Design and manufacturing of fiber optic sensors for the ITER neutral beam test facility

In the ITER neutral beam test facility, a 40 MW precursor D --/H beam will be produced neutralised and filtered along the beamline . A 18 MW D 0 /H 0 beam will be dumped onto a calorimeter corresponding to a 16.5 MW neutral beam to be injected into the ITER plasma. Heat fluxes up to 20 MW/m 2 will be exhausted through water cooled channels operating up to 1 h continuously in subcooled boiling. During beam operation, the high heat flux components along the beamline will be monitor ed using embedded thermo mechanical sensors : 642 thermocouples and 105 fiber optic sensors that includes temperature sensors, strain gauges, and accelerometers to detect implosion of vapour bubbles in the subcooled water bulk after boiling nucleation at the cooling channel inner wall. Positions of sensors have been determined through simulations of component particle beam interaction. Signals of these sensors will be processed for component protection against critical conditions, for performance monitoring through calorimetry, and for retrieving parameters of the particle beam. The fiber Bragg grating technology has been identified to manufacture temperature sensors with high dielectric strength to be installed on panels polarised up to 25 kV to electrostatically deflect the residual ions emerging from the neutralisation process. Moreover, fiber optics are compatible with strong magnetic fields produced in ITER for plasma confinement. Materials of sensors, including the optic fiber, present r adiation hardness consistent with high neutron and gamma fluxes produced from fusion reactions. The optic f iber is polyimide coated, compatible with 300 °C continuous service temperature , and inserted into a protective braided sleeve made of continuous filament fiberg lass preliminarily heat cleaned. F iber optic sensors and patch cords have been designed to be c ompatible with the vacuum environment limiting the outgassing of species contaminating the in vessel experimental environment and avoiding virtual leakages. Sensor mounting s h ave been designed considering stiffness, thermal contact, and vibration resonant frequenc y . The expected total exposure time to the environment conditions is 2780 h. Each sensor is supplied with its own calibration curve The fiber connectivity though FC/APC connectors and fusion splices has been studied and implemented considering the interrogation optical budget and the m ultiple and reliable disconnections required for installation and maintenance of in vessel components. A 8 channel configuration in the 160 nm wavelength range have been realised.