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

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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