# An Ultra-Low Temperature Transcritical R744 Refrigeration System for Future Detectors at CERN LHC
## 6 Conclusions and Outlook  
The ATLAS and CMS experiments require the primary system to be capable of providing a cooling temperature underground at \(-53\,^{\circ}\)C (corresponding to 6 bar) with a stability  
Figure 13: Investigation of COP of the proposed cycle for different cooled gas temperatures.  
of \(\pm\)0.5 K. The preliminary commissioning tests hereby presented have been conducted at 5.5 bar already anticipating the expected 500 mbar of pressure drop between the evaporator and the buildings on the surface. The results were important to validate the principle of operation and enlarge the possibilities for the final units.  
Amongst the prototypes foreseen before launching the production of the final units, only this first prototype includes two compressor slices. Therefore, this system will keep being a source of important feedback for the definition of the end control logic. This work reports the preliminary results that will precede a forethought testing campaign that will happen in the coming months. Throughout the next tests, it will be investigated how the system's control shall deal with the management of multiple slices at every case scenario, what are the optimal process setpoints given the chosen components, other different capacity control strategies and finally the reliability of the system in case of major failures. Moreover, the system has been designed so it can operate even below triple point if needed (for example, in case the pressure underground is closer to the triple point pressure of 5.2 bar). The lowest possible pressure in the evaporator underground will be dictated by the low pressure limit of the design so it will be particularly interesting to evaluate the operation limits.  
The next prototype is currently under design, benefiting from the System A preliminary results, and should be ready by the end of this year. This system will comprise one compressor slice only with twice the cooling capacity of System A (two LP compressors) and will serve mainly to investigate the amount of oil carried over to underground by connecting it to the dummy load presented in Section 3.3 through the ATLAS shaft. A third prototype with only one compressor slice should benefit from the coming testing campaign on System A and shall arrive at CERN by the middle of next year. Together with the other two prototypes, it will provide the feedback needed for the design of the ATLAS and CMS final units that shall arrive by mid 2023.  
The studies will proceed and the final return of experience on System A will soon be shared in future publishing.  
Conceptualisation, P.B., B.V. and A.H.; methodology, P.B.; software, P.B., W.H., L.Z., L.D. and D.T.; validation, P.B., A.H., B.V. and P.P.; formal analysis, P.B. and P.P.; investigation, P.B.; resources, A.H., B.V., P.P., W.H., L.Z., P.H., M.B., L.D. and D.T.; data curation, P.B.; writing--original draft preparation, P.B. and P.P.; writing--review and editing, P.B., A.H., B.V., P.P., W.H., L.Z., P.H. and M.B.; visualisation, P.B.; supervision, B.V., L.Z. and P.H.; project administration, A.H., P.P. and M.B.; funding acquisition, A.H., P.P. and M.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding and the APC was funded by CERN and NTNU.  
Not applicable.  
The authors declare no conflict of interest.