Access to an improved confinement regime with reduced turbulence by boron powder injection in the Large Helical Device

The Impurity Powder Dropper is used in the Large Helical Device to inject controlled amounts of sub-millimetric boron powder into the plasma, performing a real time boronization of the plasma facing components. During boron powder injection experiments, wall recycling and impurity content are observed to decrease both on a shot-to-shot basis, and in real time. Furthermore, a novel improved confinement regime, characterized by reduced turbulence fluctuations, is observed to be triggered by the powder injection, at constant line-averaged electron density and input power. The plasma stored energy, electron and ion temperature are increased in the order of 20%, and up to 50%. Simultaneously, the amplitude of the turbulent density fluctuations is reduced to half its value before powder injection, across most of the plasma cross section. While lower frequency ion temperature gradient-like (ITG) fluctuations are substantially damped, higher frequency modes appear in the range 100 < f [kHz] < 200. Dynamic transport analysis shows a reduction, by up to 50%, of both ion and electron thermal conductivity in the plasma edge ρ ≥ 0.5, and the energy confinement time is also observed to increase.

This new regime has been observed for different heating schemes, featuring electron and ion cyclotron resonant heating, and/or neutral beam heating, for both hydrogen and deuterium majority plasma ion, and for both directions of the magnetic fields. While neoclassical transport simulations show no significant change in the ambipolar radial electric field, the most likely mechanism is the stabilization of ITG turbulence due to both a modification of the plasma profiles, in particular to the density profile becoming steeper in the edge and more hollow in the center, and to an increased effective charge due to the boron powder injection.