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

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.

Dust and powder in fusion plasmas: recent developments in theory, modeling, and experiments

“…Recent results in LHD reported consistent observations of increased ion temperature during B powder injection (Nespoli et al 2022), in a variety of configurations and auxiliary heating schemes. The increase was sustained for time windows of multiple seconds, during B injection at flow rates of 10-50 mg/s.…”

“…The scenario showed improvement of confinement over a wide range of density (h). Reproduced from Nespoli et al (2022), ©2022 The Authors of Nespoli et al (2022) particular, the relevance of low-Z radiators has emerged as a potential solution for coupling an radiative divertor with high performance core. For instance, Pigarov Pigarov (2017) has shown through systematic modeling that, for a mid-scale tokamak, B has optimal properties in terms of maximizing divertor radiative power exhaust, while limiting the increment of core Z ef f to acceptable values.…”

Initial results from boron powder injection experiments in WEST lower single null L-mode plasmas

“…These improvements occurred with no changes to the LHCD power or plasma current and were concurrent with increases in the electron density and radiated power, suggesting improved confinement. A likely explanation is the suppression of ITGdriven turbulence due to main ion dilution, as observed in N2 seeding experiments in WEST [36], and/or modification of the density profile, as observed in powder injection experiments on AUG [15], W7-X [20], and LHD [22].…”

“…While IPDs have previously been installed and operated on other metal-walled tokamaks such as AUG [14,15] and EAST [16,17], the long pulse capabilities and full-W environment make WEST an excellent test bed to extend the evaluation of the IPD as a real-time wall conditioning technique. The present results can additionally be compared with powder injection experiments in carbon-walled machines, namely DIII-D [18], KSTAR [19], W7-X [20], and LHD [21,22].…”

Fusion: a true challenge for an enormous reward

“…A first estimate of the energy necessary to realise fusion reactions can be obtained from the potential energy of one nucleus in the presence of the electric field of another one at a distance such that, in the classical representation, the nuclei just touch each other. The maximum of the Coulomb barrier between two nuclei with radius R 1 and R 2 and charges qZ 1 and qZ 2 can be found from (9) V C…”