Effect of gamma-radiation on the transmission and return losses of FC/APC connectors

Abstract
The Fibre Optics Current Sensor (FOCS) installed at ITER will use optical fibres to perform plasma current measurements based on the Faraday effect. A connection between the sensing fibre placed on the Vacuum Vessel and the fibre guiding the signal to the data-acquisition hardware will be required. Because of practical reasons, this link contains FC/APC connectors, which guarantee the low return loss required for normal operation of the FOCS. The connectors will be located in the port cell area and will be subject to high levels of nuclear radiation. Radiation may influence both transmission and return losses of the FC/APC connectors and, as a result, degrade the FOCS performance. To address the problem, several FC/APC connectors were exposed to Co60 gamma radiation up to a 550 kGy dose. Transmission through the connectors was monitored during irradiation. The return loss of mated FC/APC connectors was measured before and after irradiation. It was found that radiation-induced changes of both transmission and reflection parameters are acceptable for the FOCS at ITER.