Research into the use of nuclear fusion has made some notable advances in recent times. For example, at the end of March, researchers in Greifswald were able to generate a hydrogen plasma with a temperature of around 50 million degrees in the plasma chamber of the Wendelstein 7-X fusion reactor and keep it stable for eight minutes.
In order for such a hot plasma to remain stable, it is kept at a distance from the vessel wall by strong magnetic fields in nuclear fusion plants. Confining the plasma for as long as possible at the highest possible temperatures is technically quite complex, however, because turbulence and disturbances form. The larger the plasma vessel, the smaller the influence of these disturbances – one of the reasons for the 40 meter diameter of the ITER reactor under construction. However, research groups and companies around the world are also working on smaller, more compact – and therefore cheaper – reactors.
Researchers at the Max Planck Institute for Plasma Physics in Garching (IPP) have now found a method to significantly reduce the distance between the plasma and the vessel wall. This could enable the construction of smaller and cheaper fusion reactors to generate energy. The work was published in the journal Physical Review Letters.
The plasma continues to heat up
The solution that the researchers at IPP found is related to another problem. When nuclear fusion actually ignites in such reactors – at even higher temperatures – not only fast neutrons are released, whose energy is ultimately used to generate electricity in the reactor. The plasma also heats up considerably – and helium-4 atoms are formed, which gradually contaminate the plasma more and more.
Particularly hot, fast ions are therefore guided by special magnetic fields to regions of the plate armored with tungsten – the so-called divertor, a kind of exhaust for the reactor. “We extract heat from the plasma at the divertor. In future power plants, the fusion product helium-4 is also to be discharged there,” explains Ulrich Stroth, head of the plasma edge and wall department at the IPP.
Despite this armor, the plasma rim must be kept at a distance from the divertor to protect it. In the ASDEX Upgrade experimental reactor, this used to be at least 25 centimetres. Researchers at the IPP have now succeeded in reducing the distance to less than five centimetres.
The so-called X-point emitter emits not only UV light but also visible blue light in a ring-shaped area above the divertor. The left image shows a camera shot (below the normal red glow of the cold plasma edge). A numerical simulation of the X-point emitter can be seen on the right.
(Image: MPI for Plasma Physics/E. Huett/O. Pan)
A volume that radiates strongly in the UV range
To do this, they used the so-called X-point emitter: the researchers add nitrogen to further cool the plasma. In specially shaped magnetic fields, when the amount of nitrogen impurity exceeds a certain level, a small, dense, particularly UV-radiating volume is formed. The researchers discovered that when the X-point emitter is used, significantly more thermal energy is converted into UV radiation than previously assumed. The plasma then radiates up to 90 percent of the energy in all directions.
Because the plasma can be moved closer to the divertor, the reactor vessel can be better utilized. Initial calculations show that if the vessel were optimally shaped, the plasma volume could almost be doubled – with the same dimensions. This would also increase the achievable fusion power. The researchers first have to verify in further experiments whether this is actually the case.
(wst)