Operation of JET NBI Optical Interlock and Compensation of Radiation Effects During DT Experiments
An optical interlock protects the JET neutral beam ducts from damage if there is a fast pressure rise in the beam duct. The ability to trip during fault conditions and avoidance of false trips is limited by background light from the plasma and the additional light from radiation effects. A false trip occurred during the JET-DTE2 experimental campaign which was due to radiation induced light in the optics. During the DTE2 experiments, the rate of neutrons produced by the JET fusion reactions was up to 4.7 × 1018 n/s. A data-base of JET DTE2 pulses was analysed, and results are presented showing the background light from Dα and Bremsstrahlung emission and from light generated by neutron radiation. Light measured in blind fibres was strongly correlated to neutron production rate. We show that compensation of the background due to radiation is possible and can significantly improve the performance of the interlock during JET fusion experiments. The interlock used annealed optical fibres heated to 200 °C to limit the attenuation from radiation damage. Results show that there was no loss of transmission over time from accumulated radiation damage.