Stabilizing the plasma that fuels fusion reactions

(Photo by Elle Starkman)
PPPL physicists Robert Lunsford, left, and Rajesh Maingi, right

Beryllium, a hard, silvery metal long used in X-ray machines and spacecraft, is finding a new role in the quest to bring the power that drives the sun and stars to Earth. Beryllium is one of the two main materials used for the wall in ITER, a multinational fusion facility under construction in France to demonstrate the practicality of fusion power. Now, physicists from the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and General Atomics have concluded that injecting tiny beryllium pellets into ITER could help stabilize the plasma that fuels fusion reactions.

Experiments and computer simulations found that the injected granules help create conditions in the plasma that could trigger small eruptions called edge-localized modes (ELMs). If triggered frequently enough, the tiny ELMs prevent giant eruptions that could halt fusion reactions and damage the ITER facility.

Scientists around the world are seeking to replicate fusion on Earth for a virtually inexhaustible supply of power to generate electricity. The process involves plasma, a very hot soup of free-floating electrons and atomic nuclei, or ions. The merging of the nuclei releases a tremendous amount of energy.

In the present experiments, the researchers injected granules of carbon, lithium, and boron carbide — light metals that share several properties of beryllium — into the DIII-D National Fusion Facility that General Atomics operates for the DOE in San Diego. “These light metals are materials commonly used inside DIII-D and share several properties with beryllium,” said PPPL physicist Robert Lunsford, lead author of the paper that reports the results in Nuclear Materials and Energy. Because the internal structure of the three metals is similar to that of beryllium, the scientists infer that all of these elements will affect ITER plasma in similar ways. The physicists also used magnetic fields to make the DIII-D plasma resemble the plasma as it is predicted to occur in ITER.

These experiments were the first of their kind. “This is the first attempt to try to figure out how these impurity pellets would penetrate into ITER and whether you would make enough of a change in temperature, density, and pressure to trigger an ELM,” said Rajesh Maingi, head of plasma-edge research at PPPL and a co-author of the paper. “And it does look in fact like this granule injection technique with these elements would be helpful.”

If so, the injection could lower the risk of large ELMs in ITER. “The amount of energy being driven into the ITER first walls by spontaneously occurring ELMs is enough to cause severe damage to the walls,” Lunsford said. “If nothing were done, you would have an unacceptably short component lifetime, possibly requiring the replacement of parts every couple of months.”

Lunsford also used a program he wrote himself that showed that injecting beryllium granules measuring 1.5 millimeters in diameter, about the thickness of a toothpick, would penetrate into the edge of the ITER plasma in a way that could trigger small ELMs. At that size, enough of the surface of the granule would evaporate, or ablate, to allow the beryllium to penetrate to locations in the plasma where ELMs can most effectively be triggered.

The next step will be to calculate whether density changes caused by the impurity pellets in ITER would indeed trigger an ELM as the experiments and simulations indicate. This research is currently underway in collaboration with international experts at ITER.

The researchers envision the injection of beryllium granules as just one of many tools, including using external magnets and injecting deuterium pellets, to manage the plasma in doughnut-shaped tokamak facilities like ITER. The scientists hope to conduct similar experiments on the Joint European Torus (JET) in the United Kingdom, currently the world’s largest tokamak, to confirm the results of their calculations. Says Lunsford, “We think that it’s going to take everyone working together with a bunch of different techniques to really get the ELM problem under control.”

Learn more: Tiny granules can help bring clean and abundant fusion power to Earth


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Fusion power pushes past 100 million degrees

Fig. 1 The plasma electron temperature over 100 million degrees achieved in 2018 on EAST. (Image by the EAST Team)

The Experimental Advanced Superconducting Tokamak (EAST), nicknamed the “Chinese artificial sun”, achieved an electron temperature of over 100 million degrees in its core plasma during a four-month experiment this year. That’s about seven times more than the interior of the Sun, which is about 15 million degrees C.

The experiment shows China is making significant progress towards tokamak-based fusion energy production.

The experiment was conducted by the EAST team at the Hefei Institutes of Physical Science of the Chinese Academy of Sciences (CASHIPS) in collaboration with domestic and international colleagues. 

The plasma current density profile was optimized through the effective integration and synergy of four kinds of heating power: lower hybrid wave heating, electron cyclotron wave heating, ion cyclotron resonance heating and neutral beam ion heating. 

Power injection exceeded 10 MW, and plasma stored energy boosted to 300 kJ after scientists optimized the coupling of different heating techniques. The experiment utilized advanced plasma control and theory/simulation prediction

Scientists carried out experiments on plasma equilibrium and instability, confinement and transport, plasma-wall interaction and energetic particle physics to demonstrate long-time scale, steady-state H-mode operation with good control of impurity, core/edge MHD stability, and heat exhaust using an ITER-like tungsten divertor.

With ITER-like operating conditions such as radio frequency wave-dominant heating, lower torque, and a water-cooling tungsten divertor, EAST achieved a fully non-inductive steady-state scenario with extension of fusion performance at high density, high temperature and high confinement.

Meanwhile, to resolve the particle and power exhaust, which is crucial for high-performance steady-state operations, the EAST team employed many techniques to control the edge-localized modes and tungsten impurity with metal walls, along with active feedback control of the divertor heat load.

Operating scenarios including the steady-state high-performance H-mode and electron temperatures over 100 million degrees on EAST have made unique contributions towards ITER, the Chinese Fusion Engineering Test Reactor (CFETR) and DEMO.

These results provide key data for validation of heat exhaust, transport and current drive models. They also increase confidence in fusion performance predictions for CFETR. 

At present, the CFETR physics design focuses on optimization of a third-evolution machine with large radium at 7 m, minor radium at 2 m, a toroildal magnet field of 6.5-7 Tesla and a plasma current of 13 MA. 

In support of the engineering development of CFETR and the future DEMO, a new National Mega Science Project – theComprehensive Research Facility – will be launched at the end of this year. 

This new project will advance the development of tritium blanket test modules, superconducting technology, reactor-relevant heating and current drive actuators and sources, and divertor materials.

EAST is the first fully superconducting tokamak with a non-circular cross section in the world. It was designed and constructed by China with a focus on key science issues related to the application of fusion power. Since it began operating in 2006, EAST has become a fully open test facility where the world fusion community can conduct steady-state operations and ITER-related physics research. 

Learn more: Chinese Fusion Tool Pushes Past 100 Million Degrees



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