Thermo-Mechanical Instrumentation of the ITER Magnet Structures

The ITER superconducting magnet system (Toroidal Field coils (TF), Central Solenoid (CS), Poloidal Field coils (PF), Correction Coils (CC) and Feeders) represents a total weight of approximately 10 000 tons. It is designed to withstand gravitational and seismic forces, stresses induced by thermal contractions during cool-down from 300 K to 4.5 K, and large Lorentz forces in the superconducting coils. Although not part of the Safety Important Class (SIC) components, the sensors (stress, displacement, thermometer) used to monitor the thermo-mechanical behavior of the structures are key diagnostic means to assess the design, support operation and survey possible fatigue effects over the 20 years´ lifetime of the Tokamak. The sensors and their wiring will be submitted to unique and severe environmental operating conditions; they will have to operate in the cryostat´s high vacuum (<;10^-4 Pa total gas pressure) at magnetic inductions of several Tesla and at low temperature (4.5 K) in the presence of a neutron fluence on the order of 10²² m^-2 - which implies a gamma dose higher than 10 MGy - for a period of 20 years. In addition, the fast and slow cycling of very large currents in the superconducting magnets and plasma will generate large eddy currents and, consequently, heat loads and electromotive forces (voltages) in the various parts of the structures, as well as a large electromagnetic noise. This paper presents the design philosophy and the chosen sensor technologies, some of which require further development in collaboration with the industry so that they can meet the objectives and the severe ITER operating conditions. In particular, only 20% of the near 1 000 measuring points for the thermo-mechanical data of the ITER magnet structures will rely on “classical” copper-wire technologies whereas 80% will rely on specifically developed optical-fiber-based sensors.