OFD 1

1
Practical Knowledge of Vacuum Windows Rev. 1.1
Oli Durney Senior Optical Engineer Steward
Observatory University of Arizona
2
Typical Geometry
Ambient Space
Mounting Bolt
Window Flange
O-ring Seal
Window
Cryostat Case
Cryostat Case
Vacuum Space
3
Example O-ring Seal
4
Example Indium Seal
5
Window Support
Case 1 Simply Supported
Pressure
Lateral translation
k 1.24
k1 0.696
Case 2 Rigidly Fixed
Pressure
k 0.75
No Lateral translation
k1 1.71
6
O-ring Groove
Window
Force
Light coating of Apiezon vacuum grease L or M
20
0.139
Cryostat Case
Cryostat Case
O-ring (2-240)
0.105
Compressed O-ring
0.150
7
Strength of Material

• Rule of Thumb Safety Factor 10
• Use reference book to get strength of material
  in PSI
• Normally the Modulus of Rupture (MOR) is used
• Safety Factor is

Strength
S.F.
Stress
Solve for Stress
8
Maximum Stress

• Outside Pressure Atm 760 Torr 14.7 PSI
• Inside Pressure 10E-6 Torr 0 PSI
• Stress of the Window

w R2
Sm
k
t2
k coefficient k for circular plates w uniform
pressure across window (outside P inside P) R
radius of Clear Aperture of window t thickness
of window
Solve for t
9
Maximum Defection

• Window bowing can affect optical design
• Deflection causes plano window to have power,
  thus creating a meniscus lens
• Optical design will govern amount of deflection
  (sag) allowable
• If window is Simply Supported and O-ring does not
  compress fully

k1 coefficient k1 for circular plates w
uniform pressure across window (outside P
inside P) R radius of Clear Aperture of
window E Youngs modulus t thickness of window
Solve for t
10
Rules of Thumb

• Typical k value for stress calculation used in
  practice is 1.00
• Reasonable (and typical) material choice for NIR
  waveband is Fused Silica or BK7
• Fused Silica Youngs modulus 1.06E07 PSI
• Modulus of Rupture
  7600 PSI
• BK7 Youngs modulus 1.19E07 PSI
• Modulus of Rupture 2400 PSI
• k1 for deflection calculations vary from 0.696 to
  0.171 depending on whether window is constrained
  by Case 1 or 2
• Typically use 0.43
• O-ring types
• Buna-N has highest permeation and retains water
• Viton has lowest permeation and minimal water
  retention

11
Real World Example
12
LBTI UBC
Vacuum windows
Gate valve windows
13
Window Specs

• Lower Gate Valve Window
• Material BK7 or Fused Silica
• Diameter 101.8mm 0.00 / -0.25mm
• Thickness 6.35mm /- 0.25mm
• Wavefront 1/4 wave across CA
• Clear Aperture gt 80 diameter
• Parallelism lt 1 arcmin
• Surface Quality 20-10

• Upper Gate Valve Windows
• Material BK7 or Fused Silica
• Diameter 137.5mm 0.00 / -0.10mm
• Thickness 8.0mm /- 0.10mm
• Wavefront 1/4 wave across CA
• Clear Aperture gt 85 diameter
• Parallelism lt 30 arcsec
• Surface Quality 40-20

14
Window Detail for Lower
15
Window Detail for Upper
16
Max Stress

• Maximum Stress (Lower Gate Valve Window)

• Maximum Stress (Upper Gate Valve Window)

17
Calculations for BK7

• Safety Factor (Lower Gate Valve Window)

• Safety Factor (Upper Gate Valve Window)

18
Calculations for BK7

• Maximum Deflection (Lower Gate Valve Window)

(14.7PSI)(46.05mm)4
w R4

(0.43)

0.009 mm
ym k1
(1.19E7PSI)(6.35mm)3
E t3

• Maximum Deflection (Upper Gate Valve Window)

(14.7PSI)(65mm)4
w R4
(0.43)

0.019 mm

ym k1
(1.19E7PSI)(8mm)3
E t3
19
Calculations for F.S.

• Safety Factor (Lower Gate Valve Window)

• Safety Factor (Upper Gate Valve Window)

20
Calculations for F.S.

• Maximum Deflection (Lower Gate Valve Window)

(14.7PSI)(46.05mm)4
w R4

(0.43)

0.010 mm
ym k1
(1.06E7PSI)(6.35mm)3
E t3

• Maximum Deflection (Upper Gate Valve Window)

(14.7PSI)(65mm)4
w R4
(0.43)

0.021 mm

ym k1
(1.06E7PSI)(8mm)3
E t3
21
Conclusions
22
Window using O-ring Seal
Figure 2 LN2 Cryostat
Figure 1 O-ring style vacuum window
23
Window using Indium Seal
Figure 3 Indium style vacuum window
Figure 4 Balloon Cryostat
24
Other Examples