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Oli Durney Senior Optical Engineer Steward Observatory University of Arizona

Practical Knowledge of Vacuum Windows Rev. 1.1. Oli Durney Senior Optical Engineer Steward Observatory University of Arizona. Typical Geometry. Ambient Space. Mounting Bolt. Window Flange. O-ring Seal. Window. Cryostat Case. Cryostat Case. Vacuum Space. Example: O-ring Seal.

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Oli Durney Senior Optical Engineer Steward Observatory University of Arizona

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  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. w R4 ym = k1 E t3 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 = Young’s 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: Young’s modulus = 1.06E+07 PSI • Modulus of Rupture = 7600 PSI • BK7: Young’s modulus = 1.19E+07 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: > 80% diameter • Parallelism: < 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: > 85% diameter • Parallelism: < 30 arcsec • Surface Quality: 40-20

  14. Window Detail for Lower

  15. Window Detail for Upper

  16. w R2 (14.7PSI)*(46.05mm)2 Sm = k = (1) ~ 773 PSI t2 (6.35mm)2 w R2 (14.7PSI)*(65mm)2 Sm = k = (1) ~ 970 PSI t2 (8mm)2 Max Stress • Maximum Stress: (Lower Gate Valve Window) • Maximum Stress: (Upper Gate Valve Window)

  17. Modulus of Rupture 2400 S.F. = = = 3.1 Max Stress 773 Modulus of Rupture 2400 S.F. = = = 2.5 Max Stress 970 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) w R4 (14.7PSI)*(46.05mm)4 ym = k1 = (0.43) ~ 0.009 mm E t3 (1.19E7PSI)*(6.35mm)3 • Maximum Deflection: (Upper Gate Valve Window) w R4 (14.7PSI)*(65mm)4 ym = k1 = (0.43) ~ 0.019 mm E t3 (1.19E7PSI)*(8mm)3

  19. Modulus of Rupture 7600 S.F. = = = 9.8 Max Stress 773 Modulus of Rupture 7600 S.F. = = = 7.8 Max Stress 970 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) w R4 (14.7PSI)*(46.05mm)4 ym = k1 = (0.43) ~ 0.010 mm E t3 (1.06E7PSI)*(6.35mm)3 • Maximum Deflection: (Upper Gate Valve Window) w R4 (14.7PSI)*(65mm)4 ym = k1 = (0.43) ~ 0.021 mm E t3 (1.06E7PSI)*(8mm)3

  21. Conclusions

  22. Window using O-ring Seal Figure 1: O-ring style vacuum window Figure 2: LN2 Cryostat

  23. Window using Indium Seal Figure 3: Indium style vacuum window Figure 4: Balloon Cryostat

  24. Other Examples

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