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Register a new userThermal and chaotic distributions of plasma in laser driven Coulomb explosions of deuterium clusters
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In this work we explore the possibility that the motion of the deuterium ions emitted from Coulomb cluster explosions is chaotic enough to resemble thermalization. We analyze the process of nuclear fusion reactions driven by laser-cluster interactions in experiments conducted at the Texas Petawatt laser facility using a mixture of D2+3He and CD4+3He cluster targets. When clusters explode by Coulomb repulsion, the emission of the energetic ions is nearly isotropic. In the framework of cluster Coulomb explosions, we analyze the energy distributions of the ions using a Maxwell- Boltzmann (MB) distribution, a shifted MB distribution (sMB) and the energy distribution derived from a log-normal (LN) size distribution of clusters. We show that the first two distributions reproduce well the experimentally measured ion energy distributions and the number of fusions from d-d and d-3He reactions. The LN distribution is a good representation of the ion kinetic energy distribution well up to high momenta where the noise becomes dominant, but overestimates both the neutron and the proton yields. If the parameters of the LN distributions are chosen to reproduce the fusion yields correctly, the experimentally measured high energy ion spectrum is not well represented. We conclude that the ion kinetic energy distribution is highly chaotic and practically not distinguishable from a thermalized one.
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The plasma density grating induced by intersecting intense laser pulses can be utilized as an optical compressors, polarizers, waveplates and photonic crystals for the manipulation of ultra-high-power laser pulses. However, the formation and evolution of the plasma density grating are still not fully understood as linear models are adopted to describe them usually. In this paper, two nonlinear theoretical models are presented to study the formation process of the plasma density grating. In the first model, a nonlinear analytical solution based on the fluid equations is presented while in the second model a particle-mesh method is adopted to investigate the kinetic effects. It is found that both models can describe the plasma density grating formation at different stages, well beyond the linear growth stage. More importantly, the second model can reproduce the phenomenon of ion wave-breaking of plasma density grating, which eventually induces the saturation of plasma density grating. Using the second model, the saturation time of the plasma density grating is obtained as a function of laser intensity and plasma density, which can be applied to estimate the lifetime of the plasma density grating in experiments. The results from these two nonlinear models are verified using particle-in-cell simulations.
The paper presents a theoretical work on the dynamics of Coulomb explosion for spherical nanoplasmas composed by two different ion species. Particular attention has been dedicated to study the energy spectra of the ions with the larger charge-to-mass ratio. The connection between the formation of shock shells and the energy spread of the ions has been the object of a detailed analysis, showing that under particular conditions the width of the asymptotic energy spectrum tends to become very narrow, which leads to a multi-valued ion phase-space. The conditions to generate a quasi mono-energetic ion spectrum have been rigorously demonstrated and verifed by numerical simulations, using a technique that, exploiting the spherical symmetry of the problem, allows one to obtain very accurate and precise results.
We explore a regime of laser-driven plasma acceleration of electrons where the radial envelope of the laser-pulse incident at the plasma entrance is strongly mismatched to the nonlinear plasma electron response excited by it. This regime has been experimentally studied with the gemini laser using f/40 focusing optics in August 2015 and f/20 in 2008. The physical mechanisms and the scaling laws of electron acceleration achievable in a laser-plasma accelerator have been studied in the radially matched laser regime and thus are not accurate in the strongly mismatched regime explored here. In this work, we show that a novel adjusted-a0 model applicable over a specific range of densities where the laser enters the state of a strong optical shock, describes the mismatched regime. Beside several novel aspects of laser-plasma interaction dynamics relating to an elongating bubble shape and the corresponding self-injection mechanism, importantly we find that in this strongly mismatched regime when the laser pulse transforms into an optical shock it is possible to achieve beam-energies that significantly exceed the incident intensity matched regime scaling laws.
Relativistic electrons generated by the interaction of petawatt-class short laser pulses with solid targets can be used to generate bright X-rays via bremsstrahlung. The efficiency of laser energy transfer into these electrons depends on multiple parameters including the focused intensity and pre-plasma level. This paper reports experimental results from the interaction of a high intensity petawatt-class glass laser pulses with solid targets at a maximum intensity of $10^{19}$ W/cm$^2$. In-situ measurements of specularly reflected light are used to provide an upper bound of laser absorption and to characterize focused laser intensity, the pre-plasma level and the generation mechanism of second harmonic light. The measured spectrum of electrons and bremsstrahlung radiation provide information about the efficiency of laser energy transfer.
The structure of driven three-dimensional complex plasma clusters was studied experimentally. The clusters consisted of around 60 hollow glass spheres with a diameter of 22 microns that were suspended in a plasma of rf discharge in argon. The particles were confined in a glass box with conductive yet transparent coating on its four side walls, this allowed to manipulate the particle cluster by biasing the confining walls in a certain sequence. In this work, a rotating electric field was used to drive the clusters. Depending on the excitation frequency, the clusters rotated (10^4 - 10^7 times slower than the rotating field) or remained stationary. The cluster structure was neither that of nested spherical shells nor simple chain structure. Strings of various lengths were found consisting of 2 to 5 particles, their spatial and temporal correlations were studied. The results are compared to recent simulations.
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