Photobleaching as a Radiation Damage Mitigation Technique for Optical Fiber Sensors in Extreme Environments

<p dir="ltr">Modern day society demands energy at an alarming rate and if that energy is to be obtained from non-renewable sources, the impact on the climate and the planet will be devastating. We are in dire need of large-scale renewable energy sources that can cope with that demand. Nuclear fusion energy has been, for decades, one of the most promising candidates to become the energy of the future. However, due to its elevated technical and scientific complexity, there are still some challenges that need to be tackled down before it can be successfully implemented into the main grid. </p><p dir="ltr">Most magnetic confinement nuclear devices rely on superconducting magnets to confine the plasma. But even High-Temperature superconductors (HTS) only show superconducting properties when they operate in cryogenic environments (below 77 K). Superconducting materials are susceptible to quenching, a phenomenon by which a localized temperature raise leads to a localized increase in the resistance (it ceases to be superconductor), which in turn, increases the temperature even further by Joule heating. This feedback loop can quickly create a vast increase in the overall resistance of the material and even lead to a fatal breakdown. Such event would cause elevated costs and disruptions in the supply and must be prevented. </p><p dir="ltr">The first strategy to avoid superconducting quenching is to monitor the temperature closely so that, if a small temperature increase is detected, the current can be dumped safely before the thermal runaway occurs. But, in order to detect this, we need temperature sensors that can detect a small (1-5 K) temperature change very fast while the magnet is cryogenically cold. On top of that, the sensors must be able to withstand immense level of ionizing radiation and electromagnetic noise and provide a very fast response. Classical voltage-based sensors do not perform well under these harsh conditions but optical fiber sensors seem to be a promising candidate for the task.</p><p dir="ltr">Optical fiber sensors can be used as distributed sensors to monitor the temperature all along the superconducting tape. They are immune to electromagnetic noise and work well in cryogenic environments. They can also provide a time response sufficiently fast for such demanding applications. However, ionizing radiation can cause a darkening of the optical fibers that can vastly reduce their transmission, to the point where they can not be used anymore. This phenomenon is called radiation-induced attenuation (RIA) and is the main obstacle towards the implementation of optical fibers in radiation-rich environments.</p><p dir="ltr">Ionizing radiation creates and activates defects in the silica matrix of the optical fibers. These defects generate new optical absorption bands that cause an increase in the attenuation. Although RIA can grow to the order of thousands of dB/km, it can be mitigated by thermal or optical annealing. In both techniques, charged carriers trapped in the defects are supplied sufficient energy to jump to the conduction or valence bands and recombine naturally. In the case of thermal annealing, the energy is supplied by heating, whereas a source of light is used for the optical annealing (also known as photobleaching). The radiation tolerance of optical fibers can also be increased by tweaking the dopants in the fiber, as the composition of the optical fiber plays a crucial role in its response to radiation.</p><p dir="ltr">In this work we study the RIA and photobleaching phenomena focusing on the effect of ionizing radiation at cryogenic temperature, which was one of the most unexplored areas in the field. We investigated the RIA dynamics under different conditions, with special focus on the relation between photobleaching efficiency and power of the photobleaching light. We investigated the RIA under neutron and X-ray irradiation at cryogenic temperatures and demonstrated the effectiveness of photobleaching as a radiation-mitigation technique in a variety of application-relevant environments. Different types of fibers (with different compositions) were also studied, including some radiation-resistant optical fibers. Finally, the effect of different wavelengths of photobleaching light were also investigated and a theoretical model was presented and validated against the experimental data. The model was also used to estimate the effect of other agents (not explored experimentally) in the RIA and photobleaching phenomena. </p><p dir="ltr">The goal of this project was to study the radiation-induced attenuation of different optical fibers subjected to harsh conditions and fill a gap in the body of knowledge that could help assess the applicability of optical fiber sensors in environments involving high doses of ionizing radiation and low temperatures, as in space or nuclear fusion reactors. It was shown that, while low temperature can have a devastating impact on RIA, the photobleaching can be effectively used as a radiation-damage mitigation technique in all the cases we investigated. Combining photobleaching with optical fiber radiation-hardening techniques could quite possibly enable the use of optical fibers even in the extremely harsh conditions imposed by a nuclear fusion reactor. Optical fibers are, therefore, still a promising candidate for quench prevention applications in these type of environments.</p>