Atmospheric air and nitrogen DC glow discharges with thermionic cathodes and swirl flow

Atmospheric air and nitrogen glow discharges have been studied in our laboratory over the past few years with an overall goal to produce nonequilibrium plasmas with high electron number densities (at least 10 ¹² cm^-3) and relatively low gas temperatures (about 2000 K), as well as to meet the challenges of low power requirements and large plasma volumes. We now introduce a novel approach to produce highly ionized atmospheric pressure air and nitrogen plasmas in DC glow discharges. We employ thermionic cathodes (LaCrO₃, LaB₆ and Mo) and place the discharges in tubes with swirl gas flow injection. As a result, an increase of the electron density by 1-2 orders of magnitude was achieved (10 ¹³-10¹⁴ cm^-3), together with higher current densities and lower electric fields. The swirl flow confines the discharge, increasing its current and electron densities, enhancing its stability, and enabling it to extend to as long as 10 cm. A new falling region in the air discharge E-j and E/N-j characteristics was revealed. Computational analysis helps to understand the stabilizing effect of the swirl flow. DC discharges with thermionic cathodes combined with the swirl flow represent a promising way for producing scaled-up highly ionized atmospheric air or nitrogen plasmas.