A cheaper more compact route to nuclear fusion?

The Z-pinch in A&A’s FuZE Lab

Nuclear fusion powers the stars and offers the promise of unlimited, clean energy on Earth.

But controlled thermonuclear fusion in the lab usually requires large and expensive magnetic field coils to stably confine burning plasma, ionized nuclei that collide to initiate nuclear fusion. A&A’s Fusion Z-Pinch Experiments (FuZE) Lab has demonstrated a smaller, cheaper method: We have measured sustained nuclear fusion for the first time from a 50-cm long plasma column called a Z-pinch. This method compresses a flowing plasma using electromagnetic forces, which drive the plasma to higher temperatures and densities.

While the Z-pinch is not a new plasma confinement concept, it was largely abandoned as a path for fusion energy because the plasma was not stable, which limited how long it could be confined. To get around this issue, we exploited the fact that flows can stabilize plasma, and our flowing plasma was maintained five thousand times longer than a static plasma. We also observed the sustained release of telltale energetic neutrons signaling nuclear fusion. Because this approach provides a path to nuclear fusion without coils, it could be used in the future for long-duration fusion burns in a compact and low-cost device.

Learn more: Plasma flows may provide the missing ingredient to a cheaper, more compact route to nuclear fusion

 

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Calling for a compact fusion pilot plant to generate electricity at the lowest possible cost

Physicist Jon Menard with concepts for a next-generation fusion facility.

Can tokamak fusion facilities, the most widely used devices for harvesting on Earth the fusion reactions that power the sun and stars, be developed more quickly to produce safe, clean, and virtually limitless energy for generating electricity?

Physicist Jon Menard of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has examined that question in a detailed look at the concept of a compact tokamak equipped with high temperature superconducting (HTS) magnets. Such magnets can produce higher magnetic fields – necessary to produce and sustain fusion reactions – than would otherwise be possible in a compact facility.

Menard first presented the paper(link is external), now published in Philosophical Transactions of the Royal Society A, to a Royal Society workshop in London that explored accelerating the development of tokamak-produced fusion power with compact tokamaks. “This is the first paper that quantitatively documents how the new superconductors can interplay with the high pressure that compact tokamaks produce to influence how tokamaks are optimized in the future,” Menard said. “What we tried to develop were some simple models that capture important aspects of an integrated design.”

“Very significant” findings

The findings are “very significant,” said Steve Cowley, director of PPPL. Cowley noted that “Jon’s arguments in this and the previous paper have been very influential in the recent National Academies of Sciences report,” which calls for a U.S. program to develop a compact fusion pilot plant to generate electricity at the lowest possible cost. “Jon has really outlined the technical aspects for much smaller tokamaks using high-temperature magnets,” Cowley said.

Compact tokamaks, which can include spherical facilities such as the National Spherical Torus Experiment-Upgrade (NSTX-U) that is under repair at PPPL and the Mega Ampere Spherical Tokamak (MAST) in Britain, provide some advantageous features. The devices, shaped like cored apples rather than doughnut-like conventional tokamaks, can produce high-pressure plasmas that are essential for fusion reactions with relatively low and cost-effective magnetic fields.

Such reactions fuse light elements in the form of plasma — the hot, charged state of matter composed of free electrons and atomic nuclei — to release energy.  Scientists seek to replicate this process and essentially create a star on Earth to generate abundant electricity for homes, farms, and industries around the world. Fusion could last  millions of years with little risk and without generating greenhouse gases.

Extends previous examination

Menard’s study extends his previous examination of a spherical design that could develop materials and components for a fusion reactor and serve as a pilot plant to produce electric power. The current paper provides a detailed analysis of the complex tradeoffs that future experiments will need to explore when it comes to integrating compact tokamaks with HTS magnets. “We realize that there’s no single innovation that can be counted on to lead to some breakthrough for making devices more compact or economical,” Menard said. “You have to look at an entire integrated system to know if you are getting benefits from higher magnetic fields.”

The paper focuses key issues on the size of the hole, defined as the “aspect ratio,” in the center of the tokamak that holds and shapes the plasma. In spherical tokamaks, this hole can be half the size of the hole in conventional tokamaks, corresponding to the cored apple-like shape of the compact design. While physicists believe that lower aspect ratios can improve plasma stability and plasma confinement, “we won’t know on the confinement side until we run experiments on the NSTX-U and the MAST upgrades,” Menard said.

Lower aspect ratios provide an attractive setting for HTS magnets, whose high current density can produce the strong magnetic fields that fusion requires inside the relatively narrow space of a compact tokamak. However, superconducting magnets need thick shielding for protection from neutron bombardment damage and heating, leaving scant room for a transformer to induce current in the plasma to complete the twisting field when the device size is reduced.  For lower aspect ratio designs, scientists would thus have to develop new techniques to produce some or all of the initial plasma current.

200-to-300 megawatts of electric power

Sustaining the plasma to generate the 200-to-300 megawatts of electric power the paper examines would also require higher confinement than standard tokamak operating regimes typically achieve.  Such power production could lead to challenging fluxes of fusion neutrons that would limit the estimated lifetime of the HTS magnets to one-to-two years of full-power operation. Thicker shielding could substantially increase that lifetime but would also lower the delivery of fusion power.

Major development will in fact be needed for HTS magnets, which have not yet been built to scale. “It will probably take years to put together a model of the essential elements of magnet size requirements and related factors as a function of aspect ratio,” Menard said.

The bottom line, he said, is that the lower aspect ratio “is really worth investigating based on these results.” The potential benefits of lower ratios, he noted, include the production of fusion power density — the crucial output of fusion power per volume of plasma  — that exceeds the output for conventional aspect ratios. “Fusion needs to become more attractive,” Menard said,  “so it’s important to assess the benefits of lower aspect ratios and what the tradeoffs are.”

Learn more; Speeding the development of fusion power to create unlimited energy on Earth

 

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Fusion power pushes past 100 million degrees

Fig. 1 The plasma electron temperature over 100 million degrees achieved in 2018 on EAST. (Image by the EAST Team)

The Experimental Advanced Superconducting Tokamak (EAST), nicknamed the “Chinese artificial sun”, achieved an electron temperature of over 100 million degrees in its core plasma during a four-month experiment this year. That’s about seven times more than the interior of the Sun, which is about 15 million degrees C.

The experiment shows China is making significant progress towards tokamak-based fusion energy production.

The experiment was conducted by the EAST team at the Hefei Institutes of Physical Science of the Chinese Academy of Sciences (CASHIPS) in collaboration with domestic and international colleagues. 

The plasma current density profile was optimized through the effective integration and synergy of four kinds of heating power: lower hybrid wave heating, electron cyclotron wave heating, ion cyclotron resonance heating and neutral beam ion heating. 

Power injection exceeded 10 MW, and plasma stored energy boosted to 300 kJ after scientists optimized the coupling of different heating techniques. The experiment utilized advanced plasma control and theory/simulation prediction

Scientists carried out experiments on plasma equilibrium and instability, confinement and transport, plasma-wall interaction and energetic particle physics to demonstrate long-time scale, steady-state H-mode operation with good control of impurity, core/edge MHD stability, and heat exhaust using an ITER-like tungsten divertor.

With ITER-like operating conditions such as radio frequency wave-dominant heating, lower torque, and a water-cooling tungsten divertor, EAST achieved a fully non-inductive steady-state scenario with extension of fusion performance at high density, high temperature and high confinement.

Meanwhile, to resolve the particle and power exhaust, which is crucial for high-performance steady-state operations, the EAST team employed many techniques to control the edge-localized modes and tungsten impurity with metal walls, along with active feedback control of the divertor heat load.

Operating scenarios including the steady-state high-performance H-mode and electron temperatures over 100 million degrees on EAST have made unique contributions towards ITER, the Chinese Fusion Engineering Test Reactor (CFETR) and DEMO.

These results provide key data for validation of heat exhaust, transport and current drive models. They also increase confidence in fusion performance predictions for CFETR. 

At present, the CFETR physics design focuses on optimization of a third-evolution machine with large radium at 7 m, minor radium at 2 m, a toroildal magnet field of 6.5-7 Tesla and a plasma current of 13 MA. 

In support of the engineering development of CFETR and the future DEMO, a new National Mega Science Project – theComprehensive Research Facility – will be launched at the end of this year. 

This new project will advance the development of tritium blanket test modules, superconducting technology, reactor-relevant heating and current drive actuators and sources, and divertor materials.

EAST is the first fully superconducting tokamak with a non-circular cross section in the world. It was designed and constructed by China with a focus on key science issues related to the application of fusion power. Since it began operating in 2006, EAST has become a fully open test facility where the world fusion community can conduct steady-state operations and ITER-related physics research. 

Learn more: Chinese Fusion Tool Pushes Past 100 Million Degrees

 

 

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An innovative solution to one of the longstanding challenges facing the development of practical fusion power plants

The ARC conceptual design for a compact, high magnetic field fusion power plant. The design now incorporates innovations from the newly published research to handle heat exhaust from the plasma.
ARC rendering by Alexander Creely

Novel design could help shed excess heat in next-generation fusion power plants.

A class exercise at MIT, aided by industry researchers, has led to an innovative solution to one of the longstanding challenges facing the development of practical fusion power plants: how to get rid of excess heat that would cause structural damage to the plant.

The new solution was made possible by an innovative approach to compact fusion reactors, using high-temperature superconducting magnets. This method formed the basis for a massive new research program launched this year at MIT and the creation of an independent startup company to develop the concept. The new design, unlike that of typical fusion plants, would make it possible to open the device’s internal chamber and replace critical comonents; this capability is essential for the newly proposed heat-draining mechanism.

The new approach is detailed in a paper in the journal Fusion Engineering and Design, authored by Adam Kuang, a graduate student from that class, along with 14 other MIT students, engineers from Mitsubishi Electric Research Laboratories and Commonwealth Fusion Systems, and Professor Dennis Whyte, director of MIT’s Plasma Science and Fusion Center, who taught the class.

In essence, Whyte explains, the shedding of heat from inside a fusion plant can be compared to the exhaust system in a car. In the new design, the “exhaust pipe” is much longer and wider than is possible in any of today’s fusion designs, making it much more effective at shedding the unwanted heat. But the engineering needed to make that possible required a great deal of complex analysis and the evaluation of many dozens of possible design alternatives.

Taming fusion plasma

Fusion harnesses the reaction that powers the sun itself, holding the promise of eventually producing clean, abundant electricity using a fuel derived from seawater — deuterium, a heavy form of hydrogen, and lithium — so the fuel supply is essentially limitless. But decades of research toward such power-producing plants have still not led to a device that produces as much power as it consumes, much less one that actually produces a net energy output.

Earlier this year, however, MIT’s proposal for a new kind of fusion plant — along with several other innovative designs being explored by others — finally made the goal of practical fusion power seem within reach. But several design challenges remain to be solved, including an effective way of shedding the internal heat from the super-hot, electrically charged material, called plasma, confined inside the device.

Most of the energy produced inside a fusion reactor is emitted in the form of neutrons, which heat a material surrounding the fusing plasma, called a blanket. In a power-producing plant, that heated blanket would in turn be used to drive a generating turbine. But about 20 percent of the energy is produced in the form of heat in the plasma itself, which somehow must be dissipated to prevent it from melting the materials that form the chamber.

No material is strong enough to withstand the heat of the plasma inside a fusion device, which reaches temperatures of millions of degrees, so the plasma is held in place by powerful magnets that prevent it from ever coming into direct contact with the interior walls of the donut-shaped fusion chamber. In typical fusion designs, a separate set of magnets is used to create a sort of side chamber to drain off excess heat, but these so-called divertors are insufficient for the high heat in the new, compact plant.

One of the desirable features of the ARC design is that it would produce power in a much smaller device than would be required from a conventional reactor of the same output. But that means more power confined in a smaller space, and thus more heat to get rid of.

“If we didn’t do anything about the heat exhaust, the mechanism would tear itself apart,” says Kuang, who is the lead author of the paper, describing the challenge the team addressed — and ultimately solved.

Inside job

In conventional fusion reactor designs, the secondary magnetic coils that create the divertor lie outside the primary ones, because there is simply no way to put these coils inside the solid primary coils. That means the secondary coils need to be large and powerful, to make their fields penetrate the chamber, and as a result they are not very precise in how they control the plasma shape.

But the new MIT-originated design, known as ARC (for advanced, robust, and compact) features magnets built in sections so they can be removed for service. This makes it possible to access the entire interior and place the secondary magnets inside the main coils instead of outside. With this new arrangement, “just by moving them closer [to the plasma] they can be significantly reduced in size,” says Kuang.

In the one-semester graduate class 22.63 (Principles of Fusion Engineering), students were divided into teams to address different aspects of the heat rejection challenge. Each team began by doing a thorough literature search to see what concepts had already been tried, then they brainstormed to come up with multiple concepts and gradually eliminated those that didn’t pan out. Those that had promise were subjected to detailed calculations and simulations, based, in part, on data from decades of research on research fusion devices such as MIT’s Alcator C-Mod, which was retired two years ago. C-Mod scientist Brian LaBombard also shared insights on new kinds of divertors, and two engineers from Mitsubishi worked with the team as well. Several of the students continued working on the project after the class ended, ultimately leading to the solution described in this new paper. The simulations demonstrated the effectiveness of the new design they settled on.

“It was really exciting, what we discovered,” Whyte says. The result is divertors that are longer and larger, and that keep the plasma more precisely controlled. As a result, they can handle the expected intense heat loads.

“You want to make the ‘exhaust pipe’ as large as possible,” Whyte says, explaining that the placement of the secondary magnets inside the primary ones makes that possible. “It’s really a revolution for a power plant design,” he says. Not only do the high-temperature superconductors used in the ARC design’s magnets enable a compact, high-powered power plant, he says, “but they also provide a lot of options” for optimizing the design in different ways — including, it turns out, this new divertor design.

Going forward, now that the basic concept has been developed, there is plenty of room for further development and optimization, including the exact shape and placement of these secondary magnets, the team says. The researchers are working on further developing the details of the design.

“This is opening up new paths in thinking about divertors and heat management in a fusion device,” Whyte says.

“All of the ARC work has been both eye-opening and stimulating of new ways of looking at tokamak fusion reactors,” says Bruce Lipschultz, a professor of physics at the University of York, in the U.K., who was not involved in this work. This latest paper, he says, “incorporates new ideas in the field with the many other significant improvements in the tokamak concept. … The ARC study of the extended leg divertor concept shows that the application to a reactor is not impossible, as others have contended.”

Lipschultz adds that this is “very high-quality research that shows a way forward for the tokamak reactor and stimulates new research elsewhere.”

Learn more: A new path to solving a longstanding fusion challenge

 

 

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“Star in a Jar” fusion process moves us closer to unlimited electric energy

(Photo by Scott Massida )
PPPL physicist Novimir Pablant, right, and Andreas Langenberg of the Max Planck Institute in front of the housing for the x-ray crystal spectrometer prior to its installation in the W7-X.

When Germany’s Wendelstein 7-X (W7-X) fusion facility set a world record for stellarators recently, a finely tuned instrument built and delivered by the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) proved the achievement. The record strongly suggests that the design of the stellarator can be developed to capture on Earth the fusion that drives the sun and stars, creating “a star in a jar” to generate a virtually unlimited supply of electric energy.

The record achieved by the W7-X, the world’s largest and most advanced stellarator, was the highest “triple product” that a stellarator has ever created. The product combines the temperature, density and confinement time of a fusion facility’s plasma — the state of matter composed of free electrons and atomic nuclei that fuels fusion reactions — to measure how close the device can come to producing self-sustaining fusion power. (The triple product was 6 x 1026 degrees x second per cubic meter — the new stellarator record.)

Spectrometer maps the temperature

The achievement produced temperatures of 40 million degrees for the ions and an energy confinement time, which measures how long it takes energy to leak out across the confining magnetic fields of 0.22 seconds. (The density was 0.8 x 1020  particles per cubic meter.) Measuring the temperature was an x-ray imaging crystal spectrometer (XICS) built by PPPL physicist Novimir Pablant, now stationed at W7-X, and engineer Michael Mardenfeld at PPPL. “The spectrometer provided the primary measurement,” said PPPL physicist Sam Lazerson, who also collaborates on W7-X experiments.

Pablant implemented the device with scientists and engineers of the Max Planck Institute of Plasma Physics (IPP), which operates the stellarator in the Baltic Sea town of Greifswald, Germany. “It has been a great experience to work closely with my colleagues here on W7-X,” Pablant said. “Installing the XICS system was a major undertaking and it has been a pleasure to work with this world-class research team.  The initial results from these high-performance plasmas are very exciting, and we look forward to using the measurements from our instrument to further understanding of the confinement properties of W7-X, which is a truly unique magnetic fusion experiment.”

Researchers at IPP welcomed the findings. “Without XICS we could not have confirmed the record,” said Thomas Sunn Pedersen, director of stellarator edge and divertor physics at IPP.  Concurred physicist Andreas Dinklage, lead author of a Nature Physics(link is external) paper confirming a key feature of the W7-X physical design: “The XICS data set was one of the very valuable inputs that confirmed the physics predictions.”

PPPL physicist David Gates, technical coordinator of the U.S. collaboration on W7-X, oversaw construction of the instrument. “The XICS is an incredibly precise device capable of measuring very small shifts in wavelength,” said Gates. “It is a crucial part of our collaboration and we are very grateful to have the opportunity to participate in these important experiments on the groundbreaking W7-X device.”

PPPL provides added components

PPPL has designed and delivered additional components installed on the W7-X. These include a set of large trim coils that correct errors in the magnetic field that confines W7-X plasma, and a scraper unit that will lessen the heat reaching the divertor that exhausts waste heat from the fusion facility.

The recent world record was a result of upgrades that IPP made to the stellarator following the initial phase of experiments, which began in December 2015. Improvements included new graphite tiles that enabled the higher temperatures and longer duration plasmas that produced the results. A new round of experiments is to begin this July using the new scraper unit that PPPL delivered.

Stellarators, first constructed in the 1950s under PPPL founder Lyman Spitzer, can operate in a steady state, or continuous manner, with little risk of the plasma disruptions that doughnut-shaped tokamak fusion facilities face. But tokamaks are simpler to design and build, and historically have confined plasma better, which accounts for their much wider use in fusion laboratories around the world.

An overall goal of the W7-X is to show that the twisty stellarator design can confine plasma just as well as tokamaks. When combined with the ability to operate virtually free of disruptions, such improvement could make stellarators excellent models for future fusion power plants.

Learn more: PPPL diagnostic is key to world record of German fusion experiment

 

 

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