Vacuum windows University of Arizona.ppt

Oli Durney Senior Optical Engineer Steward Observatory University of Arizona Practical Knowledge of Vacuum Windows Rev. 1.1 Typical Geometry Window Cryostat Case Cryostat Case Mounting Bolt Window Flange O-ring Seal Vacuum Space Ambient Space Example: O-ring Seal Example: Indium Seal Window Support Case 1: Simply Supported Lateral translation Case 2: Rigidly Fixed No Lateral translation Pressure Pressure k = 1.24 k = 0.75 k1 = 0.696 k1 = 1.71 O-ring Groove Window Cryostat Case 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 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 Maximum Stress Outside Pressure = Atm = 760 Torr = 14.7 PSI Inside Pressure = 10E-6 Torr ~ 0 PSI Stress of the Window: Sm = k t2 w R2 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 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 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: ym = k1 E t3 w R4 Solve for t 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 0.1