Petr Hoffer

Shock Waves Generated by an Electrical Discharge on Composite Electrode Immersed in Water With Different Conductivities

A generation of focused shock waves by underwater multichannel pulsed electrical discharge on a porous-ceramic-coated electrode in saline water is studied. This work describes the effect of solution conductivity of saline water on the pressure of shock waves. It was found that the amplitude of shock waves has a nonlinear dependence on water conductivity: The amplitude increases with the increase of water conductivity up to 18-20 mS/cm and then decreases again. In this paper, we show that two effects take place. First, the electrical energy dissipated in the discharge depends on the impedance of the electrode system being affected by water conductivity. Second, the velocity of streamer growth strongly depends on energy deposition time into the discharge. The two mentioned effects result in “hill-like” shape of the curve presenting the dependence of the maximum amplitude of the shock wave on water conductivity.

In vivo effects of focused shock waves on tumor tissue visualized by fluorescence staining techniques

Shock waves can cause significant cytotoxic effects in tumor cells and tissues both in vitro and in vivo. However, understanding the mechanisms of shock wave interaction with tissues is limited. We have studied in vivo effects of focused shock waves induced in the syngeneic sarcoma tumor model using the TUNEL assay, immunohistochemical detection of caspase-3 and hematoxylin-eosin staining. Shock waves were produced by a multichannel pulsed-electrohydraulic discharge generator with a cylindrical ceramic-coated electrode. In tumors treated with shock waves, a large area of damaged tissue was detected which was clearly differentiated from intact tissue. Localization and a cone-shaped region of tissue damage visualized by TUNEL reaction apparently correlated with the conical shape and direction of shock wave propagation determined by high-speed shadowgraphy. A strong TUNEL reaction of nuclei and nucleus fragments in tissue exposed to shock waves suggested apoptosis in this destroyed tumor area. However, specificity of the TUNEL technique to apoptotic cells is ambiguous and other apoptotic markers (caspase-3) that we used in our study did not confirmed this observation. Thus, the generated fragments of nuclei gave rise to a false TUNEL reaction not associated with apoptosis. Mechanical stress from high overpressure shock wave was likely the dominant pathway of tumor damage.

Characteristics of meter-scale surface electrical discharge propagating along water surface at atmospheric pressure

This paper reports physical characteristics of water surface discharges. Discharges were produced by metal needle-to-water surface geometry, with the needle electrode driven by 47 kV (FWHM) positive voltage pulses of 2 µs duration. Propagation of discharges along the water surface was confined between glass plates with 2 mm separation. This allowed generation of highly reproducible 634 mm-long plasma filaments. Experiments were performed using different atmospheres: air, N2, and O2, each at atmospheric pressure. Time- and spatially-resolved spectroscopic measurements revealed that early spectra of discharges in air and nitrogen atmospheres were dominated by N2 2nd positive system. N2 radiation disappeared after approx. 150 ns, replaced by emissions from atomic hydrogen. Spectra of discharges in O2 atmosphere were dominated by emissions from atomic oxygen. Time- and spatially-resolved emission spectra were used to determine temperatures in plasma. Atomic hydrogen emissions showed excitation temperature of discharges in air to be about 2 × 104 K. Electron number densities determined by Stark broadening of the hydrogen H β line reached a maximum value of ~1018 cm−3 just after plasma initiation. Electron number densities and temperatures depended only slightly on distance from needle electrode, indicating formation of high conductivity leader channels. Direct observation of discharges by high speed camera showed that the average leader head propagation speed was 412 km s−1, which is substantially higher value than that observed in experiments with shorter streamers driven by lower voltages.

Penetration of Gas Discharge Through the Gas–Liquid Interface Into the Bulk Volume of Conductive Aqueous Solution

Gas discharge plasma generated above the surface of conductive aqueous solutions in a glass capillary was used to study penetration of the discharge from the bubble (imitated by the space above meniscus of liquid surface in the capillary) into the bulk liquid. The experiments were conducted at both polarities with a high-voltage needle electrode placed above the liquid surface. Different aqueous solutions were examined (distilled water, conductive saline solutions). High-speed shadowgraphy was used as the main diagnostic tool for the study of the disturbances at the plasma-liquid interface. It has been found that electric field just beneath the liquid surface and the liquid/plasma conductivity ratio have a decisive effect on the development of plasma-liquid interface instabilities. Experiments with negative electrode above the liquid surface showed that this surface in the place of the largest current density recedes. This receding is caused by the reaction pressure resulting from liquid evaporation. Thus, long cavities with plasma inside can be formed. The cavity elongation speed is of the order of 1 m · s-1, and it depends on current density. The liquid surface remains smooth, when the liquid conductivity is larger than the conductivity of adjacent plasma. In the opposite case, if the liquid conductivity is smaller than the conductivity of adjacent plasma, the distribution of current density on the plasma-liquid boundary is unstable: any initial surface disturbance boosts the current density in a local surface valley simultaneously causing a detriment of the surrounding current density. Consequent stronger liquid evaporation in the valley causes its deepening, and hence, next enhancement of the inhomogeneity of current density distribution. The dips created in this way subsequently transform into negative streamers, when electric field larger than 1 MV · m-1 appears near the liquid surface. Experiments with the positive electrode above the liquid surface significantly showed more intense liquid evaporation than the experiments with the negative one - under otherwise the same conditions. Therefore, elongation speed of the gas cavities is also significantly higher. The development of spikes on liquid surface is also dependent on the liquid conductivity. However, electric field larger than 10 MV · m-1 near the liquid surface is necessary for the development of positive streamers.

Investigation of cavitations dynamics induced by tandem shock waves in water

We have developed a shock wave generator where two cylindrical pressure waves are focused to a common focal region by a metallic parabolic reflector1, and the waves can be switched on with a different time delay. Interaction of two successive shock waves (tandem shocks) in water focused to a common focal point was investigated. Amplitude of the of the pressure wave reaches up to 100 MPa at the focus, while the amplitude of the rarefaction wave falls down to −25 MPa (well above the cavitation threshold), producing thus numerous cavitations. Schlieren photography of the focal region demonstrated creation of a very complex pressure field with many secondary spherical short wavelength shocks that originate in collapsing cavitations. These secondary shocks can interact with cell scale structures and they are considered to play the main role in cell membranes damage when organic tissue is exposed to the shock waves. Therefore, we are interested in the waveform and the pressure amplitude of these secondary shocks with an ultimate goal in enhancement of cancer treatment efficiency by activation of sonosensitizers based on the effects of the collapsing cavitations.

Pressure Field Around Underwater Negative Streamers

In this paper, experimental observations of propagation of negative streamers in distilled water (conductivity of 1.7 μS·cm-1) in a needle-plane electrode geometry (gap of 6 mm) are described. The spatially resolved pressure field surrounding the tip of streamer channel in the given moment was determined by Mach-Zehnder interferometer with second harmonic of Nd:YAG laser as a source. The high-voltage needle electrode was brought on the potential of 23 kV and that remained practically unchanged during streamers propagation, while a significant noise with amplitude of several amperes was visible on the waveform of discharge current. Analysis of captured interferograms shows that propagation of negative streamers is not uniform: an active (propagating) streamer produces continuous moving pressure field (similar to that of moving object) with maximum amplitude greater than 40 MPa on the streamer tips and an inactive streamer nearly stops propagating being surrounded by a nearly spherical pressure wave moving away from the streamers tip. The inactive state can last even longer than 200 ns.

Biological effects of tandem shock waves on soft animal tissues – preliminary “in vivo” experiments

We have investigated biological effects of two successive (tandem) shock waves focused to a common focal region on soft animal tissues, including cancer tissues “in vivo” and cancer cells “ex vivo”. The tandem shock waves have been produced by our formerly developed shock wave generator where two cylindrical pressure waves are focused by a metallic parabolic reflector to a common focal region1 and the second shock can be switched on with a different time delay after the first one. The idea on application of the tandem shock waves is to localize the action of the shocks at a predictable region in an initially acoustically homogenous medium such as cancer tissues are. The first shock creates in the tissue some acoustical non homogeneity and cavitations, and the second shock dissipate on it, similarly as it is in the case of the lithotripsy of kidney stones. We have found that at some time interval between the shocks (10–15 µs) the second, originally pressure wave, reaches the focus as a rarefaction wave that produces a large number of cavitations. Collapsing cavitations create secondary, very short wavelength shocks which can interact with cell scale structures.

Experimental Study of a Long-Living Plasmoid Using High-Speed Filming

This paper describes the time evolution of ball plasmoids generated by millisecond atmospheric discharge in contact with water. The aim of this paper was to clarify an initiation and a formation mechanism of an atmospheric ball plasmoid. The use of a higher frame rate of filming and shorter exposition time, in comparison with past experiments, allows us to observe the fast processes that occur during a plasmoid evolution. We assert that an initial electrical breakdown in a cathode ceramic tube filled by a conductive liquid is essential for further plasmoid formation. It generates a shock wave resulting in a blast of conductive liquid. The resulting expansion of water vapor and bubbles with an addition of weekly ionized plasma serves as a main source of matter for plasmoid formation. We also note that an electrical arc generated after the breakdown transforms to a plasma jet. This jet accelerates plasma to the plasmoid.

Physical properties of plasma streamers produced on water surface

Summary form only given. Since high voltage plasma discharges over water surface in the air have been studied in recent years for their potential applicability in several biomedical and industrial domains. This paper reports on experimental investigations of basic physical characteristics of these discharges. The streamers were produced in a tungsten needle-to-water surface geometry, and were forced to propagate in one direction by presence of a glass streamer guide. The needle electrode was driven by 44 kV/3.5 μs voltage pulses. Experimental arrangements limited total length of the streamers to about 320 mm. The experiments were performed with different gases: air, N2, and O2, every at 1 atmosphere unit pressure. Time and spatially resolved spectroscopic measurements have been done. The emission spectra were significantly changing during life span of the produced streamers. The early spectra of the discharges in the air and nitrogen atmosphere were dominated by N2 2nd positive system. After about 150 ns the N2 radiation disappeared, and emissions from atomic hydrogen took place instead. Spectra of the discharges in O2 atmosphere provided possibility of calculation of the excitation temperature from both atomic hydrogen and oxygen emissions. The excitation temperatures were measured in different parts of the discharge channel; excitation temperature in the inner part of the discharge was 3×104 K (hydrogen emissions), excitation temperature in the outer part was 1.6×104 K (oxygen emissions). Stark broadening of the hydrogen Hβ line (486.1 nm) was used for determination of the electron densities. It reached its maximum value of 1018 cm-3 just after the discharge initiation in every case, and then it monotonically decreased. The electron densities depended only slightly on the distance from the needle electrode. Direct observation of the streamers by a high speed camera has also been done. It showed alternation of active and passive phases, where the active phase was an ionization avalanche. Whereas the average propagation speed of the streamer head was 450 km·s-1, the maximum propagation speed of the ionization avalanche exceeded 760 km·s-1.

Effect of Solution Conductivity on Shock Wave Pressure Generated by Multichannel Electrical Discharge in Water

Starting from the mid-1980s and up to now, experimental studies of the extracorporeal shock wave lithotripsy (ESWL) applications are performed in different fields of medical sciences, such as the treatment of kidney stone disease, neurosurgery, assisted drug delivery, the treatment of cerebral embolism, orthopedics and in the veterinary medicine. A generator of focused shock waves based on a high current spark discharge in water was developed in the Institute of Plasma Physics AS CR (IPP) [1, 2]. Hospitals in the Czech and Slovak Republic are equipped by such generators produced by the company MEDIPO, Brno. Number of patients undergo a course of medical treatment by these devices every year.