Speaker
Description
Detector Transfer Lines (DTLs) are a key element of the CMS Tracker two-phase CO₂ cooling system. They are responsible for distributing liquid CO₂ from the instrumented manifolds at the detector periphery into the detector volume. Their coaxial design (either one or four inner process pipes inside an outer process pipe) allows heat exchange between the liquid supply and the two-phase return, enabling the liquid CO₂ to reach saturation before expansion in the capillaries and evaporators. An insulation chamber surrounds the outer process pipe to ensure that the surface temperature remains above the ambient dew point. The qualification of the DTL design involves several engineering aspects, including thermal insulation and hydraulic performance. An experimental campaign was conducted in the Tracker Integration Facility (TIF) at CERN on two rigid DTL prototypes. The first one, for the High Granularity Calorimeter (HGCAL), was a coaxial vacuum-insulated rigid pipe with a vacuum-insulated flexible section in series. The second one, for the Tracker, was a four-into-one aerogel-insulated rigid pipe developed by the Mechanical and Materials Engineering (MME) group at CERN. The campaign combines thermal insulation testing with a pressure-drop mapping study. The thermal qualification for insulation performance validation is based on distributed pipe surface temperature measurements using PT100 sensors under nominal conditions, with a focus on the most critical locations (i.e., bellows, elbows, and spacers). To evaluate the condensation risk under final operating conditions, calculations were performed to extrapolate the measured surface-to-ambient thermal behavior in the TIF to the expected cavern ambient temperature. In parallel, the two-phase pressure drop in the DTL outer process pipe section is evaluated over multiple representative operating setpoints using temperature, absolute pressure, differential pressure, and mass-flow measurements. The aim is to determine the total ΔP of the assembly and verify that it remains within the available operating margin. The results of the prototype tests helped identify design corrections, including improvements to the aerogel filling procedure developed by EN-MME, as well as a dimensional modification consisting of an increase in the insulation chamber diameter of the Barrel Timing Layer (BTL) DTL to ensure compliant insulation performance. For the vacuum-insulated lines, the outcome was the decision to install getter pumps for active pumping in the vacuum-insulated flexible sections. The ongoing pressure-drop mapping campaign aims to validate the simulated pressure-drop budgets for the system and to support the commissioning and parameter tuning of the 2PACL equipment, such as plants and manifolds. The first results show trends consistent with the simulations. Data taking will continue in the coming months and will be used to fully validate the system assumptions.