Plasma fingers point to the taming of the ELM

High-speed video image of the MAST plasma obtained at the start of an ELM

New images from the MAST device at Culham Centre for Fusion Energy could find a solution to one of the biggest plasma physics problems standing in the way of the development of fusion power

MAST, the Mega Amp Spherical Tokamak, is the first experiment to observe finger-like lobe structures emanating from the bottom of the hot plasma inside the tokamak's magnetic chamber. The information is being used to tackle a harmful plasma instability known as the edge localized mode, which has the potential to damage components in future fusion machines, including the key next-step ITER device.

Edge localized modes (ELMs) expel bursts of energy and particles from the plasma. Akin to solar flares on the edge of the Sun, ELMs happen during high-performance mode of operation ("H-mode"), in which energy is retained more effectively, but pressure builds up at the plasma's edge. When the pressure rises, an ELM occurs—ejecting a jet of hot material. As the energy released by these events strike material surfaces, they cause erosion which could have a serious impact on the lifetime of plasma-facing materials.

In this photo the top right hand corner has been expanded to highlight the similarity of the ELM filament with a solar eruption.

One way of tackling the problem is ELM mitigation—controlling the instabilities at a manageable level to limit the amount of harm they can do. MAST is using a mitigation technique called resonant magnetic perturbation; applying small magnetic fields around the tokamak to punch holes in the plasma edge and release the pressure in a measured way. This technique has been successful in curbing ELMs on several tokamaks.

The lobe structures that have recently been observed in MAST are caused by the resonant magnetic perturbation, which shakes the plasma and throws particles off course as they move around the magnetic field lines in the plasma, changing their route and destination. Some particles end up outside the field lines, forming finger-like offshoots near the base of the plasma. Changing the shape of a small area of the plasma in this way lowers the pressure threshold at which ELMs are triggered. This should therefore allow researchers to produce a stream of smaller, less powerful ELMs that will not damage the tokamak.

False colour images of the 'X-point' region at the base of the MAST plasma captured by the high-speed camera during H-mode—without resonant magnetic perturbation .

First predicted by US researcher Todd Evans in 2004, the lobes—known as homoclinic tangles—were seen for the first time during experiments at MAST in December 2011, thanks to the UK tokamak's excellent high-speed cameras. CCFE scientist Dr Andrew Kirk, who leads ELM studies on MAST, said: "This could be an important discovery for tackling the ELM problem, which is one of the biggest concerns for physicists at ITER. The aim for ITER is to remove ELMs completely, but it is useful to have back-up strategies which mitigate them instead. The lobes we have identified at MAST point towards a promising way of doing this."

... and with resonant magnetic perturbation, showing the finger-like lobe structures emerging from the edge of the plasma.

The lobes are significant for another reason; they are a good indicator of how well the resonant magnetic perturbation is working: "The length of the lobes is determined by the amount of magnetic perturbation the plasma is seeing," explains Dr Kirk. "So the longer the 'fingers,' the deeper the penetration. If the fingers are too long, we can see that it has gone too far in and will start to disturb the core, which is what we want to avoid."

The next phase of the research will involve developing codes to map how particles will be deposited and how the lobes will be formed around the plasma.

"We already have codes that can determine the location of the fingers but we cannot predict their length due to uncertainties in how the plasma reacts to the applied perturbations. Our measurements will allow us to validate which models correctly take this plasma response into account," said Dr Kirk. "New codes will mean we can produce accurate predictions for ITER and help them tame the ELM."

Adding fuel to MAST's fire

Firing tiny deuterium pellets into the tokamak furnace is one of the most effective ways of getting fuel into the plasma, enabling fusion reactions and the unlocking of energy

They call it the "snowball in hell"—a bullet of frozen deuterium fuel heading at high speed into the furnace-like plasma of the MAST fusion device at Culham. A team at MAST is investigating this method of fuelling plasmas and how it will work in the future on the giant international experiment ITER.

Firing these tiny pellets is one of the most effective ways of getting fuel into the plasma, enabling fusion reactions and the unlocking of energy. This will become increasingly important as future fusion devices become bigger and plasmas get hotter to reach ignition, the point at which the plasma heats itself without external input—crucial for power-producing reactors. To achieve ignition, the density of the plasma core must be raised and sustained by fuelling it.

Luca Garzotti, one of the CCFE physicists studying pellet injection on MAST, explains the process: "Just like a car engine, a tokamak needs to be fuelled—the fuel goes in to the plasma and there is an exhaust to get rid of unwanted gases. In fusion, helium comes out of the exhaust via a system called the divertor. I'm looking at how we can put the fuel (deuterium and tritium) in at the start.

Princeton lab tests lithium particles to mitigate ELMs

Close-up view of the high-speed

propellor inside the injector

The Princeton Plasma Physics Laboratory (PPPL) has successfully tested a laboratory-designed device to be used to diminish the size of instabilities known as "edge localized modes (ELMs)" on the DIII-D tokamak that General Atomics operates for the US Department of Energy in San Diego. Such instabilities can damage the interior of fusion facilities.

The PPPL device injects granular lithium particles into tokamak plasmas to increase the frequency of the ELMs. The method aims to make the ELMs smaller and reduce the amount of heat that strikes the divertor that exhausts heat in fusion facilities.

The system could serve as a possible model for mitigating ELMs on ITER, the fusion facility under construction in France to demonstrate the feasibility of fusion energy.

"ELMs are a big issue for ITER," said Mickey Wade, director of the DIII-D national fusion program at General Atomics. Large-scale ELMs, he noted, could melt plasma-facing components inside the ITER Tokamak.

General Atomics plans to install the PPPL-designed device, developed by physicist Dennis Mansfield and engineer Lane Roquemore, on DIII-D this fall. Previous experiments using deuterium-injection rather than lithium-injection have demonstrated the ability to increase the ELMs frequency on DIII-D, the ASDEX-Upgrade in Germany and the Joint European Torus in the United Kingdom.

Researchers at DIII-D now want to see how the results for lithium-injection compare with those obtained in the deuterium experiments on the San Diego facility. "We want to put them side-by-side," Wade said.

PPPL-designed systems have proven successful in mitigating ELMs on the EAST tokamak in Hefei, China, and have been used on a facility operated by the Italian National Agency for New Technologies in Frascati, Italy. A system also is planned for PPPL's National Spherical Torus Experiment (NSTX), the laboratory's major fusion experiment, which is undergoing a $94 million upgrade.

The second life of Tokamak T-15

''Fusion can help fission,'' say both Englen Azizov and Oleg Filatov. While fullfilling its mission as an ''ITER-complementary machine,'' T-15 MD will explore ''hybrid concepts'' in which the fusion neutrons are used to induce fission reactions in a fertile blanket of natural uranium or thorium.

 The Soviet tokamak T-15 was a promising machine. Built at about the same time (1983-1988) as Tore Supra in Cadarache, it was the first installation to use superconducting niobium-tin conductors. Fifteen years after "economic difficulties" stopped the project's experiments, the machine's 24 Nb3Sn toroidal field coils are still the largest in the world.

T-15 produced first plasma in 1988, demonstrated the steady-state regime of its magnetic system operation, carried out about a hundred shots but never operated at full capacity. "We would have needed some $12 million to operate it annually," remember both Englen Azizov, the Director of the Moscow Institute of Tokamak Physics, and Oleg Filatov, the Director of the Efremov Institute in Saint Petersburg. "We never had enough money to start real operations ..." The machine, as a consequence, was shut down in 1995.

Now, fifteen years later, T-15 is back on stage for a spectacular upgrade aiming at ambitious results.

T-15 MD, "MD" for Modified Divertor, will use most of the original T-15's "existing infrastructure." Systems such as power, vacuum, heating and diagnostics, which account for 80 percent of the total cost of a tokamak, will be reused in the new installation.

T-15 MD will eventually trade T-15's original "circular limiter"—like the one in Tore Supra—for a graphite divertor designed to withstand heat loads in the range of 20 MW/m², comparable to that of the ITER environment. Other upgrades include modernization of the heating and current drive systems that will enable a significant increase of heating power (up to 20 MW) and pulse durations of up to 1,000 seconds.

Final design of the new machine should be complete by 2011 and by 2014 T-15 MD should produce first plasma. Experiments in the more "ITER-like" configuration could begin in 2018. The upgraded Russian tokamak will extend the operational domain of "ITER-complementary machines" and contribute to the determination of the optimal parameters required by future reactors.

"We do not want to repeat what has already been done in other machines," explain Azizov and Filatov, "we want to explore."

Hybrid concepts are among the directions T-15 MD could explore. Hybrids proponents claim they have a much better use for the highly energetic fusion neutrons than just having them "heat" the water that circulates inside the first wall's blanket. They want to use their energy to induce fission reactions in a fertile blanket of natural uranium or thorium. This is what both Azizov and Filatov mean when they say: "Fusion can help fission."

In this perspective—which is heresy for many fusion purists—T-15 MD would be a "hydrogen prototype" that would confirm some of the physics needed to launch a "very preliminary" demonstrator for a hybrid reactor. Conceptual design for this project, already named TIN-1, could begin as early as 2011.

"Whatever direction we take," say the two Russian scientists, "we need ITER to succeed." While T-15 MD will have a full-time job in support of ITER, it will also do a little work on the side for the hybrid option being contemplated by some countries.

Daniel Clery on "A Piece of the Sun"

Science writer Daniel Clery visited the Culham Centre for Fusion Energy (CCFE) recently to talk about his book on the history of fusion, A Piece of the Sun.

The book tells the story of the quest for fusion energy, from the discovery of nuclear fusion as the Sun's power source in the early 20th century through to the latest advances in magnetic and laser fusion research as the glittering prize of near-endless energy gets closer. It is a compelling account of the ups and downs of the research, the events and personalities involved, and the science of fusion.

Daniel gave a lecture at CCFE on the "Many Faces of Fusion," based on the book.

Scientists use plasma shaping to control turbulence in stellarators

Magnetic field strength in the turbulence-optimized MPX stellarator design with regions of the highest strength shown in yellow. The MPX design is named for coauthors Harry Mynick and Neil Pomphrey of PPPL and Pavlos Xanthopoulos of the Max Planck Institute of Plasma Physics.

Researchers at the US Department of Energy's Princeton Plasma Physics Laboratory (PPPL) and the Max Planck Institute of Plasma Physics in Germany have devised a new method for minimizing turbulence in bumpy donut-shaped experimental fusion facilities called stellarators. This month in a paper published in Physical Review Letters, these authors describe an advanced application of the method that could help physicists overcome a major barrier to the production of fusion energy in such devices, and could also apply to their more widely used symmetrical donut-shaped cousins called tokamaks. This work was supported by the DOE Office of Science.

Turbulence allows the hot, charged plasma gas that fuels fusion reactions to escape from the magnetic fields that confine the gas in stellarators and tokamaks. This turbulent transport occurs at comparable levels in both devices, and has long been recognized as a challenge for both in producing fusion power economically.

"Confinement bears directly on the cost of fusion energy," said physicist Harry Mynick, a PPPL coauthor of the paper, "and we're finding how to reshape the plasma to enhance confinement."

The new method uses two types of advanced computer codes that have only recently become available. The authors modified these codes to address turbulent transport, evolving the starting design of a fusion device into one with reduced levels of turbulence. The current paper applies the new method to the Wendelstein 7-X stellarator, soon to be the world's largest when construction is completed in Greifswald, Germany.

Results of the new method, which has also been successfully applied to the design of smaller stellarators and tokamaks, suggest how reshaping the plasma in a fusion device could produce much better confinement. Equivalently, improved plasma shaping could produce comparable confinement with reduced magnetic field strength or reduced facility size, with corresponding reductions in the cost of construction and operation.

The simulations further suggest that a troublesome characteristic called “stiffness” could occur in reactor-sized stellarators. Stiffness, the tendency for heat to rapidly escape as the plasma temperature gradient rises above a threshold, has been observed in tokamaks but less so in stellarators. The possibility that stiffness might be present in reactor-sized stellarators, wrote the authors, could stimulate efforts “toward further optimizing stellarator magnetic fields for reduced turbulence.” 

WEST joins family of divertor tokamaks

On 18 December 2017, the current was raised in the divertor coils and the very first X-point plasma was obtained in the WEST tokamak (France).

The current was raised in limiter configuration up to 500 kA and controlled during a couple of seconds, while the divertor coils entered into action and an ITER-like configuration was reached.

The WEST project consists in transforming the former Tore Supra tokamak in order to extend its long pulse capability and test ITER's divertor technology. The implementation of a full tungsten, actively cooled divertor with plasma-facing units that are representative of ITER's divertor targets will allow scientists and engineers to address the risks both in terms of industrial-scale manufacturing and operation.