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At the beginning of 2025, a collaboration at CERN started the development of an upgraded mechanical design for the CMS Inner Tracker TBPX L1 in preparation for its future replacement.
Building on concepts originally developed for the ALICE Inner Tracker, a new ladder design was developed to significantly reduce the material budget, achieving a reduction of about a factor of three at the mechanical level. After adapting the design to the strict geometry and integration constraints, a prototyping campaign was launched to validate the main design aspects. This included the development of new production procedures and high-precision assembly jigs to manufacture cold plates with 100 µm thickness titanium cooling pipes embedded in a high-thermal-conductivity carbon-fiber layup, combined with a carbon-filament space-frame structure to increase stiffness.
A qualification campaign was carried out on individual components, including pressure tests and CO₂ cooling tests at operating conditions (12 bar and −35 °C) to validate the pressure drop and flow parameters at different vapor quality levels. Thermal tests with dummy heaters showed agreement within 1 °C with simulations performed in Ansys Fluent, also validating the simulations for different configurations, such as TBPX L2 and TBPX L1 with planar sensors instead of 3D sensors.
Additional effort was devoted to the design of compact pipe interconnections using soft-soldering techniques derived from the CMS Outer Tracker.
Parallel studies using the tracker layout were also performed to evaluate the impact of the reduced material budget on track reconstruction precision.
The project is now approaching the end of the feasibility study, including thermal, mechanical, metrology and mass validation. The next steps include finalizing the support structure, integrating bare modules to obtain a fully equipped prototype, and evaluating the procurement of materials to be ready for the construction of a full TBPX Layer 1 structure for a potential future replacement.
This R&D activity has also been included in the DRD8 program within WP2 starting in January 2026.