MIT’s Erica Salazar reveals that sooner detection of thermal shifts can stop disruptive quench occasions within the HTS magnets utilized in tokamak fusion units.
The pursuit of fusion as a protected, carbon-free, always-on vitality supply has intensified lately, with a lot of organizations pursuing aggressive timelines for expertise demonstrations and energy plant designs. New-generation superconducting magnets are a vital enabler for a lot of of those applications, which creates rising want for sensors, controls, and different infrastructure that can enable the magnets to function reliably within the harsh circumstances of a industrial fusion energy plant.
A collaborative group led by Division of Nuclear Science and Engineering (NSE) doctoral scholar Erica Salazar not too long ago took a step ahead on this space with a promising new technique 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 heart 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 Know-how.
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 standard resistive state. This causes the huge present flowing by the coil, and saved vitality within the magnet, to rapidly convert into warmth, and probably trigger severe inside injury to the coil.
Whereas quench is an issue for all methods utilizing superconducting magnets, Salazar’s staff is targeted on stopping it in energy vegetation primarily based on magnetic-confinement fusion units. These kinds of fusion units, referred to as tokamaks, will keep 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 area 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 means of rapidly 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 referred to as VIPER, which includes assemblies of skinny metal tape coated with HTS materials, stabilized by a copper former and jacketed in copper and chrome steel, with a central channel for cryogenic cooling. Coils of VIPER can generate magnetic fields two-to-three occasions stronger than the older-generation low-temperature superconducting (LTS) cable; this interprets into vastly greater fusion output energy, but additionally makes the vitality density of the sphere greater, which locations extra onus on quench detection to guard the coil.
A concentrate on fusion’s viability
Salazar’s staff, like your complete SPARC analysis and growth effort, approached its work with a concentrate on 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 vital 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 normal quench-detection strategy for superconducting cable, led her to suppose a unique strategy 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 contemplate something that breaks high-voltage insulation to be a significant threat level, my perspective on a quench detection system was, what will we do to reduce these dangers, and the way can we make it as sturdy as doable?”
A promising various was temperature measurement utilizing optical fibers inscribed with micro-patterns referred to as fiber Bragg gratings (FBGs). When broadband gentle is directed at an FBG, many of the gentle passes by, 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 completely different intervals alongside the fiber permits unbiased temperature monitoring of every location.
Whereas FBGs have been leveraged throughout many alternative industries for measurement of pressure and temperature, together with on a lot smaller superconducting cables, that they had 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 strategy, she notes, due to its steady construction, which is designed to face up to the extreme electrical, mechanical, and electromagnetic stresses of a fusion magnet’s setting.
A brand new extension on FBGs
A novel possibility was supplied by the RRI staff 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 might 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 total size, permitting them to supply very speedy detection of temperature variation, regardless of the placement of the warmth supply.
Though which means that the exact location of sizzling spots is obscured, it really works very nicely in methods the place early identification of an issue is of utmost significance, as in an working fusion machine. And a mix of ULFBGs and FBGs may present each spatial and temporal decision.
A chance for hands-on verification got here through a CERN staff 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 nicely on the sort of cable and needed to look into it, and bought on board with the undertaking,” says Salazar.
In 2019, she and colleagues journeyed to the SULTAN facility in Villigen, Switzerland, a number one heart for superconducting cable analysis operated by the Swiss Plasma Middle (SPC), which is affiliated with Ecole Polytechnique Fédérale de Lausanne, to judge 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 underneath sensible circumstances
The researchers had been capable of rapidly and reliably detect small temperature disturbances underneath sensible working circumstances, 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 setting seen in a fusion machine, the fibers’ signal-to-noise ratio was a number of occasions higher; as well as, their sensitivity elevated as quench areas expanded, and the fibers’ response occasions could possibly be tuned. This enabled them to detect quench occasions tens of seconds sooner 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 setting, and which may result in localized injury.
“[U]sing fiber optic applied sciences for HTS magnets quench detection or as a twin verification technique with voltage present nice promise,” says the group’s write-up, which additionally cites the manufacturability and minimal technological threat of the strategy.
“The event of delicate temperature measurements with FBGs is a really promising strategy to the difficult downside of defending HTS coils from injury throughout quenches,” observes Kathleen Amm, director of the Brookhaven Nationwide Laboratory Magnet Division, who was not affiliated with the analysis effort. “That is vital 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 functions.”
Work is underway on refining the placement and set up of the fibers, together with the kind of adhesive used, and in addition on investigating how the fibers might be put in in different cables and on completely different platforms, says Salazar.
“We’re having numerous dialogue with CFS and persevering with to coordinate with the RRI staff’s ULFBG expertise, and I’m at the moment making a 3D mannequin of quench dynamics, so we are able to higher perceive and predict what quench would appear to be underneath completely different circumstances,” states Salazar. “Then we are able to develop design suggestions for the detection system, like the kind and spacing of the gratings, so it might 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 staff’s excellent collegiality, noting, “the collaboration with RRI and CERN was particular. All of us converged in Switzerland, labored onerous collectively, and had enjoyable placing our efforts in and getting nice outcomes.”
Reference: “Fiber optic quench detection for large-scale HTS magnets demonstrated on VIPER cable throughout high-fidelity testing on the SULTAN facility” by Erica E Salazar, Rodney A Badcock, Marta Bajko, Bernardo Castaldo, Mike Davies, Jose Estrada, Vincent Fry, Jofferson T Gonzales, Philip C Michael, Michael Segal, Rui F Vieira and Zachary S Hartwig, 4 February 2021, Superconductor Science and Know-how.
Funding for this analysis was supplied by CFS.