Advancing plasma physics and magnetic fusion energy

May 1, 2011

Stewart Prager, the director of the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), likens the development of fusion energy to finding a cure for cancer.

Princeton Plasma Physics Laboratory Director Stewart Prager (Photo by Elle Starkman)“Just as you need to understand the microbiology that underlies cancer to find a cure for the disease, an understanding of plasma physics is crucial to realize the goal of fusion energy,” he said. “Fusion is the big payoff--the ‘cure’ for the global energy crisis, if you will--but there’s a tremendous amount of science that needs to be done to get there.”

For this reason, PPPL, which is managed by the University, is devoted both to creating new knowledge about the physics of plasmas--ultrahot, charged gases--and to developing practical solutions for the creation of fusion energy. Through the process of fusion, which is constantly occurring in the sun and other stars, energy is created when the nuclei of two lightweight atoms, such as those of hydrogen, combine in a plasma at very high temperatures. When this happens, a burst of energy is released, which could theoretically be used to generate electricity.

In a series of experiments at the lab, PPPL researchers are expanding understanding of how plasmas behave and how they can be used to create fusion energy. The largest of these experiments, the National Spherical Torus Experiment (NSTX), began in 1999. As in many other fusion experiments, the plasmas in NSTX are confined using magnetic fields and walls designed to withstand the heat from plasmas with temperatures that exceed 100 million degrees Celsius (to date, NSTX plasmas have attained temperatures of 60 million degrees Celsius). But in contrast to most fusion experiments, which confine plasmas in a donut-like shape, the plasmas in NSTX are spherical in shape with a hole through the center.

Recent results from NSTX are advancing the understanding of plasma behavior, offering insights into what causes turbulence and how to mitigate harmful interactions between the plasma and the walls that confine it. Both turbulence and plasma-wall interactions can result in a major loss of heat from the plasma, which causes a plasma to terminate, or fall apart. When this happens, the energy in the plasma is dumped on the walls that surround it and is no longer useful for the creation of fusion energy.

PPPL researchers Wayne Solomon and Stanley Kaye have studied plasma rotation and the way that momentum is transported from one part of the plasma to another in NSTX. By measuring changes in plasma rotation at various locations in the plasma, the team calculated the components of momentum transport. Solomon and Kaye also joined PPPL theoreticians Weixing Wang, T. S. Hahm, and Greg Rewoldt in identifying a type of turbulence specific to fusion plasmas. Using computer-based simulations, members of the team discovered that forces arising from turbulence in the plasma’s electric field can transport rotation. Because rotation increases plasma stability, these advances may be beneficial for plasma performance.

In another project, PPPL physicist Ernesto Mazzucato and his colleagues have delved deeper into the nature of turbulence by scattering electromagnetic waves off of turbulent eddies in rotating plasmas. Their findings suggest that turbulence is excited by the non-uniformity of the temperature of the electrons in the plasma, and that this turbulence can lead to significant transport of heat from the plasma. When heat is lost rapidly from a plasma, it is difficult to sustain fusion, so methods to prevent heat loss are crucial for producing fusion energy. Toward this end, NSTX researchers have also shown that heat transport can be somewhat reduced by carefully adjusting the magnetic field that cages the plasma to confine the heat optimally.

In addressing the plasma-wall interaction problem, experiments in NSTX have shown that applying a thin film of lithium (the lightest metallic element) on the walls that confine the plasma can dramatically alter the properties of the plasma, such as temperature and the ability to stay hot.

In most instances when particles from a hot plasma hit the walls that confine it, some of the wall material -- often graphite in fusion experiments -- vaporizes and turns into a gas that is cooler than the plasma. This gas then enters the plasma, cooling its edges and creating a plasma that is much hotter in the center than on the periphery. This temperature difference creates turbulence, which ultimately lowers the temperature of the entire plasma.

Lithium has the special property of absorbing particles so that no cold gas enters the plasma. Thus, temperature remains more uniform, turbulence is reduced, and the whole plasma stays hot--three important characteristics for the creation of fusion energy.

Additionally, PPPL collaborator Rajesh Maingi of Oak Ridge National Laboratory found that the lithium film also stabilizes edge-localized instabilities--repetitive plasma disturbances that create large pulses of heat on the materials in contact with the plasma. Unchecked, these instabilities could damage wall surfaces in more powerful devices.

A slideshow of images from PPPL experiments is available. This close-up of a plasma shows the hot gas of charged particles. (Photo by Elle Starkman)

Watch slideshow

Beginning in 2012, NSTX will be offline for two years as it undergoes a major upgrade that will dramatically increase the machine’s capabilities by allowing for a doubling of the amount of current that flows through the plasma and a quintupling of the duration of each plasma from one second to five seconds. The upgrade, which includes the installation of superior magnets and the addition of a second neutral beam injector”
a system in which a beam of fast-moving neutral atoms are used to heat the plasmaówill also increase the heat flux experienced by the walls that confine the plasma to something on the order of 10 million watts per square meter. For a frame of reference, this is roughly equivalent to the amount of heat that a spacecraft encounters upon re-entering the Earthís atmosphere. The key difference, however, is that a spacecraft need only withstand such extreme abuse for a short amount of time whereas the walls that confine plasmas must be able to take the heat indefinitely.

“This upgrade will advance all three of the NSTX missions’to understand and control the interface between plasmas and the surrounding materials; to develop a candidate plasma for a next-generation fusion energy experimental power generation facility; and to study the basic physics of how to confine plasmas using magnetic fields,” Prager said.

Researchers at the Princeton Plasma Physics Laboratory are using the Lithium Tokamak Experiment to study how lithium walls can provide greater plasma stability and control. (Photo by Elle Starkman)In addition to NSTX, PPPL in 2008 launched another fusion energy experiment known as the Lithium Tokamak Experiment (LTX). An exploratory experiment, LTX enables the detailed study of how lithium walls can provide greater plasma stability and control.

A variety of smaller experiments that focus on basic plasma physics are also underway at the lab. The Magnetic Reconnection Experiment (MRX) enables researchers to study the phenomenon of magnetic reconnection, which converts magnetic energy to kinetic and thermal energy in plasmas, both in the laboratory and in outer space.

In plasmas, charged particles, such as ions and electrons, move along magnetic field lines like trains along railroad tracks. But these magnetic field lines can explosively tear and reconnect, as if two parallel train tracks
suddenly split and then rejoined in an overlapping pattern. In plasmas, this process, called magnetic reconnection, results in a final magnetic field that is very different from the one that existed before the magnetic explosions.

In MRX, researchers led by PPPL physicists Masaaki Yamada and Hantao Ji have made the first positive identifications of how electrons behave near the points of tearing and reconnection using detailed measurements of the reconnection site in a controlled environment. Their results suggest that the two-dimensional models used to simulate electron behavior under these conditions may be missing important three-dimensional effects that play an important role in the speed of reconnection. The work has implications both for astrophysics and the development of fusion energy.

Another project, the Paul Trap Simulator Experiment (PTSX), is a compact, three-meter-long cylindrical laboratory experiment. PTSX enables scientists to simulate conditions and experiments that take place in particle accelerators that are several kilometers long because the dynamics of particles in both systems are described by the same set of equations. The experiment is led by Professor of Astrophysical Sciences Ronald Davidson and PPPL scientist Erik Gilson.

In PTSX, experimenters can study how particles travel through a multi-kilometer accelerator by observing how the size of a beam of particles changes over time as it resides in a three-meter-long cylinder. In a particle accelerator, the magnets used to accelerate the particles inevitably are imperfectly aligned, leading to “errors” in the magnetic field. In recent PTSX experiments, these errors have been simulated by adding white noise to the electric field that is applied to a plasma. By enabling increased understanding of error-related effects, PSTX may inform the design of current and future high-energy physics experiments.

“For nearly 60 years, Princeton has been a world leader in research on magnetic fusion energy and the University has trained a large contingent of the active researchers in the field,” said Dean for Research A. J. Stewart Smith. “We are delighted that the U.S. Department of Energy continues to entrust Princeton with the management of the laboratory, especially as the need to create clean energy solutions for the future intensifies exponentially. Though the challenges going forward are daunting, the laboratory embraces them as it takes on leadership and partnership roles on the national and international scene, such as in ITER, the large international fusion experiment currently under construction in France.”