Innovative education and training in high power laser plasmas (PowerLaPs) for plasma physics, high power laser matter interactions and high energy density physics: experimental diagnostics and simulations

The second and final year of the Erasmus Plus programme "Innovative Education and Training in high power laser plasmas", otherwise known as PowerLaPs, is described. The PowerLaPs programme employs an innovative paradigm in that it is a multi-centre programme where teaching takes place in five separate institutes with a range of different aims and styles of delivery. The "in class" time is limited to four weeks a year, and the programme spans two years. PowerLaPs aims to train students from across Europe in theoretical, applied, and laboratory skills relevant to the pursuit of research in laser plasma interaction physics and inertial confinement fusion (ICF). Lectures are intermingled with laboratory sessions, and continuous assessment activities. The programme, which is led by workers from the Hellenic Mediterranean University, and supported by co-workers from Queens University Belfast, the University of Bordeaux, the Czech Technical University in Prague, Ecole Polytechnique, the University of Ioannina, the University of Salamanca, and the University of York, has just finished its second and final year. Six Learning Teaching Training (LTT) activities have been held, at the Queens University Belfast, the University of Bordeaux, the Czech Technical University, the University of Salamanca, and the Institute of Plasma Physics and Lasers (CPPL) of the Hellenic Mediterranean University. The last of these institute hosted two two-week long Intensive Programmes (IPs), whilst the activities at the other four universities were each five days in length. In addition to this a "Multiplier Event" was held at the University of Ioannina, which will be briefly described. In this second year the work has concentrated upon training in both experimental diagnostics and simulation techniques appropriate to the study of Plasma Physics, High Power Laser-Matter Interactions and High Energy Density Physics. The nature of the programme will be described in detail and some metrics relating to the activities carried out will be presented. In particular this paper will focus upon the overall assessment of the programme

Innovative Education and Training in high power laser plasmas (PowerLaPs) for plasma physics, high power laser-matter interactions and high energy density physics - Theory and experiments

The Erasmus Plus programme 'Innovative Education and Training in high power laser plasmas', otherwise known as PowerLaPs, is described. The PowerLaPs programme employs an innovative paradigm in that it is a multi-centre programme where teaching takes place in five separate institutes with a range of different aims and styles of delivery. The 'in class' time is limited to four weeks a year, and the programme spans two years. PowerLaPs aims to train students from across Europe in theoretical, applied and laboratory skills relevant to the pursuit of research in laser-plasma interaction physics and inertial confinement fusion (ICF). Lectures are intermingled with laboratory sessions and continuous assessment activities. The programme, which is led by workers from the Technological Educational Institute (TEI) of Crete, and supported by co-workers from the Queen's University Belfast, the University of Bordeaux, the Czech Technical University in Prague, Ecole Polytechnique, the University of Ioannina, the University of Salamanca and the University of York, has just completed its first year. Thus far three Learning Teaching Training (LTT) activities have been held, at the Queen's University Belfast, the University of Bordeaux and the Centre for Plasma Physics and Lasers (CPPL) of TEI Crete. The last of these was a two-week long Intensive Programme (IP), while the activities at the other two universities were each five days in length. Thus far work has concentrated upon training in both theoretical and experimental work in plasma physics, high power laser-matter interactions and high energy density physics. The nature of the programme will be described in detail and some metrics relating to the activities carried out to date will be presented

A Review of Finite Element Studies in String Musical Instruments

String instruments are complex mechanical vibrating systems, in terms of both structure and fluid–structure interaction. Here, a review study of the modeling and simulation of stringed musical instruments via the finite element method (FEM) is presented. The paper is focused on the methods capable of simulating (I) the soundboard behavior in bowed, plucked and hammered string musical instruments; (II) the assembled musical instrument box behavior in bowed and plucked instruments; (III) the fluid–structure interaction of assembled musical instruments; and (IV) the interaction of a musical instrument’s resonance box with the surrounding air. Due to the complexity and the high computational demands, a numerical model including all the parts and the full geometry of the instrument resonance box, the fluid–structure interaction and the interaction with the surrounding air has not yet been simulated

Downscaled Finite Element Modeling of Metal Targets for Surface Roughness Level under Pulsed Laser Irradiation

A three-dimensional, thermal-structural finite element model, originally developed for the study of laser–solid interactions and the generation and propagation of surface acoustic waves in the macroscopic level, was downscaled for the investigation of the surface roughness influence on pulsed laser–solid interactions. The dimensions of the computational domain were reduced to include the laser-heated area of interest. The initially flat surface was progressively downscaled to model the spatial roughness profile characteristics with increasing geometrical accuracy. Since we focused on the plastic and melting regimes, where structural changes occur in the submicrometer scale, the proposed downscaling approach allowed for their accurate positioning. Additionally, the multiscale simulation results were discussed in relation to experimental findings based on white light interferometry. The combination of this multiscale modeling approach with the experimental methodology presented in this study provides a multilevel scientific tool for an in-depth analysis of the influence of heat parameters on the surface roughness of solid materials and can be further extended to various laser–solid interaction applications

Μελέτη της δυναμικής συμπεριφοράς της ύλης υπό την επίδραση παλμών laser και εξωτερικών ισχυρών ρευμάτων

This thesis studies the physical phenomena that occur during the deposition of energy in matter and consists of two parts. In the first part, the nanosecond pulsed laser irradiation of thin metal films-substrate systems is investigated for thermoelastic, melting and ablation regimes, using the finite element method (FEM). The numerical simulations are compared and validated with experimental results, obtained by dynamic imaging interferometry and white-light tomographic interferometry. In the second part, the sequential stages of explosion of a Z-pinch copper wire from solid to plasma formation and its plasma expansion are investigated using multiphysics coupled numerical simulations, validated by experimental results obtained by interferometry, shadowgraphy, schlieren and diffraction imaging techniques. In more details, for the first part coupled thermal-structural, transient models based on FEM are developed to give a comprehensive spatiotemporal numerical solution of the physical phenomena occurring in laser matter interactions. Temperature dependent material properties of metal coatings deposited on glass substrates are used for the simulations. Initially, a 2D axisymmetric model is developed to study the generation and propagation of laser generated ultrasounds, for laser fluences below the melting threshold of metallic film-substrates. Afterwards, a 3D quarter-symmetric FEM model is developed and used for all regimes of interest. The developed FEM modeling provides a simultaneous analysis of the thermal and structural parameters, as defined by the solution of the heat conduction and wave propagation equations. The wave equation determines the displacements of the target imposed by the laser energy deposition, while the heat conduction equation predicts the temperature distribution induced by absorption of the laser pulse in the target. The 3D model computes the phase changes of matter by taking into account the latent heats of melting and vaporization, depending on the laser fluence. Ablation is simulated by the ‘killing’ of the elements that exceed the boiling temperature and are subsequently deactivated. The attenuation of the laser irradiance at the target surface due to plasma absorption is also taken into account. With regard to loading conditions, a heat source term is used, representing the laser energy absorbed by the sample and described by a Gaussian distribution in time and space. The Lagrangian mesh is locally adaptive, depending on the simulation needs. Moreover, the elastic and plastic behaviour of thin films is being investigated by taking into account stress-strains temperature-dependent curves of different materials until fracture into the 3D model. Furthermore, the finite element model was further extended to simulate the presence of surface and solid volume defects (gaps) and their subsequent influence on the generation and propagation of ultrasonic waves (surface acoustic waves). For the second part that concerns the matter behaviour dynamics, governed by the interaction with external strong currents, a 3D coupled mechanical/thermal FEM model, simulating the initial stages of explosion of a Z-pinch metallic copper wire, was initially developed. The model conveyed a simultaneous analysis of the thermal and structural parameters, as defined by the solution of the heat conduction and mechanical motion equations. The mechanical equation determines the displacements of the wire imposed by an alternative nanosecond pulsed current while the heat equation predicts the temperature distribution. The source term of the heat generation rate is the Joule heating term and is used as loading condition, while the pulsed imposed current is provided from experiments.In order to investigate the crucial physical phenomena that take place in the initial stages of wire explosion, a 3D electromagnetic-thermal-structural hydrodynamic FEM model is developed. Large volumetric deformation is considered by taking into account the material’s hydrodynamic behavior via equations of state (Gruneisen and multiphase tabular) and the elastoplastic behavior is also considered by a flow stress constitutive model (Johnson-Cook). A Lagrangian transient analysis has been carried out. The loading condition of the simulation is a nanosecond pulsed alternative current, which is provided from experiments. Τhe sequential stages of explosion of a Z-pinch copper wire from solid to plasma formation and its plasma expansion are investigated. The expansion of the exploded material is being investigated with a magnetohydrodynamic code that uses as initial condition the density distribution and radius instabilities from the above aforementioned 3D FEM model.Η παρούσα διατριβή μελετάει τα φυσικά φαινόμενα που λαμβάνουν χώρα κατά την εναπόθεση ενέργειας στην ύλη και αποτελείται από δύο μέρη στα οποία παρουσιάζονται δύο διαφορετικοί μηχανισμοί εναπόθεσης. Αναφορικά με τη δυναμική συμπεριφορά της ύλης όταν αλληλεπιδρά με παλμούς λέιζερ, η ακτινοβόληση λεπτών μεταλλικών φιλμ-υποστρωμάτων από παλμικό λέιζερ διάρκειας νανοδευτερολέπτων ερευνάται για την θερμοελαστική περιοχή, την περιοχή τήξης και την περιοχή φωτοαποδόμησης χρησιμοποιώντας τη μέθοδο των πεπερασμένων στοιχείων (ΜΠΣ). Οι αριθμητικές προσομοιώσεις συγκρίνονται και επικυρώνονται με τα πειραματικά αποτελέσματα που προκύπτουν από την τεχνική δυναμικής συμβολομετρίας απεικόνισης. Αναφορικά με τη δυναμική συμπεριφορά της ύλης όταν αλληλεπιδρά με εξωτερικά ισχυρά ρεύματα, τα διαδοχικά στάδια της έκρηξης ενός χάλκινου σύρματος Ζ-pinch από στερεά κατάσταση ως τη δημιουργία πλάσματος καθώς και η εξάπλωση του πλάσματος διερευνώνται αναπτύσσοντας multiphysics αριθμητικές προσομοιώσεις συζευγμένων πεδιών οι οποίες επικυρώνονται από πειραματικά αποτελέσματα που λαμβάνονται από οπτικές τεχνικές απεικόνισης (συμβολομετρία, σκιαγράφηση, Schlieren, απεικόνιση περίθλασης Fraunhofer).Αναλυτικότερα για το πρώτο μέρος, αναπτύσσεται συζευγμένο θερμικό-μηχανικό μοντέλο δυναμικά μεταβαλλόμενο στο χρόνο που βασίζεται στη ΜΠΣ με σκοπό να δώσει μια ολοκληρωμένη χωροχρονική αριθμητική επίλυση των φυσικών φαινομένων που συμβαίνουν κατά τη διάρκεια της αλληλεπίδραση ύλης με λέιζερ. Για τις προσομοιώσεις λαμβάνονται υπόψη θερμοκρασιακά εξαρτώμενες ιδιότητες υλικών, των μεταλλικών επικαλύψεων που εναποτίθενται σε γυάλινα υποστρώματα. Αρχικά, αναπτύχθηκε δισδιάστατο 2D αξονοσυμμετρικό μοντέλο με σκοπό να μελετηθεί η δημιουργία και διάδοση υπερήχων που παράγονται για πυκνότητες ροής ακτινοβολίας λέιζερ κάτω από το όριο τήξης του μεταλλικού φιλμ-υποστρώματος. Στη συνέχεια τρισδιάστατο 3D τέταρτο-συμμετρικό μοντέλο πεπερασμένων στοιχείων αναπτύχθηκε για όλες τις περιοχές ενδιαφέροντος (θερμοελαστική, τήξης, φωτοαποδόμησης/πλάσματος). Η αναπτυχθείσα μεθοδολογία παρέχει μια ταυτόχρονη ανάλυση των θερμικών και μηχανικών παραμέτρων όπως ορίζεται από την επίλυση των εξισώσεων της θερμικής αγωγιμότητας και κυματικής διάδοσης. Η κυματική εξίσωση καθορίζει τις μετατοπίσεις του στόχου που επιβάλλονται από την εναπόθεση ενέργειας λέιζερ, ενώ η εξίσωση αγωγής θερμότητας προβλέπει την κατανομή της θερμοκρασίας που προκαλείται από την απορρόφηση της ενέργειας του παλμικού λέιζερ στον στόχο. Το 3D μοντέλο υπολογίζει τις αλλαγές φάσης της ύλης, λαμβάνοντας υπόψη τις λανθάνουσες θερμότητες τήξης και εξάτμισης, ανάλογα με την πυκνότητα ροής της ακτινοβολίας λέιζερ. Η φωτοαποδόμηση προσομοιώνεται από την τεχνική «killing» των στοιχείων που υπερβαίνουν τη θερμοκρασία βρασμού και στη συνέχεια απενεργοποιούνται. Η εξασθένηση της ακτινοβολίας λέιζερ στην επιφάνεια-στόχο λόγω της απορρόφησης του πλάσματος λαμβάνεται επίσης υπόψη. Όσον αφορά τις συνθήκες φόρτισης, χρησιμοποιείται ως πηγή θερμότητας, η ενέργεια του λέιζερ που απορροφάται από το δείγμα και περιγράφεται από μια Γκαουσιανή κατανομή σε χρόνο και χώρο. Το Λαγκρανζιανό πλέγμα είναι τοπικά προσαρμοζόμενο ανάλογα με τις ανάγκες της προσομοίωσης. Επιπλέον, η ελαστική και πλαστική συμπεριφορά των λεπτών υμενίων ερευνάται λαμβάνοντας υπόψη, θερμοκρασιακά εξαρτώμενες καμπύλες τάσεων-παραμορφώσεων από διαφορετικά υλικά, μέχρι τη θραύση τους, στο 3D μοντέλο. Επιπλέον, το μοντέλο πεπερασμένων στοιχείων επεκτείνεται περαιτέρω, προκειμένου να προσομοιωθεί η παρουσία των επιφανειακών και των ελαττωμάτων του όγκου στο στερεό (κενά) και να μελετηθεί η μετέπειτα επιρροή τους στην παραγωγή και διάδοση των υπερηχητικών κυμάτων (επιφανειακών ακουστικών κυμάτων). Αναφορικά με τη δυναμική συμπεριφορά της ύλης όταν αλληλεπιδρά με εξωτερικά ισχυρά ρεύματα, αναπτύχθηκε αρχικά ένα 3-D συζευγμένο μηχανικό-θερμικό μοντέλο πεπερασμένων στοιχείων, που προσομοιώνει τα αρχικά στάδια της έκρηξης ενός Ζ-pinch μεταλλικού χάλκινου σύρματος. Το μοντέλο λαμβάνει υπόψη τις θερμοκρασιακά εξαρτώμενες θερμικές και μηχανικές παραμέτρους μέσω της επίλυσης των εξισώσεων της θερμικής αγωγιμότητας και της κλασσικής μηχανικής. Η μηχανική εξίσωση προσδιορίζει τις μετατοπίσεις του σύρματος που επιβάλλεται από εναλλασόμενο παλμικό ρεύμα διάρκειας νανοδευτερολέπτων, ενώ η εξίσωση αγωγής θερμότητας προβλέπει την κατανομή της θερμοκρασίας. Ο όρος που λειτουργεί ως πηγή του ρυθμού παραγωγής θερμότητας είναι ο όρος της θέρμανσης Joule και χρησιμοποιείται ως συνθήκη φόρτισης, ενώ το παλμικό εναλλασσόμενο ρεύμα παρέχεται από πειράματα. Για να αντιμετωπιστούν τα φυσικά φαινόμενα που λαμβάνουν χώρα στα αρχικά στάδια της έκρηξης του σύρματος πιο ρεαλιστικά αναπτύσσεται ένα 3D ηλεκτρομαγνητικό-θερμικό-μηχανικό υδροδυναμικό μοντέλο βασιζόμενο στη ΜΠΣ. Μεγάλες ογκομετρικές παραμορφώσεις εξετάζονται λαμβάνοντας υπόψη την υδροδυναμική συμπεριφορά του υλικού μέσω καταστατικών εξισώσεων (Gruneisen και καταστατική σε μορφή πίνακα λαμβάνοντας υπόψη τις αλλαγές φάσης), ενώ η ελαστοπλαστική συμπεριφορά του υλικού μελετάται θεωρώντας επίσης καταστατικό μοντέλο τάσεων (Johnson-Cook). Πραγματοποιείται Λαγκρανζιανή δυναμική ανάλυση στο χρόνο, ενώ η πηγή φόρτισης της προσομοίωσης είναι παλμικό εναλλασσόμενο ρεύμα, διάρκειας νανοδευτερολέπτων το οποίο παρέχεται από πειράματα. Μελετώνται τα διαδοχικά στάδια της έκρηξης χάλκινου σύρματος Ζ-pinch από στερεά κατάσταση ως τη δημιουργία πλάσματος. Στη συνέχεια η διαστολή και διάδοση του πλάσματος στο χωροχρόνο διερευνάται με μαγνητοϋδροδυναμικό κώδικα ο οποίος χρησιμοποιεί ως αρχικές συνθήκες την κατανομή πυκνότητας και τις αστάθειες στην τιμή της ακτίνας από το προαναφερθέν 3D μοντέλο με τη ΜΠΣ

FEM-BEM Vibroacoustic Simulations of Motion Driven Cymbal-Drumstick Interactions

The transient acoustic dynamics of a splash cymbal are investigated via the Finite Element Method-Boundary Element Method. Real three-dimensional motion data recorded from the interaction of drummer–drumstick–cymbal provide the initial and the loading conditions to the simulated interaction of the drumstick–cymbal Finite Element Models. Progressively intensified free strokes are used as loading conditions for both experiment and simulation. The velocity values of the moving drumstick in various drumming conditions are monitored, recorded, and analysed to provide input data into the time domain simulations. The synergy of motion capturing and numerical methods allows computing the sound generated by the combined interaction of the vibroacoustic behaviour of the cymbal with the motor-interaction of the performer. The proposed methodology promotes a novel perspective in musical instrument design, optimization, and manufacturing considering performance discrepancies intentionally introduced by performers

A Review of Finite Element Studies in String Musical Instruments

String instruments are complex mechanical vibrating systems, in terms of both structure and fluid–structure interaction. Here, a review study of the modeling and simulation of stringed musical instruments via the finite element method (FEM) is presented. The paper is focused on the methods capable of simulating (I) the soundboard behavior in bowed, plucked and hammered string musical instruments; (II) the assembled musical instrument box behavior in bowed and plucked instruments; (III) the fluid–structure interaction of assembled musical instruments; and (IV) the interaction of a musical instrument’s resonance box with the surrounding air. Due to the complexity and the high computational demands, a numerical model including all the parts and the full geometry of the instrument resonance box, the fluid–structure interaction and the interaction with the surrounding air has not yet been simulated

Characterization of an X-ray Source Generated by a Portable Low-Current X-Pinch

An X-pinch scheme of a low-current generator (45 kA, 50 ns rise time) is characterized as a potential efficient source of soft X-rays. The X-pinch target consists of wires of 5 μm in diameter—made from either tungsten (W) or gold (Au)-plated W—loaded at two angles of 55° and 98° between the crossed wires. Time-resolved soft X-ray emission measurements are performed to provide a secure correlation with the optical probing results. A reconstruction of the actual photodiode current profile procedure was adopted, capable of overcoming the limits of the slow rising and falling times due to the “slow” response of the diodes and the noise. The pure and Au-plated W deliver an average X-ray yield, which depends only on the angle of the crossed wires, and is measured to be ~50 mJ and ~70 mJ for the 98° and 55° crossed wire angles, respectively. An additional experimental setup was developed to characterize the X-pinch as a source of X-rays with energy higher than ~6 keV, via time-integrated measurements. The X-ray emission spectrum was found to have an upper limit at 13 keV for the Au-plated W configuration at 55°. The portable tabletop X-pinch proved to be ideal for use in X-ray radiography applications, such as the detection of interior defects in biological samples

Downscaled Finite Element Modeling of Metal Targets for Surface Roughness Level under Pulsed Laser Irradiation

A three-dimensional, thermal-structural finite element model, originally developed for the study of laser–solid interactions and the generation and propagation of surface acoustic waves in the macroscopic level, was downscaled for the investigation of the surface roughness influence on pulsed laser–solid interactions. The dimensions of the computational domain were reduced to include the laser-heated area of interest. The initially flat surface was progressively downscaled to model the spatial roughness profile characteristics with increasing geometrical accuracy. Since we focused on the plastic and melting regimes, where structural changes occur in the submicrometer scale, the proposed downscaling approach allowed for their accurate positioning. Additionally, the multiscale simulation results were discussed in relation to experimental findings based on white light interferometry. The combination of this multiscale modeling approach with the experimental methodology presented in this study provides a multilevel scientific tool for an in-depth analysis of the influence of heat parameters on the surface roughness of solid materials and can be further extended to various laser–solid interaction applications

Hydrodynamic computational modelling and simulations of collisional shock waves in gas jet targets

We study the optimization of collisionless shock acceleration of ions based on hydrodynamic modelling and simulations of collisional shock waves in gaseous targets. The models correspond to the specifications required for experiments with the laser at the Accelerator Test Facility at Brookhaven National Laboratory and the Vulcan Petawatt system at Rutherford Appleton Laboratory. In both cases, a laser prepulse is simulated to interact with hydrogen gas jet targets. It is demonstrated that by controlling the pulse energy, the deposition position and the backing pressure, a blast wave suitable for generating nearly monoenergetic ion beams can be formed. Depending on the energy absorbed and the deposition position, an optimal temporal window can be determined for the acceleration considering both the necessary overdense state of plasma and the required short scale lengths for monoenergetic ion beam production