Getters : From Light Bulbs to Accelerators F. Le Pimpec

Transcription

1 Getters : From Light Bulbs to Accelerators F. Le Pimpec SLAC/NLC PSI October 2004

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

3 Importance of talking about getters? Luminosity for colliders & Lifetime for Storage Rings Improvement of electronic devices, the Edison effect (thermionic emission) 1883, to polarized photocathodes at the beginning of the 3 rd millennium Torr Vacuum Gauges Vacuum Pumps Spinning Rotor gauge Penning gauge Hot Cathod Ionization gauge, Bayard Alpert Cold Cathod Discharge gauge Extractor - Ionization gauge, Modified Bayard Alpert Cryopump Diffusion pump Turbomolecular pump Titanium Sublimation pump Ion Sputter pump Non Evaporable Getter pump & Cryogenic pump F. Le Pimpec - SLAC 3

4 The Invention of the Light Bulb: Davy, Swan and Edison 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 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 F. Le Pimpec -From SLAC Ref [17] on a "deco" looking display 4 stand

5 Evaporable Getters Vacuum tube Radio valves Dull Emitter Receiving Radio valves, Triodes (1905) (Thoriated Tungsten filaments) Arcturus_UX201A Ba + Mg getter P getter Mg getter in the anode no filament 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 5 Radio Tube can produce xrays

6 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 F. Le Pimpec - SLAC 6

7 Tied Bonds and Location Tied bonds : Chemisorption ev Covalent bond (sharing of the e Ionic bonds (1 e (Mg + O - )) (sharing of the e - ) (1 e - is stolen by the most electro - elements Metallic bonds (valence electrons shared) Loose bonds : Physisorption < ev Van der Waals forces (~0.4eV) Hydrogen bonding (Polar molecules (Biology)) Stable bonds can be formed : (Polar molecules - Chemical, At the surface : Adsorption In the bulk of the material : Absorption F. Le Pimpec - SLAC 7

8 Getters are Capture Pumps Cryopumps and Sputter/getter-ion pumps are also capture pumps. Differentiation is needed Physical getters (Zeolite 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 F. Le Pimpec - SLAC 8

9 Sputter/Getter-Ion Pumps Getter-ion pump [ENGINEERING] [ENGINEERING] A high-vacuum pump that employs chemically active metal layers which are continuously or intermittently ttently 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 Developed by JPL with/for NASA Diode F. Le Pimpec - SLAC 9

10 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 : ν = α P MT ν: molecules.s -1.cm -2 α: sticking coefficient P : Pressure (Torr) 1ML : ~10 15 molecules.cm -2 F. Le Pimpec - SLAC 10

11 Getter Chemistry Dissociation of residual gases on a surface is not systematic Getter + O 2 Getter + N 2 Getter-O 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

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

13 Gettering Materials! The list of materials is quite long Barium Cesium Magnesium Columbium (Nb( Nb) Titanium Uranium - Calcium - Hafnium - Phosphorus - Tantalum - Thorium - Zirconium Alloy can be created in order to enhance some properties H 2 diffusion Aluminium - Cobalt Nickel - Vanadium Palladium Other materials including multi getter alloys F. Le Pimpec - SLAC 13

14 Choice of Getter Vapor Pressure 1 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? Zn Mg 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) 10-7 Al 700 F. Le Pimpec - SLAC 14 Ti 1200 Ta

15 How to Use Getters? There are two ways : 1. As Evaporable Getter Deposition of a fresh film of material flash evaporation 2. As Non-Evaporable Getters Use of an alloy containing one or more gettering materials F. Le Pimpec - SLAC 15

16 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

17 Evaporable Getters - Magnesium Use Mg is one of the 1 st getter used historically Good O 2 getter But physisorb most of the other gases 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 to be avoided 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] F. Le Pimpec - SLAC 17

18 Use Evaporable Getters - Phosphorus 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 MLR Courtesy of Philips This is a poisonous element, 50 mg being the average fatal dose 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

19 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 TM ) Very efficient pumping for O 2 N 2 CO 2, and good for H 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. To 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. F. Le Pimpec - SLAC 19

20 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 (77K) After several uses, the Ti film can peel off. Peeling starts ~ 50µm m. The film thickness depends on the time of the sublimation and the rate of evaporation of the Ti 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

21 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 Wide variations due to film roughness For H 2, competition between desorption and diffusion inside the deposited layers Ref. Sorption of Nitrogen by Titanium Films, Harra and Hayward, Proc. Int. Symp. On Residual Gases in Electron Tubes, 1967 F. Le Pimpec - SLAC 21

22 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 Th) 1950 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

23 Non-Evaporable Getters 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. - Restoration is achieved by activation - heating of the substrate on which the getter is deposited. Joule or bake heating - During activation, atoms migrate from the surface into the bulk, except H 2. - Heating to very high temperature will outgas the getter. This regenerates it but also damages the crystal structure. CO, N2, CO2, O2 NEG CO, N2, CO2, O2 NEG H2 H2 F. Le Pimpec - SLAC 23

24 Some Alternative Getters Depleted Uranium Thorium Tantalum 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 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( Th-Ir) ) (W-Th filaments used since post-ww I) Used for sorbing noble gases (100 times its own volume), but need high temperature degassing > 1600 C 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 F. Le Pimpec - SLAC 24

25 Non-Evaporable Getters : Uses St 707 (ZrVFe) Application of NEG are rather wide : 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 Pump cartridge for Ion Pump or as lump pumps Use of St 2002 pills to insure a vacuum of 10-3 Torr F. Le Pimpec - SLAC 25

26 NEG & Accelerators Cf. Benvenutti The LEP : 1 st major success of intensive use of NEG pumps LEP dipole chamber, getter St101 (ZrAl) ( ) ~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 DAFNE ESRF Inserted total pump SOLEIL DIAMOND RHIC (TiZrV) Surface pump / diffusion barrier LHC ILC??... F. Le Pimpec - SLAC 26

27 What Makes NEG So Attractive? GREAT Material A GREAT 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 C to 700 C) - Pyrophoricity (Zr-based alloy) Does not pump CH 4 at RT, nor noble gases Lifetime before replacement (thin film) High H 2 solubility but embrittlement (powder creation) F. Le Pimpec - SLAC 27

28 Photodesorption η at CO ε c = 194 ev 1.E-03 NEG 0% Sat ( 13 C 18 O) 13 C 18 O CO 1.E-04 NEG St707 ETA (molec/photon) 1.E-05 1.E-06 SS Sat ( 13 C 18 O) CO 1.E-07 1.E+18 1.E+19 1.E+20 1.E+21 1.E+22 1.E+23 Dose photons An activated NEG desorbs less H 2 CO CH 4 CO 2 than a 300 C baked SS A saturated NEG desorbs more CO than a baked Stainless Steel NEG 100 % F. Le Pimpec - SLAC 28

29 Electrodesorption η at CO E e- = 300 ev 1.E-01 NEG Sat ( 13 C 18 O) 13 C 18 O CO 1.E-02 NEG Sat by CO NEG St707 Eta (molec/elect) 1.E-03 1.E-04 1.E-05 Cu NEG 100 % NEG Sat ( 13 C 18 O) CO 1.E-06 1.E+16 1.E+17 1.E+18 1.E+19 1.E+20 1.E+21 Dose Electrons An activated NEG desorbs less H 2 CO CH 4 CO 2 than a 120 C baked OFE Cu surface. A saturated NEG desorbs less *C*O than a 120 C baked OFE Cu surface F. Le Pimpec - SLAC 29

30 Also True For Thin films TiZr and TiZrV 1.E-03 SS Cu Acier Inox 150 C (24H) Cuivre 150 C (24H) TiZr en l'état TiZr CO Saturation TiZr 250 C (9H) 1.E-04 Eta (molec/photon) 1.E-05 1.E-06 H2 CH4 CO CO2 F. Le Pimpec - SLAC 30

31 SEY & Electron Cloud LHC 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 Secondary Electron yield Aluminium Beryllium Titane Copper OFHC Stainless Steel NEG St 707 (activated) NEG TiZrV (activated 200 C- 2h) Electron Beam Energy (ev) SEY of technical surfaces baked at 350 C for 24hrs M. Pivi F. Le Pimpec - SLAC 31

32 δ Getter SEY & Electron Cloud α 1 cos( θ ) ( θ ) δ ( ) 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 Roughness is an issue to be considered for lowering the SEY TiZrV coating TiZrV coating 2 h at 300C, CO injected at NEG T=60C Angles of incidence, of the PE, yield the shape of the curve toward higher values cheuerlein et al. F. Le Pimpec - SLAC 32

33 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

34 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

35 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!!! F. Le Pimpec - SLAC 35

36 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 skin depth & vacuum chamber size determination A leak during an activation might lead to scrapping the chamber (2m of Be chamber, vertex detector, for LHC 10 6 CHF) Cycles of venting/activation need to be assessed for the lifetime of the machine F. Le Pimpec - SLAC 36

37 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

38 Acknowledgement SLAC : R. Kirby CERN : JM. Laurent, O. Gröbner bner,, A. Mathewson F. Le Pimpec - SLAC 38

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

40 Some More References ml ses1999/pr01.99efire.html Cs getter hcrosscompany.com/lampseal/tantalum.htm index.com/ (Thks Ed.V.. Phillips) /chapter_01.html F. Le Pimpec - SLAC 40