Background physics of isotope effect in confinement improvement plasmas
Confinement improvement is a phenomenon whereby confined plasma generates a thermal insulation layer by itself, by which central plasma temperature rises. It has been known that the confinement improvement transition occurs in an eased condition when plasma fuel gas has larger mass. In this paper, the background physics of this phenomenon, the so-called isotope effect, is experimentally investigated.
The isotope effect is a phenomenon where plasma confinement becomes better as the mass of fuel gas becomes heavier. In a simple theoretical scaling study, the confinement was considered to be worse in heavier plasmas, where a larger centrifugal force was applied. However, in reality, the isotope effect occurs. Although it is a ubiquitous phenomenon observed in plasmas throughout the world, the physical mechanism has not yet been identified. In order to better predict plasma performance in future plasma devices, where deuterium and tritium fuels are used, unveiling the background physics of the isotope effect is highly desirable.
In this study, we discovered a thermal insulation layer embedded in plasma, the so-called internal transport barrier (ITB), having a strong isotope effect. The intensity of the thermal insulation layer (ITB intensity) is known to depend on an experimental condition. When heating power divided by density is small, the ITB intensity remains small and the difference between deuterium and hydrogen plasmas is invisible. Meanwhile, the ITB intensity of deuterium plasmas becomes meaningfully larger than that of hydrogen plasmas when the heating power divided by the density is large, which is a clear isotope effect.
In order to assess the background physics, principal component analysis (PCA) is applied to this dataset. PCA statistically derives essential factors that explain the result from simultaneously varying parameters. This is a dataset that includes not only the ITB intensity and heating power divided by the density, but also density gradient, impurity density, and impurity density gradient, is analyzed. As a result, the ITB intensity has a strong correlation with the density gradient, as well as the heating power divided by the density. This observation contributes to the development of a theoretical model for the isotope effect of confinement improvement.
The paper was published on Nuclear Fusion by IOP publishing group, on June 15, 2021.