# Request for a SSD envelope modification

Lilian Martin for the SSD group - 17 January 2003

In this document the motivations for a modification of the SSD
integration envelope are detailed. In the first part, the current SSD
design is described. The reasons for this modifications are detailed in
the second section while the proposed new SSD design and envelope are
presented in the third part. A summary and the figures are grouped in
the last section.

## Current SSD design and envelope

### SSD design

The SSD is made of 20 ladders (equipped with 16 modules and a pair of
control and digitisation boards) mechanically supported by two clamshell
rings themselves attached to the SVT/SSD cone (Figure 1).

The SSD wafers are at a radius of R~wafer~ = 230 mm from the beam axis
and are tilted by 6° to allow wafer overlap from one ladder to its
neighbours (there is no wafer overlap along the beam axis) . The wafers
are mechanically supported by carbon-fiber based ladders. Their maximal
radial extension is R~ladder~ = 276 mm (Figure 2).

The clamshell structure is based on a "C" shape pieces which feature
mounting points for the ladders, mounting points for the cone and
several apertures dedicated to the SSD cooling hoses and more
specifically to avoid any interference with the SVT cooling tubes
(Figure 3).

The external shield (aluminised Mylar sheet) of the SSD is attached to
the SSD clamshells (by the mean of additional pieces forming a ring) and
extends to the SVT/SSD cone. With the shield ring, the clamshell radial
extension matches that of ladder.

The heat produced in the SSD ladders is removed by using an air cooling
system. The hot air is extracted at one end of the ladders and requires
some cool and clean air as input at the other end. Specific pieces have
been designed to be mounted on the SSD clamshell to open air hole in the
SSD shield and to use the air in the TPC inner field cage without
weakening the Mylar shield. These pieces are equipped with a metallic
grid in order to keep a continuous conductive shield (Figure 4).

### SSD envelope

The SSD external envelope is cylindrical in the active area (roughly
from z=0 to the small rings of the cone) and is defined by the SSD
external shield. Its radius is currently equal to R~envelope~ = 276 mm.
At higher z, the SSD envelope follows the cone geometry and merges with
the SVT envelope (Figure 5).

### SSD installation scheme

The mechanical structure of the SSD (two halves) is similar to the SVT
structure and designed for a similar installation scheme. The two
clamshells are first installed at a suitable location and then simply
translate in one direction to their final location to finally be coupled
and attached to the cone. A dedicated tooling has been developed for
this installation (Figure 6). Once the clamshells are in position, the
ladders are cabled (power, signal, cooling), the cooling pieces are
attached to the clamshells and finally the external shield is put in
place.

## Conclusions from the integration tests and the ladder installation

### Clamshell integration test

Dummy full scale model of the SSD clamshell have been produced and
brought to BNL for integration tests during the summer 2002 after the
SVT has been installed (Figure 7).

Although in place the clamshell does fit and does not generate any
conflict with the SVT, we concluded that it can clearly not be installed
as it was foreseen. During the horizontal translation, the clamshell
gets in contact with several SVT water fittings (the ones at the top and
bottom of the SVT water manifold).

These fitting were originally made of aluminium and have been replaced
by smaller plastic based fittings prior to the SSD clamshell integration
test (Figure 8). Their replacement (for the second time) has been
discussed and investigated with the SVT group. New fittings have not
been identified which would be able to feature a better geometry (a
different elbow for example) and to guarantee a perfectly leak free
connection to the water manifold.

The parts of the SSD clamshell which interfere with the SVT are the
mounting points of the SSD ladders. The mounting points are in fact made
of three connection points with the ladders which guarantee a minimal
sagging of the ladder. Their thickness has been reduced to the smallest
tolerable value and any further reduction would weaker the ladder
support.

The SSD clamshells can not be installed according to the original plans.
Trying to keep the two-halve structure would lead to increase the ladder
radius and consequently would increase the geometrical inefficiency of
the barrel. One should note that increasing the ladder radius implies to
increase the clamshell radius in order to keep enough material to insure
the same strength.

### Ladder installation test

Prior the SVT shielding installation, the SSD ladder has been put once
in place on the cone. Whereas the installation itself did not lead to
any particular difficulty, the ladder cabling revealed that there was
not enough room left for the SSD cables between their emerging point
from the cone and their final location on the ladder to route them at
the correct location without crossing the SSD envelope boundary. The SSD
electrical cable path can be accommodated and/or relatively packed but
the cooling efficiency would strongly suffer if the air cooling hoses
are squeezed or strongly bent.

This situation got obviously worst when the SVT shield has been put in
place and followed by the final SSD ladder installation (Figure 9). For
this year, only 1/20 of the SSD cables have been used and we did our
best to keep the unused cables within the SSD envelop. Still we were not
able to connect the air hose dedicated to the ladder and we had to use
one of the other hoses which had a smoother route from the cone to the
ladder.

One should mention that the SVT group has spent a significant amount of
time in routing and packing their cables on the small ends of the cones
and they obtained an area much compact that the previous year. Despite
this effort, there is not enough space left for the SSD cables to rout
then at their nominal location. At some specific spots for example, the
SVT water fittings are fully using the available space.

The SSD cables can not be routed at their location without violating the
SSD envelope.

### Discussions at BNL on the SSD shield and the air holes to the IFC

During the last summer, the SSD group discussed with the SVT group and
Howard Weiman about the ladder and the SSD barrel installations. The
proposal to make holes in the SSD external shield was discussed. Based
on the fact that a smooth and regular surface is the best condition to
prevent sparks to develop from the IFC, the current proposal (see above)
was evaluated as difficult to produce without creating stiff slope
changes of the surface (possibly the cause of field singularities) and
breaking the shield conductivity. Several ideas were proposed such as a
full cylindrical grid instead of discrete holes or their relocation
further away from the interaction point where the TPC field is smaller.

The current proposal to take a fraction of the IFC air for the SSD
cooling is not satisfactory and requires an alternative solution.

## New SSD design and proposed envelope

### The SSD design

In order to be able to install the SSD without increasing the wafer
radius, the two-halve structure has been abandoned in favour of a
four-part structure (Figure 10). The parts are asymmetric and are
equipped with 7 or 3 SSD ladders. Two parts supporting 7 ladders each
are installed on the left and right side of the beam axis whereas two
parts equipped with 3 ladders each are inserted on the top and the
bottom to complete the barrel.

The 7 ladder pieces can be brought in place by a horizontal translation
without interfering with the SVT water fittings. They are attached to
the existing mounting points on the cone (the tooling developed for the
previous structure may be recycled to install these new pieces). The 3
ladder pieces are then installed by a vertical translation and attached
to the 7 ladder pieces previously installed.

The advantages of this new SSD structure are two-fold :

-   The interference areas with the SVT are reduced and allow us to
    slightly decrease the SSD wafer radius in order to reduce the
    geometrical inefficiency and to increase the ladder to ladder
    overlap which is desirable for the software SSD wafer alignment. The
    position of each SSD ladder has been updated leading to a specific
    location (radius and orientation) for each ladder (Figure 11). The
    three top and bottom ladders have a slightly bigger radius than the
    average resulting from a compromise between the ladder to ladder
    overlap and interference with the rods of the cone.

-   Groups of 3 or 7 ladders will be assembled in France and the new
    structure will give us more flexibility in the case of a gradual
    installation of the SSD in STAR. Depending on the ladder production,
    we can anticipate a partial installation of 3, 7, 10, 13, 14 or 17
    ladders before completion of the full barrel.

### The SSD envelope

In order to cope with the amount of cables and connectors in the small
end cone areas and to be able to connect the SSD cables and cooling
hoses to the ladders, we propose to increase the external SSD envelope
in the active area R~envelope~ to 318 mm which will stay constant from
one cone to the other (Figure 5). A fully cylindrical shield will be
built in order to have a smooth and continuous surface which is the best
to prevent electrical discharges to develop from the IFC to the SSD.

The SSD external shield will be supported by the SSD clamshell
structure, its radius will be increased accordingly. Consequently, the
SSD clamshell parts will recover some extra strength.

The end parts of the SSD external shield which will be attached to the
cone will not be completely closed and will be used as entrance for the
input air taken within the IFC (Figure 12). Small rectangular or
circular holes will be made in the shield near the cone. Short hoses
will be attached to the cone, connect to the ladder ends and near the
holes produced in the SSD shield.

## Summary of the proposed modifications and figures 

+---------------------+-----------------------+------------------------+
| **Items**           | #### Old Proposal     | **New Proposal**       |
+---------------------+-----------------------+------------------------+
| Wafer radius from   | R~wafer~ = 230 mm     | R~wafer~ = 226, 228    |
| the beam axis       |                       | and 230 mm             |
+---------------------+-----------------------+------------------------+
| Barrel Structure    | 2 × 10 ladders        | 2 × 7 ladders          |
|                     |                       |                        |
|                     |                       | 2 × 3 ladders          |
+---------------------+-----------------------+------------------------+
| Clamshell and       | R~envelope~ = 276 mm  | R~envelope~ = 318 mm   |
| envelope radius     |                       |                        |
+---------------------+-----------------------+------------------------+
| Input air holes     | R = 276 mm            | R = 318 mm             |
|                     |                       |                        |
|                     | Z = 566 mm            | Z = 950 mm             |
+---------------------+-----------------------+------------------------+

![](media/image1.png){width="6.2965277777777775in"
height="5.089583333333334in"}

Figure 1 : The SSD clamshells (equipped with only 3 ladders)

![](media/image2.png){width="6.292361111111111in"
height="8.900694444444444in"}

Figure 2 : Drawing of the SSD in the x-y plane featuring the two
clamshells equipped with 20 ladders

![](media/image3.png){width="3.5368055555555555in"
height="4.5in"}![](media/image4.png){width="2.3645833333333335in"
height="4.179166666666666in"}

Figure 3 : Schematic views of a \"C\" shape piece of the clamshell
structure : equipped with a ladder (left) and equipped with the
additional ring (in yellow) supporting the SSD shield and the survey
marks (right)

![](media/image5.png){width="5.247916666666667in"
height="4.086805555555555in"}

Figure 4 : Schematic view of the pieces (without the metallic grid)
develop to make holes in the shield and to allow the IFC air to be use
as input for the ladder air cooling

![](media/image6.png){width="6.288888888888889in"
height="8.685416666666667in"}

Figure 5 : SVT and SSD integration envelopes in between the cones and
along the cones

![](media/image7.png){width="6.2875in" height="4.996527777777778in"}

Figure 6 : Schematic views of the tooling developed to install the SSD
half clamshell on the cone

![](media/image8.jpeg){width="6.316666666666666in"
height="8.355555555555556in"}

Figure 7 : The dummy SSD clamshell installed on the cone

![](media/image9.jpeg){width="6.3in" height="4.7125in"}

Figure 8 : Closer view of the dummy clamshell (top right) and of a SVT
water fitting (center)

![](media/image10.jpeg){width="6.3in" height="8.377777777777778in"}

Figure 9 : Routing of the SSD cables on the cone after the SVT shield
and SSD ladder installation (left). The glass fiber ring (center) is a
temporary support used to attached the external shield. The SVT shield
is completed and only the SSD cables are visible (right).

![](media/image11.png){width="6.290972222222222in"
height="8.88888888888889in"}

Figure 10 : the new SSD mechanical structure featuring four parts

### ![](media/image12.png){width="6.290972222222222in" height="8.88888888888889in"}

Figure 11 : The new design of the SSD ladders on the new clamshell (top)
and with the position and orientation (bottom)

![](media/image13.png){width="6.290972222222222in"
height="8.88888888888889in"}

Figure 12 : Location of the SSD cooling hoses on the cone used to take
input air from the IFC