Absolute single and multiple charge exchange cross sections for highly charged C, O, and Ne ions on H2O, CO, and CO2

Reported herein are measured absolute single, double, and triple charge exchange (CE) cross sections for the highly charged ions (HCIs) Cq+ (q=5,6), Oq+ (q=6,7,8), and Neq+ (q=7,8) colliding with the molecular species H2O, CO, and CO2. Present data can be applied to interpreting observations of x-ray emissions from comets as they interact with the solar wind. As such, the ion impact energies of 7.0q keV (1.62–3.06 keV/amu) are representative of the fast solar wind, and data at 1.5q keV for O6+ (0.56 keV/amu) on CO and CO2 and 3.5q keV for O5+ (1.09 keV/amu) on CO provide checks of the energy dependence of the cross sections at intermediate and typical slow solar wind velocities. The HCIs are generated within a 14 GHz electron cyclotron resonance ion source. Absolute CE measurements are made using a retarding potential energy analyzer, with measurement of the target gas cell pressure and incident and final ion currents. Trends in the cross sections are discussed in light of the classical overbarrier model (OBM), extended OBM, and with recent results of the classical trajectory Monte Carlo theory.

Plasma Ionization in Low-Pressure Radio-Frequency Discharges—Part II: Particle-in-Cell Simulation

Plasma ionization in the low-pressure operation regime ( $«$ 5 Pa) of RF capacitively coupled plasmas (CCPs) is governed by a complex interplay of various mechanisms, such as field reversal, sheath expansion, and wave–particle interactions. In a previous paper, it was shown that experimental observations in a hydrogen CCP operated at 13.56 MHz are qualitatively well described in a 1-D symmetrical particle-in-cell (PIC) simulation. In this paper, a spherical asymmetrical PIC simulation that is closer to the conditions of the highly asymmetrical experimental device is used to simulate a low-pressure neon CCP operated at 2 MHz. The results show a similar behavior, with pronounced ionization through field reversal, sheath expansion, and wave–particle interactions, and can be exploited for more accurate quantitative comparisons with experimental observations.

High-pressure deformation mechanism in scolecite: A combined computational-experimental study

Pressure-induced structural modifications in scolecite were studied by means of in situ synchrotron X-ray powder diffraction and density functional computations. The experimental cell parameters were refined up to 8.5 GPa. Discontinuities in the slope of the unit-cell parameters vs. pressure dependence were observed; as a consequence, an increase in the slope of the linear pressure-volume dependence is observed at about 6 GPa, suggesting an enhanced compressibility at higher pressures. Weakening and broadening of the diffraction peaks reveals increasing structural disorder with pressure, preventing refinement of the lattice parameters above 8.5 GPa. Diffraction patterns collected during decompression show that the disorder is irreversible. Atomic coordinates within unit cells of different dimensions were determined by means of Car-Parrinello simulations. The discontinuous rise in compressibility at about 6 GPa is reproduced by the computation, allowing us to attribute it to re-organization of the hydrogen bonding network, with the formation of water dimers. Moreover we found that, with increasing pressure, the tetrahedral chains parallel to c rotate along their elongation axis and display an increasing twisting along a direction perpendicular to c. At the same time, we observed the compression of the channels. We discuss the modification of the Ca polyhedra under pressure, and the increase in coordination number (from 4 to 5) of one of the two Al atoms, resulting from the approach of a water molecule. We speculate that this last transformation triggers the irreversible disordering of the system.

One-dimensional particle simulation of the filamentation instability: Electrostatic field driven by the magnetic pressure gradient force

Two counterpropagating cool and equally dense electron beams are modeled with particle-in-cell simulations. The electron beam filamentation instability is examined in one spatial dimension, which is an approximation for a quasiplanar filament boundary. It is confirmed that the force on the electrons imposed by the electrostatic field, which develops during the nonlinear stage of the instability, oscillates around a mean value that equals the magnetic pressure gradient force. The forces acting on the electrons due to the electrostatic and the magnetic field have a similar strength. The electrostatic field reduces the confining force close to the stable equilibrium of each filament and increases it farther away, limiting the peak density. The confining time-averaged total potential permits an overlap of current filaments with an opposite flow direction.

Radiation-Pressure Acceleration of Ion Beams from Nanofoil Targets: The Leaky Light-Sail Regime

A new ion radiation-pressure acceleration regime, the "leaky light sail," is proposed which uses sub-skin-depth nanometer foils irradiated by circularly polarized laser pulses. In the regime, the foil is partially transparent, continuously leaking electrons out along with the transmitted laser field. This feature can be exploited by a multispecies nanofoil configuration to stabilize the acceleration of the light ion component, supplementing the latter with an excess of electrons leaked from those associated with the heavy ions to avoid Coulomb explosion. It is shown by 2D particle-in-cell simulations that a monoenergetic proton beam with energy 18 MeV is produced by circularly polarized lasers at intensities of just 10(19) W/cm(2). 100 MeV proton beams are obtained by increasing the intensities to 2 x 10(20) W/cm(2).

Simulation of a collisionless planar electrostatic shock in a proton-electron plasma with a strong initial thermal pressure change

The localized deposition of the energy of a laser pulse, as it ablates a solid target, introduces high thermal pressure gradients in the plasma. The thermal expansion of this laser-heated plasma into the ambient medium (ionized residual gas) triggers the formation of non-linear structures in the collisionless plasma. Here an electron-proton plasma is modelled with a particle-in-cell simulation to reproduce aspects of this plasma expansion. A jump is introduced in the thermal pressure of the plasma, across which the otherwise spatially uniform temperature and density change by a factor of 100. The electrons from the hot plasma expand into the cold one and the charge imbalance drags a beam of cold electrons into the hot plasma. This double layer reduces the electron temperature gradient. The presence of the low-pressure plasma modifies the proton dynamics compared with the plasma expansion into a vacuum. The jump in the thermal pressure develops into a primary shock. The fast protons, which move from the hot into the cold plasma in the form of a beam, give rise to the formation of phase space holes in the electron and proton distributions. The proton phase space holes develop into a secondary shock that thermalizes the beam.

Stable ion radiation pressure acceleration with intense laser pulses

We have demonstrated the promising radiation pressure acceleration (RPA) mechanism of laser-driven ion acceleration at currently achievable laser and target parameters through a large number of two-dimensional particle-in-cell simulations and experiments. High-density monoenergetic ion beams with unprecedented qualities such as narrow-peaked spectrum, lower-divergence and faster energy-scaling are obtained, compared with the conventional target normal sheath acceleration. The key condition for stable RPA from thin foils by intense circularly polarized lasers has been identified, under which the stable RPA regime can be extended from ultrahigh intensities > 10(22) W cm(-2) to a currently accessible range 10(20)-10(21) W cm(-2). The dependences of the RPA mechanism on laser polarization, intensity and on the target composition and areal density have been studied.

Dominance of Radiation Pressure in Ion Acceleration with Linearly Polarized Pulses at Intensities of 10(21) W cm(-2)

A novel regime is proposed where, by employing linearly polarized laser pulses at intensities 10(21) W cm(-2) (2 orders of magnitude lower than discussed in previous work [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)]), ions are dominantly accelerated from ultrathin foils by the radiation pressure and have monoenergetic spectra. In this regime, ions accelerated from the hole-boring process quickly catch up with the ions accelerated by target normal sheath acceleration, and they then join in a single bunch, undergoing a hybrid light-sail-target normal sheath acceleration. Under an appropriate coupling condition between foil thickness, laser intensity, and pulse duration, laser radiation pressure can be dominant in this hybrid acceleration. Two-dimensional particle-in-cell simulations show that 1.26 GeV quasimonoenergetic C6+ beams are obtained by linearly polarized laser pulses at intensities of 10(21) W cm(-2).

Eradication of Pseudomonas aeruginosa Biofilms by Atmospheric Pressure Non-Thermal Plasma

Bacteria exist, in most environments, as complex, organised communities of sessile cells embedded within a matrix of self-produced, hydrated extracellular polymeric substances known as biofilms. Bacterial biofilms represent a ubiquitous and predominant cause of both chronic infections and infections associated with the use of indwelling medical devices such as catheters and prostheses. Such infections typically exhibit significantly enhanced tolerance to antimicrobial, biocidal and immunological challenge. This renders them difficult, sometimes impossible, to treat using conventional chemotherapeutic agents. Effective alternative approaches for prevention and eradication of biofilm associated chronic and device-associated infections are therefore urgently required. Atmospheric pressure non-thermal plasmas are gaining increasing attention as a potential approach for the eradication and control of bacterial infection and contamination. To date, however, the majority of studies have been conducted with reference to planktonic bacteria and rather less attention has been directed towards bacteria in the biofilm mode of growth. In this study, the activity of a kilohertz-driven atmospheric pressure non-thermal plasma jet, operated in a helium oxygen mixture, against Pseudomonas aeruginosa in vitro biofilms was evaluated. Pseudomonas aeruginosa biofilms exhibit marked susceptibility to exposure of the plasma jet effluent, following even relatively short (~10's s) exposure times. Manipulation of plasma operating conditions, for example, plasma operating frequency, had a significant effect on the bacterial inactivation rate. Survival curves exhibit a rapid decline in the number of surviving cells in the first 60 seconds followed by slower rate of cell number reduction. Excellent anti-biofilm activity of the plasma jet was also demonstrated by both confocal scanning laser microscopy and metabolism of the tetrazolium salt, XTT, a measure of bactericidal activity.

Ion Acceleration in Multispecies Targets Driven by Intense Laser Radiation Pressure

The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to 3 X 10(20) W cm(-2). The ion spectra show the appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the 5-10 MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than 100 MeV/nucleon are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.

Radiation-Pressure Acceleration of Ion Beams Driven by Circularly Polarized Laser Pulses

We present experimental studies on ion acceleration from ultrathin diamondlike carbon foils irradiated by ultrahigh contrast laser pulses of energy 0.7 J focused to peak intensities of 5×1019  W/cm2. A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell simulations reveal that those C6+ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

Rayleigh-Taylor Instability of an Ultrathin Foil Accelerated by the Radiation Pressure of an Intense Laser

We report experimental evidence for a Rayleigh-Taylor-like instability driven by radiation pressure of an ultraintense (1021W/cm2) laser pulse. The instability is witnessed by the highly modulated profile of the accelerated proton beam produced when the laser irradiates a 5 nm diamondlike carbon (90% C, 10% H) target. Clear anticorrelation between bubblelike modulations of the proton beam and transmitted laser profile further demonstrate the role of the radiation pressure in modulating the foil. Measurements of the modulation wavelength, and of the acceleration from Doppler-broadening of back-reflected light, agree quantitatively with particle-in-cell simulations performed for our experimental parameters and which confirm the existence of this instability. © 2012 American Physical Society.

Eradication of marine biofilms by atmospheric pressure non-thermal plasma: A potential approach to control biofouling?

Although the antimicrobial activity of atmospheric pressure non-thermal plasmas, including its capacity to eradicate microbial biofilms, has been gaining an ever increasing interest for different medical applications, its potential utilisation in the control of biofouling and biodeterioration has, to date, received no attention. In this study, the ability of atmospheric pressure plasma to eradicate biofilms of four biofouling bacterial species, frequently encountered in marine environments, was investigated. Biofilms were grown on both polystyrene and stainless steel surfaces before being exposed to the plasma source. Viability and biomass of biofilms were evaluated using colony count method and differential Live/Dead fluorescence staining followed by confocal laser scanning microscopy. Rapid and complete eradication of all biofilms under study was achieved after plasma exposures ranging from 60 to 120 s. Confocal microscopy examination showed that plasma treatment has mediated not only cell killing but also varying degrees of physical removal of biofilms. Further investigation and tailored development of atmospheric pressure non-thermal plasma sources for this particular application could provide an additional powerful and effective weapon in the current anti-biofouling armamentarium.

Microscopic Examination of Biofilm Destruction by Atmospheric Pressure Plasma

Objective: The aim of this study is to examine microscopically the destruction of bacterial biofilms mediated by atmospheric pressure non-thermal plasma (APNTP) at cellular level as well as at the level of biofilm structure as a whole. Methods: 3-day old bacterial biofilms were grown on polycarbonate coupons in a dual channel flow cell and were treated with an in-housed designed atmospheric pressure non-thermal plasma jet for up to 4 minutes of exposure before being examined by both confocal laser scanning microscopy (CLSM), preceded by Live/Dead bacterial viability staining, and scanning electron microscopy (SEM). Results: Differential live/dead staining followed by confocal microscopy examination revealed that biofilm eradication by APNTP was mediated by varying levels of both cell killing and physical removal. Relative extent of each mechanism was dependent on plasma operating conditions, bacterial species, growth conditions and biofilm thickness. On the other hand, SEM examination of plasma-exposed biofilms revealed a series of morphological changes exhibited by biofilm cells ranging from increased roughness of cell surface to complete cell lysis. Conclusions: Interesting mechanistic insights have been revealed by microscopic examination of plasma-treated bacterial biofilms that, when coupled with more specific biochemical studies, will not only contribute significantly to our understanding of the mechanism of plasma mediated biofilm destruction but also will help in better application-guided development of this novel anti-biofilm approach.