1. FP420 Detector Cooling Thermal Considerations
Berend Winter

2. The Detector Array Flexible Link (electronics signals)
Prox. Electronics
Tracker Cell
Detector Plane (CE7)
Beam Line
Thermal Interface to Cold Sink
The detector array is moved together with the beam line tube (not shown here) which has flexible links (preserving primary vaccum) with the main beam line. This means that the interface between the detector array cold sink and the vacuum enclosure (secondary vacuum) moves together (no relative displacement).
Thermal Design Goal: tracker cell operating at -20°C

3. Dissipated Heat The detector consists of 5 super modules
Each super module consists of 2 detector planes (Al/Si) with each holding 2 detectors on each side (4 in total per super module)
Each super module has an ASIC underneath each detector and the 4 silicon detectors within each super module share one proximity electronics module
Each detector/ASIC combination dissipates 0.5 W or less
Each proximity electronics module dissipates 1.0 W
This makes for a total of 3 W per super module and 15 W in total for each detector array head (5 super modules)
As we will see later on 5 W needs to be added due to parasitic heatloads

4. Heat Paths
Each super module has two thermal interfaces to a cold sink (5 on each side)
Each detector (10 in total for a detector array) is connected to (relatively) warm front end electronics via a flexible link
The detector array is mounted on an interface (vacuum enclosure) at room temperature (20 to 30 deg-C)
The detector array is radiation coupled with the vacuum enclosure
The thermal hardware (straps) are radiation coupled with the vacuum enclosure
The cold sink has to interface through the vacuum enclosure and is somehow conductively coupled with it

5. Thermal Network Around the Detectors (one detector plane)
Conducted Heat
Radiated Heat
Cold Sink Interface
Si tracker cell 0.5 W
Flex link
Al/Si detector plane (support structure)
Prox. Electronics (0.5 W)
Si tracker cell 0.5 W
Flex link
cables
Cold Sink Interface
support
Vacuum Enclosure

6. Parasitic Heat Loads
The detector array as a whole is supported by the vacuum enclosure, heat will flow from the warm vacuum enclosure to the cold detector array.
Using Torlon support rods the parasitic heat load can be limited to less than 1.5 mW/Celcius (6 rods, 5x5x50 mm) with the detector array running at -30°C and the vacuum vessel at +30°C this gives less than 0.1 W parasitic heat load for the detector array.
The proximity electronics are connected through the vacuum vessel wall, 30 conductors (28 AWG)
30 wires, 28 gauge with a minimum length of 100 mm and a thermal jump of -30°C to +30°C gives 0.5 W per proximity electronics PCB (one for each super module). This is 2.5 W for the whole detector array.

7. Parasitic Heat Loads (Continued)
Radiated Heat
Detector Array absorbs 2.2 W total (worst case)
Thermal straps absorb 0.05 W total (worst case)

8. Tracker Cell Model
In 2006 an ESATAN analysis was performed to assess the thermal gradient over the tracker cell. With updated materials and dimensions the results are still the same. Gradient between tracker cell and the CE7 carrier plate is 2.5°C (worst case)

9. Thermal analysis of Al/Si CE7 structure
A thermal analysis was performed on the CE7 support structure.
Applied heatloads:
1 W for two tracker cells
0.5 W for prox. elec.
0.5 W for parasitics
Applying a boundary condition of -29.3°C the interface with the tracker cell is -22.5°C
As a result the tracker cell itself is sitting at -20°C.
-29.3°C
-22.5°C

10. Thermal Gradients
Since we know the heat loads we can calculate the thermal gradient at each stage of the design.
Analysis has shown a gradient of 2.5°C from the active part of the tracker cell to the support structure (CE7 plane) with a heatload of 0.5 being dissipated in the tracker cell. This is worst case (but not extreme) since most of the heat will be dissipated in the ASIC underneath.
The heat dissipated in the proximity electronics and the parasitics (radiation and conduction) were analysed and resulted in a gradient of 6.8°C between the CE7 area where the tracker cell is mounted and the cold interface. The total heat flow per CE7 plane is 2 W (dissipated and parasitic)
The gradient between the tracker cell and the cold interface is 9.3°C in total.
For the rest of the design we need a cold interface to the CE7 planes at -30°C and we need to conduct 2 W per interface. (Two sinks on each side of a super plane makes ten in total, conducting 20 W in total)

11. Peltier Cooling
Peltier Cooling forces a thermal gradient over a series of solid state elements capable of pumping heat
Not a very efficient way of cooling. About 2% efficient for the first stage of detector cooling in our case (with updated heat loads)
Maximum gradients in order of 70°C (without pumping capacity)

12. Peltier Cooling no longer Viable
Last year the assumptions on the total amount of heat dissipated and the total amount of parasitics were seriously underestimated (factor of 3). Simple hand calculations now show that the Peltier cooling option is no longer viable. The efficiency has dropped by a factor of ~4. So we need to dump significantly more heat with less efficient Peltier elements (around 2% efficiency).
We need to dump an awful lot of heat to the outside of the vacuum vessel and use fins with forced convection as a heat sink to go all electric (peltier and cooling fans) on the cooling. For example computer CPU fans only get rid of a Watt or two in optimal conditions. That is by using a significant thermal gradient (about 70°C)
Using Peltier devices we estimate the total heat we need to dump (somehow) into the tunnel to be significant more than 1 kW…

13. Fluid Cooling
Using a simple fluid cooling loop with a cold plate on the inside of the vacuum vessel we should be able to dump the heat efficiently. This is by means of a copper tube soldered to the back of a copper plate. The copper plate (gold plated) has holes where thermal straps from the detector can be bolted on firmly. This is a technique routinely used in cryogenics/vacuum chambers. The interface through the wall is via a standard (off the shelf) thin wall stainless tube feed through.

14. Heat Transfer
The heat flows per CE7 plane into a copper shim and then into a copper block where the heat of two CE7 planes combines before it flows into the cold plate.
-29.3°C CE W
Cu block
2 W
Fluid Cooled Copper Plate
-29.3°C CE W

15. Heat Transfer Continued
The thermal jump between the CE7 planes and the cooling plate is 25.7°C making the total jump between tracker cell and cooling plate 35°C.
Conclusion: The cooling plate has to be at least -55°C and be able to pump 10 W (one cooling plate on each side of the detector arrays)

16. Cooling Thermostats Cooling thermostats come in wide range.
Lauda is a prime supplier of cooling thermostats capable of cooling down to -90°C, most likely not radiation hard
Can CERN provide for a cooling fluid line?