In order to contribute toward the development of a highly-repetitive TEA-CO2 laser, small-signal gains are measured for
a double-pulse operation of a laser medium in a supersonic flow at a Mach number of 2. It is found that the time interval
of the double-pulse operation should be longer than 60 μs in order to have the gain of the subsequent pulse comparable
to that of the preceding one. It is also found that the gain is enhanced with a low-temperature laser medium owing to the
concentration of excited CO2 molecules in the state of a specific rotational quantum number. The results suggest the
possibility that the output power of a TEA-CO2 laser device can be increased by utilizing the supersonic flow.
A TEA-CO2 laser with a supersonic gas flow has been developed, and the small-signal gain of a laser medium is
measured. During the generation of the supersonic gas flow, the laser medium is cooled by the adiabatic expansion
through a supersonic nozzle. Since excited CO2 molecules in the cooled laser medium are concentrated within a specific
range of the rotational quantum numbers J, the laser output and its wavelength should be changed by the cooling. An
investigation is conducted with a single-pulse excitation discharge in the laser medium flowing at Mach number 2. The
laser medium is cooled to 133 K under this condition. It is found that the breakdown voltage, the current, and the power
density of the excitation discharge are estimated to be 22 kV, 3.8 kA, and 1.2 MW/cm3, respectively. The small-signal
gain for this cooled laser medium is measured to be 2.2 %/cm, which is 1.3 times as high as that obtained for the laser
medium at room temperature. It suggests that the TEA-CO2 laser with a supersonic gas flow has a potential for higher
laser output. We also find that the wavelength for the maximum gain is 10.494 μm (J = 10) at 133 K, while is 10.551 μm
(J = 16) at room temperature.
A double-pulse discharge method is used to simulate high-repetition-rate excitation discharge in TEA gas laser with
supersonic gas flow. The supersonic gas flow is generated using a Ludwieg tube with a two-dimensional shock-free
nozzle. A solid electrode with UV pins is used to generate the discharge. The test gas is a mixture of He and Ar (He:Ar =
9:1) with the density of 0.52 kg/m3 in a discharge cavity. In supersonic gas flow with the Mach number M = 2 (v = 860
m/s), not only gas density depression but also shock wave produced by the previous pulsed discharge has a key effect on
stability of the subsequent discharge. For pulse repetition rate f = 60-25 kHz, the gas density depression has already been
removed from the discharge cavity, whereas the traveling shock wave against the supersonic gas flow still remains.
Hence the subsequent discharge becomes arc discharge. For f≤17 kHz, on the other hand, the subsequent discharge
becomes glow discharge because both the shock waves and gas density depression have already been removed from the
discharge cavity. A formula for estimating the maximum repetition rate of stable excitation discharge train in supersonic
gas flow is proposed.
The repetition rate of excitation discharge in TEA gas lasers is limited with various factors such as shock waves, heated gas and contaminations. Use of a high-speed flow is essential to achieve stable discharge in high-repetitive operation. In the present paper, the characteristics of single pulse discharge in a supersonic flow using spark UV pre-ionization and solid electrode are studied as the first step for the high-repetitive excitation discharge. A Ludwieg tube with a two-dimensional shock free nozzle is used to generate a supersonic flow with Mach number 2 in the discharge cavity, and several supersonic flow channel configurations chosen from the viewpoints of electric field and aerodynamics are tested. Shadowgraph technique is applied to visualize the gas density disturbance, which is caused by shock waves and boundary layers inside the discharge cavity. It is confirmed that the uniformity of gas, which can be achieved by suppressing the generation of shock waves, is important for the achievement of stable excitation discharge. Within the scope of the present investigation, the downstream-widened channel with upstream covered solid electrode is the best selection for the excitation discharge pre-ionized by spark UV pins in a supersonic flow.
Influences of shock waves with Mach number of 1.1-1.35 on excitation discharge in transversely excited atmospheric gas laser have been investigated eliminating the other factors. The shock waves are produced by using a shock tube with the gas mixture of helium and argon. The schlieren photographs of shock wave and direct images of light emitted from the discharge are recorded simultaneously by a streak camera. It is found that the discharge does not collapse with the shock wave of 1.1 in Mach number. The shock waves at Mach number above 1.2 tend to collapse the glow discharge in spite of no halogen gas. If the shock wave does not reach to the center of the electrode, glow discharge occurs only in front of the shock wave. Even if the shock wave passes through the center of the electrode, the glow discharge occurs, however, the discharge concentrates in the tight space between the shock front and the edge of the electrode. A streamer exists in the shock front when the shock wave just reaches the edge of electrode. It becomes clear that the discharge characteristics depend on the Mach number of shock wave and the position of shock wave.
The influence of shock waves on the excitation discharge for TEA gas laser have been investigated eliminating the other factors which may affect the discharge instabilities, such as gas density depletion, discharge products, residual ions and electrode heating. A shock wave of 1.2 in Mach number is produced by a shock tube with gas mixture of helium and argon in order to simulate the ArF excimer laser. The schlieren photographs of shock wave and direct images of light emifted from the discharge are recorded simultaneously by a high-speed image-converter camera. It is found that, if the shock wave does not reach to the discharge region, glow discharge occurs only in front of the shock wave. Even if the shock wave passes through the middle of discharge region, the glow discharge occurs only in front of the shock wave. However, an arc-like filament through the shock front is also produced. If the shock wave passes through the discharge region, the weak glow discharge can be produced again, however, a surface discharge is also produced between the main electrode and the pre-ionization electrode. It becomes clear that the discharge instabilities depend on the location of shock wave in the discharge region.
The floating particles produced by the excitation discharge for TEA gas laser have been visualized by the pulsed-laser reflection method. The double-pulse discharge experiments have also been carried out to study the effects of the floating particles on the discharge instabilities. Two kinds of gas mixture are used to simulate KrF excimer laser; F2/Kr/He/Ne (gas A) and Kr/He/Ne (gas B). The particles with diameter of the order of 100 jtm are observed in the discharge region. In both gas mixture, the number density ofthe particles increases to ~ 3 particle/cm3 at 200 ms after the discharge, and then decreases to ~ 1 particle/cm3 at 500 ms. On the other hand, the double-pulse discharge characteristics are very different between gases A and B. In the gas A, the second discharge tends to be an arc when the pulse interval decreases to 300 ms. However, the arc does not occur in the gas B. It is deduced that the floating particles of the order of 100 ?m in diameter do not strongly affect the discharge instabilities.
The influences of gas density depletion on the highly- repetitive, high-pressure, pulsed glow discharge for excitation of excimer laser have been investigated eliminating the other instabilities, such as shock waves, residual ions, discharge products and electrode heating. The gas density depletion is simulated by utilizing a subsonic flow between the curved electrodes. The comparison has been made on the discharge occurred in the presence of the gas density depletion with the second discharge on the double-pulse experiment. We have found that the big gas density non uniformity, (Delta) (rho) /(rho) 0 approximately 3.6% corresponding to a pulse repetition rate (PRR) of approximately 20 Hz, tends to cause the arc discharge without the shocks, ions, discharge products and electrode heating. On the other hand, the second discharge on the double-pulse experiment becomes arc discharge in much smaller non uniformity ((Delta) (rho) /(rho) 0 approximately 1.2% corresponding to PRR approximately 3 Hz). The arc discharge in the double-pulse experiment might be driven by the residual ions and/or discharge products other than gas density depletion except for PRR greater than 20 Hz.
In excimer lasers, the excitation discharge causes various instabilities in subsequent discharge which collapse the highly-repetitive operation. The present study has investigated the effects of the gas density depletion on the excitation discharge instability eliminating the shock waves, the residual ions and the discharge products. The gas density depletion is simulated by utilizing a subsonic flow between the curve electrodes. We have compared the discharge that occurred in the gas density depletion with that by the double- pulse experiment in the stable gas. The gas density distribution is observed by using a Mach-Zehnder interferometer combined with a high-speed image-converter camera. We have found that the big density non-uniformity tends to cause the arc without the shocks, ions and products. The transition from glow to arc with respect to the gas density depletion occurs almost discontinuously. On the other hand, the second discharge on the double-pulse discharge becomes arc in much smaller non-uniformity, where the transition occurs very slowly. Therefore, it can be suggested that the discharge instability is also caused by some factors other than gas density depletion.
Shock waves and disturbed gas generated by an excitation discharge in an excimer-laser cavity have been visualized by the shadowgraph technique. The influences of HCl and Xe concentration on the generation of shock waves have been investigated in a He/Ne gas mixture. At the higher HCl concentration, the distribution of gas density in the discharged region, where the gas is rarefied due to the expansion, seems to be strongly jagged, and the strong shock waves propagate toward the upstream and the downstream directions along the flow axis. At the lower HCl concentration even with the high Xe concentration, on the other hand, the density distribution in the heated column is fairly smooth and the shock waves are very weak. We have also successfully demonstrated the double-pulse glow discharge of a pulse interval of 200 microsecond(s) using a high-speed He-Ar gas flow of approximately 200 m/s. At the pulse interval of 100 microsecond(s) , however, the second discharge, which seems to be arc, occurs through the heated column which is swept downstream but still contains the waste products of the first discharge.
The shock waves generated by an excitation discharge on XeCl- excimer laser have been visualized by a shadowgraph technique. The propagation velocity of the shock waves has been measured by using an image converter camera. The propagation velocity is estimated to be approximately 530 m/s for the shock wave beating between the main electrodes, which persists for a long period between the discharge region. On the other hand, the shock wave propagating along the flow axis, which originates from the boundary between the heated column and non-heated region, propagates initially with the velocity of approximately 730 m/s. It is clearly explained by using a plane blast wave theory that the propagation velocity of this shock wave slightly decreases as the shock wave propagates.
The breakdown characteristics of a short-distance discharge gap in an atmosphere by TEA- CO2 laser have been studied to control the lightning artificially. It is efficient to enhance the probability of electrical breakdown induced if the focal point is set behind the discharge gap axis or near the negative high-voltage electrode. The length of the optical-breakdown plasma channel is elongated by using the micro-particles diffused in an atmosphere. Using 3 micrometers -diam. aluminum particles, the optical-breakdown threshold is lowered to 15 MW/cm2 compared to 0.5 GW/cm2 in the absence of the micro-particles.
A high-speed wind tunnel, made by using a Ludwieg tube, has been successfully developed for a highly repetitive discharge-pumped excimer laser. This apparatus allows the gas flow of velocity approximately 204 m/s, pressure approximately 293 kPa, temperature approximately 254 K, and duration time approximately 48 ms. The rate constant for the recombination process of Xe+ + Cl- + Ne yields XeClX + Ne is found to increase to a maximum of 4.2 X 10-6 cm3/s at 180 K in a gas pressure of 294.2 kPa. The kinetic simulation of XeCl excimer laser using such a rate constant indicates the enhancement of the laser output in the lower gas temperatures.
To reduce the load on switching devices, a new type of all-solid-state excitation circuit has been successfully developed, where only one GTO thyristor is utilized in the switching device and a saturable transformer is used as the magnetic pulse compressor. We have used a saturable transformer with a winding ratio of 1:7. The initial pulse of 5.5 kV, 1.7 kA, 1.0 kA/microsecond(s) was transferred to the output pulse of 32.7 kV, 8.7 kA, 282 kA/ns. Employing this circuit for XeCl-excimer laser, we have succeeded in lasing the output energy of approximately 30 mJ/pulse.