No more zigzags: Scientists uncover mechanism that stabilizes fusion plasmas

July 18, 2018 by John Greenwald, Princeton Plasma Physics Laboratory
Physicist Isabel Krebs. Credit: Elle Starkman/PPPL Office of Communications

Sawtooth swings—up-and-down ripples found in everything from stock prices on Wall Street to ocean waves—occur periodically in the temperature and density of the plasma that fuels fusion reactions in doughnut-shaped facilities called tokamaks. These swings can sometimes combine with other instabilities in the plasma to produce a perfect storm that halts the reactions. However, some plasmas are free of sawtooth gyrations thanks to a mechanism that has long puzzled physicists.

Researchers at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have recently produced complex simulations of the process that may show the physics behind this , which is called " pumping." Unraveling the process could advance the development of fusion energy.

Fusion drives the sun and stars

Fusion, the power that drives the sun and stars, is the fusing of light elements in the form of —the hot, charged state of matter composed of free electrons and atomic nuclei—that generates massive amounts of energy. Scientists are seeking to replicate fusion on Earth for a virtually inexhaustible supply of power to generate electricity.

The flux pumping mechanism limits the current in the core of the plasma that completes the that confines the hot, charged gas that produces the reactions. This development, found in some fusion plasmas, keeps the current from becoming strong enough to trigger the sawtooth instability.

Spearheading the research that uncovered the process was physicist Isabel Krebs, lead author of a Physics of Plasmas paper describing the mechanism that was published last September and made into a DOE Office of Science highlight in June that summarizes the findings. Krebs, a post-doctoral associate, used the PPPL-developed M3D-C1 code to simulate the process on the high-performance computer cluster at PPPL, working closely with theoretical physicists Stephen Jardin and Nate Ferraro, developers of the code. "The mechanism behind magnetic flux pumping had not been understood," Jardin said. "Isabel's paper describes the process."

Hybrid scenarios

In the PPPL simulations, magnetic flux pumping develops in "hybrid scenarios" that exist between standard regimes—which include high-confinement (H-mode) and low-confinement (L-mode) plasmas—and advanced scenarios in which the plasma operates in a steady state. In hybrid scenarios, the current remains flat in the core of the plasma while the pressure of the plasma stays sufficiently high.

This combination creates what is called "a quasi-interchange mode" that acts like a mixer that stirs up the plasma while deforming the magnetic field. The mixer produces a powerful effect that maintains the flatness of the current and prevents the sawtooth instability from forming. A similar process maintains the magnetic field that protects the Earth from cosmic rays, with the molten liquid in the iron core of the planet serving as mixer.

The mechanism also regulates itself, as the simulations show. If the flux pumping grows too strong, the current in the core of the plasma stays "just below the threshold for the sawtooth instability," according to Krebs. By remaining below the threshold, the current keeps the plasma temperature and density from zigzagging up and down.

The simulations could lead to measures to avoid the troublesome swings. "This mechanism may be of considerable interest for future large-scale fusion experiments such as ITER," Krebs said. For ITER, the major international fusion experiment under construction in France, creation of a hybrid scenario could produce flux pumping and deter sawtooth instabilities

One way to develop the hybrid scenario will be for operators of ITER to experiment with the timing of the neutral beam power that will heat the ITER plasma to temperatures. Such experiments could lead to the combination of plasma current and pressure that produces sawtooth-free operation.

Explore further: Simulations of magnetically confined plasmas reveal a self-regulating stabilizing mechanism

More information: I. Krebs et al, Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations, Physics of Plasmas (2017). DOI: 10.1063/1.4990704

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4.8 / 5 (5) Jul 18, 2018
That's cool. I hope that this will also work in practice. Fusion reactors might be the real solution for clean energy in the future.
2.5 / 5 (11) Jul 18, 2018
Probably not. While fusion is good it still does create some radioactive waste.
Then there's the second issue: fusion reactors still drive steam turbines (just like fission reactors). Which in turn means: You need water. A lot. Which has to come from somewhere.

Usually this water is taken from rivers and the warmed water at the end of the cycle is discharged back into the river - heating the river up in the process. This has already lead to fission power plants having to shut down in summer because otherwise the rivers would get so warm that it would kill off all life downstream.
I.e.: fission (and by extension fusion) reactors aren't suitable for assured base power generation.

Another issue is availability. Reactors have to be taken off-line for occasional maintenance (or due to unforeseen occurrences). With such a huge concentration of power generating capability per plant this means that a lot of (very costly) redundancy would be needed.
3 / 5 (6) Jul 18, 2018
Fission and Fusion steam engines meet nature
This has already lead to fission power plants having to shut down in summer because otherwise the rivers would get so warm that it would kill off all life downstream.

that's an interesting point after all the effort getting fusion working adequately such a basic fact of nature shuts production down, it puts fusion in the same position as wind mills when wind stops, stopping production
Why are we using steam engines in the nuclear age, it's starting to make me think of Newcomen's steam engine
4.5 / 5 (2) Jul 18, 2018
Other than thermometric materials, that to my knowledge would struggle to power a cell phone, are there any methods or mechanisms for creating plentiful sustainable electricity that don't utilize a turbine, on the horizon? Other than solar of course?

I'm not sure why we(humanity) haven't invested a sizable portion of the world resources into solar energy R&D, and infrastructure. Hell, lets build that Japanese lunar solar ring.
2.3 / 5 (6) Jul 18, 2018
Sb2, I'm trying to be upbeat for you. I would agree with your assertion about building that Lunar Solar-Power Ring on one condition. Okay two conditions.

First, using automated production facilities and robots for gathering and processing space resources for the construction.

Second, you promise {pinky-swear!} the energy output is never aimed at the Earth. But is used strictly for powering space facilities and projects.

The last thing we need is to afflict poor ol'momma earth with incessant blasts of heat energy getting pumped into the atmosphere. Just to keep the lights on in Los Vegas or the Akihabara Denki Gai. As important as that is to Modern Civilization. {still need that sarcasm font!}

Considering the investment required to build the Ring? We would get a hell of a lot more profitable ROI if we invested in developing more efficient and reliable earthside machinery.

"Conservative is not a political slogan but rather a progressive activity."
4 / 5 (4) Jul 18, 2018
Thermocouples can have very high efficiencies, but they are expensive and need rare materials.

The other option, one which I rather like, is direct conversion power, but that needs other (higher energy level) forms of fusion.
1.8 / 5 (5) Jul 18, 2018
Wanna hear a funny joke?

"E. Hergen
July 17, 2018 at 8:11 AM
Dear Mr. Rossi,

in a former reply you said you have reached a power density with the ecat SK you have never reached before.

What was the highest temperature you reached with the ecat SK ever"

"Andrea Rossi
July 17, 2018 at 11:58 AM
The max T we reach is around 20 000 °C."

-Punchline: Andrea Rossi has invented hot fusion! haha
not rated yet Jul 18, 2018
Think Wendelsheim-7X !!
5 / 5 (2) Jul 19, 2018
Clearly fusion and fission share some serious issues. I'm not too worried, though, solar panels have been produced that convert 43% of the energy they receive into electricity. That kind of efficiency was unheard of just a decade ago.

Soon we'll have the ability to harvest all the energy daylight produces, rain or shine. And with solar stations positioned around the Earth, we'll be able to maintain and transport energy 24/7. We're a long way from having the infrastructure to do all of this but the need is certainly great enough to warrant more and better research, and that's what will get us there.
3 / 5 (2) Jul 19, 2018
Fission, Fusion, nuclear and coal as hot rock - put it near a kettle of water will produce steam, have we advanced that far? can't we use electrons producing electricity directly without steam kettles!

3.4 / 5 (5) Jul 19, 2018
it puts fusion in the same position as wind mills when wind stops

It's not quite as extreme and there are mitigating measures like closed-loop cooling (but those would reduce efficiency considerably)

Fusion is something we need for space exploration, so there's no question that research/development needs to go full...erm...steam ahead.

Fission, Fusion, nuclear and coal as hot rock - put it near a kettle of water will produce steam, have we advanced that far?

Well, we do have solar and wind which mostly don't go via the steam cycle (solar concentrator power plants do).
Steam itself is a bit of an issue. For every GWh produced roughly an additional 2GWh are put into the environment as waste heat (and the GWh produced eventually also winds up as heat). So we should really shift towards the types that produce power directly (PV/wind/wave) and deploy adequate battery storage/net connectivity to cancel out variability.
4.5 / 5 (2) Jul 19, 2018
...water is taken from rivers and the warmed water at the end of the cycle is discharged back into the river.

Not in a closed loop system.

A closed loop system doesn't even rely on water, it uses chemicals with a lower boiling point than water to achieve greater efficiency.
5 / 5 (1) Jul 19, 2018
Induced currant by fusion electron flow in reactors
the sun is basically heating the earth as glowing hot sphere, but the plants are converting by photosynthesis and there is Albert's photo-electric effect; so looking at all the comments their does appear to be a way of converting fusion energy directly into electron flow
Is this the death knell for conventional fusion steam generators in favour of new types of fusion generators that rely on the photo-electric effect; supplying a steady stream of moving electrons where their magnetic fields can induce electric currents directly in field coils by passing steam engine generators and the dreaded neutron and unstable plasmas
4 / 5 (4) Jul 19, 2018
It's customary for people to think that there would be problems with fusion. Put the word "nuclear" in front of it and there will be much less support. People need to teach themselves by learning and asking questions.
However silly some thoughts are about energy, we must realize that we all go through learning stages.
Some people are afraid of fusion because there is still some radioactivity in the core. They think that it will consume large amounts of fresh water, for cooling, just as coal and fission do.
The radioactivity is no big deal.
And the only other issue, that of large amounts of water for cooling is more than made up for by virtue of almost unlimited clean energy. How could all the clean energy in the world be good if it heats up too much of our freshwater supplies?
Desalination, enough to cool all the fusion reactors and to green entire deserts, needed to sequester the excess CO2 and crow more crops, build 3d cities and become a space based race, to boot!!
4.5 / 5 (2) Jul 19, 2018
Aneutronic fusion, although harder to achieve, will deliver power without having to go through the conventional steam process. It would eventually be cheaper than "regular" bulky fusion or fission.Might even compete with molten salt fission reactors!
That would be great, not only for the removal of excess CO2 by providing enough power to actually do so.
That would be great for graphene ships...
5 / 5 (1) Jul 20, 2018
Yeah even my computer uses non-water liquid cooling now. You might have a giant exhaust port somewhere pumping out exhaust hot air, but that's not any worse than any of the technology-bloc call centers and server farms and whatnot.

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