Guiding center | EPFL Graph Search

In physics, the motion of an electrically charged particle such as an electron or ion in a plasma in a magnetic field can be treated as the superposition of a relatively fast circular motion around a point called the guiding center and a relatively slow drift of this point. The drift speeds may differ for various species depending on their charge states, masses, or temperatures, possibly resulting in electric currents or chemical separation. Gyration If the magnetic field is uniform and all other forces are absent, then the Lorentz force will cause a particle to undergo a constant acceleration perpendicular to both the particle velocity and the magnetic field. This does not affect particle motion parallel to the magnetic field, but results in circular motion at constant speed in the plane perpendicular to the magnetic field. This circular motion is known as the gyromotion. For a particle with mass m and charge q moving in a magnetic field with st

Heavy impurity transport in tokamaks with plasma flows and saturated 3D perturbations

Wilfred Anthony Cooper, Jonathan Graves, Samuel Lanthaler, Eduardo José Lascas Neto, Madhusudan Raghunathan, Cristian Sommariva

Observations in JET hybrid scenarios show that early termination of the plasma can be caused by tungsten accumulation events, sometimes preceded by large living MHD perturbations [1]. Only recent investigations have been made into understanding the effects of 3D background rotation profiles and 3D MHD equilibria on the transport of heavy impurities. The VENUS- LEVIS PIC code [2] has been used to follow heavy impurities in the presence of a 1/1 kink saturated 3D MHD equilibrium and strong toroidal rotation [3]. In the present work, a now self- consistent treatment of the same problem is pursued. An implementation of the 3D centrifugal effects and 3D electrostatic potential correction on the VENUS-LEVIS code, based on the neo- classical 3D flow theory in [4] and the guiding center theory in [5], is presented. The code is used to follow heavy impurities on a 3D magnetic geommetry with 3D centrifugal effects, in order to understand how these 3D neoclassical effects will affect the overall neoclassical trans- port of heavy impurities. In particular, we focus our study on a saturated 1/1 internal kink mode as it has been seen in experiments that a correlation between this 3D magnetic structure and the inward flux of heavy impurities may exist. Simulations are performed to study this phenomena in both high (Pfirsch-Schlüter) and low (banana) collisionality regimes for the background ions in order to understand the impact of the saturated 1/1 internal kink mode on the peaking of tungsten distributions in JET hybrid scenarios exhibiting continuous 1/1 activity.

European Physical Society2019

A moment-based model for plasma dynamics at arbitrary collisionality

Rogério Manuel Cabete De Jesus Jorge

Despite significant development over the last decades, a model able to describe the periphery region of magnetic confinement fusion devices, extending from the edge to the far scrape-off layer, is still missing. This is because this region is characterized by the presence of turbulent fluctuations at scales ranging from the Larmor radius to the size of the machine, the presence of strong flows, comparable amplitudes of background and fluctuating components, and a large range of collisionality regimes. The lack of a proper model has undermined our ability to properly simulate the plasma dynamics in this region, which is necessary to predict the heat flux to the vessel wall of future fusion devices, the L-H transition, and ELM dynamics. These are some of the most important issues on the way to a fusion reactor. In the present thesis, a drift-kinetic and a gyrokinetic model able to describe the plasma dynamics in the tokamak periphery are developed, which take into account electrostatic fluctuations at all relevant scales, allowing for comparable amplitudes of background and fluctuating components. In addition, the models implement a full Coulomb collision operator, and are therefore valid at arbitrary collisionality regimes. For an efficient numerical implementation of the models, the resulting kinetic equations are projected onto a Hermite-Laguerre velocity-space polynomial basis, obtaining a moment-hierarchy. The treatment of arbitrary collisionalities is performed by expressing the full Coulomb collision operator in guiding-center and gyrocentre coordinates, and by providing a closed formula for its gyroaverage in terms of the moments of the plasma distribution function, therefore filling a long standing gap in the literature. The use of systematic closures to truncate the moment-hierarchy equation, such as the semi-collisional closure, allows for the straightforward adjustment of the kinetic physics content of the model. In the electrostatic high collisionality regime, our models are therefore reduced to an improved set of drift-reduced Braginskii equations, which are widely used in scrape-off layer simulations. The first numerical studies based on our models are carried out, shedding light on the interplay between collisional, using the Coulomb collision operator, and collisionless mechanisms. In particular, the dynamics of electron-plasma waves and the drift-wave instability are studied at arbitrary collisionality. A comparison is made with the collisionless limit and simplified collision operators used in state-of-the-art simulation codes, where large deviations in the growth rates and eigenmode spectra are found, especially at the levels of collisionality relevant for present and future magnetic confinement fusion devices.

EPFL2019

Fast ion transport in the TORPEX experiment

Alice Burckel

The simulation of the fast ions dynamics experiment in the TORPEX device is presented. The experiment consists of a fast particles source that injects lithium ions in the TORPEX plasma, characterized by interchange-driven turbulence. The simulations are performed by integrating the particles equations of motion in the turbulent fields provided by the simulations of Dr. Ricci et al. [15]. The accuracy of the integration of the eq. of motion has been established by testing the conservation of energy, and comparing the trajectories computed by the simulation with the predictions of the guiding center model. The study of a single particle motion has revealed chaotic behavior, and especially high sensitivity to the initial conditions. An approximation for the motion of the center of mass of a distribution of a statistically meaningful number of particles has been obtained for specific conditions. Finally the dependence of the spreading of the distribution on the fast particles energy, on the spreading in the initial velocities of the particles and on the spreading in the initial angles of the particles has been studied. It has revealed that the effects of turbulence on the spreading of a distribution are visible in the vertical and parallel directions as long as the spreading in the initial velocities is small (else, the effect of turbulence is masked by the effect of the magnetic field geometry) and in the radial direction for any values of the spreading in the initial velocities.

2009

A moment-based model for plasma dynamics at arbitrary collisionality

Rogério Manuel Cabete De Jesus Jorge

EPFL2019