Neutral atom dynamics and plasma turbulence in the tokamak periphery

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.

Impact of neutral atoms on plasma turbulence in the tokamak edge region

Rogério Manuel Cabete De Jesus Jorge, Federico David Halpern, Paola Paruta, Paolo Ricci, Fabio Riva, Christoph Wersal

A novel first-principles self-consistent model that couples plasma and neutral atom physics suitable for the simulation of turbulent plasma behaviour in the tokamak edge region has been developed [1] and implemented in the GBS code [2]. While the plasma is modelled by the drift-reduced two fluid Braginskii equations, a kinetic model is used for the neutrals, valid in short and in long mean free path scenarios. The model includes ionization, charge-exchange, recombination, and elastic collisional processes. The model was used to study the transition from the sheath to the conduction limited regime by increasing the plasma density in the system. We compared the simulation results with the predictions of an expanded and refined two-point model, estimating the drop of electron and ion temperature along the magnetic field lines in the SOL. The model is now being applied to investigate the impact of neutrals on turbulent edge features, such as the broad shoulder observed in the far SOL at high plasma density. References [1] C. Wersal and P. Ricci 2015 Nucl. Fusion 55 123014 [2] P Ricci et al 2012 Plasma Phys. Control. Fusion 54 124047

2016

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

The role of the sheath in magnetized plasma turbulence and flows

Joaquim Loizu Cisquella

Controlled nuclear fusion could provide our society with a clean, safe, and virtually inexhaustible source of electric power production. The tokamak has proven to be capable of producing large amounts of fusion reactions by confining magnetically the fusion fuel at sufficiently high density and temperature, thus in the plasma state. Because of turbulence, however, high temperature plasma reaches the outermost region of the tokamak, the Scrape-Off Layer (SOL), which features open magnetic field lines that channel particles and heat into a dedicated region of the vacuum vessel. The plasma dynamics in the SOL is crucial in determining the performance of tokamak devices, and constitutes one of the greatest uncertainties in the success of the fusion program. In the last few years, the development of numerical codes based on reduced fluid models has provided a tool to study turbulence in open field line configurations. In particular, the GBS (Global Braginskii Solver) code has been developed at CRPP and is used to perform global, three-dimensional, full-n, flux-driven simulations of plasma turbulence in open field lines. Reaching predictive capabilities is an outstanding challenge that involves a proper treatment of the plasma-wall interactions at the end of the field lines, to well describe the particle and energy losses. This involves the study of plasma sheaths, namely the layers forming at the interface between plasmas and solid surfaces, where the drift and quasineutrality approximations break down. This is an investigation of general interest, as sheaths are present in all laboratory plasmas. This thesis presents progress in the understanding of plasma sheaths and their coupling with the turbulence in the main plasma. A kinetic code is developed to study the magnetized plasma-wall transition region and derive a complete set of analytical boundary conditions that supply the sheath physics to fluid codes. These boundary conditions are implemented in the GBS code and simulations of SOL turbulence are carried out to investigate the importance of the sheath in determining the equilibrium electric fields, intrinsic toroidal rotation, and SOL width, in different limited configurations. For each study carried out in this thesis, simple analytical models are developed to interpret the simulation results and reveal the fundamental mechanisms underlying the system dynamics. The electrostatic potential appears to be determined by a combined effect of sheath physics and electron adiabaticity. Intrinsic flows are driven by the sheath, while turbulence provides the mechanism for radial momentum transport. The position of the limiter can modify the turbulence properties in the SOL, thus playing an important role in setting the SOL width.