plasma physics

PPPL researchers combine quantum mechanics and Einstein’s theory of special relativity to clear up puzzles in plasma physics (Phys. Rev. A)

Posted on August 1, 2016August 1, 2016 by Catherine Zandonella

Sketch of a pulsar, center, in binary star system (Photo credit: NASA Goddard Space Flight Center)

By John Greenwald, Princeton Plasma Physics Laboratory Communications

Among the intriguing issues in plasma physics are those surrounding X-ray pulsars — collapsed stars that orbit around a cosmic companion and beam light at regular intervals, like lighthouses in the sky. Physicists want to know the strength of the magnetic field and density of the plasma that surrounds these pulsars, which can be millions of times greater than the density of plasma in stars like the sun.

Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have developed a theory of plasma waves that can infer these properties in greater detail than in standard approaches. The new research analyzes the plasma surrounding the pulsar by coupling Einstein’s theory of relativity with quantum mechanics, which describes the motion of subatomic particles such as the atomic nuclei — or ions — and electrons in plasma. Supporting this work is the DOE Office of Science.

Quantum field theory

Graduate student Yuan Shi Graduate student Yuan Shi (Photo by Elle Starkman/PPPL Office of Communications) — Graduate student Yuan Shi (Photo by Elle Starkman/PPPL Office of Communications)

The key insight comes from quantum field theory, which describes charged particles that are relativistic, meaning that they travel at near the speed of light. “Quantum theory can describe certain details of the propagation of waves in plasma,” said Yuan Shi, a graduate student at Princeton University in the Department of Astrophysics’ Princeton Program in Plasma Physics, and lead author of a paper published July 29 in the journal Physical Review A. Understanding the interactions behind the propagation can then reveal the composition of the plasma.

Shi developed the paper with assistance from co-authors Nathaniel Fisch, director of the Princeton Program in Plasma Physics and professor and associate chair of astrophysical sciences at Princeton University, and Hong Qin, a physicist at PPPL and executive dean of the School of Nuclear Science and Technology at the University of Science and Technology of China. “When I worked out the mathematics they showed me how to apply it,” said Shi.

In pulsars, relativistic particles in the magnetosphere, which is the magnetized atmosphere surrounding the pulsar, absorb light waves, and this absorption displays peaks. “The question is, what do these peaks mean?” asks Shi. Analysis of the peaks with equations from special relativity and quantum field theory, he found, can determine the density and field strength of the magnetosphere.

Combining physics techniques

The process combines the techniques of high-energy physics, condensed matter physics, and plasma physics. In high-energy physics, researchers use quantum field theory to describe the interaction of a handful of particles. In condensed matter physics, people use quantum mechanics to describe the states of a large collection of particles. Plasma physics uses model equations to explain the collective movement of millions of particles. The new method utilizes aspects of all three techniques to analyze the plasma waves in pulsars.

The same technique can be used to infer the density of the plasma and strength of the magnetic field created by inertial confinement fusion experiments. Such experiments use lasers to ablate — or vaporize —a target that contains plasma fuel. The ablation then causes an implosion that compresses the fuel into plasma and produces fusion reactions.

Standard formulas give inconsistent answers

Researchers want to know the precise density, temperature and field strength of the plasma that this process creates. Standard mathematical formulas give inconsistent answers when lasers of different color are used to measure the plasma parameters. This is because the extreme density of the plasma gives rise to quantum effects, while the high energy density of the magnetic field gives rise to relativistic effects, says Shi. So formulations that draw upon both fields are needed to reconcile the results.

For Shi, the new technique shows the benefits of combining physics disciplines that don’t often interact. Says he: “Putting fields together gives tremendous power to explain things that we couldn’t understand before.”

Read the abstract

Yuan Shi, Nathaniel J. Fisch, and Hong Qin. Effective-action approach to wave propagation in scalar QED plasmas. Phys. Rev. A 94, 012124 – Published 29 July 2016.

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 Princeton 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.

PPPL physicists simulate innovative method for starting up tokamaks without using a solenoid (Nuclear Fusion)

Posted on January 7, 2016January 7, 2016 by Catherine Zandonella

PPPL Scientist Francesca Poli. Photo Credit: Elle Starkman / PPPL Office of Communications. PPPL, located on Princeton University’s Forrestal Campus and managed by the University, is devoted to developing practical solutions for the creation of sustainable energy from fusion and to creating new knowledge about the physics of ultra-hot, charged gases known as plasmas.

By Raphael Rosen, PPPL Office of Communications

Scientists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have produced self-consistent computer simulations that capture the evolution of an electric current inside fusion plasma without using a central electromagnet, or solenoid.

The computer simulations of the process, known as non-inductive current ramp-up, were performed using TRANSP, the gold-standard code developed at PPPL. The results were published in October 2015 in the journal Nuclear Fusion. The research was supported by the DOE Office of Science.

In traditional donut-shaped tokamaks, a large solenoid runs down the center of the reactor. By varying the electrical current in the solenoid scientists induce a current in the plasma. This current starts up the plasma and creates a second magnetic field that completes the forces that hold the hot, charged gas together.

But spherical tokamaks, a compact variety of fusion reactor that produces high plasma pressure with relatively low magnetic fields, have little room for solenoids. Spherical tokamaks look like cored apples and have a smaller central hole for the solenoid than conventional tokamaks do. Physicists, therefore, have been trying to find alternative methods for producing the current that starts the plasma and completes the magnetic field in spherical tokamaks.

One such method is known as coaxial helicity injection (CHI). During CHI, researchers switch on an electric coil that runs beneath the tokamak. Above this coil is a gap that opens into the tokamak’s vacuum vessel and circles the tokamak’s floor. The switched-on electrical current produces a magnetic field that connects metal plates on either side of the gap.

Researchers next puff gas through the gap and discharge a spark across the two plates. This process causes magnetic reconnection — the process by which the magnetic fields snap apart and reconnect. This reconnection creates a magnetic bubble that fills the tokamak and produces the vital electric current that starts up the plasma and completes the magnetic field.

This current must be nurtured and fed. According to lead author Francesca Poli, the new computer simulations show that the current can best be sustained by injecting high-harmonic radio-frequency waves (HHFWs) and neutral beams into the plasma.

HHFW’s are radio-frequency waves that can heat both electrons and ions. The neutral beams, which consist of streams of hydrogen atoms, become charged when they enter the plasma and interact with the ions. The combination of the HHFWs and neutral beams increases the current from 300 kiloamps to 1 mega amp.

But neither HHFWs nor neutral beams can be used at the start of the process, when the plasma is relatively cool and not very dense. Poli found that HHFWs would be more effective if the plasma were first heated by electron cyclotron waves, which transfer energy to the electrons that circle the magnetic field lines.

“With no electron cyclotron waves you would have to pump in four megawatts of HHFW power to create 400 kiloamps of current,” she said. “With these waves you can get the same amount of current by pumping in only one megawatt of power.

“All of this is important because it’s hard to control the plasma at the start-up,” she added. “So the faster you can control the plasma, the better.”

PPPL is managed by Princeton University for the U.S. Department of Energy’s Office of Science.

Read the abstract.

F.M. Poli, R.G. Andre, N. Bertelli, S.P. Gerhardt, D. Mueller and G. Taylor. “Simulations towards the achievement of non-inductive current ramp-up and sustainment in the National Spherical Torus Experiment Upgrade.” Nuclear Fusion. Published October 30, 2015. DOI: 10.1088/0029-5515/55/12/123011

Identifying new sources of turbulence in spherical tokamaks (Physics of Plasmas)

Posted on December 1, 2015December 1, 2015 by Catherine Zandonella

By John Greenwald, Princeton Plasma Physics Laboratory Communications

Turbulence 1 — Computer simulation of turbulence in a model of the NSTX-U, a spherical tokamak fusion facility at the U.S. Dept. of Energy’s Princeton Plasma Physics Laboratory. Credit: Eliot Feibush

For fusion reactions to take place efficiently, the atomic nuclei that fuse together in plasma must be kept sufficiently hot. But turbulence in the plasma that flows in facilities called tokamaks can cause heat to leak from the core of the plasma to its outer edge, causing reactions to fizzle out.

Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have for the first time modeled previously unsuspected sources of turbulence in spherical tokamaks, an alternative design for producing fusion energy. The findings, published online in October in Physics of Plasmas, could influence the development of future fusion facilities. This work was supported by the DOE Office of Science.

Spherical tokamaks, like the recently completed National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL, are shaped like cored apples compared with the mushroom-like design of conventional tokamaks that are more widely used. The cored-apple shape provides some distinct characteristics for the behavior of the plasma inside.

The paper, with PPPL principal research physicist Weixing Wang as lead author, identifies two important new sources of turbulence based on data from experiments on the National Spherical Torus Experiment prior to its upgrade. The discoveries were made by using state-of-the-art large-scale computer simulations. These sources are:

Instabilities caused by plasma that flows faster in the center of the fusion facility than toward the edge when rotating strongly in L-mode — or low confinement — regimes. These instabilities, called “Kelvin-Helmholtz modes” after physicists Baron Kelvin and Hermann von Helmholtz, act like wind that stirs up waves as it blows over water and are for the first time found to be relevant for realistic fusion experiments. Such non-uniform plasma flows have been known to play favorable roles in fusion plasmas in conventional and spherical tokamaks. The new results from this study suggest that we may also need to keep these flows within an optimized level.

Trapped electrons that bounce between two points in a section of the tokamak instead of swirling all the way around the facility. These electrons were shown to cause significant leakage of heat in H-mode — or high-confinement — regimes by driving a specific instability when they collide frequently. This type of instability is believed to play little role in conventional tokamaks but can provide a robust source of plasma turbulence in spherical tokamaks.

Most interestingly, the model predicts a range of trapped electron collisions in spherical tokamaks that can be turbulence-free, thus improving the plasma confinement. Such favorable plasmas could possibly be achieved by future advanced spherical tokamaks operating at high temperature.

Findings of the new model can be tested on the NSTX-U and will help guide experiments to identify non-traditional sources of turbulence in the spherical facility. Results of this research can shed light on the physics behind key obstacles to plasma confinement in spherical facilities and on ways to overcome them in future machines.

Read the abstract:

Weixing X. Wang, Stephane Ethier, Yang Ren, Stanley Kaye, Jin Chen, Edward Startsev, Zhixin Lu, and Zhengqian Li. “Identification of new turbulence contributions to plasma transport and confinement in spherical tokamak regime.” Physics of Plasmas, published October 2015. doi:10.1063/1.4933216.

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

Posted on April 28, 2015 by Catherine Zandonella

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

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)

Posted on March 17, 2015March 17, 2015 by Catherine Zandonella

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.

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