Radiation effect in optical fibres measured in fusion environment
Neutron and gamma radiation is known to have deleterious effects on the properties of optical components. Optical fibres are especially vulnerable due to the long distances the light typically travels though them.
In 2021 and 2023 JET has completed two D-T experimental campaigns where a significant amount of 14MeV neutrons were released. In order to quantify the effect of fusion relevant neutron irradiation on the optical fibres, a 60 metres long fused silica fibre has been coiled and placed near one of the JET machine horizontal ports in order to achieve high neutron fluences. In this experimental setup, transmission of the irradiated fibre could be measured in real time during JET plasma pulses.
Different radiation effects were readily observed with this setup. Radiation induced luminescence was visible dominantly in the blue range of the visible spectrum. Prompt darkening (loss of optical transmission) was also observed stronger in the blue than in the red/infrared parts. Although, the damage to the fibre quickly recovered and the transmission were seen to return almost to the pre-exposure levels at the time frame of about 1 hour. Only ~5% of the prompt damage remained permanent and accumulated throughout the experimental campaigns.
Neutron fluences at the fibres have been modelled with MCNP code and been measured by means of material activation techniques.