Past Experiences and Future Trends on Vertex Detector Cooling at LHC

Substantially different approaches have been ad opted for the refrigeration plants of the first generation of vertex detectors at LHC: those of ALICE, ATLAS and CMS use PFC fluids, either in single phase or in a traditional Joule-Thomson cycle, while carbon dioxide in a pumped two-phase loop has been selected for the LHCb VELO. For what concerns the on-board thermal management of the sensors and related electronics, a traditional design has been followed, based on a common general approach and only differing in the specific choices related to the local configuration. Although the global performance of the detectors in this first phase of LHC operation can be claimed as fully satisfactory, it appears that the additional challenges posed by the coming upgrade phases can only be tackled through an effort on technology innovation and, in particular on much stronger and earlier integration of all the cooling-related aspects in the detector conception. Carbon dioxide seems to be the preferred choice for the refrigerant fluid of the future cooling plants (at least for cold operation), while advanced design approaches are pursued for the on-detector thermal management, integrating extended use of innovative materials and technologies. In this review paper, after a summary of the different approaches selected for the detectors presently in operation at LHC, the principal trends under development for the coming generations ar e discussed, for both off-det ector and on-detector issues.