Corner Raft Tower Module

Steady-State Thermal Analysis

Maria Krutikova, Kirk Arndt

Purdue

August 11, 2014

1

4

Input Parameters – Materials and Conductances

Material Part K value ( )½ oz. Cu in CCD Cable CCD Flex Cables 22.44

AlN Sensor Base 160

CE7 2mm spacers 180

CeSiC – HB srp 2 Corner Raft 145

Copper CryoCage, Cold Bars, Thermal

Strap blocks, REB Core400

Cu Braided Wire Thermal Strap Wire Braid 250

G10 Warp Direction REB core buffer .75

G10 + 3.38% Cu Electronic board 14

Invar WFS Step Plate and Riser 10

Si Sensors 350

SS304 CCD Cable Connectors 15

6

Input Parameters – CRSA Heat Loads

Average power (W)

Comment

IR load on exposed area of Corner Raft

0.48IR load on area of the front-side of the Corner Raft uncovered by

sensors (~20 cm^2).

Wavefront sensor electronics + IR load*

0.71Average WFS power (=284 mW) is 11% read and 89% quiescent duty-cycle (same duty-cycle as Science Sensor CCD).

Guide sensors electronics + IR load*

1.5 Same as two Science Sensor CDD with 100% read duty-cycle.

* IR heat load per sensor = 17.64cm^2 (area of 4kx4k sensor) x 24mW/cm^2 = 423mW per sensor(Gordon Bowden’s analysis of IR heat load on

the focal plane in the Corner Raft locations)

Note: no makeup heat applied to Corner Raft baseplate

5

Input Parameters – CREB Heat Sink and Loads

WREB GREB

• -40C contacts with cold plate

• WREB heat loads for ITL WFS

and GREB heat loads for e2v

guide sensors from Sven H.

• No radiative or conductive

losses from REBs

11

Results – CRSA

∆T of 5.5C within entire CRSA

• WFS: -109.3C Max

∆T of 1.3C

• Guider: -105.6C Max

∆T of 4.0C

LSST Camera Review • SLAC National Accelerator Lab, Menlo Park, CA • 2014 6

Overall Temperature Comparison to Science Raft

Science Raft Corner Raft

Trim Heat On0.5W x 2 heaters

Trim Heat OffTrim Heat Off

LSST Camera Review • SLAC National Accelerator Lab, Menlo Park, CA • 2014 9

Q1 Sensors/Spacers

Raft

Strap

Tower

Cryoplate

Q2

Q3

30

27

25

5

0

ΔT (K) from cryoplate

R1

R2

R3

R4

1-D model (electrical analog) for first-order calculation of thermal gradients

Ignore lateral heat flows

Rn = bulk + interface thermal impedance of component

Strategy: make R3 dominant, keep R4 minimum

Makeup heater to compensate for time variation of L3 and cryoplate temperatures, and for controlled warm-up

0.65 K/W

0.36 K/W

3.57 K/W

0.89 K/W

3.0 W

1.0 W

1.6 W

Radiation from L3 + CCD source follower dissipation

Makeup heat

FEE power dissipation

through flex cables

Corner Raft-Tower simplified thermal model3

Simplified CRTM Model

• No fasteners – all part connections are bonded

• Simplified CREBs – no bushings, BE connectors, thermal vias

• Simplified sensor packages – consist of base and top, no epoxy layers

• No conductance barrier or raft hold downs

LSST Corner Raft PDR September 30, 2014 1

Structural analysis: supports, applied forces

Thermal contraction is centered on the

intersection of V-grooves

no contribution to stress in Corner Rafts

due to V-groove-on-balls kinematic mounts

Fixed supports (balls),

gravity and hold-down spring

(40N per arm) loads

Degrees of

freedom at

ball/V-block

interfaces

• CRSA mass estimate 0.6-0.8 kg(includes sensors, raft plate, mounting hardware, heaters,

and hold-downs)

• 40N force required to ensure CRSA

remains in contact with balls on Grid

during 5g (seismic) acceleration (0.8kg x 9.81 m/s^2 x 5 = 40N)

hold-down spring force applied

= 40N per arm

LSST Corner Raft PDR September 30, 2014 3

Z-axis deformation of corner raft surface and sensors <1 micronsolution includes structural AND thermal loads

Raft Surface Deformation Focal Plane Deformation

Structural analysis: directional deformation

Deformation < 1 micron Deformation ~ 1 micron

displacement ~12 microns

(same as science array, corrected by pistoning)

Note: frictional connection applied at the sensor-to-spacer and spacer-to-corner-raft interfaces

g

r

a

v

i

t

y

GREB and WREB Design Revision

Old Cold Bar straps

Old Cu braid position locates the straps “underneath” the cold plate fingers. The braids are flexible and will move away from screws for fastening the straps to cold fingers.

New straps take advantage of the flexibility of the braids. Cu blocks at the ends of the straps are retracted within the profile of the REBs during insertion of the CRTM into the cryostat, then extended and fastened to the cold fingers.

WREBGREB

2

WREBGREB

New Cold Bar straps

REB straps retracted during insertion

3

Installation of CRTM into the Cryostat

REB straps extended and fastened to cold fingers

4

REB Strap Hardware (shown restraining Block to REB in Retracted Position)

Cu Braids

M2.5 Captive Screw

Cu Block

Screw “Anchor” (glued to board)