LHC thermometry: laboratory precision on an industrial scale in a hostile environment: tutorial 52

The measurement of cryogenic temperatures is essential for operating large-scale cryogenic installations and in experimental set-ups used for understanding low temperature physics phenomena. Recommendations on how to select and operate temperature sensors can be found in [1]-[2]. This paper presents how the CERN's Large Hadron Collider (LHC) [3] requirements for measurement of cryogenic temperatures were met [4]-[9]. The main challenge was a temperature uncertainty of ±0.01 K in the range 1.8 K to 2.2 K that should take into account the background radiation field provoked by the LHC circulating proton beams, the sheer quantity of the measuring channels, and an operational lifetime of at least 20 years. The LHC superconducting magnets' temperature measurements are used in feedback closed control loops to adjust the cooling power. The measurement uncertainty has thus a direct impact on the width of the temperature control range that shall be as wide as possible to increase the availability of appropriate operation conditions for permitting the circulation of particle beams around the LHC accelerator.