simulation of the lhc vacuum system with the …simulation of the lhc vacuum system with the vasco...
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G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
SIMULATION OF THE LHC VACUUM SYSTEM WITH THE VASCO CODE IN PRESENCE OF ELECTRON
CLOUDS
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G. Bregliozzi* & G. Lanza
CERN, European Organization for Nuclear Research1211 Geneva 23, Switzerland
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
THE LHC: AN OVERVIEW
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G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
LHC VACUUM LAYOUTRoom temperature
beam vacuum
6 km of RT beam vacuum in the long straight sections
Extensive use of NEG coatings
Pressure lower than 10-9 Pa after vacuum activation
2 independent beam pipes per arc: 8 arcs 2.8 km per arc
Pressure lower than 10-10 Pa
Cold beam vacuum
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
VASCO CODE: VACUUM MODEL FOR LHC
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The changing rate of the number of molecules per unit volume:
Molecular diffusion Beam induced dynamic effects: ion, electron and photon induced
molecular desorption. Gas pumping distributed along the beam pipe: NEG and Cryo Gas lumped pumping: Sputtered ion pumps
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
VASCO CODE: GENERAL EQUATIONS
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j
gegephgphgggg
jbjgji
gg
g qAnCvA
neI
xn
Dat
nV ,,,2
2
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Time variation of particles in
volume V
Diffusion through
surface a
Ionization by beam and
desorption by the ions
Distributed pumping of NEG or Cryo
Desorption by photons
Desorptionby electron
Thermaldesorption
Multi Gas Model
MULTI GAS MODEL Dominant gas species present in a vacuum system: H2, CH4, CO and CO2
The “multi gas” model takes into account that each of the gas species, once ionized, can desorbs any species both from the wall beam pipes or the condensed gas layer in a cryogenic system
The equation of each species depends on the gas densities of other species, and all the equations results inter-dependent
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
VASCO CODE: INPUT PARAMETERS FOR LHC
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Independent of Gas Species:
Geometry (cylindrical symmetry), length, temperature, electron and photons flux.
Local Pumps and Local Gas Source:
For the LHC Sputter ion pumps in l·s-1 and local source of gas in torr·l·s-1 .
Distributed Pumping:
For the LHC it represent the distributed pumping speed in l·s-1·m-1 of NEG coating and beam screen in the cryogenic sectors.
Ionization Cross Section:
Ionization cross section in m2 of molecules interacting the beam particles.
Desorption Yields:
Ions, electrons and photons stimulated desorption yields.
Thermal Outgassing
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
LHC REQUIREMENTS
The LHC, as all the particle accelerators, need an theestimation of residual gas density profiles to verify and confirmvacuum stability and beam lifetime.
In the experimental insertion regions density profiles areimportant to estimate machine background effects in thedetectors generated by proton or ion-gas scattering.
Beam induced dynamic effects such as ion, electron andphoton-stimulated gas desorption are the main source ofresidual gas.
For all the experiments of the LHC a pressure profile of eachLong Straight Sections (LSS) of the LHC is requested: about500 meters of the machine.
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G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
DYNAMIC PRESSURE IN THE LHC: ELECTRON CLOUD CASE
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G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
MEASURED PRESSURE RISE @ DIFFERENT LOCATIONS
SP ElectronsElectrons
Measured pressure over the LHC 9
Electron Cloud Effect
50 ns bunch spacing
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
SIMULATED STATIC PRESSURE PROFILE IN LSS1
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TCTATLASIP
D1
D2-Q4Q1-Q2-Q3 Q5 Q6 Q7
TCL XRPATCT D1
D2-Q4 Q1-Q2-Q3Q5Q6Recombination
chambers
Q7
TCLXRPA
RecombinationchambersARC 8-1 ARC 1-2
Interaction Point
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
SIMULATION: PRESSURE RISE IN THE INTERACTION POINT
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ATLASIP
Inner Triplet
Electron flux of 1.1016 [e/ms] Cold Warm Transition
Measuring Point
Inner Triplet
Cold Warm Transition
Measuring Point
Maximum Pressure Increase
CryoCryo NEG Coated Beam Pipes
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
SIMULATION: GAS COMPOSITION IN THE INTERACTION POINT
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CH4 Highest gas because lowest pumping speedGas not pumped by the NEG coating
NEG Coated Beam Pipes CryoCold-Warm TransitionSS Vacuum Module
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
VASCO: INPUT PARAMETERS FOR LHC
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Independent of Gas Species: OK
Geometry (cylindrical symmetry), length, temperature, electron and photons flux.
Local Pumps and Gas Source: OK
For the LHC Sputter ion pumps in l·s-1 and local source of gas in torr·l·s-1 .
Distributed Pumping: OK
For the LHC it represent the distributed pumping speed in l·s-1·m-1 of NEG coating and Bean screen in the cryogenic sectors.
Ionization Cross Section: OK
Ionization cross section in m2 of molecules interacting the beam particles.
Thermal Outgassing: OK
Desorption Yields: Electron Stimulated Desorption Yield Need to perform detailed study; Dose dependent.
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
LHC MACHINE DEVELOPMENTS STUDIES
FOR ELECTRON CLOUD
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G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
MAXIMUM PRESSURE WITH 1020 BUNCHES - 50 NS
Stainless Steel Unbaked Transition PMAX ≈ 410-8 mbar
Stainless Steel Baked Transition PMAX ≈ 210-9 mbar
TCTATLASIP
D1
D2-Q4Q1-Q2-Q3 Q5 Q6 Q7
TCL XRPATCT D1
D2-Q4 Q1-Q2-Q3Q5Q6Recombination
chambers
Q7
TCLXRPA
NEG ATLAS IP- PMAX ≈ 510-10 mbar
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G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
SIMULATED ELECTRON CLOUD EFFECTS
Uniformly distributed electron flux = 1 1016 [e/sm]
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
DISCUSSION & SUMMARY
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• VASCO code very fruitful to estimates the residual gas density for LHCoperations: case of dynamic vacuum;
• The code could be easily used to simulate 500m of the machine bysplitting it in about 200 segments;
• Pressure and gas density distribution could be used for estimatemachine background effects;
• Pressure estimates depends on the input parameters: snapshot in time;
• Calculation of residual gas pressure during stable beam may alsodepend on a transient during the beam cycle that causes particlelosses, beam displacement, collimator setting, magnetic filed: In orderto have a precise and detailed gas density profile it is necessary tostudy case by case
G. Bregliozzi - 64th IUVSTA Workshop - May 16-19. 2011 - Leinsweiler, Germany
OTHER POINTS OF INTEREST
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Where simulations could been used:
• NEG lifetime:• Determine and foreseen air leaks and internal leaks in long NEG
coated beam pipes;
• Degassing due to collimators jaws movements, desorption due tobeam impingement and or possible temperature rise;
• Synchrotron radiation: photon reflectivity;
• Beam screen temperature oscillation;
• For next shutdown there will be a complete new design of someexperimental beam pipes: critical current and vacuum stability.
• Deep study of transient during beam cycles.