28 Janvier – Thesis defense - Emilian-Dragos Tatomirescu

11 h Room F206 - University of Timisoara (Romania)

Laser-plasma acceleration at ultra high intensity - numerical modeling.

Due to the present enhancements in maximum attainable laser intensity through high power short pulses (femtosecond range) an increased focus has grown in developing potential laser plasma sources with applications in proton radiography, fast ignition for inertial confinement fusion, hadrontherapy, radioisotope production and laboratory astrophysics. Because of these advances in technology, numerous new laser facilities have been created, allowing the popularization of particle acceleration from laser-plasma interaction as a research field. In recent years, the progress achieved in the field was very rapid, showing that the understanding of the physics that stands behind the interaction of a high intensity laser pulse with plasma has been greatly improved.
The problems that are sought to be solved within this research field require the use of plasmas. In this kind of environment, one can state that the dominant processes are of a kinetic and collective nature, with an abundance of non-linear phenomena. This makes the use of analytical or fluid models very difficult, therefore, Particle-In-Cell codes have been crucial in the modelling of such parameters and also in experimental work optimization.
The typical Particle-In-Cell codes in use for laser-plasma interaction numerical modelling solve the electromagnetic fields using Maxwell's equations coupled with the Vlasov equations. The method uses macroparticles (a group of real particles that can be depicted as a reduced portion of the phase space) to solve Vlasov equations by the method of characteristics for each species of particle present in the case under study, conserving the charge and the mass of the species that is under study.
The key factor in the development of all the laser-based applications previously mentioned is the need for collimated ion beams that exhibit an adjustable energy bandwidth. The high repetition requirement can be overcome with the new laser facilities either available such as the BELLA laser, or under construction, such as the ELI-NP facility.

The thesis contains a short introduction followed by 6 chapters described in the following:
Chapter 1, named "Laser plasma acceleration mechanisms", is dedicated to the description of the physical processes that occur during the interaction of a strong electromagnetic field with a charged plasma. We talk about the main acceleration mechanisms as reported in the specialty literature. Their occurrence depends on the area of the plasma target, respectively on the parameters of both the target and the laser pulse (i.e. density, intensity, shape). This chapter is structured in three sub-chapters. In the first two we deal with the most common acceleration schemes, while in the third we discuss the mechanisms that are less common or require special parameters. In the first sub-chapter we describe the Target Normal Sheath Acceleration and its electron and ion components. The second sub-chapter deals with characteristics of the Radiation Pressure Acceleration. The last sub-chapter presents short descriptions for the related acceleration schemes and the mechanisms that are particular cases of the ones discussed in the first two sub-chapters.

Chapter 2, named "Basic introduction to the Particle-In-Cell method for laser-plasma interaction numerical modeling", is envisioned to present the basis on how the Particle-In-Cell method is being utilized in the numerical modeling of laser plasma interaction. The first part starts with two sub-chapters that differentiate and describe the classes of plasma interacting systems. The chapter then continues with a description of the Particle-Particle approach, and reasons on why it is not suited for the kind of phenomena studied in this work. In the third sub-chapter, we begin by describing the Particle-In-Cell method, its basic iteration cycle, how the integration of the equations involved in this method can be performed and the interpolations required by this approach. The chapter ends with a few practical considerations that are being handled when running Particle-In-Cell simulations.

Chapter 3, named "Laser ion acceleration from micro-structured solid targets", is structured into three sub-chapters and deals with the first of the studies performed during the last three years. We start with a description of the simulation parameters. The study has been focused on the particularities of several types of density profiles with a proton-rich microdot: flat, curved and cone target with a concave tip. We study the advantages and disadvantages of each of these target augmentations (curvature, microdot and cone structure), and determine if a composite target featuring all three attributes has the potential to produce higher quality proton and ion beams. By comparing the cases of flat versus curved targets, we intend to determine what effect has the addition of a slight curvature to the target, in particular how it affects beam collimation. In this chapter we also want to determine the effects of a pulse focusing structure on the laser pulse electric field. For this purpose, we added into our simulation a conical structure before the main target to investigate its effects. The second sub-chapter deals with the results of our simulations and their interpretation, while in the last part we draw the conclusions regarding this study.

Chapter 4, named "Target curvature influence on particle beam characteristics resulted from laser ion acceleration with microstructured enhanced targets at ultra-high intensity", continues the studies performed in Chapter 3 on structured targets. The first subchapter presents the simulation parameters, which are intended to determine how effective the manipulation of target curvature is in determining the beam collimation and maximum ion energies. The chapter continues with the presentation of the obtained results and their interpretation, culminating with the conclusions drawn from this study.

The work done in Chapter 5, named "Laser ion acceleration and high energy radiation generation from near-critical gas jets", focuses on the effects resulted from the manipulation of the density of a gaseous Xe target in interaction with a high intensity ultra-short laser pulse in order to prepare first ultra-high intensity experiments on facilities like BELLA, CETAL, APOLLON and the ELI high power lasers. Similar to the structure of the last chapter, it starts with presenting the study parameters. We want to ascertain if the ion energy spectrum features can be controlled by changing the peak density of the gas jet when using ultra high intensity laser pulses, in correlation with the results for lower intensity pulses reported in the literature. In this study we have the pulse interacting, at normal incidence, with the target composed of Xenon ions and electrons with an increasingly higher maximum density. The gas target has a cos^2 density profile with a 50 microns FWHM in the x direction and uniform in the y direction. The last two sub-chapters are dedicated to interpreting the results obtained through numerical modeling and the drawing of conclusions on the study.
The last chapter of this work, Chapter 6, contains the final conclusions that were drawn from the studies presented within the thesis.
The thesis concludes with three sections: References, Acknowledgements and Annexes. The References section contains the relevant papers, books and various sources that made it possible to complete the studies performed throughout this work. The funding that permitted me to accomplish my studies is presented in the Acknowledgements section. The last section, Annexes, lists some of the contributions that have been added to the Particle-In-Cell code PICLS used during this thesis.

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