Shinichiroh Kosugi

Measurement of Gas Temperature Profile in Discharge Region of Excimer Laser with Laser Schlieren Method

Shock waves are generated by pulse discharges in the cavity of excimer lasers. The shock waves cause arcing, nonhomogeneous excitation of laser gas and limitation of repetition rate of a high-repetition-rate excimer laser. Distribution of temperature rise by pulse discharge is an essential factor for generation and propagation of shock waves. Gas temperature profiles in the discharge region of the excimer laser cavity are measured by a laser schlieren method for single-pulse operations. The results show that the temperature distribution depends on the xenon concentration. In the cases of pure helium and higher xenon concentration, the temperature distributions are steeper than those in the cases of lower xenon concentration.

5 kHz high repetition rate and high power XeCl excimer laser

A 5 kHz high repetition rate excimer laser has been developed. An average laser power of 0.56 kW at 5 kHz operation, which remains stable for 1 s, is obtained. This duty time is limited only by overheating of the switching thyratrons. The 5 kHz operation is attained by some improvements to the gas conditions, the UV‐preionization scheme, and the gas flow speed in the discharge region. The gas is circulated at an effective flow velocity of 137 m/s by a newly developed two‐stage axial blower. The gas conditions are optimized experimentally to achieve 5 kHz operation. To obtain uniform preionization, the locations of pin electrodes for the preionization are determined on the basis of numerical results.

Continuous and pulsed discharges in gas flow lasers

The discharge effects on gas flow lasers have been treated from both aspects of the activating features to the laser performance and of demerits by electrical heating and disturbances in the laser medium. The approach has been from two types of studies, namely on the continuous discharge in CO2 supersonic mixing electric discharge laser (EDL) and on the pulsed discharge in excimer laser. For CO2 supersonic mixing EDL, small scale experiments have been performed on the small signal gain coefficient and on the laser power, together with the numerical analysis by using the quasi-one-dimensional and vibrationally nonequilibrium equation system. Discharge effect is included by solving the Boltzmann equation for electron energy distribution function, and power extraction analysis also is carried out. By comparing the experimental results, various characteristics have been clarified on this type of supersonic discharge laser. As for the pulsed discharge, flow visualization experiments have been conducted in the model cavity of excimer laser, along with a numerical calculation on the one- dimensional Euler equation system by TVD approach. There have been three types of waves in the laser cavity, and Mach numbers of horizontally propagating main waves have been discussed from both numerical and experimental aspects.

Effects of xenon gas on generation and propagation of shock waves in the cavity of excimer laser

High repetition rate excimer lasers are expected for wide industrial application. The power of excimer laser, however, decreases rapidly in a higher repetition rate operation. Shock or acoustic waves, which are caused by the periodic pulse discharge, may limit the repetition rate of an excimer laser up to 2.5 kHz. Such waves cause inhomogeneity of gas density in the discharge region of the excimer laser. In high repetition rate operation this inhomogeneity remains at the next discharge. Arcing may be generated by this inhomogeneity and the homogeneous excitation of the laser gas is obstructed. Although these phenomena have been reported, the research for the effects of shock waves has remained insufficient. And the relation between these shock waves and discharge phenomena has not been clarified. To resolve this problem, we developed a scaling model chamber of a UV preionized excimer laser cavity with windows for flow visualization. We report the first result by using this model and Schlieren technique in a pure helium gas case. In our experiment three types of shock waves are found in the discharge cavity. Those shock waves are generated from the boundary of the main discharge area, from sparking pin gaps, and from the main electrode surfaces. In this study we focus on the effect of xenon gas on the generation and the propagation of shock waves. Components of the Xe-Cl excimer laser gas are helium, xenon, and hydrogen chloride. In those gases xenon has the largest molecular weight of 131.29. So we conclude xenon plays an important role in the shock wave propagation and in discharge phenomenon.

Characteristics of pressure waves in the cavity of a pulsed excimer laser

Shock waves, which are generated by pulsed discharges in excimer lasers, cause a complicated time history of density of laser medium, and successively causing arcing and non- homogeneous excitation in the laser cavity. To clarify the characteristics of the generation and propagation of shock waves by these strong pulse-discharges in an excimer laser, experimental study and numerical analysis using a TVD scheme and Grid-distortion Splitting Method have been carried out. The shock waves are visualized using a CCD color schlieren technique. Numerical calculation by Yees symmetric TVD scheme is performed for the conditions that corresponds to the experiments. The initial conditions of our calculations are determined by measured results by the laser schlieren method, and from schlieren photographs. The propagation and attenuation of the shock waves in our experimental operation are also visualized from the numerical results and compared to the experimental results.

Influence of shock waves in the discharge region of a TEA CO2 laser

Transverse and longitudinal density disturbances in the discharge volume of a transversely excited atmospheric carbon-dioxide laser are measured using a laser schlieren method, and two-dimensional aerodynamic calculations qualitatively reproduce the temporal histories of the disturbances. Exponential decay of the transverse disturbance between the flattened electrodes lasts for several milliseconds. An efficient damping of the shock wave after discharge is confirmed using an acoustic attenuator based on the Helmholtz resonator design. Accumulated pressure disturbances in the volume due to successive laser shots induce a gradual decrease to the steady-state energy level in a pulse-periodic operation, and it is deduced that the modulational instability in the shot to shot pulse energy results from the longitudinal standing wave.

Shock Waves in the Cavity of a Xe-He Excimer Laser

Shock waves, which are generated by pulse discharges in an excimer laser, cause arcing and nonhomogeneous excitation in the laser cavity. Experiments and numerical calculations are conducted to clarify the generation and propagation of shock waves in the cavity of an excimer laser. The shock Waves are visualized by using a CCD color Schlieren method. Numerical calculations using Yee’s symmetric TVD scheme are carried out for conditions which correspond to the experiments. The initial conditions of the calculations are determined from measured results with the laser Schlieren method, and from Schlieren photographs. Color Schlieren photographs are constructed from the numerical results. The numerical Schlieren photographs are found to be in good agreements with the experimental Schlieren photographs.

Numerical simulation of shock waves in cavity of excimer laser

To clarify the characteristics of the generation and propagation of shock waves generated by pulse discharges in an excimer laser, numerical simulation using a TVD scheme and a grid- distortion splitting method is carried out. The calculations are conducted in conditions corresponding to our visualization experiments and high-frequency operations. The propagation and attenuation of the shock waves in the high-frequency operation are visualized from the numerical results.

FLOW VISUALIZATION OF SHOCK WAVES IN A EXCIMER LASER

Experimental study on shock waves in a high repitation rate excimer laser has been carried out. In a high repitation rate excimer laser. shock waves in subsonic loop are induced by the active medium excitation by means of pulse discharge. Shock waves in He gas by a pulse discharge are visualized by schlieren method using a scale discharge section model, CCD camera, stroboscope, and digital image memory which used to take photographer. Shock waves from anode surface, cathode surface. discharge region, and pin sparks for preionization are observed. Shock waves reflect at anode and cathode surfaces alternately for several times. Shock waves from dischrage region travel through channel.