Publications by Laurent Villard

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The magnetic topology and particle drift orbits are analysed in a monotonic q profile and in a reversed shear TEXTOR equilibrium that is subject to a magnetic perturbation driven by the Dynamic Ergodic Divertor (DED). The main results prove that there exists a transport barrier for magnetic field lines and for circulating particles in the reversed shear case when the DED is applied. This transport barrier occurs near the surface of minimum q value where the KAM theory may be invalid. Moreover, it should be

Turbulent transport dynamics and level are investigated with the 5D gyrokinetic global code GYSELA, modelling the Ion Temperature Gradient instability with adiabatic electrons. The heat transport exhibits large scale events, propagating radially in both directions at velocities of the order of the diamagnetic velocity. The effective diffusivity is in agreement with that reported in other gyrokinetic codes such as ORB5. Transition from Bohm to gyroBohm scaling is observed on the turbulence correlation length

Observations of nearly zero toroidal current in the central region of tokamaks (the "current hole") raises the question of the existence of toroidal equilibria with very low or reversed current in the core. The solutions of the Grad-Shafranov equilibrium equation with hollow toroidal current density profile including negative current density in the plasma center are investigated. Solutions of the corresponding eigenvalue problem provide simple examples of such equilibrium configurations. More realistic equi

The pivotal role played by radial electric fields in the development of turbulence associated with anomalous transport is examined by means of global gyrokinetic simulations. It is shown that the stabilizing effect of E x B flows on ion temperature gradient (ITG) modes is quadratic in the shearing rate amplitude. For a given shearing rate it leads to an increase in the critical gradient. The electric fields (zonal flows) self-generated by ITG modes interact in a nonlinear way and it is shown that a saturate

In global gyrokinetic simulations it takes a long time for the turbulence to reach a quasisteady state, and quantitative predictions about the quasisteady state turbulence have been difficult to obtain computationally. In particular, global particle-in-cel ...

In this work, we explore the triangularity effects on turbulent transport employing global gyrokinetic simulations performed with the ORB5 code. Numerous experiments on the Tokamak á Configuration Variable (TCV) and, more recently, on the DIII-D machine, h ...

The modelling of Alfvenic instabilities is discussed from the point of view of mode-conversion, showing how the development of the theory affects the predictions as the limitations of the models are gradually relaxed. Conventional tokamak plasmas are relatively well understood and are used for the case of a kinetic Alfven eigenmode (AE) to assess the resonant wave-particle interactions along the magnetic field. The large safety factor in the core of deeply reversed shear plasmas and the low magnetic field o

The effects of applied radial electric fields on the stability of ion temperature gradient (ITG) modes and trapped particle modes are investigated with a full radius gyrokinetic formulation in both axisymmetric and helically symmetric configurations. The validity of a simplified stabilization criterion often applied to experimental analysis (the shearing rate larger than the linear growth rate) is examined for various profiles of ExB flows using an expression for the shearing rate derived for arbitrary quas

In this article an approximated version of the multi-species, non-linear Coulomb collision operator is derived via the use of a truncated moment expansion of the distribution function to compute the Rosenbluth potentials. The evolution of the distribution ...

In the scope of the study of low-frequency electrostatic microinstabilities in tokamak plasmas, attention has been focused on the effect of trapped ions. The ballooning transform has been applied to the gyrokinetic equation, for the case of a large aspect ratio plasma with circular magnetic surfaces. A new eigenvalue code has been developed to solve the resulting integral equation, for the case of adiabatic electrons and full ion dynamics, thus taking into account both circulating and trapped ions. The goal

We present a fully-global linear gyrokinetic simulation code (GYGLES) aimed at describing the unstable spectrum of the ion-temperature-gradient modes in toroidal geometry. We formulate the Particle-In-Cell method with finite elements defined in magnetic coordinates, which provides excellent numerical convergence properties. The poloidal mode structure corresponding to k(parallel to) = 0 is extracted without approximation from the equations, which reduces drastically the numerical resolution needed. The code

The European Integrated Modelling effort (EU-IM) provides the European Transport Simulator (ETS) [1] which was designed to simulate arbitrary tokamak plasma discharges. Two new 1D Fokker-Planck solvers have recently been implemented within ETS: StixRedist ...

Multiple weakly damped Alfven eigenmodes (AE) have been excited and detected by means of a dedicated active diagnostic in JET tokamak plasmas heated by ion cyclotron resonance heating, neutral beam injection heating, lower hybrid heating. and high plasma current Ohmic heating. This phenomenon is interpreted in terms of a new class of AE, the kinetic AE. predicted in theoretical models which include finite Larmor radius and toroidicity effects.

Kinetic effects such as finite ion Larmor excursions and resonant Landau interactions are studied in the toroidicity Alfven eigenmode (TAE) range of frequencies. A new Alfven mode conversion mechanism is described taking place away from the resonances through toroidal mode coupling. It generates a group of kinetic Alfven eigenmodes (KAE) in the neighbourhood of each fluid eigenmode, with wavefields that extend throughout the plasma radius and could lead to substantial a particle losses. Comparisons carried

A semi-Lagrangian code for the solution of the electrostatic drift-kinetic equations in straight cylinder configuration is presented. The code, CYGNE, is part of a project with the long term aim of studying microturbulence in fusion devices. The code has been constructed in such a way as to preserve a good control of the constants of motion, possessed by the drift-kinetic equations, until the nonlinear saturation of the ion-temperature-gadient modes occurs. Studies of convergence with phase space resolution

The effect of plasma shape on confinement has been experimentally explored in the TCV tokamak revealing that the core electron heat transport is significantly reduced by a negative triangularity configuration, which could indicate a (partial) stabilization ...

The global stability of toroidicity induced Alfven eigenmodes (TAEs) in the presence of fast ions in realistic tokamak fusion-grade plasmas is analysed with a global, perturbative approach. Volume averaged fast particle betas for marginal stability are obtained and analysed for a wide range of plasma parameters such as the fast ion radial density profile width, the ratio of birth velocity to the Alfven velocity on axis and the bulk plasma bets. There is evidence of different stability behaviour of two types

Fluctuating quantities in magnetic confinement geometries often inherit a strong anisotropy along the field lines. One technique for describing these structures is the use of a certain set of Fourier components on the tori of nested flux surfaces. We descr ...

The unique flexibility of TCV in plasma shaping has been exploited to address different aspects of tokamak physics in which the plasma shape may play a role. This paper summarizes the experiments undertaken in the TCV tokamak ("Tokamak à Configuration Variable") over the last decade in stability and transport and offers a comprehensive and integrated view of the various effects of plasma shape observed. For each of these effects, the relation between experimental results and theoretical predictions is stres

Electromagnetic waves that resonate with the cyclotron motion of electrons in a magnetized plasma can efficiently transfer their momentum and energy to the plasma. This is routinely used to heat or drive current in tokamak plasmas. The impact of this local ...

Laurent Villard

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Magnetic topology and guiding centre drift orbits in a reversed shear tokamak

Global gyrokinetic simulations of rho* and nu* scalings of turbulent transport

Tokamak equilibria with reversed current density

The role of radial electric fields in linear and nonlinear gyrokinetic full radius simulations

Long global gyrokinetic simulations: Source terms and particle noise control

System size scaling of triangularity effects on global temperature gradient-driven gyrokinetic simulations

Gyrokinetic modelling of macro-instabilities in high performance tokamak plasmas

Radial electric fields and global electrostatic microinstabilities in tokamaks and stellarators

Moment approach of the multi-species non-linear Coulomb collision operator adapted to particle-in-cell codes

Applicability of the ballooning transform to trapped ion modes

Finite element approach to global gyrokinetic Particle-In-Cell simulations using magnetic coordinates

Modeling ICRH and ICRH-NBI Synergy in High Power JET Scenarios Using European Transport Simulator (ETS)

Observation of multiple kinetic Alfven eigenmodes

Kinetic Alfven eigenmodes in a hot tokamak plasma

A semi-Lagrangian code for nonlinear global simulations of electrostatic drift-kinetic ITG modes

The effect of plasma triangularity on turbulent transport:modelling TCV experiments by linear and non linear gyrokinetic simulations

Global Marginal Stability of Taes in the Presence of Fast Ions

Rapid Fourier space solution of linear partial differential equations in toroidal magnetic confinement geometries

Physics Insight and Performance Benefit from Plasma Shaping Experiments in MHD and Energy Transport in the TCV Tokamak

Electron-cyclotron resonance heating and current drive source for flux-driven gyrokinetic simulations of tokamaks