PPPL researcher maps magnetic fields in first physics experiment on W7-X
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. 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.”
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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