VASCO (VAcuum Stability COde) : multi-gas code to calculate gas - - PowerPoint PPT Presentation

VASCO (VAcuum Stability COde) : lti d t l l t d it multi-gas code to calculate gas density profile and vacuum stability in a UHV system

Adriana Rossi

• General equation

q

• VASCO code assumptions and solution
• Comparison between Single and Multi-Gas models
• Comparison between VASCO and MC (Pedro Costa-Pinto)

Comparison between VASCO and MC (Pedro Costa Pinto)

• Discussion on input parameters and example of IR8 results (with real

data)

• VASCO documentation and installation

VASCO documentation and installation

Equation q

• Level of water in a sink depends on:

Flow of water from the tap = source – 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

α qth

depends on

• Sources :
• Net contribution from diffusion
• Thermal desorption.

+ p ηi

e-

ηe

ηph

AD AD

SR

p

• Beam induced phenomena:

ion, electron and photon induced molecular desorption.

• Localised sources

dx

• Localised sources
• Sink:
• Localised pumps
• Distributed pumps (NEG or cryo)

2

p p ( y )

Equation describing the gas density for each gas species for each gas species

{ }

g e g e ph g ph g g g g j j b j g j i g g g

q A n C v A n e I x n D a t n V ⋅ + Γ ⋅ + Γ ⋅ + ⋅ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + ⋅ ⋅ − ⋅ + ∂ ∂ ⋅ ⋅ = ∂ ∂

• →

∑

+

, , , 2 2

4 η η α σ η 123 14243 1442443 14424443 123

123 123

Time variation Diffusion Ionisation by beam Distributed pumping Desorption

• f 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

α

Single gas model

+ p ηi α

e-

ηe

ηph

qth AD AD

⎟ ⎞ ⎜ ⎛ ⋅

b n

I σ η

dx

SR

⎟ ⎠ ⎜ ⎝

→

+

g g g g i

n eσ η ,

3

VASCO code

• Cylindrical symmetry

( )

t x n n

g g

, =

Average density across the area

• Time invariant parameters

( )

, ∂ t x n

Time invariant parameters (snapshot in time at steady state)

Surface parameters (sticking and desorption

( )

, ≈ ∂ ∂ t t x n V

g

coefficients) constant (not dependent on dose , selected for a specific incident energy)

• Maxwell-Boltzmann distribution
• f molecular velocity

Assumption of uniform

g B g

m T k v ⋅ ⋅ = π 8

D 2 ( )

Assumption of uniform distribution in space

Dg = 2 3 vg ⋅ r(x)

4

g

v A⋅

diffusion coefficient average number of particle hitting the surface area

4

VASCO input file p

• Vacuum chamber divided in segments:

g

– Geometry (length and diameter) – Temperature – Distributed and localised pumps – Distributed and localised sources

Thermal outgassing

• Thermal outgassing
• Ion, electron, photon stimulated desorption

5

Boundary conditions (steady state) y ( y )

Gk G1 Gk+1 G1

k+1

GN+1

C ti it f th d it f ti

• Continuity of the density function:

at the segment boundary xk the solution from segment (k-1) must equal the solution from segment (k)

N G x n c x n c x n S x n x n

k k k spec k k spec k k k k k k k

, 2 k ) ( ) ( ) (

1 1 1

= ⎪ ⎩ ⎪ ⎨ ⎧ + ∂ ∂ − ∂ ∂ = =

− − −

g ( )

• Continuity of the flow function :

the sum of flow of molecules coming from the two side of one boundary must

1 1 1 1 1 1

) ( + ∂ ∂ =

spec

G n c x n S x x

x x

k k

⎩ ∂ ∂

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

1 1 1

1 1

) (

+ + +

+ ∂ ∂ − = ∂

N x N N spec N N N x p

G x n c x n S x

N

6

1 +

xN

Solution

• Density vector (per each segment k) . . . . . . . .

[ ]′

=

2 4 2

CO CO CH H k

n n n n n

• Coefficient vectors or matrices

examples:

2 4 2

⎥ ⎥ ⎥ ⎤ ⎢ ⎢ ⎢ ⎡ =

− − − − − − − −

+ + + + + + + + 4 2 4 4 4 4 2 2 2 2 2 4 2 2

CH CO CH CO CH CH CH H H CO H CO H CH H H k i

η η η η η η η η η η η η η

– Ion stimulated desorption yield . . . . . . . . – Electron SDY . . . . . . . . . . . . . . . . . . . . . . .

⎥ ⎥ ⎥ ⎦ ⎢ ⎢ ⎢ ⎣

− − − − − − − −

+ + + + + + + + 2 2 2 2 4 2 2 2 4 2

CO CO CO CO CO CH CO H CO CO CO CO CO CH CO H

η η η η η η η η

[ ]′

=

− − − −

2 4 2

CO e CO e CH e H e k e

η η η η η

– Sticking coefficient . . . . . . . . . . . . . . . . . . .

⎥ ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ =

4 2

CO CO CH H k

α α α α α

• Change of variables

⎥ ⎦ ⎢ ⎣

2

CO

α

⎪ ⎩ ⎪ ⎨ ⎧ = =

• k

k k k

n y n y

, 2 , 1

( )

( )

( )

{ }

τ τ d b z M Y z M z Y

k k z k k k

exp exp

,

− + =

∫

7

( )

∫

“Single-gas model” against “Multi-gas model” g g g g

Gas density as a function of the beam current for

a) b)

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)

8

Comparison VASCO - MC p

2.5

i bl ti ki ffi i t 4 (80 di t ) t b 1E-10 torr.l/s/cm2 outgassing

2 y

MC, stick=0 VASCO, stick=0 MC, stick=1E-3 VASCO, stick=1E-3

variable sticking coefficient over 4m (80mm diameter) tube 10 l/s 10 l/s

1.5 d gas density

MC, stick=1E-2 VASCO, stick=1E-2 MC, stick=1E-1 VASCO, stick=1E-1 MC, stick=1 VASCO, stick=1 Series11 Series12

0 /s 10 l/s

1 normalised

Series11 Series12

0.5 1 2 3 4 5 distance (m)

Thanks to Pedro Costa-Pinto for running MC simulation

9

Thanks to Pedro Costa-Pinto for running MC simulation

VASCO with localised source

1E-3 torr.l/s

7m chamber Ø80 NEG coated 7m chamber - Ø80, NEG coated

1 E 01 1.E+00

stick=5E-3 stick=1E-2 stick=1E-1 stick=5E-1

1.E-02 1.E-01 density

5.00E-03 1.00E-02 1.00E-01 5.00E-01

1.E-03 normalised

Transmission probability as from Smith & Lewin – JVST 3 (92)1966

1 E 05 1.E-04

JVST 3 (92)1966

1.E-05 1000 2000 3000 4000 5000 6000 7000 distance from source (mm)

10

Photon Induced gas Desorption g p

[ Gröbner et al. Vacuum, Vol 37, 8-9, 1987] [ Gómez-Goñi et al., JVST 12(4), 1994]

Evolution with dose Energy dependence

11

Electron Induced Gas Desorption p

• J. Gómez-Goñi et al., JVST A 15(6), 1997

Copper baked at 150º C

• G. Vorlaufer et al., Vac. Techn. Note. 00-32

Copper Unbaked

10 10

10

10

η / (molec./e−)

Evolution with dose

10

10

C2H6 CH4 CO

Evolution with dose

50 100 150 200 250 300 350 10

E / eV CO CO2 H2 H2O Fit

E l ti ith d E d d Evolution with dose Energy dependence

12

NEG properties p p

[ P . Chiggiato, JVC-Gratz-06-2002] [ P . Chiggiato, JVC-Gratz-06-2002]

101 100 1013 1014 1015 1016 [molecules cm

• 2]

TiZrV on rough Cu

CO

10

3 Heating duration 24 hours

TiZrV/ St. Steel heated at 200°C

100 10-1 g Speed [l s

• 1 cm2]

Sticking facto TiZrV on smooth Cu TiZrV on rough Cu coated at 300 °C

10

2

ing speed [l s-1 m -1 ]

200°C

TiZrV/ Al heated at 200°C

10-2 10-1 10-3 10-2 Pumping

• r

coated at 100 °C

10

1

H

2 pump

beam pipe diameter = 80 mm TiZrV/ Al heated at 180°C

10 10-7 10-6 10-5 10-4 10-3 CO Surface Coverage [Torr l cm

• 2]

10 5 10 15 20 Number of heating/venting cycles

Pumping speed Aging

13

H2 CH4 CO CO2

penning ion gauges

N equivalent

1 E+ 15 1.E+ 16

Q1-Q2-Q3

mbar a 293K

• gauges

N2 equivalent

MKI MSI Q1-Q2-Q3

1.E+ 14 1.E+ 15

es/ m 3)

D1 D2/ Q4 Q5 Q6 recom b. ch.

293K

Q7 MKI MSI D1 D2/ Q4 Q5 Q6 Q7

1.E+ 13

molecule

1.E-09

TCTH TCLI B

VGPB.623.4L8.R

1.E+ 12

Density (

1.E-10 1.E-11

TDI

leak 2E-6 torr.l/ s

VGPB.123.4L8.X

1.E+ 11

D

1.E 11 1.E-12 1.E+ 10

• 280
• 210
• 140
• 70

70 140 210 280

IR8 red beam - B2 (distance from IP8 - m)

14

VASCO documentation

\\Srv2_div\div_lhc\VACUUM\Rossi\VASCO

Input file in manual.xls Code description in VASCO_brief1.pdf

15

Installation

• To install the program, copy the whole VASCO directory onto your

p g py y y C:\ drive

• From your START menu go to CONTROL PANEL -> SYSTEM ->

ADVANCE -> ENVIRONMENT VARIABLES 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)

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Example of input file p p

VMBGA.C4R8.X VCTCN.4R8.X _TDI.4R8

H2 CH4 CO CO2 H2 CH4 CO CO2 21 22 212.7 212 400 500 77233 77633 300 300 8 90E- 23 8 90E- 23 H2 CH4 CO CO2 H2 CH4 CO CO2 H2 CH4 CO CO2 23 24 26 212 212 212 1350 1350 1350 78133 79483 82183 300 300 300 1900 509.1 900 129.8 700 200.4 560 161.9 2.00E- 06 8 90E- 23 8 90E- 23 8 90E- 23 % in_Segment = [ % in_d = [ [ mm] % in_L = [ [ mm] % in_dist _ref = [ mm] % in_T = [ [ K] % in_S = [ [ l/ s] (H2) % [ l/ s] (CH4) % [ l/ s] (CO) % [ l/ s] (CO2) % in_g = [ [ t orrl/ s] % in sigma = [ [ m2] % 8.90E 23 8.90E 23 6.36E- 22 6.36E- 22 5.50E- 22 5.50E- 22 8.58E- 22 8.58E- 22 1.00E- 07 5.00E- 03 0.00E+00 0.00E+00 1.00E- 07 0.5 1.00E- 07 0.5 8.90E 23 8.90E 23 8.90E 23 6.36E- 22 6.36E- 22 6.36E- 22 5.50E- 22 5.50E- 22 5.50E- 22 8.58E- 22 8.58E- 22 8.58E- 22 1.00E- 07 1.00E- 07 1.00E- 07 0.00E+00 0.00E+00 0.00E+00 1.00E- 07 1.00E- 07 1.00E- 07 1.00E- 07 1.00E- 07 1.00E- 07 in_sigma = [ [ m2] % % % % in_alpha = [ % % % % in_alpha_p = [ % % % % 0.54 0.54 0.54 0.54 0.05 0.05 0.05 0.05 0.04 0.05 0.07 0.11 0.01 0.01 0.01 0.25 0.29 0.29 0.33 0.03 0.03 0.03 0.03 0.14 0.14 0.14 0.14 0.01 0.01 0.01 0.01 1.77E- 03 6.46E- 05 4.52E- 04 3.87E- 04 3.33E- 05 8.33E- 07 1.67E- 05 1.67E- 05 1.50E- 04 4.00E- 06 1.50E- 05 2.50E- 05 2.50E- 07 2.50E- 09 1.25E- 08 1.25E- 08 0.54 0.54 0.54 0.54 0.54 0.54 0.54 0.54 0.54 0.54 0.54 0.54 0.04 0.05 0.07 0.11 0.04 0.05 0.07 0.11 0.04 0.05 0.07 0.11 0.25 0.29 0.29 0.33 0.25 0.29 0.29 0.33 0.25 0.29 0.29 0.33 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 1.77E- 03 6.46E- 05 4.52E- 04 3.87E- 04 1.77E- 03 6.46E- 05 4.52E- 04 3.87E- 04 1.77E- 03 6.46E- 05 4.52E- 04 3.87E- 04 1.50E- 04 4.00E- 06 1.50E- 05 2.50E- 05 1.50E- 04 4.00E- 06 1.50E- 05 2.50E- 05 1.50E- 04 4.00E- 06 1.50E- 05 2.50E- 05 in_et a_i = [ % % % % in_et a_p_i = [ % % % % in_et a_e = [ % in_et a_p_e = [ % in_et a_ph = [ % in et a p ph = [ % 0.00E+00 0.00E+00 0.00E+00 0.00E+00 1.00E- 12 5.00E- 15 1.00E- 14 5.00E- 15 5.00E- 14 3.00E- 17 1.00E- 14 1.00E- 14 1.20E+14 6.00E+13 3.00E+15 3.00E+15 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 5.00E- 12 1.00E- 13 1.00E- 12 1.00E- 12 5.00E- 12 1.00E- 13 1.00E- 12 1.00E- 12 5.00E- 12 1.00E- 13 1.00E- 12 1.00E- 12 1.20E+14 1.20E+14 1.20E+14 3.00E+15 3.00E+15 3.00E+15 in_et a_p_ph = [ % in_Cbs = [ [ l/ s/ m] % % % % in_Qt h = [ % in_n_e = [ % in_N_e = [ [ e- / m/ s] % in_Gamma_ph [ ph/ m/ s] % in_S_Nplus1 [ l/ s] (H2) % [ l/ s] (CH4) % [ l/ s] (CO) % [ l/ s] (CO2) % in_g_Nplus1 [ t orrl/ s] %

17