A Collaborative National Center for Fusion & Plasma Research

APS 2012

These press releases were prepared by the APS with the assistance of the scientists quoted as well as writers in the PPPL Office of Communications including Kitta MacPherson, John Greenwald and Jeanne Jackson DeVoe.

What a cup of coffee tells scientists about solar storms

A new theory asserts that a key astrophysical process parallels what happens at the breakfast table.

Solar stormCoffeeSolar storm, cream in coffee.

Magnetic fields lines in a highly conducting gas called plasma can be an explosive mix. Solar storms, which involve rapid intermixing of magnetic field lines from different parts of the outer plasma layers of the sun, have disrupted power systems on Earth and interrupted satellite communications. The sudden intermixing of the lines from different sources of magnetic field, which is called magnetic reconnection, has been recognized as a central element in many astrophysical phenomena for more than 60 years.

“Existing models of magnetic reconnection can be misleading,” says Allen Boozer, a Columbia University professor and long-term visitor at PPPL. Such models view space as if it had only two dimensions, or focus on too little of the region where reconnection takes place, as he states in a paper published in the September issue of Physics of Plasmas.

When magnetic field lines from different sources are pushed together, such as lines from the sun and the Earth, electric currents naturally arise around the contour—or boundary— that separates the two sets of lines. These currents prevent the lines from intermixing but allow large amounts of energy to be stored, which is released if the field lines ever do intermix.

As these currents act to prevent intermixing, they also cause the contour between the magnetic field lines to become contorted and exponentially longer—even billions of times longer—than the width of the region in which the current flows. This happens in three dimensions but not in two. Even reconnection experts are surprised by this enormous increase in the contour length, Boozer says. In a paper that has been accepted for publication in the Physics of Plasmas, he explains why this extreme contour lengthening occurs in realistic three-dimensional space but is missed by two-dimensional models.

Boozer compares magnetic reconnection to what happens with cream in coffee. There is little interdiffusion—or intermixing—of the coffee and cream at first. But even a gentle stirring makes the contour between the coffee and cream grow longer and longer. Soon the length of the contour multiplied by the small distance over which coffee and cream interdiffuse equals the area across the cup—and the coffee and cream completely mix.

As in the cream in coffee example, Boozer notes, the field lines in plasmas of high electrical conductivity can interdiffuse only a small distance across the contour that separates them. Nevertheless, a sudden reconnection of the lines takes place when the product of this small interdiffusion distance times the exponentially increasing length of the contour becomes comparable to the area of the region in which the electric currents flow.

The primary difference between these two mixing examples is that the contour between cream and coffee lengthens as time advances. But for magnetic reconnection, the contour lengthens as the field lines are followed, at any instant of time, through the region in which the electric currents are flowing.

Boozer says, gently stir your coffee and understand magnetic reconnection.

Paper author: A. Boozer

Halo-current effects in tokamak reactors: hardly heavenly

Physicists at PPPL decipher the shape and movement of reactor-squeezing ropes of current.

Plasma physicists and fusion reactor engineers call them “halo currents,” but they are hardly angelic.

These powerful currents occur under certain rare fault conditions known as “disruptions.” If unchecked, they can damage components located inside reactor vacuum vessels. But their shape and form are not well known. Do they form thick ropes or are they more like wide ribbons? How fast do they fly around the tokamaks used to confine hot ionized gases known as plasmas?

While reliable methods have been developed to reduce these currents to acceptable levels, scientists want to learn more about the halo currents to improve the design of tokamak fusion reactors.

Now, physicists at PPPL, using a ring of specially designed detectors on the National Spherical Torus Experiment, are closing in on some answers. Experiments show that halo currents flow in concentrated bands and move quickly, rotating as many as eight times around the vacuum vessel’s inner chamber.

“Improving the understanding of these currents can have important implications for reactor designs,” said Stefan Gerhardt, who led the effort to install the sensors that measured the currents. “Engineers will be able to design better experiments if they understand the shape of the currents and the forces they exert on the vacuum vessel.”

Doughnut-shaped fusion devices, known as tokamaks, use strong electrical currents to generate the magnetic fields used to confine the plasma. When key components fail or tokamak operators push against known limits, the heat and current in the plasma can suddenly dissipate, a phenomenon known as a “disruption.” The resulting halo currents flowing in the hot ionized gases can strike the wall of the vessel, possibly causing local damage. If the halo currents flow around the chamber wall many times, and at a rate similar to the natural vibration frequencies of the chamber, they can distort the vessel.

Evolution of the plasma during a disruption, as captured by a fast color camera. The plasma is centered in the left-hand frame, moves downward in the center frame, and lands on the floor of the vacuum chamber in the right frame. Large “halo currents” are observed in the indicated ring of detectors at this time.

“Fortunately, we have observed that the instances with the strongest currents are often those with the smallest motion of the currents,” Gerhardt said. “This trend, along with the observations from other tokamaks that these currents can be significantly reduced with strong gas puffing, should diminish the potential for ‘resonance’ damage to the fusion plant from these currents.”

Paper authors: S. P. Gerhardt, J. Breslau, E. Fredrickson, S. Jardin, R. Kaita, J. Manickam, J. Menard, S. Sabbagh, F. Scotti, H. Takahashi, A. H. Boozer

Scientists find a shortcut to map conditions for sustainable fusion

Results derived from a new computer code are the first to map the full range of conditions required to reliably and safely sustain fusion reactions by alpha particle heating, and could facilitate the development of fusion as a clean and abundant source of energy.

Fusion takes place when the atomic nuclei—or ions—in hot, electrically-charged plasma fuse and release energy. The temperatures in these fusion plasmas can reach more than 100 million degrees, igniting the plasma and producing high-energy alpha particles so the plasma heats itself. One challenge for a fusion reactor is how to contain the alpha particles in the vessel long enough for them to efficiently heat the plasma. These fast particles can excite waves in the plasma and be lost, or transported, to the vessel wall rather quickly, much in the same way a surfer rides waves to the beach.

Scientists at PPPL have collaborated with colleagues from other leading U.S. research institutions to develop a method for rapidly distinguishing among various plasma conditions—forming a type of map that highlights regions where alpha particles will likely be well confined and fusion can safely and reliably take place. The new method could help pave the way to the design and construction of fusion devices that can produce a steady flow of fusion energy for generating electricity. Such devices include ITER, a huge international project that is being built in France to demonstrate the practicality of fusion power.

The new study illustrates for the first time the full range of plasma conditions needed to maintain a self-sustaining fusion reaction, and delineates the regions where fast ion driven waves can occur inside the plasma causing unacceptable alpha particle transport.

“We have found a relatively simple way to quantify these requirements in terms of such easily accessible parameters as plasma pressure, temperature and density,” said Nikolai Gorelenkov, an author of the paper that describes the method, which was published online in the journal Physics of Plasmas in August.

The researchers used a new computer code to calculate and plot the regions where fusion reactions can and cannot be easily maintained. The results demonstrate the correct combinations of temperature and a quantity called “beta”—the ratio of the pressure of the plasma to the pressure of the magnetic field that confines it—that are required to have good control of the alpha particles and keep the fusion reaction going.

The importance of identifying the regions where fusion can take place cannot be overstated when building new devices,” said Gorelenkov.

In the past, however, a single point on that figure would have taken the largest supercomputers in the world to calculate self- consistently. Now, with this simplified model, the entire “map” can be produced on a personal computer.

Note: Collaborative work on this paper came from the Institute for Fusion Studies at The University of Texas at Austin; the DIII-D research project at General Atomics in San Diego; and the University of California, Irvine.

Paper authors: K. Ghantous, N. N. Gorelenkov H. L. Berk, W. W. Heidbrink, M. A. Van Zeeland

Fusion plasma works best just where you least expect it

Scientists measure a surprising increase in fusion plasma stability at high performance.

A key challenge for fusion researchers has been to maintain the stability of the magnetically confined plasma gas that fuels fusion reactions as scientists elevate the plasma pressure to generate very large fusion power. Now scientists at PPPL have measured an increase in plasma stability where it is least expected—at the high pressure that produces high fusion plasma performance.

This work challenges past scientific thought that efficient, high fusion power performance needs to be sacrificed to sustain continuous energy production.

“Up to this point, scientists have believed that fusion power production needed to be reduced to sustain the energy output,” said Steven Sabbagh of Columbia University, who is on long-term appointment at PPPL’s National Spherical Torus Experiment (NSTX). “This result shows that you can indeed have both. The present result is somewhat like finding just the right way to stir a cooking pot to keep it hot, but prevent it from boiling over.”

Fusion powers the sun and stars. The process takes place when the atomic nuclei—or ions—in the plasma fuse and release energy. Scientists seek to reproduce this process on Earth by creating and controlling plasma inside powerful magnetic fields in fusion devices called tokamaks. Harnessing fusion in this way could produce a virtually limitless supply of clean energy for generating electricity.

Sabbagh and John Berkery of Columbia University, who also is on long-term appointment at PPPL, demonstrated their surprising result with the assistance of co-workers, and now are studying the process theoretically. The researchers performed their experiments on the NSTX before the facility was closed last year for an upgrade that is scheduled for completion in 2014.

The experiments turned up the plasma performance to a very high level that had been thought to be less stable. By using a special stability measurement technique, the scientists surprisingly found that these high performance plasmas in fact became more stable. Present analysis shows that a specific way the plasma rotates inside the machine creates the improved stability.

PPPL Director Stewart Prager remarked, “An historic challenge has been to elevate the plasma pressure to generate very large fusion power and keep the plasma stable. We are encouraged by this fascinating new physics regime in which the pressure is very high, but the plasma stability increases.”

The result is especially good news, as it may have uncovered a new way to design devices with both high fusion performance and high stability. “Understanding this result may allow us to create similar rotation conditions in new machine designs that may finally produce more energy than they use,” Sabbagh said. “Fusion devices have been significantly limited by stability, and creating the conditions needed to produce this enhanced stability may allow these devices to run more economically.”

Paper authors: S.A. Sabbagh, J. W. Berkery

Elements Duke It Out to Penetrate Hot Plasma

Scientists get a better understanding of why some atoms get into the core of their magnetic fusion experiments more easily than do others.

Scientists at PPPL used fast cameras that take up to 100,000 frames per second and computer codes to understand how impurities are produced and can penetrate the core of the hot ionized gas known as plasma during magnetic fusion experiments in the National Spherical Torus Experiment (NSTX).

Impurities can cool the plasma so their?accumulation has to be avoided in order to?heat the plasma to at least 100 million?degrees Celsius. These temperatures are?needed in order for the fuel ions to collide?into each other at a high speed and fuse?together to create magnetic fusion. To?understand the sources of impurities, ?scientists placed high-speed cameras viewing?the interior of NSTX during fusion?experiments, along with spectrometers that?capture the unique light wave produced by?the atoms. They were then able to trace what?happens when plasma hits the walls of the?vessel that holds the plasma inside a?magnetic field. In NSTX, in particular, the?walls of the tokamak are made of graphite?and are sprayed with a coating of lithium?before each plasma experiment. When the?plasma hits the walls of the tokamak, it?causes lithium and carbon atoms to bounce off the walls and potentially penetrate the core plasma.

“The idea was to start looking at all the processes that cause impurities to be released at the wall and eventually penetrate the plasma,” explained Filippo Scotti, a scientist on the project.

Photo of the interior of the NSTX fusion experiment showing light emitted by the hydrogen, carbon, and lithium atoms in the cooler edge regions of the plasma near where it contacts interior surfaces.

The researchers found there were numerous carbon atoms in the plasma core but very few lithium atoms, despite the layer of lithium on top of the graphite. The lithium erodes fairly quickly from the tokamak walls, exposing the graphite beneath it, but travels only a short distance before bouncing back to the tokamak floor (called a divertor). The lithium ions are then “trapped” more efficiently than carbon in the divertor region. The carbon atoms can escape more easily, travel along the magnetic lines that contain the plasma and then penetrate the plasma core.

Scientists not only used a fast camera and spectrometers, they also used computer codes to explain how these processes work, leading to the identification of other processes which limit the penetration of lithium ions in the core. In particular, the few lithium ions that make it to the core are dispersed by collisions of carbon ions, which have a higher charge, thereby leading to lower accumulation.

Understanding the processes that generate and transport impurities into the plasma could help scientists find ways to further improve plasma performance. They could investigate using other materials to replace the graphite used in the tokamak, for example, or they could investigate other plasma configurations that would reduce the number of atoms that hit the tokamak walls and floor. In particular this work also highlights the potential of lithium as a material for a reactor first wall thanks to the very low core penetration of lithium ions.

Paper authors: F. Scotti, V. A. Soukhanovskii, R. Kaita

Shedding light on the explosive process behind space plasmas and solar flares

Experiments show the impact of a key magnetic-field component on the speed of magnetic reconnection—the process that triggers solar outbursts.

PPPL scientists have shown quantitatively for the first time in a laboratory plasma how the presence of an external guide field affects the rate of magnetic reconnection—one of the most common but least understood phenomena in the universe, and one that gives rise to such events as auroras, solar flares and geomagnetic storms.

Magnetic reconnection takes place when the magnetic field lines in electrically charged gas called plasma snap apart and reconnect with violent force. To date, laboratory experiments have reproduced this process by focusing on field lines that merge in an anti-parallel manner as shown below. But actual reconnection takes place in most cases when the merging field lines come together at distinct angles rather than in an anti-parallel way.

 Oppositely directed field lines (measured) merge and reconnect.

Magnetic reconnection seen in MRX: Oppositely directed field lines (measured) merge and reconnect.

The PPPL researchers set out to explore the impact of guide fields on the speed of reconnection by introducing them into the Magnetic Reconnection Experiment (MRX), which recreates reconnection in the laboratory. The studies found that guide fields can sharply reduce the reconnection rate in plasmas. This finding helps to explain the fact that reconnection in the plasma in the Earth’s magnetosphere—a magnetic region above the Earth where strong field guides are present—takes place far more slowly than in anti-parallel reconnection. The results also shed light on solar and astrophysical plasmas and could lead to better predictions of disruptive solar flares and geomagnetic storms.

The researchers tested different strengths of guide fields by systematically altering the angle at which they intersected the reconnecting field lines. Strengthening the guide fields led to smaller merging angles and significantly slowed the rate at which reconnection took place. The results can be quantitatively compared with 2-D numerical simulations in the future.

The MRX experiment at PPPL produces magnetic field-line reconnection similar to what happens in solar flares and in the Earth’s magnetosphere.

“Lots of theories have been developed for zero guide-field reconnection,” said PPPL physicist Masaaki Yamada, the principal researcher for the MRX and coauthor of a paper on the experiment that has been accepted for publication by Physical Review Letters. “But here for the first time we have systematically added the guide field and studied its effects quantitatively. In nature reconnection always has some component of the guide field, so this makes the experiment more realistic.”

Paper authors: T. Tharp, M. Yamada, H. Ji, E. Lawrence, S. Dorfman, C. Myers and J. Yoo

Here comes the sun: How solar flares happen faster than forecast

Simulations show that magnetic reconnection can take place in dense plasmas more quickly than the conventional wisdom holds.

Scientists have struggled to predict how quickly solar flares can detach from the sun and launch the coronal mass ejections that create spectacular auroras and can disrupt communication systems and the nationwide electrical power grid. New computer simulations performed by scientists at the University of New Hampshire (UNH) and PPPL suggest that conventional theory misses an important effect that drastically speeds up the process.

Solar flares detach from the sun through a process called magnetic reconnection, a common occurrence in space plasmas. The process takes place when the magnetic field lines in the electrically charged plasma of solar flares are stretched, break apart, and reconnect—leaving the plasma to fly off into space.

“Conventional wisdom asserts that reconnection happens slowly when the plasma density is high,” said Yi-Min Huang, a UNH research scientist and principal author of the paper. “What our computer simulation tells us is that slow reconnection basically doesn’t exist.”

At left,a plasmoid observed as a bright moving blob in the current sheet formed after a coronal mass ejection event, or massive release of plasma from the sun. At right, a computer simulation of plasmoid formation in a similar coronal mass ejection.

The new calculations reveal that the region in space where the reconnection takes place is much less stable than conventional theory predicts. The numerical simulations show the formation of a chain of plasmoids, or separate cylinders of plasma, during the reconnection process (see figure). Though such plasmoids have been seen in images of solar flares, according to Huang their possible role in speeding up reconnection had not been recognized.

Paper authors: A. Bhattacharjee, Lijia Guo, Y-M Huang and B. P. Sullivan

U.S. Department of Energy
Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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