A magnetic bottle for a star

What fusion is

In fusion, two light atomic nuclei join to form a heavier one and release energy. It is what happens inside the Sun, which according to ITER turns about 600 million tonnes of hydrogen into helium every second. The products of fusion weigh slightly less than the nuclei that went in, and that missing mass is released as energy, as Einstein’s E = mc² describes.

On Earth, the reaction that researchers consider easiest to achieve combines two kinds of hydrogen: deuterium and tritium. Together they produce a helium nucleus and a neutron, and the neutron carries most of the energy away. Of the reactions considered, this one gives the most energy at the lowest temperature.

Deuterium can be extracted from water. Tritium is rare in nature, so the plan is to produce it inside the reactor itself, from lithium in its wall.

Why it has to be so hot

Atomic nuclei are positively charged and repel each other. To fuse, they have to collide fast enough to overcome that repulsion, which means the fuel must be extremely hot. The core of the Sun is at about 15 million degrees Celsius. A laboratory has to go much hotter: for deuterium and tritium, ITER gives a temperature of 150 million degrees Celsius, ten times hotter than the core of the Sun.

At such temperatures the gas becomes a plasma: the electrons separate from the nuclei, and the fuel is a mixture of charged particles. This is the state of matter the Swiss Plasma Center studies. The amount of plasma is small: EPFL notes that the hot plasma in a tokamak weighs just a few grams.

A bottle made of magnetic fields

No material could withstand contact with such a plasma, and the plasma would cool down instantly if it touched a wall. Charged particles, however, follow magnetic field lines. A tokamak uses this to build a magnetic cage. EUROfusion explains that the word is a Russian acronym describing a torus-shaped vacuum chamber surrounded by magnetic coils, and calls the tokamak the most advanced and best investigated fusion device design available today.

What the plasma looks like in cross-section, round, stretched, D-shaped or something stranger, depends on the coils around it. That cross-section is exactly what TCV was built to vary.

Three kinds of coils

A tokamak’s magnetic cage is built from several sets of coils working together. ITER, the international tokamak being built in France, shows the pattern clearly:

• Toroidal field coils are D-shaped coils standing around the ring; they create the toroidal field, which runs the long way round the torus. ITER has 18 of them, each weighing about 330 tonnes, with a maximum field of 11.8 tesla.
• The central solenoid sits in the hole of the ring and drives an electric current through the plasma itself. In ITER it can drive a plasma current of 15 megaamperes.
• Poloidal field coils are rings around the machine that shape the plasma. ITER has six; TCV, built for shaping, has sixteen.

In ITER the large coils are superconducting, made of niobium-tin or niobium-titanium and cooled with helium to about 4 kelvin, roughly −269 °C. Superconductors for such magnets are tested at the SPC’s SULTAN facility (see Beyond the tokamak).

Why the shape matters

The shape of the plasma affects how well it holds its heat and how much turbulence carries that heat out. It also decides how the hot edge of the plasma meets the wall. In most tokamaks the edge is led into a separate region, the divertor, so that contact with the wall happens at a distance from the hot plasma and fewer impurities get in; the SPC notes that plasmas which touch a limiter directly suffer from a high influx of impurities and poorer confinement.

Experiments on TCV with the snowflake divertor and with negative triangularity are about these questions: how to keep the heat in, and how to get the exhaust out without damaging the machine.

Why bother: the case for fusion

ITER summarises why fusion is pursued. Fusing atoms releases nearly four million times more energy than a chemical reaction such as burning coal, oil or gas. Deuterium can be distilled from any water, and ITER estimates that land-based reserves of lithium would allow fusion power plants to run for more than 1,000 years. The reaction emits no carbon dioxide; its main by-product is helium.

There are safety arguments too. According to ITER, there is no risk of a chain reaction in a fusion device, and the vessel holds only enough fuel for a few seconds of operation at any time. Fusion produces no high-activity, long-lived nuclear waste, although parts of the machine become activated and, depending on the wall materials, are expected to be recyclable or reusable within 100 years.

There is no fusion power plant yet. ITER is the experiment meant to show that fusion is feasible as an energy source, and ITER itself says that the cost of fusion electricity cannot yet be estimated.

From experiments to a power plant

Medium-sized tokamaks like TCV test ideas that are then used on larger machines. ITER, under construction in Cadarache in France, is the large international experiment; DEMO is the name used in Europe for a demonstration power plant after it. The SPC takes part in work on both through international collaborations; its story is on A fusion laboratory by the lake.