SIMULATION OF THE LHC VACUUM SYSTEM WITH THE …SIMULATION OF THE LHC VACUUM SYSTEM WITH THE VASCO...

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

* [email protected]


THE LHC: AN OVERVIEW

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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


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


VASCO CODE: GENERAL EQUATIONS

5

j

gegephgphgggg

jbjgji

gg

g qAnCvA

neI

xn

Dat

nV ,,,2

2

4

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


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


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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DYNAMIC PRESSURE IN THE LHC: ELECTRON CLOUD CASE

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MEASURED PRESSURE RISE @ DIFFERENT LOCATIONS

SP ElectronsElectrons

Measured pressure over the LHC 9

Electron Cloud Effect

50 ns bunch spacing


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


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


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


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.


LHC MACHINE DEVELOPMENTS STUDIES

FOR ELECTRON CLOUD

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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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SIMULATED ELECTRON CLOUD EFFECTS

Uniformly distributed electron flux = 1 1016 [e/sm]


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


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.

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