Abstract
Behavior of pressure waves generated due to a laser ablation of a metal plate in water has been investigated with shadowgraph technique and measurement of their pressure histories. A second harmonic radiation of a Q-switched Nd:YAG laser (wavelength; 532 nm, pulse duration (FWHM); 7 ns) was focused onto a water-immersed plate made of type 304 stainless steel (SUS304). The plate thickness was 0.1 mm, 1 mm and 10 mm, and the laser energy was varied from 49 to 470 mJ. High-pressure ablation plasma was generated on the plate surface by the laser irradiation, and three types of pressure waves, i.e., blast wave, transmitted wave and leaky wave, were visualized. The blast wave, which was directly driven in water by expansion of the high-pressure plasma, propagated as an-almost-hemispherical wave without being influenced by the plate thickness. Behavior of other waves was influenced by the thickness. In case of 1-mm and 10-mm-thick plates, the transmitted waves were emitted as a result of transmission of pressure waves repeatedly propagating between the plate surfaces. Their frequency was consistent with a round travel period through the plate thickness. For the thick plate, the leaky waves, which were caused by longitudinal and Rayleigh or transversal waves propagating along the plate surface, were observed. Especially, leaky wave produced by Rayleigh or transversal wave had a width approximately the same as a spot diameter of the laser irradiation. For the sufficiently thin plate, the leaky waves produced by lamb waves propagating along the plate surface were observed.
QスイッチNd:YAGレーザーの第2高調波（波長；532 nm，パルス幅；7 ns）を，水中に設置したSUS304試験板に照射し，水中を伝播する圧力波の挙動をシャドウグラフ法によって可視化するとともに，圧力測定を行った。試験板の厚さは0.1 mm，1 mm，10 mmとし，レーザーエネルギーは49〜470 mJとした。レーザーの照射により試験板の表面には高圧のアブレーションプラズマが発生し，その結果としてプラズマが直接駆動する波（ブラスト波），試験板の板厚方向に伝播する波が水中に誘起する波（透過波），試験板表面に沿う方向に伝播する波が水中に誘起する波（漏洩波）の3種類の波が水中を伝播した。ブラスト波の性質は板厚に依存せず，他2種類の圧力波の挙動は板厚によって変化した。板厚1 mmおよび10 mmにおいて，透過波が繰り返し発生するのが観測され，その発生周期は板厚の2倍をSUS304の縦波の伝播速度で除した値と一致した。また，板厚が厚い場合，試験板表面に沿って伝播する縦波とレイリー波または横波による漏洩波が形成され，板厚が十分薄い場合は，ラム波による漏洩波が観測された。レイリー波または横波による漏洩波は，レーザー光照射径と同程度の幅を持って観測された。
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