Vacuum Stability Issues

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Vacuum Stability Issues in the New Inner Triplet
V. Baglin
CERN AT-VAC, Geneva
1. Vacuum physics in LHC 2. Vacuum requirements
for IT phase 1 3. Beam screens 4. Conclusions
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• 1. Vacuum physics in LHC
• Photon stimulated desorption
• photon stimulated desorption
• Electron multipacting (electron cloud)
• electron stimulated desorption
• heat load onto the beam screen
• emittance growth
• Ion induced pressure instability
• Ion stimulated desorption
• pressure runaway
• beam loss

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• Ion induced pressure instability ISR
• Beam current stacking to 1 A
• Pressure increases to 10-6 Torr
• (x 50 in a minute)
• Beam losses

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Ion induced pressure instability model
Reduction of the net pumping speed

• Increase pumping speed (large conductance)
• Decrease desorption yield (cleanliness)

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• 2. Vacuum requirement for IT phase 1
• Ensure vacuum stability
• Provide an equivalent gas density less than 1013
  H2/m3 i.e. 2.5 10-11 Torr at RT (A. Rossi, LHC
  PR 674).
• IT characteristics
• An operating temperature of 1.9 K
• A gradient 120 T/m gradient
• A length of 10 m long for each quadrupole and a
  total of 40 m for the IT
• A cold bore diameter of 15 cm
• Due to the crossing angle, the beam are 5 mm
  off-axis in the IT. This generates a synchrotron
  radiation of 3 eV critical energy and 1/10 of
  the nominal LHC arc photon flux.
• D1 generates also synchrotron radiation in the
  IT of 6 eV critical energy and 1/4 of the
  nominal LHC arc photon flux (I.R. Collins, O.B.
  Malyshev, LHC PN 274).
• Even in the case of the absence of multipacting,
  a gas load will be due to photon stimulated
  desorption
• This gas will condense on the cold bore

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• Parameters at cryogenic temperature
• ?H2 1 000 at 1 keV and 1 monolayer
• (N. Hilleret, R. Calder, IVC, 1977).
• The critical current is given by
• It is driven by the geometry, the gas species,
  the sticking probability, the primary and the
  recycling desorption yields

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• Option without beam screen
• Critical current
• Given the IT diameter (15 cm), a vacuum chamber
  longer than 5 m is considered to be infinite so
  that the pumping speed at the ends remain
  negligible.
• We assume a sticking coefficient of unity
  (optimistic case)
• Critical current

?H2 2 000 at 1 monolayer
?co 200 at 1 monolayer
When the surface coverage approaches a few
monolayers, the vacuum may become unstable
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Option without beam screen When is the pressure
runaway reached ?
H2
CO
The pressure runaway could be reached within 100
days and
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Option beam screen When is the gas density limit
reached ?
? 1, photon flux 1/10 of arc
The gas density limit is reached in a few days !!
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So, we need to control the gas density
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• Option with beam screen

V. Baglin et al., LHC PR 435
A perforated beam screen allows to control the
gas density
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• 3. Beam screens
• Functionalities
• Provide an equilibrium density and an
  equilibrium surface coverage defined by the
  holes pumping speed, C
• Beam screens have a critical current of
• In the arcs (?ion I)crit 30 A for CO2 to 103
  A for H2
• Actively cooled beam screens avoid thermal
  transients and therefore vacuum transients
• Beam screens heaters allow a warm up to remove
  the condensed gas
• The transparency of the beam screen defines the
  level of the pressure and of the surface coverage

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• Operation of beam screens
• Beam screens operate in the range of 5 to 20 K
  (operation above 20 K must be avoided)
• The cold bore at 1.9 K provides large pumping
  capacity for all gases except helium.
• To avoid vacuum transients during operation, the
  beam screen temperature must be held above the
  cold bore temperature during cool down
• After a quench, or in the case of oscillation in
  temperature, the beam screen must be warm up to
  flush the gas towards the cold bore
• In the case of large heat load and / or pressure
  increase due to thick coverage of gases, the beam
  screen must be warm up to flush the gas towards
  the cold bore

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• Beam screen short term developments for IT phase
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• Define the required transparency i.e. estimate
  of the gas load
• Define the geometry, the material
• Built prototypes
• Validate
• Production

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• Beam screen long term developments for LHC
  upgrade
• Learn from LHC operation
• Investigate and validate electron cloud
  mitigations (clearing electrodes, grooved
  chambers )
• Define beam screen parameters
• Built prototypes
• Scientific and engineering validation
• Production

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• 4. Conclusion
• The vacuum group recommends the installation of
  beam screens in the IT
• A scaling of the present design is proposed in
  a first phase
• After acquisition of know-how, upgrading of the
  beam screen technology could be foreseen for the
  LHC upgrade

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• Some references
• Ion induced desorption coefficients for titanium
  alloy, pure aluminium and stainless steel. A.G.
  Mathewson. ISR-VA/76-5, March 1976.
• Ion desorption of condensed gases. N. Hilleret,
  R. Calder. IVC 7, 1977.
• Overview of the LHC vacuum system. O. Gröbner.
  Vacuum 60 (2001) 25-34.
• Ion desorption stability ion the LHC. O.B.
  Malyshev, A. Rossi. VTN 99-20, December 1999.
• First results from COLDEX applicable to the LHC
  cryogenic system. V. Baglin et al. LHC PR 435,
  September 2000.
• Dynamic gas density in the LHC interaction
  regions 15 and 28 for optic version 6.3. I.R.
  Collins, O.B. Malyshev. LHC PN 274, December
  2001.
• Synchrotron radiation studies of the LHC dipole
  beam screen with COLDEX. V. Baglin et al. LHC PR
  584, July 2002.
• Running-in commissioning with beam. V. Baglin.
  LHC Performance Workshop Chamonix XII, January
  2003
• Residual gas density estimations in the LHC
  experimental interaction regions. A. Rossi, N.
  Hilleret. LHC PR 674, September 2003.
• Vacuum transients during LHC operation. V.
  Baglin. LHC Project Workshop Chamonix XIII,
  January 2004.
• Performance of a cryogenic vacuum system
  (COLDEX) with an LHC type beam. V. Baglin et al.
  Vacuum 73 (2004) 201-206.
• Gas condensates onto a LHC type cryogenic vacuum
  system subjected to electron cloud. V. Baglin, B.
  Jenninger. LHC PR 742, August 2004.
• Results from the scrubbing run 2004. N.
  Hilleret. LHC MAC December 2004.

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• SEY of gas condensate

N. Hilleret. LHC MAC December 2004