1. Cryostat Structural, Thermal, and Vacuum Systems 14 October 2008 Martin Nordby, Gary Guiffre, John Ku, John Hodgson

2. LSST Camera Systems Integration 2 Contents Cryostat design Thermal design –Heat paths –Heat transfer Radiation shielding Sensitivity to emissivity values Radiation heat transfer assumptions and calculations –Radiation through L3 lens Radiation heat flux profile on focal plane Temperature profile across window—L3 lens and detectors Thermal-mechanical analysis –Grid heat loads –Distortion analysis L3 lens analysis –FEA analysis of atmospheric pressure and thermal distortion Vacuum system design

3. LSST Camera Systems Integration 3 Cryostat Design Sensor raft Pumping chimney Feedthrough flange Pump flange Cryostat housing L3 lens assembly Raft Control Crate Cold plate Radiation shield Pumping plenum Front end electronics module Cryo shroud Cryo plate

4. LSST Camera Systems Integration 4 Cryostat Exploded View L3 Flange Assembly Grid with Raft and Flexures Cryo Plate and Pumping Chimneys (Shroud not shown) Cold Plate Back end MLI Cage Feedthrough Flange, Pump Plate, and Pumping Plenum Cryostat Housing

5. LSST Camera Systems Integration 5 Cryostat and Utility Trunk Components Valve box—vacuum-insulated Pneumatically actuated valves System electronics crates Inlet/outlet lines for cryogens— vacuum-insulated Support tubeStructural support to camera back flange Cryostat vacuum pumping, valves, gauges

6. LSST Camera Systems Integration 6 Cryostat Thermal Design Cryo shroud plate Isolates Grid picture frame structure from radiant heating from front of cryostat Mat’l: nickel-plated copper Cryo shroud cone Isolates Grid perimeter from radiant heating from cryostat housing Mat’l: nickel-plated copper, wrapped in MLI Cryo plate Sinks and removes heat from front end Isolates Grid back side from radiant heating from cold plate Mat’l: copper plate / stainless steel ribs Cold plate Sinks and removes heat from back end Mat’l: copper Pump chimney/back end MLI shield Isolates RCC crates from radiant heating from cryostat housing Mat’l: nickel-plated stainless steel

7. LSST Camera Systems Integration 7 Heat Sources in the Cryostat Front End Heat loads: 714 W total 1.Radiation through L3 lens –Heat load: 98 W 2.FEE process heat –Heat load: 600 W (24 W/bay) 3.Heat leak up the flexure –Heat leak: 4 W (1.33 W/flexure) 4.Radiation on perimeter picture frame –Heat load: 3.7 W (16 W/m^2) 5.Radiation around perimeter –Heat load: 8 W (16 W/m^2) 6.Heat leak up flex cables –Heat load: not included yet Radiation heat transfer –Emissivities Analysis used  = 0.07 for nickel- plated surfaces This is 2x the value for aluminized mylar MLI –The above heat loads assumed there was no MLI Adding 15 layers of insulation reduces heat flux 16x 3. Heat leak up the flexure 5. Radiation around perimeter 4. Radiation on perimeter picture frame 1. Radiation through L3 lens 6. Heat leak up the RCC-FEE flex cables 2. FEE process heat

8. LSST Camera Systems Integration 8 Radiation Through the L3 Lens Emissivities used and sensitivity to values –Fused silica is opaque to IR radiation with wavelengths longer than 5 microns –Lens:  = 0.91 = surface emissivity based on index of refraction of fused silica –Total heat load = 98 W across focal plane –Heat load = 171 W for radiation from a black body at 300 K without L3 (worst-case) Radiation heat flux profile on focal plane –Center raft heat flux: 21 mW/cm^2 = 3.28 W, total –Corner raft heat flux: 28 mW/cm^2 = 3.75 W, total Temperature profile across window –Window temperature varies from 266K-295K –CCD temperature is controlled to 173K Temperature Contour Plot of L3 Lens IR Heat Flux on the Focal Plane as a Function of Radius

9. LSST Camera Systems Integration 9 L3 Lens Design L3 lens structural design –Fused silica allowable stress of 7 MPa includes a factor of safety of 7.5 –Deflection = 197 microns Gasket—lt blue Lens retainer—dk blue/black Flange—orange Instr ring—tan Shroud plate—lt orange P atm at 8000 ft Deflection of 782 mm diameter lens P atm at sea level Stresses in lens L3 Lens Assembly Design Details

10. LSST Camera Systems Integration 10 Grid Thermal-Mechanical Analyses Total heat load on Grid = 31.4 W –Compare with 98 W I.R. heating through L3 and 600 W process heat load from FEE Worst-case Grid distortion = 0.22 microns Maximum out-of-plane motion of Rafts on the focal plane = 0.023 microns Grid and Flexure temperatures 116 K temperature gradient up the flexures to the Grid mount Grid front face Z-deflection Grid front face Z-deflection (Heat leak from Rafts) 80 nm total z-distortion

11. LSST Camera Systems Integration 11 Cryostat Vacuum System Design Two-zone vacuum to reduce gas load on focal plane –Front-end zone: minimal electronics –Back-end zone: contains circuit boards, flex cables, MLI Pump path Conductance: 195 l/sec Eff. Pump speed: 130 l/sec Pumping chimney connects front end vacuum region to pumping plenum— this is attached to the Cryo Plate Sheet metal F.P. pumping plenum drops into Feedthrough Plate 400 l/sec turbo pump