Publication

Numerical simulations of gas puff imaging using a multi-component model of the neutral-plasma interaction in the tokamak boundary

Paolo Ricci
2022
Journal paper
Abstract

A three-dimensional simulation of gas puff imaging (GPI) diagnostics is carried out by using a self-consistent multi-component model of the neutral-plasma interaction. The simulation, based on the drift-reduced Braginskii model for the plasma and a kinetic model for the neutrals, is performed in a toroidally limited plasma with gas puff sources located at the low field side equatorial midplane. In addition to electrons, the simulation evolves the turbulent dynamics of D+ and D-2(+) ions as well as D and D-2 neutral species. The D-alpha emission arising from the excitation of D atoms and the contributions from dissociation of D-2 molecules and D-2+ ions are considered. The simulation points out the importance of considering D-alpha emission due to molecular dissociation in the far scrape-off layer (SOL), since it is the dominant source of D-alpha emission at distances from the gas puff considerably smaller than the mean free path of D-2 molecules. The correlation functions between the D-alpha emission rate and the plasma and neutral quantities, namely, the electron density, n(e), electron temperature, T-e, and density of neutral atoms, n(D), are evaluated considering each contribution to D alpha emission and analyzing the correlation functions between these quantities. The correlation functions strongly depend on the location considered within the edge and SOL with an important impact on the interpretation of GPI measurements. The statistical moments and the turbulence properties computed for different components of the D-alpha emission as well as for the relevant plasma and neutral quantities are also investigated. While neglecting neutral density fluctuations is a reasonable approximation that is widely used in the analysis of GPI measurements, this work reveals a 20%-30% influence of neutral fluctuations on most of the quantities measured through the GPI diagnostics with a possibly larger impact for some quantities in specific regions. These results, therefore, suggest the importance of considering neutral fluctuations for the accurate quantitative interpretations of GPI measurements. (C )2022 Author(s)

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Related concepts (37)
Tokamak
A tokamak (ˈtoʊkəmæk; токамáк) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. , it was the leading candidate for a practical fusion reactor. Tokamaks were initially conceptualized in the 1950s by Soviet physicists Igor Tamm and Andrei Sakharov, inspired by a letter by Oleg Lavrentiev. The first working tokamak was attributed to the work of Natan Yavlinsky on the T-1 in 1958.
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Neutral-beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas.
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