https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5715769/

Significance
Plasmas at atmospheric pressure conditions are ubiquitous, in natural form, such as the familiar lightning, or produced through artificially created electromagnetic or electrostatic fields for industrial and scientific applications. One distinctive feature of these cold-type plasmas is their lack of a topologically defined shape, concurrent with spatial unsteadiness and nonuniformity. Here, we report the observation of a coherent and stable toroidal plasma that spontaneously forms under extreme hydrodynamic shear, without external electromagnetic action. The confined and chamberless nature of this plasmoid has potential implications for the investigation of plasma–matter interactions, in the development of plasma-based deposition techniques for the microelectronics industry, in the emerging field of plasma medicine, or as a model for energy-storing self-maintained plasmoids.
Keywords: water jet, hydrodynamic shear, tribo-electricity, luminescence, toroidal plasmoid
Abstract
Saint Elmo’s fire and lightning are two known forms of naturally occurring atmospheric pressure plasmas. As a technology, nonthermal plasmas are induced from artificially created electromagnetic or electrostatic fields. Here we report the observation of arguably a unique case of a naturally formed such plasma, created in air at room temperature without external electromagnetic action, by impinging a high-speed microjet of deionized water on a dielectric solid surface. We demonstrate that tribo-electrification from extreme and focused hydrodynamic shear is the driving mechanism for the generation of energetic free electrons. Air ionization results in a plasma that, unlike the general family, is topologically well defined in the form of a coherent toroidal structure. Possibly confined through its self-induced electromagnetic field, this plasmoid is shown to emit strong luminescence and discrete-frequency radio waves. Our experimental study suggests the discovery of a unique platform t…