Residual electron momentum and energy in a gas ionized by a short high-power laser pulse

A study is made of the nonadiabatic dynamics of photoelectrons produced during interaction of an elliptically polarized, high-power laser pulse with a gas. Expressions for the so-called residual momentum and energy of the electrons (i.e., the mean electron momentum and energy after the passage of the pulse through the gas) are derived. The residual electron momentum and energy are investigated analytically as functions of the gas and laser parameters. A relationship is established between the residual energy and the electron temperature tensor.     

The thermodynamic response of soft biological tissues to pulsed ultraviolet laser irradiation.

The physical mechanisms that enable short pulses of high-intensity ultraviolet laser radiation to remove tissue, in a process known as laser ablation, remain obscure. The thermodynamic response of biological tissue to pulsed laser irradiation was investigated by measuring and subsequently analyzing the stress transients generated by pulsed argon fluorine (ArF, lambda = 193 nm) and krypton fluorine (KrF, lambda = 248 nm) excimer laser irradiation of porcine dermis using thin-film piezoelectric transducers. For radiant exposures that do not cause material removal, the stress transients are consistent with rapid thermal expansion of the tissue. At the threshold radiant exposure for ablation, the peak stress amplitude generated by 248 nm irradiation is more than an order of magnitude larger than that produced by 193 nm irradiation. For radiant exposures where material removal is achieved, the temporal structure of the stress transient indicates that the onset of material removal occurs during irradiation. In this regime, the variation of the peak compressive stress with radiant exposure is consistent with laser-induced rapid surface vaporization. For 193 nm irradiation, ionization of the ablated material occurs at even greater radiant exposures and is accompanied by a change in the variation of peak stress with radiant exposure consistent with a plasma-mediated ablation process. These results suggest that absorption of ultraviolet laser radiation by the extracellular matrix of tissue leads to decomposition of tissue on the time scale of the laser pulse. The difference in volumetric energy density at ablation threshold between the two wavelengths indicates that the larger stresses generated by 248 nm irradiation may facilitate the onset of material removal. However, once material removal is achieved, the stress measurements demonstrate that energy not directly responsible for target decomposition contributes to increasing the specific energy of the plume (and plasma, when present), which drives the gas dynamic expansion of ablated material. This provides direct evidence that ultraviolet laser ablation of soft biological tissues is a surface-mediated process and not explosive in nature.     

Generation of a wakefield during gas ionization

One-dimensional equations are derived that describe the hydrodynamic and electrodynamic properties of a plasma created through gas ionization by a short intense laser pulse. Different approaches (in particular, the particle-in-cell method) are used to show that, with ionization processes included, the excitation of a wakefield by an intense laser pulse can be described by the method of slowly varying amplitudes. It is shown that ionization processes enhance the wakefield excited by a moderate-intensity laser by about 10% in the case of a linearly polarized laser and by about 50% in the case of a circularly polarized laser. Ionization processes in light gases irradiated with high-intensity laser pulses have essentially no effect on the wakefield during the resonant excitation of a plasma wave by the ponderomotive force and play a governing role far from the resonance.     

ECR heating power modulation as a means to ease the overcoming of the radiation barrier in fusion devices

A method is proposed to ease the overcoming of the impurity radiation barrier during current drive in tokamaks, as well as in alternative fusion and plasmochemical systems with ECR plasma heating. The method is based on the fact that the dependence of the ionization rate on the electron temperature is strongly nonlinear and the dependence of the recombination rate on the latter is weaker. The result is that, during temperature oscillations, the effective temperature for ionization-recombination processes is higher than that in a steady state, so the ionization equilibrium is shifted and strongly emitting ions are stripped more rapidly. Thereby, ECR plasma heating in the initial discharge stage can be made more efficient by modulating the heating power at a low frequency. The evolution of the electron temperature in a homogeneous hydrogen plasma with a carbon impurity and in small ISX-scale tokamaks is simulated numerically, as well as the evolution of the electron and ion temperatures and of the current during discharge startup in the ITER device. Numerical simulations of the effect of modulation of the ECR heating power on the rate of heating of nitrogen, oxygen, and argon plasmas were also carried out. The assumption of coronal equilibrium is not used. It is shown that the low-frequency modulation of the heating power can substantially ease the overcoming of the radiation barrier.     

Cumulative gas-compression configuration intermediate between spherical and cylindrical

A two-dimensional axisymmetric configuration of a cumulative gas compression that can be applied to inertial microfusion is proposed. Both adiabatic gas compression and compression with energy losses are considered. The limiting gas temperature that can be attained during shock stopping of the cavity wall is estimated. It is pointed out that the energy losses during the compression cannot be ignored when considering the cumulative effects occurring in microscopic regions of a gas target of any configuration under the action of a loading pulse of any shape.     

Physical model of the formation of a low-temperature dense plasma during the irradiation of a target by a picosecond laser pulse

Results are presented from numerical simulations of the breakdown of a dense noble gas by the electrons of a boundary layer that forms during the irradiation of a metal target by a high-power picosecond laser pulse. It is shown that, when the electric field of the boundary layer is taken into account, the density of the seed electrons near the target surface increases substantially, so that the ionization process occurs much faster. The dependence of the time of the onset of breakdown on the electric field of the incident wave and on the concentration of gas atoms is calculated.     

A molecular dynamics investigation into the mechanisms of material removal and subsurface damage of nanoscale high speed laser-assisted machining

Molecular dynamics is employed to study the mechanism of material removal and subsurface damage of monocrystalline silicon when it is under a nanoscale high-speed laser-assisted grinding of a diamond tip. Laser-assisted machining (LAM) is that the workpiece is locally heated by an intense laser beam before material removal. The effects of laser moving speed, laser pulse intensity and laser spot radius are thoroughly investigated in terms of atomic trajectories, phase transformation, temperature distribution, grinding temperature, grinding force and friction coefficient. The investigation shows that a higher laser moving speed reduces the subsurface damage and improves the material remove rate because of fewer atoms with five and six coordination atoms and more chips. Besides, both tangential grinding force (Fx) and normal grinding force (Fy) decrease as the laser moving speed increases. The distribution of high-temperature zone strongly depends upon the effect of laser pulse intensity and laser spot radius. Larger laser pulse intensity can make the material more fully softened before being removed. Moreover, as the laser pulse intensity becomes larger, the friction coefficients became smaller, the material remove rate improves and the depth of grinding increases. However, larger laser pulse intensity may result in a larger thermal deformation of workpiece. A larger laser spot radius reduces the grinding depth but increases the width of laser irradiation zone on machined surface. Thus, a suitable laser spot radius can improve the material removal rate. These results indicate that it is possible to control and adjust the laser parameters according to laser moving speed, laser pulse intensity and laser spot radius, and it provides a potential technology to improve a surface integrity and a smoothness of ground surface.     

Exploring gas permeability of cellular membranes and membrane channels with molecular dynamics

Aquaporins are a family of membrane proteins specialized in rapid water conduction across biological membranes. Whether these channels also conduct gas molecules and the physiological significance of this potential function have not been well understood. Here we report 140 ns of molecular dynamics simulations of membrane-embedded AQP1 and of a pure POPE bilayer addressing these questions. The permeability of AQP1 to two types of gas molecules, O2 and CO2, was investigated using two complementary methods, namely, explicit gas diffusion simulation and implicit ligand sampling. The simulations show that the central (tetrameric) pore of AQP1 can be readily used by either gas molecule to permeate the channel. The two approaches produced similar free energy profiles associated with gas permeation through the central pore: a -0.4 to -1.7 kcal/mol energy well in the middle, and a 3.6-4.6 kcal/mol energy barrier in the periplasmic vestibule. The barrier appears to be mainly due to a dense cluster of water molecules anchored in the periplasmic mouth of the central pore by four aspartate residues. Water pores show a very low permeability to O2, but may contribute to the overall permeation of CO2 due to its more hydrophilic nature. Although the central pore of AQP1 is found to be gas permeable, the pure POPE bilayer provides a much larger cross-sectional area, thus exhibiting a much lower free energy barrier for CO2 and O2 permeation. As such, gas conduction through AQP1 may only be physiologically relevant either in membranes of low gas permeability, or in cells where a major fraction of the cellular membrane is occupied by AQPs.     

Dynamics of the ion energy spectrum in EUV-induced hydrogen plasma

The dynamics of the ion energy spectrum in low-pressure (10–100 Pa) hydrogen plasma induced by extreme ultraviolet (EUV) pulses in the wavelength range of 10–20 nm was studied experimentally. The plasma was generated under cathode irradiation due to both direct gas ionization by EUV photons and impact ionization by high-energy secondary electrons. The dynamics of the spectra of ions incident on the cathode was measured using a time-resolved retarding field energy analyzer. It is shown that the ion spectrum dynamics is completely determined by the time evolution of the cathode sheath. At low gas pressures (<20 Pa), the ion spectrum at early moments after the EUV pulse has a peaked shape, typical of a collisionless plasma sheath, and is mainly determined by the cathode voltage. As the pressure increases, the peak broadens and low energy ions appear in the spectrum due to ion collisions in the cathode sheath. An increase in the role of collisions with decreasing plasma density is also observed in the time evolution of ion spectra.     

Electron temperature in a decaying molecular nitrogen plasma in the presence of weak electric fields

The electron energy distribution function in an afterglow molecular nitrogen plasma is studied both experimentally and theoretically under the conditions of weak electric fields such that the electron gas is heated by superelastic collisions of electrons with vibrationally excited molecules. Based on the mean electron energy balance, it is established that, depending on the degree of plasma ionization and the vibrational temperature of nitrogen molecules, an afterglow plasma may evolve into two states, differing in electron temperature. This kind of bistability is found to stem from the difference in the main mechanisms for electron energy losses in the two stable states. The prediction that the shape of the electron energy distribution function should change in a jumplike manner when a weak electric field is imposed has been confirmed experimentally.     

Metastability of microtubules induced by competing internal forces

Recent modeling efforts to estimate energies of tubulin-tubulin bonds shed light on a delicate balance between competing mechanical forces maintaining microtubule walls. Here we formulate two important refinements to the explanation of bond energetics. First, energy surface calculations in the elastic filament approximation reveal a finite stabilizing barrier assumed a simple Lennard-Jones-like potential for protein bonds. The presence of a guanosine triphosphate (GTP) cap represented by straight segments is necessary, as it is predicted for a long time. In the lack of such a cap, the protofilaments are either in an absolutely stable or absolutely unstable state. Second, our calculations show that this barrier appears only if the mechanical energy associated with the conformational change after GTP hydrolysis (curling energy) is larger than the strength of lateral bonds. The overall energy balance we propose supports continuous assembly of GTP dimers, a metastable state in the presence of a finite GTP cap and energetically driven disassembly of guanosine diphosphate protofilaments.     

Gramicidin channel selectivity. Molecular mechanics calculations for formamidinium, guanidinium, and acetamidinium.

Empirical energy function calculations were used to evaluate the effects of minimization on the structure of a gramicidin A channel and to analyze the energies of interaction between three cations (guanidinium, acetamidinium, formamidinium) and the channel as a function of position along the channel axis. The energy minimized model of the gramicidin channel, which was based on the results of Venkatachalam and Urry (1983), has a constriction at the channel entrance. If the channel is not allowed to relax in the presence of the ions (rigid model), there is a large potential energy barrier for all three cations. The barrier varies with cation size and is due to high van der Waals and ion deformation energies. If the channel is minimized in the presence of the ions, the potential energy barrier to formamidinium entry is almost eliminated, but a residual barrier remains for guanidinium and acetamidinium. The residual barrier is primarily due, not to the expansion of the helix, but, to the disruption of hydrogen bonds between the terminal ethanoloamine and the next turn of the helix which occurs when the carbonyls of the outer turn of the helix librate inward toward the ion as it enters the channel. The residual potential energy barriers could be a possible explanation for the measured selectivity of gramicidin for formamidinium over guanidinium. The results of this full-atomic model address the applicability of the size-exclusion concept for the selectivity of the gramicidin channel.     

Measurements of the electron density and degree of plasma ionization in a plasma target based on a linear electric discharge in hydrogen

Laser interferometry methods were used to measure the density of free electrons and degree of plasma ionization in a hydrogen target intended for experiments on determining energy losses of heavy ion beams in an ionized matter. It is shown that the linear electron density can be varied in the range from 3.3 × 10¹⁷ to 1.3 × 10¹⁸ cm^?2 by varying the initial plasma parameters (the hydrogen pressure in the target and the discharge current). The error in measuring the linear electron density in the entire range of the varied plasma parameters was less than 1%. The maximum degree of plasma ionization achieved at the initial gas pressure of 1 mbar was 0.62 ± 0.05.     

Feasibility of using laser ion accelerators in proton therapy

The feasibility of using laser plasma as a source of high-energy ions for the purposes of proton therapy is discussed. The proposal is based on the efficient ion acceleration observed in recent laboratory and numerical experiments on the interaction of high-power laser radiation with gaseous and solid targets. The specific dependence of proton energy losses in biological tissues (the Bragg peak) promotes the solution of one of the main problems of radiation therapy, namely, the irradiation of a malignant tumor with a sufficiently strong and homogeneous dose, ensuring that the irradiation of the surrounding healthy tissues and organs is minimal. In the scheme proposed, a beam of fast ions accelerated by a laser pulse can be integrated in the installations intended for proton therapy.     

You Do the Math: Coding of Bets and Outcomes in a Gambling Task in the Feedback-Related Negativity and P300 in Healthy Adults

The feedback-related negativity (FRN) is an event-related potential (ERP) component associated with processing of performance feedback, with more negative amplitudes for losses relative to wins. The amplitude of the FRN following near misses, i.e. the experience of coming close to winning, is between the amplitude elicited by losses and wins. In gambling, however, outcome value may not always be obvious since initially placed bets need to be taken into account when evaluating wins or losses. It is still unclear if initial bet size is reflected in the FRN or the later P300 component. The present study applied a virtual card gambling task to investigate the sensitivity of FRN and P300 to the manipulation of outcome magnitude as implemented through the presence or absence of initial bets, resulting in wins, losses or ambivalent outcomes, with the latter representing losses with and wins without bets. The FRN was larger for trials with bets compared to trials without bets. Wins were associated with a smaller FRN than losses or ambivalent outcomes, while losses and ambivalent outcomes did not differ. P300 amplitudes were larger for trials without bets, and wins were associated with a larger P300 than losses or ambivalent outcomes. Crucially, P300 amplitudes were also smaller for ambivalent outcomes compared to losses. Thus, the different dimensions determining outcome value appear to be integrated in early and late stages of feedback processing. However, only at later stages reflected in the P300 were ambivalent outcomes with and without bet clearly distinguished from other outcomes.     

Rapid estimation of the energy charge from cell lysates using matrix-assisted laser desorption/ionization mass spectrometry: Role of in-source fragmentation

Nucleotides are key players in the central energy metabolism of cells. Here we show how to estimate the energy charge from cell lysates by direct negative ion matrix-assisted laser desorption/ionization mass spectrometry (MALDI–MS) using 9-aminoacridine as matrix. We found a high level of in-source decay of all the phosphorylated nucleotides, with some of them producing considerable amounts of adenosine-5′-diphosphate (ADP) fragment ions. We investigated the behavior of adenosine-5′-monophosphate (AMP), ADP, and adenosine-5′-triphosphate (ATP) as well as the cofactors coenzyme A (CoA) and acetyl-coenzyme A (ACoA) and nicotinamide adenine dinucleotides (NAD⁺ and NADH) in detail. In-source decay of these compounds depends strongly on the applied laser power and on the extraction pulse delay. At standard instrument settings, the 9-aminoacridine (9-AA) matrix resulted in a much higher in-source decay compared with 2,4,6-trihydroxyacetophenone (2,4,6-THAP). By adding ¹³C-labeled ATP to a cell lysate, we were able to determine the degree of in-source decay during an experiment. Analyzing a cell extract of the monocytic cell line THP-1 with [¹³C]ATP as internal standard, we were able to obtain values for the energy charge that were similar to those determined by a reference liquid chromatography electrospray ionization coupled to mass spectrometry (LC–ESI–MS) method.     

Comparative Study on the Uniform Energy Deposition Achievable via Optimized Plasmonic Nanoresonator Distributions

Plasmonic nanoresonators of core–shell composition and nanorod shape were optimized to tune their absorption cross-section maximum to the central wavelength of a short laser pulse. The number density distribution of randomly located nanoresonators along a laser pulse-length scaled target was numerically optimized to maximize the absorptance with the criterion of minimal absorption difference between neighboring layers illuminated by two counter-propagating laser pulses. Wide Gaussian number density distribution of core–shell nanoparticles and nanorods enabled to improve the absorptance with low standard deviation; however, the energy deposited until the overlap of the two laser pulses exhibited a considerable standard deviation. Successive adjustment resulted in narrower Gaussian number density distributions that made it possible to ensure almost uniform distribution of the deposited energy integrated until the maximal overlap of the two laser pulses. While for core–shell nanoparticles the standard deviation of absorptance could be preserved, for the nanorods it was compromised. Considering the larger and polarization independent absorption cross-section as well as the simultaneously achievable smaller standard deviation of absorptance and deposited energy distribution, the core–shell nanoparticles outperform the nanorods both in optimized and adjusted nanoresonator distributions. Exception is the standard deviation of deposited energy distribution considered for the complete layers that is smaller in the adjusted nanorod distribution. Optimization of both nanoresonator distributions has potential applications, where efficient and uniform energy deposition is crucial, including biomedical applications, phase transitions, and even fusion.

     

Cage opening and fragmentation of C60 fullerene induced by an ultrashort laser pulse

Response of C₆₀ fullerene to a 40 fs full-width at half-maximum laser pulse with a photon energy of 2.0 eV and different laser intensities is studied by semiclassical dynamics simulation technique. The simulation results show that soon after the irradiation with a strong laser pulse, many C–C bonds abruptly break but no fragments are produced. The breaking of multiple C–C bonds induces a quick increase in the kinetic energy and potential energy and a decrease in electronic energy. These results suggest that the opening of the C₆₀ cage is an effective channel for the conversion of electronic energy to kinetic energy for the electronically excited C₆₀ fullerene.     

Acceleration of an electron bunch injected in front of a laser pulse generating an accelerating wake wave

A study is made of a promising method for injecting an electron bunch into an accelerating laser-plasma system. A bunch is injected ahead of the front of a laser pulse generating a wake wave that propagates in a direction collinear with the pulse and has a velocity lower than the pulse group velocity. The influence of the initial nonmonoenergetic character of the bunch on its trapping and acceleration is investigated. By appropriately choosing the laser pulse parameters and the bunch injection energy, it is possible to create such conditions for the trapping of an initially nonmonoenergetic bunch by the wake wave that, over a certain acceleration distance, there will be no energy spread of the bunch due to its initial nonmonoenergetic character, a circumstance that allows compact electron bunches to be accelerated to high energies, with a minimum energy spread.     

Microwave and Optical Gas Discharges in Superstrong Pulsed Fields

A review is given of theoretical papers on gas breakdown in high-power pulsed microwave and optical fields under conditions such that the electron oscillatory energy in the wave field is much higher than the ionization energy of gas atoms. In microwave fields, which are much weaker than the atomic field, the ionization mechanism for gas atoms is governed by electron-impact avalanche ionization. In high-power optical fields that are comparable in strength to the atomic field, the gas atoms are ionized via the tunneling of the bound electrons. It is shown that, in both cases, the electrons obey similar, highly anisotropic distributions, thereby strongly affecting the stability of the discharge plasma.