Effect of high-energy neutron flux on fiber optics in an active diagnostic on TFTR (abstract)a)

Abstract

A bundle of 1-mm-diam fused silica optical fibers on an existing TFTR diagnostic has been exposed to 11 high-power DT discharges. Each shot subjected the fibers to a peak fast (14.7 MeV) neutron flux of ≊2×1012 n/cm2/s and a γ-dose rate of 500 rad(Si)/s for 0.75–1.0 s. The total fast-neutron fluence for these shots was ≊5×1012 n/cm2. A 15-m-long section of the bundle ran along the tokamak’s toroidal field coils and the remaining 15 m ran radially away from the reactor. Fiber luminescence at 660 nm was ≊1010 photons/s/sr/cm2/Å for the above flux (≊5%–10% of the bremsstrahlung emission), and varied linearly with DT neutron rate. Luminescence at 530 nm was 50% stronger, consistent with a Cerenkov radiation spectrum. Sensitivity to 3.5 MeV DD neutrons was ≊1/3 to 1/2 of that for DT neutrons. Fiber transmission decreased with the time integral of the neutron source rate and was reduced by 4% for the above flux. The fiber recovered rapidly: within 10 s, the transmission loss was only 2.5%. Shortly thereafter, the rate of recovery slowed to ≊0.05% per minute, but was sufficient to restore 75% of the transmission loss within two to four discharges. Recovery continued at ≊0.1% per hour and slowed overnight to ≊0.1% per day. Within the relative error of <0.2%, full transmission was recovered after five days.