Getters : From Light Bulbs to Accelerators

F. Le Pimpec

SLAC/NLC

PSI October 2004

A Demonstration !

Bulb Installed in 1901 at the Livermore’s (CA) Fire station.

C filament - 4 W Phosphorus-pumped lamps tend to have a red color cast.

From Ref [1] CERN : LEP/LHC aerial Picture Vacuum insured by St101 (Zr-Al) NEG F. Le Pimpec - SLAC 2

Importance of talking about getters ?

Luminosity for colliders & Lifetime for Storage Rings

Torr 10 -14 10 -13 10 -12 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2

Vacuum Gauges Spinning Rotor gauge Penning gauge Hot Cathod Ionization gauge, Bayard Alpert Cold Cathod Discharge gauge Extractor - Ionization gauge, Modified Bayard Alpert

Improvement

of electronic devices, the Edison effect (thermionic emission) 1883, to polarized photocathodes at the beginning of the 3 rd millennium Vacuum Pumps Ion Sputter pump Cryopump Diffusion pump Turbomolecular pump Titanium Sublimation pump Non Evaporable Getter pump & Cryogenic pump F. Le Pimpec - SLAC 3

The Invention of the Light Bulb: Davy, Swan and Edison

1800 1878 1879 Edison

filed the patent after

Swan

and still made the

$$$ An advertisement for x ray apparatus by the Edison Decorative and Miniature Lamp Department. From an issue of the Scientific American 1898.

Diagram of Edison’s vacuum system for the production of incandescent lamps (1870s).

Thomas A Edison 1878 Edison Light bulbs last longer !!

True until 1910 (invention of the W filament)

Small Dental x-ray tube mounted

Evaporable Getters –

Vacuum tube – Radio valves …

Dull Emitter Receiving – Radio valves, Triodes (1905) (Thoriated Tungsten filaments) Arcturus_UX201A 1924-1927 1927-1933 Ba + Mg getter P getter A Ba getter can give a blackish coloration – Customers did not like that in the products.

Addition of Mg gave the silvery effect – and the product became salable

F. Le Pimpec - SLAC

Mg getter in the anode – no filament

Radio Tube – can produce xrays 5

What is a Getter Material ?



A priori, any clean surfaces are getter materials.

– Bakeout, SR or e scrubbing… – Clean surfaces have pumping properties 

To be named getter, the material

must

form tied and stable bonds with molecules from the residual gas

6 F. Le Pimpec - SLAC



Tied Bonds and Location Tied bonds

: Chemisorption ≥ eV – Covalent bond – Ionic bonds (1 e (sharing of the e ) is stolen by the most electro (Mg + O )) – Metallic bonds (valence electrons shared) elements 

Loose bonds

: Physisorption < eV – Van der Waals forces – Hydrogen bonding (~0.4eV) (Polar molecules - Chemical, (Biology)) 

Stable bonds can be formed

: – At the surface : Adsorption – In the bulk of the material : Absorption 7 F. Le Pimpec - SLAC

Getters are Capture Pumps

 Cryopumps and Sputter/getter-ion pumps are also capture pumps.

 Differentiation is needed – – Physical getters (Zeolite) – Work at LN 2 temperature by trapping air gases (including water vapor). Cheap primary dry pump.

– Recycling by warming up the zeolite Chemical getters or simply : getters – “Entertainment of the moment” 8 F. Le Pimpec - SLAC

Sputter/Getter-Ion Pumps

Getter-ion pump [ENGINEERING] A high-vacuum pump that employs chemically active metal layers which are continuously or intermittently deposited on the wall of the pump, and which chemisorb active gases while inert gases are "cleaned up" by ionizing them in an electric discharge and drawing the positive ions to the wall, where the neutralized ions are buried by fresh deposits of metal. Also known as sputter-ion pump

ref. [3].

Courtesy of Varian Diode F. Le Pimpec - SLAC Developed by JPL with/for NASA 9

How Do Getters Work ?

Whatever the getter is, the same principle applies :

The use of a clean surface to form chemicals bonds

When is the getter surface saturated :  : molecules.s

-1 .cm

-2   3 .

5 10 22 

P MT

 : sticking coefficient P : Pressure (Torr) 1ML : ~10 15 molecules.cm

F. Le Pimpec - SLAC -2 10

Getter Chemistry

Dissociation of residual gases on a surface is not systematic

Getter + O 2 → Getter-O Getter + N 2 → Getter

-

N Getter + CO 2 → CO + Getter

-

O Getter + CO 2 → Getter-C + Getter-O Getter + CO → Getter-C + Getter-O Getter + H 2 O → H + Getter-O → Getter-O + H (bulk) Getter+ H 2 → Getter + H (bulk) Getter + Hydrocarbons, C x H y → Getter-C + H (bulk) Getter + He, Ne, Ar, Kr, Xe (inert gases) → No reaction

From P. Danielson Ref [4] F. Le Pimpec - SLAC 11

Getter Chemistry

t From Ref [19] E f E i E f E i E p C-H : 415 kJ/mol C-C : 348 kJ/mol C-O : 358 kJ/mol F. Le Pimpec - SLAC 12

Gettering Materials !



The list of materials is quite long…

– Barium Calcium – Cesium – Magnesium – Columbium (Nb) – Titanium – Uranium - Hafnium - Phosphorus - Tantalum - Thorium - Zirconium 

Alloy can be created in order to enhance some properties – H

2

diffusion

– Aluminium – Nickel - Cobalt - Vanadium – Palladium – Other materials including multi getter alloys F. Le Pimpec - SLAC 13

1

Choice of Getter – Vapor Pressure

P When choosing a material to be used for a vacuum application. One question which need to be asked is : At Which temperature my system is going to be running ?

10 -7 50 Zn Mg 200 700 Al 1 The elements of your vacuum system must not limit the pressure you are aiming at. Their vapor pressure must be taken into account in the design. That is also true for your getter pump After Honig and Kramer (1969) Al 10 -7 F. Le Pimpec - SLAC 700 Ti 1200 Ta 14

How to Use Getters ?

There are two ways :

1.

2.

As Evaporable Getter

– Deposition of a fresh film of material – flash evaporation

As Non-Evaporable Getters

– Use of an alloy containing one or more gettering materials F. Le Pimpec - SLAC 15

Evaporable Getters Deposition of a film of getter material

- This is achieved by evaporating the getter (alloy) or by thermal or by electron heating. - When the pumping speed is no longer adequate (saturated film), a new layer must be evaporated (Ti SP - Ba dispenser).

- In some applications, e.g. vacuum tubes, the evaporable getter is deposited by the bake of the system and should hold the vacuum for the life of the device (P - Mg - Ba).

- Temperature of evaporation depends on the material in use.

- As getters are usually highly reactive to oxygen, care must be taken. Especially if the getter is hot.

F. Le Pimpec - SLAC 16

 

Evaporable Getters - Magnesium

Use – Mg is one of the 1 – Good O 2 to be avoided st getter used historically getter – But physisorb most of the – High vapor pressure precludes use in small vacuum tubes (P~10 -5 Torr at 275°C) – Mg can be used when other types of getters with higher evaporation temperatures have Precautions – Mg metal is highly flammable in its pure form, especially when it is a powder – Magnesium metal quickly reacts exothermically upon contact with air or water and should be handled with care – Water should not be used to extinguish magnesium fires From Ref [2] 17 F. Le Pimpec - SLAC

 

Evaporable Getters - Phosphorus

Use – Phosphorus (white or red) has also a high vapor pressure. Hence, it is not used in high vacuum discharge tubes – Inexpensive and simple to handle, it is used for high-vacuum tubes and gas-filled lamps – Extremely efficient at gettering O 2 Precautions Philips MLR160 -1984 Courtesy of Philips – This is a poisonous dose element, 50 mg being the average fatal – The white form ignites spontaneously in air – The red form is more stable, and is obtained by sunlight or when heated in its own vapor to 250 °C. The red form reverts to white phosphorus in some temperature ranges and it also emits highly toxic fumes that consist of phosphorus oxides when it is heated.

F. Le Pimpec - SLAC 18

 Use

Evaporable Getters - Barium

– Ba was and still is one of the most used flash getters for (high) vacuum tubes (CRT tubes -TV) and lamps. Ba flash getters are mainly evaporated from alloys Ba-Al (Ba 43, Mg 20, Al 37 : Kemet TM ) – Very efficient pumping for O – CO 2 , and good for H 2 2 – N 2 and CO  Precautions

Ba flash getters for glass bulbs (upper row) and getter strip assemblies (1950)

– Ba and P are so reactive to air that you cannot find them in their pure form. T o remain pure, Ba should be kept under a petroleum based fluid (kerosene) or other oxygen-free liquids, or produced and kept under vacuum/inert atmosphere.

– All water or acid soluble Ba compounds are extremely poisonous .

19 F. Le Pimpec - SLAC

Evaporable Getters - Titanium

 Use & Limitation – One of the new comers – The pumping speed of a freshly evaporated film (from Ti filaments or Ti-balls), can be enhanced by cooling down the coated vessel. Allows physisorption of CH 4 peel off. Peeling starts ~ 50 rate of evaporation of the Ti (77K) – After several uses, the Ti film can  m . The film thickness depends on the time of the sublimation and the – For mechanical strength at sublimation T, the Ti filament has to be alloyed (Mo) or formed onto a rigid structure (W or Ta) Photo courtesy of Thermionics Laboratory, Inc Varian, Inc ♦ Precaution − This is a safe product F. Le Pimpec - SLAC 20

Titanium vs. Other Getters For Accelerator Use

Ba - Ca - Mg : High vapor pressure. Trouble if bake out is requested Zr - Nb - Ta : Evaporation temperature too high Typical required sublimation rate 0.1 to 0.5 g/hr 1cm 2  

Ref. “Le Normand CERN vacuum note” Ref. “Sorption of Nitrogen by Titanium Films,” Harra and Wide variations due to film roughness Hayward, Proc. Int. Symp. On Residual Gases in Electron Tubes, 1967 For H 2, competition between desorption and diffusion inside the deposited layers

F. Le Pimpec - SLAC 21

Evaporable Getters : Generalities

Designers must pay attention to accidental coating over insulators by the evaporated film - Poisoning by the getter, limitation of the life time of cathodes (polarizable e sources) or filaments (W-Th) For Accelerators Ti SP : - Large pumping speed and capacity  Low pressure - Inexpensive and easily operated - No noble gas or methane pumping,

methane production ???

- localized pumping (conductance limitation on their effectiveness) F. Le Pimpec - SLAC 22 1950

Non-Evaporable Getters

CO, N2, CO2, O2 NEGs are pure metals or are alloys of several metals - Unlike evaporable getters, pumping speed of the surface is not restored by depositing a new layer.

NEG - Restoration is achieved by “activation” heating of the substrate on which the getter is deposited. Joule or bake heating CO, N2, CO2, O2 H2 - During activation, atoms migrate from the surface into the bulk, except H 2 .

NEG - Heating to “very high” temperature will outgas the getter. This regenerates it but also damages the crystal structure.

F. Le Pimpec - SLAC H2 23



Some Alternative Getters

Depleted Uranium – Very good getter (UO 3 ) – – Slightly radioactive and very pyrophoric (CERN Accident January 1999, HEP target) Still used in some laboratories around the world. Even in custom ion pumps, instead of Ti   Thorium – – – – Used during WW II for the production of vacuum tubes Ceto getters (alloy) : 20% mischmetal, (Ce and other rare earth) and 80% Th - Low Secondary Electron Yield, when compared to Ba Pumps well at 300°C, but highly pyrophoric Used for UHV gauges filaments (Th-Ir) (W-Th filaments used since post-WW I) Tantalum – Used for sorbing noble gases (100 times its own volume), but need high temperature degassing > 1600°C : Noble gas ion pumps – – – No H 2 firing, because of embrittlement In vacuum furnace, used to capture O 2 and H 2 Also used to getter the contaminants outgassed by Nb or Ti during heat treatment of those materials  Titanium & Zirconium – Basic elements in the making of NEG of today 24 F. Le Pimpec - SLAC

Non-Evaporable Getters : Uses

St 707 (ZrVFe)

Pump cartridge for Ion Pump or as lump pumps Application of NEG are rather wide : Use of St 2002 pills to insure a vacuum of 10 -3 Torr NEG is used in UHV (accelerators - tokamak) Used for purifying gases (noble gas) Used for hydrogen storage, including isotopes (near embrittlement regime) Lamps and vacuum tubes … F. Le Pimpec - SLAC 25

NEG & Accelerators

Cf. Benvenutti The LEP : 1 st major success of intensive use of NEG pumps LEP dipole chamber, getter St101 (ZrAl) (1989-2000) ~24 km of NEG  P~10 -12 Torr range Lump pumping Inserted “linear” pump Thin film getter is the new adopted way of insuring UHV in colliders or SR light sources TiZrV NEG Coating Setup at CERN Inserted “total” pump (TiZrV) Surface pump

DAFNE ESRF SOLEIL DIAMOND RHIC LHC ILC ??...

26

 

What Makes NEG So Attractive?

A

GREAT

Material

– High distributed pumping speed – Initial photo, electron-desorption coefficient lower than most technical material (Al - Cu - SS) – Secondary Electron Yield (SEY) lower than that of common technical materials Drawbacks – Needs activation by heating (200°C to 700°C) - Pyrophoricity (Zr-based alloy) – Does not pump CH High H 2 4 at RT, nor noble gases – Lifetime before replacement (thin film) solubility but embrittlement (powder creation) 27 F. Le Pimpec - SLAC

Photodesorption

h CO

at

 c

= 194 eV

1.E-03 NEG 0% Sat ( 13 C 18 O) 13 C 18 O

CO

1.E-04 1.E-05 SS 1.E-06 1.E-07 1.E+18 1.E+19 Sat ( 13 C 18 O) CO 1.E+20

Dose photons

1.E+21 NEG 100 % 1.E+22 An activated NEG desorbs less H 2 A saturated NEG desorbs more CO CH 4 CO 2 than a 300°C baked SS 28 1.E+23

Electrodesorption

h CO

at E

e 1.E-01

= 300 eV

NEG Sat ( 13 C 18 O) 13 C 18 O

CO

1.E-02 NEG Sat by CO 1.E-03 Cu 1.E-04 1.E-05 1.E-06 1.E+16 NEG 100 % 1.E+17 NEG Sat ( 13 C 18 O) CO 1.E+18

Dose Electrons

1.E+19 1.E+20 1.E+21 An activated NEG desorbs less H 2 surface.

CO CH 4 CO 2 than a 120°C baked OFE Cu A saturated NEG desorbs less *C*O than a 120 °C baked OFE Cu surface F. Le Pimpec - SLAC 29

Also True For Thin films TiZr and TiZrV

SS Cu F. Le Pimpec - SLAC 30

LHC

SEY & Electron Cloud

Electron cloud can exist in p + / e + beam accelerator and arise from a resonant condition (multipacting) between secondary electrons coming from the wall and the kick from the beam, (PEP II - KEK B - ISR LHC).

NLC Fast Head tail straight 10 12

1.5

1 3 2.5

2 Aluminium Beryllium Titane Copper OFHC Stainless Steel NEG St 707 (activated) NEG TiZrV (activated 200°C- 2h) 0.5

0 200 400 600 800 1000 1200

Electron Beam Energy (eV)

1400 1600 1800 SEY of technical surfaces baked at 350 °C for 24hrs 2000 F. Le Pimpec - SLAC 31 M. Pivi



Getter SEY & Electron Cloud

Low SEY : Choice for the NEG of the activating T and t . Conditioning (photons e ions) Contamination by gas exposure, or by the vacuum residual gas, increases the SEY; even after conditioning.

  

e

  1  cos    Roughness is an issue to be considered for lowering the SEY TiZrV coating TiZrV coating Angles of incidence, of the PE, yield the shape of the curve toward higher values 2 h at 300C, CO injected at NEG T=60C F. Le Pimpec - SLAC 32 Scheuerlein et al.

Pumping Speed

H 2 Ti 32 Zr 16 V 52 (at.%) CERN/EST group 2 Hours Heating T ( °C) Pumping speed plots for getter are everywhere in the literature • From sample to sample, pumping speed plots vary • Many geometric cm 2 are needed to see the pumping effects. Roughness (true geometry) •Temperature and/or time of activation is critical to achieve the pumping speed required •Capacity of absorption of the NEG is determined by its thickness 33

Installing a NEG : Yes or No ?

You want to answer the terms of this formula : The tunneling ionization of molecules is not included, but should be for very short and intense bunched beams (29 GV/m for CO ~7fs) F. Le Pimpec - SLAC 34

Installing NEG : Yes ! … Which NEG and where ?

– Linear pumping via ribbons ?

– Thin film coating on the accelerator chamber itself ?

- “Ribbons” are reliable and have a good capacity time before saturation, few replacements over the years (PEP II - LEP) - Thin films allow easy reach of XHV (<10 -12 Torr). The lifetime can be long depending on the thickness, 3 years of use at ESRF in some sections.

Yes to all of that, BUT you need to activate !!!

35 F. Le Pimpec - SLAC

But !

In accelerator Cu, Al or SS are the technical materials of choice, high conductivity – Cu and SS, can be baked at high temperature, Al cannot (200°C)  special design, or ways, to activate the NEG – SS and NEG coating have a lower conductivity compared to Cu or Al, wakefield issues vertex detector, for LHC 10 6 CHF) – Cycles of venting/activation need to be  – A leak during an activation might lead to assessed for the lifetime of the machine skin depth & vacuum chamber size determination scrapping the chamber (2m of Be chamber, 36 F. Le Pimpec - SLAC

Conclusion

What is the requirement of the vacuum system ?

Pressure wise Pumping speed Vapor Pressure Bakeout of the system Vapor Pressure Design to allow bake Contamination Issues Getter Element to use Evaporable, Non-Evaporable : Design Lifetime of the vacuum device Capacity of the getter Activation cycle - NEG Evaporation cycle – EG … F. Le Pimpec / SLAC-NLC 37

Acknowledgement SLAC :

R. Kirby

CERN :

JM. Laurent, O. Gröbner, A. Mathewson – … F. Le Pimpec - SLAC 38

References

1. CERN web site and Summer lecture 2. AVS 50 – … Th 7. USPAS conference 3. Mc Graw-Hill Access Science 4. P. Danielson : Vacuum Lab 5. Electronics magazine: October 1950 6. CAS Vacuum Technology: CERN 99-05 June 2002 8. SAES getters 9. Web surfing for the beautiful pictures F. Le Pimpec - SLAC 39

Some More References

11. http://www.nasatech.com/Briefs/Sept99/NPO20436.ht

ml 12. http://info.web.cern.ch/info/Press/PressReleases/Relea ses1999/PR01.99Efire.html

13. http://www.metall.com.cn/cemm.htm

14. http://education.jlab.org/itselemental/ele055.html

- Cs getter 15. http://hcrosscompany.com/lampseal/tantalum.htm

16. http://www.fact-index.com/ 17. http://www.bulbcollector.com/ (Thks Ed.V. Phillips) 18.

http://www.centennialbulb.org/index.htm

19. http://wps.prenhall.com/wps/media/objects/724/74157 6/chapter_01.html

– … 40 F. Le Pimpec - SLAC