Laser pulses are largely used for processing and analysis of materials and in particular for nano-particle synthesis. This paper addresses fundamentals of the generation of nano-materials following specific thermodynamic paths of the irradiated material. Computer simulations using the hydro code MULTI and the SESAME equation of state have been performed to follow the dynamics of a target initially heated by a short laser pulse over a distance comparable to the metal skin depth.

A method to design the composite insulation structures in pulsed power systems is proposed in this paper. The theoretical bases for this method include the Weibull statistical distribution and the empirical insulation formula. A uniform formula to describe the reliability (R) for different insulation media such as solid, liquid, gas, vacuum, and vacuum surface is derived. The dependence curves of the normalized applied field on R are also obtained. These curves show that the normalized applied field decreases rapidly as R increases but the declining rates corresponding to different insulation media are different. In addition, if R is required to be higher than a given level, the normalized applied field should be smaller than a certain value. In practical design, the common range of the applied fields for different insulation media should be chosen to meet a global reliability requirement. In the end, the proposed method is demonstrated with a specific coaxial high-voltage vacuum insulator.

Interaction of a linearly polarized Gaussian laser pulse (at relativistic intensity of 2.0 × 1020 Wcm−2) with a multi-layer foam (as a near critical density target) attached to a solid layer is investigated by using two-dimensional particle-in-cell simulation. It is found that electrons with longitudinal momentum exceeding the free electrons limit of meca02/2 so-called super-hot electrons can be produced when the direct laser acceleration regime is fulfilled and benefited from self-focusing inside of the subcritical plasma. These electrons penetrate easily through the target and can enhance greatly the sheath field at the rear, resulting in a significant increase in the maximum energy of protons in target normal sheath acceleration regime. The results indicate that the maximum proton energy is enhanced by 2.7 times via using an assembled target arrangement compared to a bare solid target. Furthermore, by demonstration of this assembly, the maximum proton energy is improved beyond the optimum amount achieved by a two-layer target proposed by Sgattoni et al. (2012).

The interaction of a cylindrical converging shock wave with an initially perturbed gaseous interface is studied experimentally. The cylindrical converging shock is generated in an ordinary shock tube but with a specially designed test section, in which the incident planar shock wave is directly converted into a cylindrical one. Two kinds of typical initial interfaces involving gas bubble and gas cylinder are employed. A high-speed video camera combined with schlieren or planar Mie scattering photography is utilized to capture the evolution process of flow structures. The distribution of baroclinic vorticity on the interface induced by the cylindrical shock and the reflected shock from the center of convergence results in distinct phenomena. In the gas bubble case, the shock focusing and the jet formation are observed and the turbulent mixing of two fluids is promoted because of the gradually changed shock strength and complex shock structures in the converging part. In the gas cylinder case, a counter-rotating vortex pair is formed after the impact of the converging shock and its rotating direction may be changed when interacting with the reflected shock for a relatively long reflection distance. The variations of the interface displacements and structural dimensions with time are further measured. It is found that these quantities are different from those in the planar counterpart because of the shock curvature, the Mach number effect and the complex shock reflection within the converging shock tube test section. Therefore, the experiments reported here exhibit the great potential of this experimental method in study of the Richtmyer-Meshkov instability induced by converging shock waves.

A theoretical model of soft X-ray emission from laser irradiated clusters is developed. An intense short pulse laser of Gaussian radial and temporal profiles impinged on a clustered gas jet, heats the cluster electrons, leading to Bremsstrahlung emission of X-rays. As the clusters expand under hydrodynamic pressure, plasma frequency of the cluster electrons ωpe decreases. When plasma frequency of a cluster approaches plasma resonance ${\rm \omega}_{\,pe} = \sqrt{3} {\rm \omega}$ (where ω is the laser frequency), the electrons are resonantly heated by the laser and a rapid rise in X-ray emission occurs. After a while, when cluster expansion detunes the plasma resonance, X-ray emission falls off.

A fluid description is used to study the properties of two-stream instability due to interaction of a non-relativistic electron beam with quantum magnetized plasma and transverse wiggler magnetic field. It is assumed that the background plasma provides charge and current neutralization of the electron beam. The dispersion relation is obtained by solving and linearizing fluid-Maxwell equations. The resulting dispersion equation is analyzed numerically over a wide range of system parameters. The results of quantum and classical treatments are compared numerically, with including the effects of wiggler on the dispersion relation. It is found that the transverse wiggler magnetic field can strongly improve the instability of quantum plasma as well as classical plasma.

A novel approach of beam cleanup based on stimulated Brillouin scattering with a large core fiber is proposed to improve the laser beam quality. The fusion splice scheme from a single-mode fiber to a very large core fiber (105 µm) is first employed in stimulated Brillouin scattering to steadily excite the fundamental mode of the Stokes beam. As a result, the output beam achieves a measured M2 value of around 1.3 meanwhile the pump conversion efficiency is up to 90%, which is the best in the reports of stimulated Brillouin scattering cleanup to our knowledge.

We consider the acceleration of electrons in vacuum by means of the circularly-polirized electromagnetic wave, propagating along a magnetic field. We show that the electron energy growth, when using ultra-short and ultra-intense laser pulses (1 ps, 1018 W/cm2, CO2 laser) in the presence of a magnetic field, may reach up to the value 2,1 GeV. The growth of the electron energy is shown to increase proportionally with the increase of the laser intensity and the initial energy of the electron. We find that for some direction of polarization of the wave, the acceleration of electrons does not depend on the initial phase of the electromagnetic wave. We estimate the laser intensity, necessary for the electron acceleration. In addition, we find the formation length of photon absorption by electrons, due to which one may choose the required region of the interaction of the electrons with the electromagnetic wave and magnetic field. We also show that as a result of acceleration of electrons in the vacuum by laser radiation in a magnetic field one may obtain electron beam with small energy spread of the order δε/ε ≤ 10−2.

The one-dimensional behavior of a thin plasma foil heated by laser is studied, emphasizing on the fully kinetic effects associated with initial energetic electrons using a relativistic kinetic 1D3V Particle-In-Cell code. For this purpose, the generalized Lorentzian (Kappa) function inclusive the high energy tail is employed for initial electron distribution. The presence of the initially high-energy electrons leads to a different ion energy spectrum than the initially Maxwellian distribution. It is shown for the smaller Kappa parameter k where the high energy tail of the electron distribution function becomes more significant, the electron cooling rate increases. Moreover, the spatiotemporal evolution of electric field is strongly affected by the initial super-thermal electrons.

A space-resolved charge density of ions is derived from a time-resolved current of ions emitted from laser-produced plasma and expanded into the vacuum along collision-free and field-free paths. This derivation is based on a similarity relationship for ion currents with “frozen” charges observed at different distances from the target. This relationship makes it possible to determine a map of ion charge density at selected times after the laser plasma interaction from signals of time-of-flight detectors positioned at a certain distance from the target around a target-surface normal. In this work, we present maps of the charge density of ions emitted from Cu and polyethylene plasmas. The mapping demonstrates that bursts of ions are emitted at various ejection angles ϕn with respect to the target-surface normal. There are two basic directions ϕ1 and ϕ2, one belonging to the fastest ions, i.e., protons and carbon ions, and the other one to the slowest ions being a part of each plasma plume.

In the present paper, measurement of various plasma parameters during pulsed laser deposition of ruby thin film on quartz substrate is reported. The variation of electron temperature and ion density with laser fluence and ambient pressure is recorded via Langmuir probe technique. The structural and optical properties of ruby thin films were analyzed using photo-luminescence and atomic force microscopy, and then correlated with the plasma parameters to find optimum conditions for deposition of high quality ruby thin film.

It is shown that it is possible to accelerate micro-foils to velocities from 108 cm/s up to relativistic velocities without the disturbance of the Rayleigh-Taylor instability. The acceleration occurs due to the radiation pressure of proper high power lasers. In these systems, the ablation force is negligible relative to the ponderomotive force that dominates the acceleration. The laser irradiances of 1017 W/cm2 < IL < 1021 W/cm2 with a pulse duration of the order of 10 picoseconds can accelerate a micro-foil by the laser radiation pressure to velocities as high as 109 cm/s before breaking by Rayleigh Taylor (RT) instability. Similarly, laser irradiances of IL > 1021 W/cm2 with pulse duration of the order of 10 femtoseconds can accelerate a micro-foil to relativistic velocities without RT breaking. Due to the nature of the accelerating ponderomotive force, in both the relativistic and non-relativistic cases, the structure of the accelerated target contains a double layer (DL) at the interface of the laser-target interaction. The DL acts as a piston during the acceleration process. The influence of the DL surface tension on the RT instability is also analyzed in this paper.

A formalism for investigation of the propagation characteristics of various order short duration (pico second) Gaussian/dark hollow Gaussian laser pulse (DHGP) in a tunnel ionized plasma has been developed, which takes into account the electron-ion recombination. Utilizing the paraxial like approach, a nonlinear Schrödinger wave equation characterizing the beam spot size in space and time has been derived and solved numerically to investigate the transverse focusing (in space) and longitudinal compression (in time) of the laser pulse; the associated energy localization as the pulse advances in the plasma has also been analyzed. It is seen that in the absence of recombination the DHGP and Gaussian pulse undergo oscillatory and steady defocusing respectively. With the inclusion of recombination, the DHGP and Gaussian pulse both undergo periodic self-focusing for specific parameters. The DHGPs promise to be suitable for enhancement of energy transport inside the plasma.

Large energy fluxes and energy deposition are often found during off normal events in fusion reactors both magnetic and inertial confinement. Great temperature excursions can be found in the plasma facing components materials resulting in melting, evaporation, and expansion of the target material vapor cloud and successive plasma interaction. A general methodology for the computational solution of this very complex problem is proposed in this paper using a novel approach of the Particle-in-Cell method. This novel approach preserves many of the original Particle-in-Cell methodology proposed by Harlow's but introduces several changes to the method, which makes it particularly suitable in plasma dynamics applications as Tokamaks devices. Benchmarking is done using results from the very well-known and benchmarked HEIGHTS computer package and using available results from the MK200 plasma gun experiments. The general scheme is explained in detail and results and discussion related to the main computational aspects are presented. In particular, the critical importance of the starting computational mesh versus the aimed accuracy is studied and explained. The methodology is different from any other presented in literature because the dynamics of the physical problem dictates that as enough energy is being dumped on the target plate, melting and evaporation of the material takes place. As time progresses, new sample particles are introduced in the system making this boundary a dynamic one along time. Computer time is discussed in relation to the initial sample particle loading, aimed accuracy and importance of the time used for solving the radiation transport at each time step.

This paper elaborates the effect of unmatched stored energy in high-voltage high-energy pulsed power systems. High-voltage insulation failure of KALI system is analyzed thoroughly for its occurrence. According to the simulations and analysis energy mismatch of MARX generator and Blumlein transmission line is found to be the most significant cause for high-voltage failure of the system. MARX generator and Blumlein of KALI are redesigned to attain better energy balance at same voltage level. Observations, simulation and analytical results are illustrated in the following sections.

Hundred picoseconds laser pulse with high energy and high peak power has broad application prospects such as inertial confinement fusion shock ignition. But it is hard to get effective amplification through MOPA or chirped pulse amplification method. Through simulated Brillouin scattering method, 100 picoseconds laser pulse can be amplified efficiently. To be able to meet the need of high energy and high-intensity laser pulse amplification, scalable two cell structure and four different FC series liquid were used to fulfill this experiment. The results indicate that the magnification of Stokes energy and efficiency of energy extraction are closely related to medium parameters and energy parameters. The minimum width of 340 ps Stokes pulse was amplified by 13.5 times in this experiment.

Non-equilibrium heat transfer described by a two-step temperature model was developed to study the thermal evolution through target materials irradiated by femtosecond laser pulse. Two-dimensional heat transfer equations were solved numerically. The temperature dependent thermo-physical properties of the electron and the lattice are considered in the model. The Gaussian spatial and temporal distribution of the heat and temperature of the electron and the lattice is presented. The effect of reflectivity, electron — lattice coupling factor, and the spot size was studied using copper targets.

Laser-induced fusion in ultra-dense deuterium D(-1) is reported in several studies from our group, using ns- and ps-pulsed lasers. The ejection of ultra-dense hydrogen particles with thermal distributions and energy up to 20 MeV u−1 was studied previously by time-of-flight measurements. The investigations of the new processes continue now by studying the interaction of these particles with metal surfaces. In the present experiments, such particles penetrate in two steps through 1 mm of metal and reach three levels of collectors at distances up to 1 m. Only the fastest particles penetrate and move to the next level. The thermal time-of-flight distributions together with tests with strong magnetic fields exclude electrons as the particles observed. The sign of the signals to the metal collectors depends on the bias (negative bias gives positive signal and conversely) while the time variations of the signals for positive and negative bias are similar. The rapid variation of the signals indicates electrons and positrons ejected from the collectors, thus lepton-pair production. An increase in bias up to ± 400 V increases the peak signal up to 1 A with no observed limiting. A thick metal plate removes slow particles and most gamma photons. The number of lepton-pairs produced is > 4 × 1012 sr−1 in the forward direction per laser shot.

We present a new simulation method to predict the maximum possible yield of X-rays produced by electron beams accelerated by petawatt lasers irradiating thick solid targets. The novelty of the method lies in the simulation of the electron refiluxing inside the target implemented with the Monte Carlo code Fluka. The mechanism uses initial theoretical electron spectra, cold targets and refiluxing electrons forced to re-enter the target iteratively. Collective beam plasma effects are not implemented in the simulation. Considering the maximum X-ray yield obtained for a given target thickness and material, the relationship between the irradiated target mass thickness and the initial electron temperature is determined, as well as the effect of the refiluxing on X-ray yield. The presented study helps to understand which electron temperature should be produced in order to generate a particular X-ray beam. Several applications, including medical and security imaging, could benefit from laser generated X-ray beams, so an understanding of the material and the thickness maximizing the yields or producing particular spectral characteristics is necessary. On the other more immediate hand, if this study is experimentally reproduced at the beginning of an experiment in which there is an interest in laser-driven electron and/or photon beams, it can be used to check that the electron temperature is as expected according to the laser parameters.

Resonant excitation of terahertz (THz) radiation by nonlinear coupling of two filamented spatial-Gaussian laser beams of different frequencies and wave numbers is studied in plasma having transverse static electric field. The static ponderomotive force due to filamented lasers is balanced by the pressure gradient force which gives rise to transverse density ripple, while, the nonlinear ponderomotive force at frequency difference of beating lasers couples with density ripple giving rise to stronger transverse nonlinear current which results into the excitation of THz radiation at resonance. The coupling is further enhanced by the presence of static electric field and spatial-Gaussian nature of laser beams. An increase of six-fold in the normalized amplitude of THz is observed by applying a direct current field of about 50 KV. Effects of frequency, laser beam width, and periodicity factor of modulated laser amplitude are studied for the efficient THz radiation generation. These results can be utilized for generating controlled tunable THz sources for medical applications using low filament intensities (~ 1014 W/cm2) of beating lasers.