Andrea Polimadei

Low quench propagation velocity (QPV) in high-temperature superconductors (HTS) undermines conventional voltage-based quench detection, since damaging temperatures can develop locally before appreciable voltages appear. This work investigates an ENEA aluminum slotted-core cable featuring a BRAided STack (BRAST) of REBCO tapes, instrumented with capillary-mounted fiber Bragg gratings (FBGs) in direct thermal contact with the stack, alongside conventional voltage taps. Quench experiments were carried out in liquid nitrogen flow at 77 K, with the current held constant at about 500 A. Electrical and optical signals were processed with a standardized pipeline for the onset quench: statistical thresholds for electric field, and temperature-based arrivals at FBGs. From arrival times at known positions, QPV is estimated via two independent methods based on resistive front and thermal front, respectively. Both methods yield consistent values in the range of 20–60 mm/s. Downstream, where the front is fully developed, the two estimates converge within uncertainty. These results demonstrate that embedded optical fibers provide local temperature tracking, electromagnetically immune, and minimally invasive complement to voltage taps for quench detection and QPV estimation in HTS cables for fusion-magnet applications.

Fiber Bragg Grating (FBG) sensor systems are widely used to monitor mechanical parameters for structural health monitoring (SHM) of civil engineering structures. Among many applications, FBG technology allows the development of weighing pads with a fully passive optical working principle, with no need for electrical power at the measuring point. In this contribution, a technical solution for monitoring the navigability of the “Po” river, proposed in the framework of the European project CRISTAL (Climate resilient and environmentally sustainable transport infrastructure, with a focus on inland waterways) is investigated. The preliminary laboratory test to assess the feasibility of a procedure based on FBG technology to monitor sand accumulation on the riverbed is exposed, by using a weighing pad instrumented with FBGs. Indeed, its design and prototype realization are under development in the ENEA FOS (Fiber Optic Sensor) laboratory.

The high environmental electromagnetic noise characteristic of fusion reactors and low quench propagation velocity of HTS conductors designed for such applications drive the quest for quench detection solutions alternative to voltage methods. Techniques based on optical fiber sensors represent a promising solution, but their integration in cables is not straightforward due to their intrinsic fragility. In this work, we report on the production and testing of an HTS aluminum slotted core cable exploiting a BRAided STack (BRAST) of REBCO tapes. The braid also encloses two stainless steel capillaries containing fiber-optic sensors responsible for monitoring the stack temperature. The sample was subjected to quench experiments at liquid nitrogen temperature, evaluating its electrical and thermal response. The experimental results demonstrate that the integration of optical sensors within the BRAST enables monitoring of stack temperature and detection of quench events, highlighting the greater sensitivity achieved by detecting temperature changes directly on the superconducting stack compared with sensors placed on the outer jacket. This approach represents thus a simple and effective solution for the integration of optical sensors in fusion cables.

Distributed Fiber Optic Sensors (DFOS) are being considered as an alternative solution for Quench Detection in HTS fusion magnets. Their successful application is not straightforward due to the simultaneous need for high spatial and temporal resolution over long-length conductors operating in demanding cryogenic environments. In this work, we explore the potential of two different commercially available distributed sensing technologies, respectively based on the analysis of Rayleigh-backscattering through Optical Time Domain Reflectometry (OTDR) and Optical Frequency Domain Reflectometry (OFDR). The two techniques were used first to characterize the cryogenic temperature response of similar fibers between room temperature and 4 K. The temperature sensitivities were computed and compared to those of Fiber Bragg Grating sensors, showing comparable results. The performance of the investigated techniques in detecting local quenches was then experimentally assessed in operating conditions on a 2-meter-long, aluminum-stabilized HTS sample cooled by Liquid Nitrogen flow. While OFDR needs improvements in temporal resolution, the limited scale of the hot spot represents a challenge for the OTDR technique due to its lower spatial resolution.

In this article, we report the results from two test campaigns conducted to evaluate the feasibility of implementing a commercially available optical fiber-based distributed sensing technology for Quench Detection (QD) purposes. We successfully characterized the temperature response and sensitivity achievable by this technique when using a bare single mode fiber within a cryogen free cooling system operating in the temperature range from 4 K to 300 K. A fiber was also incorporated into a sample featuring an High-Temperature Superconductor (HTS) stack mounted onto an extruded Aluminum slotted-core cable. Quench-like events were triggered by locally lowering the critical current using the magnetic field produced by a set of permanent magnets placed near the center of the stack. The experiments were carried out at 77 K by cooling the sample through the central channel of the Aluminum core with a forced flow of liquid nitrogen. The test results indicate the viability of this technology as a suitable alternative for QD in long-length HTS cables and motivate further work to develop distributed sensing systems able to work in the conditions foreseen for HTS fusion coils.

Quench caused by electromechanical instabilities is a major concern in high-temperature superconducting (HTS) magnets, and quench detection methods, alternative to common voltage-based systems, are currently under study. In this work, we describe the experimental design and set-up, and we report the results of studies carried out on a cable equipped with bismuth strontium calcium copper oxide (BSCCO) tapes. The sample layout comprises an aluminum slotted core, cooled via a forced flow of liquid nitrogen in the central channel, and liquid nitrogen cooled termination. To trigger a quench in a well-defined position within the stack, a localized small magnetic field is employed to reduce the critical current density in the central zone of the cable. Various configuration of sensors including temperature sensors, voltage taps, and fiber Bragg grating (FBG) are installed, with the final aim of developing a fast and reliable quench detection set-up for the stacked-tapes HTS cable. It is shown that the response for both temperature and FBG signals are comparable for what concerns sensitivity and velocity, and both follow the voltage evolution in time. All in all, the developed setup with the multi-sensor allowed characterizing the electrical and thermal aspects of an HTS slotted-core superconducting cable during a localized magneto-thermal quench.

Embedding optical fibers with fiber Bragg grating (FBG) sensors in 3D-printed samples can effectively facilitate the systematic use of smart materials in many fields such as civil, bio-medical and soft robotics applications. The aim of the study is to analyze different combinations of filament materials and FBG coatings, and to assess their metrological characteristics. Eighteen samples are fabricated and tested under different mechanical and thermal conditions. The repeated tests allow to perform an evaluation of the measurement repeatability for each sample, along with an analysis of the samples sensitivity. The filaments employed are acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and thermoplastic polyurethane (TPU). The fiber coatings are acrylate, Ormocer® and polyimide. The fiber coating has no significative influence on the performance of the sensors. The tests for temperature sensitivity highlight a good performance of ABS (116 pm/°C) and TPU samples (32 pm/°C) up to 60 °C, whereas the fabricated PLA samples (139 pm/°C for polyimide, 55 pm/°C for acrylate, 14 pm/°C for Ormocer®) cannot be used above 40 °C. The tests for strain sensitivity in axial elongation show an average sensitivity of 3.049 nm/mm for ABS, 1.991 nm/mm for PLA, 3.726 nm/mm for TPU. The bending tests show that all specimens materials have different sensitivity to elongation (2.994 nm/mm for ABS, 0.668 nm/mm for PLA, 0.149 nm/mm for TPU). Only for acrylate in PLA samples an effective difference for bending sensitivity resulted (1.241 nm/mm for the acrylate coating vs 2.366 nm/mm for the other coatings).

In this work, we report novel relative humidity sensors realized by functionalising fibre Bragg gratings with chitosan, a moisture-sensitive biopolymer never used before for this kind of fibre optic sensor. The swelling capacity of chitosan is fundamental to the sensing mechanism. Different samples were fabricated, testing the influence of coating design and deposition procedure on sensor performance. The sensitivity of the sensors was measured in an airtight humidity-controlled chamber using saturated chemical salt solutions. The best result in terms of sensitivity was obtained for a sensor produced on filter paper substrate. Tests for each design were performed in the environment, lasted several days, and all designs were independently re-tested at different seasons of the year. The produced sensors closely followed the ambient humidity variation common to the 24-h circadian cycle.

Among several innovative procedures used in the fight against cancers, thermal treatments are gaining acceptance in clinical settings. In these approaches, heat is used to damage cancer cells. Therefore, it is useful to monitor the temperature reached in the tissue undergoing the treatment, in order to improve the outcome of the procedure. Despite the variety of available techniques investigated for temperature monitoring during thermal treatments, Linearly Chirped Fiber Bragg Grating (LCFBG) sensors can be seen as a good choice thanks to their small size, the flexibility of the fiber optic, as well as the immunity to electromagnetic interferences and the Magnetic Resonance- (MR-) compatibility. In this work a commercial LCFBG has been tested under different thermal gradients that typically occur during the thermal treatments (up to 30 °C). The sensor's response has been analysed under linear and non-linear thermal gradients. The shift and the width of the LCFBG's spectrum have been related to both the average temperature (Tmean) applied on its active area, and to the temperature difference (ΔT) experienced by the sensor between its two extremities. The analysis has showed promising results: actually with the sensor under test is possible to find a linear relationship between the spectrum shift and Tmean (with a sensitivity of about 10 pm°·C-1), and also between the spectrum width and the ΔT (5 pm°·C-1). These findings may allow extrapolating temperature information from the LCFBG spectrum, in order to monitor in real-time the effects of thermal treatment on biological tissue. Because the effects of thermal ablation depend on temperature, the knowledge of temperature may improve the treatments outcome, thus patient safety.