Plan-B results
for the LHCb VELO upgrade
Paweł Jałocha
University of Oxford
Plan-B concept
• Replace and match the VELO silicon
microchannels characteristics:
–
–
–
–
Mechanics: pipes, size, thickness, stability
Radiation length
CO2 flow and pumping pressure
Thermal conductance: delta-T
• Classic (cheap) technology
– Pipes of steel ~0.6/0.3mm O/I diameter embedded in
a substrate: CF, TPG, Al, ceramic, silicon, ...
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Plan-B prototypes
• Half-size prototypes where substrate was carbon fibre sheet and TPG
– Proof of principle for piping, flow/pressure, dissipated heat limit
– Poor thermal performance because no grooves in the substrate
• Full size prototypes
– Aluminium prototypes
• proof of principle for piping glued into the grooves
– Shapal ceramic prototypes
• a realistic material: good Cte match to silicon
– Silicon prototypes
• an ideal (microchannel-like) material
• mechanically symetric design
• a challenge to assemble: not resolved by now
– Acceptable thermal performance
• the choice of glue (or other method) for joining pipes to the substrate
– Indestructable piping
• (hard) brazed and welded, no (soft) soldering
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Plan-B piping
• CO2 supply for the module is split into four
capilary pipes: 0.65/0.32mm O/I diameter
• Each capilary has an 0.13mm orifice for triggering
the boiling and even flow distribution.
• About 20 pipings set are to be produced in
Oxford
– 7 produced this far
• The pipings produced this far show very close
flow/pumping pressure characteristics
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Plan-B piping
flow/pumping pressure characteristics
• Hardly any change whether cold or warm
– Here two different pipings at +10 and -30°C
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Plan-B piping
flow/pumping pressure characteristics
• But when heat-loaded there is a significant difference
– Here 32W of heat applied at +10 and -30°C (Al+Stycast)
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Plan-B piping
flow/pumping pressure characteristics
• Another prototype pipings (Al+Hysol)
– 32W of heat applied at +10 and -30°C
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Plan-B piping
flow/pumping pressure characteristics
•
But... I seem to have problem with the flow measurement (!)
– 32W of heat applied at -33°C to Al+Stycast
– Flow gets to zero at 3bar and then positive again at 2bar ?
– This I don’t really believe, as by the cooling capability I could see there was still a good flow.
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Flow verification by the dry-out limit
An attempt to verify the flow by measuring the dry-out limit.
• Set TRACI to given temperature and pump RPM, adjust the local box by-pass for
given pumping pressure: 2 bar
• Measure the dissipated power level at which the outlet temperature rises by given
amount: 3°C
• Note the changes in the indicated pumping pressure and flow
TRACI
setpoint
Dry-out Pumping pressure
power (change)
Flow
(change)
Outlet temp.
(change)
+10°C
69W
2.00→2.13bar
0.37→0.25g/s 9.5 → 12.7°C
0°C
69W
2.00→2.20bar
0.22→0.09g/s 0.5 → 2.7°C
-10°C
68W
2.01→2.25bar
0.40→0.30g/s -9.5 → -7.0°C
-20°C
64W
2.15→2.31bar
0.03→0.13g/s -19.7 → -17.0°C
-30°C
50W
1.95→2.23bar
0.02→0.15g/s -30.1 → -27.2°C
-33°C
41W
2.08→2.25bar
0.15→0.29g/s -33.2 → -31.2°C
only 2°C rise
-33°C
66W
3.08→3.22bar
0.31→0.18g/s -33.6 → -29.2°C
higher ΔP
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Single pipe dry-out
• Here the piping is
flowing 0.25g/sec (flow
reduced on purpose)
• The heater on the back
dissipates 22W
• The IR camera is
watching from the
pipes side
• The most inner pipe,
which is most heatloaded experiences a
dryout: simply stops
taking heat anymore
and „melts” into the
substrate
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Single pipe dry-out
• Here the attempt to
measure temperature
along the pipe which
expriences the dryout
– The vertical line on
the IR picture and the
green line on plot
below
• The dryout appears to
be a relatively sudden
transition from „fully
taking the heat” to
„taking no heat”
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Plan-B Shapal substrate
• Current best tested
• Material: strong, stable and good match to
silicon
• Can be directly ordered £450/piece
– Cut to shape, grooves done, all ready for
assembly.
• Two prototypes built for thermal performance
evaluation with two different glues used for
the pipes.
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Plan-B Shapal substrate
•
Pipes just inserted
into the grooves, not
glued yet.
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Plan-B Shapal substrate
Shapal prototype glued
with Stycast, with dummy
ASICs and sensors
attached.
In this IR picture: just one
set of the ASICs is powered
with 2W/ASIC
TRACI is set to produce CO2
at 10°C
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Shapal prototype under IR camera
No power
ASICs powered
ASICs+sensor powered
Sensor
tip
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Plan-B Shapal substrate
ΔT measured in vacuum at -30°C
• 2W/ASIC (+ 1W/sensor)
– ΔT = 6.7°C (9.6°C) at the tip (white trace)
• There is virtually no change in ΔT whether at +10°C or -30°C
ASIC’s and sensor powered
Only ASIC’s powered
Mid-ASIC
Sensor-tip
CO2 outlet
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Summary
• Several plan-B prototypes has been produced
– The most tested and best candidate for now is the Shapal ceramic substrate
• The flow characteristics are „better” than microchannel
– At 4bar the piping flows 0.7g/sec
– Hardly any reduction of flow at low temperature
– Significant reduction in flow when fullly heat loaded
• But the flow measurements are in question
– Nearly identical characteristics of all full size pipings produced this far
• The cooling capability well above the nominal 30W even at pumping
pressures below nominal: 66W@3bar
• The thermal characteristics are useable, but with little margin as for now:
– Assuming 2W/ASIC and no sensor heat, the ΔT at the tip is 6.7°C, assuming 1W
of sensor heat it goes up to 9.6°C
– As the margin is currently almost none it would be safe to consider running
the CO2 at -32 or -33°C
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Backup
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Plan-B Shapal prototype
(almost) all ASICs powered
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Why ΔT ?
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Shapal prototype:
Compare Stycast (1W/mK) with high conductive glue (3.6 W/mK)
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