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)

3 Single gas model Equation describing the gas density for each gas species        Time variation Diffusion Ionisation by beam Distributed pumping Desorption of particles in through and desorption by by NEG or by photons by electron thermal volume V surface a the ions by beam screen Multi gas model

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

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

6 Boundary conditions (steady state) Continuity of the density function: at the segment boundary x k 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 GkGk G1G1 G k+1 G N+1

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

8 “Single-gas model” against “Multi-gas model” a)b) Gas density as a function of the beam current for single-gas model - multi-gas model 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)

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

10 VASCO with localised source 1E-3 torr.l/s 7m chamber - Ø 80, NEG coated Transmission probability as from Smith & Lewin – JVST 3 (92)1966

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

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 Copper Unbaked Evolution with dose Evolution with dose Energy dependence

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

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

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

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)

17 Example of input file