Hiroki Honma

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.

An Investigation on the Behavior of Laser Induced Bubble in Cryogenic Liquid Nitrogen

There have been few experimental reports on cryogenic two-phase fluids and cavitation phenomena using the irradiation of pulsed high-power laser. This paper describes an investigation of the behavior of laser induced cavitation bubble in cryogenic liquid nitrogen. The bubble is produced by a pulsed ruby laser focused in the special cryostat. The production, growth, and rebound phenomena of the bubbles are visualized by diffusive shadowgraph technique with an image-converter camera. To compare with the experimental results, a numerical study has also been performed on the dynamics of a single spherical bubble in liquid nitrogen under the conditions of nonequilibrium phase change.

Numerical simulation for nonstationary Mach reflection of a shock wave: A kinetic-model approach

A numerical simulation was performed for the process of formation of single Mach reflection on a wedge by solving a BGK type kinetic equation for the reduced distribution function with a finite difference scheme. The calculations were carried out for a shock Mach number 2.75 and wedge angle 25° in a monatomic gas, which corresponds to the conditions of single Mach reflection in the classical von Neumann theory. The calculations were performed for both diffuse and specular reflection of molecules at the wall surface. It is concluded that the diffuse reflection of molecules at the wall surface or the existence of the viscous or thermal layer is an essential factor for a nonstationary process at the initial stage of Mach reflection. Furthermore, the numerical results for diffuse reflection are found to simulate the experimental results very well, such as a transient process from regular reflection to Mach reflection along with shock propagation.

Observation of Nonequilibrium Radiation behind Strong Shock Waves in Low-density Air

Nonequilibrium radiation phenomena behind strong shock waves in low-density air are observed by using a couple of CCD camera systems in a shock tube experiment. The simultaneous observation for total radiation and its spectral radiation is carried out in order to elucidate spaced-ependent contribution of an individual radiation spectrum to the total radiation intensity. The results are shown for the shock velocity range from 9.0 km/s to 12.1 km/s at the initial pressure 13.3 Pa. Wavelength range is selected from 300 nm to 445 nm to investigate mainly the contributions from UV radiation. It is found that the band spectra due to the molecular species N2+ and CN mainly contribute to the first-peak, while the spectra due to the atomic species O+ and N mainly contribute to the formation of the second-peak. It is also found that the Balmer series in H spectra strongly contributes to the second-peak. The radiation along the tube wall surfaces is composed of the same constituents as those around the tube axis as well as the spectra coming from the impurities.

DSMC approach to nonstationary Mach reflection of strong incoming shock waves using a smoothing technique

The direct simulation Monte Carlo (DSMC) method is applied to simulation of nonstationary Mach reflection of strong shock waves. Normally the DSMC method is very time consuming in solving unsteady flow field problems especially for high Mach numbers, because of the necessity of iterative calculations to eliminate the inherent statistical fluctuation caused by a finite sample size. A central weighted smoothing technique is introduced to process the DSMC results, so that the iteration time can be significantly reduced. In spite of some relaxations of the shock wave structure, the smoothing technique is verified to be useful to estima te the flow fields qualitatively and even quantitatively by using a relatively small sample size. The comparison between the present approach and the kineticmodel approach (Xu et al. 1991a, 1991b) on the application to unsteady rarefied flow fields was also carried out.

Irregular reflections of weak shock waves in polyatomic gases

In this paper experiments with weak shocks in carbon dioxide are described. The incident shocks were arranged to reflect off the sloping surfaces of rigid ramps of various corner angles. Regular and Mach reflectionlike phenomena were observed, but because the incident shocks were so weak, all the waves in the reflecting systems showed evidence of either partial or complete dispersion. Consequently, the wave systems were different from the classical perfect gas theory systems described by the von Neumann theory. These wave systems were named ‘‘dispersed irregular reflections.’’

Two‐dimensional features of strong shock waves in gases

Strong shock wave around 10 km/s are generated in low‐density (13.3 Pa) air and nitrogen by using a free‐piston, double‐diaphragm shock tube with the test section of 40mm×40mm. Two‐dimensional features of radiation intensity behind the shock waves are observed at the test section by using a combined system of an image converter camera and an image processor. The pseudo‐colour representation and the contour plot of radiation intensity clearly real various types of two‐dimensional profile of radiation. In addition to the regular type, two irregular types of oblique and two‐step profile are observed. Furthermore, the time histories of radiation intensity are derived from an image processing, and give us various informations about the radiative characteristics of the shocked air and nitrogen.

Kinetic‐model approach to nonstationary Mach reflection of shock waves

The formation of Mach reflection of a shock wave at a wedge is numerically investigated by using a kinetic‐model approach. A MacCormack difference scheme is applied to a BGK model equation which is described by the reduced distribution functions. The calculations are carried out for a shock Mach number 2.75 and a wedge angle 25° in a monatomic gas. In order to simulate flows with and without boundary layer, diffuse and specular relfections of molecules are assumed respectively at the wall surface. In the case of diffuse reflection, a nonstationary process from regular to Mach reflection appears clearly at the initial stage of the reflection, while in the case of specular relfection a quasi‐stationary process appears as soon as the incident shock wave reaches the wedge. The numerical results for diffuse reflection exhibit a qualitative agreement with the experimental data.

Front Structures of Strong Shock Waves in Air

Front structures of strong shock waves in air were experimentally investigated by means of the laser Schlieren technique. Shocks in air at 13 Pa were generated with Mach numbers ranging from 2 to 34. The detector response to a single pulse of a ruby laser was obtained. This enabled the response correction of the Schlieren signals and the correction is greater at high Mach numbers (M > 16). Density profiles were obtained by integrating the corrected Schlieren signals. We present data of the reciprocal shock thickness for the investigated Mach numbers.

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.