From The Conversation: “Fusion energy: A time of transition and potential”

Conversation
The Conversation

November 29, 2016
Stewart Prager
Professor of Astrophysical Science, former director of the Princeton Plasma Physics Laboratory, Princeton University

Michael C. Zarnstorff
Deputy Director for Research, Princeton Plasma Physics Laboratory, Princeton University

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fusion energy. murrayashmole

For centuries, humans have dreamed of harnessing the power of the sun to energize our lives here on Earth. But we want to go beyond collecting solar energy, and one day generate our own from a mini-sun. If we’re able to solve an extremely complex set of scientific and engineering problems, fusion energy promises a green, safe, unlimited source of energy. From just one kilogram of deuterium extracted from water per day could come enough electricity to power hundreds of thousands of homes.

Since the 1950s, scientific and engineering research has generated enormous progress toward forcing hydrogen atoms to fuse together in a self-sustaining reaction – as well as a small but demonstrable amount of fusion energy. Skeptics and proponents alike note the two most important remaining challenges: maintaining the reactions over long periods of time and devising a material structure to harness the fusion power for electricity.

As fusion researchers at the Princeton Plasma Physics Lab, we know that realistically, the first commercial fusion power plant is still at least 25 years away.

PPPLII

But the potential for its outsize benefits to arrive in the second half of this century means we must keep working. Major demonstrations of fusion’s feasibility can be accomplished earlier – and must, so that fusion power can be incorporated into planning for our energy future.

Unlike other forms of electrical generation, such as solar, natural gas and nuclear fission, fusion cannot be developed in miniature and then be simply scaled up. The experimental steps are large and take time to build. But the problem of abundant, clean energy will be a major calling for humankind for the next century and beyond. It would be foolhardy not to exploit fully this most promising of energy sources.

Why fusion power?

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Adding heat to two isotopes of water can result in fusion. American Security Project, CC BY-ND

In fusion, two nuclei of the hydrogen atom (deuterium and tritium isotopes) fuse together. This is relatively difficult to do: Both nuclei are positively charged, and therefore repel each other. Only if they are moving extremely fast when they collide will they smash together, fuse and thereby release the energy we’re after.

This happens naturally in the sun. Here on Earth, we use powerful magnets to contain an extremely hot gas of electrically charged deuterium and tritium nuclei and electrons. This hot, charged gas is called a plasma.

The plasma is so hot – more than 100 million degrees Celsius – that the positively charged nuclei move fast enough to overcome their electrical repulsion and fuse. When the nuclei fuse, they form two energetic particles – an alpha particle (the nucleus of the helium atom) and a neutron.

Heating the plasma to such a high temperature takes a large amount of energy – which must be put into the reactor before fusion can begin. But once it gets going, fusion has the potential to generate enough energy to maintain its own heat, allowing us to draw off excess heat to turn into usable electricity.

Fuel for fusion power is abundant in nature. Deuterium is plentiful in water, and the reactor itself can make tritium from lithium. And it is available to all nations, mostly independent of local natural resources.

Fusion power is clean. It emits no greenhouse gases, and produces only helium and a neutron.

It is safe. There is no possibility for a runaway reaction, like a nuclear-fission “meltdown.” Rather, if there is any malfunction, the plasma cools, and the fusion reactions cease.

All these attributes have motivated research for decades, and have become even more attractive over time. But the positives are matched by the significant scientific challenge of fusion.

Progress to date

The progress in fusion can be measured in two ways. The first is the tremendous advance in basic understanding of high-temperature plasmas. Scientists had to develop a new field of physics – plasma physics – to conceive of methods to confine the plasma in strong magnetic fields, and then evolve the abilities to heat, stabilize, control turbulence in and measure the properties of the superhot plasma.

Related technology has also progressed enormously. We have pushed the frontiers in magnets, and electromagnetic wave sources and particle beams to contain and heat the plasma. We have also developed techniques so that materials can withstand the intense heat of the plasma in current experiments.

It is easy to convey the practical metrics that track fusion’s march to commercialization. Chief among them is the fusion power that has been generated in the laboratory: Fusion power generation escalated from milliwatts for microseconds in the 1970s to 10 megawatts of fusion power (at the Princeton Plasma Physics Laboratory) and 16 megawatts for one second (at the Joint European Torus in England) in the 1990s.

PPPL NSTX
PPPL NSTX

A new chapter in research

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Under construction: the ITER research tokamak in France. ITER

ITER Tokamak
ITER Tokamak

Now the international scientific community is working in unity to construct a massive fusion research facility in France. Called ITER (Latin for “the way”), this plant will generate about 500 megawatts of thermal fusion power for about eight minutes at a time. If this power were converted to electricity, it could power about 150,000 homes. As an experiment, it will allow us to test key science and engineering issues in preparation for fusion power plants that will function continuously.

ITER employs the design known as the “tokamak,” originally a Russian acronym. It involves a doughnut-shaped plasma, confined in a very strong magnetic field, which is partly created by electrical current that flows in the plasma itself.

Though it is designed as a research project, and not intended to be a net producer of electric energy, ITER will produce 10 times more fusion energy than the 50 megawatts needed to heat the plasma. This is a huge scientific step, creating the first “burning plasma,” in which most of the energy used to heat the plasma comes from the fusion reaction itself.

ITER is supported by governments representing half the world’s population: China, the European Union, India, Japan, Russia, South Korea and the U.S. It is a strong international statement about the need for, and promise of, fusion energy.

The road forward

From here, the remaining path toward fusion power has two components. First, we must continue research on the tokamak. This means advancing physics and engineering so that we can sustain the plasma in a steady state for months at a time. We will need to develop materials that can withstand an amount of heat equal to one-fifth the heat flux on the surface of the sun for long periods. And we must develop materials that will blanket the reactor core to absorb the neutrons and breed tritium.

The second component on the path to fusion is to develop ideas that enhance fusion’s attractiveness. Four such ideas are:

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The W-7X stellarator configuration. Max-Planck Institute of Plasmaphysics, CC BY

Wendelstgein 7-X stellarator
Wendelstgein 7-X stellarator

1) Using computers, optimize fusion reactor designs within the constraints of physics and engineering. Beyond what humans can calculate, these optimized designs produce twisted doughnut shapes that are highly stable and can operate automatically for months on end. They are called “stellarators” in the fusion business.

2) Developing new high-temperature superconducting magnets that can be stronger and smaller than today’s best. That will allow us to build smaller, and likely cheaper, fusion reactors.

3) Using liquid metal, rather than a solid, as the material surrounding the plasma. Liquid metals do not break, offering a possible solution to the immense challenge how a surrounding material might behave when it contacts the plasma.

4) Building systems that contain doughnut-shaped plasmas with no hole in the center, forming a plasma shaped almost like a sphere. Some of these approaches could also function with a weaker magnetic field. These “compact tori” and “low-field” approaches also offer the possibility of reduced size and cost.

Government-sponsored research programs around the world are at work on the elements of both components – and will result in findings that benefit all approaches to fusion energy (as well as our understanding of plasmas in the cosmos and industry). In the past 10 to 15 years, privately funded companies have also joined the effort, particularly in search of compact tori and low-field breakthroughs. Progress is coming and it will bring abundant, clean, safe energy with it.

See the full article here .

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The Conversation US launched as a pilot project in October 2014. It is an independent source of news and views from the academic and research community, delivered direct to the public.
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Access to independent, high quality, authenticated, explanatory journalism underpins a functioning democracy. Our aim is to promote better understanding of current affairs and complex issues. And hopefully allow for a better quality of public discourse and conversation.

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From Princeton: “An explanation for the mysterious onset of a universal process (Physics of Plasmas)”

Princeton University
Princeton University


PPPL

November 23, 2016
John Greenwald, Princeton Plasma Physics Laboratory Communications

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Magnetic reconnection happens in solar flares on the surface in the sun, as well as in experimental fusion energy reactors here on Earth. Image credit: NASA.

Scientists have proposed a groundbreaking solution to a mystery that has puzzled physicists for decades. At issue is how magnetic reconnection, a universal process that sets off solar flares, northern lights and cosmic gamma-ray bursts, occurs so much faster than theory says should be possible. The answer, proposed by researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University, could aid forecasts of space storms, explain several high-energy astrophysical phenomena, and improve plasma confinement in doughnut-shaped magnetic devices called tokamaks designed to obtain energy from nuclear fusion.

Magnetic reconnection takes place when the magnetic field lines embedded in a plasma — the hot, charged gas that makes up 99 percent of the visible universe — converge, break apart and explosively reconnect. This process takes place in thin sheets in which electric current is strongly concentrated.

According to conventional theory, these sheets can be highly elongated and severely constrain the velocity of the magnetic field lines that join and split apart, making fast reconnection impossible. However, observation shows that rapid reconnection does exist, directly contradicting theoretical predictions.

Detailed theory for rapid reconnection

Now, physicists at PPPL and Princeton University have presented a detailed theory for the mechanism that leads to fast reconnection. Their paper, published in the journal Physics of Plasmas in October, focuses on a phenomenon called “plasmoid instability” to explain the onset of the rapid reconnection process. Support for this research comes from the National Science Foundation and the DOE Office of Science.

Plasmoid instability, which breaks up plasma current sheets into small magnetic islands called plasmoids, has generated considerable interest in recent years as a possible mechanism for fast reconnection. However, correct identification of the properties of the instability has been elusive.

The Physics of Plasmas paper addresses this crucial issue. It presents “a quantitative theory for the development of the plasmoid instability in plasma current sheets that can evolve in time” said Luca Comisso, lead author of the study. Co-authors are Manasvi Lingam and Yi-Ming Huang of PPPL and Princeton, and Amitava Bhattacharjee, head of the Theory Department at PPPL and Princeton professor of astrophysical sciences.

Pierre de Fermat’s principle

The paper describes how the plasmoid instability begins in a slow linear phase that goes through a period of quiescence before accelerating into an explosive phase that triggers a dramatic increase in the speed of magnetic reconnection. To determine the most important features of this instability, the researchers adapted a variant of the 17th century “principle of least time” originated by the mathematician Pierre de Fermat.

Use of this principle enabled the researchers to derive equations for the duration of the linear phase, and for computing the growth rate and number of plasmoids created. Hence, this least-time approach led to a quantitative formula for the onset time of fast magnetic reconnection and the physics behind it.

The paper also produced a surprise. The authors found that such relationships do not reflect traditional power laws, in which one quantity varies as a power of another. “It is common in all realms of science to seek the existence of power laws,” the researchers wrote. “In contrast, we find that the scaling relations of the plasmoid instability are not true power laws – a result that has never been derived or predicted before.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by Princeton University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

See the full article here .

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Princeton University Campus

About Princeton: Overview

Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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From PPPL: “Princeton graduate student Imène Goumiri creates computer program that helps stabilize fusion plasmas”


PPPL

April 14, 2016
John Greenwald
Raphael Rosen

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Imène Goumiri led the design of a controller. (Photo by Elle Starkman/Office of Communications)

Imène Goumiri, a Princeton University graduate student, has worked with physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) to simulate a method for limiting instabilities that reduce the performance of fusion plasmas. The more instabilities there are, the less efficiently doughnut-shaped fusion facilities called tokamaks operate. The journal Nuclear Fusion published* results of this research in February 2016. The research was supported by the DOE’s Office of Science.

The new method uses feedback from sensors for real-time control of the rotation of plasma that swirls within a tokamak and fuels fusion reactions. Goumiri, a student in the Princeton Department of Mechanical and Aerospace Engineering, led the design of a controller that employs two different kinds of actuators. The first provides a torque — or twisting force that causes rotation — by injecting high-energy neutral particles into the plasma. The second uses a tokamak’s three-dimensional coils to create a magnetic field that generates a torque by acting as a drag on the rotation and slowing it down.

Goumiri built a model of plasma rotation from data collected from PPPL’s National Spherical Torus Experiment (NSTX) before it was upgraded, and used it to construct the program in MATLAB software.

PPPL/NSTX
PPPL/NSTX

She then translated the program into a predictive model based on PPPL’s TRANSP code, the global standard for analyzing plasma performance. The TRANSP model found the new approach to be effective at controlling rotation.

“This confirmed the validity of our model and the efficacy of the controller,” said Goumiri, the lead author of the paper. Coauthors included Clarence Rowley, Princeton professor of mechanical and aerospace engineering, and David Gates, principal research physicist at PPPL and stellarator physics leader, who served as her academic advisors; and Steve Sabbagh, senior research scientist and adjunct professor of applied physics at Columbia University on long-term assignment to PPPL, a member of her doctoral committee who served as a scientific advisor.

“Shear” lessens instabilities

The new program, which adapts quickly to feedback from the plasma, draws on the fact that rotating different sections of a plasma at different speeds creates a force called “shear” that lessens instabilities. Rotation can also disrupt transport, a process that leaks heat from the plasma and interferes with fusion reactions.

A unique aspect of the new model is its use of three-dimensional magnetic fields to manipulate the torque produced by the neutral beam injector. The drag created by these magnetic fields, technically known as “neoclassical toroidal viscosity,” gives researchers more precise and continuous control of the plasma rotation.

Looking ahead, researchers noted that the upgraded NSTX, called the National Spherical Torus Experiment-Upgrade (NSTX-U), has a second neutral beam injector that can affect a broader region toward the edge of the plasma. This broadened region could alter the shear and enable greater control of plasma instabilities.

Researchers also noted that this new class of controllers can be developed from simulations based on experimental data, with no need for additional experiments for calibration. The new method could replace classical controllers like proportional-integral-derivative (PID) systems, which use experiments to tune their parameters. The new method would necessitate fewer experiments and would provide a way to predict requirements for adjusting plasma rotation in future fusion facilities.

*Science paper:

Modeling and control of plasma rotation for NSTX using neoclassical toroidal viscosity and neutral beam injection

Science team:

I.R. Goumiri1, C.W. Rowley1, S.A. Sabbagh2, D.A. Gates3, S.P. Gerhardt3, M.D. Boyer3, R. Andre3, E. Kolemen3 and K. Taira4

Author affiliations

1 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA

2 Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA

3 Princeton Plasma Physics Laboratory, Princeton, NJ 08544, USA

4 Department of Mechanical Engineering, Florida State University, Tallahassee, FL 32310, USA

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Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

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From PPPL: “PPPL physicists simulate innovative method for starting up tokamaks without using a solenoid”


PPPL

Temp 1
PPPL Scientist Francesca Poli. (Photo by Elle Starkman/PPPL Office of Communications)

Scientists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have produced self-consistent computer simulations that capture the evolution of an electric current inside fusion plasma without using a central electromagnet, or solenoid. The simulations of the process, known as non-inductive current ramp-up, were performed using TRANSP, the gold-standard code developed at PPPL. The results were published in October 2015 in Nuclear Fusion. The research was supported by the DOE Office of Science.

In traditional donut-shaped tokamaks, a large solenoid runs down the center of the reactor.

PPPL NSTX
PPPL/NSTX-U tokamak

By varying the electrical current in the solenoid scientists induce a current in the plasma. This current starts up the plasma and creates a second magnetic field that completes the forces that hold the hot, charged gas together.

But spherical tokamaks, a compact variety of fusion reactor that produces high plasma pressure with relatively low magnetic fields, have little room for solenoids. Spherical tokamaks look like cored apples and have a smaller central hole for the solenoid than conventional tokamaks do. Physicists, therefore, have been trying to find alternative methods for producing the current that starts the plasma and completes the magnetic field in spherical tokamaks.

One such method is known as coaxial helicity injection (CHI). During CHI, researchers switch on an electric coil that runs beneath the tokamak. Above this coil is a gap that opens into the tokamak’s vacuum vessel and circles the tokamak’s floor. The switched-on electrical current produces a magnetic field that connects metal plates on either side of the gap.

Researchers next puff gas through the gap and discharge a spark across the two plates. This process causes magnetic reconnection — the process by which the magnetic fields snap apart and reconnect.

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Magnetic reconnection animation

This reconnection creates a magnetic bubble that fills the tokamak and produces the vital electric current that starts up the plasma and completes the magnetic field.

This current must be nurtured and fed. According to lead author Francesca Poli, the new computer simulations show that the current can best be sustained by injecting high-harmonic radio-frequency waves (HHFWs) and neutral beams into the plasma.

HHFW’s are radio-frequency waves that can heat both electrons and ions. The neutral beams, which consist of streams of hydrogen atoms, become charged when they enter the plasma and interact with the ions. The combination of the HHFWs and neutral beams increases the current from 300 kiloamps to 1 mega amp.

But neither HHFWs nor neutral beams can be used at the start of the process, when the plasma is relatively cool and not very dense. Poli found that HHFWs would be more effective if the plasma were first heated by electron cyclotron waves, which transfer energy to the electrons that circle the magnetic field lines.

“With no electron cyclotron waves you would have to pump in four megawatts of HHFW power to create 400 kiloamps of current,” she said. “With these waves you can get the same amount of current by pumping in only one megawatt of power.

“All of this is important because it’s hard to control the plasma at the start-up,” she added. “So the faster you can control the plasma, the better.”

See the full article here .

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Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

#applied-research-technology, #fusion-technology, #pppl, #tokamak-technology

From EPFL: “Swiss Plasma Center to harness the sun’s energy”

EPFL bloc

Ecole Polytechnique Federale Lausanne

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The heart of EPFL’s tokamak

22.09.15
Emmanuel Barraud

At EPFL, the Center for Research in Plasma Physics (CRPP) has become the Swiss Plasma Center (SPC), and for good reason: the Center is upgrading its facilities and expanding its scope of activities. These improvements strengthen the role the Lausanne-based tokamak will play as one of three research facilities selected by the EUROfusion consortium to develop nuclear fusion as part of the international project known as ITER.

ITER icon

Once mastered, nuclear fusion will be able to produce enough energy – clean, reliable energy – to meet the needs of mankind for centuries to come. Unlike fission, fusion does not create radioactive waste with a long lifespan, and it is based on abundant materials that are easier to extract than uranium.

Numerous international research projects are under way, and one of the most crucial challenges they face is plasma confinement. This refers to confining a gas that is heated to more than a hundred million degrees – considerably hotter than the core of the sun – so that the component hydrogen atoms will fuse and release huge amounts of energy. But these extreme temperatures must not damage the reactor, which means the plasma must be kept away from the walls. This is done using a magnetic field that is contained inside a ring-shaped chamber called a tokamak.

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One-of-a-kind research facility

The Variable Configuration Tokamak, which was built in 1992 at the Swiss Federal Institute of Technology in Lausanne (EPFL, Switzerland), has always been on the leading edge among research facilities in this field. The TCV tokamak, as it is known, is operated by the Center for Research in Plasma Physics (CRPP) and is unique because – as its name indicates – it can produce plasma in various shapes. This feature allows scientists to determine the most appropriate configuration for use in an energy-producing reactor. And it was thanks to this feature that in late 2013 the TCV tokamak was selected by the EUROfusion consortium as one of three national facilities on the European continent to be used to help design the international power plant ITER, currently being built in the south of France, and develop its successor, DEMO, a prototype commercial reactor.

The Lausanne-based lab recently received 10 million francs from the Swiss government to upgrade certain aspects of its facility. Thanks to these funds, the Center will soon be equipped to carry out new experiments on the TCV tokamak, particularly in relation to extracting energy and particles from the plasma. New mechanisms for heating the plasma with microwaves and with the injection of neutral particles may also be installed. At the same time, the Center is expanding its sector for lower density and lower temperature plasmas in order to explore new applications for plasma, such as in the medical field, the food industry and astrophysics. These improvements will encourage many Swiss and European researchers to visit Lausanne and conduct new experiments.

The “Swiss Plasma Center”, a new international reference

Alongside these developments, the Lausanne-based lab is changing its name. It is now the Swiss Plasma Center that will impress its credentials on Switzerland, Europe and the rest of the world as a leading institution in this field. The renamed Center was officially inaugurated today in Lausanne. Attendees included Bernard Bigot, Director-General of ITER, along with officials from the EUROfusion consortium, who emphasized the importance of the research being carried out in Switzerland in support of the objective of the reactor being built in Cadarache. The reactor, using nuclear fusion, aims to generate ten times more power than was injected into it.

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EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

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From PPPL: “Giant structures called plasmoids could simplify the design of future tokamaks”


PPPL

June 2, 2015
Raphael Rosen

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Left: Plasmoid formation in simulation of NSTX plasma during CHI / Right: Fast-camera image of NSTX plasma shows two discrete plasmoid-like bubble structures. (Photo by Left: Fatima Ebrahimi, PPPL / Right: Nishino-san, Hiroshima University)

Researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have for the first time simulated the formation of structures called “plasmoids” during Coaxial Helicity Injection (CHI), a process that could simplify the design of fusion facilities known as tokamaks. The findings, reported in the journal Physical Review Letters, involve the formation of plasmoids in the hot, charged plasma gas that fuels fusion reactions. These round structures carry current that could eliminate the need for solenoids – large magnetic coils that wind down the center of today’s tokamaks – to initiate the plasma and complete the magnetic field that confines the hot gas.

PPPL Tokamak
PPPL Tokamak

“Understanding this behavior will help us produce plasmas that undergo fusion reactions indefinitely,” said Fatima Ebrahimi, a physicist at both Princeton University and PPPL, and the paper’s lead author.

Ebrahimi ran a computer simulation that modeled the behavior of plasma and the formation of plasmoids in three dimensions thoughout a tokamak’s vacuum vessel. This marked the first time researchers had modeled plasmoids in conditions that closely mimicked those within an actual tokamak. All previous simulations had modeled only a thin slice of the plasma – a simplified picture that could fail to capture the full range of plasma behavior.

Researchers validated their model by comparing it with fast-camera images of plasma behavior inside the National Spherical Torus Experiment (NSTX), PPPL’s major fusion facility.

PPPL NSTX
PPPL NSTX

These images also showed plasmoid-like structures, confirming the simulation and giving the research breakthrough significance, since it revealed the existence of plasmoids in an environment in which they had never been seen before. “These findings are in a whole different league from previous ones,” said Roger Raman, leader for the Coaxial Helicity Injection Research program on NSTX and a coauthor of the paper.

The findings may provide theoretical support for the design of a new kind of tokamak with no need for a large solenoid to complete the magnetic field. Solenoids create magnetic fields when electric current courses through them in relatively short pulses. Today’s conventional tokamaks, which are shaped like a donut, and spherical tokamaks, which are shaped like a cored apple, both employ solenoids. But future tokamaks will need to operate in a constant or steady state for weeks or months at a time. Moreover, the space in which the solenoid fits – the hole in the middle of the doughnut-shaped tokamak – is relatively small and limits the size and strength of the solenoid.

A clear understanding of plasmoid formation could thus lead to a more efficient method of creating and maintaining a plasma through transient Coaxial Helicity Injection. This method, originally developed at the University of Washington, could dispense with a solenoid entirely and would work like this:

Researchers first inject open magnetic field lines into the vessel from the bottom of the vacuum chamber. As researchers drive electric current along those magnetic lines, the lines snap closed and form the plasmoids, much like soap bubbles being blown out of a sheet of soapy film.
The many plasmoids would then merge to form one giant plasmoid that could fill the vacuum chamber.
The magnetic field within this giant plasmoid would induce a current in the plasma to keep the gas tightly in place. “In principle, CHI could fundamentally change how tokamaks are built in the future,” says Raman.

Understanding how the magnetic lines in plasmoids snap closed could also help solar physicists decode the workings of the sun. Huge magnetic lines regularly loop off the surface of the star, bringing the sun’s hot plasma with them. These lines sometimes snap together to form a plasmoid-like mass that can interfere with communications satellites when it collides with the magnetic field that surrounds the Earth.

While Ebrahimi’s findings are promising, she stresses that much more is to come. PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U) will provide a more powerful platform for studying plasmoids when it begins operating this year, making Ebrahimi’s research “only the beginning of even more exciting work that will be done on PPPL equipment,” she said.

See the full article here.

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Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

#applied-research-technology, #fusion-technology, #pppl, #tokamak-technology

From physicsworld: “Smaller fusion reactors could deliver big gains”

physicsworld
physicsworld.com

Feb 16, 2015
Michael Banks

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Hot topic: size may not be everything in tokamak design

Researchers from the UK firm Tokamak Energy say that future fusion reactors could be made much smaller than previously envisaged – yet still deliver the same energy output. That claim is based on calculations showing that the fusion power gain – a measure of the ratio of the power from a fusion reactor to the power required to maintain the plasma in steady state – does not depend strongly on the size of the reactor. The company’s finding goes against conventional thinking, which says that a large power output is only possible by building bigger fusion reactors.

The largest fusion reactor currently under construction is the €16bn ITER facility in Cadarache, France.

ITER Tokamak

This will weigh about 23,000 tonnes when completed in the coming decade and consist of a deuterium–tritium plasma held in a 60 m-tall, doughnut-shaped “tokamak”. ITER aims to produce a fusion power gain (Q) of 10, meaning that, in theory, the reactor will emit 10 times the power it expends by producing 500 MW from 50 MW of input power. While ITER has a “major” plasma radius of 6.21 m, it is thought that an actual future fusion power plant delivering power to the grid would need a 9 m radius to generate 1 GW.

Low power brings high performance

The new study, led by Alan Costley from Tokamak Energy, which builds compact tokamaks, shows that smaller, lower-power, and therefore lower-cost reactors could still deliver a value of Q similar to ITER. The work focused on a key parameter in determining plasma performance called the plasma “beta”, which is the ratio of the plasma pressure to the magnetic pressure. By using scaling expressions consistent with existing experiments, the researchers show that the power needed for high fusion performance can be three or four times lower than previously thought.

Combined with the finding on the size-dependence of Q, these results imply the possibility of building lower-power, smaller and cheaper pilot plants and reactors. “The consequence of beta-independent scaling is that tokamaks could be much smaller, but still have a high power gain,” David Kingham, Tokamak Energy chief executive, told Physics World.

The researchers propose that a reactor with a radius of just 1.35 m would be able to generate 180 MW, with a Q of 5. This would result in a reactor just 1/20th of the size of ITER. “Although there are still engineering challenges to overcome, this result is underpinned by good science,” says Kingham. “We hope that this work will attract further investment in fusion energy.”

Many challenges remain

Howard Wilson, director of the York Plasma Institute at the University of York in the UK, points out, however, that the result relies on being able to achieve a very high magnetic field. “We have long been aware that a high magnetic field enables compact fusion devices – the breakthrough would be in discovering how to create such high magnetic fields in the tokamak,” he says. “A compact fusion device may indeed be possible, provided one can achieve high confinement of the fuel, demonstrate efficient current drive in the plasma, exhaust the heat and particles effectively without damaging material surfaces, and create the necessary high magnetic fields.”

The work by Tokamak Energy follows an announcement late last year that the US firm Lockheed Martin plans to build a “truck-sized” compact fusion reactor by 2019 that would be capable of delivering 100 MW. However, the latest results from Tokamak Energy might not be such bad news for ITER. Kingham adds that his firm’s work means that, in principle, ITER is actually being built much larger than necessary – and so should outperform its Q target of 10.

The research is published in Nuclear Fusion.

See the full article here.

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