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Beam-Surface Interaction A Vacuum point of view. F. Le Pimpec SLAC/NLC. Cornell May 2004. Dynamic Vacuum. You want to address the terms of this formula. How to measure the Pressure ?. Outline.  Measuring and Reaching XHV  XHV with Getters

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beam surface interaction a vacuum point of view

Beam-Surface Interaction A Vacuum point of view

F. Le Pimpec


Cornell May 2004

dynamic vacuum
Dynamic Vacuum

You want to address the terms of this formula

How to measure the Pressure ?

F. Le Pimpec - SLAC


 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

reaching and measuring xhv 10 12 torr
Reaching and Measuring XHV(10-12 Torr)

Luminosity for accelerators

Lifetimein storage rings

Reaching XHV is commercially easier than measuring it

A CERN modified Helmer gauge measured 10-14 Torr

XHV is not official Pressure  10-7 Torr are called UHV

F. Le Pimpec - SLAC

why measure total pressure
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


F. Le Pimpec - SLAC

uhv xhv total pressure
Modify Extractor gauge with hidden collector (U. Magdeburg)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 : WrongThe grid will pump then release molecules
  • Installing a hot gauge in a small tube – Transpiration effectDespite a higher pressure thegauge will read lower. Solution: nude gauges – but sensitivity to stray ions from surroundings

F. Le Pimpec - SLAC

uhv xhv partial pressure
Kr Trace

Ar Trace

Pressure (mbar)

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



reaching xhv in static vacuum
Distributed PumpingReaching 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)


XHV is possible but is not easy to reach because of outgassing

F. Le Pimpec - SLAC

xhv limit outgassing vapor pressure
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

reaching static xhv with neg
C. Benvenutti

Lump pumping

LEP dipole chamber, getter St101 (ZrAl) (1989-2000)

Inserted “linear” pump

Inserted “total” pump

(TiZrV) Surface pump / diffusion barrier

Reaching Static XHV with NEG

The LEP : 1st major success of intensive use of NEG pumps

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







NLC/GLC ??...

TiZrV NEG Coating Setup at CERN

F. Le Pimpec - SLAC

what are getters getters are capture pumps
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

how do getters work
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

titanium vs other evaporable getters for accelerator use
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

Varian, Inc

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

 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

non evaporable getters
Non-Evaporable Getters

NEGs are pure metals or are alloys of several metals

- 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.

  • :

: sticking coefficient

P : Pressure (Torr)

1ML : ~1015

F. Le Pimpec - SLAC

non evaporable getters uses
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]

what makes neg so attractive
What Makes NEG So Attractive?
  • AGREATMaterial
    • 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

pumping speed
Pumping Speed



Ti32Zr16V52 (at.%)


Sticking probability





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 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
insuring dynamic uhv beam interaction
Insuring Dynamic UHVBeam 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

photodesorption h co at c 194 ev

NEG 0%

Sat (13C18O) 13C18O


NEG 100 %

Sat (13C18O) CO

Photodesorption hCO at c = 194 eV

NEG St707 (Zr70V25Fe5)

An activated NEG desorbs less H2 CO CH4 CO2 than a 300°C baked SSA saturated NEGdesorbs more CO than a baked Stainless Steel

F. Le Pimpec - SLAC

electrodesorption h co at e e 300 ev

NEG Sat (13C18O) 13C18O

NEG Sat by CO


NEG 100 %

NEG Sat (13C18O) CO

Electrodesorption hCO at Ee- = 300 eV

NEG St707

An activated NEG desorbs less H2 CO CH4 CO2 than a 120°C baked OFE Cu surface. A saturated NEGdesorbs less *C*O than a 120 °C baked OFE Cu surface

F. Le Pimpec - SLAC

ion desorption from al surfaces
Ion Desorption From Al surfaces

F. Le Pimpec - SLAC

(*) 300°C in the measurement of M.H. Achard

ion desorption by heavy energetic ions on technical surfaces
Ion Desorption by Heavy Energetic Ions on Technical Surfaces

1.5 109 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

other beam interactions
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

sey electron cloud
NLC Fast Head tail straight 1012

SEY of technical surfaces baked at 350°C for 24hrs

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).

M. Pivi

F. Le Pimpec - SLAC

thin film electron cloud
NLC: 130 eV e-beam conditioning

TiZrV coating

Scheuerlein et al.

Thin Film & Electron Cloud

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

Variability from sample to sample


F. Le Pimpec - SLAC

alternative solution playing with roughness
Al disk with triangular shape

1 mm

Real SEY Cu

 = 60°

Alternative Solution:Playing with Roughness

Very rough surfaces emits less SE, because SE can be intercepted by surrounding “walls”


SEY Al flat - grooved result


F. Le Pimpec - SLAC

G. Stupakov

ion instability pressure bumps
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

fast ion instability
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


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



R. Kirby, M. Pivi, T. Raubenheimer


V. Baglin, JM. Laurent, O. Gröbner,A. Mathewson

F. Le Pimpec - SLAC

  • 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