CERN, the European Organization for Nuclear Research, in partnership with Norwegian University and Science and Technology (NTNU), is proposing a unique environmentally friendly cooling system for research sensors that employs a Krypton-CO2 (R744) cascade refrigeration unit to deliver a temperature range of -60 to -80°C (-76 to -112°F).
Explaining the system, Luca Contiero, a doctoral student at NTNU, presented a paper, “An advanced Krypton-CO2 cascade refrigeration unit for the Phase III upgrade of the VELO detector at CERN,” at the 10th International Institute of Refrigeration (IIR) conference, held in Ohrid, North Macedonia, April 27–29. The study received funding from the European Union’s Horizon 2020 research and innovation program under a project named ENOUGH.
Based in Geneva, Switzerland, CERN is an intergovernmental organization that operates the largest particle physics laboratory in the world. CERN’s main function is to provide the particle accelerators and other infrastructure needed for high-energy physics research. It is the site of the Large Hadron Collider, the world’s largest and highest-energy particle collider.
The sensors used by CERN are silicon detector trackers that measure the position of charged particles. As occurred in the past with CO2, the small Vertex Locator detector (VELO) is the main candidate for the future cooling system.
As it undergoes a planned upgrade (phase III) to achieve a higher irradiated environment, CERN has determined that colder temperatures are required to avoid thermal overload of the sensors. The unprecedented amount of radiation is thought to produce temperature levels that would be unmanageable by the current CO2 cooling system (2PACL).
The current CO2 system, which uses evaporative cooling, can’t be implemented in an ultra-low temperature range because CO2 has a freezing point of -56°C (-69°F) and because ultra-low temperatures require subcooling. At low pressures, temperature losses become more substantial and require new cooling technology to overcome these bottlenecks.
Choice of krypton
The noble gas krypton was first proposed as a candidate for a cooling system by researcher Bart Verlaat during the Forum on Tracking Detector Mechanics, held at Cornell University in 2019. A preliminary study proved that krypton will help to manage the higher radiation, ensuring clean and efficient cooling in small-diameter channels as required by the low-mass detector design.
“The proposed solution involves a fully environmental cascade refrigeration system, where CO2 will be used in the high-temperature circuit to absorb the heat rejected by the cold krypton system, ensuring stable operation of the low-temperature system throughout the lifetime of the detectors,” said Contiero.
The high-pressure fluid krypton is a promising coolant for the thermal management of silicon detector trackers, but it has some challenges. Silicon detectors need to be kept cold at different temperature levels, starting from ambient conditions. But Krypton cooling doesn’t start in the liquid phase, but rather in the supercritical (gas/liquid) phase. Thus, a supercritical cooldown is necessary to gently bring the system into the cold area while connected to the detector.
An ejector-supported cold krypton cycle is proposed to provide cooling. The ejector acts as a high-pressure control device as well as a flow regulator through the detector.
Once the transition from supercritical to subcritical takes place, the tank that previously functioned as an accumulator turns into a phase separator. During transcritical operation the cycle works similarly to a traditional ejector vapor compression system, allowing the system to maintain the temperature between -60°C and -80°C, which is suitable for detector cooling.
“The proposed solution involves a fully environmental cascade refrigeration system, where CO2 will be used in the high-temperature circuit to absorb the heat rejected by the cold krypton system, ensuring stable operation of the low-temperature system throughout the lifetime of the detectors.”Luca Contiero, a doctoral student at Norwegian University of Science and Technology