Search Results - (Author, Cooperation:W. Kimura)

2013-04-19

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

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Alleles ; Animals ; Animals, Newborn ; *Cell Cycle Checkpoints ; Cell Proliferation ; Cyclin-Dependent Kinase Inhibitor p15/metabolism ; Cyclin-Dependent Kinase Inhibitor p16/metabolism ; Cyclin-Dependent Kinase Inhibitor p21/metabolism ; Female ; Heart/anatomy & histology/physiology ; Homeodomain Proteins/genetics/*metabolism ; Male ; Mice ; Myocardial Infarction/metabolism/pathology ; Myocytes, Cardiac/*cytology/*metabolism ; Neoplasm Proteins/deficiency/genetics/*metabolism ; Regeneration ; Transcriptional Activation

2015-06-23

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Animals ; Cell Hypoxia ; Cell Proliferation/genetics ; Female ; Hypoxia-Inducible Factor 1, alpha Subunit/genetics/metabolism ; Male ; Mice ; Mice, Transgenic ; Myocardium/*cytology ; Myocytes, Cardiac/*cytology/metabolism ; Protein Structure, Tertiary ; Recombinant Fusion Proteins/genetics/*metabolism ; Recombinases/genetics/metabolism ; Signal Transduction ; Stem Cells/cytology/metabolism

2015-12-25

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

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2018-03-13

Wiley-Blackwell

0148-0227

Geosciences

Physics

1089-7550

AIP Digital Archive

Physics

Detailed measurements of the time-dependent electron density, xenon excited-state densities, and total HCl depletion have been recently made in electron-beam- (e-beam-) pumped XeCl. This paper presents the results of extensive computer modeling of these experiments and detailed comparisons with the measured results. The model used includes updated HCl reaction cross sections and an enlarged xenon excited-state manifold. A reduced Boltzmann equation is used to calculate the high-energy electron-energy distribution function, and the low-energy distribution function assumes a quasi-Maxwellian distribution. These changes are upgrades to prior models. The model accurately predicts the total HCl depletion, the time-dependent electron density for initial HCl concentrations ≥0.16% (4.8 Torr), and the time-dependent xenon excited-state densities for lean ((approximately-equal-to)0.04%) initial HCl concentration cases. The model tends to underpredict the rate of electron-density growth after the electron density begins to run away. Since depletion of HCl is a key factor in understanding the kinetics data, possible mechanisms that can contribute to this process are also discussed.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Inverse Cerenkov acceleration (ICA) is a novel approach to electron acceleration involving the interaction of laser light with a polarizable gaseous medium. This technique may achieve very high acceleration gradients and tends to be best suited for the role as an energy booster for electrons of GeV energy or higher. A generic optimization study of ICA is performed to identify the desired properties of the ideal gaseous medium, in particular a medium in which the laser light frequency is near a resonance of the medium. Several factors that can limit the effectiveness of the interaction are taken into account. It appears that acceleration gradients exceeding 1 GeV/m are possible at low gas pressures that minimize problems with electron-beam emittance growth.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

The design and characterization of a Faraday cup utilizing modular components are presented. Design specifications were primarily tailored to satisfy the specific electron beam (e-beam) energy (∼375 keV), rise time (∼60 ns), and magnitude (30 A/cm2, peak) used in this work and permit convenient sampling of large e-beam areas up to 7 cm×7 cm. Characterization during evacuated conditions included Z-dependence measurements using beryllium, carbon, aluminum, and lead collector plates. Electron beam transmission measurements were made utilizing combinations of various metal screens and Kapton foils in both gas and evacuated conditions. Gas environments tested were air, krypton, and a Kr/Ar mixture. An attacher gas, SF6, was also added inside the Faraday cup. Results reveal decreasing current densities with increasing gas stopping power and increasing electron propagation distance in a gas. Employing a carbon collector plate and a 25-μm Kapton foil insulator, current densities measured through a 3.6-cm thick 760 Torr air slab are reduced ≤6% from the evacuated Kapton-free condition. Applying profile and full-aperture Faraday cup measurements, a consistent description of the e-beam is also presented.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Time-dependent density measurements of the lowest xenon excited states (5p56s) in electron beam (e-beam) pumped XeCl laser mixtures (nonlasing) are performed using pulsed hook interferometry. The e-beam pulse length is ≈0.45 μs (full width at half-maximum) with an average excitation rate of ≈250 kW/cm3. Density differences (ΔN*) of transitions at 823.2 nm (6s[3/2]02 –6p[3/2]2), 828.0 nm (6s[3/2]01 –6p[1/2]0), and 840.9 nm (6s[3/2]02 –6p[3/2]1) are obtained for various HCl and Xe concentrations. For a 98.3% Ne/1.5% Xe/0.16% HCl mixture at 3000 Torr, ΔN* (823.2 nm) and ΔN*(828.0 nm) are relatively constant at ≈3×1014 and ≈1.7×1014 cm−3, respectively. At lower initial HCl concentrations, the ΔN*(823.2 nm) density starts out similar to the 0.16% HCl case, but tends to increase dramatically during the e-beam pulse. For a 0.04% HCl mixture, a ΔN*(840.9 nm) density of ≈4.5×1015 cm−3 is measured at the end of the e-beam pulse. Preliminary comparisons of the data with a computer model show good agreement for HCl concentrations ≥0.16%, but disagreements at leaner concentrations.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A narrowband (5 GHz) electron beam pumped XeF oscillator/amplifier, lasing at 353.2 nm over a 1-μs pulse duration, has been demonstrated. Line selection, bandwidth narrowing, and near-diffraction-limited output are achieved by using an echelle grating and an intracavity solid étalon in a stable oscillator configuration. The amplifier features an off-axis design and yields output energies (approximately-greater-than)0.8 J. The oscillator and amplifier are located in the same gas chamber and are transversely pumped by the same electron beam. A Fabry–Perot interferometer together with a streak camera is used to verify narrowband operation over the entire pulse duration.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A new method is proposed for accelerating relativistic charged particle beams in a vacuum by a laser. The laser propagates in an overmoded waveguide interrupted periodically by thin dielectric disks spaced many wavelengths apart. The particle beam travels along the waveguide axis, passing through irises in the disks. The disks correct for slippage of the particle phase relative to the laser wave. This concept exploits the inverse of familiar radiation processes (transition radiation, diffraction radiation). Several practical accelerator issues are incorporated in a systems analysis. Acceleration gradients in the GeV/m range are projected using lasers with ∼100 GW power. This represents more than an order of magnitude stronger coupling of the laser energy compared to other laser acceleration schemes. © 1998 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The electron energy distributions for energies lower than 17 eV and their time-dependent evolution are calculated for electron-beam-excited Ne/Xe/HCl mixtures. A time-dependent Boltzmann equation including all interactions between electrons and ground– or excited-state species is solved together with a detailed full kinetics model for XeCl lasers. The effects of electron-electron collision, HCl concentration, and excitation rate on the steady-state electron energy distribution are examined. Under certain conditions, that is, for relatively high excitations and relatively low initial HCl concentrations, the low-energy electron distributions tend to be Maxwellian, and their average energies depend on various inelastic processes which cool down the electron energy. All electron reaction rates, especially those related to HCl vibrational excitation and dissociative attachment, are a function of the excitation rate and the transient HCl (v), Xe*, Xe**, and electron densities.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Understanding of the neutral channel formation kinetics in excimer laser gas mixtures has been limited by the lack of data on the pertinent excited-state populations in these mixtures. Presented are time-dependent measurements of the lower level xenon excited-state densities in electron-beam (e-beam) pumped XeCl and XeF laser mixtures (neon diluent). Measurements are obtained using hook interferometry under nonlasing conditions at an average excitation rate of ≈250 kW/cm3 and e-beam pulse lengths of 0.4 and 1 μs. The population differences, ΔN*, between four different electronic transitions [three in the Xe*(6s)-Xe**(6p) manifold, and one in the Xe**(6s')-Xe***(6p') manifold] are examined as a function of halogen concentration. For both XeCl and XeF at high initial halogen concentrations (〉4 Torr), the ΔN* densities of the Xe*(6s) and Xe**(6s') transitions are relatively constant during the 0.4-μs e-beam pulse [for ΔN*(6s-6p): ≈4×1014 cm−3 for XeCl, and ≈1.5×1014 cm−3 for XeF]. At lower initial halogen concentrations, the ΔN* densities of 6s-6p and 6s'-6p' start at the beginning of the pulse at approximately the same densities as the richer halogen mixtures, but at a certain point during the pulse, the ΔN* densities abruptly increase. This increase can be 〉10 times for very lean halogen mixtures (1–2 Torr), and occurs at earlier times as the initial halogen concentration is reduced. From other measurements, this increase appears related to the depletion of the halogen. The observed lifetime of the Xe*(6s) densities is ≈2 μs for the low initital halogen concentration mixtures. Additional density data for halogen-free Ne/Xe and Ar/Xe mixtures are also presented.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The electron energy distribution in electron beam (e beam) excited Ar/Kr and Ne/Xe gas mixtures is examined in detail. The binary rare-gas mixtures are similar to those used in excimer lasers. Cooling processes for the secondary electrons generated in the gas mixture plasma by the e beam are calculated using a reduced Boltzmann equation in which elastic and electron-electron collisions for electron energy distributions above the first excitation threshold of the rare gas are ignored. During the calculations for the Ar/Kr and Ne/Xe mixtures, all electron-related reactions and the interaction between the two different rare gases in the mixture are simultaneously considered. The high-energy secondary electrons produce a steady-state distribution within a very short time; however, it is found that the distribution is not Maxwellian. W values [eV/electron-ion pair] and yields of rare-gas excited states calculated from the steady-state high-energy electron distribution show a dependence on the mixture composition, especially for mixtures with low concentrations of the minor rare gas. This implies that the practice in excimer kinetics models of using the W values determined from pure rare gases is not entirely accurate.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

During self-break in spark-gap switches, multiple streamers can form in close proximity to one another. The rate of expansion of these streamers is sufficiently fast that they can interact during the current pulse. To help understand how these closely spaced, expanding spark columns interact, a laser-triggered spark gap has been studied in which two parallel columns (separation 1.3 mm) are simultaneously preionized, resulting in a pair of nearly identical, axisymmetric spark columns. The spark gap (electrode separation 1.2 cm) switches a 100 ns, 40–60 kV, 12-20 kA, 1.5 Ω waterline. Interferograms of the expanding arc channels are obtained with a laser interferometer having a time and spatial resolution of 5 ns and 10 μm, respectively. Voltage and current were measured with an internal capacitive-voltage divider and a current viewing resistor. The interferograms show that for initially identical axisymmetric columns, the individual channels do not merge into a single larger axisymmetric spark column. Instead, regions of high gas density remain inside the combined column long into the recovery period. The columns also do not remain axisymmetric as they grow, indicating a long-range interaction between the channels. The voltage drop and resistance of the dual channel spark gaps changes by less than 15% from that of a single spark channel. A scaling model is presented to explain the resistance measurements and to predict the change in resistance for multichannel spark gaps.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Time-dependent calculations of multiplexed short-pulse amplification are presented which show that cascaded short pulses in KrF lasers, with an appropriate time interval between the pulses, can extract energy with only a small cost in extraction efficiency as compared to a single long pulse. This dynamic characteristic of KrF* permits the design of a novel, simpler angular multiplexing system. Calculations include nonsaturable and saturable absorption, amplified spontaneous emission effects, and an end mirror for double-pass amplification. The dependence of the extraction efficiency and the energy gain on the time interval and pulse shape are calculated.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Laser preionization is a method whereby the formative lag time and jitter of spark gaps can be reduced, and breakdown can be initiated at voltages significantly below the self-breakdown value. In the spark gaps of interest (100-ns spark duration), the plasma column expands from an initial laser preionized diameter of approximately 50 μm to an arc of diameter in excess of 1 mm, and conducts greater than 12-kA peak current. Since the time required for the resistive collapse of the plasma is comparable to the spark duration, the spark gap represents a non-negligible resistive loss in the circuit. The objective of this study is to understand the basic mechanisms by which laser-triggered spark gaps develop and to provide a basis to optimize their design and minimize their resistive losses. To study the expansion and conduction phases of laser-triggered spark gaps, a first principles model has been developed. This model includes gas dynamics, electron collision kinetics, radiation transport, and external circuitry in a self-consistent formulation. The formulation of the model is discussed and results are compared to experimental data. We find that growth of the spark column is dominated by gas dynamic expansion of the hot ionized core, augmented by photoionization and thermal ionization at the plasma column boundary. The plasma column is confined within a high-density cylindrical shell of neutral gas that traps ionizing radiation in a region of low E/N (electric field divided by gas density), thereby inhibiting expansion by nonhydrodynamic means (electron avalanche).

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1089-7550

AIP Digital Archive

Physics

A KrF discharge laser (248 nm) has been used to laser trigger, by volume preionization, a spark gap switch (38–65 kV, 〉10 kA, 100 ns pulse duration) filled with 20 different gas mixtures using various combinations of air, Ar, CH4, H2, He, N2 SF6, and Xe. A pulsed laser interferometer is used to probe the spark column. Characteristics studied include the internal structure of the column, the arc expansion rate, and evidence of any photoionization precursor effect. Our results show that the rate of arc expansion varies depending on the average molecular weight of the mixtures. In this experiment, pure H2 has the highest rate (≈9.5×105 cm/s) and air has one of the lowest (≈7×105 cm/s) for the same hold-off voltage. A computer model of the spark column formation is able to predict most of the structure observed in the arcs, including the effect of mixing gases with widely different molecular weights. The work suggests that, under proper circumstances, the spark gap switch performance may be improved by using gases lighter than conventional switch gases such as SF6.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

In the use of spark gaps as switching devices, it is desirable to maximize the power delivered to the load and to minimize the power deposited in the switch; that is, it is desirable for the resistance of the switch to be negligible as compared to the load. The hydrodynamic time scale for expansion of the arc in a spark gap and hence for the reduction in its resistance to a small value is tens to hundreds of nanoseconds. Therefore, with current pulses of duration of a few hundred nanoseconds or less, the resistance of the spark gap may be a significant fraction of that of the load. In this paper, we report on measurements that determine the resistance of the arc in a fully diagnosed laser-triggered spark gap. The spark gap switches a 100-ns, 1.5-Ω waterline into a 1.5-Ω load resistor. A capacitive voltage divider housed within the switch enclosure measures the voltage drop across the switch, a current-viewing resistor measures the current, and an interferometer measures the diameter of the plasma column, a value required to calculate its inductance. The resistance of the arc is found to remain in excess of 0.1–0.2 Ω for the duration of the current pulse for a variety of switch gas mixtures. The resistance decreases with increasing charging voltage on the waterline at the time of triggering and decreases with decreasing average molecular weight of the gas mixture in which the arc is sustained.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

A system for measuring the absorptance of mirrors at angles of incidence from 0° to 〉89° for both s- and p-polarized light is described. Photoacoustic calorimetry is used in which the piezoelectric transducer is directly attached to the back of the mirror substrate. Determination of absolute absorptance is performed using a laser energy ratiometer at near normal incidence. The procedure for accurately aligning the laser beam to the mirror surface is described, as well as a method to compensate for the transducer dependence on the laser beam footprint shape. This system has been used successfully to measure the absorptance characteristics of metal mirrors and dielectric overcoated metal mirrors at several laser wavelengths. Sample results are given. Other applications of this technique are discussed.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

One important issue with regard to acceleration of electrons in free space using intense laser beams is the phase slippage of the electrons relative to the electromagnetic field. This arises from a phase velocity mismatch between the electron and light wave. Left uncontrolled this slippage can result in degradation of the e-beam characteristics (e.g., emittance) and dispersion of the electron bunches. By a method similar to microwave accelerators, multistaging offers a means to control the e-beam evolution by tuning each successive stage. The phase of the electron bunches relative to the laser field at the entrance of each section determines the acceleration and/or focusing that ensues in that section; therefore, the entrance phase is a natural tuning parameter. It is shown that by controlling the entrance phase it is possible to preserve the e-beam quality, both transverse (emittance) and longitudinal (bunching, energy spread). Calculations of the longitudinal and transverse beam dynamics are performed to determine the evolution of a finite-emittance e beam from stage to stage. By this method the conditions on entrance phase that allow successful e-beam trapping are found. It is also shown that conditions that assure e-beam trapping automatically preserve the overall beam quality.

Electronic Resource