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Particle acceleration from laser-plasma interactions has been a vibrant area of research since the discovery of electron and proton beams emitted from high intensity laser-plasma interactions in the 1980s. Since then, a large body of work has developed in pursuit of understanding and controlling the mechanisms that generate these particle beams, as well as the beams themselves. In particular, proton beams have a rich set of applications in the fields of medicine, fusion energy, and fundamental physics that motivates their study. In this thesis, we first discuss the laser technology that has enabled this research. We follow this with detailed information on the design and development of a 100 TW power upgrade to the graduate student-run GHOST laser system for the purposes of conducting repetition-rated laser-plasma experiments. We then outline the physics that drives particle acceleration during the interactions of high intensity lasers and solid targets with a particular focus on target normal sheath acceleration, the most widely studied laser-plasma particle acceleration mechanism. Finally, we describe experiments conducted on the Texas Petawatt Laser system to test an approach to enhance the yield, peak energy, and beam characteristics of proton beams generated by laser-plasma interactions with the use of focusing targets.

This thesis is dedicated to the investigation of laser-plasma particle acceleration concepts. Some of the work was focused on improving electron and proton acceleration for future applications, in terms of maximizing the particle energy and minimizing the divergence of the X-ray beams. In laser wakefield acceleration, a very intense laser pulse (> 1018 W/cm2) is focused in a gas. The leading edge of the pulse is intense enough to ionize the gas, and the main part of the pulse interacts with a plasma. Under the action of the ponderomotive force, the intense laser pulse can expel plasma electrons and form a wake in the plasma that trails the laser pulse. This charge separation leads to the formation of an ion cavity that results in electromagnetic fields several orders of magnitude stronger than those in conventional accelerators, up to TV/m. Some electrons can be accelerated to several hundred MeV over distances of less than a cm by injecting them into the plasma wake. There are also focusing forces inside the plasma wake, and the injected electrons will oscillate transversely about the optical axis, producing multi-keV betatron X-ray radiation. This radiation is directed along the optical axis with a low divergence of a few tens of mrad. One application investigated in this work was the possibility of using laser-wakefield-accelerated electrons for very high-energy electron (VHEE) radiotherapy. High-energy electrons can reach deep tumours with limited scattering, and have a more suitable dose-depth profile than photon beams. In this work, a VHEE beam was focused inside a phantom using electromagnetic quadrupoles to mimic stereotactic radiotherapy. The X-ray beam generated by LWFA was used to measure the equivalent path length and 3D liquid mass distribution in commercial fuel injectors by tomographic reconstruction. It was shown that the sensitivity (in terms of the detectable liquid mass) was comparable to that possible with large synchrotron facilities. Furthermore, the LWFA X-ray source is suitable for phase contrast imaging (PCI), a technique very sensitive to changes in the refractive index. In-line PCI was used to perform a high-resolution tomography of a small lacewing.

The research presented in this thesis is primarily focused on experimental investigations of laser-driven ion acceleration from solid targets via the target normal sheath acceleration mechanism. In particular, ways of optimizing the absorption of the laser pulse energy by free plasma electrons in the target, or modifying the shape of the accelerating electron sheath were addressed. The aim of this work was to increase the efficiency, and maximum proton energy that could be obtained with a given laser system, and to reduce the divergence of the beams of accelerated protons.The shape of the electrostatic sheath was indirectly influenced by using laser micromachining to modify the front surface of the target, on which the laser pulse is incident. The absorption of the laser pulse was enhanced by either placing nanostructures on the front side of the foil target, or by manipulating the temporal profile of the ultrafast part of the laser pulse before its interaction with an ultrathin target.It is important to ensure the survival of the target by using a laser pulse with very high temporal contrast. A double plasma mirror (DPM) was designed and implemented for this purpose. Design considerations and the optimization of the performance of the DPM, which is now used routinely at the Lund High-Power Laser Facility during laser-solid interaction studies, are discussed. Sufficiently high temporal contrast was achieved, and an increase was seen in the maximum proton kinetic energy when using targets with nanowire and foam structures on the surface. Efficient ion acceleration from ultrathin targets with a thickness down to 10 nm was observed as well.When an ultrafast laser pulse interacts with an ultrathin foil, the temporal shape of the electric field of the pulse affects the laser--solid interaction, and a slightly positively chirped pulse was found to increase the maximum kinetic energy of the accelerated protons.Laser-solid interactions at very high intensities are known to have shot-to-shot instabilities, motivating the use of single-shot diagnostics. The ion spectra in the forward direction were recorded using a Thomson parabola spectrometer, and in the backward direction with a magnetic dipole spectrometer. The intensities of the reflected and transmitted fractions of the laser pulse were also recorded on a shot-to-shot basis. In addition, a proton spatial profile monitor could be inserted to spatially characterize the proton bunch accelerated in the forward direction.

The interaction of overdense plasma with ultra-intense laser pulses presents a promising approach to enable the development of very compact ion sources. Prospective applications of high-energetic protons and ions include, but are not limited to, medical applications (in particular ion beam radiotherapy), laboratory astrophysics and nuclear fusion. However, current records for maximum proton energies (94 MeV, 2018) are still below the required values for the aforementioned applications (typically in the range of 150-250 MeV), and especially challenges such as stability and spectral control remain unsolved to this day. In particular, significant effort per experiment and a high-dimensional design space renders naive sampling approaches ineffective. Furthermore, due to the strong nonlinearities of the underlying laser-plasma physics, synthetic observations by means of particle-in-cell (PIC) simulations are computationally very costly, and the maximum distance between two sampling points is strongly limited as well. Consequently, in order to build useful surrogate models for future data generation and experimental understanding and control, a combination of highly optimized simulation codes (where we employ PIConGPU), powerful data-based methods, such as artificial neural networks (ANNs), and modern sampling approaches are essential. Specifically, we employ invertible neural networks for bidirectional learning of input (parameter) and output (observables) and convolutional autoencoder to reduce intermediate field data to a lower-dimensional latent representation.

The interaction of ultra-intense high-power lasers with solid-state targets has been largely studied for the past 20 years as a future compact proton and ion source. Indeed, the huge potential established on the target surface by the escaping electrons provides accelerating gradients of TV/m. This process, called target normal sheath acceleration, involves a large number of phenomena and is very difficult to study because of the picosecond scale dynamics. At the SPARC_LAB Test Facility, the high-power laser FLAME is employed in experiments with solid targets, aiming to study possible correlations between ballistic fast electrons and accelerated protons. In detail, we have installed in the interaction chamber two different diagnostics, each one devoted to characterizing one beam. The first relies on electro-optic sampling, and it has been adopted to completely characterize the ultrafast electron components. On the other hand, a time-of-flight detector, based on chemical-vapour-deposited diamond, has allowed us to retrieve the proton energy spectrum. In this work, we report preliminary studies about simultaneous temporal resolved measurements of both the first forerunner escaping electrons and the accelerated protons for different laser parameters.

The advancement of lasers over the last decades has allowed researchers to explore new regimes of physics and their applications. High power lasers interacting with plasmas have provided with the tools to create high frequency ultrashort pulsed radiation, near the X-ray regime and with temporal lengths on the attosecond scale, and has allowed to create compact and cheap particle accelerators. Numerical simulations are a fundamental tool in the advancement of this scientific area, since they provide with insights into the physical processes involved and they can be used to design experiments. In this thesis we present the results of numerical simulations for ultrashort pulse production, ion acceleration and other laser-plasma applications.

The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target's normal direction has been observed. The ring's opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here - in addition to the longitudinal electric fields responsible for the forward acceleration of the protons - a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.

International audience; We briefly summarize the contributions presented during the working group 2 (WG2) sessions.

International audience; Laser-driven ion acceleration by intense, ultra-short, laser pulse has received increasing attention inrecent years, and the availability of much compact and versatile ions sources motivates the study oflaser-driven sources of energetic neutral atoms. We demonstrate the production of a neutral anddirectional beam of hydrogen and carbon atoms up to 200 keV per nucleon, with a peak flow of 2.7 x10(13) atom s(-1). Laser accelerated ions are neutralized in a pulsed, supersonic argon jet with tunabledensity between 1.5 x 10(17) cm(-3) and 6 x 10(18) cm(-3). The neutralization efficiency has beenmeasured by a time-of-flight detector for different argon densities. An optimum is found, for whichcomplete neutralization occurs. The neutralization rate can be explained only at high areal densities (> 1x 10(17) cm(-2)) by single electron charge transfer processes. These results suggest a new perspectivefor the study of neutral production by laser and open discussion of neutralization at a lower density.

The development of high-intensity lasers has opened the field of nuclear reactions initiated by laser-accelerated particles. One possible application is the production of aneutronic fusion reactions for clean fusion energy production. We propose an innovative scheme based on the use of two targets and present the first results obtained with the ELFIE facility (at the LULI Laboratory) for the proton–boron-11 (p–11B) fusion reaction. A proton beam, accelerated by the Target Normal Sheat Acceleration mechanism using a short laser pulse (12 J, 350 fs, 1.056 µm, 1019 W cm−2), is sent onto a boron target to initiate fusion reactions. The number of reactions is measured with particle diagnostics such as CR39 track-detectors, active nuclear diagnostic, Thomson Parabola, magnetic spectrometer, and time-of-flight detectors that collect the fusion products: the α-particles. Our experiment shows promising results for this scheme. In the present paper, we discuss its principle and advantages compared with another scheme that uses a single target and heating mechanisms directly with photons to initiate the same p–11B fusion reaction.

Thin foils, 0.5–50 μm in thickness, have been irradiated in vacuum at Prague Asterix Laser System in Prague using 1015–16 W cm−2 laser intensity, 1315 nm wavelength, 300 ps pulse duration and different focal positions. Produced plasmas from metals and polymers films have been monitored in the forward and backward directions. Ion and electron accelerations have been investigated by using Thomson parabola spectrometer, x-ray streak camera, ion collectors and SiC semiconductor detectors, the latter employed in time-of-flight configuration. Ion acceleration up to about 3 MeV per charge state was measured in the forward direction. Ion and electron emissions were detected at different angles as a function of the irradiation conditions.

This paper presents the concept of transport and filtering of laser-accelerated proton pulses used for the first cell irradiation experiments performed with the Dresden 150 TW laser DRACO. Based on a simple non-focusing magnetic dipole equipped with two apertures the concept makes use of an energy dependent angular asymmetry of the proton spectra. For micron thin target foils protons of interest with energies above 7 MeV are observed to be significantly offset from target normal where low energy emission is dominantly centered. As the effect can be controlled via the target rotation with respect to the incoming light, it can be used to optimize the transport efficiency for high energy protons while simultaneously suppressing background radiation.