Laser-induced plasma formation in water with up to 400 mJ double-pulse LIBS

Double-pulse LIBS is a promising technique for deep-sea applications.LIBS measurements in shallow water with up to 400 mJ each pulse were done to select laser parameters which promote optimized spectral line emission from plasma even at elevated pressures,where line broadening until loss of most of the spectral information can occur.Optical emission spectroscopy,using a Czerny-Turner spectrometer,has been applied to investigate the dependence of the emitted radiation on laser parameters and hydrostatic pressure.It has been found,that higher laser pulse energies,especially with short pulse delay as required in high water pressure,can also have an adverse effect on the measured spectrum.

Derivation of Moment Equations for the Theoretical Description of Electrons in Nonthermal Plasmas

The derivation of moment equations for the theoretical description of electrons is of interest for modelling of gas discharge plasmas and semiconductor devices. Usually, certain artificial closure assumptions are applied in order to derive a closed system of moment equations from the electron Boltzmann equation. Here, a novel four-moment model for the description of electrons in nonthermal plasmas is derived by an expansion of the electron velocity distribution function in Legendre polynomials. The proposed system of partial differential equations is consistently closed by definition of transport coefficients that are determined by solving the electron Boltzmann equation and are then used in the fluid calculations as function of the mean electron energy. It is shown that the four-moment model can be simplified to a new drift-diffusion approximation for electrons without loss of accuracy, if the characteristic frequency of the electric field alteration in the discharge is small in comparison with the momentum dissipation frequency of the electrons. Results obtained by the proposed fluid models are compared to those of a conventional drift-diffusion approximation as well as to kinetic results using the example of low pressure argon plasmas. It is shown that the results provided by the new approaches are in good agreement with kinetic results and strongly improve the accuracy of fluid descriptions of gas discharges.

Analysis of Numerical Simulation Results of LIPS-200 Lifetime Experiments

Accelerator grid structural and electron backstreaming failures are the most important factors affecting the ion thruster＇s lifetime.During the thruster＇s operation,Charge Exchange Xenon（CEX） ions are generated from collisions between plasma and neutral atoms.Those CEX ions grid＇s barrel and wall frequently,which cause the failures of the grid system.In order to validate whether the 20 cm Lanzhou Ion Propulsion System（LIPS-200） satisfies China＇s communication satellite platform＇s application requirement for North-South Station Keeping（NSSK）,this study analyzed the measured depth of the pit/groove on the accelerator grid＇s wall and aperture diameter＇s variation and estimated the operating lifetime of the ion thruster.Different from the previous method,in this paper,the experimental results after the 5500 h of accumulated operation of the LIPS-200 ion thruster are presented firstly.Then,based on these results,theoretical analysis and numerical calculations were firstly performed to predict the on-orbit lifetime of LIPS-200.The results obtained were more accurate to calculate the reliability and analyze the failure modes of the ion thruster.The results indicated that the predicted lifetime of LIPS-200＇s was about 13218.1 h which could satisfy the required lifetime requirement of 11000 h very well.

Characterization of different plasma-treated cobalt oxide catalysts for oxygen reduction reaction in alkaline media

Plasma-synthesized cobalt oxide supported on carbon has been analyzed for its use for electrocatalytic oxygen reduction reaction(ORR) in alkaline anion exchange membrane fuel cells(AEMFC). This work presents the ORR activity in 0.1 mol L^(-1) KOH and 0.1 mol L^(-1) K_2CO_3at25 °C. Cyclic voltammetry(CV) was used to determine the potentials at which the ORR occurs and to evaluate the stability of catalyst. Moreover, a rotating ring-disk electrode(RRDE) was used to investigate the activity of the catalysts and the formation of the by-product hydroperoxide anion(HO2-) as well as to identify the preferred pathway of the ORR. Calculated kinetic parameters for the ORR for the cobalt catalysts are shown in this work together with a comparison to a commercial platinum catalyst. However, the cobalt oxide produced more by-products which could lead to damage of the membrane in a fuel cell through a radical attack of the polymer backbone.

Kinetics of Hydrogen Molecules in MAGNUM-PSI

Results from simulations of plasma and neutrals under conditions predictively characterizing the detached plasma regime in the linear machine MAGNUM-PSI are presented. The relaxation of the vibrationally excited hydrogen molecules is investigated in order to establish a relation between their relaxation and dwell times, and the role of the varions mechanisms of the molecular vibrational kinetics. Tile results obtained show that the individual vibrational states have to be inclllded in the transport code for neutrals as distinct species, since the relaxation time of tile vibrational states is sufficiently longer than the typical dwell time of hydrogen molecules in the detached plasma region. The parameters of plasma and neutrals are affected by the transport of the vibrationally excited hydrogen lnolecnles. Furthermore. the rate of molecular reconlbination is overestimated by a factor of - 5 provided that the transport of ilydrogen molecules only in their ground vibrational state is considered. The role of the various processes of vibrational kinetics is studied. The vibrational excitation through singlet electronic states ires a strong influence on the molecular densities for levels with vibrational quantum numbers v≥ 5. Vibration-vibration （V-V） collisions between vibrationally excited hydrogen molecules and vibration-translation （V-T） collisions between vibrationally excited hydrogen molecules and ground state molecules and atoms are of nlinor importance in MAGNUM-PSI.

Non-equiliburum Modelling of the Electrical Characteristics of a Free-burning Arc

An arc model considering deviations from thermodynamic and chemical equilibrium has been developed in order to achieve a better understanding of the arc plasma close to material surfaces.The model is based on unified consideration of the thermionic tungsten cathode,the arc region and the flat anode made of copper.The heat transfer within the electrodes is coupled with the plasma through the energy fluxes onto the electrode boundaries.Electrical characteristics of an 8 mm long free-burning arc are presented along with findings from spectroscopic measurements of the plasma emission in atmospheric pressure argon.The arc current varied from 60 A up to 200 A,and the gas flow rate was set at 12 L/min(at atmospheric pressure,room temperature).

Formation mechanism of streamer discharges in liquids:a review

Streamer discharges in liquids have received lots of attention with respect to their considerable applications invarious disciplines. Much effort has been spent to understand the basis of streamer ignition and propagation in dielectricliquids, but a comprehensive mechanism is far from conclusive. In this review, based on an introduction to the streamerphysics and experimentally observed characteristics in various liquids, the authors emphasise on the current status ofstreamer discharge ignition mechanisms and present their understanding for each theory.

First‐mode of negative streamers:Inception at liquid/solid interfaces

An experimental study of the inception of the first‐mode negative streamer at liquid/solid interfaces is presented in this article.The study is performed with a point‐plane configuration under square high voltage pulses.The electrode configuration is immersed in mineral oil and the liquid/solid interface is assembled in contact with the point electrode or in its vicinity.Four polymers and two impregnated papers have been tested as solids of the liquid/solid interface.Thus,it is possible to compare the influence of different parameter of the solid and the interface on the streamer inception.For example:Permittivity,solid surface roughness,chemical composition,etc.It has been observed that streamer inception voltages at interfaces with solids of higher permittivity to that of the mineral oil are statistically similar.Additionally,streamer inception voltages of streamer initiated free in the oil(no liquid/solid interface)are similar to that of the inception voltage of cases with solids with high permittivity.In contrast,the inception voltage of streamers initiated at permittivity matched interfaces are shown to be highest of the cases.The streamer inception voltage is also studied for different distances be-tween the liquid/solid interface and the point electrode with a permittivity matched interface.The results show a dependency of the inception voltage and the distance be-tween the point electrode and the interface.Finally,an analysis of the observation is performed to show that the Townsend‐Meek criterion cannot predict the obtained results.

Acquired cancer tyrosine kinase inhibitor resistance: ROS as critical determinants

The ability of a minority of cancer cells lineages to withstand drug-induced toxicity and re-enter cell cycling in the presence of the drug has been observed for long,but the underlying mechanisms remained elusive.New work published in Nature by Oren and colleagues used barcode lentiviral library labeling to prominently discern antioxidant profiles,among others,being the primary underlying cellular program that governs proliferation under tyrosine kinase inhibitor(TKI)drug pressure.^(1)