• laser
• plasma
• spectroscopy
• x-ray

X-ray spectroscopy plays a key role in the domain of laser-generated plasmas since it is a crucial tool for the investigation of their atomic properties and their hydrodynamic evolution. Its role is even more important when the evolution of the plasma is massively determined by radiative transfer phenomena.
This is the case for two distant domains such as stellar physics and inertial confinement fusion (ICF). Both these applications require a detailed, reliable modeling of the X-ray energetics of the plasma. These questions can be addressed from a theoretical point of view with the use of atomic and hydrodynamic models that are implemented to simulation codes. However, these codes have to be verified before being considered as reliable. An exper- imental verification is possible with the use of high-power lasers: a laser pulse is focused onto a solid target to generate a plasma.
The spectral emission and the hydrodynamic evolution of this plasma are studied through the use of different diagnostics. The experimental results are then compared to the output of the dif- ferent simulation codes (atomic kinetic and hydrodynamic ones).
The goal of this thesis is to generate and characterize a plasma of a well known element, i.e an element whose X-ray emission lines are well known and not so difficult to identify (C or Al in our case). In this first stage of a wider campaign, we searched for an optimal configuration of the target structure and the drive laser parameters in order to obtain an homogeneous plasma: this allows to study the plasma properties as a function of the ther- modynamic parameters (a single T and a single rho) in order to study the emission. This configuration, validated by our experiment, will be used in a later stage for the study of elements which are relevant as ablators for the ICF (such as Br).
The experimental campaign on which this thesis is based took place at the ELFIE laser facility of the LULI laboratory (Palaiseau, France). A picosecond laser pulse at moderate intensity (I = 10^15 - 10^16 W/cm2) was focused onto a structured target com- posed by a Si3N4 layer and by a C or Al layer. The 3D electron density of the plasma was obtained through Nomarski interfer- ometry via an Abel transform. The emission spectrum was determined by a reflection grating spectrometer. A pinhole camera was used as lens-free X-ray optical tool to study the plasma expansion.
The experimental results concerning hydrodynamic and spectral properties of the laser-generated plasma have been compared to the output of a Lagrangian hydrodynamic code and of an atomic kinetic software. The agreement between experimental and simulated results confirms the reliability of both the setup and the codes. The main experimental goal, i.e. to obtain an homogeneous plasma, has been realized.
This thesis is organized as follows:
- Chapter 1 introduces the scientific context and gives some generalities about nuclear fusion, plasmas and laser-matter interaction, with particular attention to radiative emission and X/XUV spectroscopy.
- Chapter 2 describes the experimental setup (laser, targets, etc.) and explains the functioning of the diagnostics.
- In Chapter 3 the data analysis strategy is pointed out, considering in particular the coupling between the different digital tools and the experimental data.
- Chapter 4 will be devoted to the overview of the results and their implications. Improvements of setup and digital tools will be discussed as well as long term goals.