Despite all the focus on alternative energy sources, nuclear fusion has barely rated a mention, as it has turned out to be very difficult to execute. The construction of ITER, a very large scale fusion reactor, has become a divisive issue within physics because of its size and resultant cost: �5 billion. But a team of researchers has discovered what may be a workable configuration for a different kind of reactor, a reverse field pinch that operates on a much smaller scale. Even with comparatively low energy and fields, calculations suggest it should be able to achieve fusion.

The most popular fusion reactor design has been the tokamak, a machine that produces a toroidal, or donut-shaped, magnetic field—this design is being used in ITER. The tokamak exploits the "pinch effect," a term that describes how passing a current through a gaseous plasma creates a magnetic field that can make the plasma unstable if it's not offset by a second magnetic field. To manage these fields, ITER's planned reactor chamber has to be about the size of a house.

A reversed field pinch works in a similar way to a tokamak, augmenting the magnetic field to take advantage of the way a current affects a plasma. However, because of the way it operates, the external magnetic field needed for an RFP is an order of magnitude smaller than the tokamak and the total ambient magnetic field is mostly the result of the current flowing through the plasma. These factors would allow the reactor to be much smaller than a tokamak.

The RFP was previously thought to be less viable because of instabilities that force different layers of the plasma to resonate, creating a large chaotic region that is difficult to confine (confinement of the plasma that fuels reactors is crucial). When an RFP plasma behaves this way, it is referred to as being in a "multiple-helicity state."

The newly published research has shown that it is possible to encourage one of the resonant modes to become dominant while forcing secondary modes to have a smaller and finite amplitude that doesn't interfere with the primary mode. This results in a quasi-single-helicity state, which has no chaotic region and is highly confineable. The higher the current that is passed through the plasma, the easier the QSH state is to achieve. There is some residual magnetic chaos, but the configuration is able to maintain consistent temperature gradients, which are necessary for a reactor to sustain its activity.

The increased current also encourages the formation of "islands" of magnetic field. Once the islands have appeared, it is possible for the primary mode magnetic island to merge into the main magnetic axis, causing both to disappear. On a larger scale and over time, this field cancellation allows for the reactor to transition to a more stable equilibrium.

While researchers have not yet achieved an exact single-helicity state, the emergence of a QSH through the process described above provides theoretical support for the existence of a single-helicity state, which would be even more confineable and have less magnetic chaos. Likewise, the experimental results show that a plasma in an RPF with an increasing current does evolve towards the desired single-helicity state.

Because an RPF is characteristically stable with low external fields, easily confined, and heated only by passing a current, it is technologically simple and has the potential to enable the production of a small-scale fusion reactor. However, because the exact desired state has not been experimentally confirmed, fusion-powered cell phones and toasters are still a long way off.

Nature, 2009. DOI: 10.1038/nphys1308