Neutron degradation of UV enhanced optical fibers for fusion installations in plasma diagnostics

The design of ITER and the future operation of DEMO will require high-temperature and high-neutron flux resistant materials to be used in plasma diagnostics subject to ITER requirements. A solution for remote plasma diagnostics implies the operation of various optical instruments placed apart from the critical temperature-neutron zones with the optical signal transmitted over optical channels for a reduction of radiation effects on the equipment and a higher resistance to electromagnetic disturbances. Generally, data are available on the radiation effects on optical fibers used in communication applications or for transmission in the visible range. A special problem arises when optical signals have to be transmitted in the UV region (200 nm to 450 nm), where attenuation over the distance is quite high (in the range of several meters) and the UV radiation by itself produces an increase of the attenuation after several hours of exposure. Until now, practically no data on the radiation effects on optical fibers for fusion plasma diagnostics operating in UV were published. In the frame of the EU funded Fusion Programme, we focused on the evaluation of radiation induced changes in the optical transmission for different commercially available optical fibers for their possible use in optical light guides. Pure silica optical fibers with an UV enhanced response with various cladding/jacket materials and core diameter of 200 μm and 400 μm were evaluated as they are subject to neutron irradiation. The optical fiber core was of a high hydroxyl content type and the coating was either Polyimide or aluminium. Optical fibers with low (150 deg C) and high (350 deg C) temperature jacket materials were investigated. The optical fiber samples were irradiated at fast neutron facility of the IFIN-HH (Horia Hulubei National Institute for Physics and Nuclear Engineering) U-120 Cyclotron. The neutron flux at 0 angle (i.e. in the beam direction) was found to be equivalent to 2.13 x 108 n/cm2sμA at 10 cm distance from the Be target. The neutron energy spectrum shows a mean energy of 5.2 MeV. The neutron and gamma components of the mixed radiation field give rise respectively to 138 Gy/C and 2.38 Gy/C at 90 cm distance from the Be target. The irradiation steps used correspond to about 6 x 1010 n/cm2. For the evaluation of the optical transmission degradation in optical fibers we developed a setup enabling the following operations: a - the investigation of the optical transmission of UV enhanced optical fibers in the spectral range 200 - 400 nm with spectral resolution of 1.5 nm and 12 bits amplitude resolution of the transmission readings; b - the evaluation of temperature influence on the optical transmission of optical fibers. The setup is based on a CW stabilized deuterium source, a miniature, multichannel optical fiber spectrometer coupled to a PC via the USB link and a programmable oven operating under the PC control. As the assessment of the optical transmission in optical fiber for the UV region is very difficult to carry out (source efficiency is very low, UV solarisation effects in sampling probes are quite high and detection noise is significant in the CCD array) signal averaging and box-car smoothing were used for data processing. All the measurements were done off-line, at room temperature, between the irradiation steps. Absorption peaks were observed for all the optical fibers at about 230 - 250 nm with an amplitude increasing with the neutron flux. We noticed some absorption related recovery phenomena induced by the fiber heating even at 100 - 120 deg C. (authors)