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OFD 1 Oli Durney Senior Optical Engineer Steward Observatory University of Arizona Practical Knowledge of Vacuum Windows Rev. 1.1.

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Presentation on theme: "OFD 1 Oli Durney Senior Optical Engineer Steward Observatory University of Arizona Practical Knowledge of Vacuum Windows Rev. 1.1."— Presentation transcript:

1 OFD 1 Oli Durney Senior Optical Engineer Steward Observatory University of Arizona Practical Knowledge of Vacuum Windows Rev. 1.1

2 OFD 2 Typical Geometry Window Cryostat Case Mounting Bolt Window Flange O-ring Seal Vacuum Space Ambient Space

3 OFD 3 Example: O-ring Seal

4 OFD 4 Example: Indium Seal

5 OFD 5 Window Support Case 1: Simply Supported Lateral translation Case 2: Rigidly Fixed No Lateral translation Pressure k = 1.24 k = 0.75 k 1 = 0.696 k 1 = 1.71

6 OFD 6 O-ring Groove Window Cryostat Case 0.150 0.105 ~20% Compressed O-ring Light coating of Apiezon vacuum grease L or M O-ring (2-240) 0.139 Force

7 OFD 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: S.F. = Stress Strength Solve for Stress

8 OFD 8 Maximum Stress Outside Pressure = Atm = 760 Torr = 14.7 PSI Inside Pressure = 10E-6 Torr ~ 0 PSI Stress of the Window: S m =k t2t2 w R 2 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 OFD 9 k 1 = coefficient k 1 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 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: y m =k1k1 E t 3 w R 4 Solve for t

10 OFD 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 = 1.06E+07 PSI Modulus of Rupture = 7600 PSI BK7: Youngs modulus = 1.19E+07 PSI Modulus of Rupture = 2400 PSI k 1 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 OFD 11 Real World Example

12 OFD 12 LBTI UBC Vacuum windows Gate valve windows

13 OFD 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 OFD 14 Window Detail for Lower

15 OFD 15 Window Detail for Upper

16 OFD 16 Max Stress Maximum Stress: (Lower Gate Valve Window) S m = k t2t2 w R 2 = (14.7PSI)*(46.05mm) 2 (6.35mm) 2 (1)~773 PSI Maximum Stress: (Upper Gate Valve Window) S m = k t2t2 w R 2 = (14.7PSI)*(65mm) 2 (8mm) 2 (1)~970 PSI

17 OFD 17 Calculations for BK7 Safety Factor: ( Lower Gate Valve Window ) S.F. = Max Stress Modulus of Rupture = 773 2400 = 3.1 Safety Factor: ( Upper Gate Valve Window ) S.F. = Max Stress Modulus of Rupture = 970 2400 = 2.5

18 OFD 18 Calculations for BK7 Maximum Deflection: ( Lower Gate Valve Window ) = (14.7PSI)*(46.05mm) 4 (1.19E7PSI)*(6.35mm) 3 (0.43)~0.009 mmy m = k 1 E t 3 w R 4 (14.7PSI)*(65mm) 4 (1.19E7PSI)*(8mm) 3 (0.43)~0.019 mm Maximum Deflection: ( Upper Gate Valve Window ) =y m = k 1 E t 3 w R 4

19 OFD 19 Calculations for F.S. Safety Factor: ( Lower Gate Valve Window ) S.F. = Max Stress Modulus of Rupture = 773 7600 = 9.8 Safety Factor: ( Upper Gate Valve Window ) S.F. = Max Stress Modulus of Rupture = 970 7600 = 7.8

20 OFD 20 Calculations for F.S. Maximum Deflection: ( Lower Gate Valve Window ) = (14.7PSI)*(46.05mm) 4 (1.06E7PSI)*(6.35mm) 3 (0.43)~0.010 mmy m = k 1 E t 3 w R 4 (14.7PSI)*(65mm) 4 (1.06E7PSI)*(8mm) 3 (0.43)~0.021 mm Maximum Deflection: ( Upper Gate Valve Window ) =y m = k 1 E t 3 w R 4

21 OFD 21 Conclusions Youngs Modulus Max Deflection Modulus of Rupture Max Stress Safety Factor LowerLower BK71.19E+070.00924007733.1 Fused Silica 1.06E+070.01076007739.8 UpperUpper BK71.19E+070.01924009702.5 Fused Silica 1.06E+070.02176009707.8

22 OFD 22 Window using O-ring Seal Figure 2: LN 2 CryostatFigure 1: O-ring style vacuum window

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

24 OFD 24 Other Examples


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