Sciences naturelles et de l'ingénieur

Local and global gyrokinetic simulations of microturbulence in magnetic fusion relevant plasmas using an Eulerian approach

Prof. Dr. Laurent Villard

Adjunct Professor at EPFL (CRPP)

20 January 2011

This project deals with the problem of turbulent transport of particles, heat and momentum in magnetically confined plasmas. Such systems are considered as a possible way to tap a virtually inexhaustible, environmentally benign source of primary energy, namely through fusion of hydrogen isotopes. In particular, the ITER project, now under construction in southern France, will address crucial issues regarding this approach, based on the "tokamak" concept.

The most complete model used to describe such turbulent phenomena is based on the so-called gyrokinetic theory. The problem is nonlinear in a 5-dimensional phase space and is solved with advanced numerical methods involving massively parallel algorithms. The ORB5 code is a truly global gyrokinetic turbulence simulation code based on the Lagrangian approach (PIC) and finite elements. It has been developed at CRPP-EPFL and with major contributions from the Max-Planck IPP in Garching and Greifswald under a longstanding collaboration. It features magnetic coordinates, field-aligned Fourier filtering, non-adiabatic trapped electron response, sources and noise-control algorithms that allow for true statistical steady state to be reached in long simulations. The ORB5 code has demonstrated excellent scalability up to 32768 processors on a BlueGene/P. The present project is an extension of the project that has been running since more than 4 years on the old BlueGene/L at EPFL. It aims at studying shaping, collisional and current profile effects (in particular relevant for internal transport barriers) on electrostatic turbulence in tokamak fusion plasmas.Atomic-Scale Investigation of the Defect Levels at Ge and III-V Interfaces

Prof. Dr. Alfredo Pasquarello

Full Professor

30 August 2011

High mobility materials such as Ge and GaAs (or InGaAs) are presently being investigated as possible solutions for addressing the post-silicon era in nanoelectronics. However, so far, the interfaces between these semiconductors and suitable gate-oxides show excessive amounts of defect states, which have resisted straightforward passivation techniques. In this project, a first-principles simulation study is proposed to identify the origin and the nature of the defects associated to the observed defect levels.

The study rests on structural generation techniques based on ab initio molecular dynamics and on hybrid functional methods, thereby allowing for a meaningful comparison with experimental data for band alignments and defect energy levels. This study encompasses the generation of several atomistic models of interfaces, involving either Ge and III-V materials as semiconductor and disordered Al2O3 or HfO2 as gate oxides.

Particular attention will be devoted to defects revealed by the ab initio molecular dynamics simulations, but specific ad-hoc created defect structures will also be considered in order to understand the effect of standard passivation schemes based on, e.g. H, S, or Se. The present project targets the comprehensive understanding of typical defects occurring at these interfaces, an ineluctable step for allowing the transition to the post-silicon era in nanoelectronics.

Application of classical, mixed quantum mechanical/molecular mechanical

Prof. Dr. Ursula Röthlisberger

Full Professor

30 August 2011

This project is centered around the development and application of classical, mixed quantum mechanical/molecular mechanical (QM/MM) and first-principles molecular dynamics (MD) simulations based on density functional theory (DFT) and time-dependent density functional theory (TDDFT). In particular, the LCBC has developed, and continues to develop, a hybrid QM/MM extension of Car-Parinello simulations for the investigation of extended biological systems. This is done through a partitioning of the description of the system into a detailed quantum mechanical (QM) part and a less detailed molecular mechanical (MM) part. LCBC has developed a QM/MM extension to the well established Car-Parinello scheme by creating an interface between first-principles MD code CPMD (http://www.cpmd.org) and the classical force fields of GROMOS and AMBER.

Physico-chemical processes at surfaces and in solution

Prof. Dr. Wanda Andreoni

Full Professor, Director of the CECAM

30 August 2011

This research program is centered on the simulation of processes of interest to the science of photovoltaic materials and to the capture and storage of carbon dioxide. CdTe-based photovoltaic modules have recently entered mass production and appear to offer a unique alternative to traditional expensive silicon-based solar cells. Still, intense research is on-going to improve on the efficiency of the solar cells and reduce further the cost by optimizing process techniques. It is general belief that a better knowledge of the fundamental properties of the materials is needed for progress to be made. Surprisingly, also in view of the high versatility of CdTe in diverse technologies, this knowledge is very limited. The scope of this project is to provide new and quantitative information on the microscopic behavior of CdTe and the CdTe/CdS interface with advanced computer simulation. This task is challenging because it involves the setting up of reliable atomistic models, the establishing of an accurate framework for the description of the dominant interactions, and the possibility to simulate and quantitatively characterize dynamical processes that accompany growth or deposition or interface formation. Large-scale simulations, both in size and time, are mandatory.

Large-Eddy Simulations of High Reynolds Number Incompressible Flow in Turbomachines

Prof. François Avellan

Full Professor

30 August 2011

Numerical flow simulations of complex unsteady hydrodynamics phenomena experienced by hydraulic turbomachines operating at off design conditions require advanced turbulence models for being able to investigate and to control phenomena like flow separation, rotating stall, rotor-stator interactions and cavitation. Recent progresses of both turbulence modeling and high performance computing lead us to investigate up-to-date Large-Eddy Simulation (LES) of unsteady turbulent flows with respect to available experimental results corresponding to both basic and industrial study cases. However, the geometrical complexity of the computing domain featuring rotating sub-domains and, as well, the large value of the Reynolds number requires a huge computing power. Therefore, an efficient implementation of either full LES software or hybrid LES – Reynolds Averaged Navier-Stokes software needs to be investigated in massively parallel computer architectures such as the EPFL IBM BlueGene/P supercomputer. Finally, the turbulent flow simulations will be validated with experimental data.

Simulation of complex fluids with the lattice Boltzmann Method

Dr. Jonas Lätt

Researcher at SPC UNIGE

27 November 2013

Combined theoretical and experimental studies of inorganic materials

Dr. Max Lawson Daku

Researcher at the Department of Physical Chemistry, UNIL

27 November 2013

Inorganic materials exhibit a wide range of physical and chemical properties such as those of magnetism, photochromism, luminescence, energy transfer, or catalysis. This makes them the subject of numerous multidisciplinary studies in different research areas, at the interface of chemistry and physics. Thanks to the CADMOS ressources which would be made available to us, we aim at making significant progresses in the following ongoing research activities of both fundamental and technological interest. Application of high-level ab initio calculations, molecular dynamics simulations, DFT and periodic DFT calculations to the in-depth characterization of the spin-state energetics and of the structural and vibrational properties of transition metal (TM) systems in the framework of the spin-crossover (SCO) phenomenon; SCO compounds being considered as the archetypes of bistable molecular compounds candidates for the design of devices for data display and storage at the molecular level. Periodic DFT calculations applied to the study of the stability and the structural and vibrational properties (1) of metal hydride materials for hydrogen storage and (2) of fluoride and fluorohalide materials, which when doped with luminescent rare-earth ions may find applications in pressure sensing devices, room-temperature spectrum hole burning for optical data storage, or as X-ray storage phosphors. Application of molecular dynamics simulations to the conformational analysis of flexible donor-bridge-acceptor dyads involved in the fundamental process of electron transfer.

Global gyrokinetic electrostatic turbulence simulations using particles

Dr. Stephan Brunner

Researcher at EPFL - CRPP

20 January 2011

This project deals with the problem of turbulent transport of particles, heat and momentum in magnetically confined plasmas. Such systems are considered as a possible way to tap a virtually inexhaustible, environmentally benign source of primary energy, namely through fusion of hydrogen isotopes. In particular, the ITER project, now under construction in southern France, will address crucial issues regarding this approach, based on the "tokamak" concept.

The most complete model used to describe such turbulent phenomena is based on the so-called gyrokinetic theory. The problem is nonlinear in a 5-dimensional phase space and is solved with advanced numerical methods involving massively parallel algorithms.

The ORB5 code is a truly global gyrokinetic turbulence simulation code based on the Lagrangian approach (PIC) and finite elements. It has been developed at CRPP-EPFL and with major contributions from the Max-Planck IPP in Garching and Greifswald under a longstanding collaboration. It features magnetic coordinates, field-aligned Fourier filtering, non-adiabatic trapped electron response, sources and noise-control algorithms that allow for true statistical steady state to be reached in long simulations. The ORB5 code has demonstrated excellent scalability up to 32768 processors on a BlueGene/P. The present project is an extension of the project that has been running since more than 4 years on the old BlueGene/L at EPFL. It aims at studying shaping, collisional and current profile effects (in particular relevant for internal transport barriers) on electrostatic turbulence in tokamak fusion plasmas.

Numerical Simulation of the Cardiovascular System

Dr. Simone Deparis

Researcher in CMCS at EPFL

20 January 2011

This research project aims at the development, analysis and computer implementation of mathematical models for the cardiovascular system. Our goal is to simulate the physiological response of the human cardiovascular system in healthy or diseased states. This project addresses many fundamental issues. Blood flow interacts both mechanically and chemically with the vessel walls and tissues, giving rise to complex coupled multiphysics problems. This aspect requires the understanding of transport, diffusion and reaction within the blood and organs of the body. Simulating the mechanical interaction between the blood flow and the arterial wall requires specialized algorithms and is computationally expensive. In particular, we plan to simulate atherosclerosis stenosis and aneurisms. The emphasis of this project will be put on algorithm implementation, computational efficiency, validation and verification.

HPC resources are necessary to both be able to perform the simulations, dur to the complexity of the problem, and to lower the time to solution. The simulations relies LifeV, a LGPL finite element library developed in the group in collaboration with Polytecnico di Milano and Emory University.

Numerical simulation of the fast ion dynamics in thermonuclear plasmas

Dr. Wilfred Anthony Cooper

Senior scientist at CRPP/EPFL

30 August 2011

Auxiliary plasma heating methods are required to achieve the necessary conditions for thermonuclear fusion in magnetically confined plasmas. Ion Cyclotron Resonance Heating (ICRH) and Neutral Beam Injection (NBI) constitute the most practical approaches to increase the temperatures of hydrogen and helium isotopes in a fusion reactor environment. The application of ICRH in the JET tokamak significantly distorts the energetic particle distribution function leading to anisotropy in the plasma pressure. The heat deposition at the resonance layer has the double effect of altering the guiding centre particle trajectories and modifying the underlying equilibrium state. Integrated modelling that incorporates the physical eects of heating on the particle distribution function and on the magnetohydrodynamic (MHD) equilibrium state is thus imperative to obtain accurate solutions. Previously, ICRH heating simulations were only coupled to distribution function solvers. The SCENIC package has been developed that uses an anisotropic pressure version of the VMEC MHD equilibrium code, the LEMan code to calculate the heat deposition due to ICRH and the full-f version of the VENUS code (with Monte Carlo radio frequency collision operators) to follow the guiding centre particle orbits. The fast particle distribution function that is obtained is tted with a bi-Maxwellian model for subsequent updates of VMEC equilibria and LEMan heat deposition. High power (12MW ICRH) minority heating scenarios in axisymmetric JET geometry has successfully converged a solution in which the equilibrium, the heating and the hot particle distributions are mutually self-consistent. This has demonstrated that the anisotropic pressure model we have adopted is an essential feature for the simulations of the JET experimental conditions.

The formalism in the SCENIC suite of codes allows for a three-dimensional (3D) description of the confinement geometry. We have applied SCENIC to a 3D quasiaxisymmetric stellarator system (scaled to the size of the JET tokamak) that has great potential of steady state oper- ation with particle confinement properties similar to an axisymmetric tokamak. The spatial distribution of the modulus of the magnetic eld strength mod-B obtained from VMEC, the heat deposition calculated with LEMan and the corresponding deposition from full-f VENUS are presented in Fig. 1 at four toroidal cross section that encompass one quarter of the torus. The beneficial effect of energetic particles for heating and current drive (NBCD) can be compromised by the presence of microturbulent fields. While turbulent heat redistribution is expected to be negligible, results concerning NBCD are scarce. Steady state experiments, such as ITER, profoundly rely on neutral beam injection for plasma control, stabilization and safety prole tailoring. This interaction is therefore investigated, for the ITER steady state scenario, by coupling the full-f version of the VENUS code and the nonlinear gyrokinetic code GENE. The GENE code is used to simulate the background turbulent fields characterizing tokamak plasmas. The features of the turbulent elds are then extracted and computed by a set of numerical diagnostics to provide a realistic estimate of the particle diusivity of energetic ions. The redistribution of the current driven is then calculated by simulating the neutral beam deposition, energetic ion motion (unperturbed and collisional) and turbulent transport. The numerical platform responsible for this part of the analysis is based on the VENUS code, coupled with a beam deposition module. We have introduce a Monte-Carlo scattering operator in the code for simulating the stochastic microturbulent transport. A schematic of the neutral beam particles simulated by the VENUS code is represented in Fig. 2.

Global gyrokinetic electrostatic turbulence simulations using particles

Prof. Dr. Michel Deville

Professeur ordinaire EPFL

5 September 2011

In nuclear safety most severe accident scenarios lead to the presence of fission products in aerosol form in the closed containment atmosphere where turbulent convection currents are dominant. It is important to understand the particle depletion process to estimate the risk of a release of radioactivity to the environment. Experiments have shown a higher settling rate under turbulence. In this project we examine the problem by means of large eddy simulation (LES). For LES a low-pass filter is applied to all flow variables so that the filtered velocity and temperature fields can be adequately resolved on a relatively coarse grid. The interaction between the filtered scales (“subgrid scale”) and the resolved scales and the influence of the subgrid scales on the aerosols need to be modeled. As a model for the containment we use the differentially heated cavity problem together with the Boussinesq equations. Last Numerically, the LES equations are discretized by the spectral element method. Particle trajectories are computed using the Lagrangian particle tracking methods, including the relevant forces on the particles (drag, gravity, thermophoresis, lift). The goal of this work is to run massively parallel LES simulations on the BlueGene/P at turbulence levels as high as in the real application.