Generation of focused shock waves by multi-channel discharges in water

Abstract

Summary form given only, as follows. A new method for generation of focused shock waves in water has been developed where a cylindrical pressure wave is focused by a metallic, parabolic reflector. The pressure wave is created by a high voltage multi-channel discharge in water with an increased electrical conductivity. The discharge is formed on a composite anode consisting of a cylindrical metallic electrode covered by a thin layer of porous ceramic layer. Role of the ceramic layer is to re-distribute the electric field during the pre-discharge phase. The electric field on the anode can be estimated as E=U/d/sub c/, where U is the applied voltage and d/sub c/ is the thickness of the layer. For d/sub c/=0.3 mm and U=30 kV the electric field on the anode is of the order of 1 MV/cm that results in initiation of a large number of short discharge channels distributed almost homogeneously on the anode surface. Each channel creates a semi- spherical pressure wave, and by superposition of all of the waves a cylindrical wave is formed. The discharge is fed by a power supply consisting of three low inductance capacitors 0.5 μF charged up to 30 kV and two triggered spark gap switches enabled production of either single discharge or two subsequent discharges with a variable time delay. The second shock wave interacts with an acoustical non-homogeneity created by the first wave and thus it can cause high mechanical stress even in an originally homogenous medium. Schlieren photography demonstrated focusing of the shock wave to a diameter less than 1 cm. Cavitation bubbles were produced by the shock waves in distilled water near the focus region. Results on interaction of the focused shock waves with biological objects will be presented.