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The high-fieldpath to fusion energy

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Plasma Science and Fusion Center Massachusetts Institute of Technology sparc@psfc.mit.edu

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MIT is partnering with private industry to build the world’s highest-performance, magnets for fusion energy. Once this work is successful, these magnets will be used to build SPARC, the first net-energy controlled fusion experiment.

The SPARC experiment would be the first fusion plasma to make net energy. Visualization by Ken Filar

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The Valley of Death

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Bridging The Valley of Death

Technology Readiness Level

The big picture: what are we planning to do? Through the same physical process that powers the sun, fusion offers the possibility of unlim-ited, carbon-free energy. At the MIT Plasma Science & Fusion Center (PSFC), we have started down a path that we believe can dramatically speed up the development of fusion energy and lead to early commercialization – to put power on the grid soon enough to impact global warm-ing. This work is built on solid foundations laid down by decades of publicly funded research, which established the scientific basis and trained the workforce required to take this bold new step. The project is enabled by the arrival of a breakthrough technology — high-temperature superconductors, which opens the “smaller, faster, cheaper” path we are pursuing. Our plan is to carry out R&D leading to a demonstration of a high-performance, High-Temperature Supercon-ducting (HTS) magnet at the scale required for fusion, followed by construction and operation of SPARC, which would be the world’s first net-energy fusion experiment.

How will this work be funded and carried out?The research will be funded by Commonwealth Fusion Systems (CFS), a private startup spun out from MIT’s Plasma Science & Fusion Center. The CFS resources come from a consortium of investors who understand the importance of fusion energy for addressing climate change and value its commercial potential. Over the long term, CFS seeks to commercialize fusion and lead the world’s fusion energy industry. MIT & CFS will collaborate on the enabling HTS Magnet R&D and on the SPARC experiment.

The collaboration is structured to bolster fusion research and education at MIT while building a strong industrial partner with the technical capabilities, workforce and strategic relationships required to fulfil its aspirations. The collaboration will function like other industrial sponsored re-search at MIT – scientists and engineers propose research projects and CFS funds them. In many cases, MIT and CFS personnel will work side-by-side on this research, each bringing their own unique skills and resources to bear. As intellectual property is generated during the research, CFS will have the option to license and commercialize it. As in any for-profit company, inves-tors in CFS will share in the value created from commercializing fusion energy production and in spinout technologies. MIT is augmenting, not moving away from, its fusion funding model in order to support two complementary activities: research and commercialization. It will continue to carry out fusion science research with government funding while simultaneously partnering with the private sector to accelerate fusion energy commercialization. The spin-out of govern-ment funded basic research into the commercial sector is entirely consistent with the essential roles and missions of each.

How the unique collaboration between MIT and CFS can bridge the technological valley of death for fusion energy

Innovative technologies, like fusion, can only have an impact on society if they emerge from the lab and enter the marketplace. But “hard-tech” like the development of a new energy source, faces a steep challenge in moving from basic research into commercialization. Typically governments and universities fund basic research, but stop well short of the level of development required for commercialization. Industry and inves-tors are reluctant to pick up that technology when the pay-off is longer than a few years and when substan-tial technical and economic factors are unresolved. This is the so-called “valley of death”. Having ideas that are technically sound is not enough — there must be a viable pathway to widespread, profitable deployment in order to succeed. Technologies, like fusion energy, that require substantial investments, take many years to reach fruition and that need to compete in a well-established markets face a particularly deep and broad valley. Moreover, U.S. government investments in fusion have been incommensurate with its potential to meet the existential threat posed by climate change, even in its funding of basic research on fusion science and technology. (Currently it does not directly support energy development and commercialization at all.) On a path determined by current levels of federal research funding, fusion energy would be unlikely to come online much before the end of this century. Our answer to this dilemma is for academia to hold on to the technology development longer and for industry to pick it up earlier — reaching across the valley of death. This new model seeks a much more rapid development path, one that could develop fusion power in time to address the growing threat that humanity faces from climate change.

Bridging The Valley Of Death. For “hard” tech, a gap can open up between the level of readi-ness established by academic or government research and the level required to attract private investment. The MIT-CFS model fills that gap by having MIT participate to higher levels of readiness while CFS picks up the research earlier. (Graphic by Bob Mumgaard)

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Fusion is the process by which light elements combine to form heavier elements releasing enormous amounts of energy. It is the ultimate source of energy in the universe, powering the sun and the stars and creating all the elements of the periodic table. Fusion on earth would be an entirely new source of energy for humankind, one with the poten-tial to solve a critical challenge facing our civilization — decades of intense research have been motivated by fusion’s advantages. The fuel for fusion, deuterium, a naturally occurring form of hydrogen, and lithium, is sufficient on Earth to meet humankind’s energy needs for millions of years. The energy density contained in fusion fuels is so large that a mere 0.1 g of deuterium, what is found in 3 gallons of ordinary water, would provide the domestic and industrial elec-tricity demands for a typical American for a year. Fusion does not create greenhouse gases like carbon dioxide, or pollutants like sulfur dioxide or nitrogen dioxide, nor particulates like soot. Compared to other clean energy sources like wind, solar or biomass, it would have a very small footprint and would not compete for scarce land or water resources needed for agriculture and human habitation. A fusion power plant would be about the same size as a conventional fossil fuel plant and run around-the-clock with no requirements for massive energy storage systems. The fusion reaction would simply be a new source of heat used to create steam to drive a tur-bine and generator - exactly the way that most electricity is produced today.

Decades of research, worldwide, has taught us a tremendous amount about how fusion systems work. In terms of performance, there was a period of great progress from the late 1960s through to the late 1990s. Over those 30 years, performance (as defined by a well-known and accepted metric) increased by more than a factor of more than 10,000. Unfortunately, progress has slowed in recent years because the size and cost of experiments grew and investments didn’t keep pace. The MIT initiative is aimed at changing that narrative by enabling much smaller devices and opening up a faster, cheaper pathway.

A fusion power plant would consume small amounts of deuterium and lithium while producing helium and an enormous quantity of energy. The reaction on the top takes place in a hot plasma, the reaction on the bottom occurs in the blanket, which surrounds the plasma. The other products, tritium and neutrons are continuously recycled. (Graphic by Martin Greenwald)

Why is fusion critical to our energy future?

A fusion power plant could provide clean, carbon-free energy with an essentially unlimited fuel supply. From the point of view of electrical power generation, the fusion device is just another heat source — to be used in a conventional thermal conversion cycle. (Adapted from Wikimedia Commons)

Fusion and the physics of plasmas

Fusion requires heating matter to extreme temperatures. The core of the sun is 15 million degrees and even at those temperatures the fuel fuses extremely slowly — the sun will burn for billions of years. For practical fusion on earth, we need the reactions to occur much more rapidly, so temperatures above 200 million degrees are required. At those temperatures, atoms are ripped apart, with the negatively charged electrons stripped away from the positively charged nuclei. The result is an electrically conductive fluid called a plasma — the fourth state of matter. We can only heat the fuel to these sorts of temperatures if the plasma is very well insulated from ordinary matter. But because of the temperatures involved, no ordinary insulator can be used — it would instantly cool the plasma. However, the plasma is electrically conductive and can be controlled and contained by strong magnetic fields.

Plasma is the fourth state of matter. As heat is added, solids melt to become liquid, the liquid evaporates to become a gas and then, when the matter is hot enough, the electrons are stripped from the atoms, ionizing the gas and becoming a plasma.

ADD HEAT

solid liquid gas plasma

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Why are magnets a key technology in this effort? To make fusion happen, matter has to be heated to extremely high temperatures, to roughly 200 million degrees. Heating and con-taining matter at these sorts of temperatures is the fundamental challenge for creating the new energy source. Fortunately, mag-netic fields can provide the thermal insulation needed to confine hot plasmas and isolate them from ordinary materials. The basic principle is simple — charged particles in a magnetic field do not move in straight lines, but revolve or “gyrate” in circular motion. This is only true for motion across a magnetic field — along the direction of the field lines, the particles move freely, as in the figure, and can escape out the ends of any device with that geometry. So — to fully confine the plasma, the field is arranged to eliminate the losses out the ends by eliminating the ends. This leads to configurations with the characteristic toroidal or donut shape.

A panoramic view of the interior of Alcator C-Mod, a compact, high-field tokamak operated at MIT from 1993–2016. This experiment was the third in a series of machines that demonstrated the advantages of high-field operation for fusion energy. The planned SPARC experiment would be the fourth. (Photo by Bob Mumgaard)

Magnetic fields can confine the charged particles that make up a plasma by imparting a force perpendicular to their motion. Note, the plasma is unconfined in the direction parallel to the field.

To eliminate the end losses in magnetic confinement, we eliminate the ends — wrapping the field lines into a toroidal configu-ration . (Graphic by Don Spong)

Why is the strength of the field so important? The mechanism that allows magnetic fields to contain hot fusion plasmas gets more and more effective as the field gets stronger. In effect, the quality of the thermal insulation increases with the strength of the magnetic field, meaning that you need less space for it to maintain fusion temperatures. Doubling the magnetic field would roughly allow you to make any given fusion device smaller in its linear dimension by a factor of two and thus reduce its volume by a factor of eight — almost an order of magnitude gain. So strong fields make fusion smaller, faster and cheaper.

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The breakthrough: HTS magnets make fusion smaller, faster and cheaper

The advantages of operating fusion experiments at high field have been understood for a long time. They were the basis for a series of high-field, compact experiments at the MIT-PSFC — which though small in size, set a number of plasma perfor-mance records. In the 1980s and 90s there were serious proposals by the U.S. program to build larger experiments along these lines, which aimed at demonstrating net fusion energy. However, at that time, the only technology that could produce the required strength of magnetic fields was cryogeni-cally cooled copper. And while this type of magnet would have fulfilled the mission, the technology does not extrapolate to a commercial energy source because far too much power is consumed by resistive losses in the magnet. A practical fusion power system will need to use superconducting magnets, which have zero resistance to the flow of electricity and thus lose no power to heat. But at the time, available superconductors couldn’t operate in the magnetic fields required, so these proposed machines were never built.

High temperature superconductor (HTS), available from manufacturers in the form of thin ribbon-shaped tapes has the power to revolutionize the quest for fusion energy. (Photo by Paul Rivenberg)

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The breakthrough came in a new class of superconducting compounds. These were first discovered in 1986 and were notable for their ability to retain superconducting properties at much higher tempera-tures than conventional materials — up to 90 K — still very cold but a big improvement over the older compounds, which required cooling to about 4 K. (For comparison room temperature is about 295 K.) Reflecting this property, the new compounds were called high-temperature superconductors (HTS) and contrasted with the older low-temperature superconductors (LTS). More importantly for our story, the new HTS materials retain their superconducting properties even when embedded in very strong magnetic fields. Their potential for fusion was recognized immediately — but the new superconductors were created as fragile crystals and not yet useful for building magnets. Over the intervening decades, researchers have found ways to package the superconductor by depositing it as a thin film on a strong, steel substrate. The resulting conductors — in the form of “tapes” or “ribbons” — have been used to build magnets with unprecedented performance. Conductors in this form are being produced by many vendors and have demonstrated the capabilities needed to build the class of magnets required for fusion. For us, this changes everything. We can now produce compact machines running at very high magnetic field, greatly reducing the cost of development and deployment for fusion power.

What needs to be done next? Before we can build SPARC, we need to demonstrate that the new superconductor can be incorporated into the kind of high-performance magnet needed. The high-field HTS magnets that have already been developed, demonstrate the basic capabilities of the technology, but are not suitable for fusion. The volume in which the magnet field is pro-duced is much too small — typically about 0.01 m3 com-pared to 15 m3 for the proposed SPARC device. This vastly increases the energy stored in the magnet and the mechan-ical stresses that it must withstand, changing the basic design and construction strategies which must be em-ployed. So the first phase of the SPARC project will be to undertake the R&D necessary to bridge this gap. The goal is to demonstrate an HTS magnet at the field required for SPARC and at a size and energy that will retire the risk for the SPARC magnet. And because our long-term goal is commercially practical energy, we wish to develop a fabrication scheme that scales to SPARC and that promises a pathway to a full-scale fusion power plant. Success offers the possibility of spin-offs into other applications for high-field magnets. This might provide additional revenue to fund the project, though for now, the priority is on fusion development.

The process of developing superconducting fusion magnets proceeds step by step from conductor to finished magnet set. (Graphic by Dan Brunner)

High Temperature Superconductors — the backstory

High-Temperature Superconductors (HTS) were first discovered in 1986 by IBM researchers J. Georg Bednorz and K. Alex Muller, who went on to win the 1987 Nobel Prize in physics for their work (in the shortest interval between a discovery and a Nobel award). The materials were compounded of rare earths, like lantha-num and yttrium, and barium copper oxides in a concerted effort to extend the temperature range for superconductors. The acronym ReBCO is often used to refer generically to these materials. The yttrium version, YBCO, retains its superconductivity above 90 K. While this development was extremely exciting for physics, the physical form of the new com-pound, small black crystals, was not obviously suitable for practical applications. It has taken decades to turn the discovery into engineering materials and commercial products. HTS is now available in thin steel ribbons, with an ultra-thin layer of superconducting material deposited on it. The deposition process maintains the required crystalline form of the conductor, which is then coupled to the mechanical strength of the steel substrate. Small-bore HTS magnets have been built with fields up to 42 Tesla, considerably higher than required for SPARC, proving that the super-conductor can operate at the fields needed for fusion. Only in the last three to five years has the performance of the superconductor, and the ability to manufacture it in sufficient quantities, been adequate for fusion applica-tions. Still, significant R&D will be needed to enable the fabrication of large-volume, high-field magnets that will be useful for fusion. Although “rare-earth” is the name given by chemists to the particular group of elements used in ReBCO superconductors, they are actually not so rare that scarcity of their supply would pose a problem for a fusion energy economy. You could fabricate 1,000 fusion power plants a year with YBCO super-conductors before you’d have even a 1% impact on the current rare-earth market.

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the Kitty Hawk for fusion

SPARC:

Rendering by Ken Filar

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The SPARC mission has three elements

• Heat and confine a plasma with greater than break-even fusion energy production

• Investigate the physics of burning plasmas in the high-field, high-density regime

• Demonstrate the integration of fusion- relevant HTS magnets at scale

To reach break-even, the plasma created in any fusion device must exceed a set of conditions for the plasma temperature, density and confinement time first calculated in 1955 by John Lawson, an early fusion researcher. Under these conditions, the ratio, Q, of fusion power to input power exceeds 1. Achieving Q > 1 has been a yardstick and a key milestone of fusion experiments since that time. The closest we’ve come so far has been from a pair of billion dollar class experiments, the TFTR tokamak, which reached Q = 0.3 in 1994 and in JET, with Q = 0.7 achieved in 1997. The SPARC goal is to exceed Q = 2, in a significantly smaller device. With a plasma volume of 15 m3, SPARC would be a mid-sized fusion experiment — of a size and configuration similar to many machines already in operation. With the new HTS magnet technology, it will have an average field in the plasma of 12 Tesla. SPARC is about the minimum size experiment that could make a net-energy plasma using this magnet technology. While the SPARC magnets will be super- conducting and run in steady state, the plasma will be pulsed — lasting about 10 seconds — to simplify many aspects of the device and facility engineering. This pulse length is long enough for all plasma-related processes to reach steady state, a condition that would have been difficult to achieve in a design with non-superconducting magnets. The employment of superconducting magnets, even in a pulsed machine, confers other benefits — enabling a faster repetition rate and requiring much smaller power systems — but the principal case for this choice is that it serves as a needed test bed for HTS magnet systems at full scale and fully integrated into a fusion system.

Based on data from decades of research on dozens of experiments around the world — includ-ing MIT’s Alcator C-Mod, we can be reasonably confident that a machine with these specifica-tions would reach the net energy milestone. Data from these experiments were exhaustively compiled and analyzed during the design of the ITER tokamak, a very large tokamak under construction in France by a consortium that includes most of the industrialized nations on earth (China, the EU, India, Japan, Korea, Russian and the United States.) Our performance projections use the same physics basis as ITER and are considerably less of an extrapolation from existing data in several important physics parameters. In fact, we can find discharges on existing ma-chines whose plasmas match all of the dimensionless physics variables believed to be important for determining plasma stability and confinement. This amounts, essentially, to a wind tunnel test for SPARC. (Note that while these older devices could match the SPARC dimensionless plasma parameters, they could not reach the same levels of absolute fusion performance, which involves nuclear physics as well as plasma physics. SPARC is predicted to produce about 50 times more fusion energy per pulse than any previous fusion experiment.) Looking at different operating scenarios, we find that SPARC will have robust access to the net energy regime.

It is worth considering how the plans for SPARC relate to the rest of the world’s ongoing fusion research programs, especially the ITER project. With an eventual cost estimated to be in the range of $30-60 B, ITER will be among the largest scientific facilities ever built. SPARC should be viewed as an alternate path to the same goal — practical fusion energy. The two devices differ in the magnet technology employed, size (SPARC is about 65 time smaller), mission and funding model. Pursuing different approaches is consistent with the technical challenge and the societal benefits that success would bring - fusion is simply too important for just one shot on goal. The SPARC project is built on the solid foundations of research carried out worldwide over of previous decades, including the planning and design for ITER. SPARC can return the favor by providing students, postdocs, and scientists from other institutions an opportunity for collabora-tion on a unique world-class facility.

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Beyond SPARC — how do we go about putting power on the grid?Once SPARC is successful, the next project would be to build and operate a fusion pilot plant, to put electricity from fusion power onto the grid. The purpose of this plant would be to demonstrate all of the science and technolo-gy required for economically competitive, mass produc-tion of fusion energy. The project would be carried out, mainly as a commercial endeavor, with some technical input from MIT and other Universities and Labs and with collaboration on regulation (and perhaps siting) from the U.S. Government. The SPARC team believes that this plant could be operating in as little as 15 years (2033). While ambitious, this schedule is not unlike what was accom-plished for fission energy for public and private space programs and for earlier large-scale projects in the fusion program.

While we have only an outline for such a device, we can get some idea about what it might look like from the ARC concept, a project undertaken by a group of MIT students in a fusion design course. The ARC design was intended to show the capabilities of the new magnet technology by developing a point design for a plant producing as much fusion power as ITER at the smallest possible size. The result was a machine about half the linear dimension of ITER, running at 9 Tesla and producing more than 500 megawatt (MW) of fusion power. The students also looked at technologies that would allow such a device to operate in steady state and produce more than 200 MW of electricity.

The ARC concept explores the possibilities of a high-field fusion power plant. (Rendering by Alex Creely)

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Innovations will speed the path to fusion energy

Consideration of the ARC device highlighted the opportunities to improve the fusion power concept by developing and deploying several innovative technologies. These technologies will require extensive research and development before they are ready to be used, providing oppor-tunities for future collaborations between the SPARC project team and other fusion researchers.

Demountable superconducting magnets

Another distinct advantage arising from the use of high-temperature superconductors is that the higher specific heat of materials at higher temperatures, combined with additional opera-tional margin, should allow construction of superconducting coils with demountable joints. This concept could revolutionize construction and maintenance of fusion devices, which otherwise have components trapped by their toroidal field coils.

Molten Salt Blanket

Fusion devices need a “blanket” to breed tritium, shield the magnets and allow extraction of thermal energy. Demountable magnets are synergistic with an additional innovation – the pro-vision of all blanket functions through immersion of the fusion core in a bath of molten salt. This blanket concept improves shielding, by removing all cracks and gaps; dramatically reduces the volume of solid material exposed to high neutron flux (and thus the quantity of activated ma-terial by about 50x) and further simplifies maintenance. The molten salt would be drained and the toroidal magnet disassembled to allow all core components to be accessible by a vertical lift. In doing so, this concept enables a development path for fusion materials in which replaceable cores are installed successively as part of the R&D program.

High-field-side launch Lower Hybrid Current Drive

It is possible that a successful pilot plant could be operated in a pulsed mode, with the thermal inertia of molten salt used to smooth out the power flow. But, to achieve steady state, the plant could take advantage of the improved Lower-Hybrid Current Drive (LHCD) efficiency predict-ed at higher field, especially if combined with an innovative high-field-side launch. Modeling suggests that 25% of the current could be driven with only 5% of the total power, allowing good control at high Q in steady-state scenarios. Placing the LHCD launchers on the high-field side also eases plasma-materials interactions, which challenge low-field-side structures.

Advanced divertors

Managing the interface between the ultra-hot plasma and ordinary matter will be a challenge for any practical fusion scheme. The current generation of experiments handle this issue by “divert-ing” magnetic field lines at the plasma edge onto a specially designed structure. However, these designs are already marginal and will not extrapolate into the regimes required for fusion power. Modeling indicates that tightly baffled, long-leg, advanced divertors with embedded secondary x-points have potential to provide an order-of-magnitude increase in power exhaust handling over conventional divertors, while fully suppressing plasma-material interaction.

In the ARC fusion pilot plant concept, assembly and maintenance are greatly eased by the use of demountable super-conducting magnets — only possible with HTS conductors. (Rendering by Alex Creely)

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SummaryOur goal is to accelerate the timeline for fusion energy by taking advantage of a breakthrough in magnet technology. The cornerstone of our plan is SPARC, a fusion device conceived to demonstrate net energy for the first time by about 2025. This would be a historic moment for humankind, the equivalent of the first telephone call or airplane flight. With its aggressive timeline, SPARC inspires us to move forward rapidly on the R&D that will be needed to develop an attractive fusion power system. As a key element in this plan, we have built an industry partner who can attract the investment and expertise required to bring fusion to market. The diagram sketches out the technical steps required and the hand-off from academic research to commercialization.

Renderings by Alex CreelyPrivate IndustryAcademia

Collaboration

Technical Readiness Level

Phase 1: Early R&D

Alcator C-Mod(completed)

SPARC demomagnet

ARC pilot plantFusion power > 200 MWElectricity producing

SPARC Net energy gainFusion power > 50 MWPulsedHigh magnetic

field plasma science

High fieldmagnets and engineering

EngineeringR&D

Phase 2: Demonstration Phase 3: Commercialization

High-field fusion pathway

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The PSFCMIT’s Plasma Science & Fusion Center (PSFC) is a multi-disciplinary research laboratory on the MIT campus, affiliated with several academic departments, including Nuclear Science & Engineering, Physics, Mechanical Engineering, Aeronautics and Astronautics and Electrical Engineering. With more than 200 affiliated faculty members, students researchers and technicians, substantial technical infra-structure and 250,000 square feet of laboratory, shop and office space, the PSFC is the largest and most capable university-based fusion lab in the world. The Center was formed in the 1970s as a collaboration between engineers from the National Magnet Laboratory and fusion physicists, who recognized the advantages of high-field magnet engineering for fusion. We have built the three highest field fusion experiments in the world, which have set a number of plasma performance records, validating the high-field pathway. SPARC will be the fourth.

CFSCommonwealth Fusion Systems (CFS) is a private start-up company, headed by former PSFC students and scientists that recently spun out from MIT. CFS aspires to develop commercial fusion energy on an international scale. The company is currently headquartered in Cambridge MA.

High-Temperature Superconductor (HTS) tapeAt liquid helium temperatures (4 K) this 12 mm wide tape can carry ~5,000 amperes of current.

The current is carried in a 1 micron layer of Rare-Earth Barium Copper Oxide (REBCO) superconductor, which constitutes ~100th of the tape. The bulk of the tape is a strong stainless steel, with copper coatings on both sides. It is commercially available.

These tapes routinely operate as super-conductors above 30 tesla of magnetic field strength.

CFS Co-Founders (left-to-right): Martin Greenwald, Dan Brunner (CTO), Zach Hartwig, Brandon Sorbom (CSO), Bob Mumgaard (CEO), and Dennis Whyte. (Photo by Bryce Vickmark)

REBCO superconductor(~1μm)

Buffer layers(~1μm)

2–12mm

~0.1mm

Hastelloy(~50μm)

Silver(~2μm)

Copper(~20μm)