A great deal of money is being poured into research into nuclear fusion as a possible commercial energy source. If the power of the sun can be harnessed, it would mean abundant energy with less pollution, less dangerous waste, less expense, less fuel, etc. that many other power sources. Instead of fission which generates energy by the radioactive decay of heavy atoms of uranium or plutonium, nuclear fission generates energy by the fusion of atoms of light elements into atoms of heavier elements.
Most fusion reactors being developed today are based on the creation of a cloud of hydrogen ions or a plasma which is subjected to great pressure and temperature. One of the main problems is confining the plasma so that it does not touch the walls of the containment vessel. Another problem is ensuring that the distribution of different hydrogen isotopes is uniform within the vessel.
A Japanese team has just developed an understanding of the confinement of hydrogen isotopes in plasmas in nuclear fusion that will help actually make fusion power a reality. They published a report on their work in the January 14th in Physical Review Letters.
Normal hydrogen contains one proton in the nucleus. Deuterium is an isotope of hydrogen that has one proton and one neutron in the nucleus. It has twice the weight of normal hydrogen. Tritium is an isotope of hydrogen with one proton and two neutrons in the nucleus and weights three times the weight of normal hydrogen.
The researchers were working on ratios of distribution of the different hydrogen isotopes in a plasma produced by the Large Helical Device (LHD) at the National Institute for Fusion Science. (NIFS). The plasma in the LHD consisted of normal hydrogen and deuterium. The researchers hope to be able to predict the distribution of hydrogen, deuterium and tritium in a plasma based on their work with a hydrogen-deuterium mix.
Katsumi Ida is a professor with both the National Institute for Fusion Science and the Graduate University for Advanced Studies. He is the author of the just published paper. He said, “In the core of fusion plasma , it is most desirable to have an even split between deuterium and tritium because it gives the highest fusion power," said paper author Katsumi Ida, a professor with both the National Institute for Fusion Science and the Graduate University for Advanced Studies. "However, we can only control the isotope ratio at the edge of the plasma, not in the core. We set out to investigate if the isotope ratio is uniform throughout the mixture. If it's not, can we make it uniform?”
Ida and his team discovered that the uniformity of the plasma is determined by how the different isotopes move. This plasma is in a turbulent state. Isotopes influence by ion temperature gradient (iTG) turbulence turned out to be far more uniformly distributed than isotopes influenced by trapped-electron mode (TEM) turbulence. Ida said, “The ITG-dominant state is far more favorable in fusion plasma. We saw the formation of a non-mixing profile and its transition to a uniform isotope state in the plasma, associated with the increase of turbulence propagating along the ion temperature gradient.”
ITG turbulence is related to a temperature gradient that is matched to the magnetic bottle that is confining the fusion plasma. The isotopes move faster if they are in the hotter region of temperature. This allows the isotopes to mix uniformly. This new understanding of plasma behavior could assist researchers in controlling isotopic uniformity in plasmas and increase the power of fusion isotopes mixtures.
Next, Ida and his team plan to study the uniformity of other elements in plasmas including helium which is produced by the fusion of deuterium and tritium.