Scientists propose an explanation for electron heat loss in fusion plasmas (Physical Review Letters)

By Raphael Rosen, Princeton Plasma Physics Laboratory

Elena Belova

PPPL Scientist Elena Belova
Photo Credit: Elle Starkman, PPPL

Creating controlled fusion energy entails many challenges, but one of the most basic is heating plasma – hot gas composed of electrons and charged atoms – to extremely high temperatures and then maintaining those temperatures. Now scientist Elena Belova of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and a team of collaborators have proposed an explanation for why the hot plasma within fusion facilities called tokamaks sometimes fails to reach the required temperature, even as researchers pump beams of fast-moving neutral atoms into the plasma in an effort to make it hotter.

The results, published in June in Physical Review Letters, could lead to improved control of temperature in future fusion devices, including ITER, the international fusion facility under construction in France to demonstrate the feasibility of fusion power. This work was supported by the DOE Office of Science (Office of Fusion Energy Sciences).

The researchers focused on the puzzling tendency of electron heat to leak from the core of the plasma to the plasma’s edge. “One of the largest remaining mysteries in plasma physics is how electron heat is transported out of plasma,” said Jon Menard, program director for PPPL’s major fusion experiment, the National Spherical Tokamak Experiment-Upgrade (NSTX-U), which is completing a $94 million upgrade.

Belova hit upon a possible answer while performing 3D simulations of past NSTX plasmas on computers at the National Energy Research Scientific Computing Center (NERSC), in Oakland, California. She saw that two kinds of waves found in fusion plasmas appear to form a chain that transfers the neutral-beam energy from the core of the plasma to the edge, where the heat dissipates. While physicists have long known that the coupling between the two kinds of waves – known as compressional Alfvén waves and kinetic Alfvén waves (KAWs) – can lead to energy dissipation in plasmas, Belova’s results were the first to demonstrate the process for beam-excited compressional Alfvén eigenmodes (CAEs) in tokamaks.

Her simulations showed that when researchers try to heat the plasma by injecting beams of energetic deuterium, a form of hydrogen, the beams excite CAE waves in the plasma’s core. Those waves then resonate with KAW waves, which occur primarily at the plasma’s edge. As a result, the energy is transported from the injection site deep within the plasma to the plasma’s edge.

“Originally, when scientists found that the electron temperature wouldn’t go up with increased beam power, everybody assumed that the electrons were getting heated at the plasma’s center and then were somehow losing that heat,” Belova said. “Our explanation is different. We propose that part of the beam energy goes into CAEs and then to KAWs. The energy then dissipates at the plasma’s edge.”

The simulations provided a broad perspective. “In simulations you can look everywhere in a plasma,” Belova said. “In the experiments, on the other hand, you are very limited in what and where you can measure inside the hot plasma.”

Belova’s findings reflect the growing collaboration between theoretical and experimental research at the Laboratory. “Her results uncover a novel loss mechanism for electron energy that could be important for NSTX-U plasmas,” said Amitava Bhattacharjee, head of the Theory Department at PPPL.

Belova plans to run more simulations to determine whether the mechanism she identified is the primary process that modifies the electron heating profile. She will also look for ways in which physicists can avoid this wave-induced change in the profile. In the meantime, she is driven by her desire to learn more physics. “We want to understand how these waves are excited by the beam ions,” she said, “and how to avoid them in the experiments.”

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, visit science.energy.gov.

Read the abstract.

Belova, E.V., N.N. Gorelenkov, E.D. Fredrickson, K. Tritz and N. A. Crocker. “Coupling of Neutral-Beam-Driven Compressional Alfvén Eigenmodes to Kinetic Alfvén Waves in NSTX Tokamak and Energy Channeling.” Physical Review Letters. Published June 29, 2015. DOI: 10.1103/PhysRevLett.115.015001

 

X marks the spot: Researchers confirm novel method for controlling plasma rotation to improve fusion performance (Physical Review Letters)

Representative plasma geometries, with the X-point location circled in red. (Reprinted from T. Stoltzfus-Dueck et al., Phys. Rev. Lett. 114, 245001, 2015. Copyright 2015 by the American Physical Society.)

Representative plasma geometries, with the X-point location circled in red. (Reprinted from T. Stoltzfus-Dueck et al., Phys. Rev. Lett. 114, 245001, 2015. Copyright 2015 by the American Physical Society.)

By Raphael Rosen, Princeton Plasma Physics Laboratory

Rotation is key to the performance of salad spinners, toy tops, and centrifuges, but recent research suggests a way to harness rotation for the future of mankind’s energy supply. In papers published in Physics of Plasmas in May and Physical Review Letters this month, Timothy Stoltzfus-Dueck, a physicist at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), demonstrated a novel method that scientists can use to manipulate the intrinsic – or self-generated – rotation of hot, charged plasma gas within fusion facilities called tokamaks. This work was supported by the DOE Office of Science.

Such a method could prove important for future facilities like ITER, the huge international tokamak under construction in France that will demonstrate the feasibility of fusion as a source of energy for generating electricity. ITER’s massive size will make it difficult for the facility to provide sufficient rotation through external means.

Rotation is essential to the performance of all tokamaks. Rotation can stabilize instabilities in plasma, and sheared rotation – the difference in velocities between two bands of rotating plasma – can suppress plasma turbulence, making it possible to maintain the gas’s high temperature with less power and reduced operating costs.

Today’s tokamaks produce rotation mainly by heating the plasma with neutral beams, which cause it to spin. In intrinsic rotation, however, rotating particles that leak from the edge of the plasma accelerate the plasma in the opposite direction, just as the expulsion of propellant drives a rocket forward.

Stoltzfus-Dueck and his team influenced intrinsic rotation by moving the so-called X-point – the dividing point between magnetically confined plasma and plasma that has leaked from confinement – on the Tokamak à Configuration Variable (TCV) in Lausanne, Switzerland. The experiments marked the first time that researchers had moved the X-point horizontally to study plasma rotation. The results confirmed calculations that Stoltzfus-Dueck had published in a 2012 paper showing that moving the X-point would cause the confined plasma to either halt its intrinsic rotation or begin rotating in the opposite direction. “The edge rotation behaved just as the theory predicted,” said Stoltzfus-Dueck.

A surprise also lay in store: Moving the X-point not only altered the edge rotation, but modified rotation within the superhot core of the plasma where fusion reactions occur. The results indicate that scientists can use the X-point as a “control knob” to adjust the inner workings of fusion plasmas, much like changing the settings on iTunes or a stereo lets one explore the behavior of music. This discovery gives fusion researchers a tool to access different intrinsic rotation profiles and learn more about intrinsic rotation itself and its effect on confinement.

The overall findings provided a “perfect example of a success story for theory-experiment collaboration,” said Olivier Sauter, senior scientist at École Polytechnique Fédérale de Lausanne and co-author of the paper.

Along with the practical applications of his research, Stoltzfus-Dueck enjoys the purely intellectual aspect of his work. “It’s just interesting,” he said. “Why do plasmas rotate in the way they do? It’s a puzzle.”

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.

Read the abstract.

Stoltzfus-Dueck, A. N. Karpushov, O. Sauter, B. P. Duval, B. Labit, H. Reimerdes, W. A. J. Vijvers, the TCV Team, and Y. Camenen. “X-Point-Position-Dependent Intrinsic Toroidal Rotation in the Edge of the TCV Tokamak.” Physical Review Letters 114, 245001 – Published 17 June 2015.

Giant structures called plasmoids could simplify the design of future tokamaks (Physical Review Letters)

Plasmoid formation in plasma simulation

Left: Plasmoid formation in simulation of NSTX plasma during CHI. Credit: Fatima Ebrahimi, PPPL / Right: Fast-camera image of NSTX plasma shows two discrete plasmoid-like bubble structures. Credit: Nishino-san, Hiroshima University

By Raphael Rosen, Princeton Plasma Physics Laboratory

Researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have for the first time simulated the formation of structures called “plasmoids” during Coaxial Helicity Injection (CHI), a process that could simplify the design of fusion facilities known as tokamaks. The findings, reported in the journal Physical Review Letters, involve the formation of plasmoids in the hot, charged plasma gas that fuels fusion reactions. These round structures carry current that could eliminate the need for solenoids – large magnetic coils that wind down the center of today’s tokamaks – to initiate the plasma and complete the magnetic field that confines the hot gas.

“Understanding this behavior will help us produce plasmas that undergo fusion reactions indefinitely,” said Fatima Ebrahimi, a physicist at both Princeton University and PPPL, and the paper’s lead author.

Ebrahimi ran a computer simulation that modeled the behavior of plasma and the formation of plasmoids in three dimensions thoughout a tokamak’s vacuum vessel. This marked the first time researchers had modeled plasmoids in conditions that closely mimicked those within an actual tokamak. All previous simulations had modeled only a thin slice of the plasma – a simplified picture that could fail to capture the full range of plasma behavior.

Researchers validated their model by comparing it with fast-camera images of plasma behavior inside the National Spherical Torus Experiment (NSTX), PPPL’s major fusion facility. These images also showed plasmoid-like structures, confirming the simulation and giving the research breakthrough significance, since it revealed the existence of plasmoids in an environment in which they had never been seen before. “These findings are in a whole different league from previous ones,” said Roger Raman, leader for the Coaxial Helicity Injection Research program on NSTX and a coauthor of the paper.

The findings may provide theoretical support for the design of a new kind of tokamak with no need for a large solenoid to complete the magnetic field. Solenoids create magnetic fields when electric current courses through them in relatively short pulses. Today’s conventional tokamaks, which are shaped like a donut, and spherical tokamaks, which are shaped like a cored apple, both employ solenoids. But future tokamaks will need to operate in a constant or steady state for weeks or months at a time. Moreover, the space in which the solenoid fits – the hole in the middle of the doughnut-shaped tokamak – is relatively small and limits the size and strength of the solenoid.

A clear understanding of plasmoid formation could thus lead to a more efficient method of creating and maintaining a plasma through transient Coaxial Helicity Injection. This method, originally developed at the University of Washington, could dispense with a solenoid entirely and would work like this:

  • Researchers first inject open magnetic field lines into the vessel from the bottom of the vacuum chamber. As researchers drive electric current along those magnetic lines, the lines snap closed and form the plasmoids, much like soap bubbles being blown out of a sheet of soapy film.
  • The many plasmoids would then merge to form one giant plasmoid that could fill the vacuum chamber.
  • The magnetic field within this giant plasmoid would induce a current in the plasma to keep the gas tightly in place. “In principle, CHI could fundamentally change how tokamaks are built in the future,” says Raman.

Understanding how the magnetic lines in plasmoids snap closed could also help solar physicists decode the workings of the sun. Huge magnetic lines regularly loop off the surface of the star, bringing the sun’s hot plasma with them. These lines sometimes snap together to form a plasmoid-like mass that can interfere with communications satellites when it collides with the magnetic field that surrounds the Earth.

While Ebrahimi’s findings are promising, she stresses that much more is to come. PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U) will provide a more powerful platform for studying plasmoids when it begins operating this year, making Ebrahimi’s research “only the beginning of even more exciting work that will be done on PPPL equipment,” she said.

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.

Read the abstract

Ebrahimi and R. Raman. “Plasmoids Formation During Simulations of Coaxial Helicity Injection in the National Spherical Torus Experiment. Physical Review Letters. Published May 20, 2015. DOI: http://dx.doi.org/10.1103/PhysRevLett.114.205003

An improvement to the global software standard for analyzing fusion plasmas (Nuclear Fusion)

By Raphael Rosen, Princeton Plasma Physics Laboratory

The gold standard for analyzing the behavior of fusion plasmas may have just gotten better. Mario Podestà, a staff physicist at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), has updated the worldwide computer program known as TRANSP to better simulate the interaction between energetic particles and instabilities – disturbances in plasma that can halt fusion reactions. The program’s updates, reported in the journal Nuclear Fusion, could lead to improved capability for predicting the effects of some types of instabilities in future facilities such as ITER, the international experiment under construction in France to demonstrate the feasibility of fusion power.

Podestà and co-authors saw a need for better modeling techniques when they noticed that while TRANSP could accurately simulate an entire plasma discharge, the code wasn’t able to represent properly the interaction between energetic particles and instabilities. The reason was that TRANSP, which PPPL developed and has regularly updated, treated all fast-moving particles within the plasma the same way. Those instabilities, however, can affect different parts of the plasma in different ways through so-called “resonant processes.”

The authors first figured out how to condense information from other codes that do model the interaction accurately – albeit over short time periods – so that TRANSP could incorporate that information into its simulations. Podestà then teamed up with TRANSP developer Marina Gorelenkova at PPPL to update a TRANSP module called NUBEAM to enable it to make sense of this condensed data. “Once validated, the updated module will provide a better and more accurate way to compute the transport of energetic particles,” said Podestà. “Having a more accurate description of the particle interactions with instabilities can improve the fidelity of the program’s simulations.”

Schematic of NSTX tokamak at PPPL with a cross-section showing perturbations of the plasma profiles caused by instabilities. Without instabilities, energetic particles would follow closed trajectories and stay confined inside the plasma (blue orbit). With instabilities, trajectories can be modified and some particles may eventually be pushed out of the plasma boundary and lost (red orbit). Credit: Mario Podestà

Schematic of NSTX tokamak at PPPL with a cross-section showing perturbations of the plasma profiles caused by instabilities. Without instabilities, energetic particles would follow closed trajectories and stay confined inside the plasma (blue orbit). With instabilities, trajectories can be modified and some particles may eventually be pushed out of the plasma boundary and lost (red orbit). Credit: Mario Podestà

Fast-moving particles, which result from neutral beam injection into tokamak plasmas, cause the instabilities that the updated code models. These particles begin their lives with a neutral charge but turn into negatively charged electrons and positively charged ions – or atomic nuclei – inside the plasma. This scheme is used to heat the plasma and to drive part of the electric current that completes the magnetic field confining the plasma.

The improved simulation tool may have applications for ITER, which will use fusion end-products called alpha particles to sustain high plasma temperatures. But just like the neutral beam particles in current-day-tokamaks, alpha particles could cause instabilities that degrade the yield of fusion reactions. “In present research devices, only very few, if any, alpha particles are generated,” said Podestà. “So we have to study and understand the effects of energetic ions from neutral beam injectors as a proxy for what will happen in future fusion reactors.”

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, visit science.energy.gov.

Read the paper

Podestà, M. Gorelenkova, D.S. Darrow, E.D. Fredrickson, S.P. Gerhardt and R.B. White. Nucl. Fusion 55 053018
doi:10.1088/0029-5515/55/5/053018

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.