MIT’s Erica Salazar exhibits that quicker detection of thermal shifts can forestall disruptive quench occasions within the HTS magnets utilized in tokamak fusion gadgets.
The pursuit of fusion as a protected, carbon-free, always-on power supply has intensified in recent times, with quite a few organizations pursuing aggressive timelines for know-how 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 business fusion energy plant.
A collaborative group led by Division of Nuclear Science and Engineering (NSE) doctoral scholar Erica Salazar lately 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 Techniques (CFS), together with members of the Swiss CERN analysis heart and the Robinson Analysis Institute (RRI) at Victoria College in New Zealand to realize 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 standard resistive state. This causes the huge present flowing by way of the coil, and saved power within the magnet, to rapidly convert into warmth, and doubtlessly trigger severe inner injury to the coil.
Whereas quench is an issue for all techniques utilizing superconducting magnets, Salazar’s staff is concentrated on stopping it in energy vegetation based mostly on magnetic-confinement fusion gadgets. These kind of fusion gadgets, often known as tokamaks, will preserve a plasma at extraordinarily excessive temperature, much like the core of a star, the place fusion can happen and generate net-positive power 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 discipline 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 method of rapidly recognizing temperature modifications within the superconductors, which may point out nascent quench incidents. Their take a look at mattress was a novel superconducting cable developed within the SPARC program often known as VIPER, which contains 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 instances stronger than the older-generation low-temperature superconducting (LTS) cable; this interprets into vastly increased fusion output energy, but additionally makes the power density of the sphere increased, 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 improvement effort, approached its work with a concentrate on eventual commercialization, usability, and ease of manufacture, with an eye fixed towards accelerating fusion’s viability as an power 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 normal quench-detection method for superconducting cable, led her to suppose a distinct 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 take into account something that breaks high-voltage insulation to be a serious 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 different was temperature measurement utilizing optical fibers inscribed with micro-patterns often known as fiber Bragg gratings (FBGs). When broadband mild is directed at an FBG, many 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 sequence of gratings with completely different durations alongside the fiber permits impartial 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 take a look at on our cable design,” says Salazar. VIPER cable was well-adapted for this method, she notes, due to its secure construction, which is designed to face up to the extraordinary electrical, mechanical, and electromagnetic stresses of a fusion magnet’s atmosphere.
A brand new extension on FBGs
A novel choice was offered by the RRI staff within the type of ultra-long fiber Bragg gratings (ULFBGs) — a sequence 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 will be meters lengthy as a substitute of millimeters. Whereas typical FBGs can monitor temperature modifications at localized factors, ULFBGs can monitor concurrently occurring temperature modifications alongside their total 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 scorching 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 chance for hands-on verification got here through a CERN staff working with normal FBGs on accelerator magnets on the CERN facility in Geneva, Switzerland. “They thought FBG know-how, together with the ULFBG idea, would work effectively on any such cable and needed to look into it, and obtained 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 situations
The researchers had been capable of rapidly and reliably detect small temperature disturbances underneath sensible 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 atmosphere 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 troublesome for voltage faucets to detect within the tokamak atmosphere, and which may result in localized injury.
“[U]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 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 crucial to the event of game-changing applied sciences like compact fusion, the place sensible, high-field, high-temperature superconducting magnets are a key know-how. It additionally has the potential to unravel 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 in addition on investigating how the fibers will be put in in different cables and on completely different platforms, says Salazar.
“We’re having a variety of dialogue with CFS and persevering with to coordinate with the RRI staff’s ULFBG know-how, and I’m at the moment making a 3D mannequin of quench dynamics, so we will higher perceive and predict what quench would appear to be underneath completely different situations,” states Salazar. “Then we will develop design suggestions for the detection system, like the sort and spacing of the gratings, so it could possibly detect within the desired size of time. That may enable the controls engineers and the engineers engaged on quench detection algorithms to jot 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 Expertise.
Funding for this analysis was offered by CFS.