Fusion power has been held up by our inability to get the reaction hot and dense enough to self-sustain, or maintain its own temperature without externally-provided heat. By studying the magnetic fields of planets and other bodies in space, scientists have found a new configuration of magnetic fields that could help concentrate the density of the fuel. The setup uses a levitating magnet to help the plasma to move across magnetic field lines, which maintains its high density, eliminates particle loss, and may even allow for the use of cheaper helium-3 as fuel.

Fusion requires the reacting plasma to be well-confined, or densely packed, and the particles involved have to remain in that area to keep energy losses at a minimum. This helps the plasma achieve the Lawson criterion, the set of conditions needed for a plasma to not only reach ignition, but remain sufficiently confined to continue reacting and releasing energy. For many reactor designs, this means confining the plasma using magnetic fields.

What's that got to do with planets? Scientists have discovered that solar activity can create fluctuations in the magnetic fields present in space. These fields can help drive streams of particles, even hot and active ones such as those from solar wind, into a narrow area, creating a high density. Normally, particles like to travel along the same magnetic field lines, like fish swimming with the current of a stream. However, the fluctuations in a field can push the particles to change field lines and move along planes of similar magnetic strength, like fish swimming across the stream current.

In the case of solar wind and Earth, random fluctuations of the solar wind can cause its particles to move across the magnetic field of Earth, rather than along it, equalizing the number of particles that sit on each thread of the magnetic field. The net effect is a denser population of particles in those areas that have equal magnetic strength—precisely the sort of thing we'd want to see in a fusion reactor.

The problem is this cross-field transport of particles is difficult to replicate—when a particle is on a field line, it wants to stay there. In the newly described experiments, researchers were able to create fluctuations in a lab that were similar to ones seen in space, and use them to produce a higher density plasma.

The scientists involved magnetically levitated a superconducting current ring, which created a dipole magnetic field similar to the one generated by a planet like Earth. In this setup, the magnetic field lines pass through the center of the current ring and produce a large amount of closed field lines that the particles can travel along and, most importantly, between, when given the right fluctuations.

Once the fuel was in the system, the researchers heated it using microwaves, and found that the configuration of magnetic fields produced and sustained a very strong turbulent pinch, or compression of the fuel by magnetic forces. The compression force was so strong, in fact, that the system may be able to use a fuel cycle based on deuterium and less-reactive helium-3, rather than the usual, more expensive fusion fuel, deuterium and tritium.

In typical fusion configurations, an issue that usually needs to be dealt with is a rogue population of hot electrons created by the initial heating; these make the fuel unstable and more likely to disperse. However, the levitating dipole configuration kept the the rogue electron population to a minimum—the apparatus didn't work nearly as well with the device locked in place.

While the experiment successfully replicated the conditions necessary to generate a tighter pinch from particle motions across the magnetic field, there are many details of how and why the setup works that must be sorted out. They're not currently sure how the microwaves help create the field fluctuations, or the structure of the plasma that gets produced. Nonetheless, the ability to create a more well-defined pinch, and therefore use less expensive fuel, may be helpful steps on the road to viable fusion power.

Nature, 2010. DOI: 10.1038/nphys1510