Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

New finding may explain heat loss in fusion reactors

22.01.2016

One of the biggest obstacles to making fusion power practical -- and realizing its promise of virtually limitless and relatively clean energy -- has been that computer models have been unable to predict how the hot, electrically charged gas inside a fusion reactor behaves under the intense heat and pressure required to make atoms stick together.

The key to making fusion work -- that is, getting atoms of a heavy form of hydrogen called deuterium to stick together to form helium, releasing a huge amount of energy in the process -- is to maintain a sufficiently high temperature and pressure to enable the atoms overcome their resistance to each other. But various kinds of turbulence can stir up this hot soup of particles and dissipate some of the intense heat, and a major problem has been to understand and predict exactly how this turbulence works, and thus how to overcome it.


This is a view inside the Alcator C-Mod tokamak.

Courtesy of the Plasma Science and Fusion Center/MIT

A long-standing discrepancy between predictions and observed results in test reactors has been called "the great unsolved problem" in understanding the turbulence that leads to a loss of heat in fusion reactors. Solving this discrepancy is critical for predicting the performance of new fusion reactors such as the huge international collaborative project called ITER, under construction in France.

Now, researchers at MIT's Plasma Science and Fusion Center, in collaboration with others at the University of California at San Diego, General Atomics, and the Princeton Plasma Physics Laboratory, say that they have found the key. In a result so surprising that the researchers themselves found it hard to believe their own results at first, it turns out that interactions between turbulence at the tiniest scale, that of electrons, and turbulence at a scale 60 times larger, that of ions, can account for the mysterious mismatch between theory and experimental results.

... more about:
»Atomics »Electrons »Plasma »fusion reactors »heat »physics

The new findings are detailed in a pair of papers published in the journals Nuclear Fusion and AIP Physics of Plasmas, by MIT research scientist Nathan Howard, doctoral student Juan Ruiz Ruiz, Cecil and Ida Green Associate Professor in Engineering Anne White, and 12 collaborators.

"I'm extremely surprised" by the new results, White says. She adds that it took a thorough examination of the detailed results of computer simulations, along with matching experimental observations, to show that the counterintuitive result was real.

Persisting eddies

The expectation by physicists for more than a decade had been that turbulence associated with ions (atoms with an electric charge) was so much larger than turbulence caused by electrons -- nearly two orders of magnitude smaller -- that the latter would be completely smeared out by the much larger eddies. And even if the smaller eddies survived the larger-scale disruptions, the conventional thinking went, these electron-scale whirls would be so much smaller that their effects would be negligible.

The new findings show that this conventional wisdom was wrong on both counts. The two scales of turbulence do indeed coexist, the researchers found, and they interact with each other so strongly that it's impossible to understand their effects without including both kinds in any simulations.

However, it requires prodigious amounts of computer time to run simulations that encompass such widely disparate scales, explains Howard, who is the lead author on the paper detailing these simulations. Accomplishing each simulation required 15 million hours of computation, carried out by 17,000 processors over a period of 37 days at the National Energy Research Scientific Computing Center -- making this team the biggest user of that facility for the year. Using an ordinary MacBook Pro to run the full set of six simulations that the team carried out, Howard estimates, would have taken 3,000 years.

But the results were clear, and startling. Far from being eliminated by the larger-scale turbulence, the tiny eddies produced by electrons continue to be clearly visible in the results, stretched out into long ribbons that wind around the donut-shaped vacuum chamber that characterizes a tokamak fusion reactor. Despite the temperature of 100 million degrees Celsius inside the plasma, these ribbon-like eddies persist for long enough to influence how heat gets dissipated from the swirling mass -- a determining factor in how much fusion can actually take place inside the reactor.

Previously, scientists had thought that simply simulating turbulence separately at the two different size scales and adding the results together would give a close enough approximation, but they kept finding discrepancies between those predictions and the actual results seen in test reactors. The new multiscale simulation, Howard says, matches the real results much more accurately. Now, researchers at General Atomics are taking these new results and using them to develop a simplified, streamlined simulation that could be run on an ordinary laptop computer, Howard says.

Independent evidence

In addition to the theoretical simulations, MIT graduate student Ruiz Ruiz, lead author of the second paper, has analyzed a series of experiments at the Princeton Plasma Physics Laboratory, which provided direct evidence of electron-scale turbulence that supports the new simulations. The results offer clear, independent evidence that the electron-scale turbulence really does play an important role, and they show that this is a general phenomenon, not one specific to a particular reactor design.

That's because Howard's simulations were based on MIT's Alcator C-Mod tokamak reactor, whereas Ruiz Ruiz's results were from a different type of reactor called the National Spherical Torus Experiment, which has a significantly different configuration.

Understanding the details of these different mechanisms of turbulence has been "an outstanding challenge" in the field of fusion research, White says, and these new findings could greatly improve the understanding of what's really going on inside the 10 tokamak research reactors that exist around the world, as well as in future experimental reactors under construction or planning.

"The evidence from both of these papers, that electron energy transport in tokamaks has a significant contribution from both ion and electron-scale turbulence and that multiscale simulations are needed to predict the transport, is profoundly important," says Gary Staebler, a researcher at General Atomics who was not involved in this work. "Both of these papers are very high quality," he adds. "The execution and analysis of the experiments is first class."

###

The research was supported by the U.S. Department of Energy.

Media Contact

Sarah McDonnell
s_mcd@mit.edu
617-253-8923

 @MIT

http://web.mit.edu/newsoffice 

Sarah McDonnell | EurekAlert!

Further reports about: Atomics Electrons Plasma fusion reactors heat physics

More articles from Physics and Astronomy:

nachricht Pulses of electrons manipulate nanomagnets and store information
21.07.2017 | American Institute of Physics

nachricht Vortex photons from electrons in circular motion
21.07.2017 | National Institutes of Natural Sciences

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Manipulating Electron Spins Without Loss of Information

Physicists have developed a new technique that uses electrical voltages to control the electron spin on a chip. The newly-developed method provides protection from spin decay, meaning that the contained information can be maintained and transmitted over comparatively large distances, as has been demonstrated by a team from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute. The results have been published in Physical Review X.

For several years, researchers have been trying to use the spin of an electron to store and transmit information. The spin of each electron is always coupled...

Im Focus: The proton precisely weighted

What is the mass of a proton? Scientists from Germany and Japan successfully did an important step towards the most exact knowledge of this fundamental constant. By means of precision measurements on a single proton, they could improve the precision by a factor of three and also correct the existing value.

To determine the mass of a single proton still more accurate – a group of physicists led by Klaus Blaum and Sven Sturm of the Max Planck Institute for Nuclear...

Im Focus: On the way to a biological alternative

A bacterial enzyme enables reactions that open up alternatives to key industrial chemical processes

The research team of Prof. Dr. Oliver Einsle at the University of Freiburg's Institute of Biochemistry has long been exploring the functioning of nitrogenase....

Im Focus: The 1 trillion tonne iceberg

Larsen C Ice Shelf rift finally breaks through

A one trillion tonne iceberg - one of the biggest ever recorded -- has calved away from the Larsen C Ice Shelf in Antarctica, after a rift in the ice,...

Im Focus: Laser-cooled ions contribute to better understanding of friction

Physics supports biology: Researchers from PTB have developed a model system to investigate friction phenomena with atomic precision

Friction: what you want from car brakes, otherwise rather a nuisance. In any case, it is useful to know as precisely as possible how friction phenomena arise –...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Closing the Sustainability Circle: Protection of Food with Biobased Materials

21.07.2017 | Event News

»We are bringing Additive Manufacturing to SMEs«

19.07.2017 | Event News

The technology with a feel for feelings

12.07.2017 | Event News

 
Latest News

NASA looks to solar eclipse to help understand Earth's energy system

21.07.2017 | Earth Sciences

Stanford researchers develop a new type of soft, growing robot

21.07.2017 | Power and Electrical Engineering

Vortex photons from electrons in circular motion

21.07.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>