Scientists make breakthrough in understanding how to control intense heat bursts in fusion experiments (Physical Review Letters)

Computer simulation

Computer simulation of a cross-section of a DIII-D plasma responding to tiny magnetic fields. The left image models the response that suppressed the ELMs while the right image shows a response that was ineffective. Simulation by General Atomics.

By Raphael Rosen, Princeton Plasma Physics Laboratory

Researchers from General Atomics and the U.S. Department of Energy (DOE)’s Princeton Plasma Physics Laboratory (PPPL) have made a major breakthrough in understanding how potentially damaging heat bursts inside a fusion reactor can be controlled. Scientists performed the experiments on the DIII-D National Fusion Facility, a tokamak operated by General Atomics in San Diego. The findings represent a key step in predicting how to control heat bursts in future fusion facilities including ITER, an international experiment under construction in France to demonstrate the feasibility of fusion energy. This work is supported by the DOE Office of Science (Fusion Energy Sciences).

The studies build upon previous work pioneered on DIII-D showing that these intense heat bursts – called “ELMs” for short – could be suppressed with tiny magnetic fields. These tiny fields cause the edge of the plasma to smoothly release heat, thereby avoiding the damaging heat bursts. But until now, scientists did not understand how these fields worked. “Many mysteries surrounded how the plasma distorts to suppress these heat bursts,” said Carlos Paz-Soldan, a General Atomics scientist and lead author of the first of the two papers that report the seminal findings back-to-back in the March 12 issue of Physical Review Letters.

Paz-Soldan and a multi-institutional team of researchers found that tiny magnetic fields applied to the device can create two distinct kinds of response, rather than just one response as previously thought. The new response produces a ripple in the magnetic field near the plasma edge, allowing more heat to leak out at just the right rate to avert the intense heat bursts. Researchers applied the magnetic fields by running electrical current through coils around the plasma. Pickup coils then detected the plasma response, much as the microphone on a guitar picks up string vibrations.

The second result, led by PPPL scientist Raffi Nazikian, who heads the PPPL research team at DIII-D, identified the changes in the plasma that lead to the suppression of the large edge heat bursts or ELMs. The team found clear evidence that the plasma was deforming in just the way needed to allow the heat to slowly leak out. The measured magnetic distortions of the plasma edge indicated that the magnetic field was gently tearing in a narrow layer, a key prediction for how heat bursts can be prevented.  “The configuration changes suddenly when the plasma is tapped in a certain way,” Nazikian said, “and it is this response that suppresses the ELMs.”

Paz-Soldan and Nazikian

Carlos Paz-Soldan, left, and Raffi Nazikian at the DIII-D tokamak. (Photo by Lisa Petrillo/General Atomics)

The work involved a multi-institutional team of researchers who for years have been working toward an understanding of this process. These researchers included people from General Atomics, PPPL, Oak Ridge National Laboratory, Columbia University, Australian National University, the University of California-San Diego, the University of Wisconsin-Madison, and several others.

The new results suggest further possibilities for tuning the magnetic fields to make ELM-control easier. These findings point the way to overcoming a persistent barrier to sustained fusion reactions. “The identification of the physical processes that lead to ELM suppression when applying a small 3D magnetic field to the inherently 2D tokamak field provides new confidence that such a technique can be optimized in eliminating ELMs in ITER and future fusion devices,” said Mickey Wade, the DIII-D program director.

The results further highlight the value of the long-term multi-institutional collaboration between General Atomics, PPPL and other institutions in DIII-D research. This collaboration, said Wade, “was instrumental in developing the best experiment possible, realizing the significance of the results, and carrying out the analysis that led to publication of these important findings.”

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 the University for the U.S. Department of Energy’s Office of Science, which is the largest 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.

General Atomics has participated in fusion research for over 50 years and presently operates the DIII-D National Fusion Facility for the U.S. Department of Energy Office of Science with a mission “to provide the physics basis for the optimization of the tokamak approach to fusion energy production.”  The General Atomics group of companies is a world renowned leader in developing high-technology systems ranging from the nuclear fuel cycle to electromagnetic systems; remotely operated surveillance aircraft; airborne sensors; advanced electronic, wireless, and laser technologies; and biofuels.

Read the articles:

C. Paz-Soldan, R. Nazikian, S. R. Haskey, N. C. Logan, E. J. Strait, N. M. Ferraro, J. M. Hanson, J. D. King, M. J. Lanctot, R. A. Moyer, M. Okabayashi, J-K. Park, M. W. Shafer, and B. J. Tobias. Observation of a Multimode Plasma Response and its Relationship to Density Pumpout and Edge-Localized Mode Suppression. Phys. Rev. Lett. 114, 105001 – Published 12 March 2015.

R. Nazikian, C. Paz-Soldan, J. D. Callen, J. S. deGrassie, D. Eldon, T. E. Evans, N. M. Ferraro, B. A. Grierson, R. J. Groebner, S. R. Haskey, C. C. Hegna, J. D. King, N. C. Logan, G. R. McKee, R. A. Moyer, M. Okabayashi, D. M. Orlov, T. H. Osborne, J-K. Park, T. L. Rhodes, M. W. Shafer, P. B. Snyder, W. M. Solomon, E. J. Strait, and M. R. Wade. Pedestal Bifurcation and Resonant Field Penetration at the Threshold of Edge-Localized Mode Suppression in the DIII-D Tokamak. Phys. Rev. Lett. 114, 105002 – Published 12 March 2015.





PPPL scientists take key step toward solving a major astrophysical mystery

By John Greenwald, Princeton Plasma Physics Laboratory

Magnetic reconnection in the Earth and sun’s atmospheres can trigger geomagnetic storms that disrupt cell phone service, damage satellites and blackout power grids. Understanding how reconnection transforms magnetic energy into explosive particle energy has been a major unsolved problem in plasma astrophysics.

Scientists at the Princeton Plasma Physics Laboratory (PPPL) and Princeton University have taken a key step toward a solution, as described in a paper published this week in the journal Nature Communications. In research conducted on the Magnetic Reconnection Experiment (MRX) at PPPL, the scientists not only identified how the mysterious transformation takes place, but measured experimentally the amount of magnetic energy that turns into particle energy. The work is supported by the U. S. Department of Energy as well as the NSF-funded Center for Magnetic Self-Organization.

Fast-camera image of plasma during magnetic reconnection with rendering of the field lines, shown in white, based on measurements made during the experiment. The converging horizontal lines represent the field lines prior to reconnection. The outgoing vertical lines represent the field lines after reconnection. Image courtesy of Jongsoo Yoo.

Fast-camera image of plasma during magnetic reconnection with rendering of the field lines, shown in white, based on measurements made during the experiment. The converging horizontal lines represent the field lines prior to reconnection. The outgoing vertical lines represent the field lines after reconnection. Image courtesy of Jongsoo Yoo.

Magnetic field lines represent the direction, and indicate the shape, of magnetic fields. In magnetic reconnection, the magnetic field lines in plasma snap apart and violently reconnect. The MRX, built in 1995, allows researchers to study the process in a controlled laboratory environment.

The new research shows that reconnection converts about 50 percent of the magnetic energy, with one-third of the conversion heating the electrons and two-thirds accelerating the ions — or atomic nuclei — in the plasma. In large bodies like the sun, such converted energy can equal the power of millions of tons of TNT.

“This is a major milestone for our research,” said Masaaki Yamada, a research physicist, the principal investigator for the MRX and first author of the Nature Communications paper. “We can now see the entire picture of how much of the energy goes to the electrons and how much to the ions in a proto-typical reconnection layer.”

The findings also suggested the process by which the energy conversion occurs. Reconnection first propels and energizes the electrons, according to the researchers, and this creates an electrically charged field that “becomes the primary energy source for the ions,” said Jongsoo Yoo, an associate research physicist at PPPL and co-author of the paper.

The other contributors to the paper were Hantao Ji, professor of astrophysical sciences at Princeton; Russell Kulsrud, professor of astrophysical sciences, emeritus, at Princeton; and doctoral candidates in astrophysical sciences Jonathan Jara-Almonte and Clayton Myers.

If confirmed by data from space explorations, the PPPL results could help resolve decades-long questions and create practical benefits. These could include a better understanding of geomagnetic storms that could lead to advanced warning of the disturbances and an improved ability to cope with them. Researchers could shut down sensitive instruments on communications satellites, for example, to protect the instruments from harm.

Next year NASA plans to launch a four-satellite mission to study reconnection in the magnetosphere — the magnetic field that surrounds the Earth. The PPPL team plans to collaborate with the venture, called the Magnetospheric Multiscale (MMS) Mission, by providing MRX data to it. The MMS probes could help to confirm the laboratory’s findings.

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, 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. Fusion takes place when atomic nuclei fuse and release a burst of energy. This compares with the fission reactions in today’s nuclear power plants, which operate by splitting atoms apart.

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 single largest 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.

Read the abstract.

Yamada, M; Yoo J.; Jara-Almonte, J.; Ji, H.; Kulsrud, R.M.; Myers, C.E. Conversion of magnetic energy in the magnetic reconnection layer of a laboratory plasma. Nature Communications. Article published online Sept. 10, 2014. DOI: NCOMMS5774


A promising concept on the path to fusion energy (IEEE Transactions on Plasma Science)

by John Greenwald, Princeton Plasma Physics Laboratory

QUASAR stellerator design

QUASAR stellerator design (Source: PPPL)

Completion of a promising experimental facility at the U.S. Department of Energy’s Princeton Plasma Laboratory (PPPL) could advance the development of fusion as a clean and abundant source of energy for generating electricity, according to a PPPL paper published this month in the journal IEEE Transactions on Plasma Science.

The facility, called the Quasi-Axisymmetric Stellarator Research (QUASAR) experiment, represents the first of a new class of fusion reactors based on the innovative theory of quasi-axisymmetry, which makes it possible to design a magnetic bottle that combines the advantages of the stellarator with the more widely used tokamak design. Experiments in QUASAR would test this theory. Construction of QUASAR — originally known as the National Compact Stellarator Experiment — was begun in 2004 and halted in 2008 when costs exceeded projections after some 80 percent of the machine’s major components had been built or procured.

“This type of facility must have a place on the roadmap to fusion,” said physicist George “Hutch” Neilson, the head of the Advanced Projects Department at PPPL.

Both stellarators and tokamaks use magnetic fields to control the hot, charged plasma gas that fuels fusion reactions. While tokamaks put electric current into the plasma to complete the magnetic confinement and hold the gas together, stellarators don’t require such a current to keep the plasma bottled up. Stellarators rely instead on twisting — or 3D —magnetic fields to contain the plasma in a controlled “steady state.”

Stellarator plasmas thus run little risk of disrupting — or falling apart — as can happen in tokamaks if the internal current abruptly shuts off. Developing systems to suppress or mitigate such disruptions is a challenge that builders of tokamaks like ITER, the international fusion experiment under construction in France, must face.

Stellarators had been the main line of fusion development in the 1950s and early 1960s before taking a back seat to tokamaks, whose symmetrical, doughnut-shaped magnetic field geometry produced good plasma confinement and proved easier to create. But breakthroughs in computing and physics understanding have revitalized interest in the twisty, cruller-shaped stellarator design and made it the subject of major experiments in Japan and Germany.

PPPL developed the QUASAR facility with both stellarators and tokamaks in mind. Tokamaks produce magnetic fields and a plasma shape that are the same all the way around the axis of the machine — a feature known as “axisymmetry.” QUASAR is symmetrical too, but in a different way. While QUASAR was designed to produce a twisting and curving magnetic field, the strength of that field varies gently as in a tokamak — hence the name “quasi-symmetry” (QS) for the design.  This property of the field strength was to produce plasma confinement properties identical to those of tokamaks.

“If the predicted near-equivalence in the confinement physics can be validated experimentally,” Neilson said, “then the development of the QS line may be able to continue as essentially a ‘3D tokamak.’”

Such development would test whether a QUASAR-like design could be a candidate for a demonstration — or DEMO —fusion facility that would pave the way for construction of a commercial fusion reactor that would generate electricity for the power grid.

Read the paper.

George Neilson, David Gates, Philip Heitzenroeder, Joshua Breslau, Stewart Prager, Timothy Stevenson, Peter Titus, Michael Williams, and Michael Zarnstorff. Next Steps in Quasi-Axisymmetric Stellarator Research IEEE Transactions on Plasma Science, vol. 42, No. 3, March 2014.

The research was supported by the U.S. Department of Energy under contract DE-AC02 09CH11466. Princeton University manages PPPL, which is part of the national laboratory system funded by the U.S. Department of Energy through the Office of Science.