A review is presented of the issues associated with the generation of large-volume, high-pressure, nonequilibrium plasmas, as well as the approaches that have been developed for generating these plasmas using electrical discharges in gases. The various instabilities that have been overcome to obtained these plasmas as well as the techniques for quenching them are also reviewed. Last, recent efforts to obtain atmospheric pressure plasmas are discussed with particular emphasis on the capillary plasma electrode discharge, which we have used to obtain high (electron) density nonequilibrium plasmas.

Generation of large-volume, atmospheric-pressure, nonequilibrium plasmas

This paper presents a complete study of the temporal and spatial characteristics of the preionization of a XeCl rare-gas halide laser. The detailed study was made possible using the technique of laser-induced preionization which utilizes the uv radiation from a KrF laser to preionize a second rare-gas halide laser. In addition to the preionization study, high spatial and temporal resolution framing camera photographs have been used to investigate the growth of discharge instabilities which can lead to the premature termination of the XeCl optical pulse. The roles played by HCl, Xe, the buffer gas as well as the discharge energy loading in the development of discharge instabilities have been determined experimentally.

Preionization and discharge stability study of long optical pulse duration UV-preionized XeCl lasers

Excimer formation in high-pressure microhollow cathode discharge plasmas in helium initiated by low-energy electron collisions

A one‐dimensional longitudinal model of a XeCl discharge including the cathode region, the plasma, and the external circuit has been used for conditions close to experiments for 50–100 ns laser pulse durations and electron power deposition in the MW/cm3 range in a 300 cm3 chamber. This model provides the space and time variations of the electric field, electron and positive‐ion densities in the cathode region, as well as the time variations of the charged‐particle densities and excited species concentrations in the plasma obtained with a simplified kinetic model. Results show that under normal conditions of operation the cathode electric field can reach values as high as several 106 V/cm. The influence of photoemission and secondary emission due to ion impact on the cathode is discussed. A transition from capacitive to resistive behavior of the sheath is seen to occur rapidly due to the multiplication of cathode‐emitted electrons in the large sheath electric field, leading to a fast increase in the ion co...

Cathode sheath formation in a discharge-sustained XeCl laser

An investigation was undertaken of the electrical, gasdynamic, and radiative properties of surface discharges intended as intense optical pump sources for lasers. Large‐area (≂20–300 cm2), short‐pulse (∼1 μs) planar surface discharges were produced across various ceramic and polymer substrates and operated in all of the rare gases, N2, and air at absolute pressures up to 4 atm. Parametric studies of the electrical circuitry and discharge dimensions demonstrate the importance of proper source‐driver impedance matching to short‐pulse operation and efficient electrical‐to‐optical energy conversion. Characteristic times for establishment of a conductive discharge gap in the heavy rare gases are consistent with breakdown by the single‐stroke leader mechanism. The electrical strength of the gap following discharge remains low for a time which is much greater than the current duration. The delay in recovery is believed to be controlled by continued substrate ablation occurring after the discharge rather than to ...

Electrical, gasdynamic, and radiative properties of planar surface discharges

Optimization of the pumping regime and of the resonator parameters, as well as the correct choice of easily ionizable additives to the active mixture of a CO2 laser excited by an SSVD ensured a specific output energy of 51 J/l and a conversion efficiency of electrical into optical energy of 22 % in an active medium volume of 16 l.

High-Energy Electric-Discharge CO 2 Laser with Easily Ionizable Substances Added to the Mixture

The basic principles that underlie the initiation of a volume self-sustained discharge, which is to be used for pumping CO2 and N2O lasers, are discussed. The experimental results for discharge stability, optical homogeneity, and scalability of the laser active medium are reported. Characteristics of CO2 lasers with apertures of up to 70 cm and active volumes of up to 400 L, as well as of N2O lasers with apertures of up to 20 cm and active volumes of up to 60 L, are presented.

High-power molecular lasers pumped by a volume self-sustained discharge

An investigation was made of the influence of the voltage risetime on the characteristics of a self-sustained volume discharge in CO2–N2–He mixtures. When the region near the cathode was subjected to intense and prolonged ultraviolet irradiation, the use of a pumping regime with a deliberately prolonged voltage risetime made it possible to obtain a volume discharge in an electrode system having a highly inhomogeneous electric field at its edges, and to raise the CO2 laser efficiency.

Formation of a self-sustained volume discharge with intense ultraviolet irradiation of the cathode region

An investigation was made of the influence of readily ionized tripropylamine (TPA) admixtures on the discharge and optical characteristics of a CO2 laser. It was established that ionization of TPA via the metastable electronic states of nitrogen in the Penning process is the main mechanism responsible for the decrease in E/p in a self-sustained discharge. Tripropylamine admixtures result in efficient depopulation of the upper active level. The rate constant for deactivation of the 001 level of CO2 by TPA molecules at low energy inputs was estimated from an analysis of the temporal behavior of the gain. As a result of the decrease in E/p in the discharge, induced by the TPA admixtures, the helium concentration in the mixture could be reduced to 30–40% as compared with 50-60% in discharges without organic admixtures with similar pump sources. A small-signal gain of 5%/cm was obtained in a CO2:N2:He = 1:2:2 mixture at an energy input of 270 Jliter–1atm–1.

Carbon dioxide laser with an active medium containing tripropylamine

An investigation was made of the parameters of an N2O laser pumped by a self-sustained volume discharge with the aim of improving the output characteristics of this laser. The discharge was ignited as a result of a preliminary filling of the discharge gap with electrons and preionization of the gas in the gap by a low-current beam of accelerated electrons. In this way it was possible to reach an output energy up to 465 J, a specific output energy up to 16.4 J liter − 1 atm − 1, and an efficiency of 6.7%, all of which exceeded the earlier values reported for N2O lasers pumped by a self-sustained volume discharge.

V A Yamshchikov

Papers

High-Energy Electric-Discharge CO 2 Laser with Easily Ionizable Substances Added to the Mixture

High-power molecular lasers pumped by a volume self-sustained discharge

Formation of a self-sustained volume discharge with intense ultraviolet irradiation of the cathode region

Carbon dioxide laser with an active medium containing tripropylamine

N2O laser pumped by a self-sustained volume discharge