A Vacuum point of view

Transcription

1 Beam-Surface Interaction A Vacuum point of view F. Le Pimpec SLAC/NLC SSRL April

2 Dynamic Vacuum You want to address the terms of this formula How to measure the Pressure? F. Le Pimpec - SLAC 2

3 Outline Measuring and Reaching XHV XHV with Getters Beam Interaction with Technical surfaces - Desorption Induced by Electronic Transition - Electron Cloud - Ion instabilities Summary and Conclusion F. Le Pimpec - SLAC 3

4 Reaching and Measuring XHV Luminosity for accelerators Lifetime in storage rings Reaching XHV is commercially easier than measuring it A CERN modified Helmer gauge measured Torr XHV is not official Pressure 10-7 Torr are called UHV ( Torr) Torr Vacuum Gauges Vacuum Pumps Spinning Rotor gauge Hot Cathod Ionization gauge, Bayard Alpert Cold Cathod Discharge gauge Extractor - Ionization gauge, Modified Bayard Alpert Ion Sputter pump Cryopump Diffusion pump Turbomolecular pump Titanium Sublimation pump Non Evaporable Getter pump & Cryogenic pump Penning gauge F. Le Pimpec - SLAC 4

5 Why Measure Total Pressure? Partial Pressure gives information on the contents of the vacuum Total pressure can be computed from the partial P measurements BA SVT305 Operational in the same range (UHV) The use of hot and cold gauge style device need calibration for every single species for accurate readings chemistry sensitivity RGA s electronics are sensitive to the beam passage! And are still not cheap compared to gauges! F. Le Pimpec - SLAC 5 RGA

6 UHV - XHV Total Pressure - Xray limitation due to the e - hitting the grid : Ions are desorbed from the Collector. Remedies : Modulation - ESD from the gauges elements Reducing emission current : Wrong The grid will pump then release molecules - Installing a hot gauge in a small tube Transpiration effect Despite a higher pressure the gauge will read lower. Solution: nude gauges but sensitivity to stray ions from surroundings Modify Extractor gauge with hidden collector (U. Magdeburg) P = F A F. Le Pimpec - SLAC 6

7 UHV - XHV Partial Pressure The instrument of predilection is the Quadrupole Mass Analyzer The Ion source is identical to that of an ion gauge - Same ESD problem as for a gauge. ESD ion have higher energy than ionized gas Need to apply RF on the rod - Resolution, and the price, is dependent on the RF supply Kr Trace Sensitivity (A/Torr) is non-linear over few decades of pressure space charge & collision at HV At XHV range, there is no absolute calibration standard Ar Trace 10-7 Pressure (mbar)

8 Reaching XHV in Static Vacuum Reaching UHV from high vacuum is easy : Sputter/getter Ion pump To reach XHV Adding extra capture pumps Diode Cryopump : lump or distributed pumping (LHC cold bore) Evaporable Getter : Ti sublimator (lump pumping) Non Evaporable Getter pump (distributed pumping) Distributed Pumping XHV is possible but is not easy to reach because of outgassing F. Le Pimpec - SLAC 8

9 XHV Limit : Outgassing & Vapor Pressure At which temperature is my system going to be running? To minimize outgassing : Find a material with a low D coefficient Provide a diffusion d barrier Installed a vacuum cryostat Degass the material After Honig and Hook (1969) Vapor Pressure : True also for getters and cryosystem F. Le Pimpec - SLAC 9

10 Reaching Static XHV with NEG C. Benvenutti The LEP : 1 st major success of intensive use of NEG pumps LEP dipole chamber, getter St101 (ZrAl) ( ) ~24 km of NEG Ł 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 NLC/GLC??... F. Le Pimpec - SLAC 10

11 What are Getters? 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 Includes Evaporable and Non Evaporable Getter F. Le Pimpec - SLAC 11

12 How do Getters Work? Whatever the getter is, the same principle applies : Dissociation of residual gases on a surface is not systematic The use of a clean surface to form chemicals bonds Covalent bond (sharing of the e-) e Tied bonds : Chemisorption ev Ionic bonds (1 e-e is stolen by the most electro- elements (Mg+O( Mg+O-)) Metallic bonds (valence electrons shared) F. Le Pimpec - SLAC 12

13 Titanium vs. Other Evaporable Getters for Accelerator Use Ba - Ca - Mg : High vapor pressure. Trouble if bake out is requested Zr - Nb - Ta : Evaporation temperature too high Photo courtesy of Thermionics Laboratory, Inc Ref. Sorption of Nitrogen by Titanium Films, Harra and Hayward, Proc. Int. Symp. On Residual Gases in Electron Tubes, 1967 Wide variations due to film roughness For H 2, competition between desorption and diffusion inside the deposited layers Peel off of the film ~50µm Varian, Inc Typical required sublimation rate 0.1 to 0.5 g/hr F. Le Pimpec - SLAC 13

14 Non-Evaporable Getters NEGs are pure metals or are alloys of several metals ν = α P MT ν: molecules.s -1.cm -2 α: sticking coefficient P : Pressure (Torr) 1ML : ~10 15 molecules.cm -2 CO, N2, CO2, O2 H2 - 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. NEG CO, N2, CO2, O2 - Heating to very high temperature will outgas the getter. This regenerates it but also damages the crystal structure. NEG H2 F. Le Pimpec - SLAC 14

15 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 Lamps and vacuum tubes Ref [7] 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 15

16 What Makes NEG So Attractive? u u GREAT Material A GREAT High distributed pumping speed Initial photo, electro-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 - Pyrophoricity (200 C C to 700 C) Does not pump CH 4 at RT, nor noble gases Lifetime before replacement (thin film) F. Le Pimpec - SLAC 16

17 Pumping Speed 0.6 H 2 Ti 32 Zr 16 V 52 (at.%) Sticking probability CERN/EST group Pumping speed plots for getter are everywhere in the literature From sample to sample, pumping speed plots vary Hours Heating T ( C) 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 17

18 Insuring Dynamic UHV Beam Interaction Dynamic Outgassing should be studied for every surfaces susceptible of being used No existing coherent theory Source of gas are induced by photons (SR), electrons and ions bombardment F. Le Pimpec - SLAC 18

19 Photodesorption η at CO ε c = 194 ev 1.E-03 NEG St707 (Zr 70 V 25 Fe 5 ) ETA (molec/photon) 1.E-04 1.E-05 1.E-06 NEG 0% Sat ( 13 C 18 O) 13 C 18 O CO 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 19

20 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 20

21 Electrodesorption η CO at 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 21

22 Ion Desorption From Al surfaces Ion induced desorption yield A.G. Mathewson N 2+ at 2 kev 15 N 2+ at 2 kev M.H. Achard 1976 K + at 2 kev M.H. Achard-R. Calder-A.G. Mathewson 1978 K + at 1.4 kev M.P. Lozano 2001 Ar + at 3 kev Aluminium as received H 2 CH CO CO Aluminium after 24 hours baking at C (*) H 2 CH CO CO (*) 300 C in the measurement of M.H. Achard F. Le Pimpec - SLAC 22

23 Ion Desorption by Heavy Energetic Ions on Technical Surfaces Pb 53+ ions (per shot) under 89.2 grazing incidence and 4.2 MeV/u E. Mahner et al. NEG Ti 30 Zr 18 V 52 Measure at CERN for the LHC F. Le Pimpec - SLAC 23

24 Other Beam Interactions Electron cloud & multipacting Free electron trapping in a p + / e + bunch Ion instabilities link to the pressure - Pressure bump - Fast beam-ion collective instability Electron Cloud F. Le Pimpec - SLAC 24

25 SEY & Electron Cloud NLC Fast Head tail straight 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). M. Pivi 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 F. Le Pimpec - SLAC 25

26 Thin Film & Electron Cloud NLC: 130 ev e - beam conditioning Low SEY : Choice for the NEG of the activation Temperature and time. Conditioning (photons e - ions) Contamination by gas exposure, or by the vacuum residual gas, increases the SEY; even after conditioning. Angles of incidence, of the PE, change the shape of the curve at higher energy Roughness changes the SEY of a material TiZrV coating Variability from sample to sample TiN/SS Scheuerlein et al. F. Le Pimpec - SLAC 26

27 Alternative Solution: Playing with Roughness Very rough surfaces emits less SE, because SE can be intercepted by surrounding walls Al disk with triangular shape Experiment SEY Al flat - grooved result 1 mm Real SEY Cu α = 60 Simulation G. Stupakov F. Le Pimpec - SLAC 27

28 Ion Instability Pressure Bumps Ionized molecules are accelerated toward the wall by e + /p beam Linked directly to η I Dependant on surface cleanliness Dependant on the beam pulse structure e σ ( η I ) = S i critical i Ion impact energy as a function of beam current, LHC - Gröbner Runaway condition is possible above a certain threshold Surface with a low η Reduce the Pressure ( S)( Use of clearing electrodes F. Le Pimpec - SLAC 28

29 Fast Ion Instability Fast ion instability can arise in e - beam accelerator from ionization and trapping of the residual gas. The amplitude of displacement y b must be kept as small as possible due to requested luminosity Diminishing the pressure It is not, so far, a critical issue T. Raubenheimer F. Le Pimpec - SLAC 29

30 Conclusion Reaching and measuring static XHV is possible and will become necessary, as we push for higher luminosity A NEG barrier diffusion solution provides pumping speed, low (η ph η e- η i ), low SEY and will insure dynamic UHV Ion instability Pressure reduction Electron Cloud Issue The vacuum solution has to be beam-dynamic friendly Wakefield (electrical conductivity) due to a film thickness or surface roughness (or both) Lifetime of the solution (NEG) - % lifetime of the vacuum device Heat Load in a cryogenic system (e-cloud) F. Le Pimpec / SLAC-NLC 30

31 Acknowledgement SLAC : R. Kirby, M. Pivi,, T. Raubenheimer CERN : V. Baglin,, JM. Laurent, O. Gröbner bner, A. Mathewson F. Le Pimpec - SLAC 31

32 References 1. CAS Vacuum Technology: CERN H. Brinkmann Leybold Vacuum 3. R. Reid Daresbury Vac group 4. CERN Colleagues & web site 5. P. Danielson : Vacuum Lab 6. USPAS - June SAES getters 8. SLAC colleagues 9. Web request for the beautiful pictures F. Le Pimpec - SLAC 32