June 10-14, 2002 Lawrence Livermore National Laboratory...
Transcript of June 10-14, 2002 Lawrence Livermore National Laboratory...
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The US Particle Accelerator SchoolNon-evaporable Getter Pumps
The US Particle Accelerator SchoolNon-evaporable Getter Pumps
Lou Bertolini Lawrence Livermore National Laboratory
June 10-14, 2002
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Non-evaporable Getters (NEG)
• NEG is available only from:SAES Getters S.p.A.Via Gallarate, 21520151 Milano Italy
SAES Getters U.S.A., Inc.1122 E. Cheyenne Mountain Blvd.Colorado Springs, CO 80906
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Non-evaporable Getters (NEG)
• Bulk Getters - gases diffuse into the interior of the getter material.
• Gases are categorized into four families based on their interactions with NEGs:1. Hydrogen and its isotopes - sorbed reversibly.2. CO, CO2, O2, and N2 - sorbed irreversibly.3. H2O, hydrocarbons - sorbed in a combination of
reversible and irreversible processes. Hydrocarbons are sorbed very slowly.
4. Rare gases - not sorbed at all.
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NEG Pumping Characteristics
Hydrogen� Hydrogen does not form a stable chemical composition with a
NEG alloy. It diffuses rapidly into the bulk of the getter and is stored as a solid solution.
� Sievert’s Law describes the relationship between H2concentration within its NEG and its equilibrium pressure.
TBq log 2AP Log −−−−++++====
q = H2 concentration in NEG, Torr - liters/gram
p = H2 equilibrium pressure, Torr
T = getter temperature, K
A, B constants for different NEG alloys
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NEG Pumping Characteristics
CO, CO2, O2, N2• Active gases are chemisorbed irreversibly by NEGs.
• The chemical bonds of the gas molecules are broken on the surface of the NEG.
• Then the various gas atoms are chemisorbed forming oxides, nitrides, and carbides.
• High temperatures do not break these chemical bonds. High temperatures promote diffusion into the bulk of the NEG.
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NEG Pumping Characteristics
H2O and Hydrocarbons• Water vapor and hydrocarbons are “cracked” on the
surface of the NEG.
• H2, O2, and C are chemisorbed irreversibly.
• However, hydrocarbons sorption efficiency below 500°Cis extremely small.
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NEG Pumping Characteristics
Rare gases• NEGs do not sorb Ar, He, Kr, Xe.
• Ion pumps are required in combination with NEGs for pumping rare gases.
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NEG Pumping Characteristics
• Below pressures of 10-5 Torr, NEG pumping speeds do not vary.
• Pumping speeds do, however, vary with NEG temperature.
10
100
1000
10 4
0.01 0.1 1 10 100 1000
CO-25 CH2O-25 CCO-280 CH20-280 CH2-25 CH2-280 C
Pum
ping
Spe
ed (l
iters
/sec
)
Sorbed Quantity (Torr-liters)
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Activation Process for NEG
H2H2
N2
N2
O2
O2
H2O
H2O
CO CO
HEAT
Passivating Layer
Vacuumor
Inert Gas
InactiveNEGParticle
Active NEG Particle
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Venting NEG Pumps
• NEG pumping speed deteriorates due to successive exposures to air or N2.
• Further improvement can be obtained if Argon is used as a protective gas.
• NEG pumps should never be exposed to air while at temperatures greater than 50°C.
0
20
40
60
80
100
120
0 5 10 15 20 25 30 35
N2AirPe
rcen
tage
of I
nitia
l Pum
ping
Spe
ed
Number of Exposures
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SAES ST101 Non-evaporable Getter
• Metal alloy made up of 84% Zr, 16% Al.
• First Zirconium based getters alloy introduced and still widely used today after 30 years.
• The operating temperature range of ST101 is 0 to 450°C.
• ST101 chemisorbs CO, CO2, H2O, N2, and O2 at high rates.
• ST101 activates at temperatures from 550 to 900°C.
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SAES ST101 Non-evaporable Getter Sorption Plot
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SAES ST101 Non-evaporable Getter Hydrogen Equilibrium Pressure
1.00E-10
1.00E-08
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02
Hydrogen Co ncentratio n (To rr-liters /g ram)
Hyd
rog
en E
qu
ilib
riu
m P
ress
ure
(To
rr)
200oC
300oC400oC
500oC600oC
700oC
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SAES ST101 Surface Composition vs. Activation Temperature
0
10
20
30
40
50
60
70
0 200 400 600 800 1000
COZrAl
Surf
ace
Com
posi
tion
(Ato
mic
%)
Temperature (C)
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Activation Efficiency for ST101 and ST707
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SAES ST707 Non-evaporable Getter
• Metal alloy made up of 70% Zr, 24.6% Va, and 5.4% Fe.
• The operating temperature range of ST707 is 20 to 100°C.
• ST707 chemisorbs CO, CO2, H2O, N2, and O2 at high rates.
• ST707 comes in a variety of forms (pills, washes, strips).
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SAES ST707 Non-evaporable Getter
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SAES ST707 Non-evaporable Getter Sorption Plot
10
100
1000
10 4
0.01 0.1 1 10 100 1000
CO-25 CH2O-25 CCO-280 CH20-280 CH2-25 CH2-280 C
Pum
ping
Spe
ed (l
iters
/sec
)
Sorbed Quantity (Torr-liters)
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SAES ST707 Surface Composition vs.Activation Temperature
0
10
20
30
40
50
60
70
0 100 200 300 400 500 600 700 800
COZrVFe
Surf
ace
Com
posi
tion
(Ato
mic
%)
Temperature (C)
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SAES ST707 Non-evaporable GettersHydrogen Equilibrium Pressure
1.00E-10
1.00E-08
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02
Hydrogen Concentration (Torr-liters/gram)
200oC300oC400oC500oC600oC
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SAES ST172 Non-evaporable Getters
• A porous sintered structure based on a mixture of Zr and ST707 alloy (Zr-V-Fe).
• Sintering process produces a getter with large amounts of surface area, high porosity, and good mechanical strength (less likely to produce dust).
• The alloy is characterized by high diffusity of sorbed gas species.
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SAES ST172 Non-evaporable Getters
• Highest pumping speeds and capacity are achieved at 800 to 900°C activation temperatures.
• However, ST172 can be activated as low as 400 to 500°C.900oC 500oC
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SAES ST175 Non-evaporable Getters
• A porous, sintered structure based on a mixture Titanium and Molybdenum powders.
• Sintering process produces a getter with large surface areas, high porosity, and good mechanical strength.
• This alloy has even higher diffusing of sorbed gases than ST172.
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SAES ST175 Non-evaporable Getter
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NEG Cartridge Pumps Using ST101Strip
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NEG Cartridge Pumps Using Sintered Plates
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NEG Cartridge Pumps for use in Ion Pumps
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APS Beampipe with NEG strips
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LLNL NEG Pump Design
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LLNL NEG Pump in a PEP-II Vacuum Chamber
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PEP-II Interaction Region Copper Vacuum Chamber
Q4 Magnet
NEG Pump
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Combination Pumping . . .Ion Pumps with TSP or NEG
• Combination pumping produces greater pumping speeds for all gases.
- TSP and NEG provide high pumping speeds for getterablegases.
- Ion Pumps provide pumping of argon and light hydrocarbons(usually Noble Diode pumps are chosen).
• Combination pumping can be attained by:- Commercial combination pumps- Custom built combination pumps- Use of multiple types of pumps
• NEGs are used on systems where high constant pump speeds are required.
• TSPs are used on systems with sudden large gas bursts and/or frequent venting takes place.
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Commercial Combination Pumps . . .Ion Pumps with TSP or NEG
Ion Pump with TSP filaments Ion Pump with NEG cartrdge
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