Radiation effects on heated optical fibers

Abstract

Optical fibers are ubiquitous on today’s plasma devices, and will play a major role in future machines. However, radiation causes luminescence and transmission loss in fibers at troubling levels even on today’s machines when they operate in DT. We have evaluated these effects and studied the use of elevated operating temperatures to reduce them. Using high-purity UV grade silica-silica fibers at 400 °C reduces transmission loss by a factor of at least 100, but has little or no effect on radioluminescence. The radioluminescent spectrum appears to be Cerenkov radiation. The transmission loss spectrum depends on the fiber material and details of the manufacture. The mode structure of transmission loss is mainly simple path length attenuation, with a suggestion of internal reflection degradation.

Features of the Fiber Optics Application in Iter

“…Следует также отметить, что на результаты ра-диационных испытаний оптоволокна влияет начальное количество примесей и дефектов в нём. В волок-не это проявляется особенно сильно из-за его большой длины [13].…”

“…Во-вторых, следует подчеркнуть, что термический отжиг не может рассматриваться как реальный способ восстановления оптической переда-чи волокна. По результатам экспериментов нагрев плавленого кварца до температуры менее 200 ºC практически не улучшает его пропускание после нейтронного облучения, а для заметного термического отжига необходим разогрев кварцевого стекла более 400 ºC [5,13]. Такой температуры оптоволоконный коллектор не выдержит.…”

Diagnostics for steady state plasmas

“…In low fluence experiments like TFTR or JET both effects are thermally quenched at fibre temperatures above 150 °C resulting in radio luminescence being dominated by Cerenkov radiation at elevated temperatures. While the luminescence yield due to Cerenkov radiation cannot be reduced by fibre heating, it can be reduced proportional to 1/d (d: fibre diameter) by using larger diameter fibres (Figure 9) [275]. The reason for this is believed to be that the radiation interaction length is proportional to d while the throughhput of the fibre and hence of the sensitivity of the optical system increases as d 2 .…”

“…For example in one sample of 'dry' fibres a temperature increase from 20 °C to 420 °C led only to an improvement in fibre transmission by a factor of 7 and in case of high-OH fibres drawn from different batches considerable variation in fibre transmission at room temperature was found. It is therefore strongly advisable to only use fibre compensation loops drawn from the same raw material batch [275].…”

Plasma Measurements: An Overview of Requirements and Status

“…Under the peak in the light caused by the alpha particles, fluorescence causes the clear rectangular pedestal. To try to reduce the transmission loss in fibers, metal-clad heated fibers were tried on JET [56,57]. More recently, the ITER partners have carried out extensive developments of special fibers [50].…”