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

F Nespoli,S Masuzaki,A Bortolon,R Lunsford,C Suzuki,M Shoji,M Yoshinuma,T Morisaki,T Kinoshita,Y Takemura,G Motojima,G Kawamura,K Ida,A Nagy,N Tamura,Tetsutarou Oishi ,Naoki Kinoshita ,N Ashikawa,Kaoru Tanaka ,A Mollén ,N Pablant ,Erik Gilson ,D Gates

doi:10.1038/s41567-021-01460-4

Abstract

In state-of-the-art stellarators, turbulence is a major cause of the degradation of plasma confinement. To maximize confinement, which eventually determines the amount of nuclear fusion reactions, turbulent transport needs to be reduced. Here we report the observation of a confinement regime in a stellarator plasma that is characterized by increased confinement and reduced turbulent fluctuations. The transition to this regime is driven by the injection of submillimetric boron powder grains into the plasma. With the line-averaged electron density being kept constant, we observe a substantial increase of stored energy and electron and ion temperatures. At the same time, the amplitude of the plasma turbulent fluctuations is halved. While lower frequency fluctuations are damped, higher frequency modes in the range between 100 and 200 kHz are excited. We have observed this regime for different heating schemes, namely with both electron and ion cyclotron resonant radio frequencies and neutral beams, for both directions of the magnetic field and both hydrogen and deuterium plasmas.

Highlights

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
It is fundamental to reduce turbulence in order to maximise the plasma confinement, determining the amount of fusion reactions. In this Article, we report the observation of a confinement regime in the Large Helical Device (LHD) stellarator[4], characterized by widespread reduction of turbulence across the plasma cross section
The increase of ne in the edge region is consistent with the powder being vaporized around the last closed flux surface, as it results from coupled EMC3-EIRENE and DUSTT simulations (Fig. 2d)

Summary

Introduction

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. A widespread turbulence reduction and temperature increase like the one reported in this Article has not been previously observed in the powder injection experiments performed on tokamaks, or following a glow discharge boronization.

Results

Conclusion