Speaker
Description
The use of cryogenics plays a major role in the operation of modern high-energy accelerators and ensures 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. Despite its considerable contribution to the strain-state, the scarcity of commercial thermal contraction measurement devices capable of examining these materials to temperatures as low as 2 K has limited the available information on the thermal contraction within these complex systems. The Mechanical Measurement Laboratory at CERN initiated thus 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 test bench enables temperatures from room temperature down to a minimum of 1.8 K.
The initial setup underwent an extensive testing campaign with several optimization iterations to achieve accurate measurements. Simultaneously, we elaborated an appropriate sample preparation that addresses the limitations in the examinable materials caused by the optical requirements of the interferometric method. It followed an experimental validation using single-crystal silicon as a certified reference material from the National Metrology Institute of Japan (NMIJ), yielding an error of less than 0.03∙10$^{-3}$ in the relative change of length ΔL/L$_0$. However, the system’s significant repeatability demonstrated in these initial validation tests permits a correction of the dominant systematic errors, resulting ultimately in an uncertainty better than 0.01∙10$^{-3}$ in ΔL/L$_0$ over the entire temperature range. Given this level of uncertainty, this setup is well-balanced between accuracy, simplicity, and time-efficiency, facilitating dynamic thermal contraction measurements across the entire low temperature range within a 10-hour timeframe for a full measurement cycle.
| Submitters Country | Switzerland |
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