Measurement of Disruption Forces in JET using Fibre-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 fibre transducers FBGs Fiber Bragg Gratings (FBGs) were installed in addition to the currently used strain gauges. During Vertical Displacement Event VDE 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 fibre-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 (±1.5µstrain corresponding to a force of 4kN due to the geometry of the leg) and measurement range (5000µstrain or 13000kN) for this application.