Survey of Advanced Gasdynamic Laser Concepts

Abstract

Introduction E is transported between the thermal heat source and the optical cavity of a gasdynamic laser (GDL) in the vibrational modes of the nitrogen molecule. Pure nitrogen exhibits relatively long vibrational relaxation times, making it well suited to the formation of an inversion during a rapid gasdynamic expansion. The carbon dioxide and catalyst that are necessary to allow efficient extraction of this stored energy in an optical cavity act as deactivants during the gasdynamic expansion, causing significant loss of stored vibrational energy. Management of this energy loss has required the use of very rapid expansions in nozzles with very small throats. The overall efficiency of GDLs is the product of the fraction of the invested thermal energy stored in the nitrogen vibrational modes, the freezing efficiency of the nozzle, and the extraction efficiency of the optical cavity. The stored energy fraction increases with stagnation temperature; however, the freezing efficiency falls with increased stagnation temperature. Consequently, an optimum stagnation temperature occurs near 2000 K for the conventional premixed GDL. The specific energy that can be stored in the vibrational levels of nitrogen as the temperature is raised is shown in Fig. 1. The conventional premixed GDL falls far short of attaining the full potential of the gasdynamic laser concept. Nozzle freezing efficiency increases as the carbon dioxide and catalyst concentrations are lowered. However, a certain concentration of both is required to allow good extraction efficiency. A small throat size provides a rapid rate of expansion which increases the freezing efficiency. However, the viscous losses increase with the lower Reynolds numbers associated with smaller nozzles, limiting the extraction efficiency in the cavity. The development of the conventional premixed GDL has been a process of optimization within these several mutually exclusive constraints, resulting in an overall efficiency of 0.1 -1.0%. Early in the development of the GDL it was suggested that the mixing of the carbon dioxide and catalyst after expansion to low static temperature and pressure would reduce the collisional loss of stored vibrational energy and provide greater flexibility of operating conditions. Vibrational energy can thus be frozen into the pure nitrogen donor gas well upstream of the nozzle throat at levels near the stagnation conditions. Calculations for stagnation conditions of 2000 K, 10 atm show a 1% loss of vibrational energy in the expansion of pure nitrogen as compared to the 40-50% loss that occurs principally in the throat region of a conventional nitrogen/carbon dioxide/catalyst GDL expansion. Mixing of the excited nitrogen with cold carbon dioxide is accompanied by near resonant collisional transfer of the vibrational energy to the lasing gas molecules, resulting in a population inversion with respect to the two laser levels. The thermal excitation of the nitrogen with subsequent turbulent mixing of the carbon dioxide and catalyst allows an increase in stagnation temperature to above 4000 K before molecular dissociation becomes important. The stored vibrational energy can be raised by an order of magnitude to the premixed case as seen in Fig. 1. The kinetic constraints on vibrational freezing are not as critical without the carbon dioxide and catalyst in the primary gas stream, allowing the use of larger nozzle throats resulting in lower viscous losses. The flexibility of operation with various mixtures of nitrogen, carbon dioxide, and catalyst permits more efficient extraction of power in the cavity region. These characteristics enable the mixing GDL to attain 5-10 times the efficiency of the conventional premixed GDL. Overall efficiencies of as high as 6% have been reported for the mixing GDL. The origins of the mixing gasdynamic laser can be found in the early research of Patel concerning energy transfer from nitrogen excited in a low-pressure electric discharge to carbon dioxide mixed subsonically 20 cm away from the discharge. His subsonic mixing laser was 10 times as efficient as the contemporary conventional CO2 discharge laser. Several other mixing lasers excited by electric discharges were subsequently developed' using both subsonic and supersonic