Discovered: Optimal magnetic fields for suppressing instabilities in tokamaks

September 10, 2018, Princeton Plasma Physics Laboratory
The diagram shows the beneficial magnetic distortion as color-shaded regions on the smooth doughnut-shaped surface of the KSTAR plasma, together with the external 3D field coils in blue and red used to generate the distortion. Credit: Jong-Kyu Park, Princeton Plasma Physics Laboratory.

Fusion, the power that drives the sun and stars, produces massive amounts of energy. Scientists here on Earth seek to replicate this process, which merges light elements in the form of hot, charged plasma composed of free electrons and atomic nuclei, to create a virtually inexhaustible supply of power to generate electricity in what may be called a "star in a jar."

A long-time puzzle in the effort to capture the power of fusion on Earth is how to lessen or eliminate a common instability that occurs in the plasma called edge localized modes (ELMs). Just as the sun releases enormous bursts of energy in the form of solar flares, so flare-like bursts of ELMs can slam into the walls of doughnut-shaped tokamaks that house fusion reactions, potentially damaging the walls of the reactor.

Ripples control new bursts

To control these bursts, scientists disturb the plasma with small magnetic ripples called resonant magnetic perturbations (RMPs) that distort the smooth, doughnut shape of the plasma—releasing excess pressure that lessens or prevents ELMs from occurring. The hard part is producing just the right amount of this 3-D distortion to eliminate the ELMs without triggering other instabilities and releasing too much energy that, in the worst case, can lead to a major disruption that terminates the plasma.

Making the task exceptionally difficult is the fact that a virtually limitless number of magnetic distortions can be applied to the plasma, causing finding precisely the right kind of to be an extraordinary challenge. But no longer.

Physicist Jong-Kyu Park of the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), working with a team of collaborators from the United States and the National Fusion Research Institute (NFRI) in Korea, have successfully predicted the entire set of beneficial 3-D distortions for controlling ELMs without creating more problems. Researchers validated these predictions on the Korean Superconducting Tokamak Advanced Research (KSTAR) facility, one of the world's most advanced superconducting tokamaks, located in Daejeon, South Korea.

KSTAR ideal for tests

KSTAR was ideal for testing the predictions because of its advanced magnet controls for generating precise distortions in the near-perfect, doughnut-shaped symmetry of the plasma. Identifying the most beneficial distortions, which amount to less than one percent of all the possible distortions that could be produced inside KSTAR, would have been virtually impossible without the predictive model developed by the research team.

The result was a precedent-setting achievement. "We show for the first time the full 3-D field operating window in a tokamak to suppress ELMs without stirring up core instabilities or excessively degrading confinement," said Park, whose paper—written with 14 coauthors from the United States and South Korea—is published in Nature Physics. "For a long time we thought it would be too computationally difficult to identify all beneficial symmetry-breaking fields, but our work now demonstrates a simple procedure to identify the set of all such configurations."

Researchers reduced the complexity of the calculations when they realized that the number of ways the plasma can distort is actually far fewer than the range of possible 3-D fields that can be applied to the plasma. By working backwards, from distortions to 3-D fields, the authors calculated the most effective fields for eliminating ELMs. The KSTAR experiments confirmed the predictions with remarkable accuracy.

Findings provide new confidence

The findings on KSTAR provide new confidence in the ability to predict optimal 3-D fields for ITER, the international tokamak under construction in France, which plans to employ special magnets to produce 3-D distortions to control ELMs. Such control will be vital for ITER, whose goal is to produce 10 times more energy than it will take to heat the . Said authors of the paper, "the method and principle adopted in this study can substantially improve the efficiency and fidelity of the complicated 3-D optimizing process in tokamaks."

Explore further: Steady as she goes: Scientists tame damaging plasma instabilities in fusion facilities

More information: Jong-Kyu Park et al, 3D field phase-space control in tokamak plasmas, Nature Physics (2018). DOI: 10.1038/s41567-018-0268-8

Related Stories

Superconducting tokamaks are standing tall

January 17, 2018

A persistent problem has dogged the largest fusion device in South Korea. The Korean Superconducting Tokamak Advanced Research (KSTAR) device has run successfully since 2008. However, controlling the vertical position of ...

Recommended for you

Researchers study interactions in molecules using AI

October 19, 2018

Researchers from the University of Luxembourg, Technische Universität Berlin, and the Fritz Haber Institute of the Max Planck Society have combined machine learning and quantum mechanics to predict the dynamics and atomic ...

Pushing the extra cold frontiers of superconducting science

October 18, 2018

Measuring the properties of superconducting materials in magnetic fields at close to absolute zero temperatures is difficult, but necessary to understand their quantum properties. How cold? Lower than 0.05 Kelvin (-272°C).

The big problem of small data: A new approach

October 18, 2018

Big Data is all the rage today, but Small Data matters too! Drawing reliable conclusions from small datasets, like those from clinical trials for rare diseases or in studies of endangered species, remains one of the trickiest ...


Adjust slider to filter visible comments by rank

Display comments: newest first

Mark Thomas
4.7 / 5 (6) Sep 10, 2018
The findings on KSTAR provide new confidence in the ability to predict optimal 3-D fields for ITER

Sounds like good progress to me. It is a shame ITER won't be ready for D-T plasma until 2035, at the earliest.


Even if ITER is ultimately successful by say 2040, I can only imagine using weak magnetic field niobium-tin superconductors at that point will appear as antiquated as using lead-acid batteries for electric vehicles today. MIT is going in different direction with their superconductors and I like their approach much better.

Thorium Boy
3.7 / 5 (3) Sep 11, 2018
Stars have symmetric gravitational fields to contain the plasma being fused. These machines have little hope of symmetric confinement. The magnetic fields probably need to be uniform to a few more zeros than they can manage. Which leaves the NIF and their lasers.
4 / 5 (4) Sep 11, 2018
Discrepancies in magnetic field looping
Take a magnetic solenoid and bend it in a circle so it is a doughnut the magnetic field no longer goes through the solenoid, but flips 90 degrees and goes through the centre of the doughnut
This why confinement coils are intermittent coils

So why in a steel transformer coil the magnetic field goes through the steel or the equivalent of looping the solenoid and the magnetic field continuing looping inside the solenoid which in practice it doesn't
4.3 / 5 (4) Sep 11, 2018
Application of theory in compressed uniform magnetic fields
By enabling the magnetic field to go down a continuous solenoid, there are no pinch points of instability, as it will be highly compressed uniform field
As the magnetic field loops in the transformer coil, a solenoid should enable the same effect in air or the vacuum – what are the atoms and electrons doing in the atomic lattice to the mangnetic solenoids wrapped around the iron solenoid core
2.8 / 5 (9) Sep 11, 2018
So much effort in suppressing instabilities and the natural processes of plasma. Success will only come when they allow the plasma to do what it wants and only then will they succeed. These guys use the instabilities to their advantage.
3.5 / 5 (8) Sep 11, 2018
Hubble Bubble Boiling Plasma

Solar like flares:- Edge localized modes, the sun releases enormous bursts of energy in solar flares, flare-like bursts of ELMs slam into the walls of doughnut-shaped tokomaks that house fusion reactions, damaging the walls of the reactor

At last were getting somewhere seeing as there are realistic similarities of actual starry solar fusion, these mini solar flares are the energy of fusion where these solar flares are recycled in the reactor core like rising water bubbles in boiling water, where this is rising solar bubbles in boiling plasma

The reactor is obviously a mite to small for the task in hand, the plasma maybe only millimetres in diameter but the mini-solar flares just like the sun need space to expand in the reactor vessel, the reactor vessel needs to be an underground cavern deep underground so the mini solar flares only at worst strike the granite cavern wall.

Is this expedient – cavernous solar fusion deep under ground?
3 / 5 (2) Sep 11, 2018
Sounds encouraging, but not clear how much it has improved the plasma duration, temperature, density parameters.
5 / 5 (1) Sep 11, 2018
So much effort in suppressing instabilities and the natural processes of plasma. Success will only come when they allow the plasma to do what it wants and only then will they succeed. These guys use the instabilities to their advantage.

I agree, use the instabilities to be the generators of power, more direct.
not rated yet Sep 11, 2018
For those grousing impatient? Just keep in mind, a fusion reactor has to be as perfect as humanly possible. Every time, all the time.

They actually succeed in producing theoretical amounts of energy? One mistake. One bungle. One poor quality piece of the machine as a cost-cutting measure. One substandard part accepted for a bribe.

One failure to contain? And the Public will not be amused. Especially if the failure is spectacular enough to destroy clues and evidence needed to redesign and avoid future fails.

I think it is a stupid risk. But the extortionate tactics and sleazy propaganda by the Atomic Priesthood of Linn force all to meekly bow to the altar where Humanity and the Biosphere are bloody sacrifices to Radioactive Mammon and Nuclear Moloch.
Mark Thomas
5 / 5 (2) Sep 11, 2018
Humanity and the Biosphere are bloody sacrifices to Radioactive Mammon and Nuclear Moloch.

rrwillsj, you sure can wax poetic. Unfortunately it is not supported by the facts. Fusion reactors only blow up like fusion bombs in the movies. The radioactivity due to neutron damage is relatively tiny and there are forms of aneutronic or nearly aneutronic fusion for the purist, like you..

You need to keep in mind that even if we somehow perfect renewables for Earth-based power generation, we need fusion for space exploration.

1 / 5 (1) Sep 17, 2018
The biggest issue is ensuring the magnetic field does stop and let the atomic process loose.
If the star in a jar gets out. Earth will become a new sun in a millionth of a second if it does.
How can the safety of the process be assured in a cyber attack, terrorist attack, cyclone or earth quake? This could be a world maker. Could just as easily be a world breaker.
We cant guarantee current nuclear reactors safety if you look at the Russian and Japan for example.
Accidents with this technology is lot more serious.
not rated yet Sep 17, 2018
If the star in a jar gets out. Earth will become a new sun in a millionth of a second

Umm, no. Any problem with containment and the plasma goes "poof", cools catastrophically and fusion stops.

Far more people are killed by burning coal for energy every year than have died in all the nuclear mishaps combined.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.