june 10-14, 2002 lawrence livermore national laboratory...
TRANSCRIPT
USPAS June 2002NEGsPage 1
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
USPAS June 2002NEGsPage 2
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
USPAS June 2002NEGsPage 3
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.
USPAS June 2002NEGsPage 4
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
USPAS June 2002NEGsPage 5
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.
USPAS June 2002NEGsPage 6
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.
USPAS June 2002NEGsPage 7
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.
USPAS June 2002NEGsPage 8
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)
USPAS June 2002NEGsPage 9
Activation Process for NEG
H2H2
N2
N2
O2
O2
H2O
H2O
CO CO
HEAT
Passivating Layer
Vacuumor
Inert Gas
InactiveNEGParticle
Active NEG Particle
USPAS June 2002NEGsPage 10
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
USPAS June 2002NEGsPage 11
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.
USPAS June 2002NEGsPage 12
SAES ST101 Non-evaporable Getter Sorption Plot
USPAS June 2002NEGsPage 13
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
USPAS June 2002NEGsPage 14
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)
USPAS June 2002NEGsPage 15
Activation Efficiency for ST101 and ST707
USPAS June 2002NEGsPage 16
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).
USPAS June 2002NEGsPage 17
SAES ST707 Non-evaporable Getter
USPAS June 2002NEGsPage 18
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)
USPAS June 2002NEGsPage 19
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)
USPAS June 2002NEGsPage 20
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
USPAS June 2002NEGsPage 21
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.
USPAS June 2002NEGsPage 22
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
USPAS June 2002NEGsPage 23
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.
USPAS June 2002NEGsPage 24
SAES ST175 Non-evaporable Getter
USPAS June 2002NEGsPage 25
NEG Cartridge Pumps Using ST101Strip
USPAS June 2002NEGsPage 26
NEG Cartridge Pumps Using Sintered Plates
USPAS June 2002NEGsPage 27
NEG Cartridge Pumps for use in Ion Pumps
USPAS June 2002NEGsPage 28
APS Beampipe with NEG strips
USPAS June 2002NEGsPage 29
LLNL NEG Pump Design
USPAS June 2002NEGsPage 30
LLNL NEG Pump in a PEP-II Vacuum Chamber
USPAS June 2002NEGsPage 31
PEP-II Interaction Region Copper Vacuum Chamber
Q4 Magnet
NEG Pump
USPAS June 2002NEGsPage 32
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.
USPAS June 2002NEGsPage 33
Commercial Combination Pumps . . .Ion Pumps with TSP or NEG
Ion Pump with TSP filaments Ion Pump with NEG cartrdge