Experimental setup for shadowgraphy of the cavitation bubble with simultaneous imaging of the plasma emission. A tungsten halogen lamp was used as a back illumination light source for the shadowgraphy. The time delays of the double laser pulses and the ICCD gating were controlled by a delay generator. The gate delay of the ICCD was set to be 100 ns before the second pulse with the gate width being 400 ns. This means that the most intensive part of the emission caused by the second pulse is integrated into the image.
(Left column) Emission spectra obtained by the irradiation of Al metal target in water by two pulses with various pulse intervals. The integration of the signal was performed in a way equivalent to the non-gated measurement. The energies of the first and second pulses were 0.4 mJ and 1.0 mJ, respectively. (Center column) Same as those in the left column but with the second pulse energy of 10 mJ. (Right column) The shadowgraph images at different delays after the first pulse. Note that the second pulse was absent when the shadowgraphs were taken.
Peak shift of the 396 nm line for the Al target in water as a function of (a) pulse interval and (b) energy of the second pulse. (c) Similar to (a) but in air. For (a) and (c) the first and second pulse energies were 0.4 mJ and 1.0 mJ, respectively. In (b) the results with the first pulse energies of 0.4 mJ and 0.2 mJ were shown by open and closed circles, respectively. In (c) the shifts obtained by a single 0.4-mJ-pulse irradiation is indicated by a broken line.
(a) Simultaneous imaging of the emission region and cavitation bubble into a single photograph. The Al target in water was irradiated by the two pulses with the energies of 0.8 mJ (first pulse) and 1.0 mJ (second pulse) with a pulse interval of 15 μs. The photograph was obtained by the ICCD camera with the gate window from 100 ns before the second pulse to 400 ns after the second pulse. The intensity profile along the solid red line shown in (a) is given in (b) by averaging over the 10 adjacent pixels. To clearly see the boundary of the bubble, a magnified profile is also shown in (b).
Emission spectra obtained by using a multi-pulse microchip laser as an excitation source. (a) Al and (b) Cu as targets in water. The solid lines in (a) and (b) were obtained with the gate which started 40 μs before the first pulse lasts for 300 μs, and hence the entire emission was integrated. The dotted line in (b) was obtained with the gate which started 500 ns after the first pulse and lasts for 10 μs, and hence the emission induced by the first pulse only was integrated.
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