# SSD review report (3/14/08)

**Committee: R. Bellwied (chair), D. Cebra, J. Engelage, J. Landgraf, G.
Visser**

The proposal to upgrade the readout of the SSD in STAR has been
reviewed. In general our committee came to the conclusion that the
project is essential for the success of the physics program of the joint
Silicon upgrades (HFT, FGT, SSD) in STAR and therefore needs to be
supported. In certain areas this initial proposal was too sketchy to be
properly reviewed and we will point out the areas where follow-up
reviews will be necessary.

We assume that this project is sufficiently inexpensive that it will be
funded through sources which do not require the standard CD process as
set up by the Department of Energy. Some of our recommendations will
therefore reflect the necessity to keep a certain oversight on the
project's progress through additional reviews. In the following we will
address the specific charges to the committee by the STAR management.

### Issues regarding multiple re-installation plan

The committee agrees with the SSD group's decision to not re-install the
SSD in the summer of 2008. Therefore the first re-installation of the
detector can happen during the 2009 shutdown. The committee recommends
an engineering run after the 2009 shutdown, using three ladders with the
new readout configuration, and then install the remaining 17 new ladders
during the following shutdown. This two year installation schedule could
move back by one year due to funding constraints, but our main
recommendation is to not use any old readout components in any future
SSD run.

We believe the main physics argument is still the role the SSD will play
in an integrated tracker after the HFT and IST have been installed. The
only other physics driver could be usage of the SSD for displaced
vertices in Run 10, which will presumably be the first low-energy heavy
ion run. In the present scheme this run would serve as an engineering
run and the first real physics run with an upgraded SSD would be Run 11,
which is likely a pp run. The group stated that the SSD could help the
FGT physics program in Run 11, and we encourage STAR to find funding for
the SSD and FGT projects so that both projects can be completed in time
for Run 11. The first heavy ion physics results of a fully upgraded SSD
will likely be produced in Run 12, which potentially is the second low
energy physics run at RHIC.

### Technical specifications

The physics-driven requirements for this detector are well understood,
because the SSD can not be used as a stand-alone detector and therefore
its requirements are driven by a.) the HFT requirements and b.) the DAQ
requirements. There is no attempt to replace the Silicon wafers on the
SSD, therefore the resolution capabilities are fixed and have been
folded into the HFT performance estimates. This proposal exclusively
deals with an upgrade to the electronics, which also requires an upgrade
to the cooling system and the mechanical structure in order to be
integrated into the HFT/FGT support system. The electronics upgrade is
driven by the increased DAQ requirements for the higher luminosity
running of STAR in the next decade.

In general we believe that the proposed mechanical and electronics
implementations will meet those requirements, although in parts of the
proposal the necessary level of detail is missing and needs to be
provided by the group and then reviewed by a technical committee by the
end of this year.

We also believe that the proposed scope of the upgrade is justified by
the physics driven requirements, although again there is a certain level
of detail missing in the funding estimate, mostly because the actual new
readout chain has not yet been fully planned and the work breakdown
schedule has not been fully resource loaded. The manpower loading is
sufficiently detailed and realistic, but the component and material
loading has not been completed at this stage.

In the following we list a series of issues that need to be followed up
by a technical review committee later this year or as soon as the final
design is at hand.

a.) The new readout chain has not been fully defined. In particular,
although the general ADC requirements are known, the sample rate and bit
resolution are not stated in the proposal, and no candidate commercially
available ADC's are named. The proposal initially gave the impression
that the ADC and RDO electronics would be adopted from ALICE, but the
only component that is actually an ALICE component is the DDL link which
has been developed by the Budapest group in ALICE. The uncertainty in
the readout chain design leads to an uncertainty in the power
consumption budget (the proposal only states that the power budget will
be likely higher than the present SSD budget), which in turn leads to an
uncertainty in the layout of the detector cooling system, which leads to
an uncertainty in the integration and the radiation length budget of the
detector and the support system. The review committee anticipates that
the increase in the power budget (which is presently around 400 W, i.e.
close to the air system maximum for the on-detector electronics) will be
moderate and that the SSD group will be able to cool the detector with a
modified air cooling system rather than water or evaporative cooling. In
fact, we would expect that recent developments in pipeline ADC's and the
necessary support circuits for the commercial market may allow even to
decrease the power of the ADC boards relative to the old system. But the
power dissipation needs to be confirmed and reviewed in due time,
because an alternate cooling system would significantly change the
detector layout, the radiation length budget and the cost of the
upgrade.

b.) If air cooling can be used then the modification to the existing
cooling system will simply require more stable hoses and connectors. The
biggest problem in the operation of the existing SSD was due to problems
with kinked air hoses. Still, the new hose material and design for the
air supplies has to be defined and reviewed, in particular with respect
to radiation length constraints on the support cone, i.e. in forward
direction.

c.) Although there is no reason to expect the particular L2 abort
problems which plagued the existing system in a new design, the group
needs to re-evaluate the L2 abort functionality after the final design
of the RDO chain is complete. The upgraded SSD readout system is
required to conform to STAR trigger/DAQ requirements, including handling
abort commands -- we stress that this requirement presently remains in
place even if L2 might be phased out in the DAQ1000 era. Additionally,
while it is permissible for the RDO\'s to drop events in the rare case
that the trigger/SSD interface is violated, the design should ensure
that the RDO\'s do not ever become unresponsive or deliver mal-formed
packets to DAQ. The committee also recommends that, if practical, a
debugging facility in the RDO's to be able to write out a history of
received trigger commands and tokens would be tremendously useful.

d.) The final readout speed can only be evaluated after a detailed
design has been completed. The follow-up technical committee needs to
review more detailed diagrams of the RDO chain from wafer to connection
board to ADC board to RDO board to DAQ. The raw data volume (\~675 kB)
in conjunction with the anticipated low occupancy estimates could
theoretically be managed with zero-suppression alone but the committee
strongly recommends also buffering the data on the RDO boards. This will
allow for the most effective dead-time management. We propose to set the
level of \'acceptable dead time\' to less than 10% at 1 kHz trigger
rate, because we should assume that all systems in DAQ1000 should be
laid out for a 1 kHz trigger rate (even if this might not be fully
achievable for the TPC). Simulations in the proposal show that this
acceptable dead time can be accomplished with the proposed readout
chain.

e.) There was some ambiguity regarding the actual proposed layout of the
RDO chain. The committee determined that each SSD ladder will be read
out by two ADC boards, one on each side of a ladder. This means that the
signal will be routed to both sides of the TPC, which also means that
cables and cooling lines will have to be mounted on the old SVT cone on
the East side, and the new FGT support cone on the West side. The ADC
boards on each side will be routed to four (4) RDO boards which are
located in a single RDO crate residing outside the pole tips. So there
will be two RDO crates, one on the East and one on the West side, each
holding a total of four RDO boards. Each RDO board will have its own DDL
link. The committee expressed concern regarding the long-term
availability of PCI-X motherboards for handling the eight DDL links,
simply because PCI-X is not considered the latest industry standard
anymore (PCI-Express is the new standard). It is probable that the DDL
receiver board will become available in PCI Express but not likely in
time for 2010 installation. The DDL processing boards (D-RORC) and the
SIU which resides at the detector are now industry standards and
commercially available through CERNTECH in Budapest. We urge the SSD
group to inquire on the potential upgrade of the boards to PCI-E, if
necessary. We also note that at 1 kHz trigger rate, even the full raw
data volume loads the eight DDL links to 41% of their capacity. It
should be considered to use only four DDL links (one per pair of RDO's),
especially if zero-suppression will be implemented on the RDO. There
should be no impact on dead time, assuming the RDO buffers the data as
recommended.

f.) Regarding the radiation length budget, the re-location of the RDO
crates to behind the pole tips solves some of the issues in forward
direction. A detailed off-detector budget, in particular along the
support cones is not available, though. It needs to be produced as soon
as the readout chain and cooling system design issues have been
resolved, and then it needs to be reviewed by the EEMC, FGT, and FMS
detector groups. We don't expect any showstoppers in terms of radiation
length but the forward detector groups should be made aware of the
additional radiation length budget.

g.) Regarding the upgrade of the TCD in STAR we recommend that the SSD
group designs its system assuming the current TCD architecture. Any
major TCD upgrade, which might include hardware and protocol changes,
needs to be implemented in all STAR systems. A TCU/TCD design meeting is
scheduled for April 2008 at BNL, which should yield more specific
information, but any new TCD will be required to support any STAR legacy
equipment and therefore need not affect the SSD upgrade.

h.) The detector did not perform well in Run-7 (around 50% live). Many
of the problems have likely occurred because of inhibited cooling, but
the group should assess the quality of the existing ladders in tests at
Nantes prior to a technical review later this year.

### Detector Integration

The impact of integrating the new readout and cooling into STAR can only
be fully assessed after the RDO chain and cooling system has been fully
designed. There are no obvious show-stoppers, but a technical review
committee needs to thoroughly review the integration work, as soon as
the design has been completed. The committee strongly recommends a close
collaboration with the HFT/FGT engineers who are charged with developing
the new West side support cone in STAR. Cable and cooling routing as
well as mounting structures need to be integrated, ideally by a
dedicated integration engineer which should be provided by the HFT
group. Final integration plans also require review by the forward
detector groups (FGT, FMS, EEMC). A potential common cooling system
integration scheme between SSD and the IST part of the HFT needs to be
explored and defined.

The DAQ and TRG experts on our committee have reviewed the integration
issues regarding DAQ and TRG. We have not found any show-stoppers
outside the current inability of the RDO firmware to handle aborts
properly. A more detailed technical review is needed when the final RDO
design is completed, with particular attention focused on the
functionality of the TRG/SSD interface.

One major integration decision which resulted from the review
deliberations is that the SSD group has agreed to place the RDO boxes in
the new SVT-RDO location, i.e. outside the pole tips, for better
maintenance during the run. The group also anticipates using the
existing SVT water cooling for the RDO boxes, which is a sensible
proposal.

### Resources, Cost, and Schedule

The cost estimate seems reasonable based on past experiences by the
committee members. But the final cost of such a project can only be
assessed as soon as the final design is developed. More effort could
have been put into the cost estimate simply by deciding on a final,
commercially available, ADC. If the power consumption would require any
cooling other than air cooling, the cost of the project will increase
considerably. We therefore propose to have a cost and schedule review in
conjunction with the final technical review by the end of this year.

The resource loading is actually well established. Additional
information provided by the group, in particular with respect to
personnel at Nantes helped in convincing the committee that the proposed
resources are sufficient.

According to the SSD group the manpower available at Nantes is the
following: Stephane Bouvier: project engineer : 10% (ladder repair,
supervising the upgrade and testing the ladders), Gerard Guilloux:
mechanical engineer : 5% (mounting/dismounting SSD and ladders),
Christophe Renard : electronic engineer : less than 50% (upgrade
designing), and Louis-Marie Rigalleau: electronic technician: 30%
(ladder repair and testing). These resources at Nantes are essential and
we strongly urge STAR to enter into a provisional MoU with Nantes prior
to funding the project. The resources at BNL and LBNL seem to be
sufficient as well, although the project would benefit from actually
identifying the BNL engineer and LBNL resources who will be responsible
for board layout, routing and PCB fabrication, prior to funding. We also
recommend that STAR operations considers an increase in its FTE loading
by about 0.1 FTE per year due to the SSD operation in the out years.

One of the biggest concerns of the committee is the complete lack of a
management structure for the project. It is our recommendation that the
project should only go forward after a well defined structure has been
developed including not only a project leader, but also a chief
engineer, a designer, and a liaison engineer to the HFT and FGT
projects. Certainly for a project of such a limited scope, the
engineering tasks could be taken over by a single engineer, and also the
integration tasks could be handled by a HFT or FGT engineer, but all of
these responsibilities have to not only be defined but also specific
names need to be assigned in order to allow us to review the management
structure in detail, Without it the lines of responsibilities are not
defined at all, neither within the project nor with respect to STAR. We
anticipate that scientists at LBNL will take on a major management role
in the SSD, but we need to review the structure further after the group
has agreed on it.

The technically driven schedule seems feasible and it is also necessary
in order to keep the main task of the SSD, as a support detector for HFT
and FGT, on track. Therefore we propose that the project receives its
first batch of R&D funding this year and then undergoes a detailed
technical, integration and management review by the end of the year. The
rather rudimentary proposal, which has been reviewed here, should be
updated by a detailed technical and management (incl. cost and schedule)
proposal prior to this second review.