Simulation of tokamak SOL turbulence

One of the most important open issue for the realization of a fusion reactor is the understanding of the turbulence that develops at the periphery of the device, in the region called the edge. Turbulence is the major cause of particles and energy losses and it is driven by the presence of plasma and magnetic field inhomogeneities. We present non-linear self-consistent simulations of the edge plasma dynamics and we interpret them by means of linear studies of the fluid equations modeling the system. The linear study points out the dependence of the instabilities growth rate and of their properties on the physical parameters of the system. We identify the regions where each instability dominates. By using the non-linear simulations, we then study how the plasma instabilities non-linearly develop and saturate

Global two-fluid simulations of tokamak Scape-Off-Layer turbulence

We present non-linear self-consistent 3D global fluid simulations of the SOL plasma dynamics using the Global Braginskii Solver (GBS) code. The code solves the drift-reduced Braginkii equations in a collisional plasma, with cold ions. The GBS code, originally developed for an electrostatic, 2D configuration has been recently upgraded to describe the SOL turbulence with the introduction of the variable curvature along the magnetic field lines, the magnetic shear, and the electromagnetic effects. The code peculiarity lies in the capability of evolving self-consistently equilibrium and 3D fluctuations as a results of the interplay among the sources, the turbulent transport and the plasma losses at the limiter plates. The non-linear simulations have been interpreted by means of linear analysis of the fluid equations modeling the system. This points out the presence of two main instabilities driving turbulence: the Drift Wave and the Resistive Balloning instabilities. The dependence of the instabilities growth rate and of their properties on the physical parameters of the system, for example the characteristic length of the plasma density, the magnetic shear and the beta ratio have been explained and the regions where each instability dominates have been identified

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

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

The role of the sheath in magnetized plasma fluid turbulence

In the sheath region at the interface between plasmas and solid surfaces, quasi-neutrality and, in the case of magnetized plasmas, drift-ordering are violated. These two assumptions are typically made in plasma fluid models; the presence of a plasma-wall transition region, typical of all bounded systems, hampers therefore their use. This problem can be overcome by introducing a set of boundary conditions (BCs) for fluid models that properly describe the physics of the plasma-wall transition region. While the classical Bohm-Chodura BCs for the ion and electron parallel velocities have been previously derived, no consistent BCs for the other fluid quantities existed up to date. Based on a recent theory [1,2], a complete set of analytical BCs for the density, temperature, potential, vorticity, and parallel ion and electron velocities, has been provided, which is fully consistent with kinetic simulations of the plasma-wall transition region [3]. These BCs have been implemented in a three-dimensional global fluid code, which is used to simulate turbulence in basic plasma physics devices and in the tokamak scrape-offlayer. It has been shown that BCs that faithfully supply the sheath physics to the fluid codes are crucial for the understanding of the equilibrium profiles, plasma recirculation, intrinsic plasma rotation, and blob propagation in basic plasma physics and fusion devices. References [1] J. Loizu, P. Ricci and C. Theiler, Physical Review E 83, 016406 (2011) [2] J. Loizu, J. Dominski, P. Ricci and C. Theiler, Phys. Plasmas 19, 083507 (2012) [3] J. Loizu, P. Ricci, F. Halpern and S. Jolliet, Phys. Plasmas 19, 122307, (2012

Global electromagnetic simulations of tokamak scrape-off layer turbulence

In the tokamak scrape-off layer (SOL), turbulent plasma interacts with the wall, determining the boundary conditions for the core plasma, and largely governing the performance of the entire device. In this region, the amplitude of the turbulent modes is comparable to the background plasma profiles, and there is no separation between turbulent and equilibrium length scales. Therefore, a fully non-linear and global approach is necessary. The present work discusses recent studies addressing the properties of tokamak SOL turbulence using a global, electromagnetic, fluid drift-reduced Braginskii model. Three-dimensional, non-linear simulations are carried out using the Global Braginskii Solver (GBS) code [1], which is now capable of carrying out self-consistent, global three-dimensional simulations of the plasma dynamics in the tokamak SOL. The simulations evolve the plasma dynamics as the interplay of the plasma flux from the core, the turbulent radial transport, and the losses at the plasma sheath where the magnetic field lines intersect with the vessel. A gradual approach in increasing complexity has made possible (a) to determine the dominant instabilities driving the SOL turbulence, (b) to identify the mechanisms that saturate the growth of the linear modes and therefore regulate level of radial transport, (c) to characterize the plasma intrinsic rotation induced by the SOL dynamics, and (d) to study the role of electromagnetic effects in enhanced transport regimes. The non-linear dynamics revealed by the simulations agree with the analytical estimates that have been carried out. [1] P.Ricci et al., Plasma Physics and Controlled Fusion, 2012, 54, 124047

Effects of plasma shaping on tokamak scrape-off layer turbulence

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

Intrinsic toroidal plasma rotation in the scrape-off-layer

The origin and nature of intrinsic toroidal plasma rotation in the scrape-off-layer are theoretically investigated. We discuss and analytically estimate three mechanisms that give rise to SOL toroidal rotation: turbulent momentum transport associated with electrostatic instabilities, pressure gradients along the poloidal direction, and deviation of the plasma velocity at the sheath entrance with respect to the Bohm's value. The results of three-dimensional global fluid simulations of tokamak scrape-off-layer in a limiter configuration are shown and compared