1
ZTF Technical Meeting
2013-02-01
ZTF Cryostat Finite Element Analysis
Andrew Lambert
2013-02-01
2
Outline
ZTF Technical Meeting
• Fused Silica Window – Case XV
• Aluminum Focal Plate Assembly
• Single CCD Assembly
• G10 Flex Supports
2013-02-01
3
Fused Silica Window Mesh and BCs
ZTF Technical Meeting
•
Quarter model simulated
due to symmetry – Case
XV
•
Boundary conditions
– Simply supported at Orings
– Radiation to ambient
temperature
– Radiation to cold CCD
temperatures
– Vacuum loading
•
Worst case temperature
loading, however stress is
dominated by vacuum
loading, not temperature
2013-02-01
4
Fused Silica Window Temperatures
ZTF Technical Meeting
•
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
2013-02-01
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Fused Silica Window Max Stress
ZTF Technical Meeting
•
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
2013-02-01
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Fused Silica Window Deflection
ZTF Technical Meeting
• 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
2013-02-01
7
Fused Silica Window MOS and FOS
ZTF Technical Meeting
•
LBNL specifies a factor of safety = 8
–
–
•
2013-02-01
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
Yield Stress
(Mpa)
Case VX
54
Allowable Max Stress Deflection Design
Stress (Mpa) (Mpa)
(µm)
Factor MOS FOS
24
6.5
12.4
3
0.23 8.31
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Focal Plate Assembly
ZTF Technical Meeting
2013-02-01
• 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 Assembly
ZTF Technical Meeting
Focal Plate
2013-02-01
Flatteners
and
Detectors
Dewar
G10 Flex Support
10
Assembly Mesh and BCs
ZTF Technical Meeting
•
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
2013-02-01
11
Assembly Temperatures
ZTF Technical Meeting
•
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
2013-02-01
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Aluminum Focal Plate Temperatures
ZTF Technical Meeting
•
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
2013-02-01
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CCD Temperatures
ZTF Technical Meeting
• 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
2013-02-01
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Contact Lens and Frame Temperatures
ZTF Technical Meeting
•
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
2013-02-01
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Assembly Deformation
ZTF Technical Meeting
•
Aluminum focal plate
assembly experiences
shrinkage due to low
temperature operation
•
Various materials are used
–
–
•
2013-02-01
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
L  LT
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CCD Deflection
ZTF Technical Meeting
• CCD deflection is
reduced by Invar
spacers between
the CCD and
aluminum plate
• CCDs at the center
of the array
experience the
greatest deflection
2013-02-01
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Detector Surface Deflection
ZTF Technical Meeting
•
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
2013-02-01
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Single CCD Assembly
ZTF Technical Meeting
•
Single CCD assembly
used to examine
stress in the aluminum
flexures
2013-02-01
Flattener
– Small model allows
Invar
easier examination
of critical
Frame
components and
reduces solution time
•
Boundary conditions
are applied to mimic
the same behavior as
the overall plate
assembly
Focal Plate
CCD and
Detector
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Single CCD Temperatures
ZTF Technical Meeting
• 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
2013-02-01
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Aluminum Flexure Stress
ZTF Technical Meeting
•
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
2013-02-01
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G10 Support Temperatures
ZTF Technical Meeting
• 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
2013-02-01
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G10 Support Deformation
ZTF Technical Meeting
•
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
2013-02-01
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G10 Support Stress
ZTF Technical Meeting
•
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
2013-02-01
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Future Work
ZTF Technical Meeting
• 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