PPPL By Jeanne Jackson DeVoe December 14, 2015
Scientists from the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) and other U.S. institutions joined colleagues from around the world at the celebration for the first plasma of the Wendelstein 7-X (W7-X) stellarator at the Max Planck Institute in Greifswald, Germany. The Dec. 10 event heralded the start of the largest and most advanced fusion experiment of its kind in the world and could yield promising solutions to some of the most difficult challenges in developing fusion energy.
PPPL physicists David Gates, Novimir Pablant, and Samuel Lazerson, who have collaborated on the machine for the past five years, were in Greifswald to witness the celebration firsthand. They joined dozens of researchers and a dozen news crews in the W7-X control room as it counted down from 10 to the first plasma. An image of the first plasma flashed briefly on a large overhead screen and then resolved into quadruple images of the white, glowing plasma. Meanwhile, a dozen or so staffers at PPPL got up early to watch the event, which was live-streamed to the Laboratory at 7 a.m.
The first helium plasma in the machine lasted a tenth of a second and achieved a temperature of 1 million degrees, according to the Max Planck Institute website. That was enough to declare success after more than 10 years of construction and nearly 20 years of planning as well as an investment of 1 billion Euros (1.09 billion dollars) and more than a million hours of assembly.
“The energy was intense,” said Lazerson, who has been working at the W7-X since March with a team that has been designing and analyzing experiments that map the stellarator’s magnetic field. “Everyone was hopeful and very excited every time there was a new pulse. So it was fun!”
Gates, the stellarator physics leader at PPPL, was equally excited. “It’s very gratifying to have the opportunity to work on such an exciting experiment,” he said. Added Pablant, who led PPPL’s development of an X-ray crystal spectrometer for W7-X: “They’ve been very welcoming to us as part of the team. It’s a very good feeling to be here.”
Glen Wurden, a physicist at Los Alamos National Laboratory (LANL) was also at the event. “It’s great!” he said. “It’s a beautiful lab and a wonderful machine and we’re excited to be part of it.”
Nothing but a win-win
“W7-X is extremely important,” said Hutch Neilson, head of Advanced Projects at PPPL. “We are fortunate to be a part of it and they are fortunate to have us as a partner. This is nothing but a win-win. Stellarator research is that important and right now this is our opportunity to be involved at the world forefront of stellarator research.”
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. The W7-X will be the first optimized stellarator 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.
Donut-shaped tokamaks have traditionally been 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, Gates said. A major aim of the research program is to see if it can operate in a steady state at high performance without disruptions and without needing to drive a current into the plasma. The promise for this resides in the fact that its twisted internal coils provide the helical magnetic field. “Because we can now optimize stellarators for confinement, they have the potential of catching up to tokamaks in performance,” Gates said.
PPPL leads U.S. effort
PPPL leads the U.S. collaboration with W7-X, which is funded at over $4 million annually from the Department of Energy’s Fusion Energy Sciences office. The Laboratory built some key components of the machine, which was planned for nearly ten years before construction began and 1 billion Euros to build. Collaborators include researchers from LANL and Oak Ridge National Laboratory, as well as researchers and students from MIT, the University of Wisconsin, Auburn University, and Xantho Technologies, LLC.
The first contribution to the experiment made by PPPL physicists and engineers was designing and delivering the five massive 2,400-pound trim coils that fine-tune the shape of the plasma in fusion experiments. Lazerson recently used the field coils to map the magnetic field on the device, proving that the main magnet system is working as intended.
Pablant said he would look at results from the first plasma measured by a diagnostic device called an X-ray imaging crystal spectrometer that he and PPPL engineer Michael Mardenfeld designed and built. It is one of several diagnostics created by U.S. researchers that will analyze experiments on W7-X.
PPPL engineers led by Doug Loesser are building a third major contribution by PPPL: two divertor scraper units. The device, designed in collaboration with Oak Ridge, intercepts heat coming from the plasma to protect the W7-X divertor targets from damage.
Focus on different kind of stellarator
Neilson said research at PPPL has primarily focused on a different type of stellarator called a quasi-axisymmetric stellarator. PPPL built one such device several years ago but halted construction in 2008 due to funding issues. “Right now we’re just beginning to scope out the program requirements and what we call the mission-need case for a new stellarator initiative,” Neilson said.
W7-X will continue running until just before Christmas, when it will close for the holidays and reopen at the beginning of January. Its next task will be to extend the duration of the plasma and to do research to prepare for the first plasma from hydrogen fuel. Lazerson will remain at the site until March, 2016, to test the effect of the trim coils on the plasma.
Pablant, along with a student from Auburn University, will be traveling back and forth to W7-X until March to operate the X-ray crystal spectrometer. It will obtain high-resolution measurements of the temperature and velocity of plasma ions that will be used to study the plasma physics. Gates will continue overseeing PPPL work on W7-X and other stellarator projects.
Champagne was poured in the W7-X control room on Dec. 10 and more parties were on tap before work continued on Dec. 11. The PPPL physicists savored the moment. “This is awesome,” said Lazerson. “He just summarized what I said!” said Gates. “That was a very accurate summary,” Pablant added.
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 (link is external).