Experimental validation of a versatile test bench for thermal contraction measurements down to 1.8 K

Abstract The use of cryogenics plays a major role in the operation of modern high-energy accelerators and ensures, for instance, the superconducting state of beam-guiding and focusing magnets. A profound understanding of the thermo-mechanical behavior of the structures and components is therefore crucial, particularly for the strain-sensitive Nb 3 Sn-based superconducting coils. Inaccuracies in considering processes like thermal contraction can substantially impact the performance of the magnets. However, the scarcity of commercial thermal contraction measurement devices capable of examining these materials down to temperatures as low as 2 K has limited our knowledge of the thermal contraction within these complex systems. The Mechanical Measurement Laboratory at CERN has thus initiated the development of a customized dilatometric test bench in collaboration with the external company attoCUBE. The resulting setup features an optical displacement sensor based on Fiber-Optic Fabry-Pérot interferometry and, with its integration into a closed-cycle cryostat, this setup accommodates temperatures from room temperature down to a minimum of 1.8 K. Preliminary tests on known materials indicated the presence of a systematic error which was corrected by a calibration study using certified single-crystal silicon from the National Metrology Institute of Japan (NMIJ) as reference material. This study served simultaneously as the base for an uncertainty analysis to determine the remaining measurement uncertainty. Using the methods of the Guide to the expression of uncertainty in measurement (GUM), an uncertainty better than 0.01 × 10 −3 in ∆ L / L 0 over the entire temperature range was obtained. Given this level of uncertainty, this setup is well-balanced between accuracy and time efficiency, facilitating dynamic measurements across the entire low-temperature range within a 10-hour timeframe for a single measurement.