The ATLAS and CMS detectors require ultra-low, stable temperatures to protect their sensitive electronics. Traditional cooling systems rely on synthetic refrigerants like hydrofluorocarbons (HFCs), which, while effective, have a global warming potential thousands of times greater than CO₂. Seeking a sustainable alternative, CERN has turned to CO₂ itself – a natural refrigerant with a far lower environmental impact.

CERN’s journey with CO₂ cooling spans nearly two decades. It began in 2008 at LHCb with the first Two-Phase Accumulator Controlled Loop (2PACL) CO₂ system – a technology originally designed by Nikhef that circulates liquid CO₂ to maintain precise thermal conditions in particle detectors. From 2010, CERN’s Detector Technology Group (EP-DT) took over, further developing and deploying upgraded 2PACL CO₂ systems for ATLAS and CMS.

By 2016, as ATLAS and CMS sought to scale up this technology for all their silicon-based detectors, EP-DT proposed extending CO₂ cooling to the primary system – a concept they had designed. In 2017, Norway’s NTNU, experts in CO₂ refrigeration, joined the effort. Their thermodynamic expertise, combined with EP-DT’s detector-specific know-how and the large-scale engineering skills of the Engineering Department’s Cooling and Ventilation Group (EN-CV), transformed EP-DT’s vision into a fully realised system.

After years of modelling, prototyping and testing – to validate the stability and efficiency of CO₂ – the primary system (upstream of the 2PACL CO₂ loops and installed on the surface) was commissioned in May 2026. Built by Infrasolution in Germany, it integrates a novel modular refrigeration system designed by CERN. Today, eleven units are operating at CMS and seven at ATLAS, marking a major milestone in sustainable cooling for particle physics.

The primary system works alongside CERN’s 2PACL units, which circulate liquid CO₂ through the innermost layers of the ATLAS and CMS detectors, maintaining the precise thermal conditions the experiments require with a minimum amount of material in the particle tracking region. The primary system supplies those units with cooling from the surface, forming a fully CO₂‑integrated cooling chain.

“CO₂’s high-pressure properties allow efficient heat transport over long distances, enabling compressors to be placed on the surface,” explains Bart Verlaat (EP-DT). “This eliminates the need for underground thermal insulation or intermediate cooling loops, simplifying the system, boosting efficiency and reducing energy consumption.”

Extensive testing will now take place throughout Long Shutdown 3, as underground works progress to complete the full CO₂ cooling infrastructure, with the aim of it being fully operational by late 2027. This system removes HFCs from CERN’s primary cooling infrastructure for the ATLAS and CMS experiments, reducing greenhouse gas impact and aligning performance with sustainability. With expected savings of some 40 000 tonnes of CO₂ equivalent each year, this is a major step towards reaching the Organization’s 2030 objective of reducing its direct greenhouse gas emissions by 50% compared to the 2018 baseline.

From a 2017 concept to a 2026 reality, this initiative demonstrates how innovation and international collaboration can advance both scientific excellence and environmental responsibility.