Frequency sweeping interferometry for robust and reliable distance measurements in harsh accelerator environment

High radiation levels, ultra-high vacuum, cryogenic temperatures of the measured components and high electro-magnetic noise push accelerator surveyors to look for more robust and accurate solutions of alignment. In the framework of the High-Luminosity LHC project at CERN, a range of new and cost-optimized solutions using Fourier analysis based Frequency Sweeping Interferometry (FSI), are under development. The technique allows the measurement of absolute distances to multiple targets simultaneously and is less sensitive to reflected optical signal intensity variations. The advantage with respect to classical interferometers (based on the detection of interference signal phase-change and sensitive to light quality) is that even weak interference beat frequency peaks can be easily retrieved from the Fourier spectrum without significant degradation of measurement precision. Moreover, the detectability of light reflected from different types of surfaces (high and low reflectance ones) makes it possible to develop a new family of simple, universal and robust micrometric sensors for harsh environments, like particle accelerators. An application of this novel method is the monitoring of the position of magnet and crab cavity cold masses inside their cryostats. For this purpose, specially designed divergent beam FSI vacuum optics and low cost glass ball reflectors are being tested and will be used in the HLLHC project. A new family of simple and cost-optimized, single and multi-reflection sensors (levelling, inclinometer, distance) is under development in the same coordinated effort. This paper describes such a measurement system, the sensor design approach, the results obtained and their final use in the LHC accelerator.