Integrated Stave Mechanics and Cooling

1. Integrated StaveMechanics and Cooling ATLAS Upgrade Workshop December 2007 M. Cepeda, S. Dardin, M. Gilchriese, C. Haber and R. Post LBNL W.Miller and W. Miller iTi

2. Nominal Design – Short Strips 10cm detectors, each with 4 rows of chips on hybrids ~ 1 meter Carbon honeycomb or foam Carbon fiber facing Bus cable Coolant tube structure Hybrids Readout IC’s Hybrids glued to detectors and this assembly glued to mechanical/cooling support

3. What Has Been Done • Prototypes fabricated and studied • 1m long for mounting silicon modules(see talk by Haber) • Three thermal prototypes, each about 1/3m long, with heaters, silicon, cables, dummy hybrids to simulate nominal design • Three different tube types(to simulate compatibility with C3F8, C2F6/C3F8 mixtures and CO2) • Varied facing thickness and adhesives • Thermal measurements(IR imaging) before and after thermal cycling from 20C <-> -35C fifty times completed • Measured weights as input to material calculations • Preliminary demonstration of removal/replacement of silicon on stave completed. • Design studies and FEA • Primarily of nominal design(hybrids glued top of silicon, short strips) • Thermal performance and thermal runaway • Gravitational deflections and support concepts • Thermal distortion (as detector is cooled down) • Long strips ie. outer barrels. • But also some work on • Bridged hybrid • Other – hermeticity, endcaps, production, R&D plan, risks……….. • Summary only here – see Backup slides and references therein

4. Prototypes • We chose to build three prototypes to approximately simulate tube sizes that would be applicable to C3F8, C2F6/C3F8 and CO2. To answer the question - is this design compatible with all these? • Tube diameters chosen from practical concerns(ie we had tube or could easily get it) but also from calculations of fluid properties that are summarized in the table below.

5. Prototype Construction 4.9 mm tube/foam POCO foam: about 0.5 g/cc thermally conducting carbon foam 2.8mm tube/foam Carbon honeycomb Facings are K13D2U fiber laminates All tubes aluminum Flattened tube 1m prototype Note prototype width is about 7cm – set in 2006

6. Alumina hybrids Bus cable Heaters 0.3mm silicon Thermal Prototypes Water at about 20C IR images Before and after thermal cycling between 20C and -35C 50 times 3.3 W/hybrid(0.55 W/chip) No Power

7. Major Prototype Lessons • Thermal performance - T between coolant and dummy detector • Same after thermal cycling to -35C fifty times. No evidence of lost coupling of tube to facing. • Same within measurement error for facing thickness in range about 0.25 – 0.7 mm. This also validated by FEA calculation. • Same within about 15% for all three tube types, also expected from FEA. Better for round tube with foam. • T measured agrees with T FEA to within about 1.5C or better, so can have some confidence in FEA • Deflection measurements (of 1m prototype) agree with calculations within about 15%. • Multiple successful trials of gluing dummy silicon to bus cable with SE4445(thermally conducting, flexible adhesive used to attach current pixel modules), removal using simple tooling(essentially a guided wire), clean up and reattach at same spot. • See Backup for the details

8. Thermal Model – 10cm Detectors • Nominal design, hybrids glued to top of silicon • Section shown below. Detailed properties in Backup • Detector heating from parameterization of Unno at MIWG 11/07 • Vary chip power(0.125-0.5W/chip) and detector power 1 or 2 mW/mm2 at 0C • Vary wall temperature of cooling tube to simulate different coolants and to see runaway.

9. Thermal Performance Note in this design, chip temperatures are within <2C of detector temperature C3F8 C2F6/C3F8 CO2

10. More Thermal Performance • Variants to lower temperature(with either more material or more cost or both) • “Triple-U” tube(see picture below). This would decrease the T by very approximately 4C in the temperature region of interest for the nominal design. • Carbon-carbon(that has much higher thermal conductivity through thickness) but gain <2C and not worth much higher cost. • Small gain(~1C) possible with fiber(eg. K1100 instead of K13D2U) that has higher axial thermal conductivity(by about 40%) but somewhat higher cost. • Thermally conducting foam rather than honeycomb would gain 2-3C. • Multi-part bus cable(not cover all of detector area). TBD • Conclusion from these studies depends obviously on coolant, chip power and prediction of detector power • If CO2, can have significant headroom • If C3F8, headroom reduced compared to CO2(no surprise) by ≥10C but still may be acceptable. • If C2F6/C3F8 mixture somewhere in between - • In all cases, need to validate with measurements on full-scale prototypes - - Note that T is referenced to tube wall temperature in this talk

11. Thermal Model – Bridged Hybrid Wire bonds, simulated as thin solid, reduced K to 97/mK 10cm 1mm air gap for bridge Al Cooling tube 0.21mm ID Chips 0.38mm thick (148W/mK) Separation between facings 4.95mm Foam bridge support

12. Air box More Bridged Hybrid • Enclosed model in “box” to estimate effects of air conduction • Preliminary result for 0.25W/chip and zero detector power is a T of about 4C compared to about 6.5C for the nominal design(same coolant conditions) • Chip temperature about 2.5C higher than detector T • Thermally comparable to nominal design, studies continuing(also see Backup)

13. Material • The total radiation length of the mechanical/cooling structure depends on the support conditions. • If one assumes a shell-like support, then just the stave mechanical/cooling structure (for the nominal design) has a radiation length of about 0.4% (based on measured weights of the prototypes). • This does not include coolant, modules or bus cables and most importantly not the shell or supports to it. • For reference, if one assumes end-supports only (not shells) 1m meter apart, then the stave radiation length for comparable deflection would be about 0.9% because stave has to be stiffer. • In short, one needs a combined stave-support-structure design to get a realistic number.

14. Miscellany • Outer barrel, long strips, with single hybrid for 10cm detectors. • Quick look at thermal performance and T to hottest point on silicon is about 3C for 0.25W/chip and 0 detector power in nominal design. More than expected from ¼ scaling by power but <1/2 of short strips. • Distortions as detector cools down • <10 microns distortion out-of-plane calculated for about 50C temperature change but only done so far for 0.7mm facings. Need to do for thinner facings. • Gravity sag • A non-issue with shell-like support and frequent supports. Tune structures to meet requirement.

15. Hermiticity and Endcaps • Hermiticity • In nominal design, there are gaps(eg. 100 microns) between adjacent detectors on same side of stave. • Mechanically at least one could offset(in Z) the top and bottom detectors such that overlaps do not line up. • But what can you do if you want hermiticity on each side by overlapping detectors in Z? • Simplest mechanical solution is to have ½ the modules glued to bus cable and the other half with about 0.5mm carbon fiber plate(with appropriate cutouts) glued to bus cable, raising ½ of the modules to allow overlap – see cartoon below. T would increase for these <1C (for 0.25W/chip, 1mW/mm2 nominal design, not bridge) • Endcaps • Purely from a mechanical/cooling aspect, one could make pie-shaped segments, routing the cooling tube as needed to achieve the same thermal performance as barrel staves. Likely offset slightly front and back facings to avoid complete gaps. Straightforward (as would be the support of these into a disk) • But, but this doesn’t address the considerable problems associate with detector layout or bus-cable-like connections….. Z

16. Interface to Barrel Support • Have looked quickly at different support options • Current preference is for shell-like overall support with support points about every 50 cm. • More on this topic in the engineering session. • Note that this implies strong coupling of stave design with shell eg. where is the stiffness and not just the obvious support interfaces. Locating pins in rings Light weight composite sandwich rings

17. Production – Mechanical/Cooling ONLY • Rough numbers for barrel only(including guess for yields) • 250 1m staves • 300 2m staves • Materials for nominal design • 100 kg of fiber unitape • 100 m2 of honeycomb or structural foam • 105 cm3 of thermally conducting foam (if used) • 600 tube sets(tubes, fittings) • Production rate 1-2/day required • At minimum, industrial production of subcomponents (facings, honeycomb/foam pieces, bare tubes…) but full industrial production feasible (with substantial lab QC oversight). • Very rough guess for technical production labor(not including design or prototype phase or cable attach or anything related to module attachment) is 15 FTE-years

18. Risks – Mechanical/Cooling ONLY • Identify components which are ‘single-source NONE for nominal design. • List components or assembly steps which need to be developed or which need qualification before they can be adopted e.g. • Critical decisions before starting real design: what is coolant and bridged hybrid or not • Interfaces(primarily electrical, where are the connections to electrical services) • How deep is the assembly pipeline? i.e. what level of prototyping effort is required to fully validate design? Would clearly have to build full-scale prototypes for thermal performance/stability • Risk due to tight mechanical tolerances during assembly Moderate, mitigate with QC (CMM) • Grounding & shielding risks (by external experts) e.g. • See electrical department…… • Options available for repair i.e. the risk of damage being unrepairable. • Module/stave damaged during shipping or testing Replace module or toss out • Module/stave damaged during assembly to structures Replace module or toss out • Cooling channel damaged during or after assembly. During, replace, after toss out(before module attach) • Service cable damaged during/after assembly Don’t know yet • Failure after integration. If design requirement(I don’t think it is), make stave replaceable • Flexibility or options to modify the design based on experience with ATLAS. • Could the material be reduced further? And at what cost? Some but more design time and prototypes • Could additional measurement layers be introduced? Sure • Are there options to improve the mechanical stability? Easy but more material • Are there options to improve the cooling i.e. remove more power and/or lower temperature? Yes, many • Are there options to improve grounding and shielding? Talk to electrical department • Other technical, schedule or financial risks • Need more studies of pipe(Al) in potential contact with carbon, although lots of work already done for existing pixel system • Mechanical/cooling ONLY part is of moderate risk. Not easy but not hard

19. R&D Plan – Mechanics/Cooling ONLY • If this approach, then what would be development plan? • Need to design, build and test realistic prototypes(eg. 1m length) that simulate well thermal and mechanical expectations. • But it does not make sense to do this before making some essential decisions • Coolant • Hybrid type(bridged or not) • Hermeticity • At least preliminary definition of critical interfaces(mounts and electro-optical) • Given these choices, estimate it would take about one year to design, build and test full-scale prototype(s) for thermal and structural performance • Concurrently could develop preliminary production plan and associated cost

20. Next Steps • Respond to questions from review panel • Apart from this does not make sense to do much more mechanical/cooling design work or prototype fabrication for this concept in the next months. • Need feedback from making 1m functional prototype with real modules • And decisions on requirements and constraints