Publications by Laurent Villard

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In this work, role of nonadiabatic untrapped electrons in the context of a global ion temperature gradient driven mode has been investigated. In past studies, untrapped electrons have been assumed to be able to respond "instantaneously" to a disturbance. I ...

Nonlinear simulations of Alfvén modes (AMs) driven by energetic particles (EPs) in the presence of turbulence are performed with the gyrokinetic particle-in-cell code ORB5. The AMs carry a heat flux, and consequently they nonlinearly modify the plasma temp ...

Linear, fully gyrokinetic, full radius (global), large aspect ratio studies of Alfven-ion temperature gradient mode (AITG) or kinetic ballooning modes or beta-induced Alfven eigenmodes considering only "passing" species is presented. Effects hitherto completely neglected in a full radius approach such as B-parallel to-fluctuations and the ones which have been treated partly [Phys. Plasmas 10, 1424 (2003)] such as Shafranov shifts are included. To this end, an existing code EM-GLOGYSTO has been upgraded to i

Highly variable flux surface averaged heat fluxes are resolved in gyrokinetic simulations of ion temperature gradient (ITG) turbulence, even in large systems. Radially propagating fronts or avalanches are also seen. Their propagation lengths in gyroradii a ...

Electromagnetic microinstabilities in tokamak plasmas are studied by means of a linear global eigenvalue numerical code. Ion dynamics is described by the gyrokinetic equation, so that finite ion Larmor radius effects are taken into account to all orders. Nonadiabatic electrons are included in the model, with passing particles described by the drift-kinetic equation and trapped particles through the bounce averaged drift-kinetic equation. A large aspect ratio plasma with circular shifted surfaces is consider

First global linear study of electrostatic drift waves in two realistic quasisymmetric configurations, namely the Quasi-Axially symmetric Stellarator with three fields periods (QAS3) [P. Garabian and L. P. Ku, Phys. Plasma 6, 645 (1999)] and the Helically Symmetric eXperiment (HSX) [F. S. B. Anderson , Trans. Fusion Technol. 27, 273 (1995)], are presented. Effects of the shape of the plasma on the growth rate and frequency of the ion temperature gradient (ITG) driven mode are investigated by varying the qua

In tokamaks and stellarators, measurements of electromagnetic fluctuations in the presence of resonant particle drive, including fusion-produced alpha's, reveal the excitation of Alfven eigenmodes (AE), related under certain conditions to a degradation in the fast-particle confinement. The balance between the drive and the background damping is investigated using active diagnostic systems to excite and measure the AE spectrum in terms of frequencies and damping rates. At JET, saddle-coil antennae drive low

TCV experiments and novel theoretical studies have addressed key questions regarding the operational space of negative triangularity (NT) plasmas in view of an L-mode NT reactor, in terms of stability, performance properties and core-edge integration. We show that reducing the top triangularity (δtop) to more negative values induces an H-L back transition, confirming the direct dependence between H-mode existence and access to 2nd ballooning stability region. We also show that the X-point δ can prohibit H-m

Linearized operators describing inter-species and like-species collisions have been discretized and implemented in the gyrokinetic Particle-In-Cell (PIC) code ORB5 [S. Jolliet, Comp. Phys. Comm. 177, 409 ( 2007)] based on the delta-f approach. Simulation r ...

Electromagnetic wave propagation in the Alfven frequency range in two-dimensional (2-D) and three-dimensional (3-D) plasma configurations is investigated by using a recently developed full-wave code, LEMan. The code solves the Maxwell equation globally in a fully 3-D numerical equilibrium geometry with nested magnetic surfaces and implements a cold plasma model with finite electron mass. Energy balance is used for the self-consistency and convergence checks. The code is applied for several 2-D systems and a

In this work, the first nonlinear particle-in-cell simulations carried out in a stellarator with the global gyrokinetic code EUTERPE using adiabatic electrons and realistic plasma parameters are reported. Several studies are conducted with the aim of enabl ...

Neoclassical transport and a particle confinement are analysed for three 3-D reactor designs-the QAS3, the ST/sphellamak hybrid and the sphellamak-using the VENUS code. It is observed that neither the QAS3 nor the ST/sphellamak is truly aquasi-axisymmetric configuration. Thus the transport is governed by the helical deformation of the magnetic Field strength, and these configurations do not effectively confine the trapped a particles. On the other hand, the sphellamak is a nearly isodynamic structure in the

Ion temperature gradient (ITG) modes in the short wavelength regime (kθρi >1.0) known as the short wavelength ion temperature gradient (SWITG) modes, are investigated linearly and nonlinearly in the electrostatic limit for ADITYA–U tokamak, in the presence of monotonic and reversed magnetic shear using a global gyrokinetic particle–in–cell (PIC) code, ORB5. It is observed that, while the growth rate of the conventional ITG branch is reduced, the growth rate of the SWITG branch is enhanced by the reversed ma

Negative triangularity tokamak plasmas are subject of an increased interest both in existing experiments and in studies of core physics as well as power handling relevant to the fusion demonstration power reactor. The ideal MHD stability calculations for the negative triangularity plasma extend the investigation of the TCV tokamak beta limits and edge stability to double null shapes, lower aspect ratio and negative shear. In case of the negative triangularity, the 2nd stability access is closed for ballooni

Alfven waves in fusion plasmas play an important role in a number of situations. First, in Alfven Wave Heating (AWH) schemes. Second, both theory and experiment have demonstrated the existence of Global Alfven Eigenmodes (GAEs). GAEs have been observed in different tokamaks (PRETEXT, TCA, TEXTOR, etc.) and, more recently, in a stellarator (Wendelstein 7-AS) where they were shown to become unstable under intense Neutral Beam injection. Third, the existence and possible destabilization by fast ions of Toroidi

Using the Eulerian code GENE [1], gyrokinetic simulations of microturbulence were carried out under conditions relevant to electron-Internal Transport Barriers (eITB) in the TCV tokmak [2], generated under conditions of low or negative shear. For typical density and temperature gradients measured in such barriers, the corresponding simulated fluctuation spectra appears to simultaneously contain longer wavelength Trapped Electron Modes (TEM, for typically k_perp*rho_i < 0.5, k_perp being the characteristic p

The absorption of power is studied with fluid and gyrokinetic plasma models, when Alfven resonances provide for a weak damping in a partially standing wave field. Examples chosen in slab and toroidal geometry show that the fluid predictions based on resonance absorption are generally very different from the Landau damping of mode-converted slow waves. They, in particular, suggest that the continuum damping of toroidal Alfven eigenmodes (TAE) and the power deposition profiles obtained in the ion-cyclotron ra

Results from the first experiments to drive Alfven Eigenmodes (AEs) with antennas external to a tokamak plasma are presented. In ohmically heated plasma discharges, direct experimental measurements of the damping of toroidicity induced AEs (TAEs) have allowed an identification of different regimes corresponding to different dominant TAE absorption mechanisms with a wide range of damping rates, 10(-3) less than or equal to gamma/omega less than or equal to 10(-1). In plasmas heated by ion cyclotron resonance

The electromagnetic effect is studied on the short wavelength branch of the ion temperature gradient mode in the linear regime for the first time using a global gyrokinetic model. The short wavelength ion temperature gradient mode growth rate is found to b ...

Laurent Villard

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Role of nonadiabatic untrapped electrons in global electrostatic ion temperature gradient driven modes in a tokamak

Nonlinear interaction of Alfvénic instabilities and turbulence via the modification of the equilibrium profiles

A full radius gyrokinetic stability analysis for large aspect ratio finite-beta tokamaks

Avalanche-like bursts in global gyrokinetic simulations

Global-gyrokinetic study of finite beta effects on linear microinstabilities

Global linear gyrokinetic simulations in quasi-symmetric configurations

Alfven eigenmode experiments in tokamaks and stellarators

NEGATIVE TRIANGULARITY TOKAMAK OPERATION IN TCV

Negative Triangularity Tokamak Operation in TCV

Global collisional gyrokinetic simulations of ITG microturbulence starting from a neoclassical equilibrium

Three-dimensional full-wave propagation code for cold plasma

Nonlinear gyrokinetic PIC simulations in stellarators with the code EUTERPE

Neoclassical transport and alpha particle confinement in novel 3-D reactor systems

Effect of reversed magnetic shear on short wavelength ion temperature gradient modes in ADITYA–U tokamak

Stability limits for tokamak plasma with negative triangularity

Alfven wave heating and stability

Gyrokinetic Simulations of Microturbulence in TCV Electron Internal Transport Barriers

On resonance absorption and continuum damping

Overview of alfven eigenmode experiments in jet

Finite beta effects on short wavelength ion temperature gradient modes