Speaker
Description
The Compact Muon Solenoid (CMS) detector will undergo a major upgrade (Phase-2) to take advantage of the increased luminosity provided by the High-Luminosity LHC (HL-LHC). As part of this upgrade, the Tracker Endcap Pixel detector (TEPX) will be introduced as a subsystem of the Phase-2 Inner Tracker, extending the pseudorapidity coverage up to |η|<4. The TEPX consists of eight double-layer disks, split into two halves, and will incorporate a total of 1408 quad-chip hybrid pixel modules. This work presents system tests conducted on TEPX layer prototypes, including electrical and thermal characterization, as well as evaluation of data transmission performance.
Summary (500 words)
The goal of the TEPX system tests is to identify and resolve issues related to the operation of 4-chip modules integrated onto the disks, communication quality, and cooling performance before the full subsystem is produced. This ensures reliable performance over the expected 10-year operational lifetime. The system tests presented in this work are carried out on a complete setup at the University of Zurich, which includes prototypes of the mechanical structure, disk printed circuit boards (PCBs) integrating power and data lines, custom-made portcards, which transform the electrical signals into optical signals, and a two-phase CO2 cooling unit.
The mechanical structure features titanium cooling pipes embedded in a carbon foam layer, which is covered by a high thermal conductivity carbon fiber. The two-phase CO2 cooling system is designed to maintain the modules at sub-zero temperatures (°C), which is critical for ensuring electronics durability and thermal stability. Modules are mounted on PCBs that are glued to the mechanical structure and provide power, sensor bias voltage, and electrical links (e-links) for communication.
Modules are serially powered, which reduces the material budget but exposes modules to a different ground potential depending on the position in the power chain and increases the risk of failure: if a single module in the chain is damaged, it could compromise the entire power chain. This is especially critical for TEPX, which features the longest power chain in the Inner Tracker detector—11 modules per chain—making powering tests particularly important.
Electrical tests verify the integrity of the power chains and the performance of the modules on the structure. The ASICs (CROCv2) of integrated modules were successfully tuned to 1000e- thresholds with the voltage bias applied to the sensors. Electrical noise remained below 100 electrons across all modules, independently of their position on the prototype.
Data transmission quality is assessed by testing for error-free communication. Signals from the modules are transmitted via e-links with 1.28 Gb/s uplink and 160 Mb/s downlink bandwidths. A bit error rate below $10^{-12}$ was measured for the longest e-link in TEPX, approximately 49 cm in length. To minimize material, the disk PCB uses fewer data lanes in its outer rings, where hit rates are lower. Both readout and command lanes are merged in these rings. This data-merging functionality was successfully validated on the full setup at the University of Zurich.
Thermal tests assess the effectiveness of the cooling system. To prevent thermal runaway, the maximum module temperature must remain below 0°C. In the original design, modules were clamped flat directly onto the carbon fiber, but this setup was tested to have temperatures exceeding 0°C when only one side of the prototype was powered. To enhance thermal conductivity, thermal interface material (grease) was introduced between the module and carbon foil. The thin grease layer significantly improved thermal contact and reduced module temperatures by an average of 7°C. However, further investigations are needed into the feasibility of removing the grease if a module replacement is required.