A boon to fusion: Scientist finds new way to predict heat layer troublemaker

Aug 20, 2012
Robert Goldston, a Princeton University professor at the US Department of Energy's Princeton Plasma Physics Laboratory, has developed a model for predicting the outflow of heat during fusion experiments, which may help overcome a key barrier to the fusion process. Credit: Elle Starkman/PPPL Office of Communications

Researchers at a recent worldwide conference on fusion power have confirmed the surprising accuracy of a new model for predicting the size of a key barrier to fusion that a top scientist at the Princeton Plasma Physics Laboratory (PPPL) has developed.

The model could serve as a starting point for overcoming the barrier. "This allows you to depict the size of the challenge so you can think through what needs to be done to overcome it," said physicist Robert Goldston, the Princeton University professor of astrophysical sciences and former PPPL director who developed the model.

Goldston was among physicists who presented aspects of the model in late May to the 20th Annual International Conference on Plasma Surface Interactions in Aachen, Germany. Some 400 researchers from around the world attended the conference. Results of the model have been "eerily close" to the data, said Thomas Eich, a senior scientist at the Max Planck Institute for Plasma Physics in Garching, Germany, who gave an invited talk on his measurements. The agreement appears too close to have happened by chance, Eich added.

Goldston's model predicts the width of what physicists call the "scrape-off layer" in tokamaks, the most widely used fusion facilities. Such devices confine hot, electrically charged gas, or plasma, in powerful magnetic fields. But inevitably flows through the system and becomes separated, or scraped off, from the edge of the plasma and flows into an area called the divertor chamber.

The challenge is to prevent a thin and highly concentrated layer of heat from reaching and damaging the plate that sits at the bottom of the divertor chamber and absorbs the scrape-off flow. Such damage would halt fusion reactions, which take place when the , or ions, inside the plasma merge and release energy. "If nothing was done and you took this right on the chin, it could be a knockout blow," said Goldston, who published his model in January in the journal .

Solving this problem will be vital for future machines like ITER, the world's most powerful tokamak, which the European Union, the United States and five other countries are building in France to demonstrate fusion as a source of clean and abundant energy. The project is designed to produce 500 megawatts of in 400 second-long pulses, which will require researchers to spread the scrape-off heat as much as possible to protect the divertor plate.

Goldston's model could help guide such efforts. He began pondering the width of the heat flux during an international physics conference in South Korea in 2010. Looking at the latest scrape-off layer data based on improved measurements, he estimated—literally on an envelope—that the new widths could be produced without plasma turbulence, a factor that is typically considered but is notoriously difficult to calculate. This led him to search for a way to estimate the width of the surprisingly thin layer, and to gauge how the width would vary as conditions such as the amount of electrical current in the plasma varied.

The way plasma flows inside tokamaks provided the major clue. The ions within the charged gas gyrate swiftly along the magnetic field lines while drifting slowly across the lines. At the same time, the electrons also in the plasma travel very rapidly along the lines and carry away most of the heat. Goldston arrived at his prediction by determining how fast these subatomic particles flow into the divertor region, and how long it therefore takes them to reach it. The result "is what we call a 'heuristic' estimate, based on the key aspects of the physics, but not a detailed calculation," said Goldston.

His estimate confirmed what Goldston had suspected: the width of the scrape-off layer nearly matched the results of a calculation, made without considering turbulence, for determining how far the ions drift away from their field lines. "What's stunning is how closely the values correspond to the data, both in absolute value and in variation with the plasma current, magnetic field, machine size and input power," Goldston said. "This does not mean that turbulence plays no role, but it suggests that for the highest performance conditions, where turbulence is weakest, the motion of the ions is dominated by non-turbulent drift effects." This will be true in the case of ITER, he added, since it is designed to operate in high-performance conditions.

Researchers are developing techniques for widening the scrape-off layer. Such methods include pumping gas into the divertor region to keep some heat from reaching the plate. Physicists use deuterium, a form of hydrogen, to block the heat, and are injecting nitrogen to turn other parts of the heat into ultraviolet light. (While charged deuterium ions are already in the plasma, the deuterium gas that is injected into the divertor region to block the heat is not electrically charged.)

These strategies look promising. "We know that they will work," said Goldston. "The outstanding question is whether they will work completely enough" to mitigate the heat flux at ITER's highest power levels, without introducing so much gas that it cools the fuel. Physicists around the world are conducting experiments to understand the process better.

For Goldston, calculating the width of the scrape-off layer marks the latest research effort in a 40-year career at PPPL, which began when he was a graduate student. Along the way he helped to pioneer techniques for heating the plasma, and developed a widely used method called "Goldston scaling" for predicting how long heat is retained in a tokamak plasma.

"First, heat is injected into the plasma," Goldston said of how tokamaks operate. "Second, that heat is retained while much more heat is generated by fusion reactions. Finally, the resulting heat has to come out of the . Without thinking about it, I have been following heat along this trajectory throughout my whole research career," he added. "We have made great progress on the first two steps, and now the most exciting challenge, to me, is the one that comes because of our success so far. Now we need to learn to handle the the outflow of heat from a high-power fusion energy source."

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User comments : 12

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Parsec
5 / 5 (3) Aug 20, 2012
Another giant step towards fusion power! When will the estimated time until we see commercial fusion power reactors get less than 20 years? Its been about 20 years in the future for at least the last 50 years (that I am personally aware of).

Please do not misunderstand. I am as hopeful as anyone. Its just going kinda slow...
ValeriaT
1 / 5 (6) Aug 21, 2012
IMO the hot fusion will be never used due to cold fusion competition. All money invested into this research are wasted already. I hope, all researchers who willingly ignored the cold fusion finding for last twenty years will end in the hell. They don't deserve a better destiny.
TheGhostofOtto1923
4.1 / 5 (14) Aug 21, 2012
IMO the hot fusion will be never used due to cold fusion competition. All money invested into this research are wasted already.
Research into confining, manipulating, transporting, and applying materials in plasma form is essential to the future of civilization. It has been essential to promise the public whatever it takes to garner their support for this research.
I hope, all researchers who willingly ignored the cold fusion finding for last twenty years will end in the hell. They don't deserve a better destiny.
There is no hell - dont you read any comments besides your own here?

Everything in its own Time. Continued fossil fuel use was necessary to maintain western influence in the third world. If cold fusion had been developed we could not have pursued plasma physics as we would have been overrun by third worlders.

Its like a big Equation. If - then; if not - then this other thing.
TheGhostofOtto1923
3.9 / 5 (15) Aug 21, 2012
Keep your pants on - we should have some news in a few weeks;
http://www.e-catw...-or-8th/
ValeriaT
1 / 5 (3) Aug 21, 2012
Continued fossil fuel use was necessary to maintain western influence in the third world.
It lead the world into WWW II and it may lead into WWW III soon. The people, who are defending the obstructions of progress connected with many deaths and devastation of life environment don't deserve the future. It's perverse ideology and you deserve the slow death at Sudan desert for it.
TheGhostofOtto1923
3.9 / 5 (15) Aug 21, 2012
It's perverse ideology
Perhaps but it IS the truth. Exploding populations in the middle east WOULD have reformed into a vast islamist empire. They would still have been able to construct vast armies running on fossil fuel, and paid for with wholly-independent internal economies.

As it was, western influence kept all of this from happening by creating an overheated technology base which could quickly consume all that troublesome petroleum down to safe levels, and be recoverable using tech only THE WEST could supply.
and you deserve the slow death at Sudan desert for it.
But then how would you learn anything about geopolitics?
ValeriaT
1 / 5 (3) Aug 21, 2012
Exploding populations in the middle east WOULD have reformed into a vast islamist empire
You're speculating way too much. You're essentially trying to convince me, that the application of more effective and environmentally friendly energy technology would harm the civilization. Especially under the situation, when ALL problems of western world with middle civilization comes from the fact, these "evil Arabs" don't want to sell their oil for free. You know, the Arabians would fuck the America heartily, if they wouldn't be exploited for so many years. How do you think the Arabs differ from Indians, for example? The India has no oil, so it's perceived as a peaceful country. You're just manufacturing enemies from people, because you need to fight against them. You need a pretense for another oil war - that's the whole story.
ValeriaT
1 / 5 (2) Aug 21, 2012
"middle civilization" = "middle East civilization"
TheGhostofOtto1923
4.4 / 5 (14) Aug 22, 2012
You're speculating way too much. You're essentially trying to convince me, that the application of more effective and environmentally friendly energy technology would harm the civilization.
Religionist civilizations are designed to exceed their ability to support themselves. You think Islamist women will stop making and raising babies because they have a new kind of heater in the basement? More resources means more growth, more instability, more trouble. Western culture has beaten this equation. Religionist culture depends upon it.
The India has no oil, so it's perceived as a peaceful country.
You should read scouting about India. When Pakistan was formed, ethnic cleansing on both sides killed perhaps 2 million.

India has fought 2 wars with Pakistan and have ongoing tensions in Kashmir. Elsewhere Hindus are slaughtering Moslems and vice versa. In the east communist insurgents at waging war. It is among the fastest growing countries in the world.
Standing Bear
5 / 5 (2) Aug 26, 2012
Why is it that most posters in here can NOT stay on message. The topic IS fusion and plasma modeling pursuant to actualizing not only possible surplus energy reactions without excess lose of consumables, but optimizing the efficiency of those reactions.
ValeriaT
1 / 5 (2) Aug 26, 2012
A modeling? Physicists model the hot fusion for fifty years - it's a gold mine for these people. But the modeling will not solve energetic crisis, the modeling of hot fusion the less.
sirchick
not rated yet Aug 27, 2012
Why is it that most posters in here can NOT stay on message. The topic IS fusion and plasma modeling pursuant to actualizing not only possible surplus energy reactions without excess lose of consumables, but optimizing the efficiency of those reactions.


Because they probably are not what you and I call "intelligent" beings. Sadly the internet has no anti-moron software developed for it. :(

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