This paper is aimed at the investigation of the acoustic and spectral characteristics of underwater electric sparks generated between two plate electrodes, using synchronized gas bubble injection. There are two purposes served by discharge initiation in the bubble. Firstly, it creates a favorable condition for electrical breakdown. Secondly, the gas bubble provides an opportunity for the direct spectroscopy of plasma light emission, avoiding water absorption. The effect of water absorption on captured spectra was studied. It was observed that the emission intensity of the Ha line and a shockwave amplitude generated by discharge strongly depend on the size of the gas bubble in the moment of the discharge initiation. It was found that the plasma in the underwater spark channel does not correspond to a source of black-body radiation. This study can be also very useful for understanding the difference between discharges produced directly in a liquid and discharges produced in gas/vapor bubbles surrounded by a liquid.

In this study, a new finite-difference cylindrical model of long underwater spark is developed that allows us to numerically calculate the time evolution of the underwater spark channel from a given power input. A one dimensional simulation starts in the breakdown moment. The whole time development is divided into time steps of equal duration. The investigated region consists of a homogeneous cylindrical central column filled with weakly ionized vapour and its atomic fragments, and co-axial cylindrical liquid slabs of equal thickness in the beginning. In each time step, some energy (experimentally given and reduced by losses spent on dissociation, excitation, and ionization) is delivered into the central plasma column. This energy is partly irradiated, out-conducted, spent on mechanical work, and/or used for an increase of inner energy of the plasma column. This ambiguity enables us in future to fit, e.g. the plasma column diameter at the end of energy input to its experimental value. The model shows that plasma channel expansion generates a primary pressure wave propagating with supersonic velocity, and a subsequent secondary pressure wave that propagates with sound velocity. An advantage of this approach is that the present solution with constant coefficients can be relatively easily upgraded to a solution with variable coefficients.

The pressure in an underwater discharge channel is the main parameter that influences all its applications. In this study, we investigate a relatively large gap with a plane-to-plane electrode geometry that uses the assistance of an injected gas bubble for breakdown. The delay in the application of high voltage following the injection of a gas bubble from a grounded electrode determines the initial dimensions of the bubble. We examined three types of discharge: (a) a large bubble with easy triggering, (b) a medium bubble, and (c) a small bubble with difficult triggering. The main diagnostic tool is Hα line broadening. It is shown that (i) Doppler broadening plays no role; (ii) at pressure broadening, the resonance and van der Waals broadenings must be considered; (iii) the impact approximation is not applicable, and hence the quasi-static high-pressure (namely “nearest neighbour”) approximation must be used; and (iv) the Stark broadening plays the dominant role.
Because a mixture of two gases (evaporated water vapour and nitrogen from bubble injection) is present in the discharge channel, simple thermodynamic considerations are applied to estimate the ratio of the contributions of resonance and van der Waals broadening. It turns out that the larger the bubble, the smaller the pressure detected in the discharge channel. This is in agreement with the measurements of the pressure wave amplitude made at some distance from the discharge channel with a piezoelectric pressure probe.

Thin-film Al filters are very popular owing to their high transmittance in the wavelength range of 17–67 nm and low transmittance in the visible and near-UV regions; however, they are prone to oxidation. The amorphous A l 2 O 3 layers on the Al surfaces have much smaller transmittance than the bulk Al material; therefore, they strongly influence the total transmittance of the filter. This paper not only provides the transmittance of very old Al filters but also maps the transmittance development of Al filters over two years since their delivery (in 2016) in an uncontrolled atmosphere.

The results of experimental research on the acoustic and electrical characteristics of underwater spark discharges facilitated by a preliminary discharge are presented. The latter was produced through the application of a short duration high-voltage pulse formed by a Marx generator. The application of this pulse lead to the formation of a low-density region in the form of a streamer which transformed to an oscillating vapor cavity. It was shown that this method provided a breakdown of a significantly increased interelectrode gap for the same charging voltage of the main capacitor and allowed the generation of stronger shocks. The temporal development of transient discharges in a long gap and the relationships between the hydrodynamic and electrical parameters of such discharges are reported and analyzed.