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This paper presents an experimental study of an underwater pulsed plasma discharge in pin-to-pin electrode configuration. Time resolved refractive index-based techniques and electrical measurements have been performed in order to study the pre-breakdown and breakdown phenomena in water. A single high voltage pulse with amplitude of a dozen of kV and duration of [0.1-1] ms is applied between two 100 µm diameter platinum tips separated by 2 mm. This novel experimental work reports that different cases of electrical discharge in water occurs for a unique set of experimental conditions such as (i) bush-like channels from the cathode that do not span the electrode gap, (ii) bush-like channels from the cathode leading to breakdown and (iii) filamentary structures from the anode leading to a stronger breakdown. Two breakdown mechanisms, anode and cathode regimes, have been clearly identified and related to the two principal schools of thoughts to explain discharge propagation in liquid.

One of the best ways to increase the diamond growth rate is to couple high microwave power to the plasma. Indeed, increasing the power density leads to increase gas temperature the atomic hydrogen density in the plasma bulk, and to produce more hydrogen and methyl at the diamond surface. Experimental and numerical approaches were used to study the microwave plasma under high power densities conditions. Gas temperature was measured by optical emission spectroscopy and H-atom density using actinometry. CH3-radical density was obtained using a 1D model that describes temperatures and plasma composition from the substrate to the top of the reactor. The results show that gas temperature in the plasma bulk, atomic hydrogen, and methyl densities at the diamond surface highly increase with the power density. As a consequence, measurements have shown that diamond growth rate also increases. At very high power density, we measured a growth rate of 40 μm/h with an H-atom density of 5 × 1017 cm−3 which corresponds to a H2 dissociation rate higher than 50%. Finally, we have shown that the growth rate can be framed between a lower and an upper limit as a function depending only on the maximum of H-atom density measured or calculated in the plasma bulk. The results also demonstrated that increasing fresh CH4 by an appropriate injection into the boundary layer is a potential way to increase the diamond growth rates.