Optically pumped and controlled electric discharges

The paper presents experimental and theoretical results on enhancing ionization in nitrogen-oxygen plasmas by laser-induced vibrational excitation of molecules. The vibrational pumping can be done by either resonant linear infrared absorption, e.g. with CO additive pumped by a CO laser, with subsequent energy transfer to nitrogen and oxygen, or by stimulated Raman pumping of nitrogen or oxygen with a two-color laser beam. Both C.W. and pulsed pumping can be accomplished. The principal mechanisms of ionization enhancement in vibrationally excited gases are: ionization in collisions of highly excited molecules, acceleration of electron detachment from negative ions, enhancement of the high-energy tail of electron energy distribution due to superelastic collisions, and increase in Frank-Condon factors. Unconditionally stable volumetric plasmas can be sustained by either laser pumping alone, or by combining laser pumping with R.F. or electron beam plasma generation. Pulsed lasers are experimentally demonstrated to be capable of initiation, guiding, and control of both d.c. and microwave spark or streamer discharges. The microwave streamer can grow at an angle to the electric field, following the path defined by the short laser pulse.