ZTF Cryostat Finite Element Analysis Andrew Lambert ZTF Technical Meeting 1

Outline Fused Silica Window – Case XV Aluminum Focal Plate Assembly Single CCD Assembly G10 Flex Supports ZTF Technical Meeting 2

Fused Silica Window Mesh and BCs Quarter model simulated due to symmetry – Case XV Boundary conditions –Simply supported at O- rings –Radiation to ambient temperature –Radiation to cold CCD temperatures –Vacuum loading Worst case temperature loading, however stress is dominated by vacuum loading, not temperature ZTF Technical Meeting 3

Fused Silica Window Temperatures Temperature contours show that the thermal gradient is approximately 6 o C Heating occurs due to ambient radiation –Due to high emissivity of the fused silica (~0.93), the window represents a major heat leak to the CCD array and aluminum focal plate ZTF Technical Meeting 4

Fused Silica Window Max Stress Evaluation of the maximum principle stress –Glass fails in tension, thus must evaluate max. tensile stress Maximum stress occurs at center of the window ~6.5 MPa High stress around support edge are artificially high due to simulation BC’s –Care should be taken however -> angle the inside support wall to avoid glass to metal contact ZTF Technical Meeting 5

Fused Silica Window Deflection Maximum deflection in the normal direction is ~12.4 microns –Occurs at window center and gradually decrease towards the support edges Very large improvement over initial design without increasing window thickness ZTF Technical Meeting 6

Fused Silica Window MOS and FOS LBNL specifies a factor of safety = 8 –Current design satisfies this criteria –MOS = > May experience a 23% load increase before exceeding acceptable levels To protect against cryogen leakage, a 11.5 psig burst disk will be installed on back of dewar assembly –Simulation of this internal positive pressure shows that for this condition FOS remains > ZTF Technical Meeting 7 Yield Stress (Mpa) Allowable Stress (Mpa) Max Stress (Mpa) Deflection (µm) Design FactorMOSFOS Case VX

Focal Plate Assembly Previous to this design, rigorous analysis of different focal plate material choices was accomplished –Silicon Carbide –Invar –Aluminum Studies showed that aluminum was a viable material for use –Good thermal performance –Less expensive –Machine-ability Aluminum focal plate design has been through several iterations prior to its current state –Increased thickness to reduce bending –Thinner flexures to allow for differential thermal contraction –Pockets to reduce the overall mass while maintaining heat paths ZTF Technical Meeting 8

Focal Plate Assembly ZTF Technical Meeting 9 Focal Plate G10 Flex Support Flatteners and Detectors Dewar

Assembly Mesh and BCs Focal plate thermal simulation yields assembly temperature profiles Boundary conditions –Radiation to ambient on contact lens surface –Radiation to cold CCD on contact lens back –Radiation to warmer contact lens on CCD surface –Set cold temperature of -130 o C for thermal links on back of Al plate –Ambient temperature on outside of dewar frame ZTF Technical Meeting 10

Assembly Temperatures Assembly temperatures show that the contact lens temperatures are well above the focal plate temperatures –In the -30 o C to -60 o C range Also, the dewar can temperature is successfully isolated from the aluminum plate by the G10 supports The heat removed through the thermal link attach points it approximately 35 Watts, with 17.5 Watts to each cryocooler ZTF Technical Meeting 11

Aluminum Focal Plate Temperatures High thermal conductivity of aluminum enhances cooling performance –Even down to low temperatures ~ -130 o C –Temperature gradient across the focal plate of ~ 8 o C Less expensive than other material and more machine-able Due to high thermal contraction, addition of support flexures are need to accommodate thermal shrinkage ZTF Technical Meeting 12

CCD Temperatures Prime interest point is thermal gradient across the CCD array –Across all 12 CCD’s there is 3 o C temperature difference –Temperature gradient across a single CCD is less than a degree ZTF Technical Meeting 13

Contact Lens and Frame Temperatures Contact lens and frame are insulated from the CCD using G10 washers –Provides an effective barrier for heat transfer from the contact lens to the CCD array –Maximum lens temperature is -31 o C, with a minimum of -60 o C Approximately 30 o C temperature gradient across lens ZTF Technical Meeting 14

Assembly Deformation Aluminum focal plate assembly experiences shrinkage due to low temperature operation Various materials are used –Material selection has been optimized for best CTE performance –Reduce CCD deflection At -130 oC, α = 1.6e-5 –Using the analytical formula to the lower right, contraction is +/-550 microns in x & y; +/-49 microns in z –Agrees well with FEA result ZTF Technical Meeting 15

CCD Deflection CCD deflection is reduced by Invar spacers between the CCD and aluminum plate CCDs at the center of the array experience the greatest deflection ZTF Technical Meeting 16

Detector Surface Deflection Detector surface deflection is due to thermal shrinkage –Maximum deflection at the center of the array –Deflection ranges from -90 to -105 microns in the direction normal to the detector surface –Peak to peak deflection change is 15 microns ZTF Technical Meeting 17

Single CCD Assembly Single CCD assembly used to examine stress in the aluminum flexures –Small model allows easier examination of critical components and reduces solution time Boundary conditions are applied to mimic the same behavior as the overall plate assembly ZTF Technical Meeting 18 Flattener CCD and Detector Focal Plate Invar Frame

Single CCD Temperatures Temperatures mimic those found in full plate simulation almost exactly –Means contraction is similar and thus flexure stress can be evaluated Model includes every component found in the CCD assembly ZTF Technical Meeting 19

Aluminum Flexure Stress Aluminum flexure stress occurs due to differential in expansion from CCD to aluminum base –Maximum equivalent stress in the flexure of about 35 MPa, which is well below yield ~ 260 MPa Thin flexure allows bending –Due to aluminum’s larger CTE as compared with the SiC detector –Because stress is well below yield, deformation is elastic and aluminum should return to normal shape when unstressed ZTF Technical Meeting 20

G10 Support Temperatures G10 supports are used to hold the focal plate in place as well as insulate it from ambient temperatures –Connected to warm end at 22 o C and cold plate at -125 o C –Show large temperature gradient across the support ZTF Technical Meeting 21

G10 Support Deformation Support deforms due to several loads –Aluminum plate contraction –G10 support shrinkage –Gravitational loading Maximum deflection at the focal plate support point of 620 microns Must examine stresses due to thermal contraction ZTF Technical Meeting 22

G10 Support Stress Equivalent stress in MPa shows maximum stress near support locations –Max stress is in the MPa range –35 MPa stress is a localized concentration and is most likely artificial –Yield strength of ~220 MPa Notice that dewar support point on left is allowed to slide –Reduces stresses from contraction ZTF Technical Meeting 23

Future Work Integrate thermal shield Dewar structure optimization for vacuum load Back wall optimization for stiffness Safety criteria for flatteners Examination of titanium for flex support material Thermal linkage heat transfer ZTF Technical Meeting 24