Numerical simulations of plasma fuelling in tokamaks using the GBS code

Carrie Fiona Beadle, André Calado Coroado, Paola Paruta, Paolo Ricci, Fabio Riva, Christoph Wersal
2018
Poster talk

Abstract

The tokamak periphery determines the fuelling of a tokamak as the result of a complex interplay of neutral and plasma dynamics, perpendicular turbulent transport, and losses to the vessel walls. In the present work, results from first-principles numerical simulations are used in order to study the tokamak fuelling, aiming to assess the neutral penetration length, the ionization region, and the mechanisms that regulate plasma transport in the edge and Scrape-Off Layer (SOL) regions of a tokamak. Ultimately, these simulations aim at understanding the role played by the neutrals in the formation of the critical density gradient near the Last Close Flux Surface (LCFS) and the impact of neutral dynamics on cross-field transport at different plasma densities. The numerical simulations are carried out with the GBS code, which has been developed in the past years in order to simulate the tokamak edge dynamics. GBS is a 3D flux-driven code that solves the drift-reduced two-fluid Braginskii equations to simulate the plasma dynamics and a self-consistent neutral kinetic equation. Neutrals and plasma models are coupled by the presence of ionization, charge exchange, and recombination processes. Based on the simulation results, we develop a 1D radial model for plasma and neutrals balance, thus enabling a quantitative evaluation of the different mechanisms determining the tokamak fuelling.

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The understanding of the plasma dynamics in the scrape-off layer (SOL) of tokamaks is of crucial importance as we approach the ITER era. In this region, particles and heat coming from the core, through turbulent transport, flow along the magnetic field lines and are exhausted to the vessel. The processes taking place in the SOL govern the performance of the entire device, as they determine the impurity dynamics, the recycling level, the peak heat loads at the vessel, and have an important role in setting the overall plasma confinement. In the recent past, a large effort has been devoted to improve the knowledge of plasma turbulent dynamics in the tokamak SOL, achieving significant progress. In the simplest circular limited configuration, electromagnetic fluid turbulence simulations carried out with the Global Braginskii Solver (GBS) [P. Ricci et al, PPCF 2012] have pointed out the mechanisms that regulate the SOL width, the plasma toroidal rotation, and the turbulence regime transition. In the present work we generalize the magnetic geometry of GBS, to perform simulations with elongated plasmas and non-zero triangularity, and we investigate the effects of plasma shaping on tokamak SOL turbulence. Nonlinear simulations are performed, with different values of elongation and triangularity. The turbulence properties are analyzed, and the gradient removal theory [P. Ricci et al, PoP 2013] is used to estimate the SOL width. Thanks to a linear study, we elucidate the mechanisms through which the plasma shaping affects the SOL turbulence.

2015

Neutral atom dynamics and plasma turbulence in the tokamak periphery

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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.

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