Michihiro Uchiumi

Differential absorption lidar at 1.67 μm for remote sensing of methane leakage

A differential absorption lidar (DIAL) for field monitoring of methane (CH4) leakage at a wavelength of 1.67 µm was developed. Compared with traditional DIAL systems for environmental monitoring, this system has a higher distance resolution (~15 m) for determining the leak position and a shorter detection range up to 500 m. First, considering appropriate design parameters, a theoretical simulation was performed to evaluate the sensitivity and the detectable range of the system. Based on the analytical simulation, a prototype DIAL system was constructed and the detection of CH4 which had leaked into the atmosphere was demonstrated. The CH4 leakage of 6000 ppmm at a distance of 130 m was successfully detected. The detection limit was 1000 ppmm. With the improvements in the light source and the detector system, the detectable boundary can be increased in the range from 90 to 540 m for a concentration of 1500 ppmm.

Operation of rhodamine 6G dye laser in water solution

The ethanolic solution of Rhodamine 6G is used for dye lasers in the atomic vapor laser isotope separation (AVLIS) system. However, the water solution is preferable from the standpoint of easy treatment and safety. In this paper, we systematically investigated the laser efficiency of a Rhodamine 6G dye laser pumped by a frequency-doubled Nd:YAG laser, for various water-based solutions with surface-active agents and alcohols. By adding an appropriate amount of surface-active agent (Triton X or Emulgen) or alcohol (ethanol or propanol), we obtained almost the same level of laser efficiency as in the pure ethanolic solution. The improvement in the laser efficiency with these additives was attained by the deaggregation of nonradiative dimers. It was also confirmed that the water-based solution showed better performance against thermal distortion than the ethanolic solution, and was suitable for copper vapor laser (CVL) pumping.

Characteristics of space charge in polyethylene/ethylene-propylene-terpolymer laminates

This paper presents experimental results of space-charge distribution in polyethylene (PE) in contact with rubbers by the pulsed electroacoustic (PEA) method. The rubbers used were ethylene-propylene copolymers (EPM) and ethylene-propylene-terpolymers (EPDM) with diene components of dicyclospentadiene (DCPD) and 5-ethylidene-2-norbornene (ENB). The influence of cross-linking on space charge in the EP rubbers under dc stress was examined. The cross-linking caused a polarity reversal of space charge in the interface between PE and EP rubber with the diene components. However, the cross-linking did not affect space charge in the interface between PE and EPM.

Characteristics of space charge formed in a laminated LDPE/EVA dielectric under DC stress

A laser-induced pressure pulse (LIPP) method was used for measuring the space charge distribution of LDPE/EVA laminate dielectrics under DC stress. A constant voltage up to /spl plusmn/20 kV was applied to a side of laminates of 0.5 mm thickness for 30 minutes. The other side is grounded. When the amount of space charge was measured by LIPP, both sides were virtually grounded. Space charge built up in or near the interface between LDPE and EVA was mainly investigated. Positive and negative voltage was applied to the side of LDPE in the laminates. It was clarified that the space charge was larger in case of LDPE negatively biased than in case of LDPE positively biased. The density of the space charge ranged around 1 nC/mm/sup 3/. The formation of interfacial space charge is analyzed.

Characteristics of interfacial space charge in several laminated dielectrics

Interfacial space charge in several laminated dielectrics was measured by the laser induced pressure pulse (LIPP) method. A low density polyethylene sheet was laminated by several kinds of materials such as ethylene vinyl acetate, ethylene propylene copolymers, ethylene propylene-diene terpolymers with the diene components of dicyclopentadiene and ethylene propylene diene terpolymers with 5-ethylidene-2-norbornane. Interfacial space charge formed in such dielectrics by application of dc voltage and its decay process are evaluated and analyzed in terms of permittivity and electrical conductivity of the materials used. Two or three decay time constants are obtained for each of the laminates and are correlated with the electrical and dielectric properties of the dielectrics used.

Formation and annihilation characteristics of space charge in PE/EVA interfaces

Interfacial space charge was measured for laminated dielectrics by the laser-induced pressure pulse (LIPP) method. Specimens used for experiments consisted of low-density polyethylene (LDPE) and ethylene vinyl acetate (EVA) of thickness 0.25 mm each and were subjected to d.c. voltages up to 10 kV. The voltage was continuously applied until the space charge in the interface increased nearly to the saturation of the charge accumulation. It was clarified that the space charge formed at the interface between the two dielectrics was around 10 to 100 /spl mu/C/m/sup 2/, which was smaller than the induced electrode charge, and was long-lived after the cessation of voltage with more than two kinds of decay times. The process for formation and annihilation of interfacial space charge is presented.

Development of DIAL for CO2 and CH4 in the atmosphere

Development of a mid-infrared differential absorption LIDAR system around 2.0 and 2.3 tm wavelength is proposed for remote sensing of greenhouse-effect gases, such as CO2 and CH. A model calculation shows that the 1.6 to 2 ptm wavelength region is also suitable for the CO2 measurements. A widely tunable self—injection seeded Ti3: sapphire laser in combination with a Raman shifter is used for a tunable mid-infrared laser source. This combination could allow a wide tuning range between 1.6 and 2.3 .tm. The spectral width of the Ti3: sapphire laser was measured around 0.04 cnf 1 and stability of the laser source was also studied by a Fabry-Perot interferometer in order to determine theaccuracy of the DIAL measurement. Currently, output energy is limited to only a few mJ limiting the detection sensitivity. Nevertheless improved output characteristics should meet the requirements satisfactorily to monitor CO2 and CH gases in the atmosphere.

Laser-induced pressure pulse measurements of space charge in PE/EVA laminates

The LIPP method was used for measuring the charge distribution at the interface between PE/EVA laminate dielectrics under dc stress. High power laser light of 300 mJ/pulse was applied to a specially coated surface of the specimen. A short (300 ps) and a long (5 ns) laser pulse were used for thin and thick specimens, respectively. The result of the measurement shows that space charge was accumulated at the interface, When it was positive, the space charge stayed at the interface on the EVA side and was not transferred to the interface on the LDPE side. On the contrary, when it was negative, the space charge was transferred to the interface on the LDPE side.

Laser-induced discharge in a rod–plane electrode in the presence of a negative dc electric field

Discharge induction experiments were performed between a plate and a rod on a plate electrode configuration with a gap length of 1 m using a high-power CO2 laser and a dc voltage generator. The electrodes are parallel planes. Each plate electrode consists of a central flat part having a diameter of 3 m with a circular edge on a cross section. The length of the rod is 10 cm. A chain of plasma beads of length 10 cm was created on the tip of the rod by the CO2 laser and used for artificial triggering of negative high-voltage sparkover. The behavior of streamer, leader, and return stroke was observed by an image converter camera. It was found that a positive upward traveling leader can be triggered from the tip of the rod on the lower plane to the upper plane by the chain of plasma beads created by the CO2 laser. This apparatus is useful for study for realization of laser-induced lightning.

Resonant Multiphoton Ionization of H2 by an ArF Laser and Application to Atmospheric Molecules

When a tunable ArF laser beam was focused into a pulsed H2 gas jet, three-photon ionization and gas breakdown were resonantly enhanced at 193.28 nm. The ionization efficiency was calculated using a rate equation model. Based on the results, resonant multiphoton ionization of atmospheric species was discussed, considering the application of this scheme to laser-controlled lightning. N2 molecules can be efficiently ionized at a wavelength of 283.1 nm by a tunable laser. Applying a similar analytical model to N2 allowed the estimation of the laser energy required to form a long (100 m) weakly ionized (1013 cm-3) channel in the atmosphere.