Stratified gas-liquid two-phase electrohydrodynamics in horizontalpipe flow

Stratified gas-liquid two-phase electrohydrodynamics in pipe flow has been studied experimentally and numerically. Experimental studies were conducted using 1.27 and 1.9×10^-2 m-inner diameter horizontal tubes with air-water two-phase flow for the range of gas surface velocity of 10±²-10 m/s, liquid surface velocity of 10^-2-2×10^-1 m/s, and applied voltage of 0-20 kV. Experimental results are analysed by an area-averaged two-fluid one-dimensional model. The results show that the effect of the applied electric field is significantly influenced by the flow regime transition boundaries between stratified smooth-to-wavy and stratified wavy-to-intermittent flow. However, the time-averaged void fraction was not observed to be significantly influenced by applied electric fields     

Nonequilibrium electron distributions and high-field electrontransport in an AlxGa1-xAs-based p-i-nnanostructure semiconductor-a picosecond Raman probe

Electron transport in an Al_xGa_1-xAs (x=0.3) based p-i-n nanostructure semiconductor under the application of an electric field has been studied at T=80 K by picosecond transient Raman spectroscopy. Single-particle excitations associated with spin-density fluctuations were used to directly measure electron distribution functions and drift velocities under various electric field intensities. Extremely nonequilibrium electron distributions were observed. Specifically, for an injected carrier density of n≅1×10¹⁸ cm^-3, a drift velocity V_d as high as 2.5×10⁷ cm/s was measured for an electric field intensity E=18 kV/cm. These experimental results are in good agreement with Ensemble Monte Carlo calculations     

Winter lightning on Japan Sea coast-development of measuring systemon progressing feature of lightning discharge

With the aid of recent optoelectronic techniques, the authors have developed a measuring instrument named ALPS (Automatic Lightning Discharge Progressing Feature Observation System) to determine the progress of lightning flashes. The progress velocities of stepped leaders for lightning in winter seasons were observed using the ALPS in the coastal area of the Japan Sea. The progress velocity of an individual step of a stepped leader can be expressed by a lognormal distribution and its mean value is 1.6×10⁶ m/s for upward leaders and 4.0×10⁶ m/s for downward leaders. The mean progress velocity of a total leader process is given as 0.8~2.7×10⁵ m/s for upward leaders and 2.9×10⁵ m/s for downward leaders     

Molecular dielectrophoresis of biopolymers

Dielectrophoresis (DEP) has found wide applications in manipulations of micrometer-sized particles such as biological cells. However, it has been considered less effective for nanometer-sized particles such as biological macromolecules where DEP effects may be overwhelmed by Brownian motion. Here, the authors use the fluid integrated circuit (FIC) concept to fabricate micrometer-sized electrodes, with which a very high-intensity, steady-state, high-frequency field, over 1×10⁶ V/m at 1 kHz to 10 MHz, is created. The observation of molecular DEP using avidin (68 kD) and other biopolymers is made, and it is found that DEP occurs at the field strength of 0.4-1.0×10⁶ V/m, which is substantially lower than DEP theory predicts. As applications of molecular DEP, a novel molecular patterning method and “dielectrophoretic chromatography” are proposed. In the latter, biopolymer solution is moved through a high-intensity field region created in a FIC, and the polymer is analyzed and/or separated by the difference in the DEP attraction     

Image quality analysis for dual energy subtraction imaging with afemtosecond laser-based hard X-ray source

We present a study of image quality for dual energy subtraction imaging using an iodinated contrast agent and a femtosecond laser-based hard X-ray source. The INRS CPA laser (400 fs pulse focused on solid targets in a 3 μm spot at 4 × 10¹⁸ W cm^-2) was used to create a bright hard X-ray source (conversion efficiency of 10^-5 in the characteristic K_∝ line emission, 12 μm X-ray source diameter). A model of image quality has been developed and been benchmarked with specific experiments using specially made angiography phantoms     

1.047-μm Yb:Sr5(PO4)3F energystorage optical amplifier

The pumping and gain properties of Yb³⁺-doped Sr₅ (PO₄)₃F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm² based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF₄ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0×10^-20 and 1.5×10^-20 cm ² for π and σ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3×10^-20 and 1.4×10^-20 cm² for π and σ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25×25×175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics (<~1%/cm) and adequately large laser damage thresholds at 1064 nm (~20 J/cm² at 3 ns). Overall, diode-pumped Yb:S-FAP amplifiers are anticipated to offer a viable means of amplifying 1.047-μm light, and may be particularly well suited to applications sensitive to overall laser efficiencies, such as inertial confinement fusion energy applications     

Wurtzite GaN-based heterostructures by molecular beam epitaxy

Progress in plasma and reactive molecular beam epitaxy (PMBE and RMBE) grown n- and p-type GaN and GaN-AlGaN-based epitaxial films and optoelectronic devices is reviewed. The growth of GaN by RMBE (PMBE) is achieved by employing ammonia gas (plasma activated nitrogen) as the nitrogen source with resultant growth rates of about 2 μm/h (⩾1 μm/h). The structural, electrical, and optical properties of binary and ternary (Al,Ga)N and (In,Ga)N layers point to high quality. The GaN layers with Mg as the dopant atoms are p-type without any postgrowth treatment, but the hole concentrations are limited to mid 10¹⁷ cm^-3 although reports in the low 10¹⁸ cm^-3 dot the literature. The background carrier concentration, mobility, optical characteristics and ability to dope p-type depend significantly on the substrate temperature and V-III ratio employed, AlGaN-GaN, and GaN-InGaN electroluminescent devices have been realized but lack commercial quality. The AlGaN-GaN photodiodes by RMBE exhibited a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than three orders of magnitude for wavelengths longer than 390 nm. A reverse bias of -10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The noise equivalent noise power near zero bias is below the detection limit of the measurement setup. At a reverse bias of 28 V, the total noise equivalent power is 2.06×10^-11 W     

VUV laser in the Lyman band of molecular hydrogen pumped by fstitanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60° onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm^-1 and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5×10¹⁵ cm^-3. A simple model of the H₂ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping     

InGaN-based blue laser diodes

The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h. The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively. The threshold current density was 8.8 kA/cm². The carrier lifetime and the threshold carrier density were estimated to be 3.5 ns and 1.8×10²⁰/cm³, respectively. The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV. Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state. From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 5.8×10^-17 cm², 9.3×10 ¹⁹ cm^-3, 5200 cm^-1, and 43 cm^-1 respectively     

Diagnostic dielectric spectroscopy methods applied to water-treedcable

Considerable effort has gone into developing polymer formulations and cable designs to minimize failures through water tree growth. However, diagnostic techniques still are required to enable the estimation of the level of damage present within a service cable. This paper reports on progress regarding the application of dielectric spectroscopy to cable diagnostics. A 40 kV, crosslinked polyethylene (XLPE) insulated coaxial cable was used as a model power cable. Sample lengths were immersed in a potassium chloride solution and some of these were subjected to AC electrical stress. After an 8 week duration, a high density of tress was found in the electrically stressed cable. Dielectric spectra have been measured for both sample types in the frequency range of 10^-5 to 10⁵ Hz. Insertion loss measurements were also carried out in the frequency range of 3×10 ⁵ to 3×10⁹ Hz. From both types of measurement, it was possible to distinguish between the cables containing water trees and those that were free from water tree structures. These approaches could therefore be developed in order to provide diagnostics for the detection of water tree damage in electrical power cables     

Electrical conduction in fluoropolymer films

Isothermal charging, discharging and transport currents in fluoropolymer films are measured in the temperature range of 50 to 200°C at electric fields to 40 MV/m. The currents are measured as a function of time elapsed after the application of steady state voltages in the range of 1 to 10⁴ s. Charging currents increase with time to 10³ s, depending upon the electric field and temperature of the polymer. For longer duration the currents decrease with time. Discharging currents decrease monotonically with time and the discharge current-time characteristics are dissimilar to charging current-time characteristics. Transport currents which are derived by subtracting the discharging currents from charging currents are shown to arise due to Schottky emission assisted by the electric field. At shorter time intervals (<10³ s), it is suggested that the electric field at the cathode increases due to heterocharges and gives rise to currents increasing with time, while for longer times, gradual filling of the traps results in a decrease of the current. Low frequency dielectric loss factors are evaluated from the discharging current measurements     

Effects of liquid conductivity upon gaseous discharge of droplets

The effect that liquid conductivity has upon gaseous breakdown and conduction between a droplet and a sharp grounded metal point was investigated as a function of the droplet charge level and point-to-droplet gap. A uniform stream of equally spaced 1210 μm droplets was studied in passing the point at 2.05 m/s at a rate of 465/s. The negative droplet charge was set at 24, 40, 55, and 60% of the Rayleigh hydrodynamic instability limit (i.e. 3×10^-10 C) for liquid conductivity values in the 10^-4-10¹ S/m range characterizing electrostatic crop sprays. No significant conductivity effect was found for charges up to the 55% level; the most conductive liquid exhibited a significantly (α<0.10) higher discharge current at the 60% charge level. For close gaps, droplets departed the grounded point region with a reversed charge, indicating they were overneutralized by a positive-ion flux from the grounded point     

Polarization and dielectric behavior of ac-aged polyethylene

High field polarization and dielectric relaxation behavior have been used to study the aging mechanisms of crosslinked polyethylene (XLPE) and low density polyethylene (LDPE) aged at constant alternating electrical field stress in a humid environment at room temperature. For this study, the dielectric spectroscopy data in the frequency range of 10^-5 to 10⁶ Hz and their comparative analysis, have been used to provide electrical analog models of the aging. The dielectric data in the frequency range of 10^-5 to 5×10 ^-2 Hz were calculated from the desorption current, whereas bridge measurements provide the data in the frequency range of 10 Hz to 10⁶ Hz. In addition, thermal transient current and thermally stimulated discharge current measurements have been carried out in order to determine the polarization and space charge accumulation in ac aged polyethylene. A correlation has been observed in the behavior of polarization and dielectric relaxation in the present work. The dielectric behavior of the XLPE cable samples and of the LDPE flat samples show a presence of both the inter-cluster (low frequency) charge exchange and the intra-cluster (high frequency) charge motion, the former mechanism becoming more dominant as the aging progresses. For the XLPE cable samples ac aged in a humid environment at room temperature, relaxation peaks obeying the fractional power law have been observed in addition to a quasi-dc process     

Dielectric recovery of a residual SF6 plasma between twoparallel plane electrodes

The present work is a theoretical investigation into the dielectric strength of a thin column of hot SF₆ gas left between two contacts after the thermal quenching of an arc at current zero in a modern HV gas-blast circuit breaker. Because of time rapid decay of arc temperature during current zero, this hot gas is still in a plasma state (referred to as the residual plasma) with sufficiently high charge density (both electrons and ions) although electrical conductivity is low. The dielectric recovery of such a residual plasma differs from that of an infinitely large, uniform plasma in two aspects. Firstly, the presence of space charge can distort severely the local electric field and can influence strongly the generation and loss of charged particles by ionization and chemical reactions. Secondly, due to the finite size of the plasma column, electrons diffuse in an ambipolar manner, thus enhancing the loss of charged particles in the plasma. In the present investigation, the gap length between the contacts (electrodes) is 1.0×10^-2 m and the diameter of the plasma column varies between 50×10^-6 and 500×10 ^-6 m. Distribution of the number density of charged species on the axis is obtained by solving their continuity equations using the method of MacCormack with Shuman filter. It is found that formation of space charge creates strong ionization layers close to the electrodes and that the whole column could deviate from electrical neutrality due to the drift of charged particles. The breakdown field of such a bounded plasma at atmospheric pressure or above is approximately equal to the critical field for an infinite plasma within a numerical uncertainty of 5%     

1.3-μm InAsP n-type modulation-doped MQW lasers grown bygas-source molecular beam epitaxy

The effect of n-type modulation doping as well as growth temperature on the threshold current density of 1.3-μm InAsP strained multiple-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy (GSMBE) was investigated for the first time. We have obtained threshold current density as low as 250 A/cm² for 1200-μm long devices. The threshold current density per well for infinite cavity length J_th/N_w∞ of 57 A/cm² was obtained for the optimum n-doping density (N_D=1×10¹⁸ cm^-3) and the optimum growth temperature (515°C for InP and 455°C for the SCH-MQW region), which is about 30% reduction as compared with that of undoped MQW lasers. A very low continuous-wave threshold current of 0.9 mA have been obtained at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest results grown by all kinds of MBE in the long-wavelength region. The differential gain was estimated by the measurement of relative intensity noise. No significant reduction of differential gain was observed for n-type MD-MQW lasers as compared with undoped MQW lasers. The carrier lifetime was also reduced by about 33% by using n-type MD-MQW lasers. Both reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay is very attractive for laser array application in high-density parallel optical interconnection systems     

InGaN multiquantum-well-structure laser diodes with GaN-AlGaNmodulation-doped strained-layer superlattices

InGaN multiquantum-well-structure (MQW) laser diodes with Al_0.14Ga_0.86N-GaN modulation doped strained-layer superlattice (MD-SLS) cladding layers grown on an epitaxially laterally overgrown GaN substrate was demonstrated to have a lifetime of more than 2300 h under the condition of room-temperature continuous-wave operation. The self-pulsation was observed with a frequency of 3.5 GHz. The relative intensity noise less than -145 dB/Hz was obtained even at the 6% optical feedback using the high-frequency modulation of 600 MHz. The threshold carrier density of the InGaN MQW-structure laser diodes was estimated to be 3×10¹⁹/cm³ using a carrier lifetime of 1.8 ns     

Thermal and electrical analysis of alumina and beryllia coaxhigh-power windows under irradiation

Characteristics of dielectric insulators in vacuum windows of coaxial 10 to 100 MHz transmission lines in high-power steady-state use under irradiation are simulated with respect to electric, nuclear, mechanical, and thermal properties. Neutron fluence ~5×10¹⁸ n/cm² at the window is obtained to be sufficiently small to allow beryllia, but not alumina, to be used as dielectric. In beryllia (10^-3 displacements per atom (dpa) due to irradiation) or in un-irradiated alumina (97.5% purity), the temperature is found to rise by not more than 125°C with maximum stress <140 MPa for 50 kV peak voltage at 60 MHz, provided niobium, titanium or materials with similar thermal expansion coefficients are used in water cooled conductors. The tangential electric field is kept well below the surface discharge limit 2 MV/m by using potential rings together with a sufficiently large inclination angle of the conical ceramic with respect to the radial coaxial direction, but high normal fields exceeding the vacuum breakdown limit are obtained near the potential rings. Abandoning the potential rings and deforming the equipotential lines by shaping the ceramic-conductor joint can reduce both tangential and normal field components below the breakdown limit, which appears to be in agreement with recent voltage test experiments     

A new model for the primary process of electrical breakdown inliquids

A new model for the generation of electrical streamers in insulating liquids is proposed, It is based on the mechanical stress generated by the electric field and its influence on the cohesive properties of the liquid. In fields of 10⁸ to 10⁹ V m^-1 the stress is sufficient to enhance significantly the thermal generation of sub-microscopic rupture surfaces (holes) in the liquid which has solid-like properties in the short time of streamer development. Using the well-known Griffith concept of mechanically-generated crack propagation in solids, it is then argued that, when the population of sub-microscopic holes becomes sufficiently large, the same stress encourages macro-crack development which has all the hallmarks of streamer growth, In this model electrical discharges do not have a traditional primary role although they will have an important secondary role once macro-cracks have developed     

Spectral analysis of the light emitted by streamers in hydrocarbonliquids

This paper presents the results of a spectroscopic study from 200 to 850 nm of the light emitted by streamers initiated in cyclohexane and n-pentane under step voltage in point-plane geometry. Experimental spectra of the light emitted by bush-like and filamentary streamers are composed of the H_α-Balmer line, the C₂ Swan band system and a background continuum. The diagnostic method we used for evaluating rotational and vibrational temperatures of excited C₂ in streamers was first tested on high-pressure corona discharges in nitrogen. For streamers in cyclohexane and n-pentane, it was impossible to determine the rotational temperature of C₂ and consequently the effective temperature of molecules in the streamer. Moreover, we found that vibrational populations of excited C₂ do not follow Boltzmann statistics. This indicates that excitation processes are due to chemical reactions. Electron densities deduced of the Stark broadening of H_α are in the range 4×10 ¹⁶ to 7×10¹⁶ cm^-3 for filamentary streamers and 2 to 6×10¹⁷ cm^-3 during the breakdown phase. For slow bush-like streamers, the electron density is not measurable