Scientists from the P.N. Lebedev Physical Institute of the Russian Academy of Sciences, MIPT, and Moscow State University have for the first time recorded a sharp transition boundary between two mechanisms of plasma generation in an electrical discharge. The results of the study, supported by a grant from the Russian Science Foundation, were published in the journal Applied Physics Letters.
The experiments used a copper cathode 100 micrometers thick and a distance between electrodes of about 2 mm. The process was filmed using picosecond laser visualization – 18 frames per pulse with nanosecond resolution. This allowed for a detailed view of what happens at the cathode surface in the first moments after breakdown.
At pressures above 100 mm Hg, the discharge proceeds according to an "explosive" scenario: microscopic irregularities on the cathode evaporate under the action of the field, forming a cloud of metallic plasma. The electron density in it is 10 times higher than can be obtained with complete ionization of air. The entire process takes less than a nanosecond.
If the pressure drops below the threshold, the explosive mechanism disappears – the discharge is maintained only by gas ionization. As the researchers explained, this is due to the weakening of the local field at the cathode surface due to the expansion of the cathode layer.
The discovery makes it possible to control plasma formation processes in pulsed power engineering and in the synthesis of metallic nanoparticles. Depending on the pressure, it is possible to either enhance cathode destruction and obtain more nanoparticles, or minimize electrode wear, increasing the reliability of high-voltage devices. The scientists' plans include studying the properties of the resulting nanoparticles and scaling the effect to multi-tip systems.
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