1. Overview of basic issues

2.   The design of the (Outer) Tracker Upgrade for phase 2 has been ongoing for more than 4 years  So far the (implicit) assumption has been that services on YB0 will be reused  Services are interleaved with other subdetectors  Re-cabling YB0 would be a time-consuming, expensive and risky project, involving other systems o To be carried out in a radiation environment  The amount (and/or cross section) of services appears to be appropriate to the needs of the upgrade  Thanks to the use of more advanced technologies o High-speed data links o On-board power converters o Two-phase cooling  Some of the features of the installed services may however preclude some convenient design choices Introduction

3.   The Tracker has more than 47,000 installed fibers.  For the tracker upgrade, it is planned to use one LP-GBT link per module, serving all trigger, readout and control functionalities.  Each link requires two fibers  Hence the installed fibers can serve a Tracker with up to 23,000 modules  The n of links appears to be ~ sufficient for the layout options considered  Marginal for some options (… the most expensive ones!)  Sufficient with plenty of margin for some others  However… Readout (I)

4.   The current tracker fibre plant is single-mode (as required by the analog readout system) and operates at 1310nm wavelength.  The future GBT-based readout system will be digital, and could thus be based on either single-mode or multi-mode fibre  New high bandwidth multi-mode fibre types (OM3 and OM4) have been developed in the last 5 years, which support operation at 10Gbps over 100m  Most short range 10Gbps links today are based on MM fibre operating at 850nm wavelength. o Excluding the use of multi-mode fibre for the future CMS outer tracker will constrain the system design and will significantly reduce the components choice.  In particular, it is difficult (but not impossible) to find commercial low power single mode 1310nm lasers which will resist the HL-LHC radiation levels.  New cable designs are now denser, and could possibly allow to decrease the used volume in the crowded cabling channels.  The full optical link chain is expected to be ~1 MCHF cheaper with new multi- mode fibres, wrt to the option of reusing the existing optical cables  This estimate only accounts for components, not for uncabling/recabling work Readout (II)

5.   The upgraded tracker will require substantially more power than the present one  Between 75 kW and 120 kW in the front end (after irradiation), depending on the options considered o Mostly at ~1.2 V  To be compared with ~40 kW for the present tracker (extrapolation at the end of its lifetime) o Mostly at 2.5 V  Power conversion in the front-end offers a factor of ~10 reduction in the cross-section of the conductors  The cross section of copper is sufficient with comfortable margin  However… Power (I)

6.   Reusing the existing cables and connectors constrains the layout of the conductors  Some of the constraints actually come from the connectors in PP1  Some appealing solutions may be difficult or impossible to implement  E.g. controlling the DC-DC converters from the back-end, which requires separate lines  The implementation of environmental sensors will also be constrained by the layout of the available conductors  Not yet studied Power (II)

7.   For the upgraded tracker it is envisaged to use a two-phase CO 2 system, continuing the technological development of the pixel phase-1 upgrade  The conceptual study of such a big system is not yet started  Thanks to the excellent thermodynamic properties of two-phase CO 2, the required mass flow will be substantially lower than for the present liquid fluorocarbon cooling  Even considering the higher power inside the detector  Hence the cross section of the installed copper pipes will be sufficient for the new cooling system, with comfortable margin  However… Cooling (I)

8.   The operating pressures of the CO 2 system will be higher wrt the present one  The saturation pressure of CO 2 at 30°C is 70 bar o Tests done on same pipe material and brazing techniques as the existing Cu pipes have shown that the pipes have an elastic limit at ~120 bar, and rupture pressure of ~250 bar.  To qualify the installed pipes for CO 2 use they need to be all tested at 88 bar o Number currently agreed with CERN safety.  The required temperature will be most likely lower than for the present system  The insulation of the pipe bundles is likely to be a limiting factor for the allowed operating temperature of the new system o Although the exact requirement is not yet defined  Heating wires will be added on the outer surface of the cooling channels during LS1  An estimate of the lowest allowed temperature will be made  Heat exchange between in- and out-going pipes is limited by the layout of the pipe bundles  This also limits the available phase space of parameters for the operation of the system Cooling (II)

9.   Designing an upgraded tracker which makes use of the installed services on YB0 is a priori possible (as far as we can tell today)  This has been the assumption so far  New services would allow to make more optimized choices for the new detector design  For optical links, power distribution, and, most importantly, cooling  Such considerations may come on top of (more or less justified) generic concerns about long-term reliability of the existing services Additional considerations (I)

10.   It would be very unfortunate to discard convenient design options to keep compatibility with the installed services, and then decide later on to replace the services anyway, on the basis of other considerations  Cfr what happened with the pipes for the pixel CO 2 plant  It would be even much more unfortunate to design a detector that requires new services, and discover at a late stage that the recabling of YB0 is unfeasible because of risk, cost, time required, incompatibility with overall plan, etc…  Patching the tracker design at a late stage to restore compatibility with the existing services would almost certainly have a dramatic impact on the quality of the detector  We need a defined strategy very soon (i.e. this year), to be able to advance in the design Additional considerations (II)

11.   A technical review to assess the implications and the feasibility of YB0 re-cabling in the same LS as the Tracker installation, and in parallel with the other foreseen activities  If the outcome was that YB0 recabling is not feasible, a back-up option of some interest would be a reconnectorization in-situ at PP1  Based on the outcome of the review, define and freeze the corresponding requirements for the Tracker design, to be endorsed by CMS  Conclude the entire process before the end of the year  … 2013! Request