A GPU-accelerated finite-difference time-domain scheme for electromagnetic wave interaction with plasma

A GPU-accelerated Finite-Difference Time-Domain (FDTD) scheme for the simulation of radio-frequency (RF) wave propagation in a dynamic, magnetized plasma is presented. This work builds on well-established FDTD techniques with the inclusion of new time advancement equations for the plasma fluid density and temperature. The resulting FDTD formulation is suitable for the simulation of the time-dependent behaviour of an ionospheric plasma due to interaction with an RF wave and the excitation of plasma waves and instabilities. The stability criteria and the dependence of accuracy on the choice of simulation parameters are analyzed and found to depend on the choice of simulation grid parameters. It is demonstrated that accelerating the FDTD code using GPU technology yields significantly higher performance, with a dual-GPU implementation achieving a rate of node update almost two orders of magnitude faster than a serial implementation. Optimization techniques such as memory coalescence are demonstrated to have a significant effect on code performance. The results of numerical tests performed to validate the FDTD scheme are presented, with a good agreement achieved when the simulation results are compared to both the predictions of plasma theory and to the results of the Tech-X® VORPAL 4.2.2 software that was used as a benchmark

Plasma propulsion simulation using particles

This perspective paper deals with an overview of particle-in-cell / Monte Carlo collision models applied to different plasma-propulsion configurations and scenarios, from electrostatic (E x B and pulsed arc) devices to electromagnetic (RF inductive, helicon, electron cyclotron resonance) thrusters, with an emphasis on plasma plumes and their interaction with the satellite. The most important elements related to the modeling of plasma-wall interaction are also presented. Finally, the paper reports new progress in the particle-in-cell computational methodology, in particular regarding accelerating computational techniques for multi-dimensional simulations and plasma chemistry Monte Carlo modules for molecular and alternative propellan

Wave propagation and absorption in ECR plasma thrusters

Proceeding of: 35th International Electric Propulsion Conference (IEPC), October 8-12, 2017, Atlanta, Georgia, USAThe physical mechanisms involved in the generation, propagation and absorption of microwaves in electron-cyclotron-resonance plasma thrusters, and their relevance in the operation of these devices, are discussed. The features of the electromagnetic waves and electron motion near the resonance region are analyzed with a one-dimensional model. The characteristics of the two-dimensional wave-plasma problem are examined, and a trade-off of different numerical models is presented as a first step toward the development of an ECR wave-plasma interaction simulation code.The research leading to these results has received funding from the European Union H2020 Program under grant agreement number 730028 (Project MINOTOR). Mario Merino's research visit at the Plasma Science and Fusion Center of the Massachusetts Institute of Technology was funded by the Spanish R&D National Plan under grant number ESP2016-75887-P

Instabilities, anomalous transport, and nonlinear structures in partially and fully magnetized plasmas.

Plasmas behavior, to a large extent, is determined by collective phenomena such as waves. Wave excitation, turbulence, and formation of quasi-coherent nonlinear structures are defining features of nonlinear multi-scale plasma dynamics. In this thesis, instabilities, anomalous transport, and structures in partially and fully magnetized plasmas were studied with a combination of analytical and numerical tools. The phenomena studied in this thesis are of interest for many applications, e.g., plasma reactors for material processing, electric propulsion, magnetic plasma confinement, and space plasma physics. Large equilibrium flows of ions and electrons exist in many devices with partially magnetized plasmas in crossed electric and magnetic fields. Such flows result in various instabilities and turbulence that produce anomalous electron transport across the magnetic field. We present first principle, self-consistent, nonlinear fluid simulations that predict the level of anomalous current generally consistent with experimental data. We also show that drift waves in partially magnetized plasmas (which we called Hall drift waves), destabilized by the electron drift along with density gradients, tend to form (via inverse energy cascade) shear flows similar to zonal flows in fully magnetized plasmas. These flows become unstable due to a secondary instability (similar to Kelvin–Helmholtz instability) and produce large-scale quasi-stationary vortices. Then, it was shown that in nonlinear regimes, the axial mode instability due to electron and ion flows (along the electric field) forms large-amplitude cnoidal type waves. At the same time, the strong electric field produced by axial modes affects Hall drift waves stability and provides a feedback mechanism on density gradient driven turbulence, creating a complex picture of interacting anomalous transport, zonal flows, vortices, and streamers. In the case where axial modes are destabilized by boundary effects, the nonlinear dynamics result in a new nonlinear equilibrium or standing oscillating waves. The formation of shear flows (zonal flows) was also studied in the framework of the Hasegawa-Mima equation and it was established that zonal flows can saturate due to nonlinear self-interactions. Lastly, a novel approach for high-fidelity numerical simulations of multi-scale nonlinear plasma dynamics is developed which is illustrated with the example of an unmagnetized plasma

Fast GO/PO RCS calculation: A GO/PO parallel algorithm implemented on GPU and accelerated using a BVH data structure and the Type 3 Non-Uniform FFT

The purpose of this PhD research was to develop and optimize a fast numeric algorithm able to compute monostatic and bistatic RCS predictions obtaining an accuracy comparable to what commercially available from well-known electromagnetic CADs, but requiring unprecedented computational times. This was realized employing asymptotic approximated methods to solve the scattering problem, namely the Geometrical Optics (GO) and the Physical Optics (PO) theories, and exploiting advanced algorithmical concepts and cutting-edge computing technology to drastically speed-up the computation. The First Chapter focuses on an historical and operational overview of the concept of Radar Cross Section (RCS), with specific reference to aeronautical and maritime platforms. How geometries and materials influence RCS is also described. The Second Chapter is dedicated to the first phase of the algorithm: the electromagnetic field transport phase, where the GO theory is applied to implement the “ray tracing”. In this Chapter the first advanced algorithmical concept which was adopted is described: the Bounding Volume Hierarchy (BVH) data structure. Two different BVH approaches and their combination are described and compared. The Third Chapter is dedicated to the second phase of the calculation: the radiation integral, based on the PO theory, and its numerical optimization. Firstly the Type-3 Non-Uniform Fast Fourier Transform (NUFFT) is presented as the second advanced algorithmical tool that was used and it was indeed the foundation of the calculation of the radiation integral. Then, to improve the performance but also to make the application of the approach feasible in case of electrically large objects, the NUFFT was further optimized using a “pruning” technique, which is a stratagem used to save memory and computational time by avoiding calculating points of the transformed domain that are not of interest. To validate the algorithm, a preliminary measurement campaign was held at the headquarter of the Ingegneria Dei Sistemi (IDS) Company, located in Pisa. The measurements, performed on canonical scatterers using a Synthetic Aperture Radar (SAR) imaging equipment set up on a planar scanner inside a semi-anechoic chamber, are discussed

Modeling of very high frequency large-electrode capacitively coupled plasmas with a fully electromagnetic particle-in-cell code

Phenomena taking place in capacitively coupled plasmas with large electrodes and driven at very high frequencies are studied numerically utilizing a novel energy- and charge-conserving implicit fully electromagnetic particle-in-cell / Monte Carlo code ECCOPIC2M. The code shows a good agreement with different cases having various collisionality and absorbed power. Although some aspects of the underlying physics were demonstrated in the previous literature with other models, the particle-in-cell method is advantageous for the predictive modeling due to a complex interplay between the surface mode excitations and the nonlocal physics of the corresponding type of plasma discharges operated at low pressures, which is hard to reproduce in other models realistically

Drift instabilities, anomalous transport, and heating in low-temperature plasmas

Plasma is an ideal gas of charged particles (ions and electrons) in addition to neutral particles. The presence of charged particles results in the generation of electric and magnetic fields that serve as the primary mechanism of the interaction and coupling of particles. As a result, various nonlinear collective phenomena occur in the plasma, the understanding of many of which remains elusive today. On the other hand, plasmas have many applications in different branches of science and technology. Different kinds of plasmas are studied in the atmospheric and space sciences. In the semiconductor industry, the fabrication of electronic chips relies heavily on plasma etching. Plasma is used in modern electrical thrusters for producing the driving force of satellites and spacecrafts. It is also used in future fusion reactors for producing abundant clean energy. Therefore, understanding the complicated phenomena in plasma is important for predicting and controlling its behaviours in various conditions. In this regard, nonlinear phenomena, such as turbulence, are formidable barriers to understanding plasma behaviours. These phenomena are described by nonlinear differential equations that can be barely understood by analytical means and are usually investigated by numerical simulations. Because of this, it is also important to understand the effect of numerical artifacts on simulations. In this thesis, we investigate the nonlinear characteristics of drift instabilities and the role of numerical methods in our understanding of these instabilities. The drift instabilities are driven by excess free energy that exists due to the average (drift) velocities of electron and ion components in plasmas. As a result of these instabilities, the amplitude of fluctuations grows while the drift energy converts into electrostatic energy. This growth continues until the nonlinear effects, such as turbulence, trapping, and wave-wave interactions, become active. As a result of these nonlinear effects, the growth of the fluctuations saturates. In this thesis, our focus will be on two particular types of drift instabilities, namely the Buneman instability and electron-cyclotron drift instability (ECDI). The Buneman instability is driven when a beam of electrons is injected into the stationary ions, while both electrons and ions are unmagnetized. In the ECDI, however, the electrons are magnetized and are also influenced by an external electric field, perpendicular to the magnetic field. This configuration of fields leads to the E × B drift of the electrons that drives the ECDI. Many kinetic simulations are performed, and several nonlinear phenomena such as trapping, heating, anomalous transport, backward waves, and transition of magnetized plasmas to the unmagnetized regime are studied with regard to both instabilities. For the study of the nonlinear effects of drift instabilities, a grid-based Vlasov code is developed and used. The numerical method used in this code is the “semi-Lagrangian” method, which is among the most popular methods for continuum simulations of plasma. In the study of the drift instabilities, we compare the results of the semi-Lagrangian Vlasov simulations with the more traditional particle-in-cell (PIC) method. The results of these benchmarking studies reveal several similarities and discrepancies between Vlasov and particle-in-cell simulations, showing how the numerical methods can interfere with the physics of the problems

Synthetic radiation diagnostics as a pathway for studying plasma dynamics from advanced accelerators to astrophysical observations

In this thesis, two novel diagnostic techniques for the identi1cation of plasma dynamics and thequanti cation of essential parameters of the dynamics by means of electromagnetic plasmaradiation are presented. Based on particle-in-cell simulations, both the radiation signatures of micrometer-sized laser plasma accelerators and light-year-sized plasma jets are simulated with the same highly parallel radiation simulation framework, in-situ to the plasma simulation. The basics and limits of classical radiation calculation, as well as the theoretical and technical foundation of modern plasma simulation using the particle-in-cell method, are brie2y introduced. The combination of previously independent methods in an in-situ analysis code as well as its validation and extension with newly developed algorithms for the simultaneous quantitative prediction of both coherent and incoherent radiation and the prevention of numerical artifacts is outlined in the initial chapters. For laser wake1eld acceleration, a hitherto unknown off-axis beam signature is observed,which can be used to identify the so-called blowout regime during laser defocusing. Since signi cant radiation is emitted only after the minimum spot size is reached, this signature is ideally suited to determine the laser focus position itself in the plasma to below 100 _m and thus to quantify the in2uence of relativistic self-focusing. A simple semi-analytical scattering model was developed to explain the blowout radiation signature. The spectral signature predicted by the model is veri1ed using both a large-scale explorative simulation and a simulation parameter study, based on an experiment conducted at the HZDR. Identi1ed by the simulations, a temporal asymmetry in the scattered laser light, which cannot be described by state of the art quasi-static models of the blowout regime, makes it possible to determine the focus position precisely by using this radiation signature

Implementation of a X-mode multichannel edge density profile reflectometer for the new ICRH antenna on ASDEX Upgrade

Ion cyclotron resonance heating (ICRH) is one of the main heating mechanisms for nuclear fusion plas- mas. However, studying the effects of ICRH operation, such as power coupling efficiency and convective transport, requires the measurement of the local edge plasma density profiles. Two new three-strap an- tennas were designed to reduce tungsten impurity release during operation, and installed on ASDEX Upgrade. One of these ICRH antennas embedded ten pairs of small microwave pyramidal horn anten- nas. In this thesis, a new multichannel X-mode microwave reflectometry diagnostic was developed to use these embedded antennas to simultaneously measure the edge electron density profiles in front of the bottom, middle and top regions of the radiating surface of the ICRH antenna. Microwave reflectome- try is a radar technique that measures the round trip delay of probing waves that are reflected at specific cutoff layers, depending on the probing wave frequency, plasma density and local magnetic field. This diagnostic uses a coherent heterodyne quadrature detection architecture and probes the plasma in the range 40-68 GHz to measure plasma edge electron densities up to 2×1019 m-3, with magnetic fields between 1.85 T and 2.7 T, and a repetition interval as low as 25 μs. This work details the implementa- tion and commissioning of the diagnostic, including the calibration of the microwave hardware and the analysis of the raw reflectometry measurements. We study the automatic initialization of the X-mode upper cutoff measurement, which is the main source of error in X-mode density profile reconstruction. Two first fringe estimation algorithms were developed: one based on amplitude and spectral information and another using a neural network model to recognize the first fringe location from spectrogram data. Kalman filters are used to improve radial measurement uncertainty to less than 1 cm. To validate the diagnostic, we compared the density profile measurements with other electron density diagnostics on ASDEX Upgrade, and observed typical plasma phenomena like the L-H transition and ELM activity. The experimental density profile results were used to corroborate ICRH power coupling simulations under different gas puffing conditions and to observe poloidal convective transport during ICRH operation