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  • richardmitnick 7:14 am on December 3, 2016 Permalink | Reply
    Tags: , , , Stellarators,   

    From PPPL: “PPPL and Max Planck physicists confirm the precision of magnetic fields in the most advanced stellarator in the world” 


    PPPL

    December 2, 2016
    John Greenwald

    Physicist Sam Lazerson of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has teamed with German scientists to confirm that the Wendelstein 7-X (W7-X) fusion energy device called a stellarator in Greifswald, Germany, produces high-quality magnetic fields that are consistent with their complex design.

    PPPL Wendelstein 7-X, built in Greifswald, Germany
    Wendelstein 7-X, built in Greifswald, Germany

    The findings, published in the November 30 issue of Nature Communications, revealed an error field — or deviation from the designed configuration — of less than one part in 100,000. Such results could become a key step toward verifying the feasibility of stellarators as models for future fusion reactors.

    W7-X, for which PPPL is the leading U.S. collaborator, is the largest and most sophisticated stellarator in the world. Built by the Max Planck Institute for Plasma Physics in Greifswald, it was completed in 2015 as the vanguard of the stellarator design. Other collaborators on the U.S. team include DOE’s Oak Ridge and Los Alamos National Laboratories, along with Auburn University, the Massachusetts Institute of Technology, the University of Wisconsin-Madison and Xanthos Technologies.

    Twisty magnetic fields

    Stellarators confine the hot, charged gas, otherwise known as plasma, that fuels fusion reactions in twisty — or 3D — magnetic fields, compared with the symmetrical — or 2D –fields that the more widely used tokamaks create.

    PPPL NSTXII
    PPPL NSTX

    The twisty configuration enables stellarators to control the plasma with no need for the current that tokamaks must induce in the gas to complete the magnetic field. Stellarator plasmas thus run little risk of disrupting, as can happen in tokamaks, causing the internal current to abruptly halt and fusion reactions to shut down.

    PPPL has played key roles in the W7-X project. The Laboratory designed and delivered five barn door-sized trim coils that fine-tune the stellarator’s magnetic fields and made their measurement possible. “We’ve confirmed that the magnetic cage that we’ve built works as designed,” said Lazerson, who led roughly half the experiments that validated the configuration of the field. “This reflects U.S. contributions to W7-X,” he added, “and highlights PPPL’s ability to conduct international collaborations.” Support for this work comes from Euratom and the DOE Office of Science.

    To measure the magnetic field, the scientists launched an electron beam along the field lines. They next obtained a cross-section of the entire magnetic surface by using a fluorescent rod to intersect and sweep through the lines, thereby inducing fluorescent light in the shape of the surface.

    Remarkable fidelity

    Results showed a remarkable fidelity to the design of the highly complex magnetic field. “To our knowledge,” the authors write of the discrepancy of less than one part in 100,000, “this is an unprecedented accuracy, both in terms of the as-built engineering of a fusion device, as well as in the measurement of magnetic topology.”

    The W7-X is the most recent version of the stellarator concept, which Lyman Spitzer, a Princeton University astrophysicist and founder of PPPL, originated during the 1950s. Stellarators mostly gave way to tokamaks a decade later, since the doughnut-shaped facilities are simpler to design and build and generally confine plasma better. But recent advances in plasma theory and computational power have led to renewed interest in stellarators.

    Such advances caused the authors to wonder if devices like the W7-X can provide an answer to the question of whether stellarators are the right concept for fusion energy. Years of plasma physics research will be needed to find out, they conclude, and “that task has just started.”

    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.

     
  • richardmitnick 10:42 am on November 30, 2016 Permalink | Reply
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    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

    1
    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?

    2
    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

    4
    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:

    5
    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.
    Our team of professional editors work with university and research institute experts to unlock their knowledge for use by the wider public.
    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.

     
  • richardmitnick 4:34 pm on December 4, 2015 Permalink | Reply
    Tags: , , , Stellarators,   

    From PPPL: “PPPL researcher maps magnetic fields in first physics experiment on W7-X” 


    PPPL

    December 4, 2015
    Jeanne Jackson DeVoe

    PPPL Wendelstein 7-X
    Wendelstein 7-X

    2
    Sam Lazerson in front of a stellarator model at PPPL that was built for the 1958 “Atoms for Peace Conference” in Geneva. (Photo by Elle Starkman/PPPL Office of Communications).

    As excitement builds around the first plasma, scheduled for December, on the Wendelstein 7-X (W7-X) experiment in Greifswald, Germany, PPPL physicist Sam Lazerson can boast that he has already achieved results.

    Lazerson, who has been working at the site since March, mapped the structure of the magnetic field, proving that the main magnet system is working as intended. This was achieved using the trim coils that PPPL designed and had built in the United States. He presented his research at the APS Division of Plasma Physics Conference in Savannah, Georgia, on Nov. 18.

    PPPL leads U.S. laboratories that are collaborating with the Max Planck Institute for Plasma Physics in experiments on the W7-X, the largest and most advanced stellarator in the world. It will be the first optimzed stellarator fusion facility to confine a hot plasma in a steady state for up to 30 minutes. In doing so, it will demonstrate that an optimized stellarator could be a model for future fusion reactors.

    Stellarators are fusion devices that use twisting, potato chip-shaped magnetic coils to confine the plasma that fuels fusion reactions in a three-dimensional and steady-state magnetic field. Stellarators are not subject to disruption of the current that completes the magnetic confinement as are traditional donut-shaped tokamaks.

    PPPL NSTX
    PPPL/NSTX-U tokamak

    Such disruptions can halt fusion reactions.

    “W7-X is a fantastic experiment,” said PPPL Director Stewart Prager. “It’s going to be critical to the future of stellarator research in the world. We’re anxious to be a part of it since stellarators are a part of the future of fusion. We’re delighted that Sam is spending time there and we’re excited that the first experimental results are from Sam’s work.”

    Hutch Neilson, head of advanced projects at PPPL, is equally enthusiastic. “Once W7-X comes on line it will be the most advanced fusion experiment in the world,” said Neilson, who is technical coordinator for the U.S. partnership with the Max Planck Institute. “It will allow us to study 3-D plasma physics and test a concept that can be steady state and have the potential to make a simpler fusion reactor. It could be a step on a path to a new more attractive fusion reactor concept.”

    In the past, tokamaks were better than stellarators at confining plasma at the high temperature and density needed to create fusion energy. But the W7-X could potentially overcome this problem. “W7-X will meet or exceed the performance of modern tokamaks,” Lazerson predicted. “That’s why W7-X is important — because it’s ground-breaking.”

    PPPL played key role

    PPPL has played a key role in the development of W7-X and leads the U.S. collaboration on the experiment under a 2014 agreement between the U.S. Department of Energy and the Max Planck Institute for Plasma Physics. PPPL physicists and engineers designed and delivered the five 2,400-pound trim coils that fine-tune the shape of the plasma in fusion experiments.

    In addition, PPPL physicists Novimir Pablant and engineer Michael Mardenfeld designed and built an X-ray crystal spectrometer for the experiment that was one of several diagnostics created by U.S. researchers from PPPL, Los Alamos National Laboratory, and Oak Ridge National Laboratory. PPPL engineers led by Doug Loesser are building two divertor scraper units, a device designed in collaboration with Oak Ridge to intercept heat coming from the plasma to protect against damage to the W7-X divertor targets.

    Neilson was at the Max Planck Institute from July of 2014 to April and helped pave the way for American researchers as coordinator of the U.S. collaboration on W7-X. Gates, who is the stellarator physics leader at PPPL, has traveled to Germany several times to manage the U.S. research program. “Dave’s leadership is critical to ensuring that Sam and other PPPL physicists are strongly engaged in important W7-X research tasks,” Neilson said.

    Mapping the magnetic field

    Lazerson arrived last March and has been working with a team that has been designing and analyzing experiments that map the stellarator’s magnetic field. Lazerson used a diagnostic designed by physicist Matthias Otte of the Max Planck Institute. It consists of two fluorescent rods inserted into the W7-X vacuum vessel, one of which emits an electron beam. This beam causes the other fluorescent rod to glow and trace the pattern of electrons moving around the magnetic field. Cameras in W7-X capture the glowing rod as it tracing the field.

    The recorded image allows researchers to determine whether the stellarator’s massive magnets are have the required accuracy and whether the trim coils designed by PPPL are producing the intended results. The coils are designed to control “error fields” that can be used to create and manipulate a chain of magnetic islands that are located at the edge of the plasma and serve to distribute heat evenly among the 10 divertors that exhaust heat from the plasma. The trim coils can shrink or grow the magnetic islands, depending on how strong a magnetic field is applied.

    The photographs allow researchers to calculate the size of these small islands. By varying the trim coil current, researchers can check that the size of the islands is changing as expected, enabling researchers to determine if there are error fields in the main magnet system.

    “Once we make a plasma, we can perform experiments using the trim coils,” Lazerson said. “The measurements we’ve made in the absence of a plasma, with just the magnetic field, give us a basis for what the system looks like without a plasma, and an understanding of what the trim coils do to the basic magnetic structure. That’s interesting in its own right, but it’s also a stepping stone to the plasma experiments.”

    A “great opportunity”

    Lazerson said he has enjoyed working at the Max Planck Institute, which at 500 people is about the same size as PPPL. “It’s a great group of people,” he said. “This is a really unique experience. It’s a great opportunity.”

    The Lazerson family, which includes Lazerson’s wife Meghan and the couple’s five-year-old daughter Samantha, live in Greifswald, where Lazerson can bike to work and take Samantha to the local kindergarten by bicycle. Greifswald is a university town in northeastern Germany that began in the 15th century, when it was part of Sweden. It is not far from a beach on the Baltic Sea and is about two-and-a-half hours from Berlin.

    Lazerson said he has often had visits from Neilson and Gates, as well as DOE officials who have stopped in to see the project’s progress.

    Lazerson is looking forward to doing research after the first plasma. “We haven’t even touched on the interesting science that we’re going to be able to do with this device,” he said. “I think the success of W7-X will perhaps chart a new course on how we do fusion energy or what we want to do as our next experiment.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

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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.

     
  • richardmitnick 8:58 am on October 22, 2015 Permalink | Reply
    Tags: , , , Stellarators   

    From AAAS: “The bizarre reactor that might save nuclear fusion” 

    AAAS

    AAAS

    21 October 2015
    Daniel Clery

    1
    Adapted from IPP by C. Bickel/ Science

    If you’ve heard of fusion energy, you’ve probably heard of tokamaks. These doughnut-shaped devices are meant to cage ionized gases called plasmas in magnetic fields while heating them to the outlandish temperatures needed for hydrogen nuclei to fuse. Tokamaks are the workhorses of fusion—solid, symmetrical, and relatively straightforward to engineer—but progress with them has been plodding.

    Now, tokamaks’ rebellious cousin is stepping out of the shadows. In a gleaming research lab in Germany’s northeastern corner, researchers are preparing to switch on a fusion device called a stellarator, the largest ever built. The €1 billion machine, known as Wendelstein 7-X (W7-X), appears now as a 16-meter-wide ring of gleaming metal bristling with devices of all shapes and sizes, innumerable cables trailing off to unknown destinations, and technicians tinkering with it here and there. It looks a bit like Han Solo’s Millennium Falcon, towed in for repairs after a run-in with the Imperial fleet. Inside are 50 6-tonne magnet coils, strangely twisted as if trampled by an angry giant.

    Although stellarators are similar in principle to tokamaks, they have long been dark horses in fusion energy research because tokamaks are better at keeping gas trapped and holding on to the heat needed to keep reactions ticking along. But the Dali-esque devices have many attributes that could make them much better prospects for a commercial fusion power plant: Once started, stellarators naturally purr along in a steady state, and they don’t spawn the potentially metal-bending magnetic disruptions that plague tokamaks. Unfortunately, they are devilishly hard to build, making them perhaps even more prone to cost overruns and delays than other fusion projects. “No one imagined what it means” to build one, says Thomas Klinger, leader of the German effort.

    W7-X could mark a turning point. The machine, housed at a branch of the Max Planck Institute for Plasma Physics (IPP) that Klinger directs, is awaiting regulatory approval for a startup in November. It is the first large-scale example of a new breed of supercomputer-designed stellarators that have had most of their containment problems computed out. If W7-X matches or beats the performance of a similarly sized tokamak, fusion researchers may have to reassess the future course of their field. “Tokamak people are waiting to see what happens. There’s an excitement around the world about W7-X,” says engineer David Anderson of the University of Wisconsin (UW), Madison.

    2

    Wendelstein 7-X, the first large-scale optimized stellarator, took 1.1 million working hours to assemble, using one of the most complex engineering models ever devised, and must withstand huge temperature ranges and enormous forces.

    Stellarators face the same challenge as all fusion devices: They must heat and hold on to a gas at more than 100 million degrees Celsius—seven times the temperature of the sun’s core. Such heat strips electrons from atoms, leaving a plasma of electrons and ions, and it makes the ions travel fast enough to overcome their mutual repulsion and fuse. But it also makes the gas impossible to contain in a normal vessel.

    Instead, it is held in a magnetic cage. A current-carrying wire wound around a tube creates a straight magnetic field down the center of the tube that draws the plasma away from the walls. To keep particles from escaping at the ends, many early fusion researchers bent the tube into a doughnut-shaped ring, or torus, creating an endless track.

    But the torus shape creates another problem: Because the windings of the wire are closer together inside the hole of the doughnut, the magnetic field is stronger there and weaker toward the doughnut’s outer rim. The imbalance causes particles to drift off course and hit the wall. The solution is to add a twist that forces particles through regions of high and low magnetic fields, so the effects of the two cancel each other out.

    Stellarators impose the twist from outside. The first stellarator, invented by astro-physicist Lyman Spitzer at Princeton University in 1951, did it by bending the tube into a figure-eight shape. But the lab he set up—the Princeton Plasma Physics Laboratory (PPPL) in New Jersey—switched to a simpler method for later stellarators: winding more coils of wire around a conventional torus tube like stripes on a candy cane to create a twisting magnetic field inside.

    In a tokamak, a design invented in the Soviet Union in the 1950s, the twist comes from within. Tokamaks use a setup like an electrical transformer to induce the electrons and ions to flow around the tube as an electric current. This current produces a vertical looping magnetic field that, when added to the field already running the length of the tube, creates the required spiraling field lines.

    ITER Tokamak
    ITER tokamak

    Both methods work, but the tokamak is better at holding on to a plasma. In part that’s because a tokamak’s symmetry gives particles smoother paths to follow. In stellarators, Anderson says, “particles see lots of ripples and wiggles” that cause many of them to be lost. As a result, most fusion research since the 1970s has focused on tokamaks—culminating in the huge ITER reactor project in France, a €16 billion international effort to build a tokamak that produces more energy than it consumes, paving the way for commercial power reactors.

    But tokamaks have serious drawbacks. A transformer can drive a current in the plasma only in short pulses that would not suit a commercial fusion reactor. Current in the plasma can also falter unexpectedly, resulting in “disruptions”: sudden losses of plasma confinement that can unleash magnetic forces powerful enough to damage the reactor. Such problems plague even up-and-coming designs such as the spherical tokamak (Science, 22 May, p. 854).

    Stellarators, however, are immune. Their fields come entirely from external coils, which don’t need to be pulsed, and there is no plasma current to suffer disruptions. Those two factors have kept some teams pursuing the concept.

    The largest working stellarator is the Large Helical Device (LHD) in Toki, Japan, which began operating in 1998.

    LHD Large Helical Device stellarator
    LHD

    Lyman Spitzer would recognize the design, a variation on the classic stellarator with two helical coils to twist the plasma and other coils to add further control. The LHD holds all major records for stellarator performance, shows good steady-state operation, and is approaching the performance of a similarly sized tokamak.

    Two researchers—IPP’s Jürgen Nührenberg and Allen Boozer of PPPL (now at Columbia University)—calculated that they could do better with a different design that would confine plasma with a magnetic field of constant strength but changing direction. Such a “quasi-symmetric” field wouldn’t be a perfect particle trap, says IPP theorist Per Helander, “but you can get arbitrarily close and get losses to a satisfactory level.” In principle, it could make a stellarator perform as well as a tokamak.

    The design strategy, known as optimization, involves defining the shape of magnetic field that best confines the plasma, then designing a set of magnets to produce the field. That takes considerable computing power, and supercomputers weren’t up to the job until the 1980s.

    The first attempt at a partially optimized stellarator, dubbed Wendelstein 7-AS, was built at the IPP branch in Garching near Munich and operated between 1988 and 2002.

    Wendelstein 7-AS
    Wendelstein 7-AS

    It broke all stellarator records for machines of its size. Researchers at UW Madison set out to build the first fully optimized device in 1993. The result, a small machine called the Helically Symmetric Experiment (HSX), began operating in 1999. “W7-AS and HSX showed the idea works,” says David Gates, head of stellarator physics at PPPL.

    5
    HSX


    download the mp4 video here.

    That success gave U.S. researchers confidence to try something bigger. PPPL began building the National Compact Stellarator Experiment (NCSX) in 2004 using an optimization strategy different from IPP’s.

    5
    NCSX

    But the difficulty of assembling the intricately shaped parts with millimeter accuracy led to cost hikes and schedule slips. In 2008, with 80% of the major components either built or purchased, the Department of Energy pulled the plug on the project (Science, 30 May 2008, p. 1142). “We flat out underestimated the cost and the schedule,” says PPPL’s George “Hutch” Neilson, manager of NCSX.

    3
    IPP/Wolfgang Filser

    Wendelstein 7-X’s bizarrely shaped components must be put together with millimeter precision. All welding was computer controlled and monitored with laser scanners.

    BACK IN GERMANY, the project to build W7-X was well underway. The government of the recently reunified country had given the green light in 1993 and 1994 and decided to establish a new branch institute at Greifswald, in former East Germany, to build the machine. Fifty staff members from IPP moved from Garching to Greifswald, 800 kilometers away, and others made frequent trips between the sites, says Klinger, director of the Greifswald branch. New hires brought staff numbers up to today’s 400. W7-X was scheduled to start up in 2006 at a cost of €550 million.

    But just like the ill-fated American NCSX, W7-X soon ran into problems. The machine has 425 tonnes of superconducting magnets and support structure that must be chilled close to absolute zero. Cooling the magnets with liquid helium is “hell on Earth,” Klinger says. “All cold components must work, leaks are not possible, and access is poor” because of the twisted magnets. Among the weirdly shaped magnets, engineers must squeeze more than 250 ports to supply and remove fuel, heat the plasma, and give access for diagnostic instruments. Everything needs extremely complex 3D modeling. “It can only be done on computer,” Klinger says. “You can’t adapt anything on site.”

    By 2003, W7-X was in trouble. About a third of the magnets produced by industry failed in tests and had to be sent back. The forces acting on the reactor structure turned out to be greater than the team had calculated. “It would have broken apart,” Klinger says. So construction of some major components had to be halted for redesigning. One magnet supplier went bankrupt. The years 2003 to 2007 were a “crisis time,” Klinger says, and the project was “close to cancellation.” But civil servants in the research ministry fought hard for the project; finally, the minister allowed it to go ahead with a cost ceiling of €1.06 billion and first plasma scheduled for 2015.

    After 1.1 million construction hours, the Greifswald institute finished the machine in May 2014 and spent the past year carrying out commissioning checks, which W7-X passed without a hitch. Tests with electron beams show that the magnetic field in the still-empty reactor is the right shape. “Everything looks, to an extremely high accuracy, exactly as it should,” IPP’s Thomas Sunn Pedersen says.

    Approval to go ahead is expected from Germany’s nuclear regulators by the end of this month. The real test will come once W7-X is full of plasma and researchers finally see how it holds on to heat. The key measure is energy confinement time, the rate at which the plasma loses energy to the environment. “The world’s waiting to see if we get the confinement time and then hold it for a long pulse,” PPPL’s Gates says.

    Success could mean a course change for fusion. The next step after ITER is a yet-to-be-designed prototype power plant called DEMO. Most experts have assumed it would be some sort of tokamak, but now some are starting to speculate about a stellarator. “People are already talking about it,” Gates says. “It depends how good the results are. If the results are positive, there’ll be a lot of excitement.”

    See the full article here .

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