The pursuit of fusion as a protected, carbon-free, always-on vitality supply has intensified lately, with plenty of organizations pursuing aggressive timelines for expertise demonstrations and energy plant designs. New-generation superconducting magnets are a crucial enabler for a lot of of those applications, which creates rising want for sensors, controls, and different infrastructure that may enable the magnets to function reliably within the harsh situations of a industrial fusion energy plant.
A collaborative group led by Division of Nuclear Science and Engineering (NSE) doctoral pupil Erica Salazar not too long ago took a step ahead on this space with a promising new methodology for fast detection of a disruptive abnormality, quench, in highly effective high-temperature superconducting (HTS) magnets. Salazar labored with NSE Assistant Professor Zach Hartwig of the MIT Plasma Science and Fusion Middle (PSFC) and Michael Segal of spinout Commonwealth Fusion Methods (CFS), together with members of the Swiss CERN analysis middle and the Robinson Analysis Institute (RRI) at Victoria College in New Zealand to attain the outcomes, which had been printed within the journal Superconductor Science and Expertise.
Quench happens when a part of a magnet’s coil shifts out of a superconducting state, the place it has no electrical resistance, and into a traditional resistive state. This causes the huge present flowing by way of the coil, and saved vitality within the magnet, to shortly convert into warmth, and doubtlessly trigger severe inside harm to the coil.
Whereas quench is an issue for all techniques utilizing superconducting magnets, Salazar’s workforce is targeted on stopping it in energy crops primarily based on magnetic-confinement fusion units. A majority of these fusion units, often called tokamaks, will preserve a plasma at extraordinarily excessive temperature, just like the core of a star, the place fusion can happen and generate net-positive vitality output. No bodily materials can deal with these temperatures, so magnetic fields are used to restrict, management, and insulate the plasma. The brand new HTS magnets enable the tokamak’s toroidal (doughnut-shaped) magnetic enclosure to be each stronger and extra compact, however interruptions within the magnetic field from quench would halt the fusion course of—therefore the significance of improved sensor and management capabilities.
With this in thoughts, Salazar’s group sought a approach of shortly recognizing temperature modifications within the superconductors, which may point out nascent quench incidents. Their check mattress was a novel superconducting cable developed within the SPARC program often called VIPER, which contains assemblies of skinny metal tape coated with HTS materials, stabilized by a copper former and jacketed in copper and stainless-steel, with a central channel for cryogenic cooling. Coils of VIPER can generate magnetic fields two-to-three instances stronger than the older-generation low-temperature superconducting (LTS) cable; this interprets into vastly increased fusion output energy, but in addition makes the vitality density of the sector increased, which locations extra onus on quench detection to guard the coil.
A deal with fusion’s viability
Salazar’s workforce, like the whole SPARC analysis and improvement effort, approached its work with a deal with eventual commercialization, usability, and ease of manufacture, with a watch towards accelerating fusion’s viability as an vitality supply. Her background as a mechanical engineer with Basic Atomics throughout manufacturing and testing of LTS magnets for the worldwide ITER fusion facility in France gave her perspective on sensing applied sciences and the crucial design-to-production transition.
“Transferring from manufacturing into design helped me take into consideration whether or not what we’re doing is a sensible implementation,” explains Salazar. Furthermore, her expertise with voltage monitoring, the standard quench-detection method for superconducting cable, led her to assume a special method was wanted. “Throughout fault testing of the ITER magnets, we noticed electrical breakdown of the insulation occurring on the voltage faucet wires. As a result of I now think about something that breaks high-voltage insulation to be a significant danger level, my perspective on a quench detection system was, what can we do to reduce these dangers, and the way can we make it as strong as attainable?”
A promising various was temperature measurement utilizing optical fibers inscribed with micro-patterns often called fiber Bragg gratings (FBGs). When broadband mild is directed at an FBG, a lot of the mild passes by way of, however one wavelength (decided by the spacing, or interval, of the grating’s sample) is mirrored. The mirrored wavelength varies barely with each temperature and pressure, so placement of a collection of gratings with totally different durations alongside the fiber permits unbiased temperature monitoring of every location.
Whereas FBGs have been leveraged throughout many various industries for measurement of pressure and temperature, together with on a lot smaller superconducting cables, they’d not been used on bigger cables with excessive present densities like VIPER. “We needed to take good work by others and put it to the check on our cable design,” says Salazar. VIPER cable was well-adapted for this method, she notes, due to its steady construction, which is designed to resist the extreme electrical, mechanical, and electromagnetic stresses of a fusion magnet’s surroundings.
A brand new extension on FBGs
A novel choice was offered by the RRI workforce within the type of ultra-long fiber Bragg gratings (ULFBGs)—a collection of 9-milimeter FBGs spaced 1 mm aside. These primarily behave as one lengthy quasi-continuous FBG, however with the benefit that the mixed grating size may be meters lengthy as an alternative of millimeters. Whereas standard FBGs can monitor temperature modifications at localized factors, ULFBGs can monitor concurrently occurring temperature modifications alongside their whole size, permitting them to offer very fast detection of temperature variation, no matter the placement of the warmth supply.
Though because of this the exact location of sizzling spots is obscured, it really works very effectively in techniques the place early identification of an issue is of utmost significance, as in an working fusion system. And a mixture of ULFBGs and FBGs may present each spatial and temporal decision.
A possibility for hands-on verification got here by way of a CERN workforce working with customary FBGs on accelerator magnets on the CERN facility in Geneva, Switzerland. “They thought FBG expertise, together with the ULFBG idea, would work effectively on any such cable and needed to look into it, and bought on board with the challenge,” says Salazar.
In 2019, she and colleagues journeyed to the SULTAN facility in Villigen, Switzerland, a number one middle for superconducting cable analysis operated by the Swiss Plasma Middle (SPC), which is affiliated with Ecole Polytechnique Fédérale de Lausanne, to guage samples of VIPER cable with optical fibers set into grooves on their outer copper jackets. Their efficiency was in comparison with conventional voltage faucets and resistance temperature sensors.
Fast detection beneath lifelike situations
The researchers had been capable of shortly and reliably detect small temperature disturbances beneath lifelike working situations, with the fibers choosing up early-stage quench progress earlier than thermal runaway extra successfully than the voltage faucets. When in comparison with the difficult electromagnetic surroundings seen in a fusion system, the fibers’ signal-to-noise ratio was a number of instances higher; as well as, their sensitivity elevated as quench areas expanded, and the fibers’ response instances could possibly be tuned. This enabled them to detect quench occasions tens of seconds quicker than voltage faucets, particularly throughout slowly propagating quenches—a attribute distinctive to HTS which is exceptionally tough for voltage faucets to detect within the tokamak surroundings, and which may result in localized harm.
“sing fiber optic applied sciences for HTS magnets quench detection or as a twin verification methodology with voltage present nice promise,” says the group’s write-up, which additionally cites the manufacturability and minimal technological danger of the method.
“The event of delicate temperature measurements with FBGs is a really promising method to the difficult drawback of defending HTS coils from harm throughout quenches,” observes Kathleen Amm, director of the Brookhaven Nationwide Laboratory Magnet Division, who was not affiliated with the analysis effort. “That is crucial to the event of game-changing applied sciences like compact fusion, the place sensible, high-field, high-temperature superconducting magnets are a key expertise. It additionally has the potential to resolve the issue of quench safety for a lot of industrial HTS purposes.”
Work is underway on refining the placement and set up of the fibers, together with the kind of adhesive used, and likewise on investigating how the fibers may be put in in different cables and on totally different platforms, says Salazar.
“We’re having numerous dialog with CFS and persevering with to coordinate with the RRI workforce’s ULFBG expertise, and I’m presently making a 3-D mannequin of quench dynamics, so we will higher perceive and predict what quench would appear like beneath totally different situations,” states Salazar. “Then we will develop design suggestions for the detection system, like the sort and spacing of the gratings, so it may well detect within the desired size of time. That can enable the controls engineers and the engineers engaged on quench detection algorithms to write down and optimize their code.”
Salazar praised the experimental workforce’s excellent collegiality, noting, “the collaboration with RRI and CERN was particular. All of us converged in Switzerland, labored exhausting collectively, and had enjoyable placing our efforts in and getting nice outcomes.”
Erica Elizabeth Salazar et al. Fiber optic quench detection for large-scale HTS magnets demonstrated on VIPER cable throughout high-fidelity testing on the SULTAN facility, Superconductor Science and Expertise (2021). DOI: 10.1088/1361-6668/abdba8
Massachusetts Institute of Technology
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New fiber optic temperature sensing method to maintain fusion energy crops working (2021, February 5)
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