# An Ultra-Low Temperature Transcritical R744 Refrigeration System for Future Detectors at CERN LHC
### Technical Challenges  
The LHC accelerator is located in a 27 km long circular tunnel excavated at a depth of the order of 100 m from the surface. As illustrated in Figure 1, the LHC experiments are built around the accelerator and are housed in large underground halls conventionally named \"cavens\": the detector itself occupies the so-called \"experimental cavern\", while all ancillaries that are not strictly required to be placed in the immediate closeness of the detectors are housed in a parallel hall called \"service cavern\". This is the case of the secondary 2 PACL system.  
However, due to space limitations, to the amount of heat produced and to the risks of asphyxiation in case of leak, ATLAS and CMS cannot afford to install underground also the large R744 chiller required as primary system. The primary system needs thus to be installed on surface, inside a dedicated building with all the space and security features needed. Its connection to the 2 PACL system in the service cavern is indeed one of the challenges to be tackled, due to the complex pipe routing, estimated to be \(\approx\)300 m long with around 90 m of height difference and very limited space for integration. In order to cope with the space constraints, it has been decided to establish this connection with non-insulated pipes.  
The LHC beam time allocated to the experiments is extremely valuable and it is mandatory to avoid any stoppage of the detector while the beam is circulating. Therefore, the highest level of reliability is the top priority for all the experiments' detectors and their vital ancillary systems, including the cooling.  
Underground, the pressure of the R744 primary shall be as low as possible at the evaporator level to cool the liquid CO2 pumped by the 2 PACL secondary loop down to lowest possible temperature. The physical limit is of course \(-56.6\,\mathrm{\SIUnitSymbolCelsius}\), corresponding to the CO2 triple point at 5.2 bar): therefore, the challenge translates into minimising that safety margin, protecting from the risk of getting below the triple point due to the pressure drop along the line. Moreover, with non-insulated pipes at room temperature and such low return pressure, the superheating of the return line will be significantly large (\(\approx\)70 K).  
The oil circulating in the primary system will tend to migrate to the coldest point of the installation, which will be located underground in this particular case. Considering the height difference of around 90 m and the complex routing of the transfer lines, pumping the oil accumulated underground back to the system does not seem feasible. Hence, the oil separation shall be extremely efficient.  
Finally, both experiments require the temperature to be kept stable in nominal operation, at startup and during stoppage to avoid any potential damage to the sensor modules. The system is therefore also required to cope with a significant range of heat loads, which will vary from a minimum of 50 kW during stoppage up to a maximum of 920 kW at startup. The amount of cooling provided to the detector's evaporator is ultimately controlled by the secondary loop which is independent from the primary system. The primary chiller shall thus keep the process stable to finely control the return pressure at \(\pm\)250 mbar during operation (0.5 K), while the system shall also be able to react quickly to transient regimens.