A Silicon Microchannel bi-phase CO$_2$ Cooling for LHCb

The Upgrade of the LHCb’s Vertex Locator demands a very low material and yet high heat dissipation cooling solution due to its operation in vacuum and strict impact parameter requirements.The chosen technology for the LHCb VELO Upgrade is a new cooling solution, based on thin silicon microchannel cooling substrate. This technology ensures a low thermal gradient between the coolant temperature and the silicon detector, therefore minimising the reverse annealing in the silicon sensors caused by the radiation damage. The thermal cycle is based on the Carbon Dioxide (CO$_2$) evaporation cycle. This solution fulfils all the aforementioned requirements: it has an excellent thermal efficiency, no thermal expansion mismatch with the silicon ASICs and sensors, excellent radiation hardness, and very low contribution to the material budget.During the R&D; phase, special attention has been dedicated to the design and layout of the microchannels as well as the fluidic connector that connects the external cooling piping to the silicon channels themselves. The attachment of the fluidic connector to the silicon substrate is, in fact, crucial: the VELO modules are installed in a secondary vacuum (within the LHC primary vacuum), and no leaks are tolerated. For this reason, the attachment of the fluid connector must guarantee to withstand a maximum pressure of 186bar (130bar design pressure × 1.43 safety factor).The coolant flow and pressure drops have been measured as well as the thermal performance of the detector. Intensive tests and measurements have been performed using different setups at CERN, at The University of Manchester and at Nikhef.