Causes of thermal instability in externally sustained molecular discharges

Abstract

In this investigation the basic causes of glow collapse and/or glow-to-arc transition have been examined for convection-cooled, externally sustained molecular-discharge conditions. The results of this analysis show that externally sustained molecular discharges of the type used in C${\mathrm{O}}_2$ lasers are inherently unstable. In particular, it is shown that there are two distinct mechanisms which can provide the driving force for thermal instability: (i) electron vibrational excitation leading to a surge in vibrational temperature; and (ii) vibrational reservoir collapse accompanied by a decrease in vibrational temperature. The plasma conditions and basic collisional processes contributing to the occurrence of these thermal instabilities are discussed in detail, with particular emphasis directed toward charged-particle production and loss mechanisms.