If we want to master the power of the stars in our power stations, we need to get more efficient at creating high-energy plasma. An international team of researchers took some interesting steps in that direction with a new plasma heating technique.
The scientists' starting point was radio-frequency heating, which is a well-established technique. The difference was in what they were heating. Usually, the plasma is made by two types of ions, like hydrogen and deuterium (a hydrogen with an extra neutron) or deuterium and helium-3 (helium with one less neutron). Their plasma was made of all three ions, mostly hydrogen and deuterium with less than 1 percent of helium-3.
The team picked radio waves that could be absorbed best by such a tiny fraction of helium-3 atoms, and they demonstrated that in this way they could heat the particular ion to the same range of temperatures as elements actively fusing. As reported in Nature Physics, this is one order of magnitude of what had been previously achieved in the lab.
"These higher energy ranges are in the same range as activated fusion products," co-author John Wright, from the Massachusetts Institute of Technology, said in a statement. "To be able to create such energetic ions in a non-activated device – not doing a huge amount of fusion – is beneficial, because we can study how ions with energies comparable to fusion reaction products behave, how well they would be confined."
To demonstrate that the technique works, researchers used the now-retired Alcator C-Mod, a record-breaking fusion reactor developed at MIT. The follow-up test was conducted in the world's largest plasma magnetic confinement device, the Joint European Torus (JET). Both Alcator C-Mod and JET are tokamaks, donut-shaped fusion reactors, but they have different specializations. Alcator was designed to precisely measure the interactions between the radio waves and the ions, while JET can easily measure the energy of the plasma.
"The JET folks had really good energetic particle diagnostics, so they could directly measure these high energy ions and verify that they were indeed there," Wright added. "The fact that we had a basic theory realized on two different devices on two continents came together to produce a strong paper."
The finding has several technical applications and some non-technical ones too. Obviously, understanding ion behavior will be useful for plasma constraints of nuclear fusion in tokamaks, but researchers think that even the alternative fusion reactor (known as a stellarator) requires a detailed analysis of this.
This technique could also lead to some astronomical clarifications. There’s a large flux of helium-3 coming from the Sun, especially during solar flares, and understanding this in the lab could help us work out what’s happening around the Sun.