Showing papers by "Stefano Atzeni published in 2018"

Measurements of parametric instabilities at laser intensities relevant to strong shock generation

Abstract: Parametric instabilities at laser intensities in the range (2–6) × 1015 W/cm2 (438 nm, 250 ps, 100–300 J) have been investigated in planar geometry at the Prague Asterix Laser System facility via calorimetry and spectroscopy. The density scalelength of the plasma was varied by using an auxiliary pulse to form a preplasma before the arrival of the main laser beam and by changing the delay between the two pulses. Experimental data show that Stimulated Brillouin Scattering (SBS) is more effective than Stimulated Raman Scattering (SRS) in degrading laser-plasma coupling, therefore reducing the energy available for the generation of the shock wave. The level of the SBS backscatter and laser reflection is found to be in the range between 3% and 15% of the incident laser energy, while Backward SRS (BRS) reflectivity ranges between 0.02% and 0.2%, depending on the delay between the pulses. Half-integer harmonic emission is observed and provides a signature of Two Plasmon Decay (TPD) occurring around the quarter o...

Experimental evidence for the enhanced and reduced stopping regimes for protons propagating through hot plasmas.

Abstract: Our understanding of the dynamics of ion collisional energy loss in a plasma is still not complete, in part due to the difficulty and lack of high-quality experimental measurements. These measurements are crucial to benchmark existing models. Here, we show that such a measurement is possible using high-flux proton beams accelerated by high intensity short pulse lasers, where there is a high number of particles in a picosecond pulse, which is ideal for measurements in quickly expanding plasmas. By reducing the energy bandwidth of the protons using a passive selector, we have made proton stopping measurements in partially ionized Argon and fully ionized Hydrogen plasmas with electron temperatures of hundreds of eV and densities in the range 1020–1021 cm−3. In the first case, we have observed, consistently with previous reports, enhanced stopping of protons when compared to stopping power in non-ionized gas. In the second case, we have observed for the first time the regime of reduced stopping, which is theoretically predicted in such hot and fully ionized plasma. The versatility of these tunable short-pulse laser based ion sources, where the ion type and energy can be changed at will, could open up the possibility for a variety of ion stopping power measurements in plasmas so long as they are well characterized in terms of temperature and density. In turn, these measurements will allow tests of the validity of existing theoretical models.

Numerical studies on capillary discharges as focusing elements for electron beams

Abstract: Active plasma lenses are promising technologies for the focusing of high brightness electron beams due to their radially symmetric focusing and their high field gradients (up to several kT/m). However, in a number of experimental situations, the transverse non-uniformity of the current density flowing in the lens causes beam emittance growth and increases the minimum achievable spot size. To study the physics of the capillary discharge processes employed as active plasma lenses, we developed a 2-D hydrodynamic computational model. Here, we present preliminary simulation results and we compare the computed magnetic field profile with one from literature, which has been experimentally inferred. The result of the comparison is discussed.

Transition from nonlocal electron transport to radiative regime in an expanding blast wave

Abstract: We have investigated the formation, evolution, and late-time propagation of a laser-generated cylindrical blast wave (BW). The whole blast wave evolution over timescales of several nanoseconds was reconstructed experimentally (via temporally resolved interferometric measurements) and via hydrodynamic simulations that included modeling of nonlocal electron transport and radiation diffusion. Comparison between the experimental results and the simulations indicates that the early expansion phase is characterised by nonlocal electron heat transport causing energy spread on times shorter than the typical timescales for hydrodynamic expansion. Nonlocal electron transport ionizes the gas ahead of the plasma front and gives rise to a smooth radial density gradient. At later times, once the shock is launched and the BW is formed, radiation results in reduced shock velocity compared to the adiabatic case. These investigations provide a suitable and effective platform to benchmark the inclusion of kinetic and radiative effects in fluid modeling of the plasma dynamics over timescales that may be inaccessible to fully kinetic simulations.

X-ray absorption radiography for high pressure shock wave studies

Abstract: The study of laser compressed matter, both warm dense matter (WDM) and hot dense matter (HDM), is relevant to several research areas, including materials science, astrophysics, inertial confinement fusion. X-ray absorption radiography is a unique tool to diagnose compressed WDM and HDM. The application of radiography to shock-wave studies is presented and discussed. In addition to the standard Abel inversion to recover a density map from a transmission map, a procedure has been developed to generate synthetic radiographs using density maps produced by the hydrodynamics code DUED. This procedure takes into account both source-target geometry and source size (which plays a non negligible role in the interpretation of the data), and allows to reproduce transmission data with a good degree of accuracy.