Mechanical Strain Measurements in High-Field Low-Temperature Superconducting Magnets

Optical fiber sensors based on Fiber Bragg Grating (FBG) technology are used to monitor the mechanical behavior of the magnet's coils from its assembly at room temperature to its powering at cryogenic temperatures. The development of this instrumentation required several years of research and development effort to validate the precision and accuracy of FBGs based measurements in cryogenic conditions. FBGs are bonded on the coils in different locations along with the longitudinal and azimuthal directions that allow studying the average stress levels. The stainless-steel shells of the magnets and each extremity of the four rods are equipped with resistive strain gauges (RSG). This paper provides a description of the instrumentation used to monitor the strain profile of superconducting magnets. Strain measurements during magnet assembly, cool-down, and powering are presented to confirm the agreement between electrical and optical sensors.