Hirokazu Hokazono

Theoretical analysis of a self-sustained discharge pumped XeCl laser

Theoretical analysis of the discharge characteristics and the output performance of a self‐sustained discharge XeCl laser is described. Validity of the theoretical laser model including the excitation circuitry is confirmed by comparing the results with the measured discharge and output performance under lasing conditions. The dischare parameters such as E/P (E is the electrical field strength and P is the operating pressure) and discharge resistivity are theoretically studied for both Ne‐ and He‐based gas mixtures. Our model shows that the electron energy distribution functions of these two mixtures become quite equal at each quasi‐steady‐state E/P, and that the improved laser output performance with Ne‐based gas mixtures is not due to the difference of the electron energy distribution function but due to the good optical extraction caused by the faster ion‐ion recombination excimer formation channel. Moreover, the model also predicts that the depletion of HCl molecules is one of the most serious problem...

Theoretical operational life study of the closed‐cycle transversely excited atmospheric CO2 laser

By using a comprehensive theoretical model that assumes a stable excitation discharge and homogeneous plasma chemical reactions in the discharge plasma, the laser output performance and the variations of the laser gas components during the sealed‐off operation of the high‐power, closed‐cycle transversely excited atmospheric CO2 laser have been investigated. The fractional CO2/N2, molecules decomposition, and the concentration of the various minor impurities accumulated in the laser gas mixture have been theoretically calculated as a function of shots and number of repetitive discharge pulses. According to the results, the gradual reduction of the laser output energy with the successive excitation pulses was mainly due to the depletion of the CO2 molecules and the reduction of the excitation efficiency; the excitation efficiency was decreased in consequence of the increased operational E/N (E is the discharge field strength, N is the total laser gas number density) caused by the accumulation of highly electronegative impurities such as O2 and O3. The nitrogen oxides were found to show little effect on the operational E/N in spite of their large electron attachment cross sections, because these molecules were much less accumulated in the laser gas mixture than O2 or O3. The theoretical model has clarified for the first time that a trace of water (H2O) vapor in the laser chamber effectively acts as a gaseous catalyst to enhance the CO2 reforming reaction in the discharge plasma. Furthermore, this CO2 reforming reaction by H2O, rather than the other backward reactions, predominantly determines the equilibrium CO2 decomposition level in the actual laser chamber. Finally, with regard to the ultraviolet (UV) preionization, it was theoretically shown that the UV absorption depth of the laser gas mixture steeply decreased as the CO2 decomposition increased owing to the contamination of strong UV absorbing species such as O2 and O3.

Theoretical analysis of the CO2 molecule decomposition and contaminants yield in transversely excited atmospheric CO2 laser discharge

Theoretical studies on the phenomena of a CO2 molecule decomposition and contaminants yield in transversely excited atmospheric CO2 laser discharge plasma have been conducted by our comprehensive plasma kinetic model. In addition to the reliable 175 plasma kinetic rate equations, the excitation circuit and the steady‐state Boltzmann equations were included in the theoretical model in order to exactly simulate the time‐dependent discharge condition. When the total capacitance and the charging voltage of the main discharge capacitor were varied over a wide range, the amount of the CO2 molecule decomposed per discharge pulse was found to be almost in proportion to only the deposited energy density: the energy deposited in the unit laser gas volume. It was also found that the amounts of CO, O2, O3, N2O, NO2, and NO yielded per discharge pulse increased almost proportionally with the increase of deposited energy density. The amount that the CO2 molecule decomposition and contaminants yielded against the unit d...

Reduction of the CO2 decomposition in the transversely excited atmospheric CO2 laser discharge plasma by a very small amount of the water vapor

Using a comprehensive theoretical model, we have theoretically clarified for the first time that the homogeneous catalytic reaction caused by a very small amount of water vapor (<100 ppm) in the closed‐cycle, transversely excited atmospheric (TEA) CO2 laser discharge plasma could considerably reduce the saturation value of the CO2 molecules decomposition. Effects of the water vapor at this concentration level have been overlooked in the previous investigations. A small concentration variation of the residual water vapor in the experimental laser chambers is thought to be one of the reasons for the considerable disagreements among the published data about the CO2 equilibrium decomposition level.

Catalytic control requirements for the stable operation of the closed‐cycle, transversely excited atmospheric CO2 laser

Using a comprehensive theoretical CO2 laser model, we calculated the amount of the catalytic conversion from CO to CO2, which is required for the stable and long‐life operation of the closed‐cycle, transversely excited atmospheric CO2 laser. The amount of the catalytic conversion was evaluated by the fractional conversion (η) defined as η=([CO]in−[CO]out)/[CO]in, where [CO]in and [CO]out are the CO molecules number density at the entrance and exit of the CO2 regenerator in the laser chamber, respectively. For the laser gas mixture of CO2/N2/He=15/15/70(%) and the output coupler reflectivity of 80%, an η of the CO2 regenerator greater than 0.22 is theoretically required to maintain the 95% level of the initial laser output energy at an input energy density and a laser gas clearing ratio CR of 176.1 J/l and 2.0, respectively.

Plasma kinetic study of high-power, high-repetition-rate, closed-cycle transversely excited CO2 laser

We have developed a comprehensive theoretical model dealing with both variations of laser output energy and laser gas composition as a function of the repetitive laser pulse in order to evaluate the performance characteristics of the high-power, closed-cycle transversely excited (TE) CO 2 laser. According to our analysis, the number of CO 2 molecules decomposed per unit input energy of single discharge pulse was calculated to be 1.45x10 17 molecules/J for the laser gas mixture of C0 2 /N 2 /He=l5/l5/70. The number of N 2 molecules decomposed per unit input energy density was calculated to be about 24000 times smaller than that of CO 2 . Our model theoretically predicted for the first time that a very little quantity of water vapor ( 2 decomposition level, resulting in increasing the laser output energy.

High‐power, long‐pulse CO2 laser transversely excited by a damped oscillating discharge through dielectric electrodes

Applying a damped oscillating discharge to a dielectric‐electrode TE (transversely excited) CO2 laser, we obtain a high‐power and long‐pulse CO2 laser output. With a decreasing value of the inductance inserted in parallel with the discharge load so as to make the discharge condition damped oscillating, laser output energy increased remarkably. When the inductance was 500 nH, a uniform glow discharge was obtained up to gas pressures of 500–600 Torr without any preionization. At a total pressure of 700 Torr (CO2/N2/He=1/2/8), a maximum CO2 laser output energy of 270 mJ/pulse (7.3 J/l atm) was obtained with an electrical efficiency of 4.3% and with a pulse duration of 7 μs.

A high efficiency HF(H2/F2) chemical laser initiated with a surface‐spark ultraviolet flash

As a novel alternative to the conventional Xe flash lamps, a surface‐spark‐discharge ultraviolet (UV) flash with a poly‐tetra‐fluoroethylene (Teflon) resin was utilized to an initiation of a H2/F2 chemical laser. Because we employed a high‐voltage and low‐inductance power supply to drive a surface‐spark flash, a stabilized surface discharge in a high pressure (≃1 atm) laser gas and high efficiency power input to an UV flash have been realized. We have obtained a maximum HF laser output energy of 8.5 J/pulse (14.6 J/ ∂) at an electrical efficiency of 9.7%. A surface‐spark light source is experimentally found to be able to uniformly initiate a large‐volume laser gas mixture.

Discharge characteristics and output performance of a UV‐preionized XeCl laser

Discharge characteristics and output performance of a self‐sustained discharge XeCl laser have been investigated exxperimentally and theoretically. The operating E/P and discharge resistivity in a quasi‐steady state were studied with Ne‐ and He‐based mixtures; E is the electric field strength and P the operating pressure. Electron energy distribution functions of these mixture became almost equal at each E/P. Small absorption and faster gain build‐up due to faster ion‐ion recombination resulted in more efficient energy extraction in Ne‐based mixture. Moreover, our theoretical model predicted the further scaling limitation not due to the build‐up of nonsaturable absorption but due to the consumption of HC1.

Analysis of gas degradation under a closed-cycle high-repetition-rate operation of a transversely excited atmospheric CO2 laser

Using the newly developed 1 kHz closed-cycle TEA CO2 laser with the efficient CO2 regenerator including the Pt/Al2O3 solid catalyst, we experimentally determined the minimum operational performance of the CO2 regenerator which is required for the stable and long-life operation of the closed-cycle TEA CO2 laser. The operational performance of the CO2 regenerator was evaluated by the fractional conversion (eta) from CO to CO2, which is defined as the ratio of the differential CO2 concentration increased by the CO2 regenerator to the total CO concentration introduced into the CO2 regenerator. The minimum eta of 0.07 was at least required to keep the laser output power at 95 percent of the initial laser output for the laser gas mixture of CO2/N2/He = 15/15/70 (percent) and at an input energy density and a clearing ratio of 150 J/l and 6.0, respectively. When operating the CO2 regenerator at eta of 0.10, no appreciable reduction of the initial laser output of 570 W due to CO2 decomposition was observed up to 1.8 x 10 exp 7 shots (5 hrs). At this time, gas analysis showed that the CO and O2 concentration in the laser gas mixture was maintained about at 0.17 percent and 0.055 percent, respectively.