VASCO VAcuum Stability COde multigas code to calculate

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 IR 8 results (with real data) VASCO documentation and installation

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 : § § § Sink: § § 2 Net contribution from diffusion Thermal desorption. Beam induced phenomena: ion, electron and photon induced molecular desorption. Localised sources Localised pumps Distributed pumps (NEG or cryo)

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

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

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

Boundary conditions (steady state) G 1 Gk Gk+1 GN+1 • 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) • 6 Ends of segment sequence

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

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

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

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

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

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 12 Energy dependence

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

VGPB. 623. 4 L 8. R VGPB. 123. 4 L 8. X 14

VASCO documentation \Srv 2_divdiv_lhcVACUUMRossiVASCO Input file in manual. xls Code description in VASCO_brief 1. pdf 15

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 + binwin 32 (; C: VASCO binwin 32) 16

Example of input file 17