15,023 research outputs found

    The DCU laser ion source

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    Laser ion sources are used to generate and deliver highly charged ions of various masses and energies. We present details on the design and basic parameters of the DCU laser ion source (LIS). The theoretical aspects of a high voltage (HV) linear LIS are presented and the main issues surrounding laser-plasma formation, ion extraction and modeling of beam transport in relation to the operation of a LIS are detailed. A range of laser power densities (I ∼ 108–1011 W cm−2) and fluences (F = 0.1–3.9 kJ cm−2) from a Q-switched ruby laser (full-width half-maximum pulse duration ∼ 35 ns, λ = 694 nm) were used to generate a copper plasma. In “basic operating mode,” laser generated plasma ions are electrostatically accelerated using a dc HV bias (5–18 kV). A traditional einzel electrostatic lens system is utilized to transport and collimate the extracted ion beam for detection via a Faraday cup. Peak currents of up to I ∼ 600 μA for Cu+ to Cu3+ ions were recorded. The maximum collected charge reached 94 pC (Cu2+). Hydrodynamic simulations and ion probe diagnostics were used to study the plasma plume within the extraction gap. The system measured performance and electrodynamic simulations indicated that the use of a short field-free (L = 48 mm) region results in rapid expansion of the injected ion beam in the drift tube. This severely limits the efficiency of the electrostatic lens system and consequently the sources performance. Simulations of ion beam dynamics in a “continuous einzel array” were performed and experimentally verified to counter the strong space-charge force present in the ion beam which results from plasma extraction close to the target surface. Ion beam acceleration and injection thus occur at “high pressure.” In “enhanced operating mode,” peak currents of 3.26 mA (Cu2+) were recorded. The collected currents of more highly charged ions (Cu4+–Cu6+) increased considerably in this mode of operation

    Overcoming Challenges in Predictive Modeling of Laser-Plasma Interaction Scenarios. The Sinuous Route from Advanced Machine Learning to Deep Learning

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    The interaction of ultrashort and intense laser pulses with solid targets and dense plasmas is a rapidly developing area of physics, this being mostly due to the significant advancements in laser technology. There is, thus, a growing interest in diagnosing as accurately as possible the numerous phenomena related to the absorption and reflection of laser radiation. At the same time, envisaged experiments are in high demand of increased accuracy simulation software. As laser-plasma interaction modelings are experiencing a transition from computationally-intensive to data-intensive problems, traditional codes employed so far are starting to show their limitations. It is in this context that predictive modelings of laser-plasma interaction experiments are bound to reshape the definition of simulation software. This chapter focuses an entire class of predictive systems incorporating big data, advanced machine learning algorithms and deep learning, with improved accuracy and speed. Making use of terabytes of already available information (literature as well as simulation and experimental data) these systems enable the discovery and understanding of various physical phenomena occurring during interaction, hence allowing researchers to set up controlled experiments at optimal parameters. A comparative discussion in terms of challenges, advantages, bottlenecks, performances and suitability of laser-plasma interaction predictive systems is ultimately provided

    Particle-In-Cell Simulation using Asynchronous Tasking

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    Recently, task-based programming models have emerged as a prominent alternative among shared-memory parallel programming paradigms. Inherently asynchronous, these models provide native support for dynamic load balancing and incorporate data flow concepts to selectively synchronize the tasks. However, tasking models are yet to be widely adopted by the HPC community and their effective advantages when applied to non-trivial, real-world HPC applications are still not well comprehended. In this paper, we study the parallelization of a production electromagnetic particle-in-cell (EM-PIC) code for kinetic plasma simulations exploring different strategies using asynchronous task-based models. Our fully asynchronous implementation not only significantly outperforms a conventional, synchronous approach but also achieves near perfect scaling for 48 cores.Comment: To be published on the 27th European Conference on Parallel and Distributed Computing (Euro-Par 2021

    High Field Plasmonics

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    Il manoscritto riguarda lo studio di effetti di plasmonica ad alti campi, ossia nel contesto dell'interazione laser-plasma ad altissima intensità (I > 10^18 W/cm^2). Si intende per "plasmonica" lo studio di plasmoni di superficie, che consistono in modi elettromagnetici all'interfaccia fra un metallo e un mezzo dielettrico. I plasmoni di superficie vengono normalmente eccitati con impulsi laser a bassa intensità.Questo regime è ben conosciuto dal punto di vista teorico e lo studio di schemi che coinvolgono effetti di plasmonica è un campo di ricerca molto vitale, motivato dalle possibili interessanti applicazioni (ad esempio per biosensori o microchip). Al contrario, non è, ad oggi, disponibile un modello teorico completo per plasmoni di superficie in regimi di altissima intensità, dove forti effetti non lineari e relativistici possono giocare un ruolo rilevante. Anche dal punto di vista sperimentale e numerico effetti di plasmonica in questo regime di interazione sono stati solo marginalmente esplorati. I risultati contenuti nella tesi sono principalmente frutto di investigazioni numeriche con codici Particle-In-Cell e risultati sperimentali relativi a effetti di plasmonica in regime di alti campi. Sono stati studiati diversi scenari, elencati di seguito. Si è studiata l'interazione di intensi impulsi laser (I~5x10^19 W/cm^2) con bersagli la cui superficie consisteva in un reticolo microstrutturato (attività sperimentale svolta presso il centro di ricerca CEA-Saclay, Gif-sur-Yvette, Francia). Si è osservata l'accelerazione di pacchetti collimati di elettroni lungo la superficie dei reticoli se irraggiati ad angoli vicini a quello atteso per l'eccitazione di un plasmone di superficie. E' stato misurato lo spettro energetico degli elettroni emessi, osservando una distribuzione piccata a 5-8 MeV e con una coda estesa fino a oltre 20 MeV. Sono state misurate cariche totali fino a oltre 100 pC. Tali caratteristiche rendono la sorgente potenzialmente di interesse per alcune applicazioni (diffrazione di elettroni ultra-veloce, produzione di fotoneutroni). Simulazioni numeriche Particle-In-Cell hanno mostrato ottimo accordo con i risultati sperimentali. Si fornisce anche un semplice modello teorico per chiarire il ruolo dei plasmoni di superficie nell'accelerazione degli elettroni Si è studiata l'interazione di impulsi laser con intensità superiori a 10^20 W/cm^2 con bersagli solidi sottili accoppiati ad una schiuma di carbonio con densità media tale da ottenere un plasma alla densità critica se completamente ionizzata (attività sperimentale svolta presso il centro di ricerca GIST, Gwangju, Corea del Sud). L'interazione laser-plasma in tali regimi consente un efficiente accoppiamento dell'impulso laser con plasmoni di volume, incrementando l'efficienza dell'assorbimento di energia da parte del bersaglio. L'attività si inquadra nel contesto dell'accelerazione di ioni con plasmi prodotti da laser (con potenziali future applicazioni in adroterapia o trattamento e diagnostica di materiali con fasci di ioni). Durante due campagne sperimentali si è osservato che la presenza di un sottile strato di schiuma consente di incrementare le energie massime degli ioni accelerati rispetto a bersagli semplici. Simulazioni numeriche con codici Particle-In-Cell hanno permesso di chiarire il ruolo giocato dalle microstrutture della schiuma. Si è studiato il ruolo di effetti plasmonici nell'instabilità di Rayleigh-Taylor “laser-driven”, che può svilupparsi in scenari di “Radiation Pressure Acceleration”, dove sottili bersagli solidi vengono accelerati direttamente dalla pressione di radiazione di un impulso laser ultraintenso. Utilizzando un semplice modello teorico si mostra che le modulazioni autoconsistenti della presione di radiazione dovute alla deformazione sinusoidale della superficie influenzano significativamente lo spettro dell'instabilità, dipendentemente dalla polarizzazione dell'impulso. L'evoluzione nonlineare è studiata con simulazioni Particle-In-Cell, che mostrano la formazione di strutture a rete. Si è studiato con simulazioni numeriche il ruolo dell'eccitazione di plasmoni di superficie nell'emissione di armoniche di ordine elevato da reticoli solidi irraggiati con impulsi laser ultra-intensi. All'eccitazione di un plasmone di superficie corrisponde un aumento dei campi elettromagnetici alla superficie del bersaglio, che incrementa l'efficienza di generazione di armoniche. Nelle simulazioni una migliore efficienza di emissione di armoiniche è stata osservata per bersagli irraggiati ad angoli vicini a quello atteso per l'eccitazione di un plasmone di superficie. I risultati presentati in quest'ultima sezione sono preliminari e saranno oggetto di studi ulteriori. The manuscript concerns the study of plasmonic effects at high fields, that is in the framework of laser-plasma interaction at ultra-high intensities (I > 10^18 W/cm^2). “Plasmonics” is the study of surface plasmons, which are electromagnetic modes at the interface between a metal and a dielectric medium. Surface plasmons are normally excited with low intensity laser pulses. This regime is well known from the theoretical point of view and the study of plasmonic schemes is a vibrant research field, motivated by the interesting possible applications (e.g. biosensors or plasmonic chips). On the other hand, a complete theoretical model is still lacking for surface plasmons in the high intensity regime, where strong non-linear and relativistic effects might play a relevant role. Also the numerical and experimental investigation of plasmonics in this regime has been limited up to now. This thesis presents mainly numerical (Particle-In-Cell simulations) and experimental results related to the study of plasmonic effects at high fields. A few scenarios were studied. They are listed hereunder. The interaction of intense laser pulses (I~5x10^19 W/cm^2) with microstructured grating targets was studied experimentally (this activity was carried out at the research center CEA-Saclay, Gif-sur-Yvette, France). We observed thee acceleration of collimated electron bunches along the surface of grating target when irradiated at angles close to that expected for the excitation of a surface plasmon. We measured the energy spectrum of the emitted electrons, observing a distribution peaked at 5-8 MeV with a tail extending up to more than 20 MeV. Total charges up to more than 100 pC were measured. These characteristics make the source interesting for some applications (like ultra-fast electron diffraction or photo-neutron generation). Particle-In-Cell numerical simulations proved to be in very good agreement with the experimental results. A theoretical model is provided to clarify the role played by surface plasmons in the electron acceleration process. The interaction of intense laser pulses (I> 10^20 W/cm^2) with solid targets coupled with a carbon foam was studied. The average density of the carbon foam was selected in order to obtain a plasma at the critical density if fully ionized (the experimental activity was carried out at the GIST research center, Gwangju, South Korea). Laser-plasma interaction in this regime allows for an efficient coupling of the laser pulse with bulk plasmons, enhanced the efficiency of the energy absorption by the target. The activity was carried out in the framework of ion acceleration with laser-produced plasmas (with potential future applications in hadron-therapy or diagnostic and treatment of materials with ion beams). During the two experimental campaigns we observed that targets coated with a thin foam allowed to obtained higher ion energies with respect to simple targets. Numerical simulations with Particle-In-Cell codes helped to clarify the role of the micro-structuring of the foam. We studied the role of plasmonic effects in laser-driven Rayleigh-Taylor instability, which may develop in Radiation Pressure Acceleration scenarios, where thin solid foils are directly accelerated by the radiation pressure of an ultra-intense laser pulse. Using a simple model it is shown that the self-consistent modulation of the radiation pressure caused by a sinusoidal rippling affects substantially the wavevector spectrum of the instability depending on the laser polarization. The nonlinear evolution is investigated by three dimensional simulations which show the formation of net-like structures. The role of surface plasmons in high order harmonic emission from solid grating targets irradiated with ultra-intense laser pulses was studied with numerical simulations. The excitation of a surface plasmon is associated with an enhancement of the electromagnetic field close to the target surface, which should increase the efficiency of harmonic generation. In the simulations, a higher emission efficiency for high-order harmonics was observed for targets irradiated at angles close to that expected for surface plasmon excitation. The results presented in this part are still preliminary. The topic will be addressed in future works
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