Elevation-Distributed Multistage Reverse Osmosis Desalination with Seawater Pumped Storage

A seawater reverse osmosis (RO) plant layout based on multistage RO with stages located at different elevations above sea level is described. The plant uses the weight of a seawater column from pumped storage as head pressure for RO (gravity-driven multistage RO) or to supplement high-pressure pumps used in RO (gravity-assisted multistage RO). The use of gravitational force reduces the specific energy for RO compared to using high-pressure pumps. By locating the RO stages at different elevations based on demand sites, the total specific energy consumption for RO and permeate transport to different elevations above sea level is reduced from that for locating the RO process entirely at sea level followed by lifting the desalinated water. A final RO stage at sea level uses seawater pressurized by energy recovery from the residual energy of the brine generated from the preceding RO stage. Examples of the plant layout that do not include pump inefficiency and head losses in pipes are described for South Sinai, Egypt, which is a mountainous region that suffers from water scarcity. A gravity-driven multistage RO with a storage tank at 660 m above sea level is considered. For five RO stages located 316–57 m above sea level with 10% recovery at each stage, the specific energy is ~ 32% lower than that for a plant located at sea level operating at the minimum specific energy followed by lifting the same quantity of desalinated water to the elevations of the distributed RO stages. For two stages located at 222 and 57 m above sea level with 30 and 20% recovery, respectively, the reduction in specific energy is ~ 27%. For gravity-assisted five-stage RO with the first stage at 260 m above sea level, while the last stage is at sea level with 10% recovery at each stage the reduction in specific energy is ~ 32%. The proposed RO plant layouts can be adapted to other regions with comparable topography

STM Study of Pulsed Laser Assisted Growth of Ge Quantum Dot on Si(1 0 0)-(2 × 1)

Ge quantum dot formation on Si(1 0 0)-(2 × 1) by nanosecond pulsed laser deposition under laser excitation was investigated. Scanning tunneling microscopy was used to probe the growth mode and morphology. Excitation was performed during deposition using laser energy density of 25-100 mJ/cm 2. Faceted islands were achieved at a substrate temperature of ∼250 °C only when using laser excitation. The island morphology changes with increased laser excitation energy density although the faceting of the individual islands remains the same. The size of the major length of islands increases with the excitation laser energy density. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is proposed. © 2014 EDP Sciences

Electronically Enhanced Surface Diffusion During Ge Growth on Si(100)

The effect of nanosecond pulsed laser excitation on surface diffusion during the growth of Ge on Si(100) at 250 °C was studied. In situ reflection high-energy electron diffraction was used to measure the surface diffusion coefficient while ex situ atomic force microscopy was used to probe the structure and morphology of the grown quantum dots. The results show that laser excitation of the substrate increases the surface diffusion during the growth of Ge on Si(100), changes the growth morphology, improves the crystalline structure of the grown quantum dots, and decreases their size distribution. A purely electronic mechanism of enhanced surface diffusion of the deposited Ge is proposed. © 2011 American Institute of Physics. [doi:10.1063/1.3567918

Excitation-Induced Germanium Quantum Dot Formation on Si (100)-(2×1)

The effect of nanosecond pulsed laser excitation on the self-assembly of Ge quantum dots grown by pulsed laser deposition on Si (100)-(2×1) was studied. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy were used to probe the quantum dot structure and morphology. At room temperature, applying the excitation laser decreased the surface roughness of the grown Ge film. With surface electronic excitation, crystalline Ge quantum dots were formed at 250 °C, a temperature too low for their formation without excitation. At a substrate temperature of 390 °C, electronic excitation during growth was found to improve the quantum dot crystalline quality, change their morphology, and decrease their size distribution almost by half. A purely electronic mechanism of enhanced surface hopping of the Ge adatoms is proposed. © 2010 American Institute of Physics. [doi:10.1063/1.3462436

Anisotropic Response of Nanosized Bismuth Films Upon Femtosecond Laser Excitation Monitored by Ultrafast Electron Diffraction

The lattice response of 5 nm thick bismuth film to femtosecond laser excitation is probed by ultrafast electron diffraction. The transient decay time after laser excitation is greater for diffraction from (012) lattice planes compared to (110) planes and is reduced for both planes with the increased laser fluence. These results indicate that different energy coupling mechanisms to the lattice occur depending on the crystal direction. The behavior of the diffraction peak width indicates partial disorder of the film upon photoexcitation that increases together with the laser fluence. © 2011 American Institute of Physics. [doi:10.1063/1.3652919

Comment on Ultrafast Electron Optics: Propagation Dynamics of Femtosecond Electron Packets J. Appl. Phys. 92, 1643 (2002)

In a recent article 关J. Appl. Phys. 92, 1643 共2002兲兴 Siwick et al. investigated the space-charge-limited electron pulse propagation in a photoelectron gun using an analytical approach, referred to as mean-field theory, and a numerical N-body simulation. The results were compared with a one-dimensional fluid model 关J. Appl. Phys. 91, 462 共2002兲兴, and a conclusion was made that the fluid model overestimates the pulse duration after a certain propagation time. Although the mean-field theory and N-body simulation give exactly the same results for all examples studied, we point out that the expression for the on-axis potential in their mean-field model is inapplicable to investigating the electron space-charge dynamics in an ultrafast electron packet. We correct that expression and derive a two-dimensional model that is in agreement with our previous one-dimensional fluid model. We also point out several areas where Siwick et al. have misinterpreted the one-dimensional fluid model. © 2003 American Institute of Physics

Electron Pulse Broadening Due to Space Charge Effects in a Photoelectron Gun for Electron Diffraction and Streak Camera Systems

The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead to a considerable electron energy spread in the drift region. Temporal broadening is calculated for an initial electron pulse as short as 50 fs with different electron densities, final electron energies, and drift distances. The results can be used to design electron guns producing subpicosecond pulses for streak cameras as well as for time resolved electron diffraction. © 2002 American Institute of Physics. [DOI: 10.1063/1.1419209

Low Temperature Epitaxial Growth of Ge Quantum Dot on Si (100) - (2×1) by Femtosecond Laser Excitation

Low temperature epitaxy of Ge quantum dots on Si (100) - (2×1) by femtosecond pulsed laser deposition under femtosecond laser excitation was investigated. Reflection high-energy electron diffraction and atomic force microscopy were used to analyze the growth mode and morphology. Epitaxial growth was achieved at ∼70 °C by using femtosecond laser excitation of the substrate. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is proposed. © 2011 American Institute of Physics. [doi:10.1063/1.3537813

Spark Discharge Coupled Laser Multicharged Ion Source

A spark discharge is coupled to a laser multicharged ion source to enhance ion generation. The laser plasma triggers a spark discharge with electrodes located in front of the ablated target. For an aluminum target, the spark discharge results in significant enhancement in the generation of multicharged ions along with higher charge states than observed with the laser source alone. When a Nd:YAG laser pulse (wavelength 1064 nm, pulse width 7.4 ns, pulse energy 72 mJ, laser spot area on target 0.0024 cm2) is used, the total multicharged ions detected by a Faraday cup is 1.0 nC with charge state up to Al3+. When the spark amplification stage is used (0.1 μF capacitor charged to 5.0 kV), the total charge measured increases by a factor of ∼9 with up to Al6+ charge observed. Using laser pulse energy of 45 mJ, charge amplification by a factor of ∼13 was observed for a capacitor voltage of 4.5 kV. The spark discharge increases the multicharged ion generation without increasing target ablation, which solely results from the laser pulse. This allows for increased multicharged ion generation with relatively low laser energy pulses and less damage to the surface of the target. © 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4923457