Study on Coulomb explosion induced by laser-matter interaction and application to ion acceleration

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Ion acceleration driven by intense laser pulses

Laser pulses incident on plasma targets are capable of exciting very intense accelerating fields, that allow the acceleration of ions to high energies in very short distances. This is why a lot of interest has been developed on the topic of laser-driven ion acceleration over the past twenty years. Such a compact and affordable ion source would have many potential applications in physics and medicine, but several requirements are still far from being fulfilled. In this thesis two mechanisms of ion acceleration are investigated: shock wave acceleration and Coulomb explosion. Ultraintense lasers shot on plasma targets are capable of driving strong electrostatic shock waves that accelerate the plasma ions to high energies with a narrow energy spectrum. In the present work, the mechanism of shock formation and propagation in near-critical density plasmas is studied in detail. An idealized scenario where shock waves arise from the interpenetration of plasma slabs is studied. A theoretical kinetic model is derived and compared with simulation results. The conditions to accelerate ions to high energies with low energy spread are derived. The role of the laser in exciting shock waves is analyzed. The factors leading to high energy ion beams with narrow energy spectrum obtained in the simpler configuration are verified in this more complex and realistic scenario. A scaling for the ion energy with the pulse intensity is inferred for the ideal case of a plane wave and for a more realistic case of a finite size laser spot. The second mechanism of ion acceleration that has been considered is the Coulomb explosion of pure ion nanoplasmas, an important subject in the field of laser-cluster interaction. In this thesis, a detailed study of Coulomb explosion in hetero-nuclear clusters consisting of different atomic species is carried out. Numerical results indicate that, in the presence of different ion species, lighter ions are accelerated in a quasi-monoenergetic way, in contrast with the well known results on Coulomb explosion of clusters composed by a single ion species, where the energy spectrum is much broader. A study on the formation of shock shells, nonlinear structures that arises during Coulomb explosion of homo-clusters when the initial density exhibits radial non-uniformity, is also presented. The analysis is carried out comparing N-body simulation results, that represent the exact solution since no approximations have been made, to the collisionless kinetic theory. The study shows that there are consistent differences between the real dynamics and the model based on the Vlasov-Poisson equations

ORB5: a global electromagnetic gyrokinetic code using the PIC approach in toroidal geometry

This paper presents the current state of the global gyrokinetic code ORB5 as an update of the previous reference [Jolliet et al., Comp. Phys. Commun. 177 409 (2007)]. The ORB5 code solves the electromagnetic Vlasov-Maxwell system of equations using a PIC scheme and also includes collisions and strong flows. The code assumes multiple gyrokinetic ion species at all wavelengths for the polarization density and drift-kinetic electrons. Variants of the physical model can be selected for electrons such as assuming an adiabatic response or a ``hybrid'' model in which passing electrons are assumed adiabatic and trapped electrons are drift-kinetic. A Fourier filter as well as various control variates and noise reduction techniques enable simulations with good signal-to-noise ratios at a limited numerical cost. They are completed with different momentum and zonal flow-conserving heat sources allowing for temperature-gradient and flux-driven simulations. The code, which runs on both CPUs and GPUs, is well benchmarked against other similar codes and analytical predictions, and shows good scalability up to thousands of nodes

Computational study of nonlinear plasma waves

A low-noise plasma simulation model is developed and applied to a series of linear and nonlinear problems associated with electrostatic wave propagation in a one-dimensional, collisionless, Maxwellian plasma, in the absence of magnetic field. It is demonstrated that use of the hybrid simulation model allows economical studies to be carried out in both the linear and nonlinear regimes with better quantitative results, for comparable computing time, than can be obtained by conventional particle simulation models, or direct solution of the Vlasov equation. The characteristics of the hybrid simulation model itself are first investigated, and it is shown to be capable of verifying the theoretical linear dispersion relation at wave energy levels as low as .000001 of the plasma thermal energy. Having established the validity of the hybrid simulation model, it is then used to study the nonlinear dynamics of monochromatic wave, sideband instability due to trapped particles, and satellite growth

Using Kernel-Based Statistical Distance to Study the Dynamics of Charged Particle Beams in Particle-Based Simulation Codes

Measures of discrepancy between probability distributions (statistical distance) are widely used in the fields of artificial intelligence and machine learning. We describe how certain measures of statistical distance can be implemented as numerical diagnostics for simulations involving charged-particle beams. Related measures of statistical dependence are also described. The resulting diagnostics provide sensitive measures of dynamical processes important for beams in nonlinear or high-intensity systems, which are otherwise difficult to characterize. The focus is on kernel-based methods such as Maximum Mean Discrepancy, which have a well-developed mathematical foundation and reasonable computational complexity. Several benchmark problems and examples involving intense beams are discussed. While the focus is on charged-particle beams, these methods may also be applied to other many-body systems such as plasmas or gravitational systems

Multiscale numerical simulations of the magnetized plasma sheath with massively parallel electrostatic particle-in-cell code

Understanding the physics of the plasma boundary and plasma-surface interactions is one of the key scientific challenges in fusion science and engineering. Large-scale integrated simulations and high-performance computing can provide valuable insights on the dynamic phenomena involved at the interface between the plasma and the material surface. Current state-of-the-art simulations of magnetically-confined fusion devices are typically performed using gyrokinetic approximations, aimed at resolving the physics of the core plasma, scrape-of-layer, and a portion of the divertor. However, the region of plasma near to the surface, called the plasma sheath, where the plasma ions accelerate from subsonic to supersonic conditions, is typically either not handled or treated with ad-hoc approximations. The characteristic scale of the near-surface plasma (sheath and presheath) is comparable to the Debye length, which is of the order of, or smaller, than the ion gyroradius. A detailed description of the kinetic processes occurring during the supersonic acceleration across the collisional and magnetic presheaths and requires a fully-kinetic model that is not present in any current fusion code, thus limiting a detailed evaluation of the energy-angle spectrum of the ions impacting on the surface of a tokamak. We have developed and verified a new massively-parallel Particle-in-Cell code, named hPIC, solving the multi-species Boltzmann-Poisson integro-differential set of equations. We give an overview of the model equations, of the architecture of the code, and summarize the verification tests, also presenting the scalability tests performed on the Blue Waters supercomputer at the University of Illinois. The model has been used for the numerical characterization of the plasma sheath and presheath in strong magnetic fields. Thanks to the new Particle-in-Cell, we have performed a systematic analysis of the structure of the magnetized plasma sheath, in order to determine the trends of the Ion Energy-Angle Distributions (IEAD) of the particles impacting on the wall after crossing the presheath and sheath regions. The model provides the dependance of the IEAD on the level of magnetization and magnetic inclination with respect to the surface. We have found that in regimes of intermediate-to-strong magnetization, the ion flow has a characteristic three-dimensional structure, which appears in all evidence within the magnetic presheath after the ions transition from sonic to supersonic. The model also suggests the disappearance of the electrostatic (Debye) sheath at high magnetic angles, with an interesting reduction of the ion flow down to subsonic conditions. Furthermore, detailed Particle-in-Cell simulations have been compared to simplified representations of the magnetized plasma sheath based on a set of fluid equations coupled to a Monte-Carlo particle-tracer for the reconstruction of the Ion Energy-Angle Distributions (IEAD) of the particles impacting on the wall, finding qualitative agreement and suggesting strategies of model reduction which could be used in Whole-Device Modeling. Finally, the model of the magnetic and collisional presheath has been validated against three-dimensional tomographic Laser-Induced Fluorescence measurements taken at the HELIX helicon facility at WVU. Our analysis highlights the role of neutral gas pressure, background neutral flow, and ambient electric field on the structure of the collisional and magnetic presheath, finding absolute quantitative agreement between our calculated data and experimental measurements. In particular, the work gives clear evidence of the three-dimensional structure of the magnetized plasma sheath, a unique feature not present in the classical thermal sheath in unmagnetized conditions

Computational Simulation of Faraday Probe Measurements.

Electric propulsion devices, including ion thrusters and Hall thrusters, are becoming increasingly popular for long duration space missions. Ground-based experimental testing of such devices is performed in vacuum chambers, which develop an unavoidable background gas due to pumping limitations and facility leakage. Besides directly altering the operating environment, the background gas may indirectly affect the performance of immersed plasma probe diagnostics. This work focuses on computational modeling research conducted to evaluate the performance of a current-collecting Faraday probe. Initial findings from one dimensional analytical models of plasma sheaths are used as reference cases for subsequent modeling. A two dimensional, axisymmetric, hybrid electron fluid and Particle In Cell computational code is used for extensive simulation of the plasma flow around a representative Faraday probe geometry. The hybrid fluid PIC code is used to simulate a range of inflowing plasma conditions, from a simple ion beam consistent with one dimensional models to a multiple component plasma representative of a low-power Hall thruster plume. These simulations produce profiles of plasma properties and simulated current measurements at the probe surface. Interpretation of the simulation results leads to recommendations for probe design and experimental techniques. Significant contributions of this work include the development and use of two new non-neutral detailed electron fluid models and the recent incorporation of multi grid capabilities.Ph.D.Aerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60715/1/jboerner_1.pd