A reduced-turbulence regime in the Large Helical Device upon injection of low-Z materials powders

Recently an improved confinement regime with reduced turbulent fluctuations was observed in the Large Helical Device upon injection of boron powder into the plasma [F. Nespoli et al., Nature Physics 2022]. Dynamic transport analysis and modeling of neoclassical fluxes suggest the confinement increase is due to reduced turbulent transport.

Here, we report further experimental observations of increased plasma temperature and decreased turbulence on an extended database featuring different heating levels and sources, plasma densities and powder materials: boron, carbon, boron nitride.

As a general trend, the ion temperature improvement increases with powder injection rates and decreases with increasing density, while the dependence on the input power is weak. For the same plasma conditions, different powder materials (B, C, BN) produce similar turbulence response and temperature improvement. Modeling of powder trajectories shows similar penetration of B and C in the main plasma, while 1/3 of the smaller BN grains evaporates in the divertor leg. The improvement with C powder suggests wall conditioning might play a secondary role in accessing this regime, most probably due to suppression of ITG turbulence driven by plasma profiles modification and increase of plasma effective charge.