Stave Mechanical Issues

1. Stave Mechanical Issues ATLAS Upgrade Workshop Silicon Tracker

2. Basic Questions • Preliminary Indications • Radiation level increasing by factor of 10 • Potential silicon damage increases markedly • Substantial increase in leakage current • Suggesting potentially colder operation • Radiation length issue • Material in detector volume increased beyond original expectations • Impetus for considering fundamental change to detector mounting, detector support, and electronic circuit design • Long, individual stave structure with embedded cooling tubes • Key questions, not all inclusive, could well be: • Is evaporative cooling still possible? • If single-phase cooling were desirable, which coolant is rad-hard? • Will detector thermal runaway require special cooling considerations? • In satisfying the stringent requirements for detector cooling, thermal runaway, detector stability will a long stave structure be low mass? • What are the best construction materials to withstand the radiation environment? ATLAS Upgrade Workshop Silicon Tracker

3. First Stave Geometry Studied Electronic chip load 108W (Pixel stave 110W 80cm length) Module heating 1mW to 1W over life time 96cm Recommend option being a 2m version of this arrangement, with central support Coolant in and out ATLAS Upgrade Workshop Silicon Tracker

4. Analysis • Key initial considerations • Stave stiffness (sag and thermal stability) • Negative CTE materials • Composite box (facings) • Positive CTE materials • Cooling tubes (Aluminum or PEEK) • Silicon wafers • Cable bus • BeO hybrid • Ceramic dielectric • Stave Cooling • Length and size of cooling tubes • Thermal resistance of cable bus Step A: First order calculation of gravity sag Step B: Thermal model of cross-section resulting from Step A ATLAS Upgrade Workshop Silicon Tracker

5. Gravity Sag • Objective is to check proportions of composite structure • Sag affected by orientation—we address worst case • Sag affected by end support conditions and the unsupported length---we look at simple and fixed end conditions for 1m length • (pins at each end provide something in between) • Sag is affected by non-structural uniform mass distributed along length---an estimate is made • Composite material-use a very modulus graphite fiber, quasi-isotropic layer • For present ignore the negative CTE effect simple support Fixed support ATLAS Upgrade Workshop Silicon Tracker

6. Stave Sag ATLAS Upgrade Workshop Silicon Tracker

7. Sag Summary If the separation between composite facings is increased, the sag can be decreased within acceptable bounds. This option will force the consideration: how best to package the cooling tubes ATLAS Upgrade Workshop Silicon Tracker

8. Material Properties in Thermal Model Properties used in 3D model ATLAS Upgrade Workshop Silicon Tracker

9. Thermal Model • Model Makeup • Array of 6 chips, two on back and one array on front, offset by the stagger in the silicon wafers • Silicon wafers 3cm axial with 6.4cm width (280 microns) • Electronic chip, 7mm by 10mm (250microns) • BeO, 2cm by 6.4cm (380 microns) • Dielectric beneath chips, 229microns, k=5W/mK • Cable bus, 126microns, k=0.12W/mK • Aluminum cooling tubes, 12mil wall • PEEK-fiber filled k=0.9W/m*K • Objectives • First order determination of gradient • Effects of various material layers • Compare PEEK versus Al coolant tubes • Coolant film gradient ATLAS Upgrade Workshop Silicon Tracker

10. Thermal Solutions (No Wafer Heating) Aluminum coolant tubes Tube surface referenced to 0ºC 0.5W per chip Side B Side A ATLAS Upgrade Workshop Silicon Tracker

11. Thermal Solutions (No Wafer Heating) • Solution---Aluminum Cooling Tubes • Added convective cooling on interior of tube surface (3000W/m2K) • Increase in peak chip temperature of 0.98ºC. An approximate calc of this component is 0.85ºC • Notice small temperature variation for center wafer, however essentially uniform • Side A electronic chip array is used to provide proper heating of the cooling tube between the two chip arrays on Side B ATLAS Upgrade Workshop Silicon Tracker

12. Thermal Solutions (No Wafer Heating) • Solution---PEEK Cooling Tubes • With convective cooling on interior of tube surface (3000W/m2K) • Peak temperature 8.4ºC, above zero • Increase in peak chip temperature of 2.2ºC. • The small temperature variation in center wafer, among areas, suggest a more refined mesh should be considered in the final analysis ATLAS Upgrade Workshop Silicon Tracker

13. Comments • Thermal Model • Model is a slice from the stave • Unfortunate but the heat load is unbalanced between Side A and Side B • Gives the appearance that the silicon wafer temperature on Side A is not uniform • The middle wafer on Side B has electronics on both sides of a wafer and here the surface temperature is nearly uniform • Deviation of the chip temperature from wafer temperature is about 4ºC for both Al and PEEK cooling tubes. • Heat flux is highest at the chip, but even here it is not very high (0.7W/cm2) • BeO serves as heat spreader • Flux between BeO and cable bus 0.23W/cm2 average • Composite facing spreads heat along the cooling tubes • Facing thickness is about the average thickness for the Pixel stave thermal management surface, but there are now two facings for distributing heat • Thru thickness conductivity is lower, 1W/mK versus 10 to 20W/mK • Heat flux at cooling tube wall reduced further over Pixel stave because of the two pass system within one stave ATLAS Upgrade Workshop Silicon Tracker

14. Wafer Heating (Next on List) • Wafer Heating • Two sources • Leakage current: peak estimated to be 1W per module from leakage current (temperature dependency) • Free convection for surrounding gas • Open for suggestions on gas temperature to be used • Need to develop more confidence in results of thermal modeling before recommending the coolant inlet temperature • Primary issues are portions chosen for stave structure and cooling tubes • Secondary issues are the material thermal properties being used • Must maintain reality check throughout analysis • What follows: • Refinement in thermal model mesh • Calculation of tendency for thermal runaway, although at the present no strong evidence this will be an issue within present geometry ATLAS Upgrade Workshop Silicon Tracker

15. Things to Watch For • Stave cross section-2m length • Need greater separation between facings for increased stiffness • Poses interesting issue in integration of cooling tube- simple circular tube geometry not likely • Wider Staves • Again integration of coolant passages • Possible wafer thermal runaway • Temperature gradients • Thermal distortion as a general issue • CTE mismatch of materials • Spatial temperature variations both lengthwise and transverse ATLAS Upgrade Workshop Silicon Tracker

16. Things to Watch For One must be mindful that stave temperature varies in Z-direction (internal pressure drop and 2-phase convection coefficient varies along stave length) ATLAS Upgrade Workshop Silicon Tracker

17. Current Emphasis • Structural model of 1m stave • Compare to analytical solution • Set up of thermal and structural coupling solution • Predict CTE effects • Axially • Transverse • Thermal gradient established by location of cooling tubes • Tube geometry for increased separation between stave faces ATLAS Upgrade Workshop Silicon Tracker