Ilya Marinov

Dynamics of plasma evolution in a nanosecond underwater discharge

A positive discharge in water is generated by applying a 30 ns high-voltage (HV) pulse on a micrometre scale electrode. The applied voltage ranges from 6 to 15 kV and a fast plasma propagating mode is launched with a velocity of up to 60 km s−1. Time-resolved shadowgraphy and spectroscopy are performed to monitor the time evolution of the discharge structure and of the plasma emission spectra. By analysing the dynamics of the shock front velocity and the lateral expansion of the plasma channel, it is possible to estimate the pressure at the ignition of the plasma by two independent methods: very good agreement is found at 6 kV giving initial pressures of 0.4 GPa and 0.3 GPa, respectively. At 15 kV, only the shock front velocity method is applicable under our experimental conditions, giving an estimate of the initial pressure of 5.8 GPa. Such high initial pressures show that, under a nanosecond HV pulse, the plasma is ignited directly in the dense phase. Emission spectra show a strong continuum emission as well as a broad Balmer α line with a strong red shift, with an estimate of the initial plasma density of 1.3 × 1026 m−3. The relaxation of discharge pressure and plasma density is studied under a series of six successive pulses.

Time-resolved imaging of nanosecond-pulsed micro-discharges in heptane

Nanosecond-pulsed micro-discharges in heptane are studied by time-resolved imaging in pin-to-plate configuration. When a voltage of +5?kV is applied to the pin electrode, the discharge exhibits one maximum in light intensity. At +15?kV, filtered images show that up to three maxima can be identified. These maxima are associated with local electron?ion recombination and bremsstrahlung emission and attributed to the development of a complex space-charge field. In the post-discharge, the dynamics of the gas bubble can be simulated by the Gilmore model, and the pressure evolution in this bubble is predicted. From our results, it seems reasonable to think that the gas bubble develops from the post-discharge of the spark. Results obtained by using the double-pulse technique show that light emission during the post-discharge of the second discharge lasts 10 times longer than the post-discharge of the first spark. The pressure drop in the gas bubble, predicted by the Gilmore model, is used to explain this result and it provides a control method by optical diagnostics in liquids.

Cell Death Induced on Cell Cultures and Nude Mouse Skin by Non-Thermal, Nanosecond-Pulsed Generated Plasma

Non-thermal plasmas are gaseous mixtures of molecules, radicals, and excited species with a small proportion of ions and energetic electrons. Non-thermal plasmas can be generated with any high electro-magnetic field. We studied here the pathological effects, and in particular cell death, induced by nanosecond-pulsed high voltage generated plasmas homogeneously applied on cell cultures and nude mouse skin. In vitro, Jurkat cells and HMEC exhibited apoptosis and necrosis, in dose-dependent manner. In vivo, on nude mouse skin, cell death occurred for doses above 113 J/cm2 for the epidermis, 281 J/cm2 for the dermis, and 394 J/cm2 for the hypodermis. Using electron microscopy, we characterized apoptosis for low doses and necrosis for high doses. We demonstrated that these effects were not related to thermal, photonic or pH variations, and were due to the production of free radicals. The ability of cold plasmas to generate apoptosis on cells in suspension and, without any sensitizer, on precise skin areas, opens new fields of application in dermatology for extracorporeal blood cell treatment and the eradication of superficial skin lesions.

Microdischarge Ignition in Liquid Heptane

Ignition of a discharge in liquid heptane is studied by a nanosecond time-resolved imaging using microgap conditions. Using an optical filter at 656 nm, we could enhance the spatial distribution of electrons within the interelectrode gap. Intense and multiple space charges are evidenced.

Ignition of nanosecond discharge in liquids: cavitation bubble, bushy and filamentary discharge

Three scenarios can be observed in polar dielectrics (deionized water and ethanol) under a nanosecond positive HV pulse depending on HV pulse amplitude. Two discharge modes (‘bush-like’ and ‘tree-like’) and cavitation mode are characterized by different morphology, propagation velocity and initial pressure. Dynamics of cavitation mode is analysed in terms of Rayleigh model. Initial cavity size in deionized water, ethanol and n-pentane agrees with the predictions of numerical simulations performed by other authors.