The sun on the earth

Huge volumes of energy, fuel in abundance, comparatively environmentally friendly and safe: Nuclear fusion has what it takes to feed the world’s increasing hunger for energy in the long term. Initial success has been achieved. But taming solar fire remains an immense technical challenge.

Greifswald – the old university and Hanseatic city on the Mecklenburg-Vorpommern Baltic coast – has lots to offer: the biggest museum harbor in Germany, with 45 historic tall ships, the house where Caspar-David Friedrich was born, and weekly sailing regattas. But maybe the city’s biggest attraction is located off of the beaten track on the eastern edge of the city. Surrounded by fields, community garden plots and a shopping center and nestled in an outwardly unassuming complex of buildings – stands Wendelstein 7-X, the world’s newest nuclear fusion reactor. Deep in the core of the facility, researchers from the Max-Planck-Institute for Plasma Physics repeatedly ignite solar fire. Since operations began in December 2015, Wendelstein 7-X has transformed minute quantities of gas into an ultra-thin, extremely hot plasma around 2,200 times for up to six seconds, and even reaching temperatures of up to 100 million degrees Celsius for a brief period. This plasma is the visible evidence of nuclear fusion – the source of energy that makes our sun shine and might one day provide the human race with energy.

The sun has already been at this for a very long time

Nuclear fusion is actually old news. Our sun has already been doing this for around 4.6 billion years. Temperatures of 15 million degrees Celsius and up to 200 billion bars of pressure at its center cause hydrogen atoms to fuse into helium. Mass is lost as fusion occurs, and is released as energy. The sun converts around five million tons of material into energy every second, releasing it into space as heat and light. Since the 1940s, scientists have been trying to use nuclear fusion under the conditions present on the earth.The possibilities are just too tempting. If a power plant were to fuse the two types of hydrogen called deuterium and tritium into helium at some time in the future, the energy yield would be enormous. One gram of fuel could release around 90,000 kilowatt hours of energy – the equivalent of the heat released from the combustion of eleven tons of coal, and sufficient to provide 30 households with power for a year. Lack of fuel is not an issue. The types of hydrogen required for the fusion process are available on earth in almost inexhaustible quantities. Deuterium is in sea water and tritium can be obtained from the light metal lithium. Nuclear fusion is also seen as “clean nuclear energy“, as no emissions and hardly any radioactivity are produced. Neutrons whizz through the reactor as a product of nuclear fusion. Their kinetic energy is converted into heat and then used to produce power. But the neutrons also activate the walls of the plasma vessel. ”Depending on the degree of activation, the radioactive components could be reprocessed after dismantling, or would need to be stored safely for just 100 years,” says Dr. Christoph Pohl, nuclear technology expert at TÜV Rheinland. In comparison, depending on the material and without any treatment, radioactive waste from nuclear fission has to be stored for several hundred thousand years. Disasters like Chernobyl and Fukushima are out of the question. “With nuclear fusion, there is no uncontrolled chain reaction as there is with nuclear fission,“ says Dr. Pohl, who was involved in the safety assessment required for the operating license for Wendelstein 7-X.

Heated up with microwaves

The basic principle of nuclear fusion might sound simple, but technical implementation is complex. The challenges are huge. Because the pressure isn’t as great as at the sun’s core, you need 100 million degrees Celsius instead of 15 million degrees Celsius to set nuclear fusion in motion. So huge amounts of energy are expended to heat the gas with microwaves. This means that current fusion reactors consume a lot more energy than they produce. US researchers recently celebrated a breakthrough when they achieved positive energy output in the lab for the first time – even if the energy surplus was only the equivalent of two AA batteries. Taming the plasma is another problem. No material in the world can withstand direct contact with 100 million degrees Celsius. And the plasma also cools on contact with material, immediately halting the nuclear fusion. The expensive solution: scientists generate magnetic fields inside the reactor, and these hold the plasma suspended in a circle, keeping it out of contact with the vessel walls. Opinion differs on how the magnetic field is formed.

Breakthrough with ITER

International research into tokamak reactors is forging ahead throughout the world. They construct the magnetic cage with external magnetic coils to encircle the plasma. To stabilize the plasma, a flow of current is used to create a further magnetic field inside the plasma. But the plasma current flow needs to keep increasing, which is why tokamak reactors are unable to continuously maintain the fusion process in plasma continuously. ITER (Latin: route) – the world’s biggest tokamak experimental reactor at the time – was built in Cadarache in the south of France in 2007. The aim is for ITER to prove that it is technically possible to generate net energy output through nuclear fusion. The idea is for it to produce 500 megawatts from 830 cubic meters of plasma – ten times more than the requirement for running a fusion reactor. Plasma is to be ignited for the first time in 2025. ITER is a joint project between the world’s biggest industrial nations: the EU, Russia, Japan, India, China, South Korea and the USA are involved. “ITER has swallowed up 20 billion euros so far. That’s a lot if the project fails, but not much if it succeeds,“ says Dr. Christoph Pohl, who worked with TÜV Rheinland on material testing for ITER.

Bagel-shaped magnetic field

Wendelstein 7-X is mainly financed by Germany and the EU. As a stellarator reactor, the Greifswald facility can run continuously on ­nuclear fusion. It doesn’t need an increasing electric current running through the plasma, as the external coils are able to generate the stabilizing field on their own. So the design is a lot more complex than for a tokamak: calculated by high-performance computers, the shape of the Wendelstein 7-X reactor cell looks a lot like a twisted bagel. Wendelstein 7-X is the world’s biggest stellarator reactor – with 30 cubic meters of plasma, but still much smaller than ITER. Plasma discharges lasting up to 30 minutes are designed to test whether the stellarator is suitable as a power plant. Net energy output is not the aim – that task belongs to ITER. The reactor type that ultimately wins the day is open to question. But a demo reactor will come after ITER and Wendelstein 7-X, and it will have all the features required of any future power plant. A prototype based on this will ultimately serve as a blueprint for a commercially viable fusion power plant. At around 1.5 gigawatts, this would deliver the electrical power of a large nuclear power plant. If costs fall and the political impetus remains, nuclear fusion could replace energy production from nuclear fission, gas and coal and supplement renewable energy in 40 to 80 years. According to experts, nuclear fusion could cover some 20 to 30 percent of Europe’s energy needs by 2100. This would be truly sensational – and it all started somewhere on the outskirts of Greifswald.

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