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Adriana Rossi General equation VASCO code assumptions and solution

VASCO (VAcuum Stability COde) : multi-gas code to calculate gas density profile and vacuum stability in a UHV system. Adriana Rossi General equation VASCO code assumptions and solution Comparison between Single and Multi-Gas models Comparison between VASCO and MC (Pedro Costa-Pinto)

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Adriana Rossi General equation VASCO code assumptions and solution

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  1. VASCO (VAcuum Stability COde) : multi-gas code to calculate gas density profile and vacuum stability in a UHV system Adriana Rossi General equation VASCO code assumptions and solution Comparison between Single and Multi-Gas models Comparison between VASCO and MC (Pedro Costa-Pinto) Discussion on input parameters and example of IR8 results (with real data) VASCO documentation and installation

  2. Equation • Level of water in a sink depends on: • Flow of water from the tap = source • Flow of water through the drain = sink • After transient level stabilises only if source = sink Pressure (density) in a vacuum tube depends on • Sources : • Net contribution from diffusion • Thermal desorption. • Beam induced phenomena:ion, electron and photon induced molecular desorption. • Localised sources • Sink: • Localised pumps • Distributed pumps (NEG or cryo) 2

  3. Equation describing the gas density for each gas species      Time variation DiffusionIonisation by beam Distributed pumping Desorption of particles in throughand desorption by by NEG or by photonsby electronthermal volume Vsurface athe ions by beam screen Multi gas model Single gas model 3

  4. VASCO code • Cylindrical symmetry • Average density across the area • Time invariant parameters(snapshot in time at steady state) • Surface parameters (sticking and desorption coefficients) constant (not dependent on dose , selected for a specific incident energy) • Maxwell-Boltzmann distribution of molecular velocity • Assumption of uniform distribution in space diffusion coefficient average number of particle hitting the surface area 4

  5. VASCO input file • Vacuum chamber divided in segments: • Geometry (length and diameter) • Temperature • Distributed and localised pumps • Distributed and localised sources • Thermal outgassing • Ion, electron, photon stimulated desorption 5

  6. Boundary conditions (steady state) • Continuity of the density function: at the segment boundary xk the solution from segment (k-1) must equal the solution from segment (k) • Continuity of the flow function : the sum of flow of molecules coming from the two side of one boundary must equal the amount of molecules pumped (S) or generated by a local source (g) • Ends of segment sequence Gk Gk+1 G1 GN+1 6

  7. Solution • Density vector (per each segment k) . . . . . . . . • Coefficient vectors or matrices examples: • Ion stimulated desorption yield . . . . . . . . • Electron SDY . . . . . . . . . . . . . . . . . . . . . . . • Sticking coefficient . . . . . . . . . . . . . . . . . . . • Change of variables 7

  8. “Single-gas model” against “Multi-gas model” Gas density as a function of the beam current for single-gas model - multi-gas model a) b) The critical current calculated neglecting desorption by different ionised gas species is > twice bigger than what is estimated with the multi-gas model (with identical j-j coefficient) 8

  9. Comparison VASCO - MC Thanks to Pedro Costa-Pinto for running MC simulation 9

  10. VASCO with localised source 1E-3 torr.l/s 7m chamber - Ø80, NEG coated Transmission probability as from Smith & Lewin – JVST 3 (92)1966 10

  11. Photon Induced gas Desorption [Gröbner et al. Vacuum, Vol 37, 8-9, 1987] [Gómez-Goñi et al., JVST 12(4), 1994] Energy dependence Evolution with dose 11

  12. Electron Induced Gas Desorption J. Gómez-Goñi et al., JVST A 15(6), 1997 Copper baked at 150ºC G. Vorlaufer et al., Vac. Techn. Note. 00-32 Copper Unbaked Evolution with dose Evolution with dose Energy dependence 12

  13. NEG properties [P. Chiggiato, JVC-Gratz-06-2002] [P. Chiggiato, JVC-Gratz-06-2002] Pumping speed Aging 13

  14. VGPB.623.4L8.R VGPB.123.4L8.X 14

  15. Code description in VASCO_brief1.pdf VASCO documentation \\Srv2_div\div_lhc\VACUUM\Rossi\VASCO Input file in manual.xls 15

  16. Installation • To install the program, copy the whole VASCO directory onto your C:\ drive • From your START menu go to CONTROL PANEL -> SYSTEM -> ADVANCE -> ENVIRONMENT VARIABLES • Select SYSTEM VARIABLES. • Select the line PATH and edit it. • At the end of the line add a semicolon, then the path name where you have the Start-Multi-Gas.exe program + \bin\win32 (;C:\VASCO \bin\win32) 16

  17. Example of input file 17

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