Expected vacuum performance in LHCb

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Expected vacuum performance in LHCb

• Adriana Rossi
• With thanks to J.R. Knaster

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Contents

• LHCb layout
• Vacuum stability for ion induced desorption
• Expected dynamic pressure profile
• General comments to the current design
• Main open questions for LHCb
• Conclusions

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LHCb layout

• Vertex Tank (Al) - Larger lateral gap
• Approximated to cylinders of same lateral pumping
• Cone (NEG coated)
• Approximated to cylinders
• Lumped pumps
• Sputter ion pumps (SIP) on Vertex Tank
• SIP sublimation pumps at 19.7 m from IP (cone
  end)
• Geometry for calculations
• From -1.2 m to 21.5 m from IP considered
• Section upstream to be defined (freedom)

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LHCb layout
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Vacuum stability for ion induced desorption

• Assumptions
• Vertex Tank
• Unbaked
• Ion desorption yield (hi) as for unbaked surface,
  with ion incident energy 300 eV.
• Cone
• Saturated not pumping
• hi as for baked surface, ion incident energy
  300 eV.
• hi as for unbaked surface, ion incident energy
  300 eV
• To be verified - measurements on going.
• Weak dependence of ion energy with b function
  neglected

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Vacuum stability for ion induced desorption

• Results Critical current
• Calculated with the single gas model, in the
  VD closed position
• The vacuum is said to be stable if
• Icrit gt 3.4 A

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Vacuum stability for ion induced desorption

• Results (2)
• The vacuum is unstable if the saturated NEG hi is
  as large as for an not conditioned stainless
  steel.
• Baking of the Vertex Tank at T 150ºC is being
  included in the new design. Main advantages
  longer time to saturate the NEG and lower hi ?
  stability
• The pump at the end of the cone ( 19.7 m from
  IP) is indispensable for vacuum stability.

The pump _at_ cone end may be removed after
experience with ops
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Expected dynamic pressure profile

• Assumptions
• Vertex Tank
• Photon desorption yield as for unbaked surface,
  with photon incident energy 12 eV
• Cone
• Photon desorption yield as for baked and glow
  discharge cleaned surface (to be verified -
  measurements on going), with ion incident energy
  300 eV.
• Electron induced desorption not taken into
  account Assumption valid in the absence of
  electron multipacting

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Expected dynamic pressure profile

• Results
• Note There is no stringent request on vacuum
  performance from the LHCb experiment

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General comments to the current design(1)

• Visit to NIKHEF and to DESY - Hera-B
• Report to be issued
• NIKHEF
• Concerns expressed by LHC/VAC taken into
  consideration. Solutions have been proposed.
• Vacuum scenarios (pump-down, venting and beam
  operations) outlined.
• VD foil feasibility and tests on going.
• CO2 2 phases also at ambient temperature ? no
  overpressure foreseen due to T changes.
• Calculations show cooling capillaries can
  withstand several hundreds bars.

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General comments to the current design(2)

• HERA-B
• Similar concept.
• No safety system in between primary and secondary
  vacua.
• Vacuum issues less critical.
• Baking not possible.
• Long pump-down at beginning of operations (due to
  secondary vacuum pressure) but faster (1/2 day)
  after following venting.
• No vacuum accidents.

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Main open questions for LHCb (1)

• VD foil thicknessWaiting for calculations and
  test results. Critical pressure to be decided
  depending on the criteria chosen by CERN/LHC.
  Accident scenario (for plastic deformation and
  rupture) to be described, with the consequences
  on machine running.
• Gravity valvesDynamic response in realistic
  geometry to be tested. Limits to be assessed.
• Pump-down and venting scenarioPrototype and
  tests requested.
• Secondary vacuum outgassingMeasurements
  requested

Fear that 15mbar not enough
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Main open questions for LHCb (2)

• Seal between primary and secondary vacuumIn
  present design one all metal seal and one Viton
  seal.
• Baking demonstration
• Cooling system reliabilityMain concern vacuum
  brazing on 0.1 mm Stainless Steel tubes for 15
  bar CO2 circulation.
• Bellow between primary and secondary vacua
• Fast shutters foreseen after the low b quads
• Electron cloud study for the LHCb
  geometryCoating of the VD Al foil with low SEY
  materials (NEG, TiN, in situ Ti evaporation or
  others) may result necessary

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Conclusions

• The LHC/VAC group accepts
• Foil not withstanding atmospheric pressure -
  compromise between safety and physics
  performances.
• 2 phases CO2 cooling system in secondary vacuum.
• Provided that
• Risk assessment complies with LHC standards.
• Design developments in close collaboration/supervi
  sion with CERN.
• Prototype and testing prove principles.
• Replacement vacuum chamber in the case of major
  accident.
• Machine and other experiments agree.