uncoated stainless steel.
Present Solutions: Amorphous Carbon-5
Courtesy of Roberto Kersevan
Paolo Chiggiato,TE-VSC 21
Present Solutions: Amorphous Carbon-6
Courtesy of Pedro Costa Pinto
Paolo Chiggiato,TE-VSC 22
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
Paolo Chiggiato,TE-VSC 23
Present Solutions: Scrubbing (Conditioning)-2
Noël Hilleret EPAC 2000
Paolo Chiggiato,TE-VSC 24
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
Paolo Chiggiato,TE-VSC 25
- 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