Mihkel Veske

Electrodynamics—molecular dynamics simulations of the stability of Cu nanotips under high electric field

The shape memory effect and pseudoelasticity in Cu nanowires is one possible pair of mechanisms that prevents high aspect ratio nanosized field electron emitters to be stable at room temperature and permits their growth under high electric field. By utilizing hybrid electrodynamics molecular dynamics simulations we show that a global electric field of 1 GV/m or more significantly increases the stability and critical temperature of spontaneous reorientation of nanosized Cu field emitters. We also show that in the studied tips the stabilizing effect of an external applied electric field is an order of magnitude greater than the destabilization caused by the field emission current. We detect the critical temperature of spontaneous reorientation using the tool that spots the changes in crystal structure. The method is compatible with techniques that consider the change in potential energy, has a wider range of applicability and allows pinpointing different stages in the reorientation processes.

Molecular dynamics simulations of near-surface Fe precipitates in Cu under high electric fields

High electric fields in particle accelerators cause vacuum breakdowns in the accelerating structures. The breakdowns are thought to be initiated by the modification of material surface geometry under high electric fields. These modifications in the shape of surface protrusions enhance the electric field locally due to the increased surface curvature. Using molecular dynamics, we simulate the behaviour of Cu containing a near-surface Fe precipitate under a high electric field. We find that the presence of a precipitate under the surface can cause the nucleation of dislocations in the material, leading to the appearance of atomic steps on the surface. Steps resulting from several precipitates in close proximity can also form protruding plateaus. Under very high external fields, in some cases, we observed the formation of voids above or below the precipitate, providing additional dislocation nucleation sites.

Application of multiphysics and multiscale simulations to optimize industrial wood drying kilns

Timber industry and export are an important part of Estonian economy, making affordable industrial scale equipment an important investment for small or starting companies. These companies often develop on-site equipment for wood processing and drying, utilizing pre-existing infrastructure to minimize cost and risk. However, under these conditions custom design of the wood drying kilns is often required.In the present study, a finite element simulation based approach is used to simulate and optimize the industrial wood drying process and the design of the custom-made kilns in a multiscale-multiphysics modeling framework. Air flow is calculated by the Navier-Stokes equations or ?-e turbulence model followed by heat transport in the solid and gas phase and moisture dynamics in wood and air. The dense packing of the processed materials is handled by utilizing a porous media approach and homogenization procedure, leading to effective simulations of the moisture and heat balance.Multiphysics-multiscale simulations are successfully adapted to optimize the industrial design of wood drying kilns. The optimization of the kiln design is achieved by estimating the necessary ventilating power and ensuring homogeneous drying of the processed material.

Thermal runaway of metal nano-tips during intense electron emission

When an electron emitting tip is subjected to very high electric fields, plasma forms even under ultra high vacuum conditions. This phenomenon, known as vacuum arc, causes catastrophic surface modifications and constitutes a major limiting factor not only for modern electron sources, but also for many large-scale applications such as particle accelerators, fusion reactors etc. Although vacuum arcs have been studied thoroughly, the physical mechanisms that lead from intense electron emission to plasma ignition are still unclear. In this article, we give insights to the atomic scale processes taking place in metal nanotips under intense field emission conditions. We use multi-scale atomistic simulations that concurrently include field-induced forces, electron emission with finite-size and space-charge effects, Nottingham and Joule heating. We find that when a sufficiently high electric field is applied to the tip, the emission-generated heat partially melts it and the field-induced force elongates and sharpens it. This initiates a positive feedback thermal runaway process, which eventually causes evaporation of large fractions of the tip. The reported mechanism can explain the origin of neutral atoms necessary to initiate plasma, a missing key process required to explain the ignition of a vacuum arc. Our simulations provide a quantitative description of in the conditions leading to runaway, which shall be valuable for both field emission applications and vacuum arc studies.

Atomistic modeling of metal surfaces under high electric fields: Direct coupling of electric fields to the atomistic simulations

We propose a novel tool to perform electrodynamics-molecular dynamics and electrodynamics-kinetic Monte Carlo simulations. The tool generates finite elements in- and outside the atomistic domain, uses them to solve a system of linear differential equations and offers the interface to output the results into atomistic simulations. The tool shows high tolerance against crystallographic orientation in the material and robustness against dynamic atomistic processes there.

Feature article: Firmware updating systems for nanosatellites

During the course of a space mission unexpected events can occur regardless of rigorous testing. In order to ensure the ability of a spacecraft to recover and adapt to new situations, it may be necessary to update the firmware for resolving the software issues, work around hardware problems, or introduce new features. The importance of remote firmware updates as well as a method to calculate an indicative value of flexibility in space missions is summarized by R. Nilchiani [1].

Verification of a multiscale surface stress model near voids in copper under the load induced by external high electric field

In the current study we use a model of surface stress for finite element method calculations to complement existing bulk stress models. The resulting combined model improves the accuracy of stress calculations near nanoscale imperfections in the material. We verify the results by simulating differently-shaped voids in single crystal copper both with FEM and with molecular dynamics, and compare the resulting stress distributions. The compared results agree well within small uncertainties, indicating that the implemented surface stress model is able to capture all the major features of the stress distributions in the material. Discrepancies occur near surfaces, where the crystal faces were not defined explicitly in the model. The fast and accurate FEM calculations can be used to estimate the stress concentration of specific extended defects, such as voids, while studying the dislocation-mediated mechanisms near these defects in the presence of external stresses by atomistic techniques.

Dynamic coupling of a finite element solver to large-scale atomistic simulations

Abstract We propose a method for efficiently coupling the finite element method with atomistic simulations, while using molecular dynamics or kinetic Monte Carlo techniques. Our method can dynamically build an optimized unstructured mesh that follows the geometry defined by atomistic data. On this mesh, different multiphysics problems can be solved to obtain distributions of physical quantities of interest, which can be fed back to the atomistic system. The simulation flow is optimized to maximize computational efficiency while maintaining good accuracy. This is achieved by providing the modules for a) optimization of the density of the generated mesh according to requirements of a specific geometry and b) efficient extension of the finite element domain without a need to extend the atomistic one. Our method is organized as an open-source C + + code. In the current implementation, an efficient Laplace equation solver for calculating the electric field distribution near a rough atomistic surface demonstrates the capability of the suggested approach.

Nanotip evaporation under high electric fiele

Vacuum arcing (also known as breakdown), is a major limiting factor in various applications such as particle accelerators, fusion reactors etc. Although it is well-established that vacuum arcs appear after intense Field electron Emission (FE), the physical mechanism that leads from FE to the ignition of plasma is not yet understood. A common hypothesis is that intense FE leads to excessive heating of the cathode, which causes its deformation, and eventually material evaporation and plasma formation. However, this process has never been observed experimentally or fully understood theoretically. Here we present atomistic simulations of this process that give an insight to it and provide possible mechanisms that can explain the initiation of plasma. Our simulations take into account various physical processes, namely field-induced stresses, electron emission with finite-size and space-charge effects, Nottingham and Joule heating. We find that under certain conditions the cathode apex melts and the field-induced stress deforms it to become longer and sharper. This initiates a positive feedback process that leads to extremely high local temperatures, and causes evaporation of material. This mechanism might give a plausible explanation to the initiation of plasma.