Operation of JET NBI optical interlock and compensation of radiation effects during DT experiments

An optical interlock protects the JET neutral beam ducts, connecting the neutral beam injection boxes to the tokamak, from damage if there is a fast pressure rise in the beam duct. Using results from the JET deuterium-tritium experimental campaign in 2021 (JET-DTE2), we demonstrate how the interlock will trip during fault conditions while avoiding false trips caused by background light from the plasma and the additional light from neutron radiation luminescence. During the DTE2 experiments, the rate of neutrons produced by the JET fusion reactions was up to 4.7 × 10¹⁸ n s⁻¹. Light produced by neutron radiation in the system was measured with blind fibres. A false trip occurred during the experimental campaign which was due to radiation induced light in the optics. Calibration of the beam emission response to the duct pressure and the background light from D_α and Bremsstrahlung emission and the contribution from light generated by neutron radiation was done through the analysis of a database of pulses. 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. At the end of the JET-DTE2 campaign, accumulated radiation damage was at most 15% with some fibres showing no loss of transmission.