To conduct elemental analysis of seawater in situ, the method of ultrasonic spark spectroscopy was used, which was implemented as an automated complex for studying spectral optical characteristics and other hydrophysical characteristics of seawater. The complex was tested in expeditionary conditions during the voyage No. 81 of the R/V Professor Gagarinsky in the Sea of Japan in August 2022 and in the 52 voyage of the R/V Academician Boris Petrov in the Atlantic Ocean in October - December 2022. New data on the state of seawater with high spatial resolution have been obtained. The complex can be recommended for in-situ studies of the state of natural water areas.
We have used a nanosecond pulsed laser to study the dynamics of laser- induced breakdown with traditional optical detection on both nanosecond time scale and at later stages. Experiments on the induction of optical breakdown in the volume of liquid were performed using an Nd:YAG laser. It is shown that the optical breakdown in the liquid in the ultrasonic field is accompanied by an increase in the intensity of the spectral lines of potassium and oxygen with an increase in the amplitude and frequency of ultrasound. It was found that the effect of ultrasound on the intensity of the lines varies depending on the time of the breakdown evolution. Along with the optical spectra, the acoustic emission accompanying the pulsations of the cavitation bubble formed at the late stages of liquid breakdown source was studied. It was shown that the acoustic emission varies significantly with different ultrasound parameters. It is shown that an excited signal at its own switching frequency has a sufficiently high amplitude for its registration under typical experimental conditions. It is shown that the saturation effect is observed at frequencies above 200 kHz and at high ultrasound power, when the growth of the intensity of spectral lines slows down sharply. This effect indicates the possibility of using relatively small ultrasound powers for the implementation of the identified optoacoustic effects and spectroscopic properties in the laser breakdown in the liquid.
The method of laser induced breakdown spectroscopy (LIBS) was used to investigate intensive lines of sodium, calcium, potassium and magnesium in the aqueous solutions under the action of ultrasound at the frequencies of 10.7 kHz, 60 and 240 kHz. Acoustic radiation was generated within of cylinder transducers using a digital generator and powerful amplifier with maximum amplitude of 100 kPa. To excite optical breakdown in water we used a Nd:YAG laser with the wavelength of 532 and 1064nm. The changing of the delay of the control pulses allows to synchronize the time of maximum expansion of the liquid acoustic impulses and the start of the optical breakdown. The additional influence of ultrasonic radiation on the process of laser breakdown was studied. The increasing of the lines intensity under the influence of ultrasound is observed at all concentrations of the used solutions. It is shown that under the action of ultrasound on the dynamics of the cavity in a fluid the phase of the acoustic impact has a great significance. As a result it is shown that under the influence of ultrasound the strong strengthening of the intensity of spectral lines of elements dissolved in aqueous solutions is observed.
Research studies have been carried out the acoustic effects accompanying optical breakdown in water generated by focused laser and ultrasound radiation. Experiments were performed by using 532 nm pulses from a Brilliant B Nd:YAG laser. Acoustic radiation was produced by acoustic focusing systems in the form of hemispheres and rings at frequencies 10.7 kHz and 60 kHz. The experimental results show the sharply strengthened effects of acoustic emission from a breakdown zone by the joint influence of laser and ultrasonic irradiation. Various breakdown thresholds and character of acoustic emission in fresh and sea water are observed. The experimental result established that acoustic emission from optical breakdown of sea water in the presence and absence of ultrasound exceeds acoustic emission for the same experimental conditions in fresh water. Atomic lines of some chemical elements like sodium and magnesium were investigated for laser breakdown of water with the ultrasound field. The effect of magnification of these atomic lines resolution for salt water in ultrasound field was obtained. It is shown that the method of registration of acoustic emission from a breakdown zone allows to investigate thresholds and dynamics of laser breakdown which will be in accord with high-speed optical methods. The study revealed important practical applications of acoustic emission for breakdown and diagnostics of cavitation in opaque environments.
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