SIGNAL CONDITIONING FOR CRYOGENIC THERMOMETRY IN THE LHC

摘要

Temperature measurement is a key issue in the Large Hadron Collider (LHC), as it will be used to regulate the cooling of the superconducting magnets. The compromise between available cooling power and the coil superconducting characteristics leads to a restricted temperature control band, around 1.9 K. An absolute accuracy of 10 mK below 2.2 K, and 5 K above 25 K, is necessary. For resistive thermometers covering the full temperature range, and having a negative dR/dT sensitivity, this is typically equivalent to a relative accuracy ΔR/R of 3 10~(-3) over 3 resistance decades. Also, to limit the thermometer's self-heating, the sensing current must be limited to few μA. Furthermore, the radiation levels next to the accelerator are expected to significantly degrade the performance of conventional analog electronics. As these stringent requirements are not met by commercial conditioners, three different architectures have been developed at CERN. The first compresses the input dynamic range using a logarithmic transfer function; the second partitions the input range into three linear regions; the third converts resistance linearly into the frequency of a square wave. They fulfill the above specifications and provide industrial robustness in terms of thermal drift, galvanic protection, and compact packaging, while optimizing cost-to-performance ratio. This paper describes the principles of their design, compares their characteristics and shows results of field tests. Future developments include Application Specific Integrated Circuit versions, Fieldbus interfacing, and radiation tolerant re-design.