Kiichiro Kagawa

A time-resolved spectroscopic study on the shock wave plasma induced by the bombardment of a TEA CO2 laser

A TEA CO2 laser (500 mJ, 100 ns) has been focused on a Zn plate in a surrounding gas at 0.5 Torr. The characteristics of the laser plasma were analysed using a unique time-resolved spectroscopic method. Time-resolved spatial distributions of Zn I 481.0 nm and Zn II 492.4 nm emission lines were observed. It was clearly shown that the resulting plasma has a thin shell structure and expands with time, and the ionization of Zn atoms precedes at slower rate than the excitation of neutral Zn atoms. The displacement of the neutral Zn emission was proportional to the two-fifths power of time. These experimental results proved that the plasma was excited by a blast wave induced by the laser bombardment.

Characteristics of a laser plasma induced by irradiation of a normal-oscillation YAG laser at low pressures

The generation of a laser-induced shock wave plasma in air at reduced pressure was achieved by focusing a normal-oscillation Nd:YAG laser on a brass sheet. The characteristics of the plasma were analysed under three different conditions: free expansion of the plasma, confinement of the plasma by limiting the plasma region using two parallel glass plates and interruption of the plasma by placement of a wedge in front of the plasma, in order to interrupt the expansion of the plasma. Spatially and temporally integrated methods were also developed in order to develop a more detailed understanding of the excitation mechanism of the plasma. Furthermore, a new technique of shadow graphing which involves the use of a He - Ne laser as a probe light was also developed as well as a measurement of the time profile of the temperature. By using this new method the relationship between the density jump and the starting point of atomic emission was detected simultaneously. The results show that the plasma was also generated by the shock wave as for the case of the short-pulse laser (Q switched). The extremely low ion and background emission, as low as compared with neutral emission, makes this plasma potentially useful for spectrochemical analysis.

Characteristics of the Secondary Plasma Induced by Focusing a 10-mJ XeCl Laser Pulse at Low Pressures

A XeCl excimer laser (20 ns, 10 mJ) was focused on a copper target at a relatively high power density (1010 W/cm2) in a surrounding gas of air in the pressure range from 1 Torr to about 10 Torr. Plasma characteristics were examined in detail with the use of a time-resolved spatial distribution technique on the emission lines Cu(I) 5218 Å, Cu(I) 5105 Å, and Cu(II) 4674 Å. The results show that the majority of atoms are ablated in the form of neutral atoms, and that the temperature of the plasma increases with time in the initial explosion stage. These facts suggest that the secondary plasma is excited by a shock wave and the recombination emission process is not predominant. It was also demonstrated that the plasma produced at 1 Torr is due to another excitation mechanism; namely, the collision of ultrafast atoms with surrounding gas molecules. This plasma is also suitable for analytical spectrochemical application.

Liquid refractometry by the rainbow method

A new method for measuring the refractive index of liquid, proposed in a previous paper [Appl. Opt. 36, 5552-5556 (1997)], has been developed. The minimum deviation of a laser beam deflected by a liquid-filled cylindrical cell was calculated by use of geometric optics. These theoretical results were compared with experimental results, with excellent agreement. As a result, the unknown refractive index of a liquid could be obtained by use of a computer calculation to give a best fit. The computer calculation showed that the sensitivity of the refractometer increases with the cell-wall thickness until total reflection takes place. A small refractive-index difference can be detected within a precision of 1 x 10(-6) by use of a metal-oxide semiconductor linear image sensor. We show how to calibrate the refractometer with pure water at 3.98 degrees C.

Comprehensive study on the pressure dependence of shock wave plasma generation under TEA CO2 laser bombardment on metal sample

An experimental study has been carried out on the dynamical process taking place in the plasma generated by a TEA CO2 laser (400 mJ, 100 ns) on a zinc target when surrounded by helium gas of pressure ranging from 2 Torr to 1 atm. Plasma characteristics were examined in detail on the emission lines of Zn I 481.0 nm and He I 587.6 nm by means of an unique time-resolved spatial distribution technique in addition to an ordinary time-resolved emission measurement technique. The results reveal, for the first time, persistent shock wave characteristics in all cases throughout the entire pressure range considered. Further analysis of the data has clarified the distinct characteristics of laser plasmas generated in different ranges of gas pressure. It is concluded that three types of shock wave plasma can be identified; namely, a target shock wave plasma in the pressure range from 2 Torr to around 50 Torr; a coupling shock wave plasma in the pressure range from around 50 Torr to 200 Torr and a gas breakdown shock wave plasma in the pressure range from around 200 Torr to 1 atm. These distinct characteristics are found to be ascribable to the different extents of the gas breakdown process taking place at the different gas pressures. These results, obtained for a TEA CO2 laser, will provide a useful basis for the analyses of plasmas induced by other lasers.

Using minimum deviation of a secondary rainbow and its application to water analysis in a high-precision, refractive-index comparator for liquids

A new method for measuring the refractive-index difference of a liquid has been developed. The liquid to be measured is contained in a 60-mm-diameter, cylindrical glass cell, and a He-Ne laser light is passed into the cell so that the laser light incidence fulfills the condition of minimum deviation. In this condition, the beam emerging from the cell has a fine interference fringe. The position of the interference fringe is read out as a marker to measure the deflection of the laser light. Directly reading the peak shift of the interference fringe makes it easy to obtain the refractive index difference of the liquid with a fairly high precision of at least 6 x 10(-6). Further high precision is potentially expected to be realized by use of an improved data analysis treatment of the overall interference fringe pattern.

Production of artificial snow crystals

Artificial snow crystals can be produced by a fairly simple method in a small closed cylindrical chamber made by combining an aluminium tube and a plastic tube. The chamber is set horizontally at room temperature and the end of the aluminium tube is cooled by dry ice. Water vapour is supplied by a diffusion process from the end of the plastic tube for a suitable time after cooling. The snow crystals are formed on a black sheet inside the end of the aluminium tube. The artificial snow crystals were observed at room temperature using our partial cooling method.