# Presentation: 619484

## New Directions in Mechanics and Cooling forPixel Detectors

- W. O. Miller
- Innovative Technologies International (**iTi**)
- Wei Shih
- Allcomp Corporation

## Objectives

- ATLAS is fielding a large Pixel Detector which represents a significant advancement over existing silicon-based detector systems
- Achievements have been significant, the design is well established and supported with extensive testing-
- With some knowledge of the past, my objective was to step back and to look at the design, focusing on mechanical issues from a different perspective
- Hopefully, the outcome will influence future detectors
- To accomplish this objective, we first briefly review mechanical and cooling concepts embodied in the ATLAS Pixel Detector
- Large in size, composed of both barrel and disk detector elements
- Design was faced with major cooling issues; highly distributed heat loads, totaling over 15kWatts
- Tight constraints on radiation length and stability
- Some of the service issues are still emerging
- We conclude by summarizing an approach being studied that is a significant departure from the approach taken by  ATLAS

## ATLAS Pixel Detector

_*General Design Goals*_
- Stability,  ~10 microns
- Radiation length-normal incidence
- Local supports <0.7%
- Frame <0.4%
- Low mass
- High Reliability
- 10 year life cycle
- Low maintenance

- Pixel Detector
- 1744 modules
- 3 barrel layers
- 3 disks, each end

_*Heat loads*_
- ~7W/module
- 13kW in detector space
- 17.1kW in pixel volume
- 38.6kW total, balance from cables

_*Cooling*_
- Evaporative (C3F8), two-phase flow
- -20C inlet, providing -6 C at detector
- For 17kW, mass flow rate of 185g/s

## Local Supports

- Disk local support
- Composite Ring
- C-C sectors

- Coolant
- lines

- Barrel local support
- Shell
- Stave

- stave

- shell

- Coolant
-  lines

- Modularity of detector
- Side A, 35 flow circuits
- Side C, 45 flow circuits

- Layer 1 and Layer 2
- 2.6g/s

- B-Layer 3.6g/s

- 1.2g/s

## Barrel Assembly

- Mechanical design approach is mature, testing of key components complete

- Details of services, connections and routing are in being finalized

- stave coolant connections

- bi-staves

- vapor return lines

- _Courtesy of Thomas Pfaff (IVW)_

## Barrel Structure

- Screws

- CF-PEEK elements with pillars,
- bonded between outer shell
- and inner skins
- ==> Sandwich structure to allow
- screwing of SSR to halfshell

- CF-PEEK core elements

- CFRP faceplates

- _Courtesy of Thomas Pfaff (IVW)_

## View of End Section (cooling)

- End section frame (frame joint)

- Return from
- barrels

- Vapor returns

- Service panels
- (truncated in length)

*Side C*: 45 outlets and 45 inlets

- Barrel capillaries

- Capillaries for both disk and staves

## Plumber’s Nightmare

- Illustration represents the beginnings of the packaging required as the services pass out of the Pixel Detector Support tube
- 1/3rd of a quadrant bundled together

- Courtesy Fred Goozen LBNL

## Radiation Length Issue

- Total mass breakdown

- Courtesy of D. Costanzo-LBNL

- Normal Incidence

- Includes barrel pixel support tube

## Cooling System

- Concept based on controlled boiling in a stave or sector
- Saturation pressure in stave or sector nominally 1.7bar entrance, 1.3bar exit
- Capillary feed to stave or sector, with inlet to capillary of 9.3bar
- As tested, each flow circuit, i.e., bi-stave and adjacent sector pair, use upstream and down stream pressure regulator to control temperature
- Extrapolation for 80 circuits: 160 pressure regulators (80 inlet and 80 outlet), and 80 outlet temperature regulators
_*Plumbing,*_ 3 connections in each barrel cooling line and 2 each for the disk from the local support to the end of the frame (PP0*)-*
- Side C: 33*3*2+12*2*2=246 mechanical connections Side A: 23*3*2+12*2*2=186 additional mechanical connections
*Roughly, *_*432 mechanical fittings*_* *to make and check at each assembly and disassembly
- The evaporative cooling system is quite sophisticated, but at what expense to earlier goals of low-mass, low radiation length, and possibly at some impact on reliability.

## New Design Option

- Passively remove heat from modules, without using forced convection
- Transfer the heat from the modules to collection points, where the energy is removed by convection
- Circulated fluid could be either single or two phase, but presently assumed to be two phase
- Use passive heat removal system to extract heat from cables as well
- Reduce fluid connections, potentially by a factor of 10, or so
- Reduction constrained only by desire for redundant flow circuits
- Efficient passive heat removal can not be provided solely by highly conductive composite
_*Proposal uses evaporative fluids*_

## Heat Pipes (HP)

- We propose that detectors involving hundreds of cooling lines can be simplified using heat pipes
- Propose to explore this option using the ATLAS barrel system as an example
- Heat pipes transport heat efficiently from the evaporator to the condenser
- Here, the two sections are close coupled
- Design options
- 1st, HP that replaces stave, eliminating OMEGA piece, Al tube, and grease film.
- 2nd,  HP becomes part of the structure, eliminating the outer shell
- Options are introduced to lower radiation length

- Vapor core

- Heat in

- Heat out

- Very efficient, effective thermal conductivity about 1000x of Cu

- evaporation

- condensation

- Annular wick

## Typical Wick Geometries

- Heat Pipes for HEP Application
- Must achieve low radiation length
- Must be lightweight
- Conceivably can be used as part of the structure, with modules mounted directly
- Proposal
- Thin Carbon-Carbon (C-C) shell with integrated wick
- C-C wall porosity sealed by the wick material

- Slab wicks with circumferential grooves

- Axial groove wick

- Screen wick

*Process is to find the best tube size and wick configuration that minimizes mass and thickness, as a function of candidate fluids*

- simple

- More complicated

- Arteries can be added, but another degree of complexity

## “C-C” What is it?

- Denotes composite composed of carbon (or graphite) fibers in a carbon matrix
- C-C may be formed as flat laminates or into structural shapes
- Material properties can be enhanced by densification of the carbonized material, followed by heat treatment to obtain high modulus, high strength and high thermal conductivity
- Carbon infiltration by CVD , and or pitch
- C-C Material: properties are tailorable
- 2D K1100 Laminate, E~3X AL, K~2X AL, CTE ~-0.75ppm/K
- Kz= 45 W/mK, ~50-60X resin based laminate
- C-C characteristics
- Typical radiation length of 23cm
- Hydrophobic, insensitive to moisture
- Negative CTE, ranging from –0.5 to –1.5 ppm/K
- depending upon laminate layup, expands when cooled
- Strength rivals steel, but strain to failure is lower

- C-C sandwich panel

- “C-C” What is it?

## HP Fluid Options

- Basis for selection
- Low temperature two-phase fluids
- Preferably low saturation pressure in temperature range of interest, -20 to –10C
- Fluid surface tension and heat of vaporization criteria
- Very important as we will see next
- Process has just started
- Selection of fluid
- Design of HP
- Integration with structure

## Candidate Fluids-Groove Wick

- Ammonia: NH3               Methanol: CH3OH             Isobutane: C4H10              R12: CCL2F2

- Perfluoropropane: C3F8     Perfluorobutane: C4F10

_***For half stave length, 55W***_

## 1st Order Groove Wick

- Ammonia fluid satisfies the goal of a small diameter HP rather easily
- Advance notice of this comes from the fluid Figure of Merit (FOM) calculation
- FOM brings together key fluid parameters that are important to HP applications.
- Methanol does not attain the goal of 55W with a simple wick geometry as hoped
- Option to a groove wick with constant dimensions is to change shape with axial dimension, improving the wick permeability
- Changes to groove geometry are being investigated
- Other candidate fluids show major short comings with the simple groove wick
- Possible one of these fluids will work with another wick geometry, e.g., screen wicks
- Work will continue to select a fluid and wick geometry to minimize the HP profile dimensions

## 1st Structural Option

- Description
- HP stave with “step-up/step-down” geometry for axial overlap
- HP is a stiff tube, overcoming gravity sag between supports
- Modules are mounted on flat machined C-C plates
- Introduce condenser manifold
- Improvements
- Eliminates several ATLAS stave components
- Condenser manifold eliminates the 432 tube connections in the tracking volume
- Condenser manifold can be built with multiple coolant passes to add redundancy

- HP

- Module thermal management piece

- Condenser manifold

## Condenser Manifold

- Condenser manifold objectives
- Condenser manifold is an external flow circuit, replacing the 80 or so flow circuits
- Condenser is supplied with multiple flow circuits, say 3 for system redundancy
- Manifold accepts heat from ends of the heat pipes
- We choose to confine the heat transfer to nominally 40mm axial distance at end of HP
- If necessary, we will use surface area enhancements, like foam to reduce temperature drop in the manifold
- Fluid in manifold could be C3F8, using the system as developed by ATLAS

## 2nd Structural Option

- HP-Structure Integration
- Adjacent HP tubes are joined with thin composite strips, *upper and lower strips*
- The strips form a circumferential “step-up/step-down” pattern, providing a continuous shell like structure
- Structural concept is similar to the CMS barrel concept
- Module Overlapping
- Provided circumferentially by step pattern
- Step-up/step-down pattern is molded in the axial strip to permit overlapping in the axial direction

- C-C tube structure with C-C facings

- down/up, 2 directions

- cable support

## Stave Support Options

- Gravity sag solutions
- Case (a)- HP tubes tied together with rings
- Case (b)-HP tubes stave only
- Case (c)- HP tubes integral with shell
- Results
- Case (a) sags excessively
- Case (b), stave with 6 supports will sag 28m, a supporting shell <3 m
- Case (c), composite strips between HP tubes sag <8m with module and cable weights added
- Radiation length
- Preliminary results for lowest mass and radiation length favor case (c)

**case a**

**case b**

**case c**

## Benefits/Risks

- Potential Benefits of Passive Heat Extraction with Heat Pipes in the ATLAS Pixel System
- Improved local thermal stability-passive, not subject to change by individual performance of regulators
- Major reduction in number of tube connections, and associated mass
- Elimination of 160 pressure regulators
- Improved structural thermal stability, eliminate adjacent coupling of Al tubing
- Less mass, lower radiation length at >1
- Potential for modularity of 1 versus present modularity of 2
- Improved system reliability and functionality
- Risks
- Demonstrate C-C heat pipe technology with 10 year life
- Address low mass and sealability
- Address thermal design, leading to pipes of the order of 6mm diameter
- Demonstrate stability of HP fluid in radiation environment

## Conclusion

- HP Design
- Construction concepts for achieving sealed C-C are being formulated
- 1st order specimens of wall structure have been constructed
- Performance studies for wick geometries are underway
- HP Testing
- Expect to test a1/2 length 55W HP composed of carbon-carbon elements by end of CY2002
- Financial support
- Provided by DOE Phase I SBIR
- Related application
- NASA Nuclear Electric Propulsion initiative in sore need of carbon-carbon HP radiator, using C-C sandwich facings