Abstract
We experimentally examine a laser-induced underwater shock wave paying special attention to the pressure impulse, the time integral of the pressure evolution. Plasma formation, shock-wave expansion and the pressure in water are observed simultaneously using a combined measurement system that obtains high-resolution nanosecond-order image sequences. These detailed measurements reveal a distribution of the pressure peak which is not spherically symmetric. In contrast, remarkably, the pressure impulse is found to be symmetrically distributed for a wide range of experimental parameters, even when the shock waves are emitted from an elongated region. The structure is determined to be a collection of multiple spherical shock waves originating from point-like plasmas in the elongated region.
Highlights
Underwater shock waves induced by illumination with a nanosecond laser pulse are utilized in various applications including low-invasive medical treatments (Razvi et al 1996; Lee & Doukas 1999; Kodama, Hamblin & Doukas 2000; Lam, Greene & Gupta 2002; Sofer et al 2002; Sankin et al 2005; Klaseboer et al 2007)
Such an observation is challenging because each of the phenomena involved in generating the shock occur within a short time; illumination with a laser pulse first triggers the emergence of the plasma in water, which leads to rapid expansion of a bubble and emission of a shock wave (Noack & Vogel 1999; Lauterborn et al 2001)
In order to investigate a laser-induced shock wave focusing on the pressure impulse, we constructed a measurement system consisting of a combination of ultra-high-speed cameras and pressure sensors
Summary
Underwater shock waves induced by illumination with a nanosecond laser pulse are utilized in various applications including low-invasive medical treatments (Razvi et al 1996; Lee & Doukas 1999; Kodama, Hamblin & Doukas 2000; Lam, Greene & Gupta 2002; Sofer et al 2002; Sankin et al 2005; Klaseboer et al 2007). Note that Vogel et al (1996b) did show single conical plasmas Despite these considerations, a common model for the pressure impulse of laser-induced shock waves has not been developed. We report on experimental observations of a laser-induced shock wave with special attention paid to the pressure impulse. We propose a new model of the shock wave to rationalize the observations Such an observation is, challenging because each of the phenomena involved in generating the shock occur within a short time; illumination with a laser pulse first triggers the emergence of the plasma in water, which leads to rapid expansion of a bubble and emission of a shock wave (Noack & Vogel 1999; Lauterborn et al 2001). The time scale for plasma growth is of the order of nanoseconds and the shock velocity in water is approximately
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