Monthly Archives: March 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.

A more potent greenhouse gas than CO2, methane emissions will leap as Earth warms (Nature)

Freshwater wetlands can release methane, a potent greenhouse gas, as the planet warms. (Image source: RGBstock.com)

Freshwater wetlands can release methane, a potent greenhouse gas, as the planet warms. (Image source: RGBstock.com)

By Morgan Kelly, Office of Communications

While carbon dioxide is typically painted as the bad boy of greenhouse gases, methane is roughly 30 times more potent as a heat-trapping gas. New research in the journal Nature indicates that for each degree that the Earth’s temperature rises, the amount of methane entering the atmosphere from microorganisms dwelling in lake sediment and freshwater wetlands — the primary sources of the gas — will increase several times. As temperatures rise, the relative increase of methane emissions will outpace that of carbon dioxide from these sources, the researchers report.

The findings condense the complex and varied process by which methane — currently the third most prevalent greenhouse gas after carbon dioxide and water vapor — enters the atmosphere into a measurement scientists can use, explained co-author Cristian Gudasz, a visiting postdoctoral research associate in Princeton’s Department of Ecology and Evolutionary Biology. In freshwater systems, methane is produced as microorganisms digest organic matter, a process known as “methanogenesis.” This process hinges on a slew of temperature, chemical, physical and ecological factors that can bedevil scientists working to model how the Earth’s systems will contribute, and respond, to a hotter future.

The researchers’ findings suggest that methane emissions from freshwater systems will likely rise with the global temperature, Gudasz said. But to not know the extent of methane contribution from such a widely dispersed ecosystem that includes lakes, swamps, marshes and rice paddies leaves a glaring hole in climate projections.

“The freshwater systems we talk about in our paper are an important component to the climate system,” Gudasz said. “There is more and more evidence that they have a contribution to the methane emissions. Methane produced from natural or manmade freshwater systems will increase with temperature.”

To provide a simple and accurate way for climate modelers to account for methanogenesis, Gudasz and his co-authors analyzed nearly 1,600 measurements of temperature and methane emissions from 127 freshwater ecosystems across the globe.

New research in the journal Nature found that for each degree that the Earth's temperature rises, the amount of methane entering the atmosphere from microorganisms dwelling in freshwater wetlands — a primary source of the gas — will increase several times. The researchers analyzed nearly 1,600 measurements of temperature and methane emissions from 127 freshwater ecosystems across the globe (above), including lakes, swamps, marshes and rice paddies. The size of each point corresponds with the average rate of methane emissions in milligrams per square meter, per day, during the course of the study. The smallest points indicate less than one milligram per square meter, while the largest-sized point represents more than three milligrams. (Image courtesy of Cristian Gudasz)

New research in the journal Nature found that for each degree that the Earth’s temperature rises, the amount of methane entering the atmosphere from microorganisms dwelling in freshwater wetlands — a primary source of the gas — will increase several times. The researchers analyzed nearly 1,600 measurements of temperature and methane emissions from 127 freshwater ecosystems across the globe (above), including lakes, swamps, marshes and rice paddies. The size of each point corresponds with the average rate of methane emissions in milligrams per square meter, per day, during the course of the study. The smallest points indicate less than one milligram per square meter, while the largest-sized point represents more than three milligrams. (Image courtesy of Cristian Gudasz)

The researchers found that a common effect emerged from those studies: freshwater methane generation very much thrives on high temperatures. Methane emissions at 0 degrees Celsius would rise 57 times higher when the temperature reached 30 degrees Celsius, the researchers report. For those inclined to model it, the researchers’ results translated to a temperature dependence of 0.96 electron volts (eV), an indication of the temperature-sensitivity of the methane-emitting ecosystems.

“We all want to make predictions about greenhouse gas emissions and their impact on global warming,” Gudasz said. “Looking across these scales and constraining them as we have in this paper will allow us to make better predictions.”

Read the abstract.

Yvon-Durocher, Gabriel, Andrew P. Allen, David Bastviken, Ralf Conrad, Cristian Gudasz, Annick St-Pierre, Nguyen Thanh-Duc, Paul A. del Giorgio. 2014. Methane fluxes show consistent temperature dependence across microbial to ecosystem scales. Nature. Article published online before print: March 19, 2014. DOI: 10.1038/nature13164 and in the March 27, 2014 print edition.