The Princeton Plasma Physics Laboratory: The quest for clean energy continues


After a three-year, $94 million overhaul, the Princeton Plasma Physics Laboratory’s primary fusion reactor has resumed the quest for clean energy. The fusion of parts of the atom inside the reactor could release a near limitless amount of energy and reduce our dependence on fossil fuels, while generating minimal hazardous waste. The upgrade included replacing the center of the apple-shaped reactor with a new 29,000-pound magnetic core. PHOTO BY JAMES CHRZANOWSKI

FUSION — the energy-making process that powers the sun — could provide us with a near limitless source of energy, ending our dependence on fossil fuels for making electricity.

This summer, after a nearly three-year overhaul, the world-leading fusion research facility at the Princeton Plasma Physics Laboratory (PPPL) switched on its newly outfitted flagship reactor, the National Spherical Torus Experiment-Upgrade (NSTX-U). The reactor uses electrical current and heat to create a hot, charged state called a plasma, which is encased by powerful magnets so that parts of the atoms can collide and fuse, releasing massive quantities of energy in the process.

The $94 million upgrade has made the NSTX-U the world’s most powerful spherical tokamak — the name given to donut-shaped fusion reactors — while doubling its heating power and magnetic fields, and making it the first major addition to the U.S. fusion program in the 21st century.

“The upgrade boosts NSTX-U operating conditions closer to those to be found in a commercial fusion power plant,” said Stewart Prager, director of PPPL, which is managed by Princeton University for the U.S. Department of Energy and is located some three miles from the campus. “Experiments will push into new physics regimes and assess how well the spherical design can advance research along the path to magnetic fusion energy.”

Fusion reactor

The upgrade included bringing in a 70-ton machine (above) that produces beams that heat the plasma. PHOTO BY MICHAEL VIOLA

The key feature of the design is its compact, cored apple-like shape, as compared with the bulkier, donut-like form of conventional tokamaks. The compact shape enables spherical tokamaks to confine highly pressurized plasma gas — the hot, charged fuel for fusion reactions — within comparatively low magnetic fields. This capability makes spherical tokamaks a cost-effective alternative to conventional tokamaks, which require stronger and thus more expensive magnetic fields.

Building the NSTX-U posed novel challenges for engineers and technicians throughout PPPL. Tasks ranged from flying a 70-ton neutral beam machine over a 22-foot wall to building a 29,000-pound center stack. These huge components fit alongside and inside an existing facility — the original NSTX — with hair-thin precision, requiring an effort that one engineer likened to rebuilding a ship in a bottle.

Researchers now plan to test whether the NSTX-U can continue to produce high-pressure plasmas under the hotter and more powerful conditions that the upgrade allows. Also on the research agenda are tests of how effectively the NSTX-U can keep temperatures approaching 100 million degrees centigrade from dissipating, and whether its spherical design can be a strong candidate for a major next step in the U.S. fusion program.

–By John Greenwald

Download PDF

Elusive particles found

IN THE PAST YEAR, PRINCETON PHYSICISTS have detected two particles that were predicted decades ago to exist but had not been found until now. Both particles were detected using a scanning-tunneling microscope to image the particles inside a crystal. The particles may someday enable powerful computers based on quantum mechanics.

A team led by Ali Yazdani, the Class of 1909 Professor of Physics, detected the “Majorana fermion,” which behaves simultaneously like matter and antimatter and was first proposed in 1937 by Italian physicist Ettore Majorana. The team, which received funding from the National Science Foundation and the Office of Naval Research, included B. Andrei Bernevig, an associate professor of physics, and other colleagues at Princeton and at the University of Texas-Austin. They published their results in the Oct. 2, 2014, issue of the journal Science.

A few months later, an international team led by M. Zahid Hasan, professor of physics, detected another elusive particle, the “Weyl fermion,” first theorized by the mathematician and physicist Hermann Weyl in 1929. The particle is massless and can also behave like matter and antimatter. The research team, which received support from the Gordon and Betty Moore Foundation and the U.S. Department of Energy, published their work in Science on July 16, 2015.

–By Steven Schultz and Morgan Kelly

Download PDF

Striking resemblance: A physical law may govern very different biological activities


FLOCKS OF BIRDS FLY ACROSS THE SKY in shifting configurations. In the retina of an eye, millions of neurons ignite in ever-changing combinations, translating light into meaningful images. Yet both of these seemingly random behaviors have an underlying order that can be described by mathematics.

Like these cells and birds, when atoms and molecules come together they can display coordinated behaviors that are more than the sum of their parts. At a critical point, such as the boundary between liquid and gas, local interactions between molecules propagate through an entire material, changing its essential properties.

Princeton biophysicist William Bialek thinks criticality may also underlie collective behaviors in living organisms, and he’s using real-world data to test this hypothesis. Recently, Bialek and his colleagues have analyzed the flocking behaviors of birds, the genetic networks of fruit fly embryos and the activation patterns of salamander neurons.

“In physics, we use the same mathematical language to describe many seemingly different behaviors,” said Bialek, the John Archibald Wheeler/Battelle Professor in Physics and the Lewis-Sigler Institute for Integrative Genomics. “So we understand that the emergence of collective behavior from all the individual interactions has a kind of universality.”

To explore the possibility that this universality might extend to living systems, Bialek made use of a large dataset on the changing positions and velocities of thousands of individual birds in a flock of starlings. A group of Italian physicists used multiple cameras to record the birds and calculate their exact locations over time in three dimensions — “a technical triumph,” according to Bialek.

The researchers, including former Princeton postdoctoral fellows Thierry Mora and Aleksandra Walczak, analyzed the deviations of each bird from the flock’s average speed and direction of movement. They found not only that these variations were correlated between nearby birds, but also that the fluctuations from the average propagated through the group over long distances. This pattern of rapid, remote signal transmission echoes the changes that occur among molecules during a phase change from solid to liquid or liquid to gas. At a critical point, this could allow information to spread swiftly through the group, enabling the whole flock to nimbly change direction.

“The model you build just by keeping track of what each bird does relative to its neighbors predicts what happens throughout the entire flock,” Bialek said. “And it does so with an accuracy that is beyond what we had any reason to expect. It’s really a very precise prediction.”

Other biological examples of criticality play out on a microscopic scale. Bialek has an ongoing collaboration with Princeton’s Squibb Professor in Molecular Biology Eric Wieschaus, a Howard Hughes Medical Institute researcher and Nobel Prize winner, who has uncovered many of the genes involved in the embryonic development of the fruit fly — a model biological system.

Bialek has found signatures of criticality in gene activation patterns during the first few hours of fly embryo development. The synchronized actions of “gap genes” establish the fly’s 14-segment body plan. Mutations in these genes lead to gaps between segments, whose effects are reflected in the names of the genes: two examples are “hunchback” and “giant.”

Recently, Thomas Gregor, an assistant professor of physics and also a member of the Lewis-Sigler Institute, has developed experimental tools to precisely measure the activity of many gap genes at once, all along the halfmillimeter length of the fly embryo. These measurements allowed Bialek and physics graduate student Dmitry Krotov to test whether the patterns of gene activity across the embryo fit a model of criticality. Indeed, using data from 24 embryos, they found that fluctuations from the average level of gene activity at each point along the embryo were correlated between certain pairs of gap genes, which regulate one another’s activity like on/off switches. They mapped the locations of these switch points, which appear to act like signals that spread over long distances, just as changes in velocity are correlated in a flock of birds.

Bialek has also looked for signatures of criticality among the activation patterns in a patch of 160 nerve cells from a salamander retina, a model system for studying this light-sensing layer of the eye. In collaboration with Michael Berry, an associate professor of molecular biology and the Princeton Neuroscience Institute, Bialek and his colleagues showed how the coordinated activity of the neurons could be tuned to a critical state.

Bialek thinks critical systems may be common features of life that have repeatedly evolved in different organisms and at different levels — both molecular and behavioral. This could explain why, though systems of cells or groups of organisms could be organized in any number of possible ways, networks with similar properties continue to emerge.

“Is there anything special about the way nature has organized things in living systems?” Bialek wondered. He said much more work is necessary to claim criticality as a general biological principle. “But I do think we’re seeing in the data, somehow, signs of that specialness — things that it seems you can only get if the system has been set up in particular ways and not in others,” he said. “That I find very appealing.”

This work was supported in part by the National Science Foundation, the National Institutes of Health, the Howard Hughes Medical Institute, the W.M. Keck Foundation and the Swartz Foundation.

–By Molly Sharlach

New mineral: Steinhardtite


Steinhardtite is a mineral named in honor of Paul Steinhardt, Princeton’s Albert Einstein Professor in Science and a professor of physics (Image courtesy of Luca Bindi, et al)

A MINERAL DISCOVERED to be of meteoritic origin has been named “steinhardtite” in honor of Paul Steinhardt, Princeton’s Albert Einstein Professor in Science and a professor of physics. The name was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association.

The mineral was found in the Koryak Mountains in Russia’s Kamchatka Peninsula during a 2011 expedition led by Steinhardt to locate the meteoritic source of the first known naturally occurring example of a quasicrystal, now known as “icosahedrite,” in which the atoms are arranged in patterns that do not regularly repeat and include unlikely configurations such as the 20-sided shape of a soccer ball. Steinhardt and collaborators had discovered a sample of icosahedrite at the Natural History Museum at the University of Florence, Italy, a finding published in the journal Science in 2009, and later identified the Florence sample as being meteoritic in origin. The expedition to the Koryak Mountains resulted in the discovery of steinhardtite, a new crystalline form of aluminum combined with significant amounts of iron and nickel.

The international team reporting the mineral and proposing the name was led by Luca Bindi, professor of mineralogy and crystallograpy at the University of Florence. Princeton researchers included Nan Yao, director of the Imaging and Analysis Center at the Princeton Institute for the Science and Technology of Materials (PRISM); Gerald Poirier, PRISM senior research specialist; Lincoln Hollister, professor of geosciences, emeritus; and Chaney Lin, a graduate student in physics. They were joined by Glenn MacPherson, a geologist at the Smithsonian Institution; Christopher Andronicos, an associate professor at Purdue University; scientists Vadim Distler, Valery Kryachko and Marina Yudovskaya of the Russian Academy of Sciences; Michael Eddy, a graduate student at the Massachusetts Institute of Technology; Alexander Kostin, a geosciences technologist at BHP Billiton; and William Steinhardt, a graduate student at Harvard University.

-By Catherine Zandonella

A farewell to arms? New technique could aid nuclear disarmament

A Farewell to Arms?

A new method that borrows from strategies used in computer cryptography could verify the presence of nuclear warheads without collecting classified information. The technique fires high-energy neutrons at a non-nuclear target (pictured above), called a British Test Object, that will serve as a proxy for warheads. (Photo by Elle Starkman)

SCIENTISTS at Princeton University and the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) are developing a system to verify the presence of nuclear warheads without collecting classified information, as a step toward the further reduction of nuclear arms.

While efforts have been made to develop systems for verifying the content of warheads covered by disarmament treaties, no such methods are currently in use. The new method borrows from strategies used in computer cryptography to identify nuclear warheads while learning nothing about the materials and design of the warheads themselves.

The research was published in the June 26, 2014, issue of Nature and was conducted by Alexander Glaser, an assistant professor in Princeton’s Woodrow Wilson School of Public and International Affairs and the Department of Mechanical and Aerospace Engineering; Robert Goldston, former director of PPPL, a fusion researcher and a professor of astrophysical sciences at Princeton; and Boaz Barak, a senior researcher at Microsoft New England who has taught computer science at Princeton.

–By John Greenwald

Telescopes take the universe’s temperature

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescopes on a Chilean mountaintop are poised to tell us much about the universe in its infancy. They are surveying the faint temperature fluctuations left over from the explosive birth of the universe, with the goal of piecing together its early history and understanding how clusters of galaxies evolved.

The telescopes are measuring these temperature fluctuations, known as cosmic microwave background radiation or CMB for short, from their perch 17,000 feet above sea level in Chile’s desolate Atacama Desert, where a dry atmosphere permits radiation to reach Earth with relatively little attenuation. In contrast to backyard telescopes that help us see visible light from stars and planets, these telescopes collect invisible microwave radiation.

Lyman Page

Lyman Page

These invisible waves are mostly uniform but contain slight differences in intensity and polarization that hold a wealth of information for cosmologists, said Lyman Page, the Henry De Wolf Smyth Professor of Physics. Page and Professor of Physics Suzanne Staggs co-lead two telescope projects, the Atacama Cosmology Telescope (ACT) and the Atacama B-mode Search telescope (ABS), which are funded by the National Science Foundation.

“If you imagine the temperature perturbations as a distant mountain range, the peaks and valleys correspond to the temperature variations,” Page said. “By looking at the patterns — the spacing between peaks, and whether they are narrow or fat — we are able to answer questions about the composition and evolution of the universe,” Page said.

ACT, which is about 18 feet across and looks like a giant metal bowl, has already made new discoveries, and confirmed and extended the findings of other CMB surveys, including two space-based telescopes, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck mission. A new, more sensitive receiver is currently being mounted on the ACT telescope, which is a collaborative effort with David Spergel, Princeton’s Charles A. Young Professor of Astronomy on the Class of 1897 Foundation, along with researchers at the University of Pennsylvania, National Institutes of Standards and Technology, the University of British Columbia, and 10 other institutions contributing significantly to the instruments and analysis.

Suzanne Staggs

Suzanne Staggs

The CMB originated in the hot plasma soon after the Big Bang, which cosmologists consider to be the birth of the universe. As the universe expanded, the radiation propagated, carrying the secrets of the early universe with it. One of the questions is why the CMB on opposite sides of the universe is so similar in temperature. The leading explanation of this observation is the inflation model, which posits that the universe underwent a rapid acceleration of its expansion just after the Big Bang.

This is where the lower-resolution, second telescope comes in. Co-led by Staggs, the ABS is looking for signs of inflation. “Inflation should produce gravitational waves which create patterns in the CMB called ‘B modes,’” said Staggs. B modes are extremely faint — to measure them requires an instrument that can detect temperature changes of just billionths of a degree. To obtain these sensitivities, ABS mirrors, which are relatively small at about two feet across, sit inside a cryogenically cooled barrel.

The two telescopes can be operated remotely, but require frequent trips to the Chilean peak, which often include Princeton students and postdocs. The team at Princeton includes Senior Research Physicist Norm Jarosik, Associate Research Scholar Jonathan Sievers, postdoctoral researchers Matthew Hasselfield, Rénee Hložek, Akito Kusaka and Laura Newburgh, and graduate students Farzan Beroz, Kang Hoon (Steve) Choi, Emily Grace, Colin Hill, Shuay-Pwu (Patty) Ho, Christine Pappas, Lucas Parker, Blake Sherwin, Sara Simon, Katerina Visnjic and Sophie Zhang.

–By Catherine Zandonella

Quantum computing moves forward

New technologies that exploit quantum behavior for computing and other applications are closer than ever to being realized due to recent advances. These advances could enable the creation of immensely powerful computers as well as other applications, such as highly sensitive detectors capable of probing biological systems.

“We are really excited about the possibilities of new semiconductor materials and new experimental systems that have become available in the last decade,” said Jason Petta, a quantum information scientist and an associate professor of physics at Princeton, who collaborated with Andrew Houck, an associate professor of electrical engineering, on a study published in Nature in October 2012 describing a method for quick and reliable transfer of quantum information throughout a computing device.

Support for the research was provided by the National Science Foundation, the Alfred P. Sloan Foundation, the Packard Foundation, the Army Research Office, and the Defense Advanced Research Projects Agency Quantum Entanglement Science and Technology Program.

–By Catherine Zandonella

Planck mission brings universe into sharp focus

Planck mission

Planck mission reveals universe (Photo courtesy of the European Space Agency)

Princeton researchers contributed extensively to the Planck space mission that earlier this year released the most accurate and detailed map ever made of the oldest light in the universe, revealing new information about its age, contents and origins.

The results suggest that the universe is expanding more slowly than scientists thought, and is 13.8 billion years old, about 100 million years older than previous estimates. The data also show there is less dark energy and more matter — both matter that we can see and invisible dark matter — in the universe than previously thought. Princeton researchers helped design and implement an essential scientific instrument on the European Space Agency’s Planck mission through the agency’s collaboration with NASA. Princeton astrophysicists worked with scientists from the United States, Europe and Canada to analyze the Planck data.

“The Planck satellite has expanded the knowledge we gained from previous missions,” said William Jones, a Princeton assistant professor of physics who was involved in the design of a detector that surveys light at high frequencies.

-By Catherine Zandonella

Princeton’s Physical Sciences-Oncology Center


The design of the micro-environment allows bacteria (red and green) to squeeze through channels as they search for nutrients (LB) that flow into the chambers through tiny slits. When the antibiotic ciprofloxacin (CIPRO) is added, bacteria evolve resistance to the drug much more rapidly than they would in an open environment. (Image courtesy of Robert Austin)

Game theory could help researchers gain an understanding of the dynamics of cancerous-tumor evolution under stress, according to research published in the journal AIP Advances in March 2012 by researchers at Princeton and the University of California-San Francisco. To explore interactions of cells in a rapidly growing tumor, the researchers modeled non-cancerous cells as cooperators, which obey the rules of communal survival, and tumor cells as cheaters, which do not obey these rules. The researchers found that the simulation was most accurate when it included how the cells behave in localized regions of the tumor rather than the entire tumor.

The researchers are affiliated with Princeton’s Physical Sciences- Oncology Center (PPS-OC), an interdisciplinary research center aimed at exploring the physical laws that govern the emergence and behavior of cancer. The center is led by Robert Austin, a professor of physics at Princeton, and includes collaborators at the University of California-San Francisco, Johns Hopkins University, the University of California-Santa Cruz and the Salk Institute for Biological Studies. Funded by the National Cancer Institute, the PPS-OC operates within a collaborative network of 12 other physical sciences– oncology centers.

In related work, Austin and colleagues reported in the journal Science in September 2011 the creation of a silicon-based microhabitat for studying the development of antibiotic resistance in bacteria. They constructed a plate containing tiny hexagon- shaped rooms connected by microscopic channels. Compared to conventional research flasks and dishes, the microhabitat is meant to more closely resemble the environment in living organisms. Austin and colleagues found that in this special habitat, bacteria evolved to be resistant to the antibiotic ciprofloxacin much more quickly than did bacteria growing in flasks. The research could make it easier for scientists to study how bacteria evade drugs and how to prevent resistance from developing.


Expedition verifies the extraterrestrial nature of quasicrystals


Model of a quasicrystal. (Image courtesy of Paul Steinhardt)

A rare and exotic mineral so unusual that it was thought impossible to exist came to Earth on a meteorite, according to an international team of scientists led by Princeton physics professor Paul Steinhardt. The mineral, called a quasicrystal, has an intricate internal structure quite different from conventional crystals, resulting in different physical properties, such as being harder than crystals made of similar elements.

Although quasicrystals can be made in a laboratory, they were not thought to exist in nature until Steinhardt, the Albert Einstein Professor in Science, with Princeton senior research scholar Nan Yao and Luca Bindi of the Florence Natural History Museum in Italy, identified the first known natural quasicrystal in a sample from a storage box at the Italian museum. They published the finding in the journal Science in 2009. Steinhardt and Bindi then traced the origin of the sample to a remote corner of far eastern Russia where mineralogist Valery Kryachko had collected it in 1979.

After arranging a collaborative agreement between Princeton and scientists at the Russian Academy of Sciences’ Institute of Ore Mineralogy, a team including Steinhardt, Bindi, Kryachko and scientists from Cornell University and the Smithsonian Institution made an expedition to the Koryak Mountains in Russia’s Kamchatka Peninsula to search for more quasicrystals. During 2011, they also examined the original sample with the help of Princeton Professor of Geosciences Emeritus, Lincoln Hollister, as well as with collaborators at the California Institute of Technology and the Smithsonian Institution. The researchers concluded that the quasicrystal was originally from a meteorite, which they reported in January 2012 in the journal Proceedings of the National Academy of Sciences.

In August 2012, Steinhardt and Bindi reported in the journal Reports on Progress in Physics that the Russia trip yielded new samples that have allowed the scientists to verify the crystals’ meteoritic origin. The expedition also showed that the quasicrystals arrived on Earth roughly 15,000 years ago during or after the last ice age, and most likely formed during the early days of the solar system, roughly 4.5 billion years ago, making them perhaps as old as the Earth itself.


A vehicle fords a river in the northeastern part of Russia’s Kamchatka Peninsula during an expedition by a team of Princeton researchers and colleagues to the Koryak Mountains to search for quasicrystals. (Image courtesy of Paul Steinhardt)

“The finding of these new samples confirms that quasicrystals can form in nature under astrophysical conditions,” Steinhardt said.

Although quasicrystals are solid minerals that look quite normal on the outside, their inner structure makes them fascinating to scientists. A quasicrystal’s atoms can be arranged in ways that are not commonly found in crystals, such as the shape of a 20-sided icosahedron with the symmetry of a soccer ball. The concept of quasicrystals — along with the term — was first introduced in 1984 by Steinhardt and Dov Levine, both then at the University of Pennsylvania. The first synthetic quasicrystal, a combination of aluminum and manganese, was reported in 1984 by Israeli materials scientist Dan Shechtman and colleagues at the U.S. National Institute of Standards and Technology, a finding for which Shechtman won the 2011 Nobel Prize.

The samples they found have been accepted by the Meteoritical Society as evidence of a new meteorite called Khatyrka. The research was supported by NASA and the National Science Foundation’s Materials Research Science and Engineering Centers through a grant to the Princeton Center for Complex Materials and New York University.