Mauro Donega, Tim Jones, Irina Nasteva, Steve Snow, Peter Sutcliffe, Ray Thompson, Rainer Wallny, Ian Wilmut

Update on Thermal - Mechanical Performance of K5 Modules

Questions on thermal and mechanical issues

In Section 2.3 of the FDR report ATC-RI-ER-0038 the reviewers asked for “more complete testing to understand reproducibility in the alignment and thermal performance of the module mounting” with more detailed questions in section 3.2.

We have answers on:

1. New specifications for the module mounting with spring washers.

2. Out-of-plane stability of modules with the new mounting under thermal cycling and connector plug/unplug cycling.

3. Thermal grease application and QA of grease contact.

4. Thermal runaway measurements with an irradiated module.

5. Reproducibility of thermal contact with grease.

We believe that the original FDR documents already demonstrated:

• Mechanical integrity of modules at –30 C.

Module mounting with spring washersSpring washers are required to ensure that the module is reliably clamped against the cooling block. Dimensional changes of up to 50 microns are expected due to thermal expansion and to possible thinning of the grease layer. The spring should maintain a force in the range 10-50 N over this range of movement.

We have identified and tested washers which meet this specification. For the main block; Belleville CuBe D1052025. For the far block; Springmasters CuBe 508-11.

Main cooling block: Belleville washer and special nut to keep it centred.

Far end cooling block: Crinkle washer and plain nut.

Both nuts tightened to a controlled torque and then locked with a spot of glue.

Thermal cycling resultsRaw Cycling data

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Run number

Hei

gh

t w

.r.t

. ref

pla

ne

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m)

Far 11

Far 12

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Far 21

Far 22

Far 31

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Far 33

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Chip 1

Chip 2

Chip 3

Chip 4

Chip 5

Chip 6

Far Blk 1

Far Blk 2

Far Blk 3

Far Blk 4

Main blk 1

Main blk 2

Run number 1 2 3 4 5 6 7 8 9 10 11Cycles 0 1 2 3 4 5 15 25 35 55 83

Module mounted on blocks with new spring washers. Blocks are machined from solid Al frame which provides reference plane.

Thirty points measured in each run: 3x3 grid on each detector, 6 chips on the hybrid, 4 points on far block, 2 points on main block.

Between runs the module is cycled from +25 to –30 C and back.

The maximum movement between Run 1 and Run 11 was: on the detectors –m at Far11 and +24 m at Far33. On the hybrid –79 m at Chip 1.

Detector movements are small compared with the physics Z tolerance of 180 m r.m.s. and all movements are small compared with the 1mm clearance between modules.

Connector plug/unplugging results

Raw Plug/Unplugging data

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igh

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Chip 1

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Main blk 1

Main blk 2

Run number 1 2 3 4 5Cycles 0 1 3 5 10

Surveys are the same as for thermal cycling but this time the connectors are plugged in and unplugged between each run.

The maximum movement between Run 1 and Run 5 was: –m at Far31 and +3 m at Far12. There was no significant movement of the hybrid.

Detector and hybrid movements are small compared with the physics Z tolerance of 180 m r.m.s. and compared with the 1mm clearance between modules.

Grease contact –application and QADow Corning DC340 will be used to make the thermal contact of module to block. Many alternatives have been tested, from thin greases to thick pastes and solid pads, but none is better than the baseline.

Application. DC340 will be applied as a pattern of dots over the cooling block area using a volumetric dispenser. This method has proved successful with another grease and gives a repeatable contact area. The optimal pattern of dots for DC340 has not yet been chosen but the test method is established.

QA. The thermal contact of module to block will be checked by:

1. Surveying the Z position of each module after it is mounted to check that it is in the expected place.

2. Running the hybrid at a known power and checking that the temperature difference between the hybrid thermistor and the coolant has the expected value.

3. Running the disk with the coolant 10 degrees below the ambient gas temperature and recording the detector temperature with an infra red camera. The detectors will pick up enough heat from the ambient to make them significantly warmer than the coolant. If a module has poor thermal contact it will be detectably warmer than the expected value.

Irradiated module with evaporative cooling.

• An irradiated inner module, K5-313, has been tested on two versions of the cooling block with the C3F8 evaporative cooling system.

• The leakage current of this module at 500 V dissipates a power of 91 W/m2 at 0 C and the expected dissipation at the worst position in the forward SCT after 10 years is 93 W/m2 at 0 C. The specification has been set at 185 W/m2 at 0 C to allow for uncertainties in inelastic(pp), particle transport and radiation damage constants.

• The highest temperature that the evaporative rig can run is around –13 C. At this temperature we were unable to make this module go into thermal runaway.

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Inlet-14.1

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N2 atmosphere2.5

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11 Hybrid thermistor

Hybrid Power = 6.4 WDetector Bias = 500 VDetector Power = 0.5 W

Irradiated module at maximum power and temperature

Temperatures

Run 11e on PEEK split block.

Irradiated module extrapolation

--- Baseline block--- -- 0.4mm PEEK block--

Meas. Extrap. Meas. Extrap.

Parameter T=7.4 T=7.2

Hybrid Power 6.4 W 6.4 W 6.4 W 6.4 W

Detector Voltage 450 V 450 V 500 V 500 V

Detector Current 950 A 950 A 950 A 950 A

Detector Power 0.428 W 0.428 W 0.475 W 0.475 W

Coolant Temperature -13.4 C -20.8 C -11.9 C -19.1 C

Hybrid Block T -4.2 C -11.6 C -2.5 C -9.7 C

Detector Block T -6.1 C -13.5 C -6.7 C -13.9 C

Detector Temperature -2.1 C -9.5 C -1.7 C -8.9 C

Power Density at 0 C 86 Wm-2 185 Wm-2 91 Wm-2 185 Wm-2

Gas Temperature 2.5 C -4.9 C 2.6 C -4.6 C

1. Increasing the radiation damage of the detector from 86 to 185 W/m^2 at 0° C,

2. Decreasing the temperature of the coolant and the environment gas by 7.4 C,

3. Keeping the hybrid power the same.

The combination of 1 and 2 leaves the detector power unchanged. So all powers are unchanged, all Ts are unchanged and all temperatures are reduced by 7.4 C.

Exact extrapolation from measured conditions to worst case Atlas conditions is possible by;

Irradiated module runaway with chiller

Since the evaporative cooling rig can not run warm enough to make the irradiated module run away we used a chiller with the 0.4mm PEEK split block to see runaway and check the module simulation.

The chiller was set to –5 C, the hybrid power was 6.5 W and the detector bias was turned up in steps from 50 to 450 Volts. After each step the nitrogen temperature was adjusted until it was equal to the detector temperature. This ensures that the detectors are neither heated or cooled by convection. Somewhere between 425 and 450 V the module goes into thermal runaway.

The simulation used a linear fit to the measured block temperatures as input and predicted the detector temperature and power.

Real runaway: 85-94 W/m2 @ 0C

Simulated runaway: 76 W/m2 @ 0C

Runaway plot (T_chiller=-5, T_atm=T_det)

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Detector power density (W/m^2 @ 0 )

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T_det simulated

T_det_block

T_coolant

Linear (T_det_block)

Runaway power

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Detector power density (W/m^2 @ 0)

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P_det simulated

Irradiated module conclusionExact extrapolation from measurements on K5-313 shows thermal stability after the maximum Atlas radiation dose, when mounted on a baseline cooling block with coolant at –20.8 C. In Atlas there could be an extra 0.4 W load from convection, which was probably much smaller in the test box. Reducing the coolant temperature to –22.6 C (T = 0.4 W × 4.53 K/W thermal resistance of module) will compensate for this extra 0.4 W load.

One may be tempted to add further safety factors to account for; module-to-module variations, block-to-block variations, unforeseen problems with the thermal grease, etc.

But remember that; the measured module was not close to runaway (detector power < 0.5 W), the radiation dose assumed for thermal specifications is a factor 1.9 times higher than the PS standard 3×1014 protons which almost kills the FE chips.

The runaway measurement with the chiller shows that the module simulation is in reasonable agreement with reality. It also confirms the module was far from runaway in the evaporative test.

Conclusion: The statement made at the FDR that the module has a safety factor of two with the baseline cooling system provided that the coolant temperature is everywhere below –23.4 C is still valid.

We have ordered a batch of 1mm PEEK split blocks for the first disk. Although the 0.4 mm version was not significantly better than the baseline there is some evidence that we could gain around 4 degrees on the detector temperature with this design. > Backup slides.

Reproducibility of thermal contactThermal contact

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Measurement

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bri

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k T

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pe

ratu

reK5-302 grease

K5-302 pad

K5-402 pad

K5-402 grease

K5-303 pad

K5-303 grease

K5-400 pad

K5-400 grease

Note: Block is typically 3 degrees above chiller

setting

Pad mean 16.9Grease mean 12.0

Thermal contact results of four measurements on each of four modules with two thermal interfaces; DC340 grease and keratherm pad.

(note: the two points from K5-400 grease are hidden by the K5-402 grease points)

The r.m.s. spread of of the temperature is 1 degree for the grease and less for the pad.

Backup slides.

Slides beyond this point are for background information. Not part of the presentation.

Survival of temperature extremes and gradientsSpecification 6 was that the module must survive temperatures between –30 and +50 C and the expected temperature gradients at operational temperature.

We tested this by putting three modules through the sequence of; full survey, ten thermal and power cycles, full survey. We looked for differences between survey results as evidence for mechanical damage.

K5-300 cycling

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Copy of FDR slide

Detector transverse tiltDetector area tilt

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Run number

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Hybrid end

Mid detector 1

Mid module

Mid detector 2

Far end

Three temperature cycles .... cool down .... three hybrid power cycles

Total tilt of around 50 microns is getting near the specified limit (57 microns) but is tolerable. Cumulative part is probably an interaction between module and block, rather than a feature of the module itself.

Copy of FDR slide

Module mounting

Alternative block designs

1. The baseline, semi-split block.

2. A fully split block. Two separate pieces of C-C soldered onto the pipe 0.5 mm apart. Not a practical design because their surfaces must be accurately coplanar but used to test the principle of a thermal break.

3. A block made from a layer of PEEK sandwiched between two pieces of C-C. The PEEK layer was 0.4 mm thick whereas simulation suggests that 1mm is optimal.

4. A 1mm PEEK split block. Not tested in reality, only simulated.

We have confirmed that the module is safe with the baseline cooling block at coolant temperatures below –23 C. However, we may be able to add safety at little cost by introducing a more effective thermal break between the detector and hybrid parts of the block. We have looked at four designs:

Block simulations3D heat flow simulation of baseline and PEEK split blocks.

Dominant uncertainty is the heat transfer coefficient.

Results match reality well for baseline block, not so well for the PEEK split block.

HTC versus power density

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Power density (W/cm^2)

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upper

lower

Vacek et al

Baseline Block. Measurements and simulations.

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Detector simulated

0.4mm PEEK split block. Measurements and simulations.

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Split block resultsThe most important feature of a block design is the temperature of the detector side when the hybrid is at its maximum power of 7 W. The maximum detector side power is 1.5 W for outer and middle modules, 2 W for inner modules. The module specification requires the detector side to be below –12 C at 2 W.

Circles are measurements, squares are simulations. The error bars on the simulation are due the uncertainty of the heat transfer coefficient.

Detector block with 7 W on hybrid.

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Detector side power (W)

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Fully split

Baseline simulated

PEEK 0.4mm simulated

PEEK 1mm simulated

Extrapolation of irradiated module runaway curveRunaway Plot

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Power density (uW/mm^2) @0C

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Tcoolant=-5

Tcoolant=-20

Tcoolant=-15

Tcoolant=-10

The combined temperature and radiation damage extrapolation, which leaves powers and relative temperatures unchanged, can also be applied to the measurements of the irradiated module on PEEK split block with the chiller.

The result is this family of curves, where the real data is with T_coolant = -5 C and the others are extrapolations.

Thermal Interface Test Jig Thick-film / ceramic 150 Ohm resistor

embedded in aluminium block Heater block insulated from plunger

assembly by TUFNOL spacers Cooled block with multiple bores for high

heat transfer to fluid. Cooling supplied by re-circulating chiller. Plunger mechanism ensures repeatable

parallel mating of contact surfaces. 2 m division DTI for notional thickness Mating surfaces have pre-K5 cooling

block shape – no split. Area ~ 192 mm2.

Blob pattern

• Eg. 4 blobs– For 0.025 mm layer need ~ 1 mm3 per 7mm diameter blob.– Coverage ~ 160 mm2 – 83%

• Tried for Shin-Etsu grease