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-α 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-α 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-α fluctuations are correlated with the neutral density fluctuations

A comparison between a refined two-point model for the limited tokamak SOL and self-consistent plasma turbulence simulations

A refined two-point model is derived from the drift-reduced Braginskii equations for the limited tokamak scrape-off layer (SOL) by balancing the parallel and perpendicular transport of plasma and heat and taking 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. Self-consistent first-principles turbulence simulations of the SOL plasma including its interaction with neutral atoms are performed with the GBS code and compared to the refined two-point model. The refined two-point model is shown to be in very good agreement with the turbulence simulation results

A kinetic neutral atom model for tokamak scrape-off layer tubulence simulations

The first-principle understanding of the processes in the Scrape-Off-Layer (SOL) of a tokamak is crucial for the developement of a thermonuclear re- actor. Since the plasma temperature in the SOL is rather low, the plasma is typically not fully ionized, and the neutral atoms play an important role in determining the SOL regimes. The description of a simple kinetic model for neutral atoms in the SOL is presented and first results of self-consistent non-linear turbulence simulations with the GBS code [1] are shown. [1] P. Ricci, et al., Plasma Phys. Control. Fusion 54 (2012) 12404

The interaction between neutral particles and turbulent plasma in the tokamak SOL

The first-principle understanding of the processes in the Scrape-Off-Layer (SOL) of a tokamak is crucial for the development of a thermonuclear reactor. Since the plasma temperature in the SOL is rather low, the plasma is typically not fully ionized, and the neutral atoms play an important role in determining the SOL regimes. We have derived a kinetic model for neutral atoms in the SOL that contains the fundamental elements of neutral dynamics, while remaining relatively simple. The model has been coupled to the drift-reduced Braginskii equations and is implemented in GBS[1], a three-dimensional numerical code developed to simulate SOL turbulence. The code GBS is able to study the self-consistent formation of the plasma profiles as the interplay of the plasma out-flowing from the core, the parallel losses, and turbulent transport. Details of the neutrals model and the interactions with the plasma are given and we present first results indicating a transition into a regime that shows typical signatures of the conduction limited regime, e.g. significant parallel temperature gradients. [1] P. Ricci, et al., Plasma Phys. Control. Fusion 54, 124047 (2012

Scrape-off layer simulations in a Double Null magnetic configuration

Different magnetic geometries are being considered for handling the power exhaust in DEMO, among which is the double-null. In addition to doubling the exhaust area, experiments have found stark differences in the SOL on the HFS and LFS in this configuration[1], allowing the possibility of efficient heating from the HFS in addition to doubling the exhaust area. The contrast between the LFS and HFS calls for theoretical investigation. In fact, the asymmetry can help to disentangle the different driving mechanisms of the turbulence. Since the temperature in the SOL is relatively low, the plasma is sufficiently collisional for a fluid model, such as the drift-reduced Braginskii, to be used. This model has been implemented in the GBS code[2],[3]. Focusing on limited geometries, the SOL width – a crucial quantity to determine the heat load on the plasma facing components – was estimated by identifying the driving linear instability and turbulence saturation mechanism[4]. The analytical and simulation results were validated against a large number of experiments, showing good agreement[5]. Recently, a non-field aligned coordinate system has been implemented in GBS. This avoids the coordinate singularity present for field-aligned coordinates at the X-point, thus allowing any toroidally symmetric magnetic field configuration to be simulated. We will introduce GBS, discuss the implementation of the new coordinate system and show results of the first simulations in a double-null magnetic configuration. We will present the first insights on the nature of SOL turbulence and the SOL width on the HFS and LFS. References [1] B. LaBombard, et al. Nuclear Fusion 55, 052020 [2015] [2] P. Ricci, et al, Plasma Physics and Controlled Fusion 54, 112103 [2012] [3] F. Halpern, et al, Journal of Computational Physics 315, 388 [2016] [4] A. Mosetto, et al, Physics of Plasmas 20, 092308 [2013] [5] F. D. Halpern, et al. Plasma Physics and Controlled Fusion 58, 084003 [2016

A flexible numerical scheme for simulating plasma turbulence in the tokamak scrape off layer

Simulating the most external plasma region of a tokamak, the scrape-off layer (SOL), is of crucial importance in the way towards a fusion reactor as heat load on the vessel wall, impurity generation, and overall plasma confinement, all depend on the plasma dynamics in this region. In the last few years a numerical code, GBS, has been developed for solving the drift-reduced Braginskii equations, and describe turbulence in the tokamak SOL. In the present work, we re-formulate these equations in a general form that enables to treat diverted configuration with X-point. This is obtained by relaxing the requirement of using a coordinate system that follows the magnetic field lines. Within this alternative coordinate system, the drift-reduced Braginskii equations are then solved by using an high order numerical scheme. We show initial simulations carried out with GBS that describe well the expected field aligned turbulence structures

A kinetic neutral atom model for tokamak SOL turbulence simulations

The first-principle understanding of the processes in the Scrape-Off-Layer (SOL) of a tokamak is crucial for the developement of a thermonuclear reactor. Since the plasma temperature in the SOL is rather low, the plasma is typically not fully ionized, and the neutral atoms play an important role in determining the SOL regimes. We have derived a kinetic model for neutral atoms in the SOL that contains the fundamental elements of neutral dynamics, while remaining relatively simple. The model has been implemented in GBS [1], a three-dimensional numerical code developed to simulate SOL turbulence. GBS is based on the drift-reduced Braginiskii equations and is able to study the self-consistent formation of the plasma profile as the interplay of the plasma outflowing from the core, the parallel losses, and turbulent transport. Details of the numerical implementation are given and the first GBS results of coupled plasma turbulence and neutral dynamics are presented. [1] P. Ricci, et al., Plasma Phys. Control. Fusion 54 (2012) 12404

Numerical simulations of plasma fuelling in tokamaks using the GBS code

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

Plasma refuelling at the SOL simulated with the GBS code

The scrape-off layer (SOL) sets the bounday conditions of a tokamak, determining the plasma confinement, the heat exhaust, the impurity levels, and controlling the fuelling of the device. Therefore, a first principles understanding of the physical mechanisms governing SOL turbulence is crucial on the way towards fusion energy. We describe SOL simulations carried out by using GBS, a three-dimensional numerical code that solves the drift-reduced Braginskii equations for the two-fluid model of the plasma and consistently includes neutrals dynamics as well. In this work, results from GBS simulations are used to understand the tokamak fuelling

Interaction of neutral atoms and plasma turbulence in the tokamak edge region

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 and implemented in the GBS code. 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 form the sheath to the conduction limited regime, to include gas puffs in the simulations, and to investigate the interplay between neutral atoms and plasma turbulence