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.

 

 

 

 

Scientists use plasma shaping to control turbulence in stellarators (Phys. Rev. Lett.)

By John Greenwald, Princeton Plasma Physics Laboratory

Stellerator

Magnetic field strength in a turbulence-optimized stellarator design. Regions with the highest strength are shown in yellow. (Source: PPPL)

Researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) and the Max Planck Institute of Plasma Physics in Germany have devised a new method for minimizing turbulen

ce in bumpy donut-shaped experimental fusion facilities called stellarators. This week in Physical Review Letters, these authors describe an advanced application of the method that could help physicists overcome a major barrier to the production of fusion energy in such devices, and could also apply to their more widely used symmetrical donut-shaped cousins called tokamaks. This work was supported by the DOE Office of Science.

Turbulence allows the hot, charged plasma gas that fuels fusion reactions to escape from the magnetic fields that confine the gas in stellarators and tokamaks. This turbulent transport occurs at comparable levels in both devices, and has long been recognized as a challenge for both in producing fusion power economically.

“Confinement bears directly on the cost of fusion energy,” said physicist Harry Mynick, a PPPL coauthor of the paper, “and we’re finding how to reshape the plasma to enhance confinement.”

The new method uses two types of advanced computer codes that have only recently become available. The authors modified these codes to address turbulent transport, evolving the starting design of a fusion device into one with reduced levels of turbulence. The current paper applies the new method to the Wendelstein 7-X stellarator, soon to be the world’s largest when construction is completed in Greifswald, Germany.

Results of the new method, which has also been successfully applied to the design of smaller stellarators and tokamaks, suggest how reshaping the plasma in a fusion device could produce much better confinement. Equivalently, improved plasma shaping could produce comparable confinement with reduced magnetic field strength or reduced facility size, with corresponding reductions in the cost of construction and operation.

The simulations further suggest that a troublesome characteristic called “stiffness” could occur in reactor-sized stellarators. Stiffness, the tendency for heat to rapidly escape as the plasma temperature gradient rises above a threshold, has been observed in tokamaks but less so in stellarators. The possibility that stiffness might be present in reactor-sized stellarators, wrote the authors, could stimulate efforts “toward further optimizing stellarator magnetic fields for reduced turbulence.”

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.

Xanthopoulos, P.; Mynick, H.E.; Helander, P.; Turkin, Y.; Plunk, G.G.; Jenko, F.; Görler, T.; Told, D.; Bird, T.; J.H.E. Controlling turbulence in present and future stellarators. Article published in Physical Review Letters on Oct. 7, 2014.

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.

New imaging technique provides improved insight into controlling the plasma in fusion experiments (Plasma Physics and Controlled Fusion)

Graphic of fluctuating electron temperatures

Graphic representation of 2D images of fluctuating electron temperatures in a cross-section of a confined fusion plasma. (Image source: Plasma Physics and Controlled Fusion)

By John Greenwald, Office of Communications, Princeton Plasma Physics Laboratory

A key issue for the development of fusion energy to generate electricity is the ability to confine the superhot, charged plasma gas that fuels fusion reactions in magnetic devices called tokamaks. This gas is subject to instabilities that cause it to leak from the magnetic fields and halt fusion reactions.

Now a recently developed imaging technique can help researchers improve their control of instabilities. The new technique, developed by physicists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), the University of California-Davis and General Atomics in San Diego, provides new insight into how the instabilities respond to externally applied magnetic fields.

This technique, called Electron Cyclotron Emission Imaging (ECEI) and successfully tested on the DIII-D tokamak at General Atomics, uses an array of detectors to produce a 2D profile of fluctuating electron temperatures within the plasma. Standard methods for diagnosing plasma temperature have long relied on a single line of sight, providing only a 1D profile. Results of the ECEI technique, recently reported in the journal Plasma Physics and Controlled Fusion, could enable researchers to better model the response of confined plasma to external magnetic perturbations that are applied to improve plasma stability and fusion performance.

PPPL is managed by Princeton University.

Read the abstract.

B.J. Tobias; L.Yu; C.W. Domier; N.C. Luhmann, Jr; M.E. Austin; C. Paz-Soldan; A.D. Turnbull; I.G.J. Classen; and the DIII-D Team. 2013. Boundary perturbations coupled to core 3/2 tearing modes on the DIII-D tokamak. Plasma Physics and Controlled Fusion. Article first published online: July 5, 2013. DOI:10.1088/0741-3335/55/9/095006

This work was supported in part by the US Department of Energy under DE-AC02- 09CH11466, DE-FG02-99ER54531, DE-FG03-97ER54415, DE-AC05-00OR23100, DE- FC02-04ER54698, and DE-FG02-95ER54309.