PPPL QUASAR Stellerator: A concept on the path to fusion energy

QUASAR stellerator design. Credit: 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 (NCSE), 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.

George "Hutch" Neilson

"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.

ITER: the world's largest Tokamak

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.'"

More information: Neilson, G.H.; Gates, D.A.; Heitzenroeder, P.J.; Breslau, J.; Prager, S.C.; Stevenson, T.; Titus, P.; Williams, M.D.; Zarnstorff, M.C., "Next Steps in Quasi-Axisymmetric Stellarator Research," Plasma Science, IEEE Transactions on , vol.42, no.3, pp.489,494, March 2014. DOI: 10.1109/TPS.2014.2298870

Distant quasar illuminates a filament of the cosmic web

This deep image shows the nebula (cyan) extending across 2 million light-years that was discovered around the bright quasar UM287 (at the center of the image). 

The energetic radiation of the quasar makes the surrounding intergalactic gas glow, revealing the morphology and physical properties of a cosmic web filament.

The image was obtained at the W. M. Keck Observatory. 

Credit: S. Cantalupo, UC Santa Cruz

Astronomers have discovered a distant quasar illuminating a vast nebula of diffuse gas, revealing for the first time part of the network of filaments thought to connect galaxies in a cosmic web.

Researchers at the University of California, Santa Cruz, led the study, published January 19 in Nature.

Using the 10-meter Keck I Telescope at the W. M. Keck Observatory in Hawaii, the researchers detected a very large, luminous nebula of gas extending about 2 million light-years across intergalactic space.

Sebastiano Cantalupo

"This is a very exceptional object: it's huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar," said first author Sebastiano Cantalupo, a postdoctoral fellow at UC Santa Cruz.

The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which (about 84 percent) is invisible dark matter.

This web is seen in the results from computer simulations of the evolution of structure in the universe, which show the distribution of dark matter on large scales, including the dark matter halos in which galaxies form and the cosmic web of filaments that connect them.

Gravity causes ordinary matter to follow the distribution of dark matter, so filaments of diffuse, ionized gas are expected to trace a pattern similar to that seen in dark matter simulations.

Until now, however, these filaments have never been seen. Intergalactic gas has been detected by its absorption of light from bright background sources, but those results don't reveal how the gas is distributed.

In this study, the researchers detected the fluorescent glow of hydrogen gas resulting from its illumination by intense radiation from the quasar.

Computer simulations suggest that matter in the universe is distributed in a "cosmic web" of filaments, as seen in the image above from a large-scale dark-matter simulation (Bolshoi simulation, by Anatoly Klypin and Joel Primack). 

The inset is a zoomed-in, high-resolution image of a smaller part of the cosmic web, 10 million light-years across, from a simulation that includes gas as well as dark matter (credit: S. Cantalupo). 

The intense radiation from a quasar can, like a flashlight, illuminate part of the surrounding cosmic web (highlighted in the image) and make a filament of gas glow, as was observed in the case of quasar UM287. 

Credit: Background image: A. Klypin and J. Primack; Inset: S. Cantalupo

J. Xavier Prochaska

"This quasar is illuminating diffuse gas on scales well beyond any we've seen before, giving us the first picture of extended gas between galaxies. It provides a terrific insight into the overall structure of our universe," said co-author J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz.

The hydrogen gas illuminated by the quasar emits ultraviolet light known as Lyman alpha radiation.


The distance to the quasar is so great (about 10 billion light-years) that the emitted light is "stretched" by the expansion of the universe from an invisible ultraviolet wavelength to a visible shade of violet by the time it reaches the Keck Telescope.

Knowing the distance to the quasar, the researchers calculated the wavelength for Lyman alpha radiation from that distance and built a special filter for the telescope's LRIS spectrometer to get an image at that wavelength.

More information: Paper: doi.org/10.1038/nature12898

Obese Black-Hole Galaxies Could Reveal Quasar Secrets

Obese black-hole galaxies could be a stepping stone to the quasars we see today. 

CREDIT: NASA/ESA 

Gluttonous black holes in the center of some galaxies could be precursors to the brightest objects in the known universe.

A recently proposed type of galaxy with an overwhelmingly large black hole in its center could give astronomers a better understanding of the formation of quasars — bright objects in galaxies with supe-rmassive black holes.

The centers of these obese black-hole galaxies (OBGs) could harbour black holes so massive that radiation from where the black hole accretes would overwhelm that of the stars within its galaxy.

New research indicates that some of the most luminous quasars seen from Earth were likely OBGs first before something "lit up" the black hole and had it pump out energy visible from Earth.

The simulations indicate there could be a million OBGs in the observable universe, making the team behind the work ask, why don't we see them?

Tanking up
The team, led by scientists from the Max Planck Institute for Extraterrestrial Physics in Germany, was trying to figure out why astronomers can see quasars from a very far away, at the beginning of the universe. (More distant objects are older and have less metal in them.)

"People have hypothesized there could be these black holes forming from this metal-free gas reservoir ... which is completely free of stars," said Bhaskar Agarwal, a doctoral research fellow at Max Planck who led the research.

This means the black holes in OBGs could have formed separately from the stars surrounding them, contradicting recent observations by the Herschel Space Observatory.

In that earlier finding, astronomers suggested that galaxies that have black holes in their center — including the Milky Way — see the stars and black holes evolve together.

Other research, including observational research, also shows OBG-like objects with a very massive black hole at the center of the galaxy, Agarwal told reporters.

But if the black holes in OBGs formed solo from a reservoir of gas, the stars must have come from somewhere.

The simulations suggested the black hole would have remained on its own until a merger between galaxies brought stars within its vicinity.

The results were published in the Monthly Notices of the Royal Astronomical Society by Oxford University Press on May 29.

Participating researchers came from Max Planck, Yale University and the University of Edinburgh.

Novel analysis method reveals more about the enigmatic Quasar

The interaction of a supermassive black hole and a disk of accreting matter, called a quasar, can be seen at the center of a faraway galaxy in this artist's concept. 

It consists of a dusty, doughnut-shaped cloud of gas and dust that feeds a central supermassive black hole. 

As the black hole feeds, the gas and dust heat up and spray out different kinds of light, as illustrated by the white rays. 

In the nearly six decades since quasars were discovered, the list of these energetic galaxies powered by supermassive black holes has grown to more than 100,000 – enough examples to reveal important information about the quasar population as a whole.

But attempts to conduct a celestial census of these powerful objects have been limited by a fundamental problem: Although quasars are bright, they also span billions of light years in distance from Earth. Just as with stars in an urban sky, the closest quasars can be seen even if they are dim, while the oldest and most distant ones can be seen only if they are bright.

This means astrophysicists have to study a sample with big differences among individual members, including distance, age, brightness and type of radiation emitted.

Astrophysicists with the Kavli Institute for Particle Astrophysics and Cosmology, a joint SLAC-Stanford institute, found a way to reach past these limitations: They improved an algorithm that homes in on important commonalities of a population of objects while taking into account the limitations and biases for observations made in multiple types of electromagnetic radiation, such as optical light or radio waves – two of the most important wavelengths for studying quasars.

In the process they shed new light on a contentious question: Are there two types of quasars, with one "louder" in radio than the other, or is there just one type with emissions that vary widely across the electromagnetic spectrum?

A recent paper in The Astrophysical Journal details how the team, including KIPAC scientist Jack Singal, KIPAC member and Stanford professor Vahe Petrosian and KIPAC alumnus Lukasz Stawarz, improved upon an algorithm developed more than a decade ago by Petrosian and Bradley Efron, a respected statistics professor at Stanford.

After validating their algorithm with a small sample, the team turned it loose on the largest sample of quasars yet available: the Sloan Digital Sky Survey (SDSS) Data Release 7 catalog, which has optical-light measurements for more than 100,000 quasars, plus data from the FIRST radio survey, which has more than 400,000 celestial radio sources, to achieve a huge combined sample of radio and optical quasars for analysis.

The team found that quasars have, on average, grown steadily dimmer in both radio and optical light over the history of the universe, but have dimmed more in radio than in optical – dramatically so.

This analysis also supports the "one quasar population" model, and both that and the greater dimming in radio relative to optical light are sure to spark controversy.

Regardless, the work represents what the authors consider to be the most rigorous analysis yet of the evolution of quasars in both radio and optical emissions.

According to Singal, the technique is also useful for studying any population of objects that can be found at widely varying distances – for example, blazars or gamma-ray bursts.

More information: iopscience.iop.org/0004-637X/764/1/43