ZTF Cryostat Finite Element Analysis

ZTF Technical Meeting

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ZTF Cryostat Finite Element Analysis

Andrew Lambert

2013-02-01

2013-02-01


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Outline

• Fused Silica Window – Case XV• Aluminum Focal Plate Assembly• Single CCD Assembly• G10 Flex Supports

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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

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Fused Silica Window Temperatures

• Temperature contours show that the thermal gradient is approximately 6 oC

• 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

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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

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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

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Fused Silica Window MOS and FOS

• LBNL specifies a factor of safety = 8– Current design satisfies this criteria– MOS = 0.23 -> 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 > 8

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Yield Stress

(Mpa)Allowable

Stress (Mpa)Max Stress

(Mpa)Deflection

(µm)Design Factor MOS FOS

Case VX 54 24 6.5 12.4 3 0.23 8.31


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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

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Focal Plate Assembly2013-02-01

Focal Plate

G10 Flex Support

Flatteners and Detectors

Dewar


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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 oC for thermal links on back of Al plate

– Ambient temperature on outside of dewar frame

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Assembly Temperatures

• Assembly temperatures show that the contact lens temperatures are well above the focal plate temperatures– In the -30 oC to -60 oC

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

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Aluminum Focal Plate Temperatures

• High thermal conductivity of aluminum enhances cooling performance– Even down to low

temperatures ~ -130 oC– Temperature gradient

across the focal plate of ~ 8 oC

• Less expensive than other material and more machine-able

• Due to high thermal contraction, addition of support flexures are need to accommodate thermal shrinkage

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CCD Temperatures

• Prime interest point is thermal gradient across the CCD array– Across all 12

CCD’s there is 3 oC temperature difference

– Temperature gradient across a single CCD is less than a degree

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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 -31oC, with a minimum of -60 oC

• Approximately 30 oC temperature gradient across lens

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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

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TLL


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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

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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

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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

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Flattener

CCD and Detector

Focal Plate

Invar Frame


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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

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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

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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 oC and cold plate at -125 oC

– Show large temperature gradient across the support

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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

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G10 Support Stress

• Equivalent stress in MPa shows maximum stress near support locations– Max stress is in the

25-35 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

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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

2013-02-01

Documents

ZTF Cryostat Finite Element Analysis