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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Triple-Threat Method Sparks Hope for Nuclear Fusion Energy

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THE Z-MACHINE: The intense electrical discharge of New Mexico’s Sandia National Laboratories’ Z-machine is used in attempts to trigger nuclear fusion. Image: Randy Montoya/Sandia National Laboratories

The secrets to its success are lasers, magnets and a big pinch

The Z machine at Sandia National Laboratories in New Mexico discharges the most intense pulses of electrical current on Earth. Millions of amperes can be sent towards a metallic cylinder the size of a pencil eraser, inducing a magnetic field that creates a force — called a Z pinch — that crushes the cylinder in a fraction of a second.

Since 2012, scientists have used the Z pinch to implode cylinders filled with hydrogen isotopes in the hope of achieving the extreme temperatures and pressures needed for energy-generating nuclear fusion. Despite their efforts, they have never succeeded in reaching ignition — the point at which the energy gained from fusion is greater than the energy put in.

But after tacking on two more components, physicists think they are at last on the right path. Researchers working on Sandia’s Magnetized Liner Inertial Fusion (MagLIF) experiment added a secondary magnetic field to thermally insulate the hydrogen fuel, and a laser to preheat it (see ‘Feeling the pinch’). In late November, they tested the system for the first time, using 16 million amperes of current, a 10-tesla magnetic field and 2 kilojoules of energy from a green laser.

“We were excited by the results,” says Mark Herrmann, director of the Z machine and the pulsed-power science center at Sandia. “We look at it as confirmation that it is working like we think it should.”

The experiment yielded about 1010 high-energy neutrons, a measure of the number of fusion reactions achieved. This is a record for MagLIF, although it still falls well short of ignition. Nevertheless, the test demonstrates the appeal of such pulsed-power approaches to fusion. “A substantial gain is more likely to be achieved at an early date with pulsed power,” says nuclear physicist David Hammer of Cornell University in Ithaca, New York, who co-wrote a 2013 US National Research Council assessment of approaches to fusion energy.

With its relatively slim US$5-million annual budget, MagLIF is a David next to two fusion Goliaths: the $3.5-billion National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California, and the €15-billion (US$20-billion) ITER experiment under construction in France. (Sandia has about $80 million to operate the Z machine each year, but it serves other experiments in addition to MagLIF.) The NIF squashes fuel capsules using nearly 2 megajoules of laser energy, and ITER will use 10,000 tons of superconducting magnets in a doughnut-shaped ‘tokamak’ to hold a plasma in place to coax self-sustaining fusion.

Both of the big projects have run into problems.

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South Korea Makes Billion-Dollar Bet on Fusion Power

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It will generate “some 1 billion watts of power for several weeks on end”

A fusion power demonstration reactor to be built in the 2030s in collaboration with the DoE’s Princeton Plasma Physics Lab, represents a step toward commercial use

South Korea has embarked on the development of a preliminary concept design for a fusion power demonstration reactor in collaboration with the US Department of Energy‘s Princeton Plasma Physics Laboratory (PPPL) in New Jersey.

The project is provisionally named K-DEMO (Korean Demonstration Fusion Power Plant), and its goal is to develop the design for a facility that could be completed in the 2030s in Daejeon, under the leadership of the country’s National Fusion Research Institute (NFRI).

South Korea is already developing the Korea Superconducting Tokamak Advanced Research (K-STAR) project and contributing to ITER, the €15-billion (US$20-billion) experimental reactor being built in Cadarache, France, under the auspices of an international collaboration. K-DEMO is intended to be the next step toward commercial reactors and would be the first plant to actually contribute power to an electric grid.

“It is a very smart strategy to take advantage of the experience gained in constructing ITER and to immediately proceed to construct a fusion power plant like K-DEMO,” says Stephen Dean, president of Fusion Power Associates, an advocacy group in Gaithersburg, Maryland.

K-DEMO will serve as prototype for the development of commercial fusion reactors. According to the PPPL, it will generate “some 1 billion watts of power for several weeks on end”, a much greater output than ITER’s goal of producing 500 million watts for 500 seconds by the late 2020s.

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via Scientific American – Soo Bin Park and Nature magazine
 

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Fusion power: Next ITERation?

Joint European Torus

Image by Kevan via Flickr

Generating electricity by nuclear fusion has long looked like a chimera. A reactor being built in Germany may change that

AS THE old joke has it, fusion is the power of the future—and always will be. The sales pitch is irresistible: the principal fuel, a heavy isotope of hydrogen called deuterium, can be extracted from water. In effect, therefore, it is in limitless supply. Nor, unlike fusion’s cousin, nuclear fission, does the process produce much in the way of radioactive waste. It does not release carbon dioxide, either. Which all sounds too good to be true. And it is. For there is the little matter of building a reactor that can run for long enough to turn out a meaningful amount of electricity. Since the first attempt to do so, a machine called Zeta that was constructed in Britain in the 1950s, no one has even come close.

At the moment, the main bet being placed by fusion enthusiasts is on ITER, the International Thermonuclear Experimental Reactor, a research machine that can hold 840 cubic metres of hot, gaseous fuel. It is being bolted together at a projected cost of €15 billion ($22 billion) in the south of France. ITER is what is known as a tokamak, a doughnut-shaped device invented in Russia at about the same time Zeta was active. Deuterium (along with an even heavier hydrogen isotope called tritium, which is made by bombarding either deuterium or lithium with neutrons) is injected into the doughnut, heated to the point at which its electrons break free and it forms a plasma, and squeezed by magnetic fields.

If the speed of the nuclei (a consequence of their temperature) and their density (a consequence of the magnetic squeezing) can both be made high enough, that will overcome the mutual repulsion of the nuclei’s positive electric charges. This allows a short-range phenomenon called the strong nuclear force to take over and causes the nuclei to merge and form helium. The fusion of deuterium and tritium into helium in this way releases energy—enough of it, in theory, both to power the reactor and to yield a surplus that can be converted into electricity. It also releases neutrons, which engineers hope to use to make tritium and thus close the fuel cycle.

Unfortunately, there is a fundamental snag. The shape of the reactor means that the magnetic field which does the squeezing (and thus also keeps the superhot plasma away from the walls) produces different forces in the inner and outer parts of the doughnut. That would result in a turbulent release of plasma if it were not counteracted by a second magnetic field created by an electric current induced in the plasma itself.

The problem is that sustaining this second current is hard, and if its level varies too much, the system breaks down. That means the reactor is constantly starting and stopping. This is not a tenable arrangement for a commercial power station. One of ITER’s goals is to get the length of individual runs up to 50 minutes. (In ITER’s predecessor, the Joint European Torus, runs lasted for a matter of seconds.) Even that, though, is not really satisfactory. Hence the interest in another reactor design, the stellarator, a rival to the tokamak which fell behind in the 1960s but which is now making a comeback.

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