Plasma Damage Control: From Biomolecules to Cells and Skin.

The antibacterial and cell-proliferative character of atmospheric pressure plasma jets (APPJs) helps in the healing process of chronic wounds. However, control of the plasma-biological target interface remains an open issue. High vacuum ultraviolet/ultraviolet (VUV/UV) radiation and RONS flux from plasma may cause damage of a treated tissue; therefore, controlled interaction is essential. VUV/UV emission from argon APPJs and radiation control with aerosol injection in plasma effluent is the focus of this research. The aerosol effect on radiation is studied by a fluorescent target capable of resolving the plasma oxidation footprint. In addition, DNA damage is evaluated by plasmid DNA radiation assay and cell proliferation assay to assess safety aspects of the plasma jet, the effect of VUV/UV radiation, and its control with aerosol injection. Inevitable emission of VUV/UV radiation from plasmas during treatment is demonstrated in this work. Plasma has no antiproliferative effect on fibroblasts in short treatments (t < 60 s), while long exposure has a cytotoxic effect, resulting in decreased cell survival. Radiation has no effect on cell survival in the medium due to absorption. However, a strong cytotoxic effect on the attached fibroblasts without the medium is apparent. VUV/UV radiation contributes 70% of the integral plasma effect in induction of single- and double-strand DNA breaks and cytotoxicity of the attached cells without the medium. Survival of the attached cells increases by 10% when aerosol is introduced between plasma and the cells. Injection of aerosol in the plasma effluent can help to control the plasma-cell/tissue interaction. Aerosol droplets in the effluent partially absorb UV emission from the plasma, limiting photon flux in the direction of the biological target. Herein, cold and safe plasma-aerosol treatment and a safe operational mode of treatment are demonstrated in a murine model.

Atmospheric Pressure Plasma Surface Treatment of Polymers and Influence on Cell Cultivation.

Atmospheric plasma treatment is an effective and economical surface treatment technique. The main advantage of this technique is that the bulk properties of the material remain unchanged while the surface properties and biocompatibility are enhanced. Polymers are used in many biomedical applications; such as implants, because of their variable bulk properties. On the other hand, their surface properties are inadequate which demands certain surface treatments including atmospheric pressure plasma treatment. In biomedical applications, surface treatment is important to promote good cell adhesion, proliferation, and growth. This article aim is to give an overview of different atmospheric pressure plasma treatments of polymer surface, and their influence on cell-material interaction with different cell lines.

Thomson scattering from a three-component plasma.

A model for a three-component plasma consisting of two distinct ionic species and electrons is developed and applied to study x-ray Thomson scattering. Ions of a specific type are assumed to be identical and are treated in the average-atom approximation. Given the plasma temperature and density, the model predicts mass densities, effective ionic charges, and cell volumes for each ionic type, together with the plasma chemical potential and free-electron density. Additionally, the average-atom treatment of individual ions provides a quantum-mechanical description of bound and continuum electrons. The model is used to obtain parameters needed to determine the dynamic structure factors for x-ray Thomson scattering from a three-component plasma. The contribution from inelastic scattering by free electrons is evaluated in the random-phase approximation. The contribution from inelastic scattering by bound electrons is evaluated using the bound-state and scattering wave functions obtained from the average-atom calculations. Finally, the partial static structure factors for elastic scattering by ions are evaluated using a two-component version of the Ornstein-Zernike equations with hypernetted chain closure, in which electron-ion interactions are accounted for using screened ion-ion interaction potentials. The model is used to predict the x-ray Thomson scattering spectrum from a CH plasma and the resulting spectrum is compared with experimental results obtained by Feltcher et al. [Phys. Plasmas 20, 056316 (2013)].

Predictions of x-ray scattering spectra for warm dense matter.

We present calculations of x-ray scattering spectra based on ionic and electronic structure factors that are computed from a new model for warm dense matter. In this model, which has no free parameters, the ionic structure is determined consistently with the electronic structure of the bound and free states. The x-ray scattering spectrum is thus fully determined by the plasma temperature, density and nuclear charge, and the experimental parameters. The combined model of warm dense matter and of the x-ray scattering theory is validated against an experiment on room-temperature, solid beryllium. It is then applied to experiments on warm dense beryllium and aluminum. Generally good agreement is found with the experiments. However, some significant discrepancies are revealed and appraised. Based on the strength of our model, we discuss the current state of x-ray scattering experiments on warm dense matter and their potential to determine plasma parameters, to discriminate among models, and to reveal interesting and difficult to model physics in dense plasmas.

Scaling of resistive guiding of laser-driven fast-electron currents in solid targets.

The resistive magnetic field plays a crucial role in determining the laser produced fast-electron transport in solid targets. The scaling of the resistive guiding is derived and benchmarked against two-dimensional collisional particle-in-cell simulations. We study the impact of the initial state of the material (Z dependence, conductor, or insulator) on global electron-transport patterns, and conclude that the initial state of a conductor or insulator is not important. Instead, global transport patterns depend on the material Z. The fast-electron transport seen in the simulations is consistent with the derived scaling rule. Previous experimental observations [e.g., R. B. Stephens et al., Phys. Rev. E 69, 066414 (2004) and Y. Sentoku et al., Phys. Rev. Lett. 107, 135005 (2011)] that show confinement or divergence in various regimes are also explained by our scaling. The presented scaling then becomes a useful tool to design compact radiation sources or fast ignitor experiments.

Collision of counterpropagating laser-excited wake bubbles.

The collision of wake bubbles behind two counterpropagating laser pulses in rarefied plasma is investigated using particle-in-cell simulation. Special attention is paid to the highly nonlinear dynamics of the electrons in the interaction region. It is found that, as the two bubbles approach each other and collide, the electrons in the interaction region first oscillate in a periodic fashion, forming a quasistationary dense electron density ripple with fairly regular spatial structure. At longer times, the electron motion becomes chaotic, and the density grating is gradually smeared. The electrons escape in the transverse direction, and eventually the two bubbles merge to form a single one. The transition of the electron motion from regular to chaotic is confirmed by analytical modeling using test electrons moving in counterpropagating planar electromagnetic waves. The findings shed light on the dynamics of wake-bubble collisions and the complex behavior induced by multiple laser pulses in plasmas.

Measurement of the laser-pulse group velocity in plasma waveguides.

Electrically discharged plasma channels can guide laser pulses, extending the laser-plasma interaction length to many Rayleigh ranges. In applications such as the laser-plasma accelerator, the laser group velocity in the channel plays a critical role. The laser travel time (and thus the averaged group velocity) was measured through two-pulse frequency-domain interferometry and was found to depend on the on-axis plasma density and laser spot size. The data is in agreement with theory.

Pulse laser deposited nanostructured ZnO thin films: a review.

This review summarizes the work principles of pulse laser deposition (PLD) apparatus, physical processes like ablation, and plasma plume formation accompanying the deposition of un-doped ZnO from target to substrate material. Various modes of deposition and factors influencing the properties of thin films such as substrate temperature, background gas pressure, laser energy density (laser fluence), target to substrate distance, repetition rate, oxygen partial pressure in deposition chamber, deposition time and post growth annealing which control deposition parameters such as adsorption, desorption, surface diffusion, nucleation, and crystallization/re-crystallization are also discussed in this review. Moreover, various film properties such as morphology, roughness of the film surface, film thickness, grain size, optical transmittance, sensitivity, electrical conductivity, uniformity and electrical resistivity of the deposited ZnO thin films have also been enumerated in the present review.

Target charging in short-pulse-laser-plasma experiments.

Interaction of high-intensity laser pulses with solid targets results in generation of large quantities of energetic electrons that are the origin of various effects such as intense x-ray emission, ion acceleration, and so on. Some of these electrons are escaping the target, leaving behind a significant positive electric charge and creating a strong electromagnetic pulse long after the end of the laser pulse. We propose here a detailed model of the target electric polarization induced by a short and intense laser pulse and an escaping electron bunch. A specially designed experiment provides direct measurements of the target polarization and the discharge current in the function of the laser energy, pulse duration, and target size. Large-scale numerical simulations describe the energetic electron generation and their emission from the target. The model, experiment, and numerical simulations demonstrate that the hot-electron ejection may continue long after the laser pulse ends, enhancing significantly the polarization charge.

Effects of front-surface target structures on properties of relativistic laser-plasma electrons.

We report the results of a study of the role of prescribed geometrical structures on the front of a target in determining the energy and spatial distribution of relativistic laser-plasma electrons. Our three-dimensional particle-in-cell simulation studies apply to short-pulse, high-intensity laser pulses, and indicate that a judicious choice of target front-surface geometry provides the realistic possibility of greatly enhancing the yield of high-energy electrons while simultaneously confining the emission to narrow (<5°) angular cones.

Laser-driven three-stage heavy-ion acceleration from relativistic laser-plasma interaction.

A three-stage heavy ion acceleration scheme for generation of high-energy quasimonoenergetic heavy ion beams is investigated using two-dimensional particle-in-cell simulation and analytical modeling. The scheme is based on the interaction of an intense linearly polarized laser pulse with a compound two-layer target (a front heavy ion layer + a second light ion layer). We identify that, under appropriate conditions, the heavy ions preaccelerated by a two-stage acceleration process in the front layer can be injected into the light ion shock wave in the second layer for a further third-stage acceleration. These injected heavy ions are not influenced by the screening effect from the light ions, and an isolated high-energy heavy ion beam with relatively low-energy spread is thus formed. Two-dimensional particle-in-cell simulations show that ∼100MeV/u quasimonoenergetic Fe24+ beams can be obtained by linearly polarized laser pulses at intensities of 1.1×1021W/cm2.

Strong field-induced frequency conversion of laser radiation in plasma plumes: recent achievements.

New findings in plasma harmonics studies using strong laser fields are reviewed. We discuss recent achievements in the growth of the efficiency of coherent extreme ultraviolet (XUV) radiation sources based on frequency conversion of the ultrashort pulses in the laser-produced plasmas, which allowed for the spectral and structural studies of matter through the high-order harmonic generation (HHG) spectroscopy. These studies showed that plasma HHG can open new opportunities in many unexpected areas of laser-matter interaction. Besides being considered as an alternative method for generation of coherent XUV radiation, it can be used as a powerful tool for various spectroscopic and analytical applications.

Effect of an electric field on air filament decay at the trail of an intense femtosecond laser pulse.

Air plasma density decay in a filament produced by an intense femtosecond laser pulse in an external electric field was investigated experimentally and theoretically. It was demonstrated by means of the terahertz scattering technique that the rate of plasma decay decreases with increasing electric field. At the electric field of 7 kV/cm the lifetime of plasma with the density above 10(16) cm(-3) was prolonged from 0.5 ns to 1 ns. Numerical simulation of electron density decay and electron temperature evolution was performed, taking into consideration dissociative and three-body electron-ion recombination as well as formation of complex positive ions. The simulation showed that under the electric field the electron temperature evolves nonmonotonically and passes through a minimum due to varying contribution of electron-ion collisions to electron heating in the field. The rate of three-body electron recombination with O(2)(+) ions of 2×10(-19)(300/T(e))(9/2) cm(6)/s was found from the experimental measurements at electron temperatures in the 0.25-0.4 eV range and electron densities in the 10(15)-10(17) cm(-3) range.

Nonequilibrium electron dynamics in materials driven by high-intensity x-ray pulses.

We calculated the evolution of the electron system in solid-density matter irradiated by high-intensity x-ray pulses between 2 and 8 keV using molecular dynamics. For pulses shorter than 40 fs, the kinetic energy distribution of the electrons is highly nonthermal during and right after the pulse, and a large fraction of the absorbed x-ray energy resides with the fast photoelectrons which equilibrate on the timescale of the pulse length. The average ionization and electron temperature of the bulk of the electrons are significantly lower than their equilibrium values.

Pattern dynamics and filamentation of femtosecond terawatt laser pulses in air including the higher-order Kerr effects.

Plasma defocusing and higher-order Kerr effects on multiple filamentation and pattern formation of ultrashort laser pulse propagation in air are investigated. Linear analyses and numerical results show that these two saturable nonlinear effects can destroy the coherent evolution of the laser field, and small-scale spatial turbulent structures rapidly appear. For the two-dimensional case, numerical simulations show that blow-up-like solutions, spatial chaos, and pseudorecurrence can appear at higher laser intensities if only plasma defocusing is included. These complex patterns result from the stochastic evolution of the higher- or shorter-wavelength modes of the laser light spectrum. From the viewpoint of nonlinear dynamics, filamentation can be attributed to the modulational instability of these spatial incoherent localized structures. Furthermore, filament patterns associated with multiphoton ionization of the air molecules with and without higher-order Kerr effects are compared.

Nonlinear saturation of wave packets excited by low-energy electron horseshoe distributions.

Horseshoe distributions are shell-like particle distributions that can arise in space and laboratory plasmas when particle beams propagate into increasing magnetic fields. The present paper studies the stability and the dynamics of wave packets interacting resonantly with electrons presenting low-energy horseshoe or shell-type velocity distributions in a magnetized plasma. The linear instability growth rates are determined as a function of the ratio of the plasma to the cyclotron frequencies, of the velocity and the opening angle of the horseshoe, and of the relative thickness of the shell. The nonlinear stage of the instability is investigated numerically using a symplectic code based on a three-dimensional Hamiltonian model. Simulation results show that the dynamics of the system is mainly governed by wave-particle interactions at Landau and normal cyclotron resonances and that the high-order normal cyclotron resonances play an essential role. Specific features of the dynamics of particles interacting simultaneously with two or more waves at resonances of different natures and orders are discussed, showing that such complex processes determine the main characteristics of the wave spectrum's evolution. Simulations with wave packets presenting quasicontinuous spectra provide a full picture of the relaxation of the horseshoe distribution, revealing two main phases of the evolution: an initial stage of wave energy growth, characterized by a fast filling of the shell, and a second phase of slow damping of the wave energy, accompanied by final adjustments of the electron distribution. The influence of the density inhomogeneity along the horseshoe on the wave-particle dynamics is also discussed.

From short pulses to short breaks: exotic plasma bullets via residual electron control.

Plasma plumes with exotically segmented channel structure and plasma bullet propagation are produced in atmospheric plasma jets. This is achieved by tailoring interruptions of a continuous DC power supply over the time scales of lifetimes of residual electrons produced by the preceding discharge phase. These phenomena are explained by studying the plasma dynamics using nanosecond-precision imaging. One of the plumes is produced using 2 - 10 µs interruptions in the 8 kV DC voltage and features a still bright channel from which a propagating bullet detaches. A shorter interruption of 900 ns produces a plume with the additional long conducting dark channel between the jet nozzle and the bright area. The bullet size, formation dynamics, and propagation speed and distance can be effectively controlled. This may lead to micrometer- and nanosecond-precision delivery of quantized plasma bits, warranted for next-generation health, materials, and device technologies.

Spatially resolved analysis of Kα x-ray emission from plasmas induced by a femtosecond weakly relativistic laser pulse at various polarizations.

Spatially resolved K-shell spectroscopy is used here to investigate the interaction of an ultrashort laser pulse (λ=800 nm, τ=40 fs) with a Ti foil under intense irradiation (Iλ(2)=2×10(18)Wµm(2)cm(-2)) and the following fast electron generation and transport into the target. The effect of laser pulse polarization (p, s, and circular) on the Kα yield and line shape is probed. The radial structure of intensity and width of the lines, obtained by a discretized Abel deconvolution algorithm, suggests an annular distribution of both the hot electron propagation into the target and the target temperature. An accurate modeling of Kα line shapes was performed, revealing temperature gradients, going from a few eV up to 15-20 eV, depending on the pulse polarization. Results are discussed in terms of mechanisms of hot electron generation and of their transport through the preplasma in front of the target.

Optimization of the neutron yield in fusion plasmas produced by Coulomb explosions of deuterium clusters irradiated by a petawatt laser.

The kinetic energy of hot (multi-keV) ions from the laser-driven Coulomb explosion of deuterium clusters and the resulting fusion yield in plasmas formed from these exploding clusters has been investigated under a variety of conditions using the Texas Petawatt laser. An optimum laser intensity was found for producing neutrons in these cluster fusion plasmas with corresponding average ion energies of 14 keV. The substantial volume (1-10 mm(3)) of the laser-cluster interaction produced by the petawatt peak power laser pulse led to a fusion yield of 1.6×10(7) neutrons in a single shot with a 120 J, 170 fs laser pulse. Possible effects of prepulses are discussed.