Observation of a reduced turbulence regime with boron powder injection in a stellarator

In state-of-the-art stellarators, turbulence is a major cause of the degradation of plasma confinement. To maximize confinement, which eventually determines the number of nuclear fusion reactions, turbulent transport must be reduced. Here we report the observation of a confinement regime in a stellarator plasma characterized by increased confinement and reduced turbulent fluctuations. The transition to this regime is driven by injecting submillimetric boron powder grains into the plasma. With the line-averaged electron density kept constant, we observe a substantial increase in stored energy and electron and ion temperatures. At the same time, the amplitude of the turbulent plasma fluctuations is halved. While lower frequency fluctuations are damped, higher frequency modes, between 100 and 200 kHz, are excited. We have observed this regime for different heating schemes, namely with electron and ion cyclotron resonant radio frequencies and neutral beams, for both directions of the magnetic field and hydrogen and deuterium plasmas. Stellarators are one of the most promising concepts for magnetic confined nuclear fusion, which could provide a clean alternative to fossil fuels and nuclear fission for mass energy production. Unlike tokamaks, their 3D magnetic field is provided entirely by external coils, removing the need for a current to flow into the plasma, which makes it prone to instabilities and violent disruptions. This also allows the magnetic field to be tailored to minimize neoclassical (due to collisions) transport and improve confinement1. For present-day stellarators, plasma turbulence gives the biggest confinement degradation. This results in an increased ‘anomalous’ transport contribution, typically an order of magnitude higher than the neoclassical transport contribution for particles2 and accounting for more than 50% of the energy transport3. While theoretically possible, optimizing the stellarator magnetic field to reduce turbulent transport is extremely challenging due to the computational cost of 3D turbulence simulations.

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