Formation of UV-Radiating Strongly Non-Equilibrium Plasma with Multiply Charged Ions in the Expanding High-Pressure Gas Jet

摘要

In the present paper we target the physics of transition from subsonic to supersonic regimes in the expanding plasma flow. Originated form ECR ion sources, this problem is generalized to describe the stationary highly-localized plasma discharge formed under resonant microwave heating in a gas jet freely expanding after a high-pressure nozzle. The peculiar feature of such a discharge is formation of a strongly non-equilibrium plasma flow with multiply charged ions and the electron temperature much greater than the temperature of ions. We study possibilities for conjugating the slow movement of initially neutral dense gas and the supersonic flow of accelerated plasma, or, equivalently, a smooth transition from ion-acoustic barrier in the expanding flow with a varying charge state distribution. The proposed model is used to understand and optimize the recent experiments performed at IAP, Nizhny Novgorod, and aimed at demonstration of a new possibility of development of a point source of extreme ultraviolet radiation for the projective lithography. In these experiments, a line emission of multiply charged ions of noble gases has been investigated in the non-equilibrium discharge supported by high-power sub-millimeter waves in the expanding gas flow.