# Presentation: 128359

## Pixel Support TubeRequirements and Interfaces

- M.Olcese
- PST CDR: CERN Oct. 17th 2001

## Assumptions

- Pixel system and beam pipe integrated together on surface and installed as a “package”.
- Prerequisites:
- beam pipe bake-out in situ with B-layer in place
- access to any part of pixel detector requires         removal of the whole pixel detector, to bring it to the surface; this requires:
- an ID installation/removal shutdown
- breaking of the vacuum

## Outline and Definitions

- PST is a cylinder about 7 long, 480 mm OD, coaxial to the beam pipe, going all the way through the ID volume and closed on the beam pipe at the two ends with two end plugs
- PST main functions are:
- allowing an independent installation and removal of pixel detector with all the rest of ID in place
- supporting the Pixel detector, the services along the beam pipe and the beam pipe itself
- keeping the pixel volume thermally and electrically independent from the rest of ATLAS
- all the pixel services have to pass through the two end plugs

## PST Requirements: major categories

- mechanical
- environmental
- electrical
- service penetration
- installation of pixel package

## PST mechanical requirements

- geometry and envelopes
- support conditions
- alignment
- assembly
- stability
- operating loads

## Envelopes

- Radial envelopes PST-SCT defined and agreed
- Z-envelope in the end region still undefined: this is driven by: pixel service connections, PST end support, polymoderator geometry.
- envelopes account for shape errors, deflection under load: in the case of forward PST, the envelope has to account for gravity sag with no support at the ends and with the pixel system in place (VI section of beam pipe included)
- envelopes do not account for relative motions of different blocks and adjustments

## Interface to SCT barrel

- Recent decision to have an independent thermal barrier for the barrel SCT
- this implies:
- more schedule flexibility
- no temporary thermal barrier for x-ray testing of SCT barrel
- possible adjustment of PST in pit without opening the SCT thermal barrier
- PST structurally independent from SCT thermal barrier
- concept and geometry of the 4 PST support penetration through the SCT thermal barrier have been defined and agreed

## Support conditions: internal

## Support conditions: external

## Beam pipe supports

- There are four internal supports + 1 external
- all the 4 internal supports are remotely (outside the end plug) adjustable in X,Y
- the adjustment range is +/- 9 mm (nominal clearance B-Layer/beam pipe)
- however this will be mechanically limited to +/- 5 mm for the two internal supports by means of hard stops on the B-layer structure. This will avoid in any condition the risk of collision with the B-layer modules

## Alignment

- The PST central section has to be as much as possible coaxial to the theoretical beam axis (TBA): this allows for the max adjustment flexibility of the beam pipe wrt pixel B-layer
- emergency X,Y adjustments of +/-3 mm of the PST after ID is in place will be allowed within the PST barrel envelope.
- They are going to be used only in case of macroscopic (surveyable) misalignment. However this might not be enough to accommodate all foreseeable errors.
- The +/-3 mm X,Y adjustments are not included in the forward PST envelope and they have to be accounted in the 9 mm gap

## Assembly

- PST is split in three parts (one barrel and two forward) for assembly reasons
- assembly sequence:
**A**. the barrel PST is installed on surface inside the barrel SCT on nominal centre
**B**. if necessary,after the barrel ID is in place the barrel PST is adjusted to theoretical beam axis
**C**. the two forward sections of PST are connected and PST is adjusted wrt the ID rails on cryostat wall if necessary
**D**. the forward SCT and TRT are slid in, the PST end supports are installed and connected to the PST

## Stability

- PST is an intermediate support structure in between SCT and Pixel detector.
- The support points of Pixel detector - PST are located very close to the PST-SCT supports
- natural stability requirement for the PST is:
- to provide a link between SCT-PST-Pixel support points as stiff as possible with a stability of the same order as the one of  Pixel detector. Max gravity sag Pixel wrt SCT fixed points should be about 20 m, this is equivalent to a first mode at 80 Hz
- to minimize the perturbation of the pixel and SCT stability due to operating loads:
- thermal deformations,
- change of relative positions
- vibration of forward tube and services
- the injection of such loads is function of the stiffness of the PST, of the support conditions at the end of PST and of the beam pipe support conditions

## Operating loads

- As function of the support conditions of PST and beam pipe,  4 design load cases are identified

- (*) Note that these are based on assumptions from current models

## PST environmental requirements

- Pixel volume has to be maintained:
- cold (modules at 0 C)
- dry (to avoid condensation)
- dry and clean conditions are achieved by means of a continuous N2 flow (about 0.5 m3/h) entering at one side and going out from the opposite side
- cold conditions are maintained by the internal cooling system however the gas temperature is not controlled. The design range of the gas temperature  is -10/+10 °C
- PST has to be designed as a thermal and gas sealed enclosure which will separate the internal pixel volume from the rest of ATLAS.  The max leak rate budget is determined by the nitrogen flow and a nominal overpressure which has to be maintained in the pixel volume of 5 mbar
- the whole PST external surface has to be actively heated (T> 15 C) to prevent condensation: some areas during operation others during access

## PST active heating scenarios 

## PST electrical requirements

- Pixel detector need to be shielded as efficiently as possible from the external sources of electromagnetic noise
- a Faraday cage has to be built around the detector
- the most natural solution is to have an built-in Faraday cage on the PST
- this will be implemented with a continuous thin Al layer. Special care will has to be paid in the connection areas to achieve a good electrical continuity of the shielding
- electrical requirements indicate a suitable thickness range       50-100 m
- it is not clear yet whether the Faraday cage has to be extended along the beam pipe outer surface inside the pixel volume

## PST service penetration

- This is a very congested area with many constraints
- all the services need to be broken here
- beam pipe has to be floating wrt the end plugs and free to be adjusted to +/- 9 mm (but practically limited to +/-5 mm, see previous comment)
- tube feed throughs will have to be axially floating to minimize the axial loads on Pixel detector due to temperature change.
- External environment is in principle cavern air (dew point about 10°C), actions will have to be implemented to avoid condensation on cold tubes and surfaces

## Pixel package installation requirements

- The Pixel package is lowered into the pit inside an Integration & Testing Tool (ITT)
- the ITT has to be aligned to the PST for the insertion: same tool used for other ID parts?

- ITT