207 research outputs found
Nonlinearly Shaped Pulses in Photoinjectors and Free-Electron Lasers
Photoinjectors and Free Electron Lasers (FEL) are amongst the most advanced
systems in accelerator physics and have consistently pushed the boundaries of
emittance and x-ray peak power. In this paper, laser shaping at the cathode is
proposed to further lower the emittance and reduce electron beam tails, which
would result in brighter x-ray production. Using dispersion controlled
nonlinear shaping (DCNS), laser pulses and beam dynamics were simulated in
LCLS-II. The photoinjector emittance was optimized and the resulting e-beam
profiles were then simulated and optimized in the linac. Finally, the expected
FEL performance is estimated and compared to the current technology: Gaussian
laser pulses on the cathode. The e-beams produced by DCNS pulses show a
potential for 35% increase in x-ray power per pulse during SASE when compared
to the standard Gaussian laser pulses
Evidence for Microbial Enhanced Electrical Conductivity in Hydrocarbon-Contaminated Sediments
Bulk electrical conductivity of sediments during microbial mineralization of diesel was investigated in a mesoscale laboratory experiment consisting of biotic contaminated and uncontaminated columns. Population numbers of oil degrading microorganisms increased with a clear pattern of depth zonation within the contaminated column not observed in the uncontaminated column. Microbial community structure determined from ribosomal DNA intergenic spacer analysis showed a highly specialized microbial community in the contaminated column. The contaminated column showed temporal increases in bulk conductivity, dissolved inorganic carbon, and calcium, suggesting that the high bulk conductivity is due to enhanced mineral weathering from microbial activity. The greatest change in bulk conductivity occurred in sediments above the water table saturated with diesel. Variations in electrical conductivity magnitude and microbial populations and their depth distribution in the contaminated column are similar to field observations. The results of this study suggest that geophysical methodologies may potentially be used to investigate microbial activity
In-situ Apparent Conductivity Measurements and Microbial Population Distribution at a Hydrocarbon-Contaminated Site
We investigated the bulk electrical conductivity and microbial population distribution in sediments at a site contaminated with light nonaqueous-phase liquid (LNAPL). The bulk conductivity was measured using in-situ vertical resistivity probes; the most probable number method was used to characterize the spatial distribution of aerobic heterotrophic and oil-degrading microbial populations. The purpose of this study was to assess if high conductivity observed at aged LNAPL-impacted sites may be related to microbial degradation of LNAPL. The results show higher bulk conductivity coincident with LNAPL-impacted zones, in contrast to geoelectrical models that predict lower conductivity in such zones. The highest bulk conductivity was observed to be associated with zones impacted by residual and free LNAPL. Data from bacteria enumeration from sediments close to the resistivity probes show that oil-degrading microbes make up a larger percentage (5-55%) of the heterotrophic microbial community at depths coincident with the higher conductivity compared to ∼5% at the uncontaminated location. The coincidence of a higher percentage of oil-degrading microbial populations in zones of higher bulk conductivity suggests that the higher conductivity in these zones may result from increased fluid conductivity related to microbial degradation of LNAPL, consistent with geochemical studies that suggest that intrinsic biodegradation is occurring at the site. The findings from this study point to the fact that biogeochemical processes accompanying biodegradation of contaminants can potentially alter geoelectrical properties of the subsurface impacted media
The LCLS-II Photoinjector Laser Infrastructure
This paper presents a comprehensive technical overview of the Linac Coherent
Light Source II (LCLS-II) photoinjector laser system, its first and foremost
component. The LCLS-II photoinjector laser system serves as an upgrade to the
original LCLS at SLAC National Accelerator Laboratory. This advanced laser
system generates high-quality laser beams to power the LCLS-II, contributing to
the instrument's unprecedented brightness, precision, and flexibility. Our
discussion extends to the various subsystems that comprise the photoinjector,
including the photocathode laser, laser heater, and beam transport systems.
Lastly, we draw attention to the ongoing research and development
infrastructure underway to enhance the functionality and efficiency of the
LCLS-II, and similar X-ray free-electron laser facilities around the world,
thereby contributing to the future of laser technology and its applications.Comment: Submitted to High Power Laser Science and Engineerin
Enhanced ultrafast X-ray diffraction by transient resonances
Diffraction-before-destruction imaging with single ultrashort X-ray pulses
has the potential to visualise non-equilibrium processes, such as chemical
reactions, at the nanoscale with sub-femtosecond resolution in the native
environment without the need of crystallization. Here, a nanospecimen partially
diffracts a single X-ray flash before sample damage occurs. The structural
information of the sample can be reconstructed from the coherent X-ray
interference image. State-of-art spatial resolution of such snapshots from
individual heavy element nanoparticles is limited to a few nanometers. Further
improvement of spatial resolution requires higher image brightness which is
ultimately limited by bleaching effects of the sample. We compared snapshots
from individual 100 nm Xe nanoparticles as a function of the X-ray pulse
duration and incoming X-ray intensity in the vicinity of the Xe M-shell
resonance. Surprisingly, images recorded with few femtosecond and
sub-femtosecond pulses are up to 10 times brighter than the static linear model
predicts. Our Monte-Carlo simulation and statistical analysis of the entire
data set confirms these findings and attributes the effect to transient
resonances. Our simulation suggests that ultrafast form factor changes during
the exposure can increase the brightness of X-ray images by several orders of
magnitude. Our study guides the way towards imaging with unprecedented
combination of spatial and temporal resolution at the nanoscale
Attosecond Delays in X-ray Molecular Ionization
The photoelectric effect is not truly instantaneous, but exhibits attosecond
delays that can reveal complex molecular dynamics. Sub-femtosecond duration
light pulses provide the requisite tools to resolve the dynamics of
photoionization. Accordingly, the past decade has produced a large volume of
work on photoionization delays following single photon absorption of an extreme
ultraviolet (XUV) photon. However, the measurement of time-resolved core-level
photoionization remained out of reach. The required x-ray photon energies
needed for core-level photoionization were not available with attosecond
tabletop sources. We have now measured the x-ray photoemission delay of
core-level electrons, and here report unexpectedly large delays, ranging up to
700 attoseconds in NO near the oxygen K-shell threshold. These measurements
exploit attosecond soft x-ray pulses from a free-electron laser (XFEL) to scan
across the entire region near the K-shell threshold. Furthermore, we find the
delay spectrum is richly modulated, suggesting several contributions including
transient trapping of the photoelectron due to shape resonances, collisions
with the Auger-Meitner electron that is emitted in the rapid non-radiative
relaxation of the molecule, and multi-electron scattering effects. The results
demonstrate how x-ray attosecond experiments, supported by comprehensive
theoretical modelling, can unravel the complex correlated dynamics of
core-level photoionization
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