Beam surface interaction a vacuum point of view
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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

SLAC/NLC

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


Outline

Outline

 Measuring and Reaching XHV

 XHV with Getters

 Beam Interaction with Technical surfaces- Desorption Induced by ElectronicTransition

- 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

RGA

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

10-7

10-4


Reaching xhv in static vacuum

Distributed Pumping

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

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

DAFNE

ESRF

SOLEIL

DIAMOND

RHIC

LHC

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.

    • : molecules.s-1.cm-2

      : sticking coefficient

      P : Pressure (Torr)

      1ML : ~1015 molecules.cm-2

    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

    0.6

    H2

    Ti32Zr16V52 (at.%)

    0.01

    Sticking probability

    0.005

    0

    100

    350

    2 Hours Heating T (°C)

    CERN/ESTgroup

    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

    CO

    NEG 0%

    Sat (13C18O) 13C18O

    SS

    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


    Also true for thin films tizr and tizrv

    SS

    Cu

    Also True For Thin films TiZr and TiZrV

    F. Le Pimpec - SLAC


    Electrodesorption h co at e e 300 ev

    CO

    NEG Sat (13C18O) 13C18O

    NEG Sat by CO

    Cu

    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

    TiN/SS

    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”

    Experiment

    SEY Al flat - grooved result

    Simulation

    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


    Conclusion

    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


    Acknowledgement

    Acknowledgement

    SLAC :

    R. Kirby, M. Pivi, T. Raubenheimer

    CERN :

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

    F. Le Pimpec - SLAC


    References

    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


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