Ultrahigh-charge electron beams from laser-irradiated solid surface

Compact acceleration of a tightly collimated relativistic electron beam with high charge from a laser–plasma interaction has many unique applications. However, currently the well-known schemes, including laser wakefield acceleration from gases and vacuum laser acceleration from solids, often produce electron beams either with low charge or with large divergence angles. In this work, we report the generation of highly collimated electron beams with a divergence angle of a few degrees, nonthermal spectra peaked at the megaelectronvolt level, and extremely high charge (∼100 nC) via a powerful subpicosecond laser pulse interacting with a solid target in grazing incidence. Particle-in-cell simulations illustrate a direct laser acceleration scenario, in which the self-filamentation is triggered in a large-scale near–critical-density plasma and electron bunches are accelerated periodically and collimated by the ultraintense electromagnetic field. The energy density of such electron beams in high-Z materials reaches to ∼1012 J/m3, making it a promising tool to drive warm or even hot dense matter states

/Magnetized jets in Laboratory Αstrophysics/ /Μαγνητισμένοι πίδακες στην Εργαστηριακή Αστροφυσική/

Σύντομη Περίληψη H ερευνητική συνεισφορά της παρούσης διατριβής αφορά τρείς επιμέρους εργασίες. Πρώτον, τη μαθηματική και γεωμετρική μοντελοποίηση των αρχικών συνθηκών που αφορούν την πυκνότητα ύλης, τη θερμική πίεση και του αντίστοιχου μαγνητικού πεδίου στο μαγνητοϋδροδυναμικό κώδικα για τη βέλτιστη χωροχρονική εξέλιξη και συμπεριφορά του πλάσματος. Δεύτερον, τη συνθετική εργασία φυσικής μοντελοποίησης όλων των φυσικών παραμέτρων που επηρεάζουν τη δυναμική εξέλιξη του πλάσματος και του μαγνητικού πεδίου, όπως, η ηλεκτρική και θερμική αγωγιμότητα, το ιξώδες, ο λογάριθμος Coulomb, οι καταστατικές εξισώσεις, ο βαθμός ιονισμού, η μεταφορά και η απώλεια της ακτινοβολίας του πλάσματος. Η δεύτερη αυτή εργασία, προσφέρει λύση στη φυσική μοντελοποίηση που αφορά όλες τις περιοχές πυκνοτήτων και θερμοκρασιών του πλάσματος. Ο τρίτος και κύριος στόχος εστιάζει στην αριθμητική διερεύνηση και μοντελοποίηση της δυναμικής εξέλιξης του πλάσματος και του παραγόμενου μαγνητικού πεδίου παραγόμενο από οπτοηλεκτρονικές διατάξεις Ζ-pinch και Χ-pinch, μονών, διπλών και πολλών μεταλλικών καλωδίων υπό συνθήκες μεγάλης πυκνότητας κι θερμοκρασίας, για την εξαγωγή συμπερασμάτων του τρόπου δημιουργίας και της δυναμικής εξέλιξης συγκεκριμένων ασταθειών, της κεντρικής περιοχής του X-pinch αλλά κυρίως για τον τρόπο σχηματισμού των πιδάκων πλάσματος που προκύπτουν από διατάξεις Χ-pinch. Για το λόγο αυτό, ένας αρχικά αστροφυσικός κώδικας, ο PLUTO, έχει διαμορφωθεί και μοντελοποιηθεί με επιτυχία, για την μελέτη των προαναφερθέντων διατάξεων πλάσματος. Ένας δεύτερος κώδικας, ο GORGON, χρησιμοποιείται ως εργαλείο σύγκρισης και αξιολόγησης σε μοντέλα διατάξεων X-pinch. Τα συμπεράσματα που εξάγονται δίνουν πολύτιμες πληροφορίες για τον τρόπο σχηματισμού των πιδάκων πλάσματος και των μαγνητοϋδροδυναμικών ασταθειών. Καταληκτικά παρουσιάζεται η πιθανή συσχέτιση των εργαστηριακών πιδάκων πλάσματος με τους αντίστοιχους πίδακες που προκύπτουν από νεαρά αστροφυσικά αντικείμενα (YSO's) και αντικειμένων Herbig Haro (HH). Συγκεκριμένα αυτή η εργασία συνεισφέρει στην εκτίμηση των παραγόντων κλίμακας και όλων των σχετικών αδιάστατων αριθμών και παραμέτρων που αφορούν τη συσχέτιση των αστροφυσικών και εργαστηριακών πιδάκων πλάσματος που παράγονται από μία διάταξη X-pinch, δύο καλωδίων Βολφραμίου, χαμηλού παλμικού ρεύματος.Abstract The main goal of this work is the numerical investigation of the plasma dynamic evolution of Z-pinch and X-pinch devises. A more thorough investigation of the jet formation mechanisms and dynamic evolution of a two-wire tungsten X-pinch formation along with experimental validation is presented. Additionally a possible correlation to astrophysical jets of YSO and HH objects is discussed. Specifically this study contributes to the estimation of the scalability factors and all the relevant numbers and parameters for a low current X-pinch tungsten plasma configuration. In order to achieve robust, flexible and realistic computational simulation models, a variety of different numerical and physical schemes is needed. An extra study for this dissertation, is the synthesizing process of extracting physical formulas for all the dissipative terms for all density and temperature regimes. The synthesis of different physical formulas for piece-wise functions or correction factors to already known formulas or combination formulas, provide flexibility for any MHD numerical scheme and also, for any Z or X-pinch configuration at any density, thermal pressure and plasma temperature regime. The graphic presentation of these formulas, as a comparison evaluation tool with experimental/semi-empirical or other studies data, is provided and estimated by the author. The mathematical transformation of any Local plasma configuration to the Global coordinate system is a supplementary and necessary study, as it provides the appropriate spatial profiles for all the physical variables and assisting for the best spatial resolution that can be computationally executable. Moreover, the mathematical transformations through rotation and displacement matrices are presented correlating the mass density, thermal pressure and magnetic field Local components to the Global ones. The latter is important due to the novelty of the initial condition spatial implementation of the magnetic field. Examples of Z and X-pinch configurations are discussed according to these transformations. Information over the physical, mathematical and numerical implementation procedure of the new modifications to the code and the significance they have along the simulation run, is presented and discussed. The choice of the appropriate Riemann solver, the radiation transport module, the appropriate boundary conditions for the computational box, the magnetic field initial topology and the correct updating along the simulation run are, among others, subjects of this thesis analysis! Two specific simulation applications of these transformations are preliminary tested and evaluated, through the experimental and other simulation studies bibliographic data. The first is a "cold" start X-pinch configuration using two unmerged metallic tungsten wires, starting just before the ablation phase and the merging in a denser cross-point area. The focus of this simulation is at the cross-point area. The study ends when the merging is succeeded and the formation of the axial jets is initiated. Similarities with astrophysical jet mechanisms, are discussed. The second is a four tungsten, thin wire, low current Z-pinch array configuration study till the stagnation phase. Both studies present novel and very promising results for future work. The computational results study presents the dynamic behavior of 1/4 spatial model of a single tungsten Z-pinch wire evaluating the contribution of the radiation transport module to the appropriate plasma evolution. A 1/16 spatial model, of an X-pinch tungsten plasma configuration for a wide wire angle, is subsequently studied. The spatiotemporal plasma jet formation and evolution is presented and discussed. The simulated areal mass density is compared with the experimentally measured dense opaque region to enlighten the dense plasma evolution. In addition, the measured experimental areal electron density is compared to the simulation results. The main jet formation mechanisms are analyzed and discussed in relation to the influence of the J×B force and the mass momentum density. The physical and numerical modeling differences of GORGON and PLUTO and their influence on the simulation results are demonstrated and analyzed, for a sharp two-wire tungsten load X-pinch geometry. Six different computational MHD schemes are presented at a specific spatial moment only for PLUTO. The model comparison provides valuable information on the improvement of flexible and efficient X-pinch models according to the experimental data and offer crucial insights on the mechanisms of plasma evolution and jet formation. The scalability and jet astrophysical relevancy estimation, for the wide wire angle X-pinch pulsed power configuration, is attempted. All relevant dimensionless numbers are calculated for the X-pinch plasma and compared to the ones of other laboratory astrophysics experiments and to the ones of YSOs, HHs and other astrophysical objects

Dynamic fragmentation of graphite under laser-driven shocks: Identification of four damage regimes

This study presents the results of a large experimental campaign conducted on the Luli2000 laser facility. Thin targets of a commercial grade of porous graphite were submitted to high-power laser-driven shocks leading to their fragmentation. Many diagnostics were used such as high-speed time- and space-resolved imaging systems (shadowgraphy and photography), laser velocimetry (PDV and VISAR), debris collection and post-mortem X-ray tomography. They provided the loading levels into the targets, the spall strength of the material, the shape and size of debris and the localization of the subsurface cracks. The crossed data reduction of all the records showed their reliability and allowed to get a better insight into the damage phenomena at play in graphite. Thereby, four damage regimes, ranked according to their severity and loading level, were identified. It confirms that laser shocks are very complementary to classical impact tests (plates and spheres) since they ally two-dimensional loadings to the possibility of using both, in-situ and post-mortem diagnostics. Finally, the campaign shall be able to provide large and consistent data to develop and adjust reliable models for shock wave propagation and damage into porous graphite

Wechselwirkung intensiver Laserpulse mit dichten Plasmen - Ultrakurzzeitkinetik und Diagnostik

Für die Auflösung der Ultrakurzzeitkinetik der Licht-Materie-Wechselwirkung wurden in dieser Arbeit Particle-In-Cell (PIC) und strahlungs-hydrodynamische Simulationen für Wasserstoff, Helium und Kohlenstoff durchgeführt, aus deren Resultaten für die inhomogenen Dichte- und Temperaturprofile Spektren der Thomsonstreuung von Röntgenstrahlung berechnet wurden. Damit konnten in dieser Arbeit vollständige Pump-Probe-Experimente, mit einem optischen Laser zur Erzeugung von dichten Plasmen (Pump) und einem Freie-Elektronen Laser (FEL) zur Diagnostik dieser Plasmen, simuliert werden

High repetition rate laser driven proton source and a new method of enhancing acceleration

In the past few decades and with the increasing availability of high-intensity laser systems, laser ion acceleration has evolved into a mature and promising source for experiments with energetic ions. In particular, the latest laser-driven proton energy has reached nearly 100 MeV. Most applications require a stable ion source with high repetition frequency. The methods and strategies for realizing such repetitive laser-ion sources vary dramatically, in particular with respect to the employed target technology. In view of the interest of our research group on solid thin targets, the first focus of this PhD work was on an automated target positioning system that is employed in various research topics. A pilot study with one thousand targets was conducted with the nano-Foil Target Positioning System at the ATLAS 300 at Laboratory for Extreme Photonics (LEX Photonics), which was able to deliver laser pulses with a pulse energy of up to 6J and a pulse duration 25fs. Through real-time monitoring of various parameters of the laser pulses and targets, we have evaluated the stability of the proton source at a repetition rate of 0.5 Hz. During this study, we artificially varied parameters that were controllable and studied their impact on the proton yield. While scientifically interesting, the results did not clearly reveal the basis that would allow for stabilizing the proton source. It is likely that spatial-temporal contrast fluctuations contribute, which cannot yet be monitored on shot-to-shot. The request for repetition rate poses challenges to optimization strategies that rely on targets more complicated than plain foils. Among currently favored methods, which are reviewed in this thesis, are mass-limited-targets (MLT). Their lateral size is comparable to the laser focus diameter and therefore the energized electrons remain confined to a microscopically small volume such that acceleration fields are increased, as is the ion energy. However, the rapid positioning of MLTs is experimentally challenging. In order to find alternatives, we tested the generation of transient micro-targets by manipulating an initially plain foil with a Laguerre-Gaussian (LG) pre-pulse. This pre-pulse was introduced in the frontend of the CLAPA 200 TW laser at Peking University and passed through a spiral phase plate (SPP) before sending it back with a 1.7 ns advance to the main laser pulse into the laser chain. In the far-field, i.e. in the focus on the target, the LG pre-pulse results in a donut-shaped intensity distribution and initiates a ring-shaped plasma that is left to expand. The main laser pulse focuses on the center of this ring. The experimental results revealed a doubling of proton energy under the right pre-pulse intensity conditions. The evolution of the ring-pre-plasma expansion is modeled and the interaction between the main pulse with the transiently micro-plasma is studied by particle-in-cell simulations. The simulation results can recover the experimental observation, in particular, the proton energy increase in the relevant parameter range. Our understanding is in line with expectations that energetic electrons remain concentrated around the central part of the quasi-isolated micro-target, even though the target is not fully isolated but surrounded by a low-density plasma by the time of laser-plasma interaction at peak intensity