A new technique to probe the inside of a cryostat has just been tested at Fermilab, in the United States. Between March and May 2023, scientists have been able to monitor with a resolution of a few micrometres the position of a cold mass inside a cryostat during a thermal cycle.
The component subjected to this new technique was the first of the new High Luminosity LHC triplet quadrupole magnets built by the Accelerator Upgrade Project (AUP) collaboration and tested in its final cryostated configuration at Fermilab. To carry out these measurements, special thermo-optical targets were mounted on the cold mass before it was installed in the cryostat. The position of these targets was then measured very accurately in the referential frame of the cryostat after the cryostating process. In parallel, dedicated optical heads integrating an optical fibre feedthrough were installed on the vacuum vessel and measured in the cryostat referential frame. The absolute distances between the heads and the targets were then measured continuously at three locations along the length of the cryostated magnet: at the extremities and at its centre (see Figure 1). The measurement technique is based on Frequency Scanning Interferometry (FSI).
The thermal cycle lasted for more than 2 months, starting with an initial slow cooling of the cryostated magnet from 295K to 5K, with a stable cold phase lasting more than one month at 5K, and reheating to 295K at the end of the cycle.
The measurements performed show a longitudinal contraction of nearly 15 mm of both extremities of the cold mass and of 1.55 mm in the vertical direction between 295K and 5K (see Figure 2). No relevant displacements are seen along the radial direction, while a good symmetry of both extremities was observed. The transverse position of the cold mass within the vacuum vessel is determined with an accuracy below 0.1 mm and can be monitored with a resolution in the order of 10 µm. A second thermal cycle will soon be carried out on the same component, to verify the repeatability of the process and therefore the robustness of the measurement technique.
These results come after a long process of development and qualification tests at CERN. The first test measurements had already been undertaken on an LHC dipole in 2018 , highlighting the need to develop specific insulated targets and to develop more robust analysis tools and algorithms for the FSI measurements. Special calibration processes have also been developed and qualified to optimise measurement accuracy . A first version of an intra-cryostat FSI measurement system was installed on the HL-LHC crab cavity prototype inside the SPS accelerator in 2018, demonstrating very good measurement repeatability throughout thermal cycles, and the micrometric resolution of measurements .
All of the 16 inner triplet quadrupoles and the 8 crab cavities of the HL-LHC project will all be equipped with this new FSI-based measurement technique, offering a much better monitoring and understanding of the position of cold masses and crab cavities inside their cryostats. The final system will comprise some 320 measurement channels. The corresponding hardware and software are currently being industrialised through collaboration between several groups in CERN’s Beams Department. The next step is to qualify this measurement technique on the HL-LHC Inner Triplet String, which is due to start its cooldown in the second half of 2024.