Fibre Optic Sensors for the Thermo-Mechanical Instrumentation of the ITER Magnets

ITER will be the world's largest experimental facility to demonstrate the scientific and technical feasibility of fusion power. Fusion is the process which powers the sun and the stars. When light atomic nuclei fuse together to form heavier ones, a large amount of energy is released. Fusion research aims at developing a prototype fusion power plant that is safe and reliable, environmentally responsible and economically viable, with abundant and widespread fuel resources. ITER is based on the “Tokamak” concept, in which the fusion fuel is contained in a doughnut-shaped vessel. The fuel— a mixture of deuterium and tritium, two isotopes of hydrogen—is heated to temperatures in excess of 100 million degrees, forming a hot gas “plasma”. The plasma is kept away from the walls by a strong magnetic field produced by superconducting coils surrounding the vessel and an electrical current driven in the plasma. The ITER superconducting coils and structures (Toroidal Field coils (TF), Central Solenoid (CS), Poloidal Field coils (PF), Correction Coils (CC) and Feeders), representing a total weight of approximately 10 000 tons, are submitted to gravitational and seismic forces, stresses induced by constrained thermal contractions during cool-down from 300 K to 4.5 K, and large Lorentz forces in the superconducting coils. The (strain, displacement, temperature) sensors used to monitor the thermo-mechanical behaviour of the structures have to operate under unique and very severe conditions (cryogenic temperatures, large magnetic fields, vacuum, high radiation doses and electro-magnetic noise). The near 1000 measuring points for thermo-mechanical data of the ITER magnet structures will rely for 80% on industry-developed optical, fibre-based sensors. These include sensors using the following technologies: Fabry-Perot, Fibre Bragg Grating, Distributed Raman Scattering and Laser Distance Meter. ITER requires thermo-mechanical diagnostic sensors that do not exist for its specifically harsh environment, and a consistent development effort had to be undertaken by industry in collaboration with research institutes. The sensors design philosophy, arguments for the choice of specific optical sensor technologies, and significant results of the qualification programs of prototypes are presented in this paper.