From MRI [1] to motors [2], from radiation intensive tokamaks [3] to the fluctuating temperature of space the list of high temperature superconductor (HTS) applications and environments expands every year. At the Paihau-Robinson Research Institute, we are simultaneously working on developing both the applications themselves and the necessary monitoring and diagnostic systems to ensure their continued success. Given the range of potential applications, magnetic, temperature, strain and radiation sensing may be necessary and developing robust sensing systems for each of these areas is essential. However, the nature of these sensing environments can cause substantial issues. Consider, for example, the use of HTS in compact tokamaks [4] or magnetoplasmadynamicthrusters [5] in both cases the sensors must operate in a background of high electromagnetic noise, with significant radiation fields from cryogenic to high temperature [6,7].
Optical fibre sensing (OFS) may offer the ideal solution for all environments, and, with continued development, all required sensing parameters. OFS is inherently resilient to electromagnetic fields making it ideally suited to the highly electromagnetic noisy environments around thrusters and tokamaks [8,9]. Furthermore, optical fibres can be installed during initial processing or post-hoc after system fabrication. Ensuring that integration can be relatively straightforward. In addition, their small size means that they can be unobtrusive and with further optimisation multiple parameters could be sensed within a single fibre using the same diagnostic infrastructure.
One highly developed area for OFS is condition monitoring of HTS magnets in tokamak fusion reactors. Paihau-Robinson has been heavily involved in the development of a unique OFS based around continuous Fibre Bragg gratings (cFBG). Unlike standard FBGs, cFBGs utilise identical FBGs throughout the sensing region. While precise spatial data is lost, we have shown that they remain highly sensitive to hotspot generation without significant degradation of the temporal resolution [10]. In this talk, I will discuss our recent advancements outlining the advantages and disadvantages of the cFBG technique and establishing its validity.
Irrespective of the exact OFS methodology, the development of quench detection algorithms will be essential for effective condition monitoring. Quench behaviour is highly material and application dependent [11]. It is therefore expected that the precise quench signature will be unique to each system and potentially determined by the exact hotspot location. This complexity is further accentuated by the large overlapping arrays of cFBGs creating a complicated and dynamic optical response which can produce unintuitive behaviour [12]. This response requires bespoke analysis programs. An attractive approach for these is machine learning to create new highly responsive algorithms [13]. Therefore, I will introduce our preliminary work utilising convolution neural networks in creating condition monitoring software.
Finally, the influence of radiation on the behaviour of optical fibres and FBGs will be examined [14]. It is well established that radiation causes significant attenuation of the sensing light (RIA) which can render the optical fibre unusable. The RIA remains poorly characterised at cryogenic temperatures particularly at dose rates equivalent to those expected in fusion tokamaks. Our recent work on the cryogenic properties of RIA will be presented and mitigation techniques discussed. The use of additional light sources to reduce the RIA (photobleaching) appears the most viable solution for fusion energy applications. I will discuss the wavelength and power dependence of photobleaching.
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14. Schuyt, J. J. et al. Gamma irradiation of Ge-doped and radiation-hard silica fibers at cryogenic temperatures: Mitigating the radiation-induced attenuation with 1550 and 970 nm photobleaching. J. Appl. Phys. 134, (2023).
This work was supported by the New Zealand Ministry of Business, Innovation, and Employment Catalyst Fund (No. RTVU1916). This research was also supported by Commonwealth Fusion Systems (CFS).