It is known that rare-gas or metal clusters absorb incident laser energy very efficiently. However, due to the intricate dependencies on all the laser and cluster parameters it is difficult to predict under which circumstances ionization and energy absorption is optimal.
With the help of three-dimensional particle-in-cell simulations of xenon clusters (up to 17256 atoms) we find that for a given laser pulse energy and cluster an optimum wavelength exists which corresponds to the approximate wavelength of the transient, linear Mie-resonance of the ionizing cluster at an early stage of negligible expansion. In a single ultrashort laser pulse, the linear resonance at this optimum wavelength yields much higher absorption efficiency than in the conventional, dual-pulse pump-probe set-up of linear resonance during cluster expansion.
Effects of ionization injection in low and high Z gas mixtures for the laser wake field acceleration of electrons are analyzed with the use of balance equations and particle-in-cell simulations via test probe particle trajectories in realistic plasma fields and direct simulations of charge loading during the ionization process. It is shown that electrons appearing at the maximum of laser pulse field after optical ionization are trapped in the first bucket of the laser pulse wake. Electrons, which are produced by optical field ionization at the front of laser pulse, propagate backwards; some of…
The kinetic Boltzmann equation is used to model the non-equilibrium ionization phase that initiates the evolution of atomic clusters irradiated with single pulses of intense vacuum ultraviolet radiation. The duration of the pulses is < 50 fs and their intensity in the focus is < 10^{14} W/cm^2. This statistical model includes various processes contributing to the sample dynamics at this particular radiation wavelength, and is computationally efficient also for large samples. Two effects are investigated in detail: the impact of the electron heating rate and the effect of the plasma environment…
The harmonic emission from cluster nanoplasmas subject to short, intense infrared laser pulses is analyzed by means of particle-in-cell simulations. A pronounced resonant enhancement of the low-order harmonic yields is found when the Mie plasma frequency of the ionizing and expanding cluster resonates with the respective harmonic frequency. We show that a strong, nonlinear resonant coupling of the cluster electrons with the laser field inhibits coherent electron motion, suppressing the emitted radiation and restricting the spectrum to only low-order harmonics. A pump-probe scheme is suggested …
Balmer spectra are investigated which are obtained from hydrogen droplets irradiated by ultra-short intense laser pulses. A unified quantum statistical description of bremsstrahlung, the Stark broadening and the van der Waals profile of hydrogen spectral lines is used, which allows to include many-particle effects. Analyzing the line profiles, a low ionization degree of a dense plasma is inferred, where the main contribution to the spectral line shape originates from the interaction with the neutral components. Effective temperatures and densities of the radiating microplasma are deduced. A dy…
In this paper the generation of Kα X-ray produced by interaction of ultrashort laser pulses with metal targets has been studied numerically. Several targets were assumed to be irradiated by high intensity ultra-short laser pulses for the calculations. Using Maxwell Boltzmann distribution function for hot electron and applying an analytical model, the number of Kα photons were calculated as a function of hot electron temperature, target thickness and K-shell ionization cross section. Also, simulation results of Kα yield versus target thickness variations from two and three layer metals have bee…
Surfaces covered with nanostructures, such as nanowire arrays, have shown to facilitate a significantly higher absorption of laser energy as compared to flat surfaces. Due to the efficient coupling of the laser energy, highly energetic electrons are produced, which in turn can emit intense ultrafast X-ray pulses. In the present work we use full three dimensional PIC simulations to analyze the behavior of arrays of carbon nanowires $400 nm$ in diameter, irradiated by a $λ_0 = 400 nm$ laser pulse of $60 fs$ duration at FWHM and a vector potential of $a_0 = 18$. We analyze the ionization dynamics…
The ablation of solid tin surfaces by an 800-nanometer-wavelength laser is studied for a pulse length range from 500 fs to 4.5 ps and a fluence range spanning 0.9 to 22 J/cm^2. The ablation depth and volume are obtained employing a high-numerical-aperture optical microscope, while the ion yield and energy distributions are obtained from a set of Faraday cups set up under various angles. We found a slight increase of the ion yield for an increasing pulse length, while the ablation depth is slightly decreasing. The ablation volume remained constant as a function of pulse length. The ablation dep…
We study the excitation of electron currents in a transparent cell of sub-millimeter size filled by an atomic gas and illuminated by an intense two-color femtosecond laser pulse. The pulse consists of a strong fundamental component and its second harmonic of low intensity, both circularly polarized. We show that for sufficiently small $20μ$m cells the plasma oscillation excited by asymmetric ionization is almost spatially homogeneous within the interaction volume. This coherent dipole plasma oscillation results in a remarkably efficient conversion of the electron energy into that of radiation …
Non-equilibrium processes following the irradiation of atomic clusters with short pulses of vacuum ultraviolet radiation are modelled using kinetic Boltzmann equations. The dependence of the ionization dynamics on the cluster size is investigated. The predictions on: (i) the maximal and average ion charge created, (ii) ion charge state distribution, (iii) average energy absorbed per atom, (iv) spatial charge distribution, and (v) thermalization scales are obtained for spherical xenon clusters containing: 20, 70, 2500 and 90000 atoms. These clusters were exposed to single rectangular pulses of …
We investigate bulk ion heating in solid buried layer targets irradiated by ultra-short laser pulses of relativistic intensities using particle-in-cell simulations. Our study focuses on a CD2-Al-CD2 sandwich target geometry. We find enhanced deuteron ion heating in a layer compressed by the expanding aluminium layer. A pressure gradient created at the Al-CD2 interface pushes this layer of deuteron ions towards the outer regions of the target. During its passage through the target, deuteron ions are constantly injected into this layer. Our simulations suggest that the directed collective outwar…
Ionization Dynamics in Intense Laser-Produced Plasmas
Lawrence Livermore National Laboratory（劳伦斯利弗莫尔国家实验室）；Laboratory for Laser Energetics（激光能量学实验室）；University of Alberta（阿尔伯塔大学）；Department of Physics, University of Alberta（阿尔伯塔大学物理系）；Department of Physics and Astronomy, University of Rochester（罗切斯特大学物理与天文系）；University of Rochester（罗切斯特大学）
A novel radiation pressure acceleration (RPA) regime of heavy ion beams from laser-irradiated ultrathin foils is proposed by self-consistently taking into account the ionization dynamics. In this regime, the laser intensity is required to match with the large ionization energy gap when the successive ionization of high-Z atoms passing the noble gas configurations [such as removing an electron from the helium-like charge state $(\text{Z}-2)^+$ to $(\text{Z}-1)^+$]. While the target ions in the laser wing region are ionized to low charge states and undergo rapid dispersions due to instabilities,…
We develop a large scale quantum master equation approach to describe dynamical processes of practical open quantum systems driven by both coherent and stochastic interactions by including more than one thousand true states of the systems, motivated by the development of highly bright and fully coherent lasers in the X-ray wavelength regime. The method combines the processes of coherent dynamics induced by the X-ray laser and incoherent relaxations due to spontaneous emissions, Auger decays, and electronic collisions. As examples, theoretical investigation of {\it real} coherent dynamics of in…
We examine the effect of cluster size on the interaction of Ar$_{55}$-Ar$_{2057}$ with intense extreme ultraviolet (XUV) pulses, using a model we developed earlier that includes ionization via collisional excitation as an intermediate step. We find that the dynamics of these irradiated clusters is dominated by collisions. Larger clusters are more highly collisional, produce higher charge states, and do so more rapidly than smaller clusters. Higher charge states produced via collisions are found to reduce the overall photon absorption, since charge states of Ar$^{2+}$ and higher are no longer p…
PIConGPU modeling of nanoplasma formation in helium nanodroplets irradiated by intense femtosecond laser pulses
Max-Planck-Institut für Kernphysik（马克斯·普朗克核物理研究所）；Helmholtz-Zentrum Dresden - Rossendorf, Center for Advanced Systems Understanding (CASUS)（德累斯顿-罗森多夫亥姆霍兹中心，先进系统理解中心）；Helmholtz-Zentrum Dresden - Rossendorf（德累斯顿-罗森多夫亥姆霍兹中心）；Institute of Physics, University of Kassel（卡塞尔大学物理研究所）；Department of Physics and Quantum Center of Excellence for Diamond and Emerging Materials, Indian Institute of Technology Madras（马德拉斯印度理工学院物理系与钻石及新兴材料量子卓越中心）；ELI Beamlines facility, The Extreme Light Infrastructure ERIC（极亮基础设施欧洲利益协调实体 ELI光束线设施）
An enhanced ionization injection scheme using a tightly focused laser pulse with intensity near the ionization potential to trigger the injection process in a mismatched pre-plasma channel has been proposed and examined via multi-dimensional particle-in-cell simulations. The core idea of the proposed scheme is to lower the energy spread of trapped beams by shortening the injection distance. We have established theory to precisely predict the injection distance, as well as the ionization degree of injection atoms/ions, electron yield and ionized charge. We have found relation between injection …
The interaction of a free-electron-laser pulse with a moderate or large size cluster is known to create a quasi-neutral nanoplasma, which then expands on hydrodynamic timescale, i.e., $>1$ ps. To have a better understanding of ion and electron data from experiments derived from laser-irradiated clusters, one needs to simulate cluster dynamics on such long timescales for which the molecular dynamics approach becomes inefficient. We therefore propose a two-step Molecular Dynamics-Hydrodynamic scheme. In the first step we use molecular dynamics code to follow the dynamics of an irradiated cluster…
An unprecedented increase of kinetic energy of laser accelerated heavy ions is demonstrated. Ultra thin gold foils have been irradiated by an ultra short laser pulse at an intensity of $6\times 10^{19}$ W/cm$^{2}$. Highly charged gold ions with kinetic energies up to $> 200$ MeV and a bandwidth limited energy distribution have been reached by using $1.3$ Joule laser energy on target. $1$D and $2$D Particle in Cell simulations show how a spatial dependence on the ions ionization leads to an enhancement of the accelerating electrical field. Our theoretical model considers a varying charge densit…
The spatiotemporal evolution of the field and plasma in the optical breakdown induced in the volume of transparent dielectric (fused silica) by the focused fs laser pulse is studied under condition of the so-called plasma-resonance-induced ionization instability that results in the deep small-scale periodic modulation of the breakdown plasma parameters in the direction of the laser polarization. In the framework of the model used, the optical electric field was calculated with allowance for the effects influencing both its long-scale structure (the beam focusing accounted for in the given-ray-…
A relativistic petawatt laser light can heat heavy metals over keV temperature isochorically and ionize them almost fully. Copious hard X-rays are emitted from the high-Z hot plasma which acts as X-ray sources, while they work as a cooling process of the plasma. The cooling process can affect on the creation of high energy density plasma via the interaction, however, the details are unknown. The X-ray spectrum depends on the plasma temperature, so that it is worthwhile to investigate the radiation cooling effects. We here study the isochoric heating of a solid silver foil irradiated by relativ…
