INVESTIGATION OF NON-EVAPORABLE GETTER FILMS

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INVESTIGATION OF NON-EVAPORABLE GETTER FILMS

• O. B. Malyshev, K.J. Middleman,
• A. Hannah and S. Patel
• ASTeC Vacuum Science Group, STFC Daresbury
  Laboratory, UK
• J.S. Colligon, R. Valizadeh and V. Vishnyakov
• Department of Chemistry,
• Manchester Metropolitan University, UK

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What are usual considerations for vacuum

• Required pressure P is defined by gas desorption
  Q in the vessel and effective pumping speed Seff.
  
• In a simple case it is

P
Q
U (l/s)
Pump, S (l/s)
Thermal, photon, electron and ion stimulated
desorption
3
Usual accelerator vacuum chamber

• Average pressure depends on vacuum conductance u
  of the beam vacuum chamber, which depends on the
  cross section and the length L

4
Vacuum chamber with a distributed pump
B

• SIP in dipole and quadrupole magnetic field
• Does not pump when magnets off
• Requires HV supply
• Getter strip in LEP at CERN
• Does not pump Noble gases and CxHy
• Requires activation

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NEG coated vacuum chamber

• Non-Evaporable Getter (NEG) coating magnetron
  sputtered onto the inner walls recent innovation
  technique developed at CERN and is an attractive
  solution for many UHV applications.
• One such application is in the vacuum systems of
  particle accelerators that have to be designed so
  as to provide sufficiently low pressure in the
  beam pipe during machine operation
• NEG film have to be optimised to exhibit low
  photon, electron and ion stimulated desorption
  yields and reduce secondary electron emission.
• Pumping speed and capacity are important
  parameters for design.
• There are a number of issues which are still not
  yet fully understood to engineer, to optimise and
  to use such coatings.

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Why Do We Want to Coat the Chambers with NEG?

• Accelerator chambers have limited conductance of
  a few l/(s?m). Especially in the insertion device
  chambers with gaps of 10mm.
• Need 10-10 mbar to reduce Bremsstrahlung
  radiation ? Linear pumping
• Photons and charged particles will desorb
  electrons and molecules and impact the lifetime
  and stability of the beam

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Source of Gas in a Vacuum System
Vacuum Subsurface Bulk
layers

• Thermal ,photon, electron or ion stimulated
  desorption
• Molecules diffusing through the bulk material
  (mainly subsurface layers) of the vacuum chamber,
  entering the surface and desorbing from it
• Molecules adsorbed on the surface (initially or
  after the air venting) and desorbing when vacuum
  chamber is pumped
• Outgassing rate depends on many factors
  choice of material, cleaning procedure, pumping
  time, bombardment (irradiation) dose, etc...

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What NEG coating does
Vacuum NEG Subsurface Bulk
Coating Layers

• A pure metal film 1?m thick without
  contaminants.
• A barrier for molecules from the bulk of vacuum
  chamber.
• A sorbing surface of entire vacuum chamber
  surface

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Stainless steel vs NEG coated vacuum chamber
under SR
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Study and optimising the NEG coatings

• Collaboration between ASTeC and MMU was set-up
• Surface science
• NEG film deposition (existing and new
  technologies)
• NEG film surface analysis with SEM, XPS, RBS,
  etc.
• Vacuum science
• Pumping properties evaluation
• Gas dynamics modelling
• Photon, electron, ion stimulated desorption
• PEY and SEY
• Application to accelerator design (coating
  geometry, pumping scheme, activation procedure,
  etc.)
• Gas dynamic model in accelerator beam chamber

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Why Do We Want to Coat the Chambers with NEG?

• ? ? e-, M ? ?P ? ?, BS
• e- ? e- (SEY), M ? ?P ? ?, and cause multipacting
  and e-cloud
• in e hadron machine
• M ? ?P ? ?, stability
• NEG coated surface will
• reduce the surface desorption yields induced by
  photons ?, electrons e- and ions M
• provide pumping which in turn minimizing the
  desorption
• provide low SEY to suppress multipacting (which
  reduces electron stimulated desorption flux) and
  e-cloud

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NEG coating is a technology for UHV and XHV
The CO capacity of the NEG coating is about 1
monolayer for CO and CO2

• If pressure during activation is 10-9 mbar, then
  the amount of molecules hitting the wall is an
  equivalent of
• If pressure of NEG-sorbing gases (CO, CO2, H2O)
  during activation
• P gt 10-10 mbar gt
• the NEG film is continuously poisoning by these
  gases gt
• the activation is not full

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The conditions for NEG film activation

• To allow NEG film to be activated and not to be
  poisoned by residual gas molecules for the
  duration of the experiment
• The background pressure due to thermal desorption
  from uncoated part should be better than 10-11
  mbar for CO, CO2, H2O, O2 and N2
• NEG film activation must be performed only after
  the bakeout of the uncoated parts of vacuum
  chamber, when desorption from uncoated parts of
  the test system is low
• the temperature of the test chamber and the NEG
  coated sample should be maintained independently
  (separate heaters and air or water cooling).
• The area and capacity of uncoated parts should be
  much smaller than NEG coated one to avoid NEG
  saturation during and after (re-)activation for
  the duration of time until the gas injection
  experiment started.
• No short pressure increase can be tolerated
  after NEG coating activation.
• ex. to switching on the gauge and the RGA, by
  opening or closing a valve, etc.

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Sample deposition
Solenoid magnetron deposition
Planar magnetron deposition
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Set-up for NEG pumping evaluation
Sticking probability ? is calculated from
pressure measurements during gas injection using
the results of TPMC
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Usual activation procedure
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Reducing of CO, CO2 and H2O pressure in the open
geometry set-up

• There is an area where temperature changes from
  the temperature of the NEG coated sample TNEG to
  the temperature of the rest of vacuum chamber
  TVC
• During the set-up bake-out this are is
  under-baked
• During the NEG activation this area temperature
  is higher than TVC and outgases. It might be the
  main source of gas.

Area with transitional temperature
Cooling channel
TNEG TVC
Test sample with NEG coating
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ASTeC activation procedure
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NEG film density

• Four TiZrV coated cup sample were prepared
• Cup 1 thin and columnar
• Cup 2 two times thicker and columnar
• Cup 3 4 dense and thick as Cup 2

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Cap 3
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Comparison of three samples
- Thin - Thick - Dense
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SEM images of films
Cup 1 2 Cup 3 and 4 columnar dense
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NEG film composition

• Different combination of Ti, Zr, V and Hf
• Same deposition parameters
• Binary, ternary and quadruple alloys

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TiV binary alloy coating
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Triple alloy coating
TiZrV TiHfV
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TiZrHfV quadruple alloy coating
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Binary alloy coatings TiV, TiZr, ZrV
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Ternary alloy coatings
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TiZrHfV quadruple alloy coating
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Application to ILC

• Pressure along the arc
• inside an aluminium tube
• Bakeout at 220?C
• A pump with 200 l/s every 5 m
• H2, CO and CO2
• Inside NEG coated tube
• Activation at 160-180 ?C
• A pump with 20 l/s every 30-40 m
• H2 and CH4

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Conclusions

• New bakeout/activation procedure developed at
  ASTeC to minimise the NEG film poisoning from
  uncoated parts.
• Measured results dont depend on injected gas
  flow rate.
• Columnar NEG structure, required for higher
  pumping speed and sorption capacity, is formed at
  higher pressure.
• Reduced pumping speed and sorption capacity
  measured for dense films deposited either at
  lower pressures or by pulsed sputtering.
• Larger number of element in the target allows
  reducing the grain size of the film which, in
  turn, increase the molecule diffusion along grain
  boundaries and led to a lower activation
  temperature.
• All together allows engineering the films with
  different properties

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Future investigations

• New understanding of physics and chemistry of the
  NEG coating allows to the next stage
• engineering of NEG coating with necessary
  properties
• Study dynamic properties of new coatings such as
  
• NEG as low dynamic outgassing
• NEG as low SEY coating
• Combination of both
• Accumulated experience allows
• Designing vacuum systems of new accelerators
  considering NEG coating and applying it where it
  is beneficial (DLS, ILC, NLS, FAIR, CLIC)
• Using the NEG coating in the appropriate way.