1. Beam-Surface Interaction A Vacuum point of view
F. Le Pimpec
SLAC/NLC
Cornell May 2004

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

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

4. Reaching and Measuring XHV (10-12 Torr)
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
F. Le Pimpec - SLAC

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
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 !
BA –SVT305
RGA
F. Le Pimpec - SLAC

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
Modify Extractor gauge with hidden collector (U. Magdeburg)
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
F. Le Pimpec - SLAC

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
 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
Kr Trace
Pressure (mbar)
10-7
10-4

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

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 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

10. Reaching Static XHV with NEG
The LEP : 1st major success of intensive use of NEG pumps
LEP dipole chamber, getter St101 (ZrAl) ( )
Inserted “linear” pump
Inserted “total” pump
(TiZrV) Surface pump / diffusion barrier
Lump pumping
C. Benvenutti
Thin film getter is the new adopted way of insuring UHV in colliders or SR light sources
~24 km of NEG  P~10-12 Torr range
DAFNE
ESRF
SOLEIL
DIAMOND
RHIC
LHC
NLC/GLC ??...
TiZrV NEG Coating Setup at CERN
F. Le Pimpec - SLAC

11. What are Getters ? Getters are Capture Pumps
 Cryopumps and Sputter/getter-ion pumps are also capture pumps.
 Differentiation is needed
Physical getters (Zeolite)
Work at LN2 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

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

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
Varian, 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 H2, competition between desorption and diffusion inside the deposited layers
 Peel off of the film ~50m
Typical required sublimation rate 0.1 to 0.5 g/hr
F. Le Pimpec - SLAC

14. Non-Evaporable Getters
NEGs are pure metals or are alloys of several metals
: molecules.s-1.cm-2
: sticking coefficient
P : Pressure (Torr)
1ML : ~1015 molecules.cm-2
- 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 H2.
- Heating to “very high” temperature will outgas the getter. This regenerates it but also damages the crystal structure.
F. Le Pimpec - SLAC

15. Non-Evaporable Getters : Uses
St 707 (ZrVFe)
Pump cartridge for Ion Pump or as lump pumps
Use of St 2002 pills to insure a vacuum of 10-3 Torr
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 …
F. Le Pimpec - SLAC
Ref [7]

16. What Makes NEG So Attractive?
A GREAT Material
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 to 700°C)
Does not pump CH4 at RT, nor noble gases
Lifetime before replacement (thin film)
F. Le Pimpec - SLAC

17. Pumping Speed
0.6 H2
Ti32Zr16V52 (at.%)
0.01
Sticking probability
0.005
100
350
2 Hours Heating T (°C)
CERN/EST group
Pumping speed plots for getter are everywhere in the literature
From sample to sample, pumping speed plots vary
Many geometric cm2 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

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

19. Photodesorption hCO at c = 194 eV
Sat (13C18O) CO
Sat (13C18O) 13C18O SS
NEG 0%
NEG 100 %
NEG St707 (Zr70V25Fe5)
An activated NEG desorbs less H2 CO CH4 CO2 than a 300°C baked SS A saturated NEG desorbs more CO than a baked Stainless Steel
F. Le Pimpec - SLAC

20. Also True For Thin films TiZr and TiZrVSS Cu
F. Le Pimpec - SLAC

21. Electrodesorption hCO at Ee- = 300 eVCu
NEG Sat by CO
NEG Sat (13C18O) CO
NEG Sat (13C18O) 13C18O
NEG 100 % CO
NEG St707
An activated NEG desorbs less H2 CO CH4 CO2 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

22. Ion Desorption From Al surfaces
Ion induced desorption yield
A.G. Mathewson
M.H. Achard
M.H. Achard-R. Calder-A.G. Mathewson
M.P. Lozano
1976
1978
2001
15N2+ at 2 keV
K+ at 2 keV
K+ at 1.4 keV
Ar+ at 3 keV
Aluminium
as received H2
4.5 – 10
2.3 18
4 - 7
CH4
0.55 – 0.95
0.2 1
0.5 – 0.8 CO
7 – 10
2.5 7
0.9 – 1.5
CO2
1.8 – 3.2
0.5
1.2
1 – 2.5
Aluminium after 24 hours baking at 2000C (*)
3.2 – 4
3.2
0.22 – 0.23
0.32
2.8 – 2.9
2.2
1.5
0.18
0.35
F. Le Pimpec - SLAC
(*) 300°C in the measurement of M.H. Achard

23. Ion Desorption by Heavy Energetic Ions on Technical Surfaces
Pb53+ ions (per shot) under 89.2° grazing incidence and 4.2 MeV/u
E. Mahner et al.
NEG Ti30Zr18V52
F. Le Pimpec - SLAC
Measure at CERN for the LHC

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
F. Le Pimpec - SLAC
Electron Cloud

25. NLC Fast Head tail straight 1012
SEY & Electron Cloud
NLC Fast Head tail straight 1012
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).
SEY of technical surfaces baked at 350°C for 24hrs
M. Pivi
F. Le Pimpec - SLAC

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
Scheuerlein et al.
TiZrV coating
Variability from sample to sample
TiN/SS
F. Le Pimpec - SLAC

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

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

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

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 (ph e- 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

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

32. References CAS Vacuum Technology: CERN 99-05
H. Brinkmann –Leybold Vacuum
R. Reid – Daresbury Vac group
CERN – Colleagues & web site
P. Danielson : Vacuum Lab
USPAS - June 2002
SAES getters
SLAC – colleagues
Web request for the beautiful pictures …
F. Le Pimpec - SLAC