High prevalence of penicillin-nonsusceptible Streptococcus pneumoniae at a community hospital in Oklahoma.

During 1997, Oklahoma City's Hospital A reported penicillin-nonsusceptible Streptococcus pneumoniae in almost 67% of isolates. To confirm this finding, all Hospital A S. pneumoniae isolates from October 23, 1997, through February 19, 1998, were tested for antibiotic susceptibility and repeat-tested at two other hospital laboratories. Medical records of Hospital A patients with invasive S. pneumoniae infections during 1994 through 1997 were also reviewed. These data were compared with 1998 statewide sentinel hospital surveillance data for invasive S. pneumoniae. Of 48 S. pneumoniae isolates from Hospital A during October 23, 1997, through February 19, 1998, 31 (65%) were penicillin-nonsusceptible S. pneumoniae, and 23 (48%) were highly penicillin resistant. Similar prevalences were confirmed at the other hospital laboratories; however, significant interlaboratory differences were noted in the determination of third-generation cephalosporin susceptibility. During 1994 through 1997, a trend toward increasing penicillin nonsusceptibility (p <0.05) was noted among S. pneumoniae isolates from nursing home patients. During 1998, 85 (30%) of 282 invasive isolates reported to the state surveillance system were penicillin-nonsusceptible S. pneumoniae; 33 (12%) were highly resistant. The increase in resistance observed is notable; the interlaboratory discrepancies are unexplained. To respond, a vaccination program was implemented at Hospital A, and vaccination efforts were initiated at nursing homes

The effect of grading the atomic number at resistive guide element interface on magnetic collimation

Using 3 dimensional numerical simulations, this paper shows that grading the atomic number and thus the resistivity at the interface between an embedded high atomic number guide element and a lower atomic number substrate enhances the growth of a resistive magnetic field. This can lead to a large integrated magnetic flux density, which is fundamental to confining higher energy fast electrons. This results in significant improvements in both magnetic collimation and fast-electron-temperature uniformity across the guiding. The graded interface target provides a method for resistive guiding that is tolerant to laser pointing

Controlling femtosecond-laser-driven shock-waves in hot, dense plasma

Ultrafast pump-probe reflectometry and Doppler spectrometry of a supercritical density plasma layer excited by 1017-1018 W/cm2 intensity, 30 fs, and 800 nm laser pulses reveal the interplay of laser intensity contrast and inward shock wave strength. The inward shock wave velocity increases with an increase in laser intensity contrast. This trend is supported by simulations as well as by a separate independent experiment employing an external prepulse to control the inward motion of the shock wave. This kind of cost-effective control of shock wave strength using femtosecond pulses could open up new applications in medicine, science, and engineering

Electron acceleration by a transient intense-laser-plasma electrode

Rapid strides in the technology of laser plasma-based acceleration of charged particles leading to high brightness, tunable, monochromatic energetic beams of electrons and ions has been driven by potential multidisciplinary applications in cancer therapy, isotope preparation, radiography and thermonuclear fusion. Hitherto laser plasma acceleration schemes were confined to large-scale facilities generating a few tens of terawatt to petawatt laser pulses at repetition rates of 10 Hz or less. However, the need to make viable systems using high-repetition-rate femtosecond lasers has impelled recent research into novel targetry [1,2]. Of contemporary importance is the generation of supra thermal electrons, beyond those predicted by the scaling relation, reflected in both theoretical and computational work [3,4]. In this work we present evidence of generation of relativistic electrons (temperature >200 keV, maximum energies >1 MeV) at intensities that are two orders of magnitude lower than the relativistic intensity threshold. The novel targets [6] are 15 micron sized crystals suspended as aerosols in a gas interacting with a kHz, few-mJ femtosecond laser focussed to intensities of 10 PW/cm2. A pre-pulse with 1-5% of the intensity of the main pulse, arriving 4 ns early, is critical for hot electron generation. In addition to this unprecedented energy enhancement, we also characterize the dependence of X-ray spectra on the background gas of the aerosol. Intriguingly, easier the gas is to ionise, greater is the number of hot electrons observed, while the electron temperature remains the same. 2-D Radiation hydrodynamics and Particle-in-cell (PIC) simulations explain both the experimentally observed electron emission and the role of the low-density plasma in yield enhancement. We observe a two-temperature electron spectrum with about 50 and 240 keV temperatures consistent with the measurements made in the experiments. The simulations show that the following features contribute to the high-energy electron emission. The pre-pulse generates a hemispherical plasma-profile that enhances the coupling of the laser light. Overdense plasma is generated about the hemispherical cavity on the particle due to the main pulse interaction. The gradient in the plasma density in and around the cavity serves as a reservoir of low energy electrons to be injected into the particle potential and enables the hot electron generation observed in the experiments. Higher energy electron emission is dominantly from the edges of the hemispherical cavitation. The increase in total X-ray yield observed in the experiments scales with the number of electrons generated in the low density neighborhood surrounding the particle. In a simple-man picture, the laser interacts with the particle and ejects electrons from the particle. The particle acquires a strong positive potential that can only be brought down by ion expansion that occurs over 10's of picoseconds. The particle with strong positive potential acts as an 'accelerating electrode' for the electrons ionized in the low-density gas neighborhood. These results assume importance in the context of applications such as fast fuel ignition [6] or in medical applications of laser plasmas [7] where high irradiance of energetic electrons is of consequence. 1. D. Gustas et al., Phys. Rev. Accel. Beams, 21, 013401 (2018). 2. S. Feister et al , Opt. Express, 25, 18736 (2017). 3. B. S. Paradkar, S. I. Krasheninnikov, and F. N. Beg, Physics of Plasmas, 19, 060703 (2012). 4. A. P. L. Robinson, A. V. Areev, and D. Neely, Phys. Rev. Lett., 111, 065002 (2013). 5. R. Gopal, et al., Review of Scientific Instruments, 88, 023301 (2017). 6. M. Tabak et al., Physics of Plasmas, 1, 1626 (1994). 7. A. Sjogren, M. Harbst, C.-G. Wahlstrom, S. Svanberg, and C. Olsson, Review of Scientific Instruments, 74, 2300 (2003)

Enhancing relativistic electron beam propagation through the use of graded resistivity guides

We show, using three dimensional hybrid particle-in-cell simulations, that fast electron transport is improved in a resistive guide when using a linear decreasing gradient in the resistivity between the guide and substrate. We observe increased heating-at-depth along the guide and significantly reduced heating inhomogeneity. These improvements result from an increase in the width of the collimating magnetic field, improving fast electron confinement and limiting the growth of magnetic fields in the interior of the guide

Efficient transport of femtosecond laser-generated fast electrons in a millimeter thick graphite

We demonstrate efficient transport of fast electrons generated by ∼1018 W/cm2, 30 fs, 800 nm laser pulses through a millimeter thick polycrystalline graphite. Measurements of hot electron spectra at the front side of the graphite target show enhancement in terms of the electron flux and temperature, while the spectra at the rear confirm the ability of the graphite to transport large electron currents over a macroscopic distance of a millimeter. In addition, protons of keV energies are observed at the rear side of such a macroscopically thick target and attributed to the target-normal-sheath-acceleration mechanism

Evaluation of fatigue damage in steel structural components by magnetoelastic Barkhausen signal analysis

This paper is concerned with using a magnetic technique for the evaluation of fatigue damage in steel structural components. It is shown that Barkhausen effect measurements can be used to indicate impending failure due to fatigue under certain conditions. The Barkhausen signal amplitude is known to be highly sensitive to changes in density and distribution of dislocations in materials. The sensitivity of Barkhausen signal amplitude to fatigue damage has been studied in the low‐cycle fatigue regime using smooth tensile specimens of a medium strength steel. The Barkhausen measurements were taken at depths of penetration of 0.02, 0.07, and 0.2 mm. It was found that changes in magnetic properties are sensitive to microstructural changes taking place at the surface of the material throughout the fatigue life. The changes in the Barkhausen signals have been attributed to distribution of dislocations in stage I and stage II of fatigue life and the formation of a macrocrack in the final stage of fatigue

Laboratory measurements of resistivity in warm dense plasmas relevant to the microphysics of brown dwarfs

Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. The inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs

Generation of a strong reverse shock wave in the interaction of a high-contrast high-intensity femtosecond laser pulse with a silicon target

We present ultrafast pump-probe reflectivity and Doppler spectrometry of a silicon target at relativistic laser intensity. We observe an unexpected rise in reflectivity to a peak approximately9 ps after the main pulse interaction with the target. This occurs after the reflectivity has fallen off from the initially high “plasma-mirror” phase. Simultaneously measured time-dependent Doppler shift data show an increase in the blue shift at the same time. Numerical simulations show that the aforementioned trends in the experimental measurements correspond to a strong shock wave propagating back toward the laser. The relativistic laser-plasma interaction indirectly heats the cool-dense (ne 10^23 cm^-3 and Te ~10eV) target material adjacent to the corona, by hot electron induced return current heating, raising its temperature to around 150eV and causing it to explode violently. The increase in reflectivity is caused by the transient steepening of the plasma density gradient at the probe critical surface due to this explosive behavior