For decades, physicists have had a dream: to ignite a controlled solar fire on Earth by fusing hydrogen nuclei.

If the project succeeds, then – according to the imagination – an almost inexhaustible source of energy would be available.

An international research group at the Lawrence Livermore National Laboratory (LLNL) in Berkeley near San Francisco has now made major progress in fusion research.

As Alex Zylstra and his colleagues report in "Nature" (doi: 10.1038/s41886-021-04281), they have succeeded in igniting a mixture of the heavy hydrogen isotopes deuterium and tritium and keeping it burning.

In the process, more energy was released than was previously deposited in the fuel.

Manfred Lindinger

Editor in the department "Nature and Science".

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Fusion processes are the energy source of the glow of the Sun and most other stars.

Various approaches are being pursued to allow them to run in a controlled manner on Earth: Facilities such as the "Joint Eruropean Torus" (JET) in Culham, UK, or the "International Thermonuclear Experimental Reactor" (ITER) still under construction in southern France, work with a Magnetic confinement of the fuels: They are held together in a vacuum vessel as a hot plasma by strong magnetic fields.

By supplying energy, the temperature and the density of the plasma are increased until the conditions for nuclear fusion are present.

In the so-called inertial fusion, on the other hand, as researched by the National Ignition Facility at the LLNL, a pea-sized capsule with stored deuterium and tritium is briefly irradiated with 192 beams from a high-power laser.

With such a laser pulse, a pressure of around 100 billion bar and finally a temperature of 100 million degrees are reached in a small volume area - in the center of the sun it is just 15 million degrees.

Under these conditions, the plasma becomes so hot and dense that the hydrogen nuclei in it fuse.

In the past, however, it has proven to be extremely difficult to align the many laser beams with sufficient precision to their target in order to generate a uniform radiation field there.

In 2014, it was possible to fuse deuterium and tritium nuclei for the first time.

But there were still too many technical problems and only a small part of the laser energy could be used.

After further improvements, 70 percent of the supplied laser energy could be deposited in the capsule with the enclosed fuel for the first time last year.

The long road to ignition

However, that was still not enough to ignite the fusion fire.

For this, the energy released when the atomic nuclei fuse must be greater than the energy supplied.

The researchers at the National Ignition Facility have now apparently succeeded in taking this step.

In one of four experiments, energy of 170 kilojoules was released - a record in the field of inertial fusion.

The researchers led by Alex Zylstra from LLNL were able to generate a burning plasma.

In this state, there are so many fusion processes going on that the energy of the helium nuclei produced by the fusion of the hydrogen nuclei is enough to maintain the temperature of the plasma and keep the fusion fire burning for a while.

However, the researchers do not provide any information on how long the fire burned.

It may be difficult to determine, since the laser irretrievably destroys the chamber containing the fuel capsule with each pulse.

Does this pave the way for finally using fusion as an energy source?

Not that soon.

Because too much energy is lost in the current experiments.

Of the 1.9 megajoules of energy that the National Ignition Facility's high-power laser releases with each pulse, only a small portion still reaches the fuel.

The operation of the laser itself even consumes a total of 400 megajoules per pulse.

Most fusion researchers are certain that the near future of nuclear fusion will therefore belong more to facilities such as ITER.

If everything runs optimally, a fusion fire will burn there for the first time in a good ten years.

Once that is achieved, ITER could pave the way for fusion power plants, which by the middle of the century will supply large amounts of electricity on favorable terms - as dreams promise.