In this paper, we present the results from the computational study of the two competing mechanisms of precursor electrons production—conventional photoionization and preionization by fast electrons from the cathode. To discriminate between the two mechanisms we simulate a negative streamer evolution under conditions when photoionization is zero/non-zero and/or fast electrons are present/absent. In the absence of all factors producing the precursor electrons the streamer eventually stalls. We show that the effect of fast electrons on streamer evolution is similar to that of the photoionization but it is more stochastic by nature. The domination of one or another of the mechanisms considered depends on the pulse duration and voltage amplitude (other conditions being equal).

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In this paper, results from the computational investigation of active species production in an argon microwave plasma torch are presented. We address the main paths of species creation and quenching and focus on their spatial distributions. We show that with the flowing Ar/O2 the radical production is relatively small. The operation of the argon torch can be more effective with an Ar/O2/N2 or Ar/O2/N2/H2O mixture as a feed gas. It was also shown that for high applied voltages the gas temperature inside the microwave torch can be high enough to significantly disturb the flow field. This disturbance can induce depletion and rarefaction of some species near the electrode and influence their production.

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In this paper, we investigate pre-breakdown and breakdown phenomena in gas-liquid systems. Cavitation void formation and breakdown in bubbles immersed in liquids are studied numerically, while complete breakdown of bubbled water is studied in experiments. It is shown that taking into account the dependence of water dielectric constant on electric field strength plays the same important role for cavitation void appearance under the action of electrostriction forces as the voltage rise time. It is also shown that the initial stage of breakdown in deformed bubbles immersed in liquid strongly depends on spatial orientation of the bubbles relative to the external electric field. The effect of immersed microbubbles, distributed in bulk water, on breakdown time and voltage is studied experimentally. At the breakdown voltage, the slow 'thermal' mechanism is changed by the fast 'streamer-leader' showing a decrease in breakdown time by two orders of magnitude by introducing microbubbles (0.1% of volumetric gas content) into the water. In addition, the plasma channel is found to pass between nearby microbubbles, exhibiting some 'guidance' effect.