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

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)

Neutral atom dynamics and plasma turbulence in the tokamak periphery

Christoph Wersal

Understanding the physical mechanisms at play in the interaction between turbulent plasma and neutral particles is a crucial issue that we approach in this Thesis by using a first-principles self-consistent model of the tokamak periphery implemented in the GBS code. While the plasma is modeled by the drift-reduced two-fluid Braginskii equations, a kinetic model for the neutrals is developed, valid in short and in long mean free path scenarios. The model includes ionization, charge-exchange, recombination, and elastic collisional processes. The neutral kinetic equation is solved by using the method of characteristics. We identify the key elements determining the interaction between neutrals and the turbulent plasma focusing on a tokamak with a toroidal rail limiter on the high-field side equatorial midplane. For this purpose, we simulate the dynamics of the plasma and the neutrals in a domain that includes both the confined edge region and the scrape-off layer (SOL). It turns out that, in the considered plasma conditions, neither the fluctuations of the neutral moments, nor the friction between neutrals and the plasma impact the time-averaged plasma profiles significantly. Thanks to this study, we derive a simple model for the neutral-plasma interaction, which is helpful to identify and understand the principal physical processes at play in the tokamak periphery. By studying the dynamics of the neutral-plasma interplay along the magnetic field lines in the SOL, we derive a refined two-point model from the drift-reduced Braginskii equations that balances the parallel and perpendicular transport of plasma and heat, and takes into account the plasma-neutral interaction. The model estimates the electron temperature drop along a field line, from a region far from the limiter to the limiter plates. The refined two-point model is shown to be in very good agreement with the simulation results. Finally, we self-consistently simulate a diagnostic neutral gas puff, which is often used experimentally as a tool to learn about the turbulence properties in the tokamak periphery. In particular, we investigate the impact of neutral density fluctuations on the D-Î± light emission, finding that at a radial distance from the gas puff smaller than the neutral mean free path, neutral density fluctuations are anti-correlated with plasma density, electron temperature, and D-Î± fluctuations, while at distances from the gas puff larger than the neutral mean free path, a non-local shadowing effect influences the neutrals, and the D-Î± fluctuations are correlated with the neutral density fluctuations.

EPFL2017

Self-consistent simulation of multi-component plasma turbulence and neutral dynamics in the tokamak boundary

André Calado Coroado

The interaction between neutral particles and the plasma plays a key role in determining the dynamics of the tokamak boundary that, in turn, significantly impact the overall performance of the device. Leveraging the work in Wersal and Ricci [Nucl. Fusion 55, 123014 (2015)], the present thesis describes the development and implementation in the GBS code of a mass-conserving multi-component self-consistent model to simulate the interplay between neutrals and plasma in the tokamak boundary. The simulation results are shown to be a useful tool to disentangle the physics at play in the tokamak boundary, and in particular for the interpretation of gas puff imaging (GPI).Developed in the last decade, the GBS code [Ricci et al, Plasma Phys. Control. Fusion 54, 124047 (2012)] allows for self-consistent three-dimensional numerical simulations of the turbulent plasma and neutral dynamics in the tokamak boundary. In GBS, a set of Braginskii equations in the drift limit describes the plasma time evolution, and the neutrals are modelled by solving a kinetic equation using the method of characteristics. While GBS enables simulations in arbitrary magnetic configurations, here we focus on limited plasmas. We first describe the geometrical operators and proper boundary conditions to ensure that mass conservation is satisfied. In comparison to the previous non-mass-conserving model of GBS, the mass-conserving simulations capture more accurately the sharp transition of the plasma and neutral quantities between the edge and scrape-off layer regions. In addition, we show that mass conserving simulations allow for reliable quantitative studies of particle fluxes in the tokamak boundary.A multi-component model of the neutral-plasma interaction is then developed by extending the single-component model to the description of a deuterium plasma that includes electrons, D+ ions, D atoms, D2 molecules and D2+ ions. The molecular dynamics is introduced through a set of drift-reduced Braginskii equations for the D2+ species and considering a kinetic equation for D2 molecules, in addition to the kinetic equation for D atoms, thus resulting in a coupled system of kinetic equations for the atomic and molecular neutral distribution functions. The first multi-component GBS simulations show that, in the sheath-limited regime under consideration, most of the D2 molecules cross the last closed flux surface (LCFS) and are dissociated or ionized in the edge region, thus giving rise to sources of D atoms inside the LCFS. This leads to an inward radial shift of the peak of the plasma source due to ionization of D atoms with respect to the single-component simulations.The multi-component model is applied to the simulation of GPI diagnostics, where the presence of a molecular gas puff is simulated self-consistently with the plasma and neutral dynamics of the tokamak boundary. The injected molecules interact with the boundary plasma, resulting in the emission of light in the D alpha wavelength that can be measured to infer the turbulent properties of the plasma. The simulated mechanisms underlying the light emission, which include the excitation of D atoms and dissociation of both D2 and D2+, provide a reliable tool for the interpretation of GPI experimental measurements. The impact of neutral fluctuations on the D alpha emission rate is investigated, as well as the correlation between the D alpha emission and the plasma and neutral quantities.

EPFL2021

Neutral atom dynamics and plasma turbulence in the tokamak periphery

Christoph Wersal

EPFL2017

Self-consistent simulation of multi-component plasma turbulence and neutral dynamics in the tokamak boundary

André Calado Coroado

EPFL2021