Processes driving non-Maxwellian distributions in high energy density plasmas

The purpose of this thesis is to explore the driving of non-Maxwellian distributions of particles in high energy density plasmas in a few select cases, with particular reference to efforts to produce a net gain in energy via inertial confinement fusion (ICF). Non-Maxwellian distributions are typically short-lived, as distributions are forced toward equilibrium by collisions, and are rarely static as a net transfer of energy must occur to sustain them. This makes non-Maxwellian distributions challenging to study with conventional approaches to plasma physics. The strategy adopted in this work to understand their evolution, and their effects, is a kinetic approach in which particles are individually accounted for. The specific cases presented are that of degenerate electrons during the heating of the cold fuel shell in hotspot ignition schemes, ion-ion inverse bremsstrahlung absorption of laser radiation, and large-angle Coulomb collisions. New computational algorithms based on the Monte Carlo technique are presented, and are capable of modelling the salient aspects of the phenomena explored. Important results which form part of this thesis include that conventional models underestimate degenerate electron temperatures long after the plasma ceases to be degenerate, that it may be possible to induce temperatures of keV in light-ion species with high power, short pulse lasers, and that consideration of large-angle collisions changes interactions in a plasma in several significant ways. Of most interest are the ability of large-angle collisions to decrease equilibration times, drive athermal tails on distribution functions, and increase the overall yield from fusion reactions relative to small-angle only simulations.Open Acces

Attenuation and Refraction of an Electromagnetic Wave in an Electron Beam Generated Plasma

Artificially generated plasmas may be employed to alter the propagation characteristics of electromagnetic waves. The purpose of this report is to study the propagation of electromagnetic waves in an electron beam generated plasma. To understand the physics related to this concept requires the development of computational tools dealing with a plasma created by an electron beam, an assessment of the temporal and spatial evolution of the plasma, and a characterization of the refraction and attenuation of electromagnetic (EM) waves in a collisional plasma. Three computer programs were developed to characterize the effectiveness of an electron beam generated plasma in refracting and attenuating an EM wave. The spatial extent and density distribution of a plasma generated by a relativistic electron beam were determined using an axisymmetric Monte Carlo model. This plasma density distribution was used as a source term in the second code, a temporal solution of the plasma evolution based on a time dependent analysis of the plasma rate equations. The third code developed, evaluates the attenuation and refraction of an EM wave in the resulting plasma by using a ray tracing method based on the eikonal approach of Sommerfeld. The theoretical foundation and validation procedures are presented for each program. A limited exploration of the dependence of the plasma distribution on neutral densities and the electron beam energies was performed. For neutral densities corresponding to 5 km altitude, the plasma longitudinal extent ranged from 52 to 868 cm and the radial extent ranged from 18 to 292 cm for initial electron energies between 100 keV and 1 MeV respectively. Plasma chemistry plays a critical role in determining the electron plasma density and dictates the beam format required to achieve a desired level of EM wave attenuation

Contemporary particle-in-cell approach to laser-plasma modelling

Particle-in-cell (PIC) methods have a long history in the study of laser-plasma interactions. Early electromagnetic codes used the Yee staggered grid for field variables combined with a leapfrog EM-field update and the Boris algorithm for particle pushing. The general properties of such schemes are well documented. Modern PIC codes tend to add to these high-order shape functions for particles, Poisson preserving field updates, collisions, ionisation, a hybrid scheme for solid density and high-field QED effects. In addition to these physics packages, the increase in computing power now allows simulations with real mass ratios, full 3D dynamics and multi-speckle interaction. This paper presents a review of the core algorithms used in current laser-plasma specific PIC codes. Also reported are estimates of self-heating rates, convergence of collisional routines and test of ionisation models which are not readily available elsewhere. Having reviewed the status of PIC algorithms we present a summary of recent applications of such codes in laser-plasma physics, concentrating on SRS, short-pulse laser-solid interactions, fast-electron transport, and QED effects

A Computationally Efficient Moment-Preserving Monte Carlo Electron Transport Method with Implementation in Geant4

The subject of this dissertation is a moment-preserving Monte Carlo electron transport method that is more efficient than analog or detailed Monte Carlo simulations, yet provides accuracy that is statistically indistinguishable from the detailed simulation. Moreover, the Moment-Preserving (MP) method is formulated such that it is distinctly different than Condensed History (CH) methods making the MP method free of the limitations inherent to CH and proving a viable alternative for transporting electrons. Analog, or detailed, Monte Carlo simulations of charged particle transport is computationally intensive; thus, it is impractical for routine calculations. The computational cost of analog Monte Carlo is directly attributed to the underlying charged particle physics characterized by extremely short mean free paths (mfp) and highly peaked differential cross sections (DCS). As a result, a variety of efficient, although approximate solution methods were developed over the past 60 years. The most prolific method is referred to as the Condensed History method. However, CH is widely known to suffer from inconsistencies between the underlying theory and the application of the method to real, physical problems. Therefore, it is of interest to develop an alternative method that is both efficient and accurate, but also a completely different approach to solving the charged particle transport equation that is free of the limitations inherent to CH. This approach arose from the development of a variety of reduced order physics (ROP) methods that utilize approximate representations of the collision operators. The purpose of this dissertation is the theoretical development and numerical demonstration of an alternative to CH referred to as the Moment-Preserving method. The MP method poses a transport equation with reduced order physics models characterized by less-peaked DCS with longer mfps. Utilizing pre-existing single-scatter algorithms for transporting particles, a solution to the aforementioned transport equation is obtained efficiently with analog level accuracy. The process of constructing ROP models and their properties are presented in detail. A wide variety of theoretical and applied charged particle transport problems are studied including: calculation of angular distributions and energy spectra, longitudinal and lateral distributions, energy deposition in one and two dimensions, a validation of the method for energy deposition and charge deposition calculations, and response function calculations for full three-dimensional detailed detector geometries. It is shown that the accuracy of the MP method is systematically controllable through refinement of the ROP models. In many cases, efficiency gains of two to three orders of magnitude over analog Monte Carlo are demonstrated, while maintaining analog level accuracy. That is, solutions generated sufficient ROP DCS models are statistically indistinguishable from the analog solution. To maintain analog level accuracy under strict problem conditions, small efficiency gains are realized. However, loss of efficiency under these conditions is true of all approximate methods, but the MP method remains accurate where other methods may fail. That is not to say the MP method does not suffer from limitations because the MP method will result in discrete artifacts when the problems conditions are strict. However, where limitations of the method arise, they are overcome through systematic refinement of the ROP DCS models required by the method. In addition to accuracy and efficiency results, it is shown that the MP method does not require a boundary crossing or pathlength correction algorithm, which is in great contrast to the CH method. Finally, implementation and maintenance of the MP method was found to be straightforward and requires significantly less effort than CH when measured by the number of lines of code required for each method. In particular, as compared with the class II CH method utilized in the Geant4 standard electromagnetic physics list. Ultimately, the MP is shown to be accurate, efficient, versatile, and simple to implement and maintain

Frontiers, challenges, and solutions in modeling of swift heavy ion effects in materials

Since a few breakthroughs in the fundamental understanding of the effects of swift heavy ions (SHI) decelerating in the electronic stopping regime in the matter have been achieved in the last decade, it motivated us to review the state-of-the-art approaches in the modeling of SHI effects. The SHI track kinetics occurs via several well-separated stages: from attoseconds in ion-impact ionization depositing energy in a target, to femtoseconds of electron transport and hole cascades, to picoseconds of lattice excitation and response, to nanoseconds of atomic relaxation, and even longer macroscopic reaction. Each stage requires its own approaches for quantitative description. We discuss that understanding the links between the stages makes it possible to describe the entire track kinetics within a multiscale model without fitting procedures. The review focuses on the underlying physical mechanisms of each process, the dominant effects they produce, and the limitations of the existing approaches as well as various numerical techniques implementing these models. It provides an overview of ab-initio-based modeling of the evolution of the electronic properties; Monte Carlo simulations of nonequilibrium electronic transport; molecular dynamics modeling of atomic reaction on the surface and in the bulk; kinetic Mote Carlo of atomic defect kinetics; finite-difference methods of tracks interaction with chemical solvents describing etching kinetics. We outline the modern methods that couple these approaches into multiscale multidisciplinary models and point to their bottlenecks, strengths, and weaknesses. The analysis is accompanied by examples of important results improving the understanding of track formation in various materials. Summarizing the most recent advances in the field of the track formation process, the review delivers a comprehensive picture and detailed understanding of the phenomena.Comment: to be submitte

Electron Thermal Runaway in Atmospheric Electrified Gases: a microscopic approach

Thesis elaborated from 2018 to 2023 at the Instituto de Astrofísica de Andalucía under the supervision of Alejandro Luque (Granada, Spain) and Nikolai Lehtinen (Bergen, Norway). This thesis presents a new database of atmospheric electron-molecule collision cross sections which was published separately under the DOI : With this new database and a new super-electron management algorithm which significantly enhances high-energy electron statistics at previously unresolved ratios, the thesis explores general facets of the electron thermal runaway process relevant to atmospheric discharges under various conditions of the temperature and gas composition as can be encountered in the wake and formation of discharge channels

Jets in hot nuclear matter : Resumming multiple emissions in QCD

Under tung-ion-kollisjonar med høg energi smeltar protonar og nøytronar saman og dannar eit kvark-gluon-plasma. Modifiseringa av jeter, som forplantar seg gjennom det avgrensa mediet, har vorte grundig forska på ved CERN-LHC, og BNL-RHIC i kollisjonseksperiment. I løpet av det siste tiåret har man samla opp omfattande kunnskap om teorien om jet-modifikasjonar. Denne avhandlinga presenterer eit konsistent og toppmoderne perspektiv på jet-modifikasjonar basert på perturbasjons-QCD. Tatt i betraktning nylege framsteg mot ei meir nøyaktig forklaring av jeter i høgenergifysikk, vert jet-modifikasjonar i mediet gjennomgått ved å fokusera på deira perturbasjonsstruktur i alle orden og definere grannsemda til observerbare jet-strålar.In high-energy heavy-ion collisions, protons and neutrons melt and form the quark-gluon plasma. The modification of jets, propagating through this deconfined medium, has been extensively studied at the CERN-LHC, and the BNL-RHIC colliders experiments. Over the last decade, extensive knowledge has piled up in the theory of jet modification. This thesis presents a consistent and state-of-the-art perspective of jet modification based on perturbative QCD. Considering recent progress toward a more accurate description of jets in high-energy physics, jet modification in the medium is reviewed by focusing on their all-order perturbative structure and defining the accuracy of quenched jet observables.Doktorgradsavhandlin

Cherenkov radiation in the soft X-ray region : towards a compact narrowband source

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