Dynamics of the wakefield of a multi-petawatt, femtosecond laser pulse in a configuration with ultrarelativistic electrons

The wake field excitation in an unmagnetized plasma by a multi-petawatt, femtosecond, pancake-shaped laser pulse is described both analytically and numerically in the regime with ultrarelativistic electron jitter velocities, when the plasma electrons are almost expelled from the pulse region. This is done, for the first time, in fluid theory. A novel mathematical model is devised that does not break down for very intense pump strengths, in contrast to the standard approach that uses the laser field envelope and the ponderomotive guiding center averaging. This is accomplished by employing a three-timescale description, with the intermediate scale associated with the nonlinear phase of the electromagnetic wave and with the bending of its wave front. The evolution of the pulse and of its electrostatic wake are studied by the numerical solution in a two-dimensional geometry, with the spot diameter \geq 100 microns. It reveals that the optimum initial pulse length needs to be somewhat bigger than 1 micron (1-2 oscillations), as suggested by simple analytical local estimates, because the nonlocal plasma response tends to stretch very short pulses

Grid Approach to Path Integral Monte Carlo Calculations

Approach taken for the gridification of the developed Monte Carlo code for calculation of path integrals is described. Brief introduction to path integrals and Grids is given, and details on the implementation of SPEEDUP in the Grid environment are described. The numerical results obtained by the gridified version of the application are shortly presented, demonstrating its usefulness in the research in physics and related areas

Humidity-Dependent Reversible Transitions in Gold Nanoparticle Superlattices

The changes in interparticle spacing upon hydration and dehydration of drop-cast films of hydrophilic gold nanoparticles (GNP) have been measured in situ with nanometer resolution using WetSTEM and ESEM. These subtle variations correlate well with the corresponding changes in the optical spectra and perceived color as well as changes in the electrical conductivity of the films. AC impedance analysis allows us to differentiate between resistive and capacitive components and to evaluate how these depend on average particle spacing and the water content of the matrix, respectively. Thin films of this type are well-known structures used for development of sensors and diagnostics

Postural stability – a comparison between rowers and field sport athletes

Postural stability (PS) is an important function for maintaining equilibrium during periods of standing still, locomotion, and any motor activities that require high degree of balance. High PS is essential in different sports for the regulation of voluntary movement and for improving athletic physical condition and performance. Purpose: The purpose of this study was to compare the static PS of elite rowing athletes and field sport athletes. Methods: A total of 90 elite athletes (age: 23.9 ± 1.97 years; body height: 174.9 ± 8.9 cm; body weight: 67.7 ± 12.03 kg) were divided into Rowing (N = 47) and Field sport (N = 43) athlete groups. Static PS parameters were assessed with a static double-leg and single-leg standing stability test on a force plate platform. Results: The multivariate analysis of variance showed a general stability difference between the groups (F = 13.255; P ≤ 0.0001), in double leg stability (F = 16.735; P ≤ 0.0001), and left leg (F = 15.097; P ≤ 0.0001) stability parameters. When analyzing variables separately, significant statistical differences were observed in favor of the Rowing group in double leg sway area (p = 0.017; ES = −0.07), double leg center of force (COF) traveled way (p ≤ 0.0001; ES = −27.42), length function of surface (p ≤ 0.0001; ES = −26.86), right leg ML displacement (p = 0.030; ES = −0.46), left leg sway area (p = 0.030; ES = −0.44), left leg COF traveled way (p ≤ 0.0001; ES = −60.63), left leg AP displacement (p = 0.043; ES = −0.44). Conclusion: These results underline the differences in rowing and field sport athletes in terms of static PS. The characteristics of sport and competition may affect PS, and it is important to adjust training modalities for the required level of PS in every sport, especially in rowing

Multichannel electrotactile feedback with spatial and mixed coding for closed-loop control of grasping force in hand prostheses

Providing somatosensory feedback to the user of a myoelectric prosthesis is an important goal since it can improve the utility as well as facilitate the embodiment of the assistive system. Most often, the grasping force was selected as the feedback variable and communicated through one or more individual single channel stimulation units (e.g., electrodes, vibration motors). In the present study, an integrated, compact, multichannel solution comprising an array electrode and a programmable stimulator was presented. Two co ding schemes (15 levels), spatial and mixed (spatial and frequency) modulation, were tested in able-bodied subjects, psychometrically and in force control with routine grasping and force tracking using real and simulated prosthesis. The results demonstrated that mixed and spatial coding, although substantially different in psychometric tests, resulted in a similar performance during both force control tasks. Furthermore, the ideal, visual feedback was not better than the tactile feedback in routine grasping. To explain the observed results, a conceptual model was proposed emphasizing that the performance depends on multiple factors, including feedback uncertainty, nature of the task and the reliability of the feedforward control. The study outcomes, specific conclusions and the general model, are relevant for the design of closed-loop myoelectric prostheses utilizing tactile feedback

A Method of Approximate Greens Function for Solving Reflection of Particles in Plane Geometry

A method for approximate analytical solution of transport equation for particles in plane geometry is developed by solving Fredholm integral equations. Kernels of these equations are the Greens functions for infinite media treated approximately. Analytical approximation of Greens function is based on decomposition of the functions into terms that are exactly analytically solved and those which are approximately obtained by usual low order DPN approximation. Transport of particles in half-space is treated, and reflection coefficient is determined in the form of an analytical function. Comparison with the exact numerical solution and other approximate methods justified the proposed analytical technique

Amorphous carbon film deposition on inner surface of tubes using atmospheric pressure pulsed filamentary plasma source

Uniform amorphous carbon film is deposited on the inner surface of quartz tube having the inner diameter of 6 mm and the outer diameter of 8 mm. A pulsed filamentary plasma source is used for the deposition. Long plasma filaments (~ 140 mm) as a positive discharge are generated inside the tube in argon with methane admixture. FTIR-ATR, XRD, SEM, LSM and XPS analyses give the conclusion that deposited film is amorphous composed of non-hydrogenated sp2 carbon and hydrogenated sp3 carbon. Plasma is characterized using optical emission spectroscopy, voltage-current measurement, microphotography and numerical simulation. On the basis of observed plasma parameters, the kinetics of the film deposition process is discussed

Multiple scattering approach to low-energy electron collisions with the water dimer

Multiple scattering theory is applied to low-energy electron collisions with a complex target formed of two molecular scatterers. The total T-matrix is expressed in terms of the T-matrix for each isolated molecule. We apply the approach to elastic electron-(H2O)2 collisions. Following the method developed in our previous work on crystalline ice, we impose a cut-off on the dipole outside the R-matrix sphere and an energy dependent cut-off on the angular momentum components of the monomer T-matrix. An R-matrix calculation of electron-dimer collisions is performed in order to evaluate the accuracy of the multiple scattering approach. The agreement between the two calculations is very good.Comment: 15 pages, 4 figures New submission: Added references Included PACS numbers Figure 3 slightly changed Additions made to the Conclusions and Discussion sectio

Strong fragmentation of low-energy electromagnetic excitation strength in 117^{117}Sn

Results of nuclear resonance fluorescence experiments on $^{117}$Sn are reported. More than 50 $\gamma$ transitions with $E_{\gamma} < 4$ MeV were detected indicating a strong fragmentation of the electromagnetic excitation strength. For the first time microscopic calculations making use of a complete configuration space for low-lying states are performed in heavy odd-mass spherical nuclei. The theoretical predictions are in good agreement with the data. It is concluded that although the E1 transitions are the strongest ones also M1 and E2 decays contribute substantially to the observed spectra. In contrast to the neighboring even $^{116-124}$Sn, in $^{117}$Sn the $1^-$ component of the two-phonon $[2^+_1 \otimes 3^-_1]$ quintuplet built on top of the 1/2$^+$ ground state is proved to be strongly fragmented.Comment: 4 pages, 3 figure