Measurement of disruption forces in JET using fiber-optic sensors

Monitoring the forces induced on the vacuum vessel
when a disruption occurs is essential for the protection of the
machine. Since divertor coils have been installed in JET, during
disruptions a significant difference between the forces measured
on the top and the bottom of the machine has been observed. In
order to investigate these further, optical fiber transducers FBGs
(Fiber Bragg Gratings) were installed in addition to the currently
used strain gauges. During VDE (Vertical Displacement Event)
tests, these new transducers were used to measure different level
of disruptions ranging from 80T up to 230T vertical forces. The
results were then compared with the strain gauges. The FBG
measurement was carried out using a broadband light source
illuminating an array of 4 FBGs through a combination of
directional couplers, connectors and fiber-optic cable. The forces
measured both by FBG and by the strain gauges are within the
same range, except for the initial swing where large random
differences are observed. The tests have demonstrated that the
FBG-based force measurement system has adequate resolution
(±1.52strain corresponding to a force of 4kN due to the geometry
of the leg) and measurement range (50002strain or 13000kN) for
this application.